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Improved Acetabular
Cementing Techniques
Masters of Applied Science (Orthopaedics)
Queensland University of Technology
Faculty of Built Environment and Engineering
BN71
Author: Dr Bjorn N Smith BAppSc (PHTY), MBBS.
Principal Supervisor: Prof Ross Crawford.
Associate Supervisors: Dr Clive Lee (External),
Mr John Timperley (External).
1
Abstract: The most common cause for revision total hip replacement surgey is aseptic loosening of
the acetabular component. This thesis explores the effect of three techniques to improve
the depth and quality of cemented acetabular component fixation in primary total hip
replacement. This may have beneficial effects on the longevity of cemented acetabular
components and reduce the rate of revision surgery for aseptic loosening.
Aims: 1. Determine the effect of the rim cutter on cement pressure during cup insertion.
2. Examine the effect of the rim cutter on cement penetration distance. 3. Evaluate the
effect of bone grafting of the acetabular notch. 4. Determine the effect of iliac suction
during cement pressurisation. 5. Compare the behaviour of bone cement with Play
Dough®.
Materials and Methods: 1. Sawbones hemi pelvis models were fitted with pressure
transducers at the rim and apex of the acetabulum. Peak pressure was measured upon
insertion of cups with different flange sizes and when the acetabulum was prepared with
the rim cutter. 2. Foam cavities were used to measure the depth of cement penetration
when the same cups and rim cutter were used. 3. Hemi pelvis models were modified to
simulate bone grafting of the acetabular notch. Again, pressure sensors were mounted at
the apex and rim of the acetabulum. Intra-acetabular cement pressure was compared with
native acetabulae. 4. A back bleeding model of the acetabulum was fitted with a suction
catheter. The effect on cement penetration into cancellous bone was measured compared
with no suction. 5. Play Dough® pressurisation and penetration into hemi pelvises and
foam was compared to bone cement.
Results: 1. Significant increase in peak apex and rim pressures when flanged cup inserted
into an acetabulum prepared with the rim cutter compared with both flanged and
unflanged cups alone. 2. Significant increase in cement penetration at the rim of the
acetabulum when rim cutter used and flanged cup inserted when compared with flanged
and unflanged cups alone. 3. Significant increase in intra-acetabular pressure when
cement pressurised in presence of simulated acetabular notch bone grafting compared
with normal acetabulae. 4. Significant increase in cement penetration distance when
2
suction used compared with no suction. 5. Significant differences in the flow
characteristics between bone cement and Play Dough®.
Conclusion: The authors recommend preparation of the acetabular rim with the rim cutter
and bone grafting of the acetabular notch to improve the depth and uniformity of the
cement mantle in cemented primary THA. Play Dough® at room temperature is not a
suitable substitute for bone cement in in-vitro cementing studies.
Key words: hip, acetabulum, cement, flanged, cup, rim cutter, suction, pressurisation,
bone graft, acetabular notch, arthroplasty, penetration, Play Dough®.
3
Contents:
Abstract………………………………………………………………..Page 1-2.
List of Diagrams and Photographs…………………………………….Page 4-5.
Statement of Authorship……………………………………………….Page 6.
Acknowledgements…………………………………………………....Page 7.
Prologue……………………………………………………………….Page 8.
Chapter One: Background……………………………………………..Page 9-19.
Chapter Two: Materials and Methods…………………………………Page 20-33.
Chapter Three: Results…………………………………………………Page 34-60.
Chapter Four: Discussion………………………………………………Page 61-83.
Chapter Five: Conclusion……………………………………………...Page 84-85.
Disclosure……………………………………………………………...Page 86.
Appendix 1……………………………………………………………..Page 87-88.
Bibliography……………………………………………………………Page 89-93.
4
List of Diagrams and Photographs:
Photo 2.1: Sawbones hemipelvis model. Photo 2.2: Reamed hemipelvis model mounted in vice. Photo 2.3: Holes drilled in apex and rim of acetabulum. Photo 2.4: Holes drilled at apex and rim of acetabulum. Photo 2.5: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.6: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.7: Pressure transducers screwed into drill holes at apex and rim of acetabulum. Photo 2.8: Vinyl glove placed in acetabulum prior to cement insertion. Photo 2.9: Simplex bone cement. Photo 2.10: Load sensor mounted in shaft of acetabular pressuriser. Photo 2.11: Laptop computer and data acquisition unit. Photo 2.12: Patch of foam adhered to acetabular notch. Photo 2.13: Exeter Contemporary Cup prior to trimming. Photo 2.14: Exeter Contemporary Cup prior to trimming. Photo 2.15: Unflanged Exeter contemporary cup used. Photo 2.16: Flanged Exeter contemporary cup used. Photo 2.17: The Rim Cutter. Photo 2.18: The Rim Cutter assembled. Photo 2.19: Hemipelvis model reamed and prepared by using the rim cutter. Photo 2.20: Closer view of the acetabulum prepared with the rim cutter. Photo 2.21: Exeter contemporary cup with the flange trimmed to the second groove. Photo 2.22: Acetabular foam cavity used for cement penetration testing. Photo 2.23: Computer generated image of foam cavities. Photo 2.24: Schematic drawing of the foam cavities used to simulate acetabulae. Photo 2.25: Foam cavity mounted in jig. Photo 2.26: Pressurisation of the foam acetabulum. Photo 2.27: Pressurisation of the cup in the foam cavity. Photo 2.28: Closer view of cup pressurisation. Photo 2.29: Cavity prepared by rim cutter. Photo 2.30: Side view of cavity prepared by rim cutter. Photo 2.31: ‘Open cell’ foam used in the third part of the study. Photo 2.32: Closer view of ‘open cell’ foam. Photo 2.33: Foam pieces placed in metal box. Photo 2.34: Side view of foam in metal box. Photo 2.35: Pressurised cement in foam, with the various points labelled A to D. Photo 2.36: Side view of foam piece after cement pressurisation. Photo 2.37: Cement penetration measured by drawing a line between opposite corners of
the foam. Photo 2.38: The Eschmann TJ 240H portable suction machine. Photo 2.39: Motor oil fills the inverted 60mL syringe, suspended 77 cm above the metal
box.
5
Photo 2.40: Motor oil fills the metal box and the suction catheter can be seen at point C. Photo 2.41: Pressurisation of the cement whilst the foam bleeds oil and suction catheter
aspirates fluid from the metal box. Photo 2.42: Manual pressurisation of the cement in the metal box. Photo 3.1: Pressurisation of cement in the native acetabulum. Photo 3.2: Unflanged cup has been inserted into the hemipelvis model. Photo 3.3: Holes seen in extracted cement mantle. Photo 3.4: No holes seen in extracted cement mantle of Rim Cutter group. Photo 3.5: Anterior view of Rim Cutter cement mantle after cup removed. Photo 3.6: Cementing of the unflanged cup into the acetabular foam cavity. Photo 3.7: Cement extrudes out of the base of the cavity in the rim cutter tests. Photo 3.8: Closer view of cement extruding from holes. Table 3.1: Pressures recorded in the normal acetabulum. Table 3.2: Pressures recorded using Play Dough® in normal acetabulum. Table 3.3: Pressures recorded using cement in a ‘grafted’ acetabulum. Table 3.4: Pressure recordings in unflanged cup group. Table 3.5: Pressures recorded in the flanged cup group. Table 3.6: Pressures recorded in Rim Cutter group. Table 3.7: Penetration of pressurised bone cement (in millimetres). Table 3.8: Measurements of Play Dough penetration in each trial (millimetres). Table 3.9: Mean force exerted on pressuriser (N). Table 3.10: Penetration of bone cement with insertion of unflanged cup (millimetres). Table 3.11: Penetration of bone cement into cavities with a flanged cup inserted (mm). Table 3.12: Penetration of bone cement in cavities prepared by rim cutter (millimetres). Table 3.13: Force exerted on pressuriser in different phases of penetration testing. Table 3.14: Maximal force required to seat each cup type. Table 3.15: Cement penetration distance at rim of dry foam (millimetres). Table 3.16: Cement penetration distance in dry foam with suction at point ‘C’
(millimetres). Table 3.17: Cement penetration distance into wet foam with no suction (millimetres). Table 3.18: Cement penetration distance with back bleeding and suction at point ‘C’
(millimetres). Table 3.19: Forces exerted on cement in foam under various experimental conditions. Table 3.20: Mean cement penetration at various points (in mm).
6
Statement of Authorship:
The work contained in this thesis has not been previously submitted to meet requirements
for an award at this or any other higher education institution. To the best of my
knowledge and belief, the thesis contains no material previously published or written by
another person except where due reference is made.
Signature
Date
7
Acknowledgements: The author would like to acknowledge the principal supervisors of this project: Mr John
Timperley, Professor Ross Crawford and Dr Clive Lee for their invaluable assistance,
guidance and support in this project. Without their input this study would not have been
possible. Thanks are also extended to the Engineering Department of the University of
Exeter whose generous assistance with the design and loan of custom built testing
equipment is much appreciated. Gratitude is also expressed to the management of the
Princess Elizabeth Orthopaedic Centre for the use of their laboratory facilities. Assistance
with statistical methods was generously provided by Ms Sarah Whitehouse from the
Queensland University of Technology to whom I am most grateful. The suction testing in
part three required an assistant to manage the flow of oil and this role was carried out by
Ms Fiona Graham. The author thanks her for her assistance and support throughout the
whole study. A special mention must go to Emeritus Professor Robin Ling who is a
fantastic role model and an inspiration to the author. Finally thanks must go to the Stryker
Corporation without whose financial contribution towards the testing equipment and
materials this study would not have been possible.
8
“If a certain cup can, at the time of cup insertion, produce higher cement intrusion
pressure and depth than other cups, and if a cup, when completely inserted, is positioned
concentrically within the reamed acetabulum with a uniform cement thickness without
‘bottoming out’, then this feature would be highly desirable for lasting fixation of the
acetabular component.”
Oh, Sander and Trahearne (1985)
9
Chapter One: Background: The aims of the studies in this thesis are fivefold: (1) To determine the effect of the rim
cutter on cement pressure during cup insertion, (2) to examine the effect of the rim cutter
on cement penetration distance, (3) to evaluate the effect of bone grafting of the
acetabular notch, (4) To determine the effect of iliac suction during cement
pressurisation, (5) To compare the behaviour of bone cement with Play Dough®. The
hypotheses being tested is that each technique will increase the cement pressure or
penetration distance in vitro. Play Dough® is hypothesized to behave similarly to bone
cement. A graphical representation of the thesis plan is shown on page 19.
The history of cemented acetabular components is an interesting one. This chapter will
endeavour to explain the history of cemented total hip replacement, the advances that
have been made in cementing technique, the current problems with acetabular
pressurisation and discuss the various experimental techniques, which aim to improve the
long term outcome of cemented sockets.
History:
The first total hip replacement was performed in 1961 by Sir John Charnley in
Wrightington, UK. A number of different materials for the cup were trialled by Sir
Charnley including Teflon®. After much consideration and experimentation the
components he used consisted of a stainless steel femoral stem, and femoral head, which
articulated with a high molecular weight polyethylene cup (Charnley, 1995). Sir Charnley
used both press-fit and cemented cups in his initial series of 582 patients. The cemented
components were held in place with polymethylmethacrylate bone cement, otherwise
known as PMMA. The techniques for cementing of the components used by Sir Charnley
became known as first generation cementing techniques. On the femoral side this
involved broaching of the femoral canal, bowl mixing of the cement, finger packing of
the cement into the femur and inserting of one of a limited selection of different sized and
shaped components. On the acetabular side the subchondral bone was reamed, cement
finger packed into the cavity and the cup inserted. Since that time, the technique of
inserting and fixing the femoral and acetabular components have advanced significantly.
10
These changes have evolved to combat a variety of problems, which inevitably arise
when a completely new procedure is devised. Initially the results were very good, with a
high proportion of patients (>90%) experiencing relief of their debilitating arthritic
symptoms (Charnley, 1995). As the practice of cemented total hip replacement spread
over the years however, a number of the components appeared on radiographic review to
be loosening, and a (smaller) proportion of patients who developed symptoms related to
this required revision surgery. As the period of time since implantation of total hip
replacements increased, the problem of loosening of either of the components became
more pronounced. While the incidence of cemented femoral component loosening has
reduced significantly over the last twenty years, the rate of acetabular component
loosening has not shown such encouraging progress (Mulroy and Harris 1990). In fact,
countless authors over the last twenty years have stated that the most common reason for
revision total hip replacement is loosening of the acetabular component (Chandler et. al.
1981, Ranawat et. al. 1984, Wroblewski 1986, Kavanagh et. al. 1989, Schulte et. al.
1993, Ranawat et. al. 1995, Mulroy et. al. 1995, Sochart and Porter 1997, Wroblewski et.
al. 1999 and Callaghan et. al. 2000).
Each year there are more than one million hip replacement operations performed across
the globe (Flivik, 2005). According to the Australian Orthopaedic Association National
Joint Replacement Registry Annual Report of 2006, there were 20,683 total hip
replacements for the financial year 2004 – 2005, a number that is increasing each year.
Of these, only 9.5% (1965) used cemented acetabular fixation, a number that is gradually
decreasing each year. This indicates a trend in Australia away from cemented acetabular
fixation in favour of newer technologies such as cementless and hybrid fixation systems
and hip resurfacing. In other countries such as Sweden greater than 90% of the 10,000
plus hip replacements performed each year utilise cemented acetabular fixation (Flivik,
2005). Furthermore, because of its excellent long term results, cemented fixation systems
provide a level of reliability that at least matches if not exceeds that of cementless and
hybrid fixation. The Australian Joint Replacement Registry reports that in the 75 years
plus age group cemented total hip replacement has the lowest rate of revision at three
11
years of all fixation systems (1.52%). Of the 1965 cemented cups implanted in 2004-05
in Australia, just under half (46%) were Exeter Contemporary cups.
Comparing the last five years of data from the Australian Joint Replacement Registry
shows that revision surgery accounts for 12% of all total hip replacement operations. In
total 9194 cases out of 19257 revision operations (47.7%) were performed because of
loosening of a prosthesis. Of all major hip revision operations performed acetabular
revisions far outnumber femoral revisions (37.2% vs. 21.2%). This further highlights the
need for improved acetabular fixation methods. Countless authors agree that the key
determinant in ensuring adequate, long lasting cup fixation in cemented total hip
arthroplasty is the quality of the cementing technique (Flivik et. al. 2004).
Strength of the Bone-Cement Interface:
The work of Krause et. al. in 1982 showed that bone cement fixation is dependent on the
penetration of bone cement into the cancellous interstices of the bone. They also
demonstrated that cleaning of the bone with high-intensity lavage allows improved
cement penetration, which thereby increased tensile and shear strength of the bone-
cement interface. These effects of improved cement penetration have been supported by a
number of other authors (Halawa et. al. 1978, Panjabi et. al. 1986, Macdonald et. al.
1993, Juliusson et. al. 1994). However, Majkowski et. al. in 1994 suggested that
penetration beyond three millimetres does not enhance the strength of the bone-cement
interface. A later study of the effects of bone-porosity and cement penetration on the
bone-cement interface was conducted by Graham et. al. in 2003. That study concluded
that the strength of the bone cement interface is increased significantly by increased bone
porosity and consequently by increased cement penetration. Kuivila et. al. (1989) also
showed that increased cement penetration leads to increased tensile strength at the bone-
cement interface, a finding that was also reported by Eftekhar and Nercessian (1988).
Causes of Prosthesis Loosening:
Anderson et. al. postulated in 1972 that the causes of prosthesis loosening in hip
replacements could be due to a combination of factors including micro motion at the
12
bone-cement interface, thermal bone damage at the time of cement polymerisation or
chemical factors. In 1978 and 1979 a number of studies (Beckenbaugh and Ilstrup 1978,
MacBeath and Foltz 1979 and Gruen et. al. 1979) quoted the rates of prosthesis loosening
seen on radiological examination to be from 15 to 21 per cent after between one and
seven years of followup. Later, Mjoberg (1991 and 1994) suggested that loosening
resulted from early prosthetic instability and that the ‘late’ loosening being reported was
perhaps really just late detection of previously loose components. It has also been
suggested by DeLee and Charnley in 1976 that part of the loosening process may stem
from a layer of blood forming at the bone-cement interface, which could later go on to
form a fibrous tissue membrane. This concept has been supported by a number of other
authors (Lee and Ling 1981, Eftekhar and Narcessian 1988 and Bannister et. al. 1990).
Charnley suggested in 1975 that the accumulation of blood at the bone-cement interface
may be the cause of the radiolucent lines seen on post operative radiographs. Later,
DeLee and Charnley (1976) first classified the radiological appearance of those lucent
lines at the bone cement interface in their famous paper. Freeman et. al. 1982 first
suggested that radiolucency at the bone-cement interface may not be caused by micro-
motion but in fact be related to a biological cause. Later, Schmalzreid et al (1992)
claimed that the cause of aseptic loosening of the cemented acetabular component was in
fact different to that causing the femoral component to loosen. While the femoral
component is thought to loosen secondary to mechanical forces, they proposed that
acetabular loosening was indeed secondary to biologic phenomena. New et. al. (1999)
suggested that perhaps the better results seen recently are secondary to reduced fibrous
tissue between cement mantle and bone, which leads to reduced wear debris behind the
prosthesis.
Advances in Cementing Technique:
PMMA is by nature viscoelastic, which is important when considering the topic of
pressurisation of the acetabulum. The aim of pressurisation is to achieve an adequate
cement mantle with extensive cement interlock in cancellous bone interstices (Ranawat
et. al. 1997). This manifests stability at the bone-cement interface and prevents micro-
motion and subsequent loosening of the prosthesis. Pressurisation will therefore ideally
13
force PMMA far enough into cancellous bone to achieve a strong interlock and then
prevent movement of the cement until it has polymerised. Pressurisation must be capable
of overcoming all additional forces that act either on or within the cement. The work of
Markolf and Amstutz in 1976 investigating the penetration and flow of bone cement was
able to highlight a number of important factors. Firstly, they showed that the penetration
of bone cement into cancellous bone is proportional to the pressure applied to the cement
bolus. They also demonstrated that the majority of the depth of cement penetration
occurred in the first two seconds of pressurisation (76%) and that the depth of penetration
was three times greater when cement was pressurised at four minutes from mixing rather
than the standard six and a half minutes. Another of their findings was that vigorous
finger packing of cement produced very high pressures for a short period of time and
caused greater cement penetration than low pressure sustained for a longer period of time.
Bayne et. al. (1975) showed that higher pressures applied to the cement mantle during
polymerisation may lead to a reduction in cement porosity. The relationship between the
pressure and penetration of bone cement was also studied by Panjabi et. al. in 1983 and
1986. Their research suggested a positive logarithmic relationship between the pressure
applied to the cement and the depth of cement penetration. They also showed that there
was a linear relationship between cement intrusion penetration and bending and axial
stiffness of the bone-cement interface. A similar link between pressure and penetration
has been reported by a number of authors (Convery and Malcolm 1980, Bannister et. al.
1988, Graham et. al. 2003, Flivik et. al. 2004). Oh and Harris in 1982 proposed a cement
fixation system by pressurising cement in the acetabulum for five to ten seconds. This
was followed up by Oh et. al. in 1983 in their study of the effect of pressurising keying
holes in a dry cadaveric acetabulum with multiple short bursts of high pressure, which
yielded a cement penetration depth of 19.2 to 25.5 millimetres. A landmark study was
performed by Benjamin et. al. in 1987 who studied the effects of bleeding on cement
penetration. Their findings showed that water was able to displace low viscosity PMMA
at a pressure of ten centimetres of water. They also found that water was able to displace
PMMA up to seven minutes after mixing of cement. This research suggested a change in
practice to apply pressure onto the cement mantle until such time as its viscosity would
be sufficient to resist the back bleeding pressure of blood. Otherwise, the cement mantle
14
would be displaced both in the time between removing the pressuriser and inserting the
prosthesis and after insertion of the prosthesis if it was not itself pressurised, resulting in
a less adequate bone cement interlock. Majkowski et. al. in 1994 published results
suggesting that pressurisation for thirty seconds is adequate and that additional
pressurisation of cement in the dough stage after that is of no additional value at
physiological bleeding pressures. The simulated bleeding flow rates used in their study
however, were somewhat lower than those used in previous studies. These proposed
techniques were developed further by New et. al. (1999) who suggested that short
duration, high intensity pulses of pressure would be more effective when combined with
a sustained background pressure to resist back bleeding pressure and elastic recoil of the
cement.
Other aspects of cementing technique have been added and refined over the last thirty
years. In 1978, Oh et. al. found that when cementing femoral components, distal plugging
of the femoral canal with PMMA caused increased cement intrusion pressure, deeper
cement penetration and consequently increased tensile strength of the bone-cement
interface. Similar findings were also reported later by Macdonald et. al. (1993). In current
practice plastic or bioabsorbable femoral canal plugs are used. Cleaning of the cancellous
bone bed is known to remove debris and facilitate flow of cement and hence increased
cement penetration. This notion was supported by Krause et. al. (1982). Majkowski et. al.
(1993) examined cement penetration into bovine bone and reported no difference
between continuous lavage and pressurised pulsed lavage. However, jet lavage was
shown to be more effective at facilitating cement penetration than syringe lavage by
Breusch et. al. (2000) in their study of cadaveric femora, and also in the tibia by Dorr et.
al. (1984). Microcrystalline collagen was used as a topical haemostatic agent, but it was
later shown to significantly reduce the shear strength of the cement-bone interface by
Lange in 1979. Later, hydrogen peroxide was shown to reduce bleeding from cancellous
bone by Hankin et. al. (1984). This practice has since become common in cementing
techniques for all components. Halawa et. al. (1978) showed that in fact the greatest shear
strength at the cement-bone interface in the femur was achieved with pressurised low
viscosity cement. Noble and Swarts (1983) suggested that low viscosity cement would
15
allow increased cement penetration into cancellous bone. These findings were confirmed
by Macdonald et. al. (1993) who showed that the use of low viscosity cement did indeed
improve cement penetration. However, a study by Mjoberg et. al. (1987) found no
difference in cup loosening rates between low viscosity and high viscosity cement use.
Current practice reflects a compromise between cement of sufficiently low viscosity to
allow adequate cement penetration and cement that is of such a viscosity that it is too
difficult for the surgeon to handle (Dorr et. al. 1984). Hypotensive epidural anaesthesia
has been frequently applied in cases of cemented total hip arthroplasty for over ten years.
Ranawat et. al. (1991) demonstrated that hypotensive epidural anaesthesia allowed deeper
penetration of cement into cancellous bone of the acetabulum on radiographic evidence.
Problems with Pressurisation:
Much attention has been focussed on improving the acetabular cement mantle. The
majority of authors claim that the optimal depth of cement penetration is three to five
millimetres (Huiskes and Sloof 1981, Walker et. al. 1984, Askew et. al. 1984, Eyerer and
Jin 1986, Miller 1990). A greater depth of cement penetration imparts minimal increased
interfacial strength and avoids some of the major problems with a thicker cement mantle
(Dorr et. al. 1984). Cement mantles thicker than ten millimetres may cause increased heat
generation when the cement is polymerising, which can cause bone necrosis, bone loss
and reduction in the strength of the bone cement (Homsy et. al. 1972, Huiskes and Sloof
1981, Sew Hoy et. al. 1983). This concept is contested by Jefferiss et. al. in their 1975
paper in which they claim that bone necrosis is not a consequence of thermal damage
caused by polymerisation of PMMA. The problem with adequate generation of pressure
in the acetabulum stems from two main features. The acetabulum itself is an open
hemisphere, which compared with the relatively closed, cylindrical shape of the femoral
canal makes containment of cement more difficult. This makes it more difficult to
generate the pressures required for adequate cement penetration in the acetabulum. The
second feature is the acetabular notch under the transverse ligament. This results in
inconsistency in the rim of the acetabulum, and hence makes achieving a closed cavity
for pressurisation much more difficult (Flivik et. al. 2004). A number of different shaped
and styled pressurisers have been designed to combat this problem and to date, there is no
16
accepted gold standard design. The main drawback with current techniques in acetabular
cement pressurisation is extrusion of cement underneath the transverse ligament out
through the acetabular notch (Bernoski et. al. 1998). Martin et. al. (2003) support the
notion that the majority of cement extrusion occurs in this region with their report of a
series of post operative radiographs in which the main location of cement extrusion was
anteroinferiorly. Some authors (Oh et. al. 1983, Flivik et. al. 2004) have recommended
sequential pressurisation of keying holes in the acetabulum in order to overcome the high
pressures needed for adequate cement pressurisation. However, Gruen et. al. (1976) and
Saha and Pal (1984) have reported the formation of laminations in the cement mantle
with such pressurisation, which may cause weakening of the tensile and shear strength of
the cement mantle. Flivik et. al. (2004) claim that if the laminations form before 3.5
minutes from mixing of cement, that the polymerisation process is able to bridge the
laminations and hence prevent any reduction in the strength of the cement mantle.
Flanged and unflanged cups:
The problem of containing bone cement in the region of the acetabular notch has been
addressed since 1976 when the flanged socket was first introduced (Shelley and
Wroblewski 1988). The use of this socket was however, complicated by a tendency for it
to be placed in anteversion. Sir Charnley designed the Ogee flanged cup in the early
eighties when cross-linked polyethylene became available, which could be effectively
cast into an asymmetrical shape (Shelley and Wroblewski 1988). Oh et. al. (1985)
showed that cement pressures during cup intrusion are significantly elevated by the
employment of a cup with a continuous flange. This was supported by Shelley and
Wroblewski (1988) who showed that Ogee flanged cups generate three times more
pressure than standard unflanged cups. They also claimed that it is difficult to insert an
unflanged cup concentrically without it ‘bottoming out’. Bernoski et. al. (1998) also
agreed with this notion, by claiming that the use of a cup to generate pressure in the
acetabulum is not possible unless the cup is fitted with a suitably designed flange.
Beverland et. al. (1993) showed that inadequately sized and prepared flanges can actually
impair the pressurisation of cement in the acetabulum. In 2004, Parsch et. al. reported
that the inserting of Ogee flanged cups produced a higher intra-acetabular pressure than
17
unflanged cups, but did not result in significantly increased cement penetration. Similar
findings were also reported by Flivik et. al. (2004).
Bone grafting of the acetabular notch:
Bone grafting has had a role in cemented total hip arthroplasty for many years
(McCollum et. al. 1980). It has been utilised in patients suffering from developmental
dysplasia of the hip, where grafting allows a deeper and more stable socket to be formed
into which the cup can be cemented. Bone grafting in the acetabulum has also been used
in revision hip arthroplasty in cases of extensive bone loss in the form of impaction
grafting and primary hip arthroplasty in the presence of acetabular protrusio, where it is
essential to stabilise the medial wall of the acetabular cavity. McCollum et. al. reported in
1980 that in the case of acetabular protrusio bone grafts placed in the medial wall of the
acetabulum under cemented sockets had incorporated into the bone within three months
on radiographic evidence. Similar work into medial wall bone grafting by Heywood
(1980) and Mendes et. al. (1982), (1983) and (1984) suggested that the graft had united
within six to ten months. It has also been demonstrated by Xenakis et. al. in 1997 that
bone graft used with uncemented cups in the acetabulum will reliably consolidate and
incorporate in patients with developmental dysplasia of the hip. To the authors’
knowledge there have been no published studies specifically examining the effect of bone
grafting the acetabular notch in primary cemented total hip arthroplasty. However, an
unpublished study conducted in Exeter examining the effects of bone grafting of the
acetabular notch showed a reduction in cement extrusion clinically and an increase in
cement penetration radiographically.
Use of Iliac Sucker:
The use of a suction catheter or retractor has been proposed by Berend and Ritter (2002)
to maintain a dry acetabular cavity prior to and during cementing of sockets. Their
anecdotal evidence suggests that intra-pelvic suction may improve the cement mantle and
reduce the incidence of early radiolucent lines. This could be applied particularly to
DeLee-Charnley zone 1, where the presence of early radiolucency is currently the best
predictor of long-term stability of the acetabular component. Use of a suction catheter to
18
improve cancellous bone drying and cement penetration has also been reported as being
effective in total knee arthroplasty by Banwart et. al. (2000). Their study used cadaveric
tibiae and although it did not show an increase in cement penetration, it did show better
bone-cement interlock, with fewer voids.
Play Dough
Experimental studies of acetabular cementing techniques tend to place a very high
demand on materials. For each experiment, all components need to be brand new and
once cemented cannot obviously be re-used. This means that unless non-stick coatings
are used on the various components, they are all single use. It is not known what effect
coating components has on their physical and mechanical characteristics. The authors
have postulated that Play Dough® used at a particular temperature would be a reasonable
substitute for bone cement in terms of physical and mechanical behaviour. This would
allow re-use of all materials in further studies involving bone cement.
Proposed rationale behind the study:
A new device has been designed to combat the problem of cement leakage, the rim cutter.
The rim cutter is designed to attach to conventional acetabular reamers. It allows a
standardised, uniform rim to be cut in the acetabulum. A flanged cup that has been
trimmed to fit the rim precisely can then be inserted into the acetabulum, which will
hopefully reduce the volume of cement extrusion, thereby increasing the cement pressure
and increasing the depth of cement penetration. It is hoped that the improvement in
cement penetration will lead to less radiolucency in zone 1 and reduce the incidence of
cup loosening. The purpose of this study is also to compare the pressure generated by two
different acetabular conditions: a native acetabulum and one in which the acetabular
notch has been bone grafted. The study will also investigate the pressures generated when
inserting flanged and unflanged cups using conventional reaming techniques and when
using the rim cutter. The depth of penetration of the cement mantle in each case will also
be compared. The study will also compare the effect of the iliac aspirator on cement
penetration in the presence and absence of back bleeding and the physical properties of
play dough compared with PMMA.
19
20
Chapter Two: Materials and Methods: The study was broken into three trials. The first trial was designed to measure the cement
pressure generated in the acetabulum under various conditions. The second trial aimed to
measure the depth of penetration of cement into simulated acetabulae under a variety of
conditions. The third trial attempted to assess the effectiveness of the iliac suction
retractor used in cemented total hip arthroplasty.
Part One
The first part of the trial was divided into six experiments, each of which was performed
six times. For each experiment the same basic set up was used. The variation lay in the
preparation of the acetabulum. For the testing of pressure sawbones (Sawbones Inc.,
Sweden) hemi pelvis foam models were used (Item #1307), shown in photo 2.1. Each
model hemi pelvis was mounted in a standard vice (City 60mm swivel table vice stock
no. 3335), such that the acetabular concavity was directed upwards. Next, the hemi pelvis
model was reamed in the conventional manner using Stryker acetabular reamers (Stryker,
Howmedica) up to a 56mm diameter as shown in photo 2.2.
Two drill holes were made using a 9mm diameter drill at the apex (90°) of the
acetabulum and at the rim (10° angle to the plane of the acetabular lip) (see photo 2.3 and
2.4). Next, the drill holes were manually tapped with a T-handled tap. A finely calibrated
pressure transducer [RDP Group model A105, 0 to 200 psi (0 to 13.5 bar), (Grove Street,
Heath Town, Wolverhampton WV10 0PY, UK)] was then screwed into each of the two
drill holes such that their upper edges were flush with the wall of the acetabular cavity as
shown in photos 2.5, 2.6 and 2.7. The pressure transducers were then coated with silicone
grease to prevent the cement from adhering to them. These pressure sensors were then
connected up to a laptop computer [Toshiba Tecra A2 CE] via an analog to digital
converter. Finally the acetabular cavity was lined with a vinyl glove, which had been cut
open so that a layer of vinyl separated the cement from the pressure transducers as shown
in photo 2.8. This also prevented cement from adhering to the model, allowing the
cement to be removed after each test and the same model to be used for all testing.
21
The control group consisted of a model prepared in exactly the way described above. One
mix of PMMA (Simplex, Stryker Howmedica, photo 2.9) was hand mixed at 2-3Hz and
the bone cement introduced to the acetabular cavity at exactly six minutes from the
commencement of mixing, when the cement was at dough stage. The ambient
temperature in the laboratory at the time of testing ranged from 11.2°C to 13.5°C. For
this reason, the cement took longer to reach dough phase. The cement was then
pressurised at 275N using a Stryker pressuriser (Stryker, Howmedica) for one and a half
to two minutes, depending on the quality of the seal around the lip of the acetabulum. A
constant force was manually applied to the pressuriser by an examiner for the duration of
the test. The force being applied to the pressuriser was measured by a load sensor (RDP
model MLC, 0 to 500 N), which was mounted in the middle of the pressuriser (see photo
2.10). The measured force was then displayed graphically and simultaneously on the
display screen of the laptop computer. This provided the examiner real time feedback
about their force on the pressuriser and ensured that a known, constant force was applied
to the cement. During pressurisation, continuous measurements were made of the
pressure generated in the cement mantle at the apex and the rim using a custom designed
computer program on a laptop computer mentioned earlier as shown in photo 2.11. The
cement mantle was then removed from the hemi pelvis model using the glove lining the
socket and the pressure and force trace saved on the computer. The whole process was
then repeated five more times.
The next step was to compare the pressures generated in a standard acetabulum with
those in which the acetabular notch had been bone grafted. Special hemi pelvis models
(Sawbones, Sweden, item #1307-4) were requested of and constructed by Sawbones Ltd.
in which a piece of foam had been adhered to the model, which covered the acetabular
notch so that the rim of the acetabulum took the form of a continuous circle (see photo
2.12). The model was prepared in exactly the same manner as previously i.e. reamed to
56 mm diameter, two holes drilled at apex and rim and pressure transducers screwed into
the holes. Hand mixed PMMA was again introduced to the acetabular cavity at exactly
22
six minutes after mixing began (again due to the low temperature in the laboratory), when
the cement had reached dough phase. The cement was then pressurised using the same
apparatus, by the same examiner using the same force (275N). Again pressure
measurements were recorded on the computer for the duration of the test. The cement
mantle was then removed and the process was again repeated five more times.
To determine whether the physical properties of Play Dough® matched those of bone
cement, another round of pressurisation tests were conducted. In these tests, the standard
hemi pelvis models were used. There was no simulated grafting of the acetabular notch in
these models. The hemi pelvis that was used in the initial cement pressurisation test was
re used in these tests. Instead of introducing low viscosity PMMA, the same volume of
Play Dough® was placed in the acetabular cavity and pressurised in the same manner as
the previous twelve times. The duration of pressurisation was chosen to be ninety seconds
as this was the approximate length of time of all of the pressure testing in the previous
experiments. The temperature of the play dough was maintained in the same range as the
bone cement 11.3° to 13.5° Celsius. At that temperature the play dough was quite
viscous, it was not warmed because the degree of warming and rate of cooling could not
be adequately controlled. There were also no previous reports of comparisons between
bone cement and play dough at the same temperature, which would be the obvious
starting point for experimentation. The pressure generated in the acetabulum was again
measured continuously and recorded on the computer. The process was repeated five
times, with the play dough being removed from the acetabulum and re-moulded into a
bolus prior to each test. Once the apparatus had been dismantled at the end of testing, the
hemi pelvis was labelled.
These three different experiments allowed comparison of pressure generated in normal
versus grafted acetabulae and also the pressure generated in normal acetabulae using
PMMA and using Play Dough®. The results are described in chapter 3.
It was also necessary to measure the pressure generated in normal acetabulae when either
flanged or unflanged cups are inserted, and also when the acetabular rim is prepared for a
23
flanged cup using the rim cutter. For this part of the study only two standard Sawbones
hemi pelvis models (Sawbones, Sweden, item #1307) were used. None of the hemi pelvis
models used had simulated acetabular notch grafting. For the first two trials the
acetabulum was prepared in exactly the same manner as in the previous trials. The model
hemi pelvis was mounted, reamed, drilled and two pressure transducers fitted. The
temperature in the testing room was controlled such that it ranged from 20.0°C to 21.1°C.
Simplex bone cement was again hand mixed at 2-3 Hz for the first one and a half minutes
and introduced into the acetabular cavity at exactly six minutes (i.e. the total standard
time before cement insertion into the acetabulum [4 minutes] and pressurisation [2
minutes] in vitro). It was not necessary to pressurise the cement for these tests as the
foam models used featured closed pores, which would not allow interdigitation of
cement. At this point an Exeter contemporary cup (Stryker, Howmedica) seen in photo
2.13 and 2.14 with either the flange completely removed flush with the edge of the cup
itself (i.e. unflanged, see photo 2.15) or the flange trimmed at the first groove (i.e. flange
intact, see photo 2.16) was inserted into the acetabular cavity using the modified Stryker
acetabular pressuriser (Stryker, Howmedica). At the six minute mark, the cup was
manually inserted into the acetabulum. The duration of cup insertion lasted twenty to
thirty seconds in an attempt to reflect in vivo practice. This aspect of the experiment was
tightly controlled as pressure measurements of this nature rely on reproducible insertion
forces on the cups. The force applied to each cup was defined as the force required to seat
the cup in twenty to thirty seconds. That force varied depending on the type of cup as a
greater force was required to seat a cup with a larger flange. The cup pusher being used in
these experiments was the Stryker pressuriser mentioned earlier, but with the silicone cap
removed from the head. The load sensor mounted in the centre allowed the same
measurements of force on the cup as in the earlier pressurisation tests. The forces applied
to the cups were recorded and displayed in real time on the computer screen. The
pressure generated in the acetabulum at the rim and the apex was again constantly
measured and recorded on the computer. After the cup had been seated, a constant force
of 100N was applied to the cup via the pressuriser for the duration of the recording trace,
which, in the case of our computer program was three and a half minutes (ten minutes
total trace on computer). The experiment was repeated six times for the flanged cup and
24
six times for the unflanged cup. The same model and cups were used for all of the
experiments.
In the final experiment in Part One, the acetabulum underwent extra preparation. Again a
hemi pelvis model was mounted and the two holes were drilled for the pressure
transducers. The acetabulum was reamed to 56 mm as it had been in all of the prior
experiments, however the rim of the acetabulum was also modified in this case. The rim
cutter attachment to the standard Stryker reamer was used (see photo 2.17 and 2.18). The
acetabular cavity was still reamed to 56 mm, but an additional 3 mm wide ledge was cut
into the acetabular rim, 3 mm deep (see photo 2.19 and 2.20). It is then easily to calculate
that the diameter of the rim of the acetabular cavity is exactly 56 + 6 mm. The flange of
the Exeter contemporary cup was then trimmed along the second groove, to fit the
acetabular cavity precisely as shown in photo 2.21. After the two pressure transducers
were fitted and one mix of hand mixed PMMA was inserted into the acetabular cavity,
the specifically flanged cup was inserted. It was pressurised in the same manner as in the
two previous experiments, with the load sensor in the middle of the pressuriser providing
continuous feedback to the examiner as to how much force they were applying to the cup.
Both the force being applied and the pressure generated in the acetabulum were recorded
and displayed on the computer.
25
A flow chart showing the timing of each part of the experiment is shown below:
These data allowed comparison of cup intrusion pressures between a flanged cup versus
an unflanged cup and a flanged cup versus a flanged cup with the rim of the acetabulum
cut. The results are again described in chapter three.
Mixing Starts Trace Begins
Mixing ceases
Cement In Hand
Cement In Cavity
Cement Not Pressurised
Cup Inserted
Cup Is Seated Pressurisation at 100N
Cup pressurisation Ceases
Time: 0 secs 1min30 3min30 4min 6 min 6 min30 10 min 3min50
26
Part Two
Part Two of the study was aimed at investigating cement penetration under various
conditions. Firstly a comparison was made between bone cement and Play Dough®, then
tests were conducted to compare the cement penetration when using flanged and
unflanged cups and also a flanged cup with the acetabular rim cut. A simulated
acetabular cavity was used for all experiments. These cavities were made from Sawbones
closed cell foam blocks (Sawbones Ltd., Sweden). Each rectangular block was machined
into a hemispherical cavity with an outer diameter of 75 millimetres and an inner
diameter of 60 millimetres1 (see photo 2.22). This was specially designed to fit a 56
millimetre cup with a 4 millimetre cement mantle. Each cavity was further prepared by
drilling 1 millimetre diameter holes through it at predetermined angles. Holes were
drilled at 5° from the vertical axis of the cavity, 45° and 85°. At each angle, four holes
were drilled in each quadrant of the cavity at each of the three aforementioned angles (as
shown in figure 2.23 and 2.24). Therefore, each cavity had a total of twelve x 1
millimetre holes drilled in it. This allowed calculation of an average cement penetration
at each of the three angles: 5°, 45° and 85°. The cavities were manufactured by Stryker
Europe at their plant in Northern France.
These cavities were then mounted in a specially designed jig, which consisted of a metal
box attached to the vice mentioned earlier. The metal box was fitted with a concave
rubber base on the inside and a screw down clear plastic face plate (see photo 2.25). The
cavity was placed in the metal box and the face plate screwed onto the top to hold the
cavity steadily in the box. Conveniently, the metal box was mounted such that the
concavity of the simulated acetabulum faced up obliquely towards the examiner at an
angle of 45°. All experiments were carried out in the same room, which was air
conditioned to a temperature range between 20.0°C and 21.1°C.
1 As the thickness of the blocks was a maximum of 40 mm, the cavity had only a thickness of 12.1 mm at the apex.
27
The first experiment involved setting up the apparatus as described above. PMMA was
hand mixed at 2-3 Hz and inserted into the cavity at exactly four minutes from the
commencement of mixing. The cement was then manually pressurised at a constant force
of 200 N using the same modified Stryker® pressuriser with a load sensor mounted in it
as had been used previously in Part One. The examiner was again able to watch the
computer screen to monitor the force of pressurisation to maintain a constant
predetermined level.
After two minutes, the pressurisation was reduced to 100N to simulate the forces on a cup
after it was inserted. Pressurisation of the foam acetabular cavity is shown in photo 2.26.
This force was maintained for a further one and a half minutes and removed at the eight
minute mark. When the cement had polymerised, the cavity was removed from the jig
and the depth of cement penetration was measured. This was done by using a fine one
millimetre gauge, which was inserted into each of the twelve holes drilled in the cavity.
The thickness of the cavity was pre measured and therefore known at each of the three
angles. By measuring the remaining space left in the hole, it is possible to determine the
depth of cement penetration. Using the four holes an average depth was then calculated
for each angle. This process was then repeated five more times.
In order to compare bone cement with Play Dough®, the bone cement in the previous
experiment was substituted for Play Dough®. The foam cavities were prepared and set
up in exactly the same manner as described above. However, instead of inserting bone
cement, an equal volume of Play Dough® was inserted into the cavity. It was pressurised
using the same modified pressuriser at a constant force for the same time as in the
previous experiment. The foam cavity was then removed from the jig and the holes
measured. The average depth of cement penetration was again calculated. This process
was repeated a further five times using the Play Dough®. The results of the experiments
are discussed in chapter three.
Cement penetration measurements were also required for assessment of the flanged and
unflanged cups and the rim cutter. In these experiments, the flanged and unflanged cups
28
were prepared in the same way as in part one. An Exeter Contemporary cup (Stryker,
Howmedica) was trimmed at either the first groove or flush with the edge of the cup to
create a flanged or unflanged cup respectively. The same foam cavities were used to
simulate the acetabulum. All apparatus was set up in exactly the same manner as
previously. Hand mixed PMMA bone cement at between 20.0°C and 21.1°C was
introduced at exactly four minutes after mixing commenced. The cement was then
pressurised for two minutes at the same known force (200N) and confirmed graphically
by the computer. After pressurisation, either a flanged or an unflanged cup was inserted
into the cavity. The cup was pushed in by the modified pressuriser with the load cell
fitted in it (as used in previous experiments in part one). Once again as in the pressure
tests of part one, the cup was aimed to be seated in 25 to 30 seconds to mimic in vivo
practice. Once seated, the cup was placed under a constant force of 100N by the
pressuriser for another one and a half minutes as shown in photo 2.27 and 2.28. This
again is identical to the technique used in part one. At the ten minute mark, the pressure
trace automatically stopped and the foam cavity with the cup cemented into it was
removed from the jig. The depth of the cement penetration into each of the twelve 1mm
holes was then measured and recorded. The cavity was then labelled and stored. The
process was repeated five times for each of the flanged and unflanged cups.
The final experiment in part two once again utilised the rim cutter. The rim cutter was
attached to the standard Stryker acetabular reamer and as was done in part one and a
ledge of foam three millimetres deep and three millimetres wide was cut in the rim of the
foam cavity as shown in photo 2.29 and 2.30. Again the flange of the Exeter
contemporary cup was trimmed to perfectly fit the rim, by cutting along the second
groove of the cup. This modified cavity was then mounted in the jig as had been done for
the previous twelve tests. Hand mixed bone cement was inserted and pressurised in the
same manner as before. The flanged cup was then inserted, again over a thirty second
period, and pressurised at 100N for one and a half minutes. Finally the foam cavity was
removed from the jig and the depth of cement penetration measured at each angle and
recorded. For each of the thirty experiments in part two, a new foam cavity and a new
cup was used. None of the materials were recycled. These data allowed comparison of
29
cement penetration depth using a variety of acetabular conditions and the results are
discussed in chapter three.
Above is a flow chart showing the timing of each part of the experiment.
Mixing Starts Trace Begins
Mixing ceases
Cement In Hand
Cement In Cavity
Pressurisation Begins 200N
Pressurisation Ceases Cup Inserted
Cup Is Seated Pressurisation at 100N
Cup pressurisation Ceases
Time: 0 secs 1min30 3min30 4min 6 min 6 min30 8min 3min50
30
Part Three
The final part of the study was to examine the effect of the suction retractor (a.k.a. iliac
aspirator) on the depth of cement penetration. Four different experiments were
performed, which examined cement penetration with pressurised PMMA in dry foam,
with and without the use of a suction retractor and pressurised bone cement in ‘bleeding’
foam with and without the use of a suction retractor. To simulate cancellous bone, an
open cell foam block (Sawbones, Sweden, item number 1521–59) was used (shown in
photo 2.31 and 2.32). A metal box of dimensions 7cm x 7cm x 6cm was constructed by
staff at the University of Exeter Engineering Department into which the foam would be
placed. It consisted of five closed sides and one open frame, facing upwards, which was
screwed onto the top (see photo 2.33 and 2.34). A corner of the box was engraved with
the letter ‘A’ so that all other corners could be labelled. This allowed the foam placed in
the box to be labelled after cementing to correspond with its position in the box. A clear
piece of plastic was used to cover the exposed foam in the centre of the metallic frame,
which formed the roof of the box. A hole 47 millimetres in diameter was cut in the
middle of the plastic so that the foam below could be reamed and cemented. Finally,
silicone was used to seal all joins on the inside of the box so that it was both water and
airtight on all sides. This set up was constructed in an attempt to mimic, on a small scale,
a pelvis with the circular hole in the plastic representing the acetabular rim. The box also
had a fluid inlet port installed on one side near the base, and a small four millimetre
diameter hole drilled in one corner of the metallic roof of the box, through which air or
fluid could be aspirated. The foam pieces, originally 15cm x 7.5cm x 2cm were manually
cut into 7cm x 7cm x 2cm squares by a bandsaw. Three such pieces needed to be placed
on top of one another in order to fill the metal box. The clear plastic and the metal frame
were then screwed onto the top of the box. Play dough was used around all joins in the
roof of the box to ensure that it was both air and watertight. This step was required for
practicality reasons as the continual removing and reinserting of the foam pieces from the
box displaced the original silicone (which took 24 hours to cure once reapplied). The
foam was then reamed to a diameter of 47 mm through the hole in the plastic. Once this
31
standard preparation of the foam had been carried out, a variety of experiments were
carried out.
Firstly, one mix of Simplex PMMA bone cement (Stryker, Howmedica) was hand mixed
at 2-3 Hz as in all previous experiments. The ambient temperature in the testing room
remained controlled such that it ranged between 20.0°C and 21.5°C. The pressure trace
on the computer began at the commencement of cement mixing (T=0 mins). The cement
was then manually pressurised at a constant force of 200N by the aforementioned
modified Stryker acetabular pressuriser in the same manner as described in parts one and
two. Visual feedback was provided to the examiner via the display on the computer
screen. Pressurisation at 200N lasted for two minutes as in previous tests, at which point
the force on the pressuriser was reduced to 100N for the final four minutes of the pressure
trace. After pressurisation each region of the cement mantle was labelled A to D
according to the position of each quadrant in relation to the labelled corners of the metal
box. When the cement had polymerised, the foam with its embedded cement mantle was
removed from the jig. The uppermost two pieces of foam became solidly bound together
by the cement interdigitation and had effectively become one piece (shown in photos 2.35
and 2.36). Lines were drawn on the piece of foam from corner to corner, which
highlighted the thickness of the cement mantle at each corner of the foam block as shown
in photo 2.37. The depth of cement penetration from the edge of the simulated acetabular
cavity to the outer limit of the cement in the line marked was measured and recorded.
Finally, the foam and attached cement mantle from each test was placed in a labelled
clear plastic bag and stored. This process was repeated another five times.
The next step was to determine whether the depth of the cement penetration was different
when a suction catheter was used. A small hole of four millimetres in diameter was
drilled in a corner of the frame plate of the box. Into this hole was placed a (spinal)
suction catheter to a depth of twenty millimetres. The same process as earlier was
repeated, with hand mixed bone cement introduced at four minutes and pressurised with
the modified pressuriser at 200N for two minutes, then pressurised at 100N for a further
four minutes. In these tests however, the suction catheter was switched on for the
32
duration of the cementing. The suction used in these experiments was provided by a
portable suction machine (Eschmann TJ 240H, shown in photo 2.38) as used in some
operating rooms. It provides a variable suction pressure depending on the resistance to
flow of material. It was set to provide 200mmHg of suction when the three millimetre
calibre spinal sucker is fitted (in air). This pressure increased automatically as the
viscosity of the material being aspirated increased, or the resistance to aspiration
increased. The maximum suction possible with this machine was 700mmHg, although
this suction pressure was far greater than any achieved during testing. The quadrants of
the foam block were then labelled A to D and then the foam block was removed from the
box. The cement mantle thickness was again measured in the same way as previously in
each of the four lines drawn in the four quadrants of the foam.
The third part of the trial aimed to simulate back bleeding of the cancellous bone. The
foam used in all of the experiments in part three is open cell. This means that fluid and air
is able to move easily throughout the foam. Unfortunately this foam, even though it is the
most dense foam that is possible to manufacture, is more similar to osteoporotic bone
than healthy cancellous bone. Due to this increased porosity of the cancellous bone foam,
a liquid more viscous than saline was required in order to closer mimic the flow
characteristics of blood in native acetabular cancellous bone. Motor oil (Havoline®
10w30) was chosen as a suitable fluid as it was shown on preliminary testing to flow
through the pores of the foam block in a similar pattern and at approximately the same
rate at which blood flows through the cancellous interstices of the acetabulum at
operation. The cancellous bone foam was cut to size as it had been done previously and
placed in the metal box with its plastic cover and metal frame. It was then reamed to a
diameter of 47 millimetres as had been done in all previous tests. Play dough was used as
mentioned earlier to seal all edges of the roof of the box from the inside, and a piece of
play dough was used to plug the hole in the metal face plate since the four millimetre
hole for the spinal sucker would not be used in this part of the testing. The metal box was
filled with motor oil up to a level at the apex of the reamed hemispherical cavity. A
reservoir for the oil in the form of an inverted 60mL syringe was set up, the top of which
was suspended at a point 77 centimetres above the box as shown in photo 2.39. This was
33
shown on preliminary tests to provide a (back bleeding) pressure of forty centimetres of
water (which is roughly equal to 30mmHg). The cement bolus at dough stage was
inserted at three minutes and fifty seconds from mixing and the oil clamp was released,
allowing the remainder of the box to fill with oil around the cement bolus. As in previous
tests at four minutes from mixing the cement was pressurised at 200N for two minutes
and then at 100N for a further four minutes. The oil was seen to have completely filled
the box just prior to the commencement of pressurisation. After the cement had
polymerised, the foam was dried, labelled A to D based on its position in the box and
then each quadrant marked and measured. The piece of foam was then labelled wet foam
without suction and stored in a plastic bag. This process was repeated five more times in
an identical fashion.
The final step in the experiments of part three was to examine the effect of suction on the
simulated ‘bleeding’ acetabulum. An identical set up to the previous three steps was used.
The foam pieces were placed in the metal box, the lid sealed with Play Dough® and
screwed down. The spinal suction catheter was again inserted and set to the same suction
pressure as previously used (200mmHg). The metal box was filled with motor oil, again
just to the apex of the reamed foam cavity (see photo 2.40) and filled completely at three
minutes and fifty seconds after the cement bolus had been placed in the ‘bleeding’
acetabular cavity. The cement was pressurised in the same manner as had been done for
all of the study with the load cell positioned in the middle of the pressuriser, providing
real time graphical feedback to the examiner on the force applied. Pressurisation is shown
in photo 2.41 and 2.42. When the cement had polymerised, the foam was dried, the four
quadrants labelled A to D and removed from the metal box. The depth of the cement
mantle thickness in each quadrant was recorded. The same process was then repeated five
more times. These data allowed comparison of the cement penetration in dry bone with
and without a suction catheter and bleeding bone with and without a suction catheter with
respect to its position relative to the suction catheter. The results are discussed in chapter
three.
34
Chapter 3: Results: All measurements taken in these experiments were converted from analog to digital by
the data acquisition unit and then recorded and displayed on the computer. The data was
recorded in channels such that channel one represented the pressure in the acetabulum at
the apex, channel two represented the pressure in the acetabulum at the rim and channel
seven represented the force being applied to the cement mantle via the pressuriser. The
data was displayed on the computer screen as a graph with time on the X axis and both
pressure in Bar and force in Newton on the Y axis. However, the data acquisition unit
was not capable of simultaneously converting two different units of measurement (Bar
and Newton) on two different scales. As a result, all data was recorded as a value on an
arbitrary scale and needed to be converted manually to the appropriate units after all data
had been recorded. In this study, pressure was expressed as millimetres of mercury
(mmHg) and force was expressed as Newton (N). In the following report of the results of
the experiments, in all tables and graphs, measurements will be expressed in terms of the
absolute value assigned by the computer and then converted to the appropriate units at the
end of each section. To convert the units of pressure measured to mmHg, each value was
multiplied by 2.34. Similarly for values of force each value was multiplied by 0.64. Data
was acquired and recorded in all three channels every 0.5 seconds whilst the pressure
recording computer program was running. The pressure recording program was able to
run for a maximum of nine minutes and fifty five seconds. It could be stopped earlier, but
once it had been stopped it could not be restarted again. This ensured that the pressure
recording program ran continuously for the complete duration of all experiments reported
in this paper. The only exception to this was in the Play Dough® tests (which did not
require mixing) and in the initial pressure tests, where the laboratory temperature was
very low. In these tests, the cement needed to be inserted into the acetabulum at dough
stage at six minutes from the commencement of mixing (rather than four). The pressure
recording trace was only started twenty to thirty seconds prior to the commencement of
pressurisation as it was initially uncertain how long the cement would take to reach
dough stage.
35
Statistical analyses in this paper were carried out using Microsoft® Excel for
Macintosh® using paired and unpaired t-tests and ANOVA. The t-score could then be
used to generate the p-value for each comparison between groups. This is based on 10
degrees of freedom (dF) for each comparison as each group consisted of six trials [dF =
(n1 + n2) – 2].
36
Part One: Pressure Testing.
Pressurisation of Native Acetabulum:
As mentioned earlier in chapter two, cement was pressurised in a model of a human
acetabulum, which had undergone no modification of any kind. The cement was
pressurised at 275N at a time six minutes from mixing of cement for one and a half
minutes. The pressure was recorded at the apex and rim of the acetabulum and also the
force placed on the acetabular pressuriser. The process was repeated five more times. The
pressure trace recorded for each of these six tests are shown in Appendix 1. The peak
pressure generated in the acetabulum at the rim and apex was recorded for each trial as
was the total pressure generated at the apex and rim (represented by units [in mmHg] x
seconds). The mean force applied manually to the pressuriser by the examiner was also
calculated. These results are shown in table 3.1 below. A photograph of the cement
pressurisation is shown in photo 3.1, note the extrusion of cement from around the
pressuriser.
Normal Acetabulum
Trial Peak apex pressure
Peak rim pressure
Total pressure generated at apex
Total pressure generated at rim
Mean force applied to pressuriser
1 214 194 23762 25977 405 2 156 151 24712 26171 410 3 177 165 28267 29554 413 4 153 148 16067 16738 411 5 149 142 22875 23187 419 6 150 146 24936 26207 418
Mean values
167 units = 390mmHg
158 units= 370mmHg
23437 units= 54843mmHg
24639 units = 57655mmHg
413 units= 264 N
Table 3.1: Pressures recorded in the normal acetabulum.
Pressurisation of Play Dough in Native Acetabulum:
In exactly the same way, play dough was pressurised in a model pelvis with no structural
modifications to it. The same force was applied to the pressuriser and the same
measurements of apex and rim pressure and force on the pressuriser were recorded. The
pressure trace from each of the six tests are shown in Appendix 1. The peak pressure
generated in the acetabulum at the rim and apex was recorded for each trial as was the
total pressure generated at the apex and rim (represented by the area under the curve).
37
The mean force applied to the pressuriser was also calculated. These results are shown in
table 3.2 below.
Play Dough
Trial Peak pressure at apex
Peak pressure at rim
Total pressure generated at apex
Total pressure generated at rim
Mean force applied to pressuriser
1 384 388 53428 52254 417 2 402 381 50808 51714 417 3 347 349 50681 53104 427 4 339 348 56768 58023 405 5 291 302 50016 53764 417 6 297 326 54979 55732 394
Mean values
345 units = 807 mmHg
349 units= 817 mmHg
52780 units= 123505mmHg
54099 units = 126592mmHg
413 units= 264 N
Table 3.2: Pressures recorded using Play Dough® in normal acetabulum.
Pressurisation of Cement in ‘Grafted’ Acetabulum:
The third series of tests were conducted using bone cement in a model hemi pelvis.
However, this model hemi pelvis had undergone modification in the form of simulated
bone grafting of the acetabular notch. The bone cement was introduced into the
acetabular cavity at the same time as in the initial tests (i.e. six minutes from the
commencement of mixing of the cement), and pressurised at the same force, for the same
duration. Again there were measurements taken at the rim and apex of the acetabulum as
well as the force applied to the pressuriser. Six tests were conducted and the results of the
pressure trace recorded on the computer are shown in Appendix 1. The results are
summarised in table 3.3, which shows the measured peak pressure generated at the apex
and at the rim as well as the calculated total pressure generated at both the apex and the
rim. It also shows the mean force applied to the pressuriser for each test and for all six
tests.
38
Grafted Acetabulum
Trial Peak pressure at apex
Peak pressure at rim
Total pressure generated at apex
Total pressure generated at rim
Mean force applied to pressuriser
1 307 291 46944 48529 406 2 303 296 41253 41966 371 3 261 255 44565 46408 411 4 320 303 52438 54982 440 5 355 315 51948 53669 416 6 302 302 48689 49477 421
Mean values
308 units = 721 mmHg
295 units= 690 mmHg
47640 units= 111478mmHg
49172 units = 115062mmHg
411 units= 263 N
Table 3.3: Pressures recorded using cement in a ‘grafted’ acetabulum.
Comparisons:
The initial tests in part one were designed to allow comparison between the cement
pressure generated in the normal human acetabulum and an acetabulum in which the
acetabular notch had been bone grafted. It was also designed to allow comparison
between the behaviour of bone cement and Play Dough® in the normal human
acetabulum. Firstly, the peak pressure in the normal acetabulum will be compared with
the acetabulum with the simulated bone grafting of the acetabular notch. The mean peak
pressure at the apex of the acetabulum was 390mmHg for the native acetabulum
compared with 721mmHg for the ‘grafted’ acetabulum. When the peak pressure at the
rim of the normal acetabulum is compared with the ‘grafted’ acetabulum a similar trend
is seen. The mean peak pressure at the rim is 370mmHg in the normal acetabulum and
690mmHg in the ‘grafted’ acetabulum. This represents a significant increase in mean
peak pressure in the acetabulum at the apex (p-value < 0.001) and at the rim (p-value <
0.001) when the acetabular notch is bone grafted. Chart 3.1 in the appendix shows the
differences in peak pressure at the apex and rim of the acetabulum under the three
different conditions.
Next it is necessary to compare the total pressure generated at the apex and rim of the
normal acetabulum with the ‘grafted acetabulum’. As mentioned earlier in this chapter,
the total pressure generated at each point was calculated by measuring the total area
under the curve in each trial. The mean total pressure generated at the apex of the normal
39
acetabulum was 54.8 x 103 mmHg compared with 11.1 x 104 mmHg in the ‘grafted’
acetabulum. At the rim of the acetabulum, the mean total pressure generated was 57.7 x
103 mmHg in the normal acetabulum compared with 11.5 x 104 mmHg in the ‘grafted’
acetabulum. This again represents a significant increase in the mean total pressure
generated in the acetabulum at the apex (p-value < 0.001) and at the rim (p-value <
0.001) when the acetabular notch is ‘grafted’. The mean force applied to the cement
mantle via the pressuriser was 264N for the normal acetabulum and 263N for the
‘grafted’ acetabulum. There was no significant difference between these two groups in
terms of the forces applied to the cement (p value = 0.88).
The other important comparison in this section is between bone cement and Play
Dough®. The mean peak pressure generated by cement at the apex of the normal
acetabulum is 390mmHg compared with the mean peak pressure of 807mmHg generated
by Play Dough® in the same acetabulum. At the rim of the same acetabulum a mean
peak pressure of 370mmHg is generated by cement compared with 817mmHg when Play
Dough® is used. This represents a statistically significant increase in the peak pressures
of Play dough over bone cement at the apex (p-value < 0.001) and rim (p-value < 0.001)
of the acetabulum. When the total pressures generated at the apex and the rim are
compared, a similar association is seen. The mean total pressure at the apex of the
acetabulum is 54.8 x 103 mmHg when cement is pressurised compared with 12.4 x 104
mmHg when Play Dough® is pressurised. A similarly large difference exists at the rim
with the mean total pressure generated in the cemented acetabulum being 57.7 x 103
mmHg compared with 12.7 x 104 mmHg in the Play Dough filled acetabulum. This again
represents a statistically significant difference between the two groups at both the apex
(p-value < 0.001) and at the rim (p-value < 0.001). The mean force exerted on the
pressuriser in the cement group was 265N, which is exactly the same as the Play Dough®
group. There was no statistically significant difference between the force exerted on the
pressuriser between the groups (p-value = 0.99).
Insertion of Unflanged Cup:
40
Similar pressure measurements were taken in the tests involving the insertion of the
unflanged Exeter Contemporary cup into the acetabular model. Again peak pressure was
measured and total pressure generated was calculated (based on the area under the curve)
at the rim and the apex of the acetabulum. The force required to seat each cup in the
designated time was also measured via the load sensor in the pressuriser for each trial cup
insertion. The data for each of the six trials is again displayed in the form of the pressure
trace recorded on the computer. These graphs are shown in Appendix 1. The peak and
total pressures generated in the acetabular model at the apex and the rim are shown in
table 3.4 below. This table also shows the peak force required to seat each cup and the
mean pressures and forces for the group. Photo 3.2 shows the insertion of an unflanged
cup into the acetabulum, note the significant volume of cement that has extruded around
the cup during insertion.
Unflanged cups
Trial Peak pressure at apex
Peak pressure at rim
Total pressure generated at apex
Total pressure generated at rim
Peak force required to seat the cup
1 196 67 14205 3821 163 2 239 100 8850 3589 225 3 284 94 10901 3585 234 4 269 71 10974 2637 268 5 289 132 6283 2212 282 6 216 109 7107 2834 231
Mean values
249 units = 583mmHg
96 units= 225mmHg
9720 units= 22745mmHg
3113 units = 7284mmHg
234 units= 150N
Table 3.4: Pressure recordings in unflanged cup group.
Insertion of Flanged Cup:
Pressures and cup insertion forces were again measured in the six trials conducted using
flanged Exeter Contemporary cups. Peak pressure at the apex and rim of the acetabulum,
total pressure generated at the apex and the rim and the force required to seat the cup
were all recorded for each trial. The results recorded by the pressure trace on the
computer of the six trials are again shown in Appendix 1. The data and mean values for
all six trials is summarised in table 3.5. Again the total pressure generated is calculated
based on the area under each curve.
41
Flanged cups
Trial Peak pressure at apex
Peak pressure at rim
Total pressure generated at apex
Total pressure generated at rim
Peak force required to seat the cup
1 283 198 9848 7601 3262 339 259 23060 13454 362 3 272 155 12226 4831 332 4 312 211 13547 8550 362 5 293 245 9611 5229 314 6 286 225 20867 12319 340
Mean values
298 units = 697mmHg
216 units= 505mmHg
14860 units= 34772mmHg
8664 units = 20273mmHg
339 units= 217N
Table 3.5: Pressures recorded in the flanged cup group.
Insertion of Flanged cup into Acetabulum prepared with Rim Cutter:
The final tests in part one consisted of insertion of a specially flanged cup into a hemi
pelvis model in which the acetabular rim had been prepared with the rim cutter. The same
measurements were recorded as had been done in all prior tests in part one. The pressure
traces recorded on the computer for each trial are shown in Appendix 1. Again the total
pressure generated at the apex and rim of each trial are calculated based on the area under
the respective curves. These results are summarised in table 3.6 below.
Rim Cutter
Trial Peak pressure at apex
Peak pressure at rim
Total pressure generated at apex
Total pressure generated at rim
Peak force required to seat cup
1 306 266 11789 7803 484 2 446 306 13981 8581 560 3 394 348 9585 7302 567 4 278 233 7673 5925 419 5 365 352 13382 10734 565 6 378 325 14770 10053 550
Mean values
321 units = 751mmHg
305 units= 714mmHg
11863 units= 27759mmHg
8400 units = 19656mmHg
524 units= 335N
Table 3.6: Pressures recorded in Rim Cutter group.
Comparisons:
42
The three tests described above were designed to demonstrate any differences between
the pressure generated in the acetabulum when cups with different sized flanges were
inserted. Firstly, the pressures generated when an unflanged cup is inserted into a normal
acetabulum will be compared with the pressures generated when a flanged cup is
inserted. The mean peak pressure generated at the apex when an unflanged cup is inserted
is 583mmHg compared with 697mmHg when a flanged cup is inserted. The mean peak
pressure at the rim of the acetabulum is 225mmHg for the unflanged cup group and
505mmHg for the flanged cup group. Similarly, the mean total pressure generated at the
apex was 22.7 x 103 mmHg for the unflanged cup group and 34.8 x 103 mmHg for the
flanged cup group. At the rim the mean total pressure generated was 7.3 x 103 mmHg for
the unflanged cup group and 20.3 x 103 mmHg for the flanged cup group. These data
represent a statistically significant increase in the peak intra-acetabular pressure
generated on cup insertion at the apex (p-value = 0.03) and rim (p-value < 0.001) when a
flanged cup is inserted compared with an unflanged cup. The total pressure generated at
the apex and the rim is also statistically significantly higher when a flanged cup is
inserted compared with an unflanged cup (p-value = 0.091 and 0.013, for apex and rim
respectively).
It was also necessary to compare the pressures generated by insertion of a flanged cup
alone with the larger flanged cup inserted into the acetabulum prepared with the rim
cutter. The mean peak pressure generated at the apex of the acetabulum was 697mmHg
for the flanged cup group compared with 751mmHg for the rim cutter group. A similar
difference was seen at the rim with the mean peak pressure generated being 505mmHg
for the flanged cup group compared with 714mmHg for the rim cutter group. These data
again represent a statistically significant increase in the peak pressure generated at the
apex (p-value = 0.049) and the rim (p-value = 0.005) of the acetabulum when the rim is
prepared with the rim cutter and a flanged cup is inserted. When the total pressures
generated at each point are considered, the differences are less marked. The mean total
pressure generated at the apex is 34.8 x 103 mmHg for the flanged cup group compared
with 27.8 x 103 mmHg for the rim cutter group. A small difference exists at the rim with
43
the mean total pressure generated being 20.3 x 103 mmHg for the flanged cup group
compared with 19.7 x 103 mmHg for the rim cutter group. This represents no significant
difference between the total pressure generated at the apex and the rim when comparing
the rim cutter to a standard flanged cup at the apex and the rim (p-value = 0.29 and 0.86
respectively) .
The mean peak force required to seat each cup was 150N for the unflanged cup group,
217N for the flanged cup group and 335N for the rim cutter group.
As mentioned in chapter two, the cups used to test pressure were removed from the hemi
pelvis after each test, along with the cement mantle. At that time, each cement mantle
was inspected and it was this examination that revealed an interesting discovery. The
cement mantles removed from the normal hemi pelvises i.e. all unflanged cup trials and
all flanged cup trials had four small circular holes. Examples of the cement mantles with
holes is shown in photo 3.3. However, when the cement mantles in the rim cutter group
were examined, there were no such holes seen (see photo 3.4 and 3.5).
44
Part Two: Penetration Testing.
Penetration of pressurised bone cement:
As mentioned in chapter two, the initial testing of penetration began with bone cement
alone, pressurised with no cups inserted. The results of each trial are shown in table 3.7
below, with the distance of cement penetration listed at each of the four sets of holes at
the apex, middle and rim of the cavity.
Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration Distance in mm. 1
Rim 9 5 5 5 Middle 6 5 5 6
Apex 9 11 9 10 2
Rim 8 7 7 9 Middle 12 9 12 11
Apex 10 9 9 10 3
Rim 10 9 9 9 Middle 9 8 9 10
Apex 8 10 11 10 4
Rim 4 5 5 5 Middle 8 5 4 5
Apex 9 11 10 8 5
Rim 10 9 10 9 Middle 9 10 9 10
Apex 11 10 12 11 6
Rim 9 10 10 10 Middle 7 7 9 9
Apex 9 10 10 10 Table 3.7: Penetration of pressurised bone cement (in millimetres).
45
Penetration of pressurised Play Dough:
Play Dough was also pressurised in the same manner and under the same conditions as
the bone cement. It also underwent six trials. The results are shown in table 3.8 below.
The play dough penetration described in millimetres.
Position 1 Position 2 Position 3 Position 4 Trial Number Play Dough Penetration in mm. 1
Rim 4 4 4 3 Middle 4 4 4 3
Apex 3 4 3 3 2
Rim 3 5 4 3 Middle 4 5 5 2
Apex 3 4 3 4 3
Rim 4 4 5 3 Middle 4 5 5 3
Apex 4 4 4 4 4
Rim 4 4 4 3 Middle 4 4 4 4
Apex 4 3 3 4 5
Rim 4 5 4 2 Middle 4 4 4 5
Apex 5 4 3 2 6
Rim 5 3 3 3 Middle 4 3 3 4
Apex 4 3 3 2 Table 3.8: Measurements of Play Dough penetration in each trial (millimetres).
Comparisons:
The purpose of the first two penetration tests was to determine whether Play Dough
behaved in a similar manner to bone cement. To do this the mean cement penetration was
calculated at the apex, middle and rim of each cavity for the bone cement group and for
the Play Dough group. Considering the apex, the mean cement penetration was 10mm for
the bone cement group compared with 3.5 mm for the Play Dough group. A similar result
is seen at both the middle and rim of the cavity with the mean bone cement penetration
46
being 8mm at both and the Play Dough penetrating 3.5mm at both. These results show a
statistically significant difference between the penetration of the play dough at the apex,
the middle and the rim (p-value < 0.001 for all).
The force applied to the bone cement and the Play Dough was recorded on the computer
in each trial for its duration. The mean force applied during each trial is displayed in table
3.9 below.
Material Bone Cement Play Dough
Trial 1 308 304
2 304 302
3 304 300
4 295 306
5 296 305
6 301 299
Mean values 301 units= 193N
303 units= 194N
Table 3.9: Mean force exerted on pressuriser (N).
The mean force applied to the bone cement for all six trials was 193N while the mean
force applied to the play dough was 194N. There is no statistically significant difference
between the forces applied in the two groups (p-value = 0.58).
47
Penetration of bone cement with pressurisation and insertion of unflanged cup:
The next step was to build on the data from the initial tests described above, and assess
the effect on cement penetration of inserting a cup into the cavity after cement
pressurisation. The Unflanged Exeter Contemporary cups were inserted into the cavity
after cement pressurisation and then pressurised at 100N for two minutes. Six trials were
carried out. The distance of cement penetration was again measured in millimetres at the
apex, middle and rim in each of the four sets of holes in the cavities. The results of all
trials for the unflanged cup group are shown in table 3.10 below.
Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration in mm. 1
Rim 9 8 10 9 Middle 9 7 8 9
Apex 9 9 7 10 2
Rim 9 10 10 11 Middle 11 11 10 11
Apex 12+ 12+ 12+ 12+ 3
Rim 8 9 9 9 Middle 8 9 8 8
Apex 9 10 11 8 4
Rim 10 10 11 11 Middle 11 11 11 11
Apex 12 12 12 12 5
Rim 10 10 10 10 Middle 10 9 10 10
Apex 11 9 12 10 6
Rim 11 9 10 10 Middle 9 9 8 9
Apex 10 9 9 9 Table 3.10: Penetration of bone cement with insertion of unflanged cup (millimetres).
Photo 3.6 shows the extrusion of cement from the acetabular cavity on insertion of an
unflanged cup.
48
Penetration of bone cement with pressurisation and insertion of flanged cup:
The same procedure was carried out for the flanged cup group, with the cement
pressurised in the cavity, a flanged cup inserted and then pressurised for a further two
minutes at 100N. Again the cement penetration was measured at the apex, middle and rim
of the acetabulum in each of the four sets of holes. These measurements of cement
penetration (in millimetres) are shown in table 3.11 below.
Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration in mm. 1
Rim 11 10 10 11 Middle 11 11 10 11
Apex 9 10 12 12 2
Rim 10 10 10 10 Middle 12 12 12 13
Apex 10 10 12 11 3
Rim 11 11 11 11 Middle 12 11 12 11
Apex 12 12 12 12 4
Rim 9 10 10 10 Middle 11 12 11 13
Apex 10 10 12 12 5
Rim 11 12 12 11 Middle 11 12 11 12
Apex 12 12 12 12 6
Rim 11 11 12 11 Middle 10 12 9 12
Apex 12 12 12 11 Table 3.11: Penetration of bone cement into cavities with a flanged cup inserted (mm).
Penetration of pressurised bone cement in cavity with rim cut and insertion of specially
flanged cup:
In this final test of part two, all six cavities were prepared by the rim cutter. A three
millimetre wide and deep ledge was cut in the rim of each cavity prior to cement
insertion. The rest of the procedure remained unchanged from the previous cup
49
insertions. The cement was pressurised, a cup with the flange specially trimmed for the
rim of the cavity was inserted and then pressurised at 100N for a further two minutes.
The same measurements were taken as in all previous penetration tests. Of note in the rim
cutter trials, the cement at the apex in all but one trial penetrated so far that it extruded
out from the holes as shown in photo 3.7 and 3.8. The cavity was thinner at this point due
to the design limitations described earlier in chapter two. As a result, measurement in the
case of cement extrusion from a hole was recorded as 12+mm, as it was not possible to
accurately measure the distance of cement extrusion. The results are shown in table 3.12
below.
50
Position 1 Position 2 Position 3 Position 4 Trial Number Cement Penetration Distance in mm. 1
Rim 12 14 13 15 Middle 13 14 13 14
Apex 12+ 12+ 12+ 12+ 2
Rim 15 15 14 15 Middle 13 14 13 14
Apex 12+ 12+ 12+ 12+ 3
Rim 13 8 10 11 Middle 13 9 8 15
Apex 12+ 12+ 12+ 12+ 4
Rim 14 14 14 14 Middle 15 14 14 15
Apex 12+ 12+ 12+ 12+ 5
Rim 8 10 10 14 Middle 10 6 9 6
Apex 12+ 12+ 12+ 12+ 6
Rim 10 10 9 12 Middle 9 9 10 13
Apex 8 12 12 11 Table 3.12: Penetration of bone cement in cavities prepared by rim cutter (millimetres).
51
Forces applied to the cavities:
Again the force applied to the cement mantle during pressurisation and during cup
insertion was measured and recorded for the duration of each trial. The mean force
applied during pressurisation was calculated for each trial, as was the mean force of
pressurisation after each cup had been seated. The peak force required to seat each cup
was also noted for each trial. These data are listed for each of the three groups in table
3.13 below.
Table 3.13: Force exerted on pressuriser in different phases of penetration testing.
The mean force of pressurisation was 191N for the unflanged cup group, 192N for the
flanged cup group and 195N for the rim cutter group. There was no statistically
significant difference in the mean force used to pressurise the cement in each group (p-
value = 0.30). Similarly, the mean force used to pressurise the cup after it was seated
remained reasonably constant. The mean force applied to pressurise the cup was 98N for
the unflanged cup group, 93N for the flanged cup group and 103N for the rim cutter
group. Although the rim cutter group was pressurised with slightly more force than the
other two groups, this was not significant (p-value = 0.06).
Cup Type Unflanged Flanged Rim cutter Pressurisation Cement Cup Cement Cup Cement Cup
Trial 1 303 140 296 159 307 1632 302 158 294 145 304 1683 296 141 304 130 295 1624 292 160 299 158 302 1705 294 160 307 149 303 1516 309 158 306 131 312 151
Mean Value 299 units= 191N
153 units= 98N
301units = 192N
145 units = 93N
304 units = 195N
161 units = 103N
52
Table 3.14 Shows the maximal force required to seat the cup in each group.
Maximal Insertion Force (units) Trial Unflanged Flanged Rim Cutter
1 331 422 396 2 357 497 390 3 346 591 417 4 357 693 337 5 338 725 396 6 344 656 407
Mean Values 345 units = 221N 597 units = 382 N 390 units = 250 N Table 3.14: Maximal force required to seat each cup type.
The mean force required to seat each cup was 221N for the unflanged cup group, 382N
for the flanged cup group and 250N for the rim cutter group.
53
Comparisons:
When the cement penetration with pressurisation alone is compared with cement
pressurisation and unflanged cup insertion an increase in penetration is seen. The mean
cement penetration distance at the apex is 10mm with pressurisation alone compared with
10.5mm with pressurisation and cup insertion. The mean cement penetration in the
middle was 8mm with pressurisation alone and 9.5mm with pressurisation and unflanged
cup insertion. At the rim a similar picture is seen with 8mm mean penetration with
pressurisation alone and 9.5mm mean penetration with both pressurisation and cup
insertion. These results indicate a statistically significant increase in the cement
penetration at the middle (p-value = 0.015) and the rim (p-value < 0.001) when an
unflanged cup is inserted after pressurisation. However, the difference at the apex was
not significant (p-value = 0.23).
Cement penetration distance with unflanged cup insertion was also compared with
flanged cup insertion. The mean cement penetration distance at the apex was 10.5mm in
the unflanged cup group compared with 11.5mm in the flanged cup group. Similar results
were seen in the middle and at the rim with mean cement penetration in the unflanged
cup group being 9.5mm and 9.5mm respectively compared with 10.5mm and 11.5mm
respectively in the flanged cup group. These results demonstrate a statistically significant
increase at the apex (p-value = 0.008), middle (p-value < 0.001) and rim (p-value <
0.001) of the cavity when a flanged cup is inserted instead of an unflanged cup.
The final comparison is between the cement penetration in the flanged cup group
compared with the rim cutter group. At the apex the mean cement penetration was
11.5mm for the flanged cup group compared with 12+mm in the rim cutter group. In the
middle, the difference between the two groups was again relatively small, the mean
cement penetration being 11.5mm in the flanged cup group compared with 12mm in the
rim cutter group. At the rim there was a slightly larger difference in the cement
penetration. The mean cement penetration here was 10.5mm for the flanged cup group
and 12mm for the rim cutter group. These data show no statistically significant difference
54
between cement penetration at the apex (p-value = 0.12) or middle (p-value = 0.37) with
insertion of a flanged cup alone or a flanged cup with the rim cutter.
However, there was a statistically significant increase in cement penetration at the rim
with the rim cutter used compared with the flanged cup alone (p-value = 0.003).
Charts 3.2, 3.3 and 3.4 in Appendix 1 show the cement penetration differences between
all of the groups at the apex, middle and rim respectively.
55
Part Three: Suction Testing.
The cement penetration distance was measured at the rim of the piece of foam in each
corner of the foam block. The measurement was taken on a line drawn diagonally across
the foam block between opposite corners of the block. Measurements were recorded
based on their position on the foam block with a letter corresponding to its position in one
corner of the box. This is shown in photo 2.37.
Cement pressurised in dry foam with no suction:
The first tests used to examine the effect of suction in acetabular cementing consisted of a
control group in which there was no suction. Cement was pressurised at 200N into dry
foam with no suction for two minutes followed by pressurisation at 100N for four
minutes. The force exerted on the cement for the duration of each trial was recorded on
the computer. The measured distance of cement penetration (in mm) at each point in each
of the six trials is shown in table 3.15. below.
Position A B C D Trial 1 8 9 9 9
2 7 7 7 8 3 4 5 6 6 4 5 4 6 7 5 7 6 6 7 6 5 8 6 6
Mean Value 6 6.5 6.5 7 Table 3.15: Cement penetration distance at rim of dry foam (millimetres).
Penetration of bone cement into dry foam with suction:
The next series of trials measured the distance of cement penetration in dry foam with
supplemental suction. Measurements were taken in the same way as in the previous tests
and recorded based on their position in the metal box. Note that the suction catheter was
positioned in the corner of the box labelled point ‘C’. These measurements are shown in
table 3.16 below.
56
Position A B C D Trial 1 7 9 10 9
2 4 7 9 7 3 7 7 10 10 4 7 8 10 8 5 7 9 7 6 6 1 7 10 7
Mean value 5.5 8 9.5 8 Table 3.16: Cement penetration distance in dry foam with suction at point ‘C’
(millimetres).
Cement penetration in foam with simulated back bleeding:
Cement was also pressurised in wet foam with simulated back bleeding. Measurements
were again taken in the same manner as previously and the force applied to the
pressuriser was again recorded for the duration of each of the six trials. There was no
suction applied in these trials and the hole in the ceiling of the box for the suction
catheter was blocked with Play Dough. The results are shown in table 3.17 below.
Position A B C D Trial 1 7 7 11 8
2 5 6 5 4 3 5 6 5 7 4 7 5 5 6 5 5 6 5 5 6 7 10 10 6
Mean value 6 6.5 7 6 Table 3.17: Cement penetration distance into wet foam with no suction (millimetres).
Cement penetration into foam with simulated back bleeding and suction:
In the final six trials, cement was pressurised in wet foam with the effect of back bleeding
and active suction at point ‘C’. Measurements of cement penetration were again taken in
all corners of the piece of foam as well as continuous measurement of the force applied to
the cement mantle via the pressuriser. The results are shown in table 3.18 below.
57
Position A B C D Trial 1 5 7 10 8
2 7 6 11 11 3 10 9 13 11 4 7 9 9 5 5 5 5 12 10 6 4 11 11 5
Mean value 6.5 8 11 8.5 Table 3.18: Cement penetration distance with back bleeding and suction at point ‘C’
(millimetres).
Forces exerted on cement mantle:
Once again the mean force exerted on the cement mantle during pressurisation of the
cement was calculated for each trial. The pressurisation in part three consisted of an
initial pressurisation of around 200N for two minutes (i.e. from 4 minutes to 6 minutes),
followed by a second pressurisation of 100N for four minutes (i.e. from 6 minutes to 10
minutes). The mean force applied to the pressuriser in each of these two bouts was
calculated for each trial. The results are listed in table 3.19 below.
58
Table 3.19: Forces exerted on cement in foam under various experimental conditions.
The mean force applied to the cement mantle in the dry foam group was 192N for the
first two minutes and 93N for the four minute pressurisation. The mean force applied in
the dry foam with suction group was 193N for the first pressurisation and 93N for the
second pressurisation. Similar mean forces were recorded for the wet foam group and the
wet foam with suction group with 186N and 188N respectively for the first pressurisation
(i.e. lasting two mins) and 99N and 95N respectively for the second pressurisation (i.e.
lasting four minutes). There is no statistically significant difference between the mean
force applied to the cement mantle in any of the four groups (p-value = 0.41 for the initial
pressurisation and p-value = 0.054 for the second pressurisation).
Comparisons:
The mean cement penetration at point ‘A’ (i.e. opposite the point of suction) was
relatively unchanged depending on the conditions in the foam. In dry foam the mean
cement penetration was 6mm, in dry foam with suction 5.5mm, in wet foam 6mm and in
wet foam with suction 6.5mm. However, points ‘B’ and ‘D’, which were located mid way
between point ‘A’ and the suction catheter at point ‘C’ (when it was switched on) showed
differences in mean cement penetration depth. The mean cement penetration at point B
was 6.5mm in dry foam and wet foam, and 8mm in dry foam and wet foam with suction.
Foam
Conditi
ons Dry Foam Dry Foam + Suction Wet Foam Wet Foam + Suction
Dry Foam Dry Foam + Suction Wet Foam Wet Foam + Suction Trial
1st Pressurisation
2nd Pressurisation
1st Pressurisation
2nd Pressurisation
1st Pressurisation
2nd Pressurisation
1st Pressurisation
2nd Pressurisation
1 300 139 307 151 285 148 307 150 2 299 142 313 150 273 152 313 150 3 291 149 278 143 300 168 270 144 4 302 145 303 135 292 164 287 158 5 302 153 310 146 296 151 280 150 6 305 146 293 146 298 148 304 141
Mean Values
300 units = 192 N
146 units = 93 N
301 units= 193 N
145 units= 93 N
290 units= 186 N
155 units = 99 N
293 units= 188 N
149 units= 95 N
59
At point D the mean cement penetration was 7mm in dry foam and 6mm in wet foam
compared with 8mm in dry foam with suction and 8.5mm in wet foam with suction.
The suction catheter was located at point C in the box and it did increase the penetration
of cement in its immediate vicinity. In dry foam cement penetration was 6.5mm, however
in dry foam with suction it was 9mm. Similarly, in wet foam the cement penetration was
7mm, compared with 11mm in wet foam with suction.
Comparing points in each group:
In the dry foam and wet foam groups there were no significant differences between
cement penetration at any of the points. ANOVA of all points in dry foam p-value = 0.6
and in wet foam ANOVA of all points had a p-value = 0.8.
In the dry foam with suction group point A was used as the reference point against which
the other points could be compared. Point B and D both exhibited a statistically
significant increase in the mean cement penetration over point A (p-values = 0.039 and =
0.042 respectively). An even larger increase in cement penetration was demonstrated at
point C compared with point A in the dry foam with suction group (p-value = 0.006).
In the wet foam with suction group point A was again used as the reference point. A
significant difference was not see when comparing point B and D to A (p-value = 0.13
and 0.10 respectively). However, an even larger increase was seen at point C (p-value <
0.001). Chart 3.5 in appendix 1 shows the differences in cement penetration at each point
in the four groups.
Comparing between groups:
As stated previously, there was no significant difference between the mean cement
penetration at all points in the dry foam group and the wet foam group. Comparing the
mean cement penetration distance at point A in all four different groups, there was also
no significant difference (p-value = 0.90). Point A has essentially acted as an
experimental control point across all groups. The increase in mean cement penetration
over the control (point A) observed at point C and to a lesser extent points B and D in the
60
two groups involving suction remain valid and applicable to the two groups in which
suction was not used.
The mean total cement penetration across all points was also calculated for each group.
The results are shown in table 3.20 below.
Position A B C D Total Mean Cement Penetration
Dry Foam 6 7 6 7 26 Dry Foam + Suction 6 8 9 8 31 Wet Foam 6 7 7 6 26 Wet Foam + Suction 7 8 11 8 34
Table 3.20: Mean cement penetration at various points (in mm).
It is seen from these data that the mean total cement penetration is higher in the groups
where suction was used. Analysis of variance reveals a significant difference in the wet
foam with suction group (p-value = 0.04). An observable increase in mean total cement
penetration is also seen in the dry foam with suction group, however, this difference was
not significant (p-value = 0.07).
61
Chapter Four: Discussion: This discussion will begin with some general comments regarding the equipment and
techniques common to all parts of the study. The specific details and results of the
experiments in parts one two and three will then be discussed in order.
Many previous studies into cement pressurisation in vitro have stressed the need to
consider the effect of back bleeding on cement penetration. Cadaveric bone had
previously been used for testing of pressure and penetration in the presence of back
bleeding (Breusch et. al. 2004). In our institution the authors were faced with a paucity of
cadaveric pelvises for laboratory use. The other issue of using cadaveric pelvises is the
relative inconsistency of the bone quality. The use of animal pelvises was considered,
but it was decided that the pelvic bone structures of other animals did not sufficiently
match that of the human. An ideal model for in vitro testing would have the mechanical
properties of bone, a correct anatomical shape and have open pores to simulate cancellous
bone, which would allow liquid to flow through it. Finally, large numbers of almost
identical models would be readily available. Unfortunately such a model does not exist.
For these experiments the authors chose Sawbones® Hemi pelvis models. This decision
was based on both their fidelity to the shape of the human acetabulum, which is of
particular importance in part one of the study and on their consistency of character and
mechanical behaviour. They do not, however, allow liquid to flow through them being of
“closed cell foam” construction. Therefore there was a necessity to separate the study into
three distinct parts, each of which investigated a particular aspect of the experimental
variables. The authors feel that in the absence of an ideal model for testing that
individual, controlled experiments would provide more reproducible and valid results.
Another important variable, which has been mentioned sporadically in the literature is the
force used to pressurise the acetabulum. The average force that an orthopaedic surgeon
places on a pressuriser intra-operatively has not been clearly defined, although it has been
suggested that 35 to 50 kPa pressure sustained for one minute would generate an
acceptable cement mantle (Noble and Swarts 1983). New et. al. (1999) measured an
62
average pressure of 48 kPa by two surgeons in their study. This is in stark contrast to the
paper by Juliusson et. al. in 1994 which claims that a force of 0.3 MPa is ideal to generate
a cement penetration distance of three to five millimetres. This level of force (300kPa) is
far in excess of any force able to be generated in our laboratory during the experiments in
this study. It is clear that for in vitro studies of pressurisation, the force applied to the
pressuriser must be both relatively constant and at a level which is clinically relevant.
The average surgeon is not capable of maintaining pressurisation at an exact force at
exactly the same angle for the whole duration of pressurisation. Therefore the study was
designed to simulate the effects of manual pressurisation of cement within a relevant
force range as one could expect to find in vivo. This study used a load cell mounted in the
shaft of a Stryker® acetabular pressuriser, which measured the force in Newton applied
through it. The diameter of the pressuriser head was five centimetres, so the area of the
head was 78 square centimetres (= 0.0078 m2). To convert this to one square meter we
calculate 1/0.0078 (=128). Since one Pascal (Pa) is equal to one N/m2, then the force on
the pressuriser needs to be multiplied by 128 to convert to Pascals. In this study,
pressurisation of the cement mantle alone was chosen to be 275N [35,200 N/m2 = 35.2
kPa], pressurisation of the cement mantle prior to cup insertion 200N [25,600 N/m2 =
25.6 kPa], while pressurisation of the cup was chosen to be 100N [12,800 N/m2 = 12.8
kPa].
Simplex bone cement was used for all experiments in the study. It was necessary to
specifically control the environment in which the cement was prepared to avoid
variations in cement polymerisation time and rate. Therefore, the most temperature stable
room in the hospital was chosen for testing. Unfortunately the initial three tests involving
pressurisation of a native acetabulum, pressurisation of play dough and pressurisation of
a hemi pelvis with simulated bone grafting were carried out in a laboratory whose
temperature was kept far too low 11.2° to 13.5°C. As a result the bone cement took six
minutes to reach dough phase rather than the planned four minutes. After this was noted,
a warmer room that was equally temperature stable was chosen. The temperature ranged
from 20.0° to 21.1°C, which not only allowed the cement to reach dough phase in four
minutes, but also replicates fairly well the temperature in the operating theatre during
63
surgery. The ambient temperature is of pivotal importance as the viscoelastic properties
of bone cement are very sensitive to temperature. Subtle reductions in temperature can
result in greatly increased time to polymerisation. As the cement polymerises, it becomes
rapidly more viscous. All of the experiments in this thesis rely on the cement to
polymerise at the same rate, so that the rate of change of the viscosity is constant, which
in turn ensures that the flow characteristics of the cement remain constant. Another
property of PMMA is also worthy of mention namely its mechanosensitivity. Bone
cement polymerisation rate is influenced by the degree of agitation of the cement. This
means that unless the cement is mixed at the same frequency in the same way and for the
same period of time it risks polymerising at an inconsistent rate. In order to control these
variables hand mixing was chosen over vacuum mixing, to allow the examiner more
control over the frequency of mixing. The frequency was chosen to be one to two cycles
per second as it was both effective and easily reproducible. The duration of mixing was
also kept constant at ninety seconds as it has shown on preliminary testing to provide the
most reliable cement polymerisation times. Finally, the cement was taken into the hand at
a consistent time and once in the hand not manipulated any more than was necessary to
keep it there until it was inserted as a bolus into the acetabulum, simulated acetabular
cavity or piece of foam.
In order to control the force applied to the cement and to the cups in these experiments, it
was necessary to incorporate a mechanism by which the force could be measured and
displayed. A load sensor was specially adapted and mounted in an acetabular pressuriser
by the staff in the Engineering Department of Exeter University. This sensor conveyed
force readings via the data acquisition unit mentioned in chapter two. It displayed the
force applied graphically in real time at half-second intervals. The examiner was able to
then watch the computer screen and adjust the force applied to ensure a relatively
constant pressurisation force. The laptop computer had installed on it specially designed
software to record and display the data from the force transducer and the two pressure
transducers. Since the computer program ran for the duration of each test it was able to
record all forces applied to the cement mantle. Statistical analysis of the force readings
64
across all of the tests proved that there was no significant difference between the
treatment of each group.
When considering the analysis of results in this thesis it is important to note that the
absolute values measured are not meant to be applied to in vivo practice. The absolute
values measured are important when compared relative to each other. For example it is
of no matter how far bone cement penetrates through a foam cavity when merely
pressurised. It is however important to note that if under the same conditions a flanged
cup is then inserted after pressurisation, then the cement will penetrate much further
through the foam. The materials used are not a valid representation of normal human
anatomy, but the biomechanical principles demonstrated in this thesis and their effects on
the experimental models are applicable to normal human anatomy.
65
Discussion of Part One:
The choice of materials for pressure testing has been discussed above. As discussed an
ideal model for the testing of cement pressurisation of the acetabulum would have the
same shape and strength as a human pelvis as well as the ability to adequately adhere
bone graft to it in the laboratory. A variety of different options were considered including
cadaveric pelvises, bovine pelvises and cast metal models. Previous studies of acetabular
cementing and pressurisation have used cadaveric pelvises (Parsch et. al. 2004 ), (Parsch
et. al. 2004 ) and (Oh et. al. 1983). Two problems were reported with regard to the use of
cadaveric pelvises, which were the difficulty in re-using the same specimen and the
inconsistency in acetabular size between the different cadavers. Shelley and Wroblewski
(1988) used a synthetic socket and rightly pointed out that in a comparative study a
“simple but consistent model” is valid and acceptable. Oh et. al. (1985) used polyethylene
blocks as a simulated acetabulum while Flivik et. al. (2004) and New et. al. (1999)
examined acetabular cementing pressure by using pelvises intraoperatively. Finally,
Bernoski et. al. (1998) used Sawbones® hemi pelvises in their study of acetabular
pressurisers. The authors in our study chose Sawbones® hemi pelvis models. This
ensured that the models were identical and also that the simulated bone grafting of the
acetabular notch, in this case foam, was carried out by the manufacturer prior to their
arrival in the laboratory. The foam also allowed ease of drilling holes for the pressure
transducers and ease of mounting in a vice.
Initially the examiners planned to use a separate hemi pelvis for each individual test.
However, the variability in the reaming, drilling of holes for pressure transducers and
tapping of a thread into the holes was thought to introduce confounding factors to testing
that ultimately requires very precise measurements. In order to reduce the possibility of
confounding variables, a single hemi pelvis was used for the pressurisation tests of bone
cement and play dough in a native acetabulum. The same hemi pelvis was used to
measure the pressure during unflanged cup and flanged cup insertion tests. In order to
allow the repeated use of the hemi pelvis, the reamed acetabular cavity was covered with
a latex glove to allow the cement to be removed from the model after the test. Silicone
grease was applied to the tip of the two pressure transducers to reduce the effect of any
66
shear forces created by the latex as the cement was pressurised by the cup as it was
inserted.
Pressurisation of bone cement, Play Dough® and ‘grafted’ acetabular notch:
The force of pressurisation for cement in native and grafted acetabulae and play dough
was 275N. This force was chosen as it was the maximum reliable force the examiner
could place on the pressuriser for a duration of ninety seconds. As higher forces are
generated, the reliability of the pressurisation is more difficult to maintain. The force
used in this part of the study compares with the 210N force used by Bernoski et. al.
(1998), but is far more than the 60N force used by Parsch et. al. (2004) in their
comparison of two acetabular pressurisers. Shelley and Wroblewski (1988) used an eight-
kilogram weight in their study of flanged and unflanged cup insertion pressures.
In all trials, the initial pressure generated in the acetabulum was higher than the final
pressure at the end of the ninety-second pressurisation time. This was due to cement
extruding from the acetabulum. As cement extrudes, the pressure is seen to fall until the
pressuriser has sunk far enough into the cement mantle to cover all of the inconsistencies
between the rim of the acetabulum and the pressuriser head. At this point, as there are no
pores in the foam models (in contrast to human cancellous bone, into which the cement
would be forced) the pressure trace levels out as a steady state is reached where there is
no net movement of cement. Had the pressurisation been continued for a further five
minutes, the cement would eventually polymerise and the pressure trace would fall to
zero. The acetabulum with the grafted notch has much smaller gaps and as a result allows
less extrusion of cement. Hence the volume of cement retained within the acetabulum is
greater and the pressuriser cannot be forced far into the acetabulum. This is postulated to
be the cause for the higher intra acetabular pressure measurements recorded with
simulated grafting of the acetabular notch. The results show a significantly higher
pressure generated in the cement mantle when the acetabular notch is covered. The total
cement pressurisation pressure listed is a function of the area under the pressurisation
curve and reflects the improved retention of cement in the acetabulum achieved when the
acetabular notch is covered. Since the pressure trace plateaued at a higher pressure with
67
the acetabular notch grafted than in the native acetabulum, then the total cement
pressurisation will consequently be significantly higher.
The peak pressure generated in the normal acetabulum was 390mmHg at the apex and
370mmHg at the rim. These figures fall far short of the results reported by Bernoski et.
al. (1998) of 900mmHg (120kPa) at the apex with a standard pressuriser and 1350mmHg
(180kPa) at the apex using a pressuriser with an additional flap to cover the acetabular
notch. Shelley and Wroblewski (1988) reported a peak pressure of 193mmHg when
testing with an acetabular pressuriser, however, the difference may be accounted for by
their use of Palacos® cement (rather than Simplex®) and the fact that the pressurisation
in their study was done at two minutes from the commencement of mixing. A greater
peak pressure was also measured by Parsch et. al. (2004) who reported 510mmHg
(68kPa) at the rim and 607mmHg (81kPa) at the apex with the Bernoski pressuriser and
585mmHg (78kPa) and 645mmHg (86kPa) at the rim and apex respectively with the
Exeter pressuriser. Their study used simplex bone cement inserted at 5.5 minutes from
the commencement of mixing. New et. al. (1999) measured a peak pressure of 570mmHg
(76kPa) and 698mmHg (93kPa) by the two surgeons in their study. Pressurisation in that
study began at one to two minutes from the commencement of mixing the Palacos® bone
cement. They also measured negative pressurisation on the cement mantle when the
pressuriser was removed to insert more cement into the acetabulum in the case of cement
leakage during pressurisation. Flivik et. al. (2004) measured a mean peak (Palacos®)
cement pressurisation of 647mmHg with conventional pressurisation at 2.5 minutes from
mixing in their in vivo study using two surgeons. The higher pressure mentioned in these
studies relate more closely to the pressures recorded in the acetabulum with simulated
bone grafting of the acetabular notch, which was 721mmHg at the apex and 690mmHg at
the rim. To the author’s knowledge there are no published studies reporting the pressure
generated when cementing an acetabulum with bone grafting of the acetabular notch.
Play Dough® and bone cement are not dissimilar in consistency and flow at particular
temperatures. When bone cement reaches dough stage it behaves rather like warm play
dough. As the cement continues to polymerise, its consistency mimics more that of Play
68
Dough® at room temperature and finally just prior to polymerisation it is much like cold
Play Dough®. Bone cement is a dynamic substance whose viscosity level once mixed,
follows a roughly predictable path. Play Dough® on the other hand must be manually
warmed to experience changes in its viscosity, and in fact undergoes the opposite change
to that of bone cement. Where bone cement viscosity increases with warming, Play
Dough® viscosity deceases with warming. In this study using Play Dough® at room
temperature it did not behave at all like that of bone cement. Its relatively higher viscosity
at room temperature resisted extrusion from the acetabulum and hence the intra-
acetabular pressure was much higher than that of bone cement alone. Perhaps if the Play
Dough® had been warmed, it may have performed more like bone cement. However, the
degree of warming, cooling rate and volume could not be adequately controlled for use in
the study unless extensive testing in its own right was carried out to create a valid
standard. The authors opted not to use warmed Play Dough® for this reason. It will be
seen in a subsequent section that the penetration of Play Dough® into the foam model is
also hindered by its high viscosity at room temperature.
Insertion of cups into model hemi pelvis:
Previous pressurisation tests were carried out by Bernoski et. al. in 1998 to test their new
acetabular pressuriser. Their study utilised Sawbones® hemi pelvises and also placed
pressure transducers at the apex and rim of the acetabulum. Tests of the pressuriser were
done using Boneloc® cement at four minutes from mixing, as was the cement used for
the insertion of a flanged cup. Shelley and Wroblewski (1998) inserted cups in their study
at two minutes from the commencement of mixing, whilst Oh et. al. (1985) inserted 48
millimetre cups at 3.5 minutes from mixing. Parsch et. al. (2004) used CMW 2000®
cement and inserted cups (of different sizes) at three minutes from mixing. Flivik et. al.
(2004) inserted cups at 4.5 minutes from mixing. We inserted our cups at six minutes
from mixing to be more in line with in vivo cementing technique in Exeter. At this time,
the viscosity of the cement is much higher and hence the propensity for cement extrusion
through the acetabular notch was less.
69
In this study the peak pressure generated by the insertion of an unflanged cup was
583mmHg at the apex and 225mmHg at the rim and for a flanged cup 697mmHg at the
apex and 505mmHg at the rim. These pressures are slightly higher than those measured
by Parsch et. al. (2004) which were 293mmHg (39kPa) and 150mmHg (20kPa) with an
unflanged cup at the apex and the rim respectively. In their study a flanged cup generated
pressures of 593mmHg (79kPa) at the apex and 368mmHg (49mmHg) at the rim. These
lower pressures may be accounted for by the reduced viscosity of the cement as the cups
were inserted (at three minutes from mixing) in their study. Our results are also much
higher than the pressures noted by Shelley and Wroblewski (1988) who recorded a peak
insertion pressure of 46mmHg with CMW1® cement and an unflanged cup and
185mmHg with Palacos® and a flanged cup. Cups were inserted in their study at two
minutes from mixing, which again may account for the reduced pressure if the cement
viscosity is lower. However, three studies reported much higher cement pressures when
cups were inserted. Oh et. al. (1985) recorded pressures of 848mmHg (11.3N/cm2) at the
apex and 548mmHg (7.3N/cm2) at the rim with an unflanged cup and 1080mmHg
(144N/cm2) at the apex and 788mmHg (105N/cm2) at the rim with a flanged cup. As
mentioned previously, cups in that study were inserted at 3.5 minutes from mixing. Flivik
et. al. (2004) measured a peak pressure on cup insertion (at 4.5 minutes from mixing) of
1115mmHg. Finally, Bernoski et. al. (1998) reported mean peak cup insertion pressures
of 900mmHg (120kPa) at the apex and 412mmHg (55kPa) at the rim.
After each cup was inserted it was pressurised at 100N for three and a half minutes. This
step was designed to simulate in vivo practice where pressurisation of the cup after it is
seated is necessary to resist the effect of back–bleeding which displaces bone cement
from cancellous bone interstices as discussed in an earlier section. Even though there
were no pores or back bleeding in the foam hemi-pelvis models used in the experiment,
the pressurisation of the cup was retained to better reflect in vivo practice. As we will see
later, this step highlighted some distinct differences in the behaviour of the cups and hints
at future improvements in cementing technique. Another reason for keeping this step was
to allow the experimental technique to remain the same throughout all parts of the study.
This means that in the experiments of cement pressurisation, the experimental technique
70
was the same as in the experiments involving cement penetration and those investigating
the effect of the suction catheter.
It was necessary in this study to also control the time in which the cups are inserted into
the acetabulum of the foam model. We aimed to have all cups seated within thirty
seconds. Only the study by Bernoski et. al. (1998) mentions the time in which cups are
seated. In their study cups were seated over a twenty-second time period. In our study the
volume of the acetabular cavity was held constant for all tests (i.e. reamed to 56mm).
Also, each bolus of bone cement inserted is a constant volume (i.e. one mix). For a cup to
be properly seated the edge (or flange) of the cup must be positioned just inside the rim
of the foam acetabulum. For this to occur the cup must sink a set distance into the
acetabulum. This distance is therefore also constant. The movement of the cup as it is
seated will displace bone cement from the acetabulum. It follows that a specific volume
of cement will be displaced in order for the cup to be seated. The net force required to
seat each cup must be sufficient to create intra-acetabular cement mantle pressure
sufficient to displace that volume of cement out of the acetabulum. Consider an
unflanged cup. It is possible to apply a low level force to the cup so that it displaces
cement slowly from the acetabulum. It will create low intra-acetabular cement mantle
pressure and the cup will be seated over a period of minutes. Conversely, if an extremely
high pressure was applied to the cup, it would create high intra-acetabular cement mantle
pressures and the cup would be seated swiftly over a period of seconds. So it is possible
to alter the intra-acetabular cement mantle pressure by inserting the cup faster. It was
clear that this variable also needed to be controlled in order to compare the intra
acetabular cement mantle pressure between different cup types. As can be seen from the
results of the initial tests where an unflanged cup was inserted into the acetabulum, if the
time of cup insertion is held (relatively) constant, the intra-acetabular cement mantle
pressure generated is also within a limited range.
When tests were carried out using a flanged cup, the intra-acetabular cement mantle
pressure was seen to be much higher at both the apex and the rim when compared to the
unflanged cup. The only change to the experimental conditions was the size of the flange
71
on the cup. This increase in flange had the effect of decreasing the area through which
cement could extrude from the acetabulum. As discussed above, a set volume of cement
must extrude from the acetabulum in order for the cup to be seated correctly. Since the
time of cup insertion was kept constant as is the volume of cement extruded, then the rate
of cement extrusion must also be constant. Hence the following relationships apply:
Volume of cement extruded = rate of cement extruded x time (all constant)
Rate of cement extrusion ∝ area through which cement can extrude x intra-acetabular cement mantle pressure
The rate of cement extrusion remains constant and is proportional to the intra-acetabular
cement mantle pressure multiplied by the area through which cement can extrude. It
follows that as the area through which the cement can extrude decreases, then the intra-
acetabular cement mantle pressure must increase in order to maintain the same rate of
cement extrusion. The flanged cup reduces the area through which cement can extrude by
virtue of its flange being closer to the rim of the acetabulum and covering the defects
present when the unflanged cup was used. These physical principles are postulated as the
reason for the increases in peak and rim pressures seen in the results of our study.
This phenomenon can be extrapolated to also explain why the use of the rim cutter and a
cup with an even larger flange showed even higher pressure measurements. When the rim
cutter is used to prepare an acetabulum it essentially smooths out the rim of the
acetabulum. The flange on the cup is cut such that it fits precisely into the ledge cut in the
acetabular rim by the rim cutter. This ensures that when the cup is seated, there are no
inconsistencies between the rim of the acetabulum and the flange of the cup.
It was expected that as the flanged cup engaged with the cut rim of the acetabulum, that
the pressure recorded on the pressure trace would rapidly increase at both the apex and
the rim. In reality something different was observed. For the last half of the duration of
cup insertion in the rim cutter tests, the intra-acetabular cement mantle pressure was
much higher at both the apex and the rim than the flanged cup alone. This effect was
72
most marked at the rim, suggesting that the extra size of the flange had the effect of
retaining more cement at the rim of the acetabulum by resisting its extrusion from the
acetabulum. The results from the cup insertion tests in part one showed that there is a
significantly increased intra-acetabular cement mantle pressure generated when a rim
cutter is used in conjunction with a flanged cup when compared to a flanged cup or an
unflanged cup alone. These findings are similar to those of Shelley and Wroblewski
(1988) who reported a significant increase in cement pressurisation with a flanged cup
over an unflanged cup. A significant increase in cement pressure with insertion of a
flanged cup was also reported by Oh et. al. (1985) and Parsch et. al. (2004) . However, if
that increase in pressure does not cause an increase in cement penetration distance (which
is considered to be a major benefit of using the rim cutter), then the relevance of these
findings is questionable.
An interesting finding was seen when the cement mantles were removed from the hemi-
pelvis models. Holes were seen in the cement mantles removed from the hemi-pelvis in
the flanged cup and unflanged cup groups (see photo 3.3 previously). These holes
corresponded with the size, orientation and location of the pods on the back of the Exeter
Contemporary cup. The holes appeared to have been formed by the pods impacting on
the floor of the acetabulum as the cup continued to be displaced into the acetabulum after
it had been seated. This phenomenon is also termed ‘bottoming out’ of the cup and is
generally associated with a poor position of the cup in the acetabulum. This ‘bottoming
out’ of the cup is believed to be caused by the 100N force applied to the cup after it was
seated in each trial cup insertion. However, these holes were not present in the cement
mantles removed from the hemi pelvis whose acetabulum had been prepared with the rim
cutter, as shown in photo 3.4. This is believed to be due to the flange impacting on the
ledge cut into the acetabular rim by the rim cutter. This implied that flanged cups
inserted into the acetabulum that had been prepared by the rim cutter do not ‘bottom out’,
since there is a mechanical block to the cup displacing further posteriorly. When a cup
‘bottoms out’, the pods on the back of the cup are in direct contact with the floor of the
acetabulum. Hence any further force applied to the cup will be transmitted via the pods
directly into the acetabulum, bypassing the cement mantle. Therefore, no additional
73
pressure is generated in the cement mantle after the cup has ‘bottomed out’. If the cup is
physically unable to ‘bottom out’, then any additional forces applied to the cup would be
transmitted into the rim of the acetabulum. Parsch et. al. (2004) “avoided” bottoming out
in their study of cup insertion whereas all cups bottomed out in the study by Bernoski et.
al. (1998).
It is important to consider the mechanical properties of the region of the cup that
transmits force into the acetabulum after it has been seated. The pods on the posterior
surface of the cup are solid pieces of UHMWPE and it is through these very rigid
hemispherical structures that forces in a ‘bottomed out’ cup are transmitted. In the case of
cups seated in a ledge created by the rim cutter, forces are transmitted through a thin
flange of UHMWPE, which is directed obliquely posterior. The flange of the Exeter
Contemporary cup has been designed to be deformable in order to allow the cement
mantle beneath to be pressurised if the surgical situation allows. Therefore, it is
reasonable to suggest that it may be possible for the surgeon to apply effective pressure to
the cement mantle after a flanged cup has been seated on the ledge created by the rim
cutter. The results of the rim cutter trials described in chapter three do not however
support this. The pressurisation force after cup insertion was only 100N, which was not
sufficient to generate detectable pressure within the acetabulum. However, the maximum
force that could be generated by the investigators was in the order of 280N. A final test
was carried out to determine whether there would be any effective pressurisation of the
cement mantle after cup insertion if a surgeon’s maximum force was applied to the cup.
The result of this trial is displayed in the form of the super pressure trace shown in
appendix 1. In this trial it is clear that pressure can be generated in the cement mantle
after a cup has been seated if the force applied is of sufficient magnitude. Future studies
will be required to further investigate this phenomenon and determine whether it is
unique to cups inserted in conjunction with the rim cutter.
In our study we measured a mean peak cup insertion force of 150N for the unflanged cup,
217N for the flanged cup and 335N for the rim cutter group. The result of the unflanged
cup group is similar to those obtained by Oh et. al. (1985) who reported a mean cup
74
insertion force of 113N – 171N for the unflanged cup group. However, their results with
regard to the flanged cup group was much higher at 2167N – 2912N. Cup insertion forces
recorded by Parsch et. al. (2004) were between 60N and 100N.
75
Discussion of Part Two:
Part two of the study used closed cell foam blocks, which had been machined into the
shape of an acetabular cavity. This was unfortunately the most troublesome feature of the
experimental design. The foam blocks made by Sawbones® are made in a uniform
thickness of forty millimetres. The foam blocks were available in a range of sizes, but
only one thickness. As a result it was necessary for the thickness of the cavity at its apex
to be restricted to twelve millimetres. In hindsight it would have been better to use a foam
that was thick enough to allow the cavity to be a uniform fifteen millimetre thickness.
The interior diameter of the cavity was made 56 millimetres to match the size of the
hemi-pelvis cavities cemented in part one. This would allow a degree of correlation
between the pressure and penetration of the cement under similar pressurisation forces.
The holes drilled, through which penetration would be measured, were one millimetre in
diameter. This size was chosen to best mimic the calibre of the cancellous bone in the
floor of the acetabulum and yet be large enough to be easily measured by inserting a
small probe. Holes were drilled at 5°, 45° and 85° to the axis of the acetabulum to
roughly correlate with the penetration at the sites of the pressure transducers in the
previous models in part one. All preparation of the foam cavities was carried out prior to
delivery to the testing laboratory (except preparing six for use in the rim cutter trials). To
the author’s knowledge, no prior studies using pre-made foam cavities have been
reported.
The shape of the rim of the acetabular cavities was perfectly flat having been machined
from a solid block of foam. In this respect it differs from the hemi-pelvis model used in
the first part of the study where the rim of the acetabulum mimicked the shape of the
normal human acetabulum with its myriad of subtle irregularities and the large acetabular
notch. Therefore the seal of the pressuriser onto the rim of the cavity was much better
than in part one and the fit of the various cups used was also much better. This fact must
be considered when evaluating the absolute values obtained from measuring the cement
penetration in part two. It again underscores that the individual values of the results in
76
this paper are primarily designed to be compared relative to one another rather than as
absolute values.
77
Bone cement and play dough tests:
From the initial comparisons of the behaviour of bone cement and Play Dough® in part
one, it was clear that the difference in viscosity plays a major role in the respective flow
characteristics. The same cavity was re-used for each of the Play Dough® tests as it was
easy to remove the play dough from the cavity after each test. The results of the Play
Dough® penetration tests revealed significantly less penetration distance compared to the
bone cement tests. This is a valid result as the experimental conditions were kept the
same for each testing group and only the substance pressurised changed. These results are
consistent with those in part one, where much greater Play Dough® pressures were
recorded indicative of the relative resistance of Play Dough® to flow when at the same
temperature as bone cement. The results of this part of the study show that bone cement
and Play Dough® do not behave similarly in experimental conditions. When compared to
the results seen in part one of the study, it would appear that even though a greater intra-
acetabular Play Dough® pressure is generated, it in fact does not lead to an increase in
Play Dough® penetration.
Cup insertion tests:
In our study the mean cement penetration depth at the apex was 10.5 millimetres in the
unflanged cup group, 11.5 millimetres in the flanged cup group and 12 millimetres in the
rim cutter group. These results differ from those obtained by Oh et. al. (1985) who
recorded a cement penetration distance of 17 millimetres with an unflanged cup and 93.6
millimetres with a flanged cup. In their study cups were inserted at three minutes from
mixing, whereas in our study cups were inserted at six minutes, which may account for
the differences in results. However, our results are similar to those of Flivik et. al. (2004)
who measured a mean cement penetration radiologically of 10.3 millimetres. As
mentioned previously, their cup insertion time was 4.5 minutes from mixing.
Previous studies (Oh et. al. 1985, Shelley and Wroblewski 1988) have suggested that the
use of an unflanged cup to generate pressure in the acetabulum is not a viable possibility.
However, the results of this study have shown that even an unflanged cup is able to exert
a pressurisation force on the cement mantle at the middle and the rim. The results of this
78
study have also shown that that increase in cement pressure leads to an increase in
cement penetration distance. When the pressurised cement mantle alone is compared to
the pressurised cement mantle and an unflanged cup inserted, the cement penetration
increases significantly at the middle and the rim of the acetabulum. This finding implies
that the viscosity of the bone cement at six minutes is perhaps still low enough for
increases in cement mantle pressure to displace the cement. This finding is significant in
light of the other results that compare flanged cups with the rim cutter.
It was thought that six minutes after mixing, bone cement would simply be forced out of
the cavity and not into the small holes as a cup was inserted. In fact, the engineer behind
the design of the rim cutter stated that he felt no difference would be seen in cement
penetration distance with the rim cutter. It can be seen from the results that the cement
penetration is significantly increased at the apex, middle and the rim when a flanged cup
is compared to an unflanged cup. This increase in cement penetration distance is thought
to be due to the same physical properties of cement extrusion from the cavity through a
smaller space around the cup. The result is not surprising in light of what we know about
the flow characteristics of acrylic bone cement discussed in chapter one. It is also
supported by the work done by Shelley and Wroblewski (1988) who showed that an Ogee
flanged cup can be used to generate increases in cement penetration as it is inserted.
Increases in cement mantle pressure lead to increases in cement penetration and in these
experiments that is what we have seen. The cement penetration distance is globally
increased when a flanged cup is used compared with an unflanged cup, which suggests
that the addition of the flange is pivotal if not solely responsible for the improvement in
cement retention in the cavity. It follows that if more cement is retained in the cavity
then the pressure will increase and the cement will flow preferentially down the path of
least resistance i.e. into the twelve holes drilled in the cavity.
The same physical principles are postulated to be the cause for the improved cement
penetration seen when the rim cutter is used. Since the contact between the rim of the
cavity and the flange of the cup are so intimately matched, more of the cement is retained
in the cavity and hence more is forced into the measuring holes. The results show a
79
significant increase in cement penetration distance at the rim of the cavity. This region
corresponds with DeLee-Charnley zones 1 and 3, where lucent lines on initial post
operative radiographs are correlated with an increased rate of aseptic loosening (Breusch
and Malchau, 2005). Therefore any device or mechanism that improves cement
penetration in this area potentially reduces the incidence of radiolucent lines and also
may improve the long-term survival of the cup.
Previous studies are divided on whether cement penetration can be increased with cup
insertion. Oh et. al. (1985) reported a significant increase in cement penetration distance
when a flanged cup is inserted compared with an unflanged cup. However, even though
they reported a higher cement mantle pressure during cup insertion, Parsch et. al. (2004)
reported no significant increase in cement penetration distance with insertion of a flanged
cup compared to an unflanged cup.
Unfortunately the design characteristics of the foam cavity used may have limited the
power of the study to highlight other differences. The results of the cement penetration
measurements of the rim cutter group at the apex of the cavity show that in five out of the
six trials cement extruded right through the cavity. As mentioned previously, the cavities
could only be machined to a maximum thickness of twelve millimetres at the apex.
Ideally this would have been fifteen millimetres as was the case at the middle and rim
positions. As a result, the cement penetration distance could not reliably be measured
once the cement had exuded through the holes and the maximum distance was therefore
twelve millimetres. In reality the cement penetrated much further than that at the apex. It
is postulated that had the cavity been manufactured at a uniform thickness of fifteen
millimetres then the distance of cement penetration at the apex would have been
measurable. It may have also shown a significant increase in cement penetration depth at
the apex in the rim cutter group compared with the flanged cup group.
80
Discussion of Part Three:
The effect of back bleeding of cancellous bone on cementing technique was first
highlighted by Benjamin et. al. in 1987 as previously mentioned. It has been stated often
in world literature that in order to be valid, studies of cement penetration should take into
account the effect of back bleeding pressure (Breusch et. al. 2004). Majkowski et. al. in
1994 in their study of cement penetration utilised a bleeding model of cancellous bone
formed by cancellous bone discs of bovine femora enclosed in an aluminium chamber.
More recently, Parsch et. al. (2004) used human cadaver pelvises to assess the effect of
back-bleeding.. This study used Sawbones® “open cell” foam. This foam has a density of
0.12 g/cc and a strength of 0.28 MPa. It is in reality a closer match to osteoporotic
cancellous bone rather than normal human cancellous bone. It did, however, allow liquid
to flow through it although rather more readily than normal cancellous bone. This foam
was used as although its physical characteristics were not identical to normal cancellous
bone, it was thought that the relationship between the cement and the suction was the
relevant factor rather than the absolute value of the depth of cement penetration. Other
materials were proposed for use such as flower arranging foam, stainless steel foam,
animal cancellous bone and trabecular metal but the respective strengths, ease of
manipulation, lack of fidelity and cost made these materials unsuitable for the scale of the
testing involved.
To compensate for the increased porosity of the cancellous bone foam being used, a
substitute for blood was required. The author sought a liquid that, although thicker than
blood, would have similar flow characteristics to blood when allowed to flow through the
cancellous bone foam. Previous studies had used dyed saline but as mentioned
previously, while it flows well through cadaveric pelvises, it flows rather more readily
through foam. Motor oil (Havoline® 10w30) was found on preliminary testing to flow
through the foam at a rate and fashion similar to blood through cancellous bone in vivo.
As the temperature of the experimental work station was controlled, the changes in motor
oil viscosity due to temperature were not experienced. The spinal suction catheter was
inserted twenty millimetres into the cancellous bone foam in keeping with the reported
81
depth of suction retractor insertion by Berend and Ritter (2002) in their technical note on
the use of suction to aid cement penetration.
The height of the column of oil above the metal box was of pivotal importance. It must be
placed at a height such that it generates a back bleeding pressure equivalent to 30mmHg.
Parsch et. al. (2004) used dyed saline held at a point one metre above the pelvis to
provide a back-bleeding pressure of 25 to 30 mmHg. Their results yielded cement
penetration of two to 3.6 millimetres with cement introduced five and a half minutes from
mixing. In comparison, our experiments found a mean cement penetration distance of 6.5
millimetres in the wet foam group. Shelley and Wroblewski (1988) used a simulated back
bleeding pressure of 25mmHg in their study of simulated acetabular cementing. One
mmHg is equal to 1.33cm of water. Therefore the column of motor oil was set such that it
was in equilibrium with 40cm of water. This height was measured to be 77cm and
remained constant throughout all testing.
In this part of the study, pressurisation of the cement mantle was chosen to be 200N
initially for two minutes, followed by 100N for four minutes in keeping with the previous
parts of the study. These times were chosen in an attempt to recreate faithfully the in vivo
pressurisation of cement in the acetabulum for two minutes, followed by a period of
pressurisation at 100N for four minutes representing the more gentle pressurisation of a
prosthesis in the acetabulum to resist back bleeding until the cement had polymerised.
This pressurisation appears to have been effective in that no reduction in penetration of
cement was noted between the dry foam and foam with simulated back bleeding (p-value
= 0.33). These results further underscore the importance of correct cement pressurisation
technique to prevent the displacement of cement by back bleeding in the acetabulum.
Parsch et. al. (2004) claim that simulation of back bleeding is mandatory in experiments
involving acetabular cementing. However, our results show that adequate pressurisation
of the cement mantle and the cup after seating is able to resist the effects of back
bleeding. Our methodology was unchanged in each of the three parts of the study. Since
our pressurisation of cement and cup was able to nullify the effects of back bleeding (as
seen in the comparison between dry foam and wet foam) it is reasonable to assume that it
82
was not mandatory to include the effect of back bleeding in parts one and two of the
study.
Most importantly, a significant increase in cement penetration distance was noted when a
suction catheter was introduced at point C. This effect was seen both in dry foam and also
in foam with simulated back-bleeding. However, the effect is greater in foam with
simulated back-bleeding than in dry foam (p-value = 0.03). This difference is likely to be
due to the added pressure of back-bleeding on the cement mantle, which was the only
factor added to the experimental system. It remains unclear as to exactly how this effect
is brought about. The most likely explanation is that in the back-bleeding tests the cement
mantle opposite the suction catheter is subject to two forces, the force of the pressuriser
on one side and the force of the back-bleeding on the other. In the tests using dry foam
and suction there existed the force on the cement of the pressuriser alone. These findings
suggest that bone cement is propelled deeper into cancellous bone by two distinct
mechanisms. First is the direct effect of the suction on the cement (as seen in the dry
foam) and secondly by removing the opposing force of back-bleeding in the cancellous
bone (as seen in the foam with simulated back-bleeding) in the region of the suction
catheter. The effect of the suction catheter in improving cement penetration was not
apparent at all points in the metal box. The cement penetration at point A remained
unchanged during all experiments. This implies that the effect of the suction catheter is
confined to the region in which it is located and that its beneficial effects are reduced as
the distance from the suction catheter is increased.
Interestingly, both the dry foam group and the wet foam group had the same mean total
cement penetration, and in fact more cement penetration was noted in the wet foam with
suction group over the dry foam with suction group. The cause for this increase in cement
penetration is not known. It is therefore reasonable to suggest that if a suction catheter
was inserted under the iliac crest of a patient’s pelvis during cementing of an acetabular
prosthesis, it may serve to increase the cement penetration around the rim. The
importance of improved cement penetration at the rim of the acetabulum was mentioned
83
previously with regard to reducing the incidence of radiolucent lines on the initial post
operative radiograph.
84
Chapter Five: Conclusion:
The majority of the study was designed to explore the in vitro effect of three techniques
that may improve cement penetration into the acetabulum in total hip arthroplasty namely
1) bone grafting of the acetabular notch, 2) use of the rim cutter and 3) use of a suction
retractor in the ilium. This study aimed to help answer the question of whether these
techniques improve cement penetration and cement mantle thickness. The results
reported in this paper are valid and show that there is indeed an increase in cement
pressurisation and hence penetration when the various techniques are employed in vitro.
In the first part of the study the more uniform and constricting rim of the acetabulum in
the hemi pelvis with simulated bone grafting of the acetabular notch experiments
contained bone cement more effectively than the native acetabulum. This resulted in
higher pressures generated within the cement mantle. The pressure generated within the
acetabulum also increased with the size of the flange on the cup inserted and also when
the rim cutter was used. In the second part of the study the insertion of the flanged cup
resulted in a global increase in cement penetration distance when compared to an
unflanged cup. The effect of the rim cutter was to increase the cement penetration at the
rim only. In the third part of the study the suction catheter was used to show that suction
in simulated acetabular cementing results in an increase in cement penetration in the
immediate vicinity of the catheter with a gradually diminishing effect as distance from
the catheter increases.
What has been demonstrated by this study is that acetabular notch bone grafting, the use
of the rim cutter and suction catheters exhibit some effect on the cement penetration into
foam. This effect is supplemental to current acetabular cementing technique when used
on the same type of foam. Whether that effect applies to human cancellous bone in vivo
remains to be seen.
It is possible that in isolation or in concert these acetabular cementing techniques may
lead to a thicker and more uniform cement mantle, particularly around the rim of the
acetabulum. This may lead to a reduction in the rates of aseptic loosening of the
85
acetabular component and a lower revision rate for aseptic loosening. This study has
highlighted improvements in vitro relative to current cementing techniques and it is not
unreasonable to extrapolate this to in vivo practice. Further clinical studies are required to
properly assess the effectiveness of the new techniques in vivo.
The first element of this study compared the behavior of Play Dough® with Simplex®
bone cement in an attempt to find an alternative substance to use in future testing. It was
shown that at the same temperature, the two materials behave quite differently.
Therefore, this study does not recommend the use of room temperature Play Dough® as a
substitute for bone cement in experimental studies. Further studies may show more
similar biomechanical characteristics between the two materials when the Play Dough®
is warmed to a specific temperature.
86
Disclosure: This study was performed with the assistance of the Stryker group who provided the bone
cement, acetabular pressuriser, jig for mounting foam cavities and Exeter Contemporary
cups for testing. They also financed the purchase of all foam and foam models, the vice
and the pressure transducers. The Stryker plant in Cedex, France prepared the foam
blocks by machining them into cavities and drilling the measurement holes. They also
supplied the testing jig for the foam cavities. Exeter University generously loaned the
laptop computer, data acquisition unit and software. They also constructed the metal box
for the testing of the suction catheter. The patent on the Rim Cutter is jointly held by
Stryker Corporation, the French engineer who designed the Rim Cutter (also a Stryker
employee) and the hip surgeons at the Exeter Hip Centre (one of whom was a principal
supervisor of this project). No past or future financial or non-financial benefits have been
offered to or accepted by the author.
87
Appendix 1 1. Pressure Tracings:
Native Acetabulum Cement Trial 1 Native Acetabulum Cement Trial 2 Native Acetabulum Cement Trial 3 Native Acetabulum Cement Trial 4 Native Acetabulum Cement Trial 5 Native Acetabulum Cement Trial 6 Native Acetabulum Play Dough Trial 1 Native Acetabulum Play Dough Trial 2 Native Acetabulum Play Dough Trial 3 Native Acetabulum Play Dough Trial 4 Native Acetabulum Play Dough Trial 5 Native Acetabulum Play Dough Trial 6 ‘Grafted’ Acetabulum Cement Trial 1 ‘Grafted’ Acetabulum Cement Trial 2 ‘Grafted’ Acetabulum Cement Trial 3 ‘Grafted’ Acetabulum Cement Trial 4 ‘Grafted’ Acetabulum Cement Trial 5 ‘Grafted’ Acetabulum Cement Trial 6 Unflanged Cup Trial 1 Unflanged Cup Trial 2 Unflanged Cup Trial 3 Unflanged Cup Trial 4 Unflanged Cup Trial 5 Unflanged Cup Trial 6 Flanged Cup Trial 1 Flanged Cup Trial 2 Flanged Cup Trial 3 Flanged Cup Trial 4 Flanged Cup Trial 5 Flanged Cup Trial 6 Rim Cutter Trial 1 Rim Cutter Trial 2 Rim Cutter Trial 3 Rim Cutter Trial 4 Rim Cutter Trial 5 Rim Cutter Trial 6 Comparison of all three cup types Super Pressure Test
2. Charts: Mean Peak Pressure Generated: Cemented, Play Dough and Grafted Acetabulum Mean Peak Pressure Generated: Unflanged Cup, Flanged Cup, Rim Cutter Mean Cement Penetration at Apex
88
Mean Cement Penetration in Middle Mean cement Penetration at Rim Average Total Cement Penetration Per Cavity Comparison of Cement Mantle Thickness Under Various Conditions
89
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