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Design of a Hip Screw for
Injection of Bone Cement
Caroline Ann Grant, B.E. (Medical)
Submitted for the award of the degree of Master of
Engineering in The Centre for Built Environment and
Engineering Research, School of Engineering Systems,
Queensland University of Technology
Design of a Hip Screw for Delivery of Bone Cement
ii
Keywords
Bone Cement; Cement Augmentation; Compression Hip Screw; Fracture
Fixation; Head of Femur; Lag Screw; Modified Hip Screw; Modified Lag
Screw; Sliding Hip Screw
Design of a Hip Screw for Delivery of Bone Cement
iii
Abstract
Project Title: Design of a hip screw for injection of bone cement
Author: Caroline Ann Grant
Supervisors: Prof. Mark Pearcy (Primary)
Prof. Ross Crawford (Secondary)
Fracture to the neck of femur is frequently stabilised with a hip screw
system, however the host bone is often weak or osteoporotic. This causes
premature failure of the system, commonly by cut-out of the lag screw
through the head of the femur. While augmentation of the fixation with
bone cement improves the holding power and decreases failure rate,
current methods of administering the cement are messy and inaccurate.
This project proposes a lag screw design which allows for direct injection
of the cement, via the lag screw itself, after the screw has been inserted
and correctly positioned in the femur. A method is also suggested to
reduce the risk of cement leakage into the joint space when the guide wire
has punctured the head of the femur.
The design uses a system of holes in the threaded section of a cannulated
screw to allow delivery of cement to the desired area; the modified screw
was also tested with and without the tip of the screw closed. These design
Design of a Hip Screw for Delivery of Bone Cement
iv
and implantation techniques were compared to the standard design lag
screw both with and without bone cement augmentation by traditional
methods.
Initial testing in a synthetic bone analogue looked promising. The modified
screw with closed end performed better in push out tests than the standard
screw alone and comparably with the standard screw with cement
augmentation. A second phase of testing with the synthetic material was
then conducted to more closely represent physiological loading conditions.
In this case again the closed ended modified screw with cement
augmentation outperformed the original screw and was comparable with
the augmented original screw.
However, during this phase of testing problems were observed with the
synthetic testing material and it was decided to conduct further testing in
paired porcine cadaveric femurs. Several further problems occurred in this
phase of testing, including the bending of the test screws.
It was concluded that the modified screw showed potential in being a more
accurate and consistent method of cement augmentation, however neither
the synthetic bone analogue or the porcine material was an adequate
model of an osteoporotic human femur. If a suitable testing material could
be found, continued study of this prototype may prove beneficial.
Design of a Hip Screw for Delivery of Bone Cement
v
Table of Contents
1. Introduction 1
1.1. Injury – description, rates, causes, effects 1.2. Fixation 1.3. Bone Cement 1.4. Previous Work 1.5. Aims
2. Background and Literature review 7
2.1. Conclusions
3. Initial Testing 17
3.1. Background – Sliding Compression Hip Screw System
3.2. Screw Modifications 3.3. Test material – Bone or Analogue? 3.4. Bone Cement
3.4.1. Standard or low Viscosity Bone Cement 3.4.2. Curing requirements – 24hr @37oC
3.5. Apparatus 3.6. Screw Fixation Strength Studies
3.6.1. Rationale 3.6.2. Methodology 3.6.3. Cement Delivery to the Original Screw 3.6.4. Cement Delivery to the Modified Screw
3.7. Clinical risk of guide pin puncturing the head of the femur
3.7.1. Testing the modified screw with a temporary plug in the guide wire hole
3.8. Testing 3.9. Results
Design of a Hip Screw for Delivery of Bone Cement
vi
3.10. Statistics 3.11. Discussion 3.12. Conclusion
4. Continued Testing 47
4.1. Rationale/Introduction
4.2. Method 4.2.1. Original screw with no cement 4.2.2. Original screw with alternative cement augmentation
method 4.2.3. Results 4.2.4. Discussion 4.2.5. Conclusions
4.3. Modified Plugged Screw with augmentation 4.3.1. Modified screw Method 1
4.3.1.1. Sample 1 4.3.1.2. Sample 2 4.3.1.3. Discussion
4.3.2. Modified screw method 2 4.3.2.1. Sample 3 4.3.2.2. Results
4.3.3. Modified screw method 3 4.3.3.1. Sample 4 4.3.3.2. Results
4.3.4. Modified screw method 4 4.3.4.1. Sample 5 4.3.4.2. Discussion 4.3.4.3. Sample 6 4.3.4.4. Discussion
4.4. New information from the manufacturer of the bone analogue – 95% closed cell
4.4.1. Bone Cement Penetration into Open and Closed cell foams
4.4.1.1. Method 4.4.2. Results 4.4.3. Discussion
Design of a Hip Screw for Delivery of Bone Cement
vii
4.5. All Results
4.6. Discussion
4.7. Conclusions
5. Pig Testing 81
5.1. Porcine cadaveric material 5.1.1. Method Development 1
5.1.1.1. Discussion 5.1.2. Method Development 2
5.1.2.1. Discussion 5.1.3. Method Development 3
5.1.3.1. Discussion 5.1.4. Numerical Results 1
5.1.4.1. Results 5.1.4.2. Discussion
5.1.5. Numerical Results 2 5.1.5.1. Results 5.1.5.2. Discussion
5.1.6. Conclusions
5.2. New Modified Screw Pig Testing 5.2.1. Numerical Results 3
5.2.1.1. Results 5.2.1.2. Discussion
5.2.2. Numerical Results 4 5.2.2.1. Results 5.2.2.2. Discussion
5.3. Summary of All Pig Testing Results 5.3.1. Cement Distribution 5.3.2. Force – Displacement Data 5.3.3. Stiffness Data 5.3.4. Comparison to Sawbones samples
5.4. All Pig Discussion
5.5. Conclusion
Design of a Hip Screw for Delivery of Bone Cement
viii
6. General Discussion 129
7. Conclusions 131
8. References 133
Appendix 1: Technical Drawings 139
1.1. Slotted Screw design 1.2. Final Screw design 1.3. Closed End Screw design 1.4. Hounsfield Adaptor 1.5. 45o Alignment Jig 1.6. Pig Test Rig 1.7. Pressure Transducer T-piece Adaptor
Appendix 2: Ringers Foam Compression Test 147
2.1. Toad Ringers Solution Formula
Appendix 3: Statistical analysis of Sawbones 90o Data 148
Appendix 4: Sawbones website details 149
Design of a Hip Screw for Delivery of Bone Cement
ix
Figures and Tables
Figure 1.1. Stryker Howmedica Osteonics Omega+Plus Standard 85mm
lag screw and side plate, implanted in a Sawbones Femur
with cut-out sections to reveal fixation detail
Figure 3.1. A schematic diagram showing the modifications used by
Kramer et al. and Augat et al. The screw had set of three
rectangular slots placed axially through the threaded section
of the screw.
Figure 3.2. Photos of all screw designs, A: The slotted screw used in pilot
testing; B: A screw with holes similar to the final design, also
used in pilot testing; C: The Original unmodified screw; D: The
modified screw with holes for cement delivery.
Figure 3.3. A sample of Sawbones polyurethane foam with bone cement
penetration
Figure 3.4. Bone Cement Compression Data, samples with a mix ratio of
2:1 are shown in red when cured at 22oC and blue at 37oC,
samples with a mix ratio of 4:3 are shown in green when
cured at 22oC and yellow at 37oC
Figure 3.5. The yield load of bone cement samples with two different mix
ratios and curing temperatures, the mean value is shown in
column one in red with plus or minus one standard deviation
marked
Figure 3.6. Compression data of foam samples after soaking in ringers
solution (red) or in the standard dry state, the range of the
data is shown with error bars at selected data points
Figure 3.7. Diagrammatic representation of the Hounsfield Injection
Apparatus, the screw inserted in the foam block is supported
by the syringe, which in turn is supported in the metal tube on
the base plate
Design of a Hip Screw for Delivery of Bone Cement
x
Figure 3.8. A Photograph of the Hounsfield Testing Apparatus with
sample in place
Figure 3.9. Bone Cement distribution around the modified screw once
removed from the test block
Figure 3.10. Failure curves for all Sawbones 90o tests, the original screw
uncemented samples are shown in green, the original screw
augmented with cement in black, the modified augmented
samples in blue and the modified screw samples with closed
guide wire hole in red
Figure 3.11. Cracks visible in the base of the foam blocks after testing
Figure 4.1. A cut-away view of a hip screw implanted in a Sawbones
femur showing the force application angle.
Figure 4.2. Schematic diagram of screw placement in the Sawbones foam
blocks
Figure 4.3. Diagrammatic view of the testing procedure
Figure 4.4. Method of block sectioning, first cut in red, second in blue and
third in green, the screws placement in the block is marked by
the block oval.
Figure 4.5. Failure Curves for the original screw tested at 45o with and
without cement, the original uncemented screw samples are
shown in green and the original screw cemented samples are
in blue. The two samples in red were original cemented
samples that were removed from the analysis because of
material batch differences
Figure 4.6. Bone Cement distribution in Samples 1 and 2 of the modified
screw tested at 45o is shown on the left, on the right is a
schematic representation of the cement delivery holes in the
screw, the large centre hole is the guide wire hole, while the
three sets of holes for cement delivery are shown radially
representing their distance from the tip of the screw, shaded
holes have been closed to prevent cement flow
Design of a Hip Screw for Delivery of Bone Cement
xi
Figure 4.7. The failure pattern of Sample 2, the foam was seen to
separate from the top of the screw, while crushing underneath
it
Figure 4.8. Cement distribution in Sample 3 of modified screw at 45o with
schematic view of the closed holes
Figure 4.9. Cement distribution and hole closure pattern in sample 4, with
schematic view of the closed holes
Figure 4.10. Results of Samples 1-4 (methods 1, 2 and 3) for the
Sawbones 45o tests, Sample 1 is shown in dark blue, Sample
2 in pink, Sample 3 in yellow and Sample 4 in light blue
Figure 4.11. Cement distribution and hole closure pattern of sample 5, with
schematic view of the closed holes
Figure 4.12. Cement distribution and hole arrangement for sample 6, with
schematic view of the closed holes
Figure 4.13. Bone Cement Penetration, the foam is in the top 10mL of the
syringe with plasticine coating, with cement below it
Figure 4.14. Foam samples with plasticine edges to prevent leakage, the
left is the open cell foam prior to testing with the post testing
view on the right.
Figure 4.15. Cement Penetration Testing apparatus post testing, the
modified syringe is supported on a base plate with a hole in
the centre slightly smaller than the diameter of the syringe
Figure 4.16. Sectioned view of cement penetration into a closed cell foam
sample (top) and an open cell foam sample (bottom), in the
open cell foam the cement was seen to flow out of the foam
before curing leaving gaps in the foam
Figure 4.17. Pressure required to inject the bone cement into the foam, the
closed cell foam is shown in red and the open cell foam in
black
Figure 4.18. Failure loads of all sawbones samples, the mean and
standard deviation for each group are shown in red, each
sample within a group is shown in a different colour
Design of a Hip Screw for Delivery of Bone Cement
xii
Figure 4.19. Stiffness of all sawbones samples, the mean and standard
deviation of each group are shown in red, each sample within
a group is shown in a different colour
Figure 5.1. Method Development 1, the holes for cement delivery were
seen to clog with bone material preventing cement flow out of
all but one hole, where the cement flowed back up the shaft of
the screw
Figure 5.2. Method Development 2, the cement delivery holes were seen
to clog with bone prior to cement injection, limiting the flow of
cement
Figure 5.3. Method Development 2, cement (white) pooled at the tip of the
screw as it was not inserted to the full depth of the hole
Figure 5.4. Schematic diagram of the testing method used in Method
Development 3, the femoral head was supported using a
dental acrylic ring and the force applied to push out the screw
Figure 5.5. Photograph of the stainless steel angled testing rig with
sample in place
Figure 5.6. The modified screw after testing of Numerical Results 2, The
shaft of the screw was bent at the edge of the angled test rig,
supporting the sample at 45o, this also occurred to the original
screw in Numerical Results 2
Figure 5.7. Bone Cement injection pressure as recorded in the head of
the femur from Numerical Results 2
Figure 5.8. Photograph of the cement delivery holes in the new modified
screw
Figure 5.9. Photograph of the new modified screw with sealed guide wire
hole in the tip of the screw
Figure 5.10. Photograph of the Injection pressure recording apparatus
used in Numerical Results 3 and 4. The barrel of the screw
(bottom left) and the pressure transducer (top) are attached to
Design of a Hip Screw for Delivery of Bone Cement
xiii
the brass T-piece adaptor, the cement injection gun attaches
to the right hand side of the T-piece.
Figure 5.11. Numerical Results 3, bone cement injection pressure as
measured inline with the delivery
Figure 5.12. Comparison of injection pressures measure in the head of the
femur (Numerical Results 2, shown in Dark Blue) and inline
with the injection (Numerical Results 3, shown in Pink).
Figure 5.13. Sectioned view of the modified cemented sample from
Numerical results 3, a bone void can be seen filled with
cement
Figure 5.14. Sectioned view of the modified cemented sample from
Numerical results 3, with the screw removed a bone void can
be seen filled with cement
Figure 5.15. X-ray of the Porcine femora used in Numerical Results 4 prior
to use
Figure 5.16. Numerical Results 4, bone cement injection pressure
measured inline with the injection
Figure 5.17. Comparison of the injection pressures recorded in Numerical
Results 2 (Blue), Numerical Results 3 (Pink) and Numerical
Results 4 (Yellow)
Figure 5.18. Sectioned view of Numerical Results 1, the white cement is
very hard to see
Figure 5.19. The modified screw once removed from Numerical Results
Figure 5.20. A sectioned view of the Numerical Results 2 modified
cemented sample, the bone cement is clearly visible in blue
Figure 5.21. The modified screw after removal from the bone in Numerical
Results 2
Figure 5.22. A sectioned view of Numerical Results 3, the bone void is
apparent by the large mass of cement
Figure 5.23. The same sample (Numerical Results 3) with the modified
screw removed, the size of the bone void is evident
Figure 5.24. The modified screw as removed from Numerical Results 3
Design of a Hip Screw for Delivery of Bone Cement
xiv
Figure 5.25. A sectioned view of Numerical Results 4
Figure 5.26. The Numerical Results 4 bone sample after removal of the
screw, the full extent of the bone cement penetration can be
seen
Figure 5.27. The Modified screw removed from Numerical Results 4
Figure 5.28. Force – Displacement data from the four sets of paired
cadaveric porcine femora, Numerical Results 1, Original screw
in pink and Modified screw in red, Numerical Results 2,
Original screw in light blue and Modified screw in dark blue,
Numerical Results 3, Original screw in light green and
Modified screw in dark green, Numerical Results 4, Original
screw in light orange and Modified screw in dark orange
Figure 5.29. Stiffness of the pig samples at 2, 3 and 4mm displacement,
Modified samples in blue and original samples in green, the
original samples in Numerical Results 1 failed after 3mm of
displacement
Figure 5.30. Failure or Peak loads of Sawbones foam and Porcine
Numerical Results, the mean and standard deviation of the
Sawbones samples is shown in red, the porcine Modified
screw samples are shown in blue and the Original screw
samples in green
Figure 5.31. Stiffness of all samples, mean values from the Sawbones data
are shown in red with standard deviations marked, the Porcine
Numerical results data are shown at displacements of 2, 3 and
4 mm with the Modified samples shown in blue and the
Original samples in green
Design of a Hip Screw for Delivery of Bone Cement
xv
Table 3.1. Student t test p values comparing temperature and mix ratio
for bone cement samples
Table 3.2. Number of Samples of each type tested in initial testing
Table 3.3. Maximum Load Data for Sawbones 90o tests
Table 3.4. Stiffness data (N/mm) for Sawbones 90o tests, evaluated
along the linear section of the curve
Table 3.5. Simple Student t-test results using tables of p-values,
comparing the different implantation methods tested in the
Sawbones 90o testing
Table 4.1. Number of Samples of each technique in Sawbones 45o
testing as determined statistically from the data from
Sawbones 90o testing (Appendix 3)
Table 4.2. Failure load data for all Sawbones 45o samples
Table 4.3. Stiffness Data (N/mm) for all Sawbones 45o samples
Table 5.1. Stiffness (N/mm) data calculated at displacements of 2, 3
and 4mm
Design of a Hip Screw for Delivery of Bone Cement
xvi
Abbreviations used in the text
g/cc grams per cubic centimetre
mm Millimetres
N Newtons
Ø Diameter
PMMA Polymethylmethacrylate (Bone Cement)
PU Polyurethane (PU)
Design of a Hip Screw for Delivery of Bone Cement
xvii
Statement of Originality
“The work contained in this thesis has not been previously submitted for a
degree or diploma at 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:
Design of a Hip Screw for Delivery of Bone Cement
xviii
Acknowledgments I would like to take this opportunity to thank all of the numerous people
who have helped me survive these past few years and produce what is
certainly my greatest achievement to date.
To Mark and Ross, my supervisors, for encouraging and helping me to do
this, without your help I wouldn’t have even started this. Thanks also for
all of your knowledge and insights along the long and winding road.
I would especially like to thank all of the lab staff, Greg, Kimble, Melissa,
for putting up with me and for your technical knowledge and ideas and for
helping to get my test methods actually working. The workshop staff
particularly Terry and John, for your technical knowledge and ideas, and
for modifying my hip screw, and then modifying it again after I broke it.
I’d also like to thank all the other Medical Engineering postgraduate
students and staff for you help and humour along the way. Especially to
Mr Ocean and his carbon rod, living next to you there is never a dull
moment.
I would especially like to thank my Mum and Dad for their financial support
over the last few years, it really means a lot to me to have been able to do
this. Dad, your constant supply of ideas and possible solutions to problems
and both of your continual love and support and encouragement, really
Design of a Hip Screw for Delivery of Bone Cement
xix
helped get me through. And thankyou to my brothers, simply for being my
big brothers and for always loving me.
To my friends, particularly Robert and Kate and Elise, you’ve all been
there for me with love and support and jokes and distractions and tea and
coffee and shopping… I don’t know how I’d survive without you.
Design of a Hip Screw for Delivery of Bone Cement
xx
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 1
1. Introduction
1.1. Injury – description, rates, causes, effects
Fractures to the neck or trochanteric region of the femur are frequent
occurrences in elderly populations of the world. In New South Wales,
Australia, the incidence of fracture in over 50s between 1990 and 2000
has increased from 4219 to 5648 per year (Boufous et al., 2004). This is
accentuated by an aging population. By the year 2025 the world-wide
occurrence of hip fractures will have increased to 2.6 million per year,
double the 1990 rate (Gullberg et al., 1997).
Mortality rates associated with this type of fracture are reported as being
as high as 35% in the first year post fracture (Goldacre et al., 2002). This
mortality is frequently the result of prolonged fracture healing time and
reduced mobility. The average time a patient spent in hospital with a hip
fracture was 14.2 days in 2000 (Boufous et al., 2004), during which time
the patient spends a large proportion of their day in bed. This reduction in
mobility and prolonged bed rest greatly increase the number of secondary
conditions and infections, such as pneumonia and chest infections.
Predominately this prolonged bed rest results in a general decrease in the
patients’ health and well being, as well as their standard of living.
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 2
1.2. Fixation
Fractures to the neck of the femur are most commonly corrected with
internal fixation. There are many different methods of fixation for these
types of fractures, including various screws, nails, hooks and pins. One of
the commonly used systems involves the use of a lag screw in the head of
the femur with a side plate attachment down the length of the femur with
several cortical screws (Figure 1.1). This system provides initial stability
and load bearing while allowing compression of the fracture fragments to
promote healing.
One of the most common problems with the fixation of this type of fracture
is Osteoporosis, which is often also the initial cause of the fracture.
Osteoporosis causes an often dramatic decrease in the strength and
quality of both the cortical and cancellous bone. In a severe case there
may be very little cancellous bone left in the head of the femur. This
creates a major obstacle to adequate fixation of the fracture as the host
material is too weak to hold the device, while increasing the chance of the
fixation failing prematurely.
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 3
1.3. Bone Cement
The use of a bone cement of either Acrylic or Calcium Phosphate base, is
a common method of increasing the strength of fixation. Current methods
of delivering the cement are inadequate. Some of the current methods
include placing either a runny or doughy mass of cement into the lag
screw hole prior to insertion of the lag screw (Bartucci et al., 1985, Elder et
al., 2000, Eriksson et al., 2002, Moore et al., 1997). When placing the
cement in the hole prior to screw insertion the low viscosity cement
frequently flows out of the desired placement area while the fast setting
nature of the cement makes using a doughy mass largely impractical. Low
viscosity cement runs the risk of leaking into the joint cavity if the guide pin
has punctured the head of the femur (Szpalski et al., 2004).
1.4. Previous Work
A small series of pilot tests were undertaken in order to test the feasibility
of a new design of screw to enable delivery of bone cement throughout the
threads (Grant, 2003). A comparison of the cut-out strength of the original
screw with and without bone cement and a modified screw with bone
cement augmentation was made. The addition of bone cement was found
to greatly increase the strength of fixation. A limitation of the test method
was noted in which the cracks caused by failure of one sample unduly
weakened the subsequent samples. However, the results suggested this
method had potential leading to the studies presented in this thesis.
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 4
1.5. Aims
This study aimed to address problems of fixation of a lag screw in an
osteoporotic femoral head by providing a convenient, timely and accurate
method of administering specific quantities of bone cement directly to the
area surrounding the threaded tip of the lag screw, without risk of
undesirable cement leakage.
A modified screw design is presented followed by comparative mechanical
tests of the existing screw, alone and with current cement augmentation
techniques, with the modified screw and augmentation method.
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 5
Figure 1.1 Stryker Howmedica Osteonics Omega+Plus Standard 85mm lag screw and side plate, implanted in a Sawbones Femur with cut-out sections to reveal fixation detail
Design of a Hip Screw for Delivery of Bone Cement
Section 1: Introduction 6
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 7
2. Background and Literature review
Over the past century reported rates of hip fractures around the world
have been steadily increasing, but has this trend finally plateaued?
Boufous et al. (2004), suggest in their study of hip fracture incidence rates
in NSW, Australia, over the past 10 years, that in fact the age-specific and
age-adjusted rates remain virtually unchanged over this time and that
perhaps the trend of increasing incidence has finally passed. However the
number of hospital admissions for a fractured neck of femur has still
increased quite dramatically in the same period, with the number of
fractures reported increasing by 41.9% for men and by 31.2% for women.
The increased number of fractures is primarily caused by the overall aging
of the population world wide. Gullberg et al. (1997), suggest that even
with no change in the age- or sex-specific incidence, by the year 2025 the
world-wide occurrence of hip fractures will have increased to 2.6 million,
double the 1990 rate and that by the year 2050 it will have increased to
4.5 million cases each year.
With increases in the number of patients the amount of time spent in
hospital or secondary care and the cost of treatment becomes a critical
factor. Hollingworth et al. (1996), studied these costs in the UK and
estimated that the total cost of care would increase from a total of
approximately ₤280 million per year in 1991-2 to approximately ₤500
million per year in 2031. Graves (2004), reports that in Australia the
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 8
estimated cost of hip and knee replacements in 2002 was over $500
million.
Because of this it becomes increasingly important to reduce the hospital
and rehabilitation time of each patient. If an implant could be developed
that would increase the initial stability of the system the patient could be
rehabilitated sooner. This benefits not only the health care system but
also the quality of life of the patient.
A fracture to the neck of femur, or any other region of the hip, results in a
decline in physical function and often mortality in the elderly. Marottoli et
al. (1992) report on the decline in physical function seen in elderly hip
fracture patients. He found a substantial decline in the patients’ ability to
do everyday tasks such as dress themselves, walk across a room or
ascend a flight of stairs. There was a marked decrease in the number of
patients able to do each of the tasks 6 months post fracture, with the
number able to walk across a room independently falling from 75% pre-
fracture to only 15% at 6 months. He found that the only factors that
predicted this decline in function were pre-morbid physical and mental
function. He also reported a mortality rate of 18% within 6 months of
fracture.
The standardised mortality ratios associated with fractured neck of femur
are reported as being between 20% and 35% (Goldacre et al., 2002) and
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 9
Bannister et al. (1990) reports a mortality rate of 37% in the first year post-
fracture, in his study of fixation and prognosis. It is hoped that this could
be reduced with improved fixation and shorter recovery times.
Many different methods have been used to fix these fractures over the
years. The sliding compression hip screw system or Dynamic hip screw
(DHS) is one of the most common methods. Other devices include the
Jewit nail (Harrington, 1975), Gamma nail (Rosenblum et al., 1992,
Haynes, 1998, Haynes et al., 1997b, Haynes et al., 1997a, Bridle et al.,
1991)) and the Küntscher Y-nail (Davis et al., 1990), the use of multiple
screws or pins (Goodman et al., 1998, Goodman et al., 1992, Stankewich
et al., 1996, Kubiak et al., 2004), occasionally with additions such as
reinforced struts (Baixauli et al., 1999) and the Alta dome
plunger(Choueka et al., 1995, Choueka et al., 1996).
Many studies have been done comparing fixation methods. Eriksson et al.
(2002), studied five implants, including cannulated and solid lag screws as
well as a hybrid design with a barb and the LIH hook-pin. Each was tested
in a polyurethane bone analogue in its original state and when augmented
with a calcium phosphate cement and with a PMMA cement. The cement
was applied by injecting it into the pilot hole prior to insertion of the screw.
This is the method used in this study as the current clinical augmentation
method. In all cases those augmented with PMMA had the greatest pull
out load and extraction torque, and in most cases the calcium phosphate
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 10
cemented samples were stronger than the original uncemented implant
samples.
Sommers et al. (2004) created a cut-out model using a polyurethane foam
to compare a DHS and Gamma nail with two novel blade type implant
designs. They found that the DHS and gamma nail migrated further under
cyclic loading than the blade implants. A similar model to this was used in
the angled testing in Chapter 4 of this report with a static loading system.
The augmentation of fracture fixation devices with bone cement has been
done for many years. When augmenting fixation for a neck of femur
fracture two methods are common in the literature.
The first method of augmentation is to pack doughy cement into defects, in
particular the posteromedial defect. This method has been studied by
many people since the early 1980’s (Cheng et al., 1989, Chow et al., 1987,
Lau et al., 1983, Pun et al., 1987, Yetkinler et al., 1998, Yetkinler et al.,
2002). This method was not studied because of the risks associated with
this kind of augmentation preventing fracture healing through non-union as
well as preventing the sliding of sliding hip screw devices.
The second primary method of bone cement augmentation is by placing
the bone cement in the head of the femur prior to insertion of the screw or
fixation device (Bartucci et al., 1985, Eriksson et al., 2002, Elder et al.,
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 11
2000, Moore et al., 1997). This method proves effective in increasing the
holding power of the fixation but has several undesirable aspects. If a low
viscosity cement is used, it may flow out of the desired area before the
screw has been inserted and with a cement that is already doughy the
time required to put the screw in place may be such that the cement
completely hardens before the screw is fully inserted. Neither of these
methods allow for repositioning of the screw once it is in place.
The validity of bone cement augmentation around the threads of the screw
was also studied by Lee et al. (2001) who constructed an FE model of a
hip screw with and without a cement mantle. They found an 80%
reduction in the stresses observed in the cancellous bone, suggesting that
this makes it unlikely for further fractures or cut out to occur with a cement
mantle in place.
The dangers associated with bone cement injection are highlighted by
Bartucci et al. (1985), who point out the risk of cement leakage into the
joint space should the guide wire puncture the head of the femur.
A new method of delivering the bone cement to the desired location at the
right time is required. Some novel approaches have been developed to do
this.
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 12
Szpalski et al. (2004, 2001) conducted a review of cementing techniques
and gave preliminary results of a new method of delivering the cement.
They comment on the non-reproducible nature of injecting cement into the
predrilled hole prior to insertion of the screw. The method they have
suggested involves the use of a cannulated lag screw which is inserted to
the desired location. The screw is then retracted by the length of the
threaded section of the screw and a catheter inserted down the cannula.
The bone cement is then injected into the space in front of the screw. The
screw is then returned to its original position. This method also runs the
risk of cement leaking if the guide wire has punctured the head of the
femur. No mention is made as to whether the cement catheter is left in
place during tightening of the screw or if it has been removed. With the
catheter removed the cement will flow into the area where there is least
pressure – the cannula of the screw. This becomes a problem when such
a small volume of cement (2.5mL) is used.
Another method suggested to combat the difficulties of injection is the Alta
Dome plunger (Choueka et al., 1995, Choueka et al., 1996). This device
carries a small bolus of doughy cement at the tip, which is then squeezed
out into the surrounding bone once in place. It was noted that this device
did not always contain the cement in the head of the femur, with it
sometimes found at the fracture site or at the screw barrel junction. This
acted to transform the sliding screw into a rigid nail, eliminating all
mechanical advantages associated with the sliding nature of the device.
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 13
Two papers suggest a modification to a lag screw to allow the cement to
be injected to the area around the threads of the screw once positioned.
Kramer et al (2000) and Augat et al. (2002), both suggest similar
modifications. Both have started with a traditional cannulated sliding
compression lag screw and modified it to include open channels running
axially through the threaded section of their screws. The aim of this is to
deliver bone cement through the channels, once the screw has been
positioned. Both of these devices showed promise when compared with
standard devices and augmentation procedures. It was decided to use a
similar device in this testing. However it was felt that the use of axial
channels along the length of the threads was not an ideal method of
delivery. The act of screwing the thread into the bone will act to compress
or cut the bone, this material will then be forced into the open channels in
the same way that a thread tap works. In this case the material will not be
removed, but will cause the central cannula and delivery channels to
become clogged with material preventing cement delivery.
The use of bone cement to augment fixation raises questions about which
type of cement is best. Numerous people have conducted studies of the
various mechanical properties of different types of bone cements
subjected to different mixing and curing conditions (Krause and Mathis,
1988, Dalby et al., 2001, Hansen and Jensen, 1992, Jefferiss et al., 1975,
Knepper-Nicolai, 2002, Lee et al., 1978, Lee et al., 1977, Lewis, 1997,
Linden, 1991, Nzihou, 1998, Saha and Pal, 1984, Thompson et al., 2003,
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 14
Witschger et al., 1991, Yamamoto, 1998, Yetkinler and Litsky, 1998, Lee
and Ling, 1981, Khairoun, 1999, Older, 1990, Ooms, 2003). Despite all of
this it was decided to use a standard PMMA cement as a base line for
study.
A method of determining the permeability for cement infiltration into
osteoporotic bone was suggested by Baroud et al. (2003), this method
was subsequently used to determine the differences in cement penetration
into the two different foam products.
Two options were presented by the literature for a testing material,
cadaveric bone or polyurethane foam. The vast majority of studies
undertaken in this field use cadaveric material, however there are several
that use a polyurethane foam as a bone analogue. Eriksson et al. (2002)
used three densities of foam to study implant pull-out strength and
extraction torque. The three densities were said to represent severe,
medium and mild levels of osteoporosis. The density chosen for this study
is slightly less than that used by Eriksson as a medium level of
osteoporosis. Sommers et al. (2004) also used a polyurethane foam as a
bone substitute, though they do not report the density used. Sommers
study uses a very similar test method to that established here but with
cyclic loading rather than static.
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 15
Angled testing was conducted in most studies reported. Loading
conditions and angles were typically chosen to reflect a simulated one-leg
stance with the force being applied at an angle of 45o to the screw axis
(Elder et al., 2000).
Of the studies using cadaveric material Moore et al (1997) and Witschger
et al (1991) however, used a method of supporting and testing their
samples using only the head of the femur. In both cases the heads of
cadaveric femurs were removed and had lag screws implanted. The load
was then applied to the head of the femur using a curved or moulded
surface to give an even distribution of force, while the screw barrel was
supported at an angle of 45o to the force application. This method was
used in Chapter 5 of this report when cadaveric porcine material was used
for testing.
There were also several standards that were applicable to various aspects
of the materials and testing including (ISO, 2002, ASTM, 1999, ASTM,
2002, ASTM, 2001a, ASTM, 2000, ASTM, 2001b).
Design of a Hip Screw for Delivery of Bone Cement
Section 2: Background and Literature Review 16
2.1. Conclusions
These papers have provided a lot of information on what is currently being
done in this field and an indication of where to go from here. The screw
modifications suggested by Augat and Kramer are the most relevant to this
project; however the testing methods and materials used by some of the
other papers are more appropriate at this stage of the study. It was
decided that testing would initially be carried out in polyurethane blocks
with a similar density to that used by Eriksson at the speed recommended
by the ASTM standard.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 17
3. Initial Testing
In order to test any potential modifications to the hip screw several things
were required. Firstly a prototype screw was designed and manufactured,
then a testing material was chosen as well as a bone cement and test
methodology.
3.1. Background – Sliding Compression Hip Screw
System
The fixation system used in this study was the Omega + Plus sliding
compression hip screw system from Stryker Howmedica Osteonics
(Stryker (Worldwide Headquarters), 2725 Fairfield Road, Kalamazoo, MI
49002, U.S.A.). These were generously donated by Stryker Australia.
The Omega + Plus system consists of an 85mm Lag screw, a
compressing screw and a side plate with cortical bone screws. Both the
length of the lag screw and the length of the side plate can be changed to
suit the recipient and fracture characteristics.
For a femoral neck or intertrochanteric fracture, the lag screw and side
plate would normally be inserted then the compressing screw used to pull
the lag screw back, to compress the fracture. This assists in the healing of
the fracture while providing mechanical stability. The second mechanical
advantage of the system is the ability of the lag screw to slide within the
barrel of the side plate. This allows for further compression of the fracture,
while helping to prevent cut out of the screw through the femoral head.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 18
3.2. Screw Modifications
In order to deliver the bone cement to the desired location in the head of
the femur, modifications to the lag screw design were required. The basic
principle behind the design was to use the lag screw as the delivery device
for the bone cement, allowing simple accurate and timely cement
augmentation after insertion.
Two possible designs are suggested by the literature (Augat et al., 2002,
Kramer et al., 2000). These are both very similar designs involving the
removal of three rectangular sections of material, running axially through
the threaded section of the screw, leaving slots for cement delivery (Figure
3.1).
Figure 3.1 A schematic diagram showing the modifications used by Kramer et al. (Kramer et al., 2000) and Augat et al. (Augat et al., 2002) The screw had set of three rectangular slots placed axially through the threaded section of a screw.
It was decided to avoid this style of design due to its similarity to a thread
tap – in which the debris produced during cutting is removed via the
channels along the side of the tap. In this situation, a ‘tap’ style of design
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 19
would cause the slots to fill with bony material and clog, reducing or
preventing the cement delivery.
For ease of manufacturing – both of the prototype and the final product, it
was decided to use an existing screw and make modifications to it, rather
than design a screw from scratch. As such the basic characteristics of the
screw, such as the thread shape, size and pitch, and the barrel diameter
were left unmodified. This also allows for the design modifications to be
easily transposed on to another screw design if required in the future.
The design then aimed to create an even mantle of cement around the
threaded section of the screw. Two designs were created; the first was
used only in pilot testing and involved a series of slots parallel to the
threads on opposite sides of the screw (Figure 3.2 and Appendix 1). This
design was abandoned due to a manufacturing difficulty creating weak
points in the screw. The second design was also used in the pilot testing;
it was then recreated for use in this first stage of testing. The design
consists of three sets of three holes, at 120o to each other and positioned
in alternate thread troughs. The holes were given a diameter of 1.6mm,
this gave an adequate cement flow without interfering with the thread
characteristics at all, reducing the likelihood of clogging or a significant
reduction in strength (Figure 3.2 and Appendix 1)
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 20
Figure 3.2 Photos of all screw designs, A: The slotted screw used in pilot testing; B: A screw with holes similar to the final design, also used in pilot testing; C: The Original unmodified screw; D: The modified screw with holes for cement delivery.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 21
3.3. Test material – Bone or Analogue?
In order to undertake repeatable tests a Sawbones (Pacific Research
laboratories, 10221 SW 188th St, Vashon, Washington, 98070, USA)
material was used as a bone analogue for these tests.
This material was chosen over an animal or human cadaveric bone model
because of the uniformity and repeatability it offered for testing. A bone
model may be used at a later stage of testing.
The Sawbones material (Figure 3.3) is a cellular rigid polyurethane foam,
with a density of 0.16g/cc, which is equivalent to moderate osteoporosis
and is similar to the middle of three densities used by Eriksson, (2002) in
his study of hip screw fixation.
It was decided to use the cellular rigid foam rather than the solid rigid foam
to allow for cement penetration into the area surrounding the implant. The
solid foam conforms to (ASTM, F1839-01 Standard Specification for Rigid
Polyurethane Foam for Use as a Standard Material for Testing
Orthopaedic Devices and Instruments), and so gives much more
reproducible results than animal or cadaveric material. The cellular rigid
foam however, is slightly less consistent and uniform that the solid foam.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 22
Figure 3.3 A sample of Sawbones polyurethane foam with bone cement penetration
3.4. Bone Cement
The cement used in this study was Howmedica Antibiotic Simplex ®
Radiopaque Bone Cement with Tobramycin (Howmedica International S.
de R.L., Raheen Business Park, Limerick, Ireland). It was decided to use
Polymethylmethacrylate (PMMA) cement rather than a calcium phosphate
cement as methacrylates are more commonly used clinically because of
their mechanical characteristics, though the modified screw could also be
used with a calcium phosphate cement in the future.
An antibiotic and radiopaque cement was chosen primarily because of its
availability. While cements with additives are mechanically weaker than
standard cements, this was not seen as a problem as the nature of the
study was comparative.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 23
3.4.1. Standard or low Viscosity Bone Cement
In order to utilise the design features of the modified screw and allow for
injection, low viscosity cement was required. While the chosen cement
was relatively low in viscosity, it was decided to lower the viscosity further
by modifying the powder to liquid ratio from the standard 2:1 mix, to a less
viscous 4:3. This not only enabled injection but also increased the working
time of the cement in its liquid phase.
Compression tests were conducted on the cement to determine any
changes in mechanical properties caused by this change in mixing ratio.
These test were conducted in accordance with (ASTM, 1999). The testing
compared the standard 2:1 mix ratio with the lower viscosity 4:3 ratio. It
also compared two different curing conditions, room temperature 22oC or
body temperature 37oC simulated by an oven.
Using a mould described in the standard, between five and eleven
samples of each specification were made. The final number of samples
was dependant on how many were rejected from testing due to voids or
large bubbles; however it was ensured that there was a minimum of five
for each condition, in line with the standard’s recommendations. The
cement samples were then allowed to cure at the appropriate temperature
for 24hr. The samples then had their ends machined flat and the final
height of each was recorded. The samples were compressed between
two flat plates in the Hounsfield testing apparatus at a rate of 20mm/min
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 24
until failure, as recommended by the standard. The yield load was
calculated by taking a 2% offset (Figure 3.4 and 3.5). A student t test was
conducted to determine the significance of the results (Table 3.1).
Table 3.1. Student t test p values comparing the effect of temperature and mix ratio on yield strength for bone cement samples
Student t test results using tabled p values Ratio 4:3 37oC Vs 22oC
0.0001 Highly Significant Ratio 2:1 37oC Vs 22oC
0.005 Highly Significant
Temperature 37oC 4:3 Vs 2:1 0.0001 Highly Significant
Temperature 22oC 4:3 Vs 2:1
0.15 Not Significant
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 25
Figure 3.4 Bone Cement Compression Data, samples with a mix ratio of 2:1 are shown in red when cured at 22oC and blue at 37oC, samples with a mix ratio of 4:3 are shown in green when cured at 22oC and yellow at 37oC
-1000
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Displacement (mm)
Forc
e (N
)
2 to 1 @ 22deg2 to 1 @ 37 deg4 to 3 @ 22 deg4 to 3 @ 37 deg
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 26
Figure 3.5 The yield load of bone cement samples with two different mix ratios and curing temperatures, the mean value is shown in column one in red with plus or minus one standard deviation marked
6053.33
6915.75
6454.056038.00
0.00
1000.00
2000.00
3000.00
4000.00
5000.00
6000.00
7000.00
8000.00
9000.00
4 to 3 @ 37deg 4 to 3 @ 22 deg 2 to 1 @ 37 deg 2 to 1 @ 22 deg
Yeild
Loa
d (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 27
3.4.2. Curing requirements – 24hr @37oC
All samples containing bone cement were cured at 37oC for 24hr prior to
testing. This mimics the conditions the cement would experience in vivo.
It was originally thought that in order to maintain the samples at 37oC they
would have to be kept in a water bath. This raised questions about
whether the strength of the polyurethane bone analogue would be affected
by being submerged in saline or simulated body fluid for 24hr.
Because of this it was necessary to test for any changes that might occur
to the foam during the curing. The material properties of the Sawbones
polyurethane test blocks were tested in compression when dry (normal
usage) and after being soaked in Ringers solution for several days.
Small cubes of Sawbones (10mm side) were placed in sealed jars of
freshly prepared Toad Ringers Solution (Appendix 2). They were left to
soak for a period of four days. The samples were then compressed in the
Hounsfield testing machine at a rate of 5mm/min. This is the same rate at
which the final screw samples will be tested. Force displacement data
were recorded during testing.
There was a tendency for the soaking of the samples to lower the failure
load of the wet samples but no significant difference was seen (Figure 3.6).
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 28
However at this point an oven became available in which the samples
could be maintained accurately and consistently at 37oC in a dry
environment. This was considered to be the better option.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 29
Figure 3.6 Compression data of foam samples after soaking in ringers solution (red) or in the standard dry state, the range of the data is shown with error bars at selected data points
0
50
100
150
200
250
300
350
400
450
500
550
600
0 1 2 3 4 5 6 7 8 9 10 11
Displacement (mm)
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 30
3.5. Apparatus
All testing was carried out on a Hounsfield 25kN universal testing machine
(model number H25KS, Hounsfield Test Equipment, 6 Perrywood
Business Park, Salfords, Red Hill, RH1 5DZ, UK), located in the
Biomedical Engineering Laboratory, School of Engineering Systems, QUT,
Brisbane.
Samples were cured in a Special liquid N2 Injection Chamber (Serial no
3863, Thermoline L+M Australia, Thermoline Scientific Equipment Pty Ltd,
9 Tarlington place, Smithfield, NSW 2164, Australia), which was
maintained at 37oC. All screws were implanted using a standard set of
Stryker Endoscopy Surgical Drills (Stryker (Worldwide Headquarters),
2725 Fairfield Road, Kalamazoo, MI 49002, U.S.A.).
3.6. Screw Fixation Strength Studies
3.6.1. Rationale
The Pilot testing demonstrated the possibility of greatly increased holding
strength when utilising this method. However, several problems with the
methods of augmentation and testing were identified. As all of the
samples were tested in the same block of testing foam, interference was
seen between tests. These issues were rectified in this initial phase of
testing.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 31
In this phase three different implantation techniques were tested (Table
3.2). These were then compared with the data already collected on the
original un-cemented method. The original screw, augmented with bone
cement was re-tested as well as the modified screw with bone cement.
The modified screw with bone cement augmentation and the guide pin
hole plugged was also tested. This last test was included at the end of
this series of tests.
Table 3.2. Number of Samples of each type tested in initial testing Original Screw Clinical Method Cemented Method
No. of samples 4 5 Modified Screw Cement Augmented Guide pin hole plugged
and Cement Augmented No. of samples 5 5
In order to test the modifications in a clinically relevant way it was decided
to model the tests on one of the common modes of failure of the screw.
Cut-out of the screw occurs by compression of the bone onto the tip of the
screw continuing until the screw cuts out of the femoral head. Because of
this mode of failure it was decided to test the push-through strength of the
screws rather than the more commonly tested pull-out strength. This was
also suggested by Eriksson, (2002), who noted that with the use of a
femoral plate the screw will not fail by pulling out.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 32
3.6.2. Methodology
Large blocks of Sawbones PU foam (0.16g/cc) were cut into six (6) smaller
blocks each with dimensions 60 x 60 x 40 mm. Each of the small blocks
was then labelled as to which large block it had come from, and the six
individual blocks were given a number e.g. Large Block 1, Small Block 4.
The blocks were measured to determine their final size.
A central hole was created in each block. The diagonals were marked to
determine the centre, which was then drilled with a Ø9mm drill bit in a
manual drill press. The depth of drilling was preset to 2-3mm from the
base of the block, and was the same for all of the blocks.
The chosen screw (original or modified) was manually inserted into the
central hole of the block, ensuring that the shaft of the screw remained
vertical at all times (Subsequently referred to as Sawbones 90o Samples).
Due to the limited number of test screws available, tests were conducted
in pairs, with one original cemented sample and one modified cemented
sample being prepared and tested in each pair. The original screw,
uncemented samples were tested separately as they did not need to be
cured.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 33
3.6.3. Cement Delivery to the Original Screw
The original screw was implanted and then removed from the block to tap
the hole and allow for easier insertion after cement had been added. The
guide pin hole at the tip of the screw was capped with plasticine to prevent
the bone cement from flowing back into the cannula of the screw as this
would have decreased the volume of cement left around the threads of the
screw.
The cement components were then measured, 4.7g of powder and 3.5mL
of liquid monomer. The cement was mixed and 2.5 millilitres of cement
was used to fill the pre-tapped hole. This method is consistent with that
used by Eriksson (2002) to augment hip fixation devices.
The volume of cement used was determined to be equal to the volume of
the central hole in the foam block, 2.5mL. This volume was chosen as it
was the maximum volume of cement able to be delivered to the original
cemented samples prior to re-insertion.
The original screw was then reinserted, and the sample was placed in the
oven to cure at 37oC for 24hr.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 34
3.6.4. Cement Delivery to the Modified Screw
With the modified screw in the desired location, the Hounsfield injecting
apparatus was assembled (Figure 3.7). The cement was then measured
and mixed in the same proportions as in the original cementing procedure.
The volume of cement used for the modified screw samples was
determined to be equal to the volume of the initial hole plus the internal
volume of the screw, a total of 3mL. As the original screw had its guide
pin hole capped to prevent cement reflux, this resulted in the same
external volume of cement in each case.
The required volume of cement was placed in a 10mL syringe and a luer
lock fitting attached to the end. The syringe was placed inside an
aluminium tube on top of a base plate. The implanted screw was inserted
up through the base plate and attached to the end of the syringe via the
luer lock. This arrangement was used to keep the syringe body still and
attached to the screw while the Hounsfield crosshead applied force to the
plunger (Figure 3.7). A fume extraction system was used during the
cementing procedure.
The Hounsfield injection apparatus was used to deliver the cement at a
cross-head rate of 50mm/min. This rate was chosen as the cement is a
Non-Newtonian fluid which exhibits shear thickening, however the cement
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 35
needed to be delivered within 2-4mins of mixing. This rate allowed the
desired 3mL of cement to be delivered in 30sec.
The sample was returned to the oven to cure for 24hr at 37oC
Figure 3.7 Diagrammatic representation of the Hounsfield Injection Apparatus, the screw inserted in the foam block is supported by the syringe, which in turn is supported in the metal tube on the base plate
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 36
3.7. Clinical risk of guide pin puncturing the head of the
femur
At this point it was recognised that clinically there is a risk associated with
injecting cement into the head of the femur if the guide wire has punctured
the joint space. While care is taken by the surgeon to avoid this, it does
occasionally happen, and when combined with cement augmentation, runs
the risk of fusing the joint. A case such as this was reported by Bartucci et
al. (1985) in which cement was seen to leak into the joint space when the
guide wire had punctured the head of the femur. This method is risky and
because of this it was decided to also test an additional modification to the
implant that may reduce the risk of cement leaking in this situation.
The modification would ideally involve the permanent closure of the guide
wire hole in the modified screw, possibly even with an elongated tip to
block the hole left by the guide wire in front of the screw (Appendix 1:
Technical drawings).
To test this theory before any further modifications were made to the
already modified screw, a temporary plug of plasticine was used. This
was found to be sufficient to prevent any leakage of cement. Care was
also taken that the plug sealed the end of the screw without impeding the
cement flow out of any of the cementing holes.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 37
3.7.1. Testing the modified screw with a temporary plug in the
guide wire hole
The procedure followed for the modified screw with a plugged guide wire
hole was essentially the same as for the standard modified screw.
However, a plasticine plug was used to block the guide wire hole prior to
the implantation of the screw. The bone cement was mixed and injected in
the same method as for the standard modified screws and cured at 37oC
for 24hrs.
3.8. Testing
A Hounsfield adaptor that could be screwed into the top of the lag screw
(Appendix 1) was attached to the crosshead. The construct was placed on
a suitable base, a flat doughnut shaped plate with a large internal diameter,
arranged to allow movement of the screw out of the base of the foam block.
The screw was attached to the adaptor and the crosshead lowered until
the foam was almost in contact with the base but without any load being
applied (Figure 3.8).
Testing was carried out at a rate of 5mm/min to a maximum displacement
of 10mm or until failure. Once removed from the blocks the screws were
photographed to show failure modes and cement distributions. (Figure 3.9)
The screws were then cleaned by soaking in acetone.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 38
3.9. Results
The failure curves for all of the Sawbones 90o
samples are shown in Figure 3.10 and Table
3.3. Cracks were seen in the base of the
blocks after testing (Figure 3.11).
Figure 3.8 A Photograph of the Hounsfield Testing Apparatus with sample in place
Figure 3.9 Bone Cement distribution around the modified screw once removed from the test block
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 39
Figure 3.10 Failure curves for all Sawbones 90o tests, the original screw uncemented samples are shown in green, the original screw augmented with cement in black, the modified augmented samples in blue and the modified screw samples with closed guide wire hole in red
0
500
1000
1500
2000
2500
3000
3500
4000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Displacement (mm)
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 40
Figure 3.11 Cracks visible in the base of the foam blocks after testing
3.10. Statistics
Table 3.3. Maximum Load Data for Sawbones 90o tests
Test No Original No Cement
Original Cemented
Modified Cemented
Modified Plugged Cemented
1 794.4 3476 2944 34122 933 2924 3028 34683 882 3180 2912 34324 871 3308 2708 33965 3224 3180 34006 2724 - 3156 Mean 870.1 3112.57 2954.40 3377.33Standard Deviation 57.2402 258.06 172.43 111.59Maximum 933 3476 3180 3468Minimum 794 2724 2708 3156Range 139 752 472 312
The stiffness of each sample was evaluated along the linear portion of the
failure curve. The results are shown in Table 3.4 and Figure 4.19.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 41
Table 3.4. Stiffness data (N/mm) for Sawbones 90o tests, evaluated along the linear section of the curve
Original No Cement
Original Cemented
Modified Cemented
Modified Plugged Cemented
1 485.06 2734.10 2307.10 2576.202 1007.90 2264.80 2430.50 2506.503 879.47 2512.70 2336.30 3077.404 728.82 2623.20 2245.30 2554.605 2715.20 2587.90 2419.006 2414.10 - 2824.90
Mean 775.31 2544.02 2381.42 2659.77St Deviation 224.61 183.04 133.35 245.43Maximum 1007.90 2734.10 2587.90 3077.40Minimum 485.06 2264.80 2245.30 2419.00
A simple Student t-test was carried out on the data to determine the
apparent significance of the results. A comparison was made between the
mean failure loads of the original screw in its uncemented and cemented
form. This was found to be a highly significant difference (p ≤ 0.0001). A
comparison was also made of the differences between the different
cementing methods with the associated p-values displayed in table 3.5.
Table 3.5. Simple Student t-test results using tables of p-values, comparing the change in failure load from different implantation methods tested in the Sawbones 90o testing
Student t test results using tabled p values Original Cemented Vs Original Uncemented
p ≤ 0.0001 Highly Significant Blocked Modified Vs Modified
p ≤ 0.0005 Highly Significant
Original Cemented Vs Blocked Cemented p ≤ 0.025 Significant
Original Cemented Vs Modified Cemented
p ≤ 0.15 Not Significant
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 42
3.11. Discussion
The testing method used in this series of tests was chosen to mimic cut-
out of the lag screw through the femoral head. This is one of the common
modes of failure of lag screws with side plates. It is generally the result of
impaction of the femoral head down over the tip of the lag screw. For this
reason it was decided to test the push-through strength of the screw rather
than the pull-out strength. This method was also suggested as being
appropriate for fixation systems that use a lag screw and end plate by
Eriksson et al. (2002). While this is a very simplified method involving a
coaxial force, a more clinically relevant arrangement was tested in the next
phase of testing, described in Chapter 4.
It was decided that it was necessary to block the guide wire hole at the tip
of the screw because of the clinical risk associated with bone cement
injection with a punctured head of femur. In a clinical situation the guide
wire is inserted into the head of the femur first. As the bone is often very
weak from osteoporosis, it is not unheard of for the guide wire to puncture
a hole completely or partially through the head of the femur. Normally this
would not cause a great problem, however when a low viscosity bone
cement is being introduced to the area surrounding the screw after the
removal of the guide pin, there is the potential for the cement to flow down
the hole left by the guide wire, out of the head of the femur and into the
joint itself. This would be a disastrous outcome resulting in the cementing
of the joint and dramatic if not total reduction in range of motion. Because
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 43
of this it is desirable to prevent any cement from flowing past the tip of the
screw. Closing the guide wire hole at the tip of the screw will greatly
reduce the potential for cement penetration away from the tip of the screw.
During these experiments it was only possible to use a temporary cap to
close the guide wire hole; however it is desirable to use a more permanent
cap or plug clinically. Possible designs for the plug, as well as other
possible modifications to the screw and implantation method are
discussed later in this report. The temporary plug used was made of a thin
layer of plasticine; this was inserted carefully into the tip of the screw to
ensure the hole was completely plugged without causing any interference
to the cement flow out of the first of the cement holes. This method
proved adequate in preventing cement leakage out of the tip of the screw.
In each case the plasticine plug was found to be intact during cleaning of
the screw.
One problem identified with the technique used in these tests was in the
implantation of the screw. Insertion of the screws was done manually
without the use of a guide wire. This caused the screws to deviate slightly
from the ideal position, perpendicular to the surface of the foam. The
design of the testing apparatus then meant that the bottom of the foam
block was at an angle to the base plate. The slight variation in angle of
each of the screws may account for a small degree of error in the results
and the varying size of the toe region seen in Figure 3.10. This problem
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 44
was alleviated in subsequent tests by using an alignment jig to insert the
screw.
In this phase of the study the cement was injected via a 10mL syringe
either manually or using the Hounsfield injection apparatus. Clinically a
cement gun would be used to inject the cement via a catheter however it
was felt that this was not necessary at this stage of the testing. The use of
the Hounsfield injection apparatus allowed the cement to be injected at a
controlled rate.
The testing apparatus was designed such that the movement of the screw
out of the base of the block would not be impeded, while still providing
adequate support to the rest of the block. This proved to be an adequate
arrangement, with cracks visible in the base of the blocks after testing
demonstrating the movement of the screw.
All of the results gathered exhibited a small ‘bump’ in their force-
displacement graphs at around 600N. It was determined that this was due
to backlash on the screws driving the cross-head in the Hounsfield testing
machine. This means that the initial force is applied by the weight of the
cross-head, and then as this weight is reached the slack is taken up by the
screws – creating the ‘bump’, before any additional force is applied. It is
expected that this will be seen in all compression tests conducted on this
machine.
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 45
It was decided to test a minimum of five samples for each method as the
pilot testing suggested that this would be enough to determine any
significant differences between the methods.
The force – displacement data clearly show the benefits of bone cement
augmentation. The peak load prior to failure in the cemented samples
(minimum 2950N) was more than three times that of the un-cemented
samples (870N). The use of even such a small volume of bone cement to
augment the fixation greatly improved the holding power and stability of
the system. A simple Student t-test showed that this was a highly
significant difference (p ≤ 0.0001).
The original screw augmented with bone cement had a mean failure load
of 3112N; 200N more than that of the modified screw with augmentation.
However the range of values and the standard deviation of the original
cemented samples were much greater than the modified samples. So
while the modified samples were slightly weaker they were much more
predictable with the standard deviation and range of values being half that
of the original screw with cement.
The introduction of the plug to the guide wire hole in the modified screw
had a substantial effect on the results observed. Not only did it increase
the mean load to failure to 3377N but it also decreased the standard
deviation and range of values, making it not only the strongest fixation but
Design of a Hip Screw for Delivery of Bone Cement
Section 3: Initial Testing 46
also the most reliable and predictable method of augmentation. This was
also found to be statistically significant (p ≤ 0.0005). This supports the
decision to only continue testing with the closed end screw and to
permanently close the guide wire hole in the future.
3.12. Conclusion
The results of this testing showed a highly significant (p ≤ 0.0001)
difference in mean failure load with the addition of bone cement. They
also showed a highly significant (p ≤ 0.0005) difference between the
closed end modified screw and the standard modified screw. These
findings support continued testing of this method of improvement with the
closed end modified screw.
Following these studies it was considered that a better model of the clinical
loading environment, with the primary load being applied to the screw at
an angle rather than coaxially, simulating one-leg stance, should be
investigated to more fully characterise the affect of cement augmenting on
the system, this is investigated in Chapter 4.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 47
4. Continued Testing
4.1. Rationale/Introduction
After completion of the first stage of testing, it was decided to progress to a
more physiologically accurate model. As such the second stage of testing
was designed with the force being applied to the screw as it would be
clinically. This method is also a better simulation of the cut-out mode of
failure.
In a clinical situation the force applied to the implanted screw is applied at
an angle, rather than the coaxial force used in the initial testing. For
example in a simulated one-legged stance the force is applied at an angle
of 45o to the axis of the screw (Figure 4.1), this angle was often used in
similar testing scenarios in the literature (Moore et al., 1997, Sommers et
al., 2004, Witschger et al., 1991).
The necessity of plugging the guide pin hole was shown in the initial
testing. This negated the need to test the open ended modified screw.
Only three techniques were tested, the original clinical method, the original
screw with bone cement augmentation and the modified screw with
plugged guide pin hole and augmented with cement. A statistical analysis
of the data gathered in phase one of testing was used to determine the
number of test required in this stage (Table 4.1 and Appendix 3).
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 48
Figure 4.1 A cut-away view of a hip screw implanted in a Sawbones femur showing the force application angle.
Table 4.1. Number of Samples of each technique in Sawbones 45o testing as determined statistically from the data from Sawbones 90o testing (Appendix 3)
Original Screw Clinical Method Cemented Method No. of samples 9 9 Modified Screw Guide pin hole plugged and Cement Augmented No. of samples 9
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 49
4.2. Method
Blocks of Sawbones bone analogue foam, of the same density (0.16g/cc)
and strength as those used previously, were cut into smaller cubes of
dimension 60mm x 60mm x 40mm. Pilot holes of 9mm diameter were
then drilled in the blocks at an angle of 45o and to a depth of 25mm such
that the tip of the screw, once implanted, was in the centre of the block
(Figure 4.2). An alignment jig was used to ensure the screws were
implanted in the blocks at the correct angle (Appendix 1).
Figure 4.2 Schematic diagram of screw placement in the
Sawbones foam blocks
4.2.1. Original screw with no cement
The original screw was implanted in the chosen block using the alignment
jig, and oriented such that the flat sides of the screw were vertical during
testing. This was done to ensure the screw was in the same orientation
for each of the tests. The sample was then placed in the Hounsfield
testing apparatus (Figure 4.3). The screws were implanted to a depth
such that there was a 5-6mm gap between the foam block and the sample
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 50
holder. If necessary the screw was retracted a half rotation to ensure this
clearance.
Figure 4.3 Diagrammatic view of the testing procedure
Using a flat plate attachment, testing was then carried out at a speed of
5mm/min until failure or for a maximum displacement of 5mm, at which
point the foam block would then be in contact with the sample holder. This
is consistent with the testing parameters used in phase one, with the
maximum displacement reduced to reflect the distance between the
bottom of the foam block and the top surface of the sample holder, a
distance of 5mm. If the displacement exceeded this amount the sample
holder would interfere with the loading of the block.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 51
Load-displacement data were recorded for each sample. Photographs of
each sample were also taken post testing showing any failure mode
characteristics visible. The screw was then removed from the block.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 52
4.2.2. Original screw with alternative cement augmentation
method
For the technique involving the original screw with bone cement
augmentation, the screw was inserted in the foam using the alignment jig,
and correctly oriented. The screw was removed in preparation for cement
augmentation.
The bone cement was prepared in the same way as in the initial phase of
testing, using a 4:3 ratio of 4g powder to 3ml liquid. The cement was
injected to fill the hole in the foam (~3mL) and the screw re-inserted. The
samples were left to cure for 24hr at 37oC.
The blocks from sample four onwards were placed on their sides in the
oven while curing to simulate the angle at which they would be cured
clinically i.e. with the patient lying flat on their back. This is discussed
further in the modified cemented methods and discussion.
After 24hr the samples were removed from the oven and placed in the
Hounsfield testing apparatus. Testing was then conducted in the same
manner as for the un-cemented samples.
Load-displacement data were recorded for each sample. Photographs of
each sample were also taken post testing showing any failure mode
characteristics visible. Samples were sectioned to show cement
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 53
penetration and failure characteristics. The blocks were sectioned such
that one side of the block was removed parallel with the screw, then the
base of the block and finally the opposite side of the block, each time
removing material till the first sign of bone cement (Figure 4.4).
Photographs were taken at all stages including the totally removed screw
with cement in place.
The screws were cleaned of cement by soaking in acetone.
Figure 4.4 Method of block sectioning, first cut in red, second in blue and third in green, the screws placement in the block is marked by the black oval.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 54
4.2.3. Results
The results of the original screw with and without cement are shown in
Figure 4.5. The first two samples tested with cement augmentation are
shown in red, these were found to have been from an earlier batch of
Sawbones material and have been removed from further analysis. Further
results and statistics are shown in section 4.5.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 55
Figure 4.5 Failure Curves for the original screw tested at 45o with and without cement, the original uncemented screw samples are shown in green and the original screw cemented samples are in blue. The two samples in red were original cemented samples that were removed from the analysis because of material batch differences
0
500
1000
1500
2000
2500
3000
0 1 2 3 4 5Displacement (mm)
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 56
4.2.4. Discussion
When reviewing the results of the original cemented samples it can be
seen that the first two samples failed at a much lower load than the rest of
the samples, a failure load difference of 17%. While this may have just
been random chance, closer examination of the blocks of sawbones
material that were used, indicated that the first two samples had been
taken from a previous batch to all of the others. Consultation with the
manufacturer suggested that one of the batches was of a different density
to the other, as the difference in strength was greater than what they
considered possible from batch differences. Samples of the two batches
were tested and both were found to have a density of 0.16g/cc or 10pcf.
The only explanation that could be found was the degradation of the
samples over time. The first two samples came from a batch that was
ordered eight months before the second batch of material. This was a
large difference in strength to be attributed to long term storage and it was
decided that there was sufficient uncertainty about the material that data
from the first two tests should be discarded. Two extra specimens were
tested using the new material.
As with the original 90o testing a ‘bump’ can be seen in the compression
data at around 600N. Its presence confirms that it is an external factor,
the mass of the Hounsfield cross-head. A series of small failures can be
seen in several of the samples of both the original cemented and original
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 57
uncemented samples. These failures can most likely be attributed to
minor failures in the PU foam matrix.
The first three samples were cured in the same orientation as they were
inserted, with the screw coming out of the top of the block. Due to
difficulties encountered in the Modified plugged 45o samples that were
being tested simultaneously with the Original samples, the curing
orientation of the samples was changed to reflect clinical conditions. This
meant that as soon as the bone cement was sufficiently thick not to flow
out of the hole, the blocks were placed on their sides with the screw
parallel to the floor. This reflects the clinical orientation of the patient lying
flat on their back during and after surgery. This was done for the
remaining samples, including the two replacement samples. This is
discussed further in the modified plugged 45o testing discussion (Section
4.3.4).
4.2.5. Conclusions
The benefit of bone cement augmentation can again be clearly seen in this
test. The Original Cemented samples withstand a load of two and a half
times as much as the Original uncemented samples.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 58
4.3. Modified Plugged Screw with augmentation
Initially the modified plugged screw was to be implanted and tested in a
similar manner to the original cemented 45o samples. However as this
method was employed, problems developed and had to be corrected for.
The evolution of the method is given below sequentially, with discussion of
the problems encountered, their results and the solutions implemented.
4.3.1. Modified screw Method 1
4.3.1.1. Sample 1
In the first sample, a temporary plug of plasticine was used to close the
guide wire hole, as was done in the 90o push out tests. The screw was
then inserted and correctly oriented in the block using the alignment jig.
With the screw in place the bone cement was then mixed and injected,
using the same ratio and volume of cement used in the original cemented
samples. For this series of tests the injection procedure involved using a
10mL syringe with luer lock fitting to attach to the end of the screw. The
desired volume of bone cement was then injected manually, pressure was
not recorded.
The sample was left to cure at 37oC for 24hr and testing was carried out
as for the original cemented samples. The sample was sectioned and
photographed, again in the same manner as for the original cemented
samples.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 59
The force – displacement curves from samples one through four are
shown in Figure 4.10. Sample 1 did not fail when tested. It was
determined that this was because the screw was implanted too deeply in
the block restricting movement.
Sectioning showed that the cement was not evenly distributed around the
screw. Figure 4.6 shows a large bolus of cement around the tip and
underside of the screw with very little around the top threads. This is
consistent with the large amount of force observed and non-failure of the
sample. Figure 4.6 also shows a representative diagram of the modified
screws’ hole pattern and orientation, holes that are shaded in grey have
been closed. The directions of forces applied to the sample are also
marked, with GC being the orientation of gravity while curing and the
testing force F.
Figure 4.6 Bone Cement distribution in Samples 1 and 2 of the modified screw tested at 45o is shown on the left, on the right is a schematic representation of the cement delivery holes in the screw, the large centre hole is the guide wire hole, while the three sets of holes for cement delivery are shown radially representing their distance from the tip of the screw, shaded holes have been closed to prevent cement flow
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 60
4.3.1.2. Sample 2
The method used for sample one was repeated for sample two; the depth
of implantation was checked prior to cement augmentation to ensure there
was adequate room for failure.
This sample failed under load as expected. Sectioning of the sample
showed a similar cement distribution to that seen in sample one. The
sectioning also showed that the sample failed by the separating of the
foam and cement/screw along the top surface of the threads and had
crushed the material under the screw (Figure 4.7).
Figure 4.7 The failure pattern of Sample 2, the foam was seen to separate from the top of the screw, while crushing underneath it
4.3.1.3. Discussion
The bone cement that was injected was seen to pool around the bottom
surface and tip of the screw. This is not an ideal cement distribution. To
attempt to correct this it was decided to minimise the cement flow out of
the area around the tip of the screw.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 61
4.3.2. Modified screw method 2
4.3.2.1. Sample 3
It was then attempted to fix this problem of cement pooling in sample three
by closing some of the cement holes. In this sample the hole closest to
the tip of the screw in each of the three sets of holes was plugged as well
as the guide wire hole, this is shown in Figure 4.8.
4.3.2.2. Results
This sample also failed as expected and by the same mode as sample 2.
Sectioning however showed that the distribution problems had not been
corrected. The cement had still pooled slightly at the tip of the screw and
showed an even distribution down the back of the threads however the top
surface of the screw was still not covered, the cement had also leaked out
onto the surface of the block as shown in Figure 4.8.
Figure 4.8 Cement distribution in Sample 3 of modified screw at 45o with schematic view of the closed holes
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 62
4.3.3. Modified screw method 3
4.3.3.1. Sample 4
To reduce the pooling of the cement and increase distribution around the
top threads of the screw the plug in the first hole of the top set of holes
was removed leaving only the first hole in each of the bottom sets plugged,
Figure 4.9.
4.3.3.2. Results
Failure was by the same method as for the previous samples. Sectioning
showed the cement was evenly distributed around the threads; however
there was still significant pooling at the tip of the screw (Figure 4.9).
Figure 4.9 Cement distribution and hole closure pattern in sample 4, with schematic view of the closed holes
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 63
Figure 4.10 Results of Samples 1-4 (methods 1, 2 and 3) for the Sawbones 45o tests, Sample 1 is shown in dark blue, Sample 2 in pink, Sample 3 in yellow and Sample 4 in light blue
0
500
1000
1500
2000
2500
3000
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Displacement (mm)
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 64
4.3.4. Modified screw method 4
4.3.4.1. Sample 5
It was determined that the pooling of the cement at the tip, and under the
bottom threads of the screw was due to the effects of gravity during curing.
Because of this it was necessary to cure the samples in the manner and
orientation that they would be cured clinically. This would normally be with
the patient lying flat on their back, making the screw barrel parallel to the
floor. This change in procedure was also instigated in the Original
Cemented samples at sample number four.
It was also decided to try a new arrangement of open and closed holes.
With the screw now in an upright Y position, the entire bottom row of holes
were plugged, Figure 4.11. The screw was then oriented such that the
closed holes would be pointing vertically down when the screw was in its
curing position.
It was decided not to mechanically test any more of the trial methods until
the final method had been decided upon, and so the sample was then left
to cure for only 1 hr at 37oC. After this time the cement had hardened
sufficiently to allow sectioning and observation of the cement penetration
pattern.
Sectioning of the sample showed very even coverage around the threads
of the screw, however it was evident that the screw had not been
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 65
implanted to the full depth of the pilot hole, as there was a section of
cement at the tip, that had pooled in an open area in front of the screw,
this is shown in Figure 4.11.
Figure 4.11 Cement distribution and hole closure pattern of sample 5, with schematic view of the closed holes
4.3.4.2. Discussion
It could be seen in the sectioning of the screw that it had not been
implanted to the correct depth. This is an easily avoidable mistake caused
by human error. Marking the depth of implantation of the side of the screw
prior to the final implantation will prevent this.
4.3.4.3. Sample 6
The results from sample 5 were promising however it was evident that the
screw had not been implanted correctly. Because of this sample 6 was
done as a repeat of the method used for sample 5.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 66
This sample was also sectioned after 1 hr of curing without testing. The
cement distribution was very even and showed no pooling whatsoever as
shown in Figure 4.12 Cement distribution.
Figure 4.12 Cement distribution and hole arrangement for sample 6, with schematic view of the closed holes
4.3.4.4. Discussion
The distribution of cement around the threads of the screw using this
method was excellent and it was decided to use this method for further
testing.
Experience of surgical colleagues working in this area suggested that
there would be no pooling of the cement clinically. So it was decided to
first test the standard method in porcine cadaveric femurs. It was thought
that the density of the porcine bone may act to prevent any pooling of
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 67
cement. This would eliminate the need to block any of the cementing
holes. It would also raise questions about the validity of the Sawbones
material as a bone analogue.
4.4. New information from the manufacturer of the bone
analogue – 95% closed cell
At this point further information from the manufacturer of the Sawbones
bone analogue foam came to light. A new product, 95% Open cell foam,
was released with the further information that the current cellular rigid
foam is 95% closed cell. Prior to this it was not known that the foam was
closed cell. This made it an inappropriate material for use in mechanical
studies involving bone cement. The new open cell foam also proved
inadequate for this style of testing as it did not have the necessary
mechanical properties to be considered a bone analogue. A copy of the
updated website is included in Appendix 4.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 68
4.4.1. Bone Cement Penetration into Open and Closed cell
foams
A sample of the open cell material was obtained and used to quantify the
difference in cement penetration between the open cell and the closed cell
foams. As the open cell foam is of a lower density (0.12g/cc) than the
material used in this study it was decided to test the open cell (0.12g/cc)
and compare it with a closed cell 0.12g/cc foam.
The pressure required to push a known volume of cement into the foam
was tested in a similar apparatus to that used by Baroud et al. (2003).
4.4.1.1. Method
A 50mL Syringe was modified for use as
the testing apparatus. The tip of the
syringe was removed to leave a cylinder
with a plunger.
Figure 4.13 Bone Cement Penetration, the foam is in the top 10mL of the
syringe with plasticine coating, with cement below it
Samples of the foam were then cut and shaped to fit tightly in the bottom
of the syringe. The full height of the open cell foam was used (19mm)
whereas the closed cell blocks were cut down to create samples of
approximately 20mm in height. Care was taken during this process to
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 69
minimise damage done to the edges of the foam, particularly in the open
cell foam where the filaments were weak and easily crushed or bent. Due
to the cellular nature of the foams even when the cylinders were as closely
fitting as possible there was still sufficient space between the inner wall of
the syringe and the foam to allow cement leakage. To prevent this
leakage the edges of the foam cylinders were sealed with a thin coating of
plasticine, this prevented any leakage without interfering with the
movement of cement through the rest of the foam (Figure 4.14).
Figure 4.14 Foam samples with plasticine edges to prevent leakage, the left is the open cell foam prior to testing with the post testing view on the right.
To test the sample the syringe was loaded with bone cement while held
upside down (Figure 4.13). The foam sample was then placed in the top
and any air removed. The testing apparatus was then inverted and placed
in the Hounsfield testing machine. A flat plate attachment was used to
compress the syringe plunger while measuring the force and displacement
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 70
data. The base of the syringe was supported on a flat doughnut to allow
cement flow out of the base of the foam sample (Figure 4.15). Testing
was conducted at a speed of 120mm/min to a maximum of 20mm as used
by Baroud et al. (2003).
Samples were removed from the testing apparatus immediately and
allowed to cure. The curing orientation
of the samples was the same as the
testing orientation.
Once the cement had hardened the
samples were cut in half and
photographed to show the cement
distribution (Figure 4.16).
Figure 4.15 Cement Penetration Testing apparatus post testing, the modified syringe is supported on a base plate with a hole in the centre slightly smaller than the diameter of the syringe
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 71
4.4.2. Results
The recorded injection pressures are shown in Figure 4.17.
Figure 4.16 Sectioned view of cement penetration into a closed cell foam sample (top) and an open cell foam sample (bottom), in the open cell foam the cement was seen to flow out of the foam before curing leaving gaps in the foam
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 72
Figure 4.17 Pressure required to inject the bone cement into the foam, the closed cell foam is shown in red and the open cell foam in black
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0 2 4 6 8 10 12 14 16 18 20Displacement (mm)
Pres
sure
(Mpa
)
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 73
4.4.3. Discussion
The force required to push the cement into the closed cell foam was much
greater than that required for the open cell foam. It was noted that, if
merely placed on the surface, the cement would flow down through the
open cell foam. The force required for the closed cell foam is greater than
that which would be applied manually with a syringe. It is expected that
the force required to push the cement into the 0.16g/cc foam would be
much higher still, however this has not been tested.
This result suggests, as expected, that in the previous testing the cement
flowed only into those cells that had been ruptured by the drilling or
implantation procedure, giving a consistent thickness to the cement mantle.
This confirmed the unsuitability of the Sawbones material for use in the
mechanical testing of implants with bone cement augmentation.
It has been noted that the appearance of the screw samples with cement
augmentation created in the closed cell foam are very similar to those
shown by Eriksson (2002) in his results using a polyurethane foam. No
mention is made is his paper as to wether the foam is open or closed cell,
however his cement distributions suggest that in fact the foam he was
using was also of a closed cell nature.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 74
From this testing it is apparent that the cement distribution that would be
seen around a screw augmented in an open cell environment would be
much greater than that seen in the closed cell foam.
4.5. All Results
A summary of the failure loads of all Sawbones samples is shown in table
4.2. The stiffness of the samples was also evaluated (Table 4.3)
Table 4.2. Failure load data for all Sawbones 45o samples Original No Cement Original Cement Modified Cemented
1 1053.75 2282.5 1554 2 914 2162.5 2077.5 3 1002.5 2475 1882 4 917 2482.5 5 903 2470 6 978 2365 7 915 2620 8 1038.75 2177.5 9 957
Mean 964.33 2379.38 1837.83 St Dev 57.23 161.81 264.53 Maximum 1053.75 2620.00 2077.50 Minimum 903.00 2162.50 1554.00 n 9 8 3
A simple Student t-test was carried out on the data to determine the
significance of the results. A comparison was made between the mean
failure loads of the Original screw in its uncemented and cemented form.
This was found to be a highly significant difference (p ≤ 0.0001). A
comparison was also made of the differences between the Original
cemented screw samples and the modified screw with plugged end
samples, this difference was also found to be highly significant (p ≤ 0.005).
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 75
Table 4.3. Stiffness Data (N/mm) for all Sawbones 45o samples Original No Cement Original Cement Modified Cemented
1 506.43 813.11 605.65 2 353.34 613.25 1130.40 3 421.62 839.66 709.41 4 518.03 820.72 5 485.74 743.48 6 393.45 800.41 7 288.44 766.32 8 418.87 603.62 9 396.62
Mean 420.28 750.07 815.15 St Dev 74.51 92.58 277.90 Maximum 518.03 839.66 1130.40 Minimum 288.44 603.62 605.65 n 9 8 3
The failure loads and stiffness of all of the Sawbones samples tested at
90o and at 45o are shown in Figures 4.18 and 4.19.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 76
Figure 4.18 Failure loads of all sawbones samples, the mean and standard deviation for each group are shown in red, each sample within a group is shown in a different colour
870.10964.33
3112.572954.40
1837.83
2379.38
3377.33
0
500
1000
1500
2000
2500
3000
3500
4000
Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 77
Figure 4.19 Stiffness of all sawbones samples, the mean and standard deviation of each group are shown in red, each sample within a group is shown in a different colour
2659.77
2381.42
2544.02
775.31
815.15
750.07
420.28
0
500
1000
1500
2000
2500
3000
3500
Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45
Stiff
ness
(N/m
m)
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 78
4.6. Discussion
The angled load testing of the implant was intended to be a better model
of physiological loading conditions than the original coaxial load. This
testing however proved mainly to highlight problems with the testing
materials and methods.
The closed cell nature of the Sawbones foams combined with the batch
degradation problems experienced combine to make an unreliable and
inappropriate testing material for orthopaedic implants, particularly those
involving the use of bone cements.
This became more of a problem with the modified screw because of the
method of cement augmentation. In the original screw samples that were
augmented with bone cement, the cement was placed in the pilot hole
prior to reinsertion of the screw. As the screw was then inserted into the
hole the cement was forced to flow around the threads and pushed into
the surrounding foam, creating an even coverage. With the modified
screw samples, the screw was already in place when the cement was
injected and gravity played a much greater role in determining the flow
pattern of the cement. The cement was seen to flow down the central
cannula and out of the lower holes with no reason to flow out of the upper
holes or coat the threads of the screw.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 79
Despite the problems experienced with the test material, the results of this
angled phase of testing appear promising. The cemented samples of the
original screw were still consistently stronger than their uncemented
counterparts. The stiffness of the samples was also seen to almost double
with the addition of a cement mantle, though this is a much smaller
increase than that seen in the initial testing.
4.7. Conclusions
The Sawbones polyurethane foam was shown not to be suitable as a
testing material due to its predominantly closed cell structure. However
despite this, the method of injection with the modified screw and the use of
bone cement to augment the implant showed significant improvement in
load to failure. To further test the hypothesis a new testing material was
required. The most appropriate material to switch to at this point was
Porcine Cadaveric femora, maintained in pairs from individual animals. It
was predicted that there would not be problems with cement pooling in
bone samples, allowing a return to the original design modifications, with
the only addition being the closure of the guide wire hole in the tip of the
screw.
Design of a Hip Screw for Delivery of Bone Cement
Section 4: Continued Testing 80
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 81
5. Pig Testing
Owing to the problem of cement pooling and the fact that the Sawbones
material was a closed cell foam it was decided to undertake testing with
Porcine Cadaveric material. This material is more variable than the
Sawbones foam, but by conducting tests with paired femora, the effect of
this variability is reduced. Despite this variability the porcine material is a
more appropriate material for studying bone cement due to the open cell
trabeculae and presence of bone marrow and body fluids, giving a better
representation of the clinical environment.
As the bone samples to be used were taken from young animals the bone
was much denser and stronger than that of a typical hip screw recipient.
This will not only result in higher than expected yield strengths but may
also act to reduce the penetration of the bone cement into the bone. It is
predicted that in osteoporotic bone the cement penetration would be
greatly increased and despite weak bone, create a much stronger implant
construct.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 82
5.1. Porcine cadaveric material
Owing to problems encountered in previous methods it was decided not to
test any of the samples until a suitable method of cement injection was
determined. It was also decided not to create any samples using the
original uncemented screw until the method was established as this would
be a waste of resources.
5.1.1. Method Development 1
A porcine femur was acquired and sectioned leaving only the head and
proximal end of the shaft intact. Care was taken to ensure that the
remaining length of the femur was sufficient for correct insertion of the lag
screw.
The implantation procedure then closely followed the standard clinical
procedure. A set of Stryker Endoscopy Surgical drills (Stryker (Worldwide
Headquarters), 2725 Fairfield Road, Kalamazoo, MI 49002, U.S.A.) were
used to first create a guide wire hole through the head of the femur, it was
necessary to have the guide wire puncture the head of the femur to ensure
correct alignment and position without the use of x-rays.
A 9mm diameter hole was created over the guide wire to a depth of 55mm,
such that the threaded section of the screw would be located centrally in
the head of the femur.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 83
The guide wire was removed prior to insertion of the lag screw such that
the end hole of the screw could be plugged to prevent cement leakage.
The modified lag screw, with temporary end plug, was inserted to the
desired depth. The area around the screw barrel and the guide wire hole
were both plugged with plasticine to prevent any cement leakage, cement
restrictors would be used clinically. Bone cement was mixed as per
previous samples and injected using a syringe.
Upon injection it was evident that there was a problem with the method as
a large force was required to start the injection, whereupon the cement
immediately flowed back up the barrel of the screw and out around the
plasticine barrier. The cement was left to cure for one hour before the
sample was sectioned.
Removal of the screw showed that all of the cement holes in the modified
screw had been clogged with bony material, and that the cement had only
been delivered though one of the holes. The clogging of the holes and the
density of the bone was such that the cement had flowed directly up the
barrel of the screw with very minimal penetration into the surrounding bone
(Figure 5.1).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 84
Figure 5.1 Method Development 1, the holes for cement delivery were seen to clog with bone material preventing cement flow out of all but one hole, where the cement flowed back up the shaft of the screw
5.1.1.1. Discussion
The sectioning of the sample showed that all of the holes had been
clogged with debris, most likely created from the drilling of the base hole.
In this sample the base hole was not tapped prior to insertion of the screw
as this is standard clinical procedure.
5.1.2. Method Development 2
The first test in the porcine material demonstrated the need to tap the base
hole prior to insertion of the modified lag screw. This technique was
expected to reduce the clogging of the cement holes with bony debris.
The procedure used for this sample was predominantly the same as for
the previous sample. After drilling of the base hole, with the guide wire still
in place, the modified screw was used to tap the hole. The modified screw
itself was used as no other suitable tap was available. After tapping the
modified screw was removed and any material was cleaned from the
cement holes. Removal of the screw after tapping showed that all of the
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 85
cement holes had been thoroughly clogged with bony material, as was
seen to be the problem in the first pig bone sample. The guide wire was
removed and the temporary plug inserted into the tip of the screw. The
modified screw was reinserted and cemented as per the previous sample.
This sample was not mechanically tested. Sectioning occurred after the
cement had been allowed to cure.
Sectioning of the sample showed that a large number of the cement holes
had become clogged with bony material. Some of the cement had
managed to form a thin coating around the threads, however most of the
cement was found to have flowed back up the shaft of the screw, or
pooled around the tip (Figure 5.2 and 5.3).
Figure 5.2 Method Development 2, the cement delivery holes were seen to clog with bone prior to cement injection, limiting the flow of cement
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 86
Figure 5.3 Method Development 2, cement (white) pooled at the tip of the screw as it was not inserted to the full depth of the hole
5.1.2.1. Discussion
The tapping of the base hole prior to insertion was not sufficient to prevent
the holes from becoming clogged with material. A better method of
cleaning the hole or another method of preventing clogging was required.
The sectioning of the sample showed pooling at the tip of the screw
(Figure 5.3) where it was evident that the hole had been tapped to a
greater depth than the screw was finally inserted. This can easily be
avoided by marking the screw with the depth of the hole prior to insertion.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 87
5.1.3. Method Development 3
At this point a new method for inserting the lag screw needed to be
developed to counteract the clogging of the holes. It was decided to drill a
larger base hole and to tap it prior to insertion. This technique was
expected to create a large enough gap between the screw barrel and the
bone to allow for compression of the material during the tapping without
impinging on the cement holes.
As it was expected that this method would work, a pair of porcine femora
were obtained and the original screw implanted in one without cement.
This was done using standard clinical implantation procedure.
In the contralateral femur the modified screw was implanted. In this case
after the guide wire was inserted a base hole of Ø10mm was drilled rather
than the standard Ø9mm. This left a gap of 0.5mm around the barrel of
the screw. With the guide wire still in place, the modified screw was again
used to tap the hole. The depth of the hole was marked on the side of the
screw to ensure correct placement. It was removed and cleaned and the
guide wire removed. With the end plugged the screw was reinserted to
the marked depth.
The modified screw was cemented in the same manner as for the previous
samples. Both samples were then cured at 4oC for 24hr. This was done
to ensure the samples did not decay during curing.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 88
The samples were tested by supporting the femoral head in a dental
acrylic ring, with the force applied to the screw to push it out of the head
(Figure 5.4). However, on testing, the femoral heads collapsed pushing
through the acrylic ring. A new method was devised for subsequent
testing.
Figure 5.4 Schematic diagram of the testing method used in Method Development 3, the femoral head was supported using a dental acrylic ring and the force applied to push out the screw
5.1.3.1. Discussion
The insertion method was quite successful in preventing the holes from
becoming clogged with debris. This method was used as the standard
method for subsequent tests.
It was also seen that the cementing of samples using a syringe was not
sufficient. In order to more accurately represent clinical practice a cement
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 89
gun should be used. This would also allow the cement to be pressurised
in the femur, giving a better distribution.
The testing method used did not work and a new method was developed
for subsequent tests.
5.1.4. Numerical Results 1
With a successful implantation method now developed an adequate
testing procedure was required. The method used by both Witschger
(1991) and Moore (1997) in their studies was adopted. To facilitate this, a
stainless steel test rig was manufactured (Figure 5.5 and Appendix 1).
Two fresh femora were then implanted, one with the original screw and the
other with the modified screw augmented with bone cement. After
placement of the modified screw, the guide wire hole in the head of the
femur was closed with a self tapping screw and washer. A cement gun
was loaded with bone cement and attached to the end of the modified
screw using a luer lock fitting. Ten millilitres of bone cement was injected
into the screw. The luer lock and cement gun were removed and cleaned
immediately. The self tapping screw in the guide wire hole was left in
place for a few minutes to ensure there was no leakage of cement, it was
then also removed and cleaned.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 90
Both samples were allowed to cure at 4oC for 24hr. The new angled
testing rig was then assembled (Figure 5.5). A Hounsfield adaptor
originally designed for use with an acetabular cup was used to apply an
even load to the head of the femur. The samples were tested at a cross-
head speed of 5mm/min until failure or to a maximum displacement of
10mm. They were subsequently sectioned and photographed.
Figure 5.5 Photograph of the stainless steel angled testing rig with sample in place
5.1.4.1. Results
The modified cemented screw (2732N) proved stronger than the original
uncemented screw (1815N). Full results for all pig tests are given in
section 5.3: Summary of all Pig testing results.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 91
5.1.4.2. Discussion
The cement injection procedure using the cement gun worked well and
gave a better cement penetration than was achieved previously.
Recording the pressure at which the cement is applied would be a useful
addition to the procedure.
The sectioning of the samples highlighted another problem with the bone
cement, that of visibility in the bone. As the cement is effectively the same
colour as the bone, it becomes difficult to differentiate between the two,
preventing an adequate determination of cement penetration. A solution
to this problem is to dye the monomer of the cement prior to mixing. The
addition of a few crystals of Crystal Violet allowed to dissolve in the
monomer will create blue cement while only very marginally affecting the
mechanical properties.
The testing procedure worked very well, and was used for subsequent
testing.
5.1.5. Numerical Results 2
In this pair of samples the method followed was very similar to that of the
previous pair. However, the cementing procedure for the modified screw
was altered slightly.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 92
Once the modified screw was correctly positioned a pressure transducer
(RS 256 – 736, 10 Bar G, 0-100mV) was screwed into the guide wire hole
in the head of the femur. This was set up to record the pressure in the
head of the femur during bone cement injection. A few crystals of Crystal
Violet were added to the bone cement monomer and allowed to dissolve
completely prior to mixing. The injection then continued as normal.
The samples were cured and tested in the same manner as for the
previous samples. They were then sectioned and photographed as usual.
5.1.5.1. Results
The modified cemented screw (4340N) again proved stronger than the
original uncemented screw (3152N). This pair of samples proved much
stronger than the previous pair of samples (All results shown in section
5.3).
The pressure observed in the head of the femur during injection was much
higher than anticipated with peak pressures exceeding 30Bar or 3.5MPa,
the saturation point of the transducer. The injection pressure was applied
using an injection gun with a trigger system; this caused the pressure
applied to appear cyclic in nature (Figure 5.7).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 93
5.1.5.2. Discussion
Sectioning of the samples showed immediately that both of the screws had
been bent during testing (Figure 5.6). The bend had occurred at the edge
of the sample supporting apparatus. This tells us that in this case the
strength of the bone/screw or bone/cement interface was greater than the
bending strength of the stainless steel screw.
Figure 5.6 The modified screw after testing of Numerical Results 2, The shaft of the screw was bent at the edge of the angled test rig, supporting the sample at 45o, this also occurred to the original screw in Numerical Results 2.
The coloured cement was much easier to see than the original white
cement, giving a very definite cement distribution. However this also
made it evident that the large pressures applied by the cement gun had
removed the temporary plug in the tip of the screw.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 94
Figure 5.7 Bone Cement injection pressure as recorded in the head of the femur from Numerical Results 2
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0.50
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2.00
2.50
3.00
3.50
4.00
0 10 20 30 40 50 60
Time (Sec)
Pres
sure
(Mpa
)
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 95
5.1.6. Conclusions
Both the modified and the original screw were deformed and could not be
reused for testing. Two further screws were obtained and one was
modified. It was decided to take this opportunity to review the design of
the modified screw and permanently close the tip of the screw. This
modification would prevent the leakage of bone cement out of the guide
wire hole.
As a precaution, to prevent screws bending it was decided that further
tests would be limited to a maximum load of 3000N or to a maximum
displacement of 5mm.
At this point the modified cemented samples were consistently stronger
than the original uncemented samples. This is consistent with results
obtained with the Sawbones material despite its unsuitability to this kind of
testing. It was thought that this new screw with a permanently closed tip
will continue this trend, and possibly improve on the results as seen
previously.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 96
5.2. New Modified Screw Pig Testing
5.2.1. Numerical Results 3
The new modified screw was created from a standard Stryker Howmedica
Omega + Plus sliding compression hip screw. It was noted that the new
screw used for modification was of a different batch number to the one that
was originally modified. Some slight differences were seen in the thread
characteristics of the two screws; however these could be attributed to
standard wear of cutting tools and manufacturing equipment and would be
well within the tolerance of the design. The effect of these differences on
the mechanical characteristics of the screw would be negligible.
The new screw was modified in house to include the nine cementing holes
from the original design (Figure 5.8). The guide wire hole at the tip of the
screw was permanently closed (Figure 5.9). This will prevent the flow of
cement out of the tip and along the guide wire hole, while increasing the
flow of cement out of the side cementing holes and so increase the radial
penetration of cement into the bone.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 97
Figure 5.8 Photograph of the cement delivery holes in the new modified screw
Figure 5.9 Photograph of the new modified screw with sealed guide wire hole in the tip of the screw
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 98
The standard implantation procedure was followed for both screws;
however the cementing procedure for the modified screw was altered
slightly. In this instance the pressure of the cement delivery was
measured in-line with the delivery by way of a T piece adaptor (Figure 5.10
and Appendix 1). This was designed to give a more accurate measure of
the pressure of delivery rather than the final pressure experienced in the
head. Because of this arrangement the guide wire hole in the head of the
femur was closed using a self tapping screw and washer as was done
previously.
The samples were cured at 4oC for 24hr and tested in the same manner
as the previous sample. They were then sectioned and photographed.
Figure 5.10 Photograph of the Injection pressure recording apparatus used in Numerical Results 3 and 4. The barrel of the screw (bottom left) and the pressure transducer (top) are attached to the brass T-piece adaptor, the cement injection gun attaches to the right hand side of the T-piece.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 99
5.2.1.1. Results
The stiffness and final load (3000N or at a displacement of 5mm) of the
modified screw sample (1447.5N) was much lower than that of the Original
uncemented sample (3060N) (Results in section 5.3).
The pressures observed during the cement injection were also much lower
than were previously seen in the head of the femur during injection.
Figure 5.11 shows the pressure recorded in-line with the injection, and
then compared with the first pressure measurement in the head of the
femur in Numerical Results 2 (Figure 5.12).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 100
Figure 5.11 Numerical Results 3, bone cement injection pressure as measured inline with the delivery
-0.05
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0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Time (sec)
Pres
sure
(Mpa
)
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 101
Figure 5.12 Comparison of injection pressures measure in the head of the femur (Numerical Results 2, shown in Dark Blue) and inline with the injection (Numerical Results 3, shown in Pink).
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2.00
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4.00
0 10 20 30 40 50 60
Time (Sec)
Pres
sure
(Mpa
) Injection Pressure in Head of FemurInjection Pressure in line 1
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 102
5.2.1.2. Discussion
Sectioning of the modified cemented sample showed immediately why the
cementing pressure had been much lower than previously and why the
final load was much lower than that of the uncemented sample. A large
void in the bone had caused most of the cement to flow away from the
head of the femur and down towards the shaft of the femur (Figures 5.13
and 5.14). This void not only acted to channel the cement away from the
desired area but suggests that the remaining bone in the head of the
femur would have been diseased and weakened. The only way to avoid
this happening in the future is to x-ray the samples prior to testing to check
for any pathology. It does however highlight what might happen clinically if
the patient has any pre-existing pathology.
As anticipated neither of the bone samples failed before the safety cut-offs.
Instead comparisons of the maximum load reached and the stiffness could
be made (Section 5.3).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 103
Figure 5.13 Sectioned view of the modified cemented sample from Numerical results 3, a bone void can be seen filled with cement
Figure 5.14 Sectioned view of the modified cemented sample from Numerical results 3, with the screw removed a bone void can be seen filled with cement.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 104
5.2.2. Numerical Results 4
Due to the possibility of voids or other pathology in the porcine cadaveric
material the samples were X-rayed prior to use (Figure 5.15).
Figure 5.15 X-ray of the Porcine femora used in Numerical Results 4 prior to use
After any major pathology of the specimens was excluded the implantation
and cementing of the screws proceeded by the same methods used for
the previous samples.
Samples were cured for 24hr at 4oC and tested as for previous samples.
Specimens were then sectioned and photographed.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 105
5.2.2.1. Results
While the modified cemented sample was much stronger than in the
previous pair it was still dramatically lower than the Original uncemented
sample (Full results in Section 5.3).
The pressures measured during cement injection were very similar to
those recorded when the pressure was measured in the head of the femur,
Numerical Results 2 (Figures 5.16 and 5.17).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 106
Figure 5.16 Numerical Results 4, bone cement injection pressure measured inline with the injection
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0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0 10 20 30 40 50 60
Time (s)
Pres
sure
(MPa
)
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 107
Figure 5.17 Comparison of the injection pressures recorded in Numerical Results 2 (Blue), Numerical Results 3 (Pink) and Numerical Results 4 (Orange)
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0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0 10 20 30 40 50 60
Time (Sec)
Pres
sure
(Mpa
)
Injection Pressure in Head of FemurInjection Pressure in line 1Injection Pressure in line 2
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 108
5.2.2.2. Discussion
The testing of this sample went completely according to plan. The bones
were x-rayed prior to implantation and were found to be of good quality.
The Injection pressures recorded in line with the injection were the same
as when measured in the head of the femur. Sectioning showed that the
cement was evenly distributed (Figures 5.25 and 5.26). In spite of this the
failure load of the modified cemented screw was again much lower than
that of the uncemented original fixation. The difference between this pair
is much less than for the previous pair, however there is no apparent
reason for this as there was in the previous samples.
The stiffness of the modified cemented sample was also consistently lower
than that of the original sample at 2mm, 3mm and 4mm of displacement.
This result appears so unusual because of the results of samples one and
two. In both of these pairs the modified screw was consistently stronger
and stiffer than the original screw. The only difference between pairs one
and two, and pair four, is the closed end of the screw. The cement
patterns seen in pair two and four are quite similar, with a large proportion
of the cement around the threads and no reflux down the barrel.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 109
5.3. Summary of All Pig Testing Results
5.3.1. Cement Distribution
Pictures of the bone cement distribution of each of the modified samples
are shown in figures 5.18 – 5.27. The limit of bone cement penetration is
marked with a black line. It is difficult to see the bone cement in sample
one as it is white. Crystal violet was added to subsequent samples for
better visibility. The approximate area of penetration has been marked on
each of the photographs. The void present in sample three can clearly be
seen filled with bone cement; this is believed to be responsible for the
greatly reduced strength in this sample.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 110
Figure 5.18 Sectioned view of Numerical Results 1, the white cement is very hard to see
Figure 5.19 The modified screw once removed from Numerical Results 1
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 111
Figure 5.20 A sectioned view of the Numerical Results 2 modified cemented sample, the bone cement is clearly visible in blue
Figure 5.21 The modified screw after removal from the bone in Numerical Results 2
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 112
Figure 5.22 A sectioned view of Numerical Results 3, the bone void is apparent by the large mass of cement
Figure 5.23 The same sample (Numerical Results 3) with the modified screw removed, the size of the bone void is evident
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 113
Figure 5.24 The modified screw as removed from Numerical Results 3
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 114
Figure 5.25 A sectioned view of Numerical Results 4
Figure 5.26 The Numerical Results 4 bone sample after removal of the screw, the full extent of the bone cement penetration can be seen
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 115
Figure 5.27 The Modified screw removed from Numerical Results 4
5.3.2. Force – Displacement Data
The force – displacement curves for all of the samples tested in porcine
material are shown in Figure 5.28. The pairs of bones have been grouped
by colour with the darker of the colour representing the modified screw
sample and the lighter representing the original screw sample.
In pairs one and two the modified screw samples are much stronger than
the original screw samples, however the reverse is true for samples three
and four.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 116
Figure 5.28 Force – Displacement data from the four sets of paired cadaveric porcine femora, Numerical Results 1, Original screw in pink and Modified screw in red, Numerical Results 2, Original screw in light blue and Modified screw in dark blue, Numerical Results 3, Original screw in green and Modified screw in black, Numerical Results 4, Original screw in light orange and Modified screw in dark orange
2732
1815
4340
3152
1447.5
3060
2330
3012
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 1 2 3 4 5 6 7 8 9 10
Displacement (mm)
Forc
e (N
)
Numerical Results 1 Modified ScrewNumerical Results 1 Original ScrewNumerical Results 2 Modified ScrewNumerical Results 2 Original ScrewNumerical Results 3 Modified ScrewNumerical Results 3 Original ScrewNumerical Results 4 Modified ScrewNumerical Results 4 Original Screw
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 117
5.3.3. Stiffness Data
The stiffness of each of the samples was calculated at three
displacements (Table 5.1 and Figure 5.29). This was done because the
limitations placed on the testing to prevent the bending of the screws also
prevented failure of the samples and as such there are no true failure
loads to compare.
In sample one it can be seen that both of the samples have failed, with the
original sample failing before a displacement of 3mm had been reached.
In the third pair the effect of the bone void on the stiffness can clearly be
seen by the marked differences in stiffness between the two samples at
each point.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 118
Table 5.1. Stiffness (N/mm) data calculated at displacements of 2, 3 and 4mm. * Specimen failed before this displacement
2mm 3mm 4mm Pair 1 Mod 747.06 657.74 490.75 Orig 698.53 40.428 * 2mm 3mm 4mm Pair 2 Mod 856.15 911.83 754.88 Orig 746.67 611.21 418.8 2mm 3mm 4mm Pair 3 Mod 247.66 390.11 540.51 Orig 863.22 930.35 808.19 2mm 3mm 4mm Pair 4 Mod 522.35 697.99 512.87 Orig 603.26 784.42 694.36
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 119
Figure 5.29 Stiffness of the pig samples at 2, 3 and 4mm displacement, Modified samples in blue and original samples in green, the original samples in Numerical Results 1 failed after 3mm of displacement
0
100
200
300
400
500
600
700
800
900
1000
1 2 3 4
Numerical Results
Stiff
ness
(N/m
m)
Modified @ 2mm Original @ 2mm
Modified @ 3mm Original @ 3mm
Modified @ 4mm Original @ 4mm
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 120
5.3.4. Comparison to Sawbones samples
A comparison of the stiffness and failure loads of all Sawbones and
Porcine samples tested are shown in figures 5.30 and 5.31. The means of
the Sawbones data are represented by the red bar at the start of each set,
with the error bars showing one standard deviation in each direction. For
the porcine samples the peak or failure load was used as appropriate.
It is interesting to note the stiffness values of the Pig bone and the
Sawbones 45o samples as well as the Original uncemented 90o Sawbones
samples, which are all very close to one another (Figure 5.31).
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 121
Figure 5.30 Failure or Peak loads of Sawbones foam and Porcine Numerical Results, the mean and standard deviation of the Sawbones samples is shown in red, the porcine Modified screw samples are shown in blue and the Original screw samples in green
870.10
3377.33
2379.38
1837.83
2954.40
3112.57
964.33
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45 Pig 1 Pig 2 Pig 3 Pig 4
Forc
e (N
)
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 122
Figure 5.31 Stiffness of all samples, mean values from the Sawbones data are shown in red with standard deviations marked, the Porcine Numerical results data are shown at displacements of 2, 3 and 4 mm with the Modified samples shown in blue and the Original samples in green
2659.77
2381.42
2544.02
775.31
815.15
750.07
420.28
0
500
1000
1500
2000
2500
3000
3500
Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45 Pig 1 Pig 2 Pig 3 Pig 4
Stifn
ess
(N/m
m)
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 123
5.4. All Pig Discussion
The use of porcine cadaveric material was not as successful for testing as
was hoped. Several problems were encountered with implantation,
cementing and testing methodology.
After several trials a method was developed in which the cement delivery
holes did not become clogged with material upon insertion. This method
involved the creation of a base hole 1mm larger than the barrel of the
screw, leaving a 0.5mm gap in each direction. This creates a space for
the bone to be compressed into from the tapping process, with a reduced
chance of blocking any of the delivery holes. Questions were raised as to
whether this would weaken the fixation by decreasing the purchase on the
surrounding bone, however as the gap is filled with solid cement, the
cement then penetrates further into the surrounding bone.
The second pair of pig bone samples then created further problems.
While the initial failure curves looked good, with the modified sample
having a failure load 27% greater than the original sample, it soon became
apparent that the failure had in fact been in the screws rather than the
bone. Both of the screws had bent at the edge of the supporting part of
the testing apparatus. While this does happen clinically, it was not
expected in this trial and so safe guards were not in place. However it
does indicate that the weakest part of the construct was the screw itself.
This is not unreasonable as the bone samples were taken from young
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 124
healthy pigs. To prevent this from happening again limitations were put
into the testing, the maximum displacement was reduced to 5mm and the
maximum applied force to 3000N. This was sufficient to prevent this from
happening again in subsequent testing. However the secondary effect of
these limitations was to prevent any of the subsequent samples from
failing under load. For comparison the stiffness of the samples was
compared at three displacements.
The use of a cement injection gun greatly increased the penetration of the
cement into the area around the threads. As the gun pressurises the
cement it is useful to know what pressure is actually being delivered by the
gun. Initially the pressure transducer was set up to measure the pressure
experienced in the head of the femur, the pressure recorded was much
larger than expected, exceeding the maximum of the transducer. However
it was then decided that knowing the pressure of delivery was more
relevant than the final pressure experienced. In the second pressure
recording the transducer was inserted inline with the delivery, between the
cement gun and the screw, using a T-piece adaptor (Appendix 1:
Technical Drawings). This gave an accurate measure of the pressure the
cement was being applied at. However the value was much lower than
the previous measurement. Upon sectioning it became evident that this
was caused by the cement flowing into a large void in the bone, which
prevented the cement gun from pressurising the cement to any great
degree. The third attempt to record the injection pressure was more
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 125
successful. Again the transducer was placed inline with the injection, this
time the pressure measured was very large, exceeding the maximum of
the transducer. As both the first and third recordings exceeded the
maximum value recorded by the transducer it is impossible to say what the
actual pressure of delivery was, however it is evident that in intact bone
the pressure easily exceeds 3.5MPa. This could create a problem in weak
bone, however as this bone is normally also of a very low density, the
pressures experienced would be much lower than recorded here.
While the radiopaque nature of the bone cement makes it highly visible
under x-rays, the white colour of the cement makes it very hard to see in
reality. This is evident in the first pair of pig bones tested, where it is very
difficult to determine the extent of cement penetration into the bone. This
problem was corrected by the addition of Crystal Violet to the monomer of
the cement prior to mixing. The larger amount of Crystal Violet used, the
brighter blue the cement became, and the easier it was to see.
The presence of the bone void in sample 3 was unexpected because of
the age of the bone samples. However in order to prevent this in the
future samples, all remaining pairs of cadaveric femurs were X-rayed. The
films showed no obvious signs of pathology and all pairs of bones were
cleared for use.
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 126
As the initial modified screw and the original screw were both bent beyond
further use in sample 2, a new pair of screws were required. A pair of lag
screws had already been acquired in case they were needed. These new
screws were of the same type as the initial screws but were of a different
batch number. The bending of the old screws provided an opportunity to
review the current design of the screw before another was modified. From
looking at the results up to this point the current design was looking very
good, but one adjustment was made to the design. Because of the clinical
risk of the guide wire puncturing the head of the femur and subsequent
cement leakage, and the improvement of the results in the Sawbones
material with the use of a temporary plug, it was decided to permanently
close the guide wire hole in the tip of the screw. The new modified screw
was then manufactured and used for subsequent testing. However,
because of the closed tip it was no longer possible to tap the base hole
with the modified screw and still have the guide wire in place. Instead the
remaining original screw was used to tap the hole with the guide wire in
place, both were then removed and the modified screw inserted to the
correct depth. This created only a minor inconvenience and it is expected
that should this design be adopted clinically a specific tapping device
would be created for this purpose.
The results of the pig testing - the bending of the screws and the better
results without cement, raise questions about the validity of using young
porcine material as an analogue for Osteoporotic human bone. In
Design of a Hip Screw for Delivery of Bone Cement
Section 5: Pig Testing 127
Osteoporotic bone it is rare for the implant to be the weakest component
and fail through bending. In samples three and four, the final load of the
original uncemented samples was considerably higher than that of the
modified cemented sample. While in sample three this can be attributed to
the bone pathology, in sample four there were no extenuating
circumstances. Whereas the literature consistently shows an increase in
failure load with the addition of bone cement in human cadaveric material.
This suggests that the porcine material is both too strong and too dense
for this style of testing to be an accurate analogy for Osteoporotic bone.
5.5. Conclusion
While the porcine material is a more accurate model than the Sawbones
foam, problems still exist in using it as an osteoporotic bone analogue.
And while it is thought that the modified screw with the permanently closed
tip, would be at least as good as the initial modified screw, it was unable to
be shown in the porcine material.
Design of a Hip Screw for Delivery of Bone Cement
Section 6: General Discussion 128
Design of a Hip Screw for Delivery of Bone Cement
Section 6: General Discussion 129
6. General Discussion
Many problems have been encountered and overcome over the course of
this study. Most of the problems centre on the appropriateness of testing
materials.
The Sawbones polyurethane foam was found to be 95% closed cell. This
made it completely unsuitable for use in testing implants with bone cement
augmentation. As well as having degradation problems resulting in a 17%
decrease in strength over a period of eight months.
While the Porcine cadaveric material offered a better model of human
bone, it did not represent the Osteoporotic nature of most patients who
require fixation. The porcine bone had a far greater strength and density
than would be seen clinically in an osteoporotic patient. What this did
demonstrate was that there is no point in augmenting fixations in healthy
bone as it gives no mechanical advantage.
Testing the modified screw in a better analogue of osteoporotic bone may
be beneficial. Currently an Osteoporotic Sheep is being developed in
Adelaide; if this is successful then suitable cadaveric test material may be
able to be obtained.
Design of a Hip Screw for Delivery of Bone Cement
Section 6: General Discussion 130
Once a suitable testing material becomes available the modified screw
could be tested in a cyclic manner to better represent the clinical failure
mechanisms.
Design of a Hip Screw for Delivery of Bone Cement
Section 7: Conclusions 131
7. Conclusions
The Sawbones Polyurethane foam was found to be completely
inappropriate for use as a testing material for implants augmented with
bone cement. Care should also be taken when using this product for other
mechanical testing due to its propensity to degrade over time.
The testing methodology was suitable for preliminary testing of the
modifications, however in order to more fully model the cut-out behaviour
of the lag screw, cyclic testing would need to be carried out.
In terms of the modifications made to the lag screw it is difficult to make
any definitive conclusions. In the Sawbones material the failure load was
increased significantly with the addition of bone cement, this finding is
supported by the literature where cement augmentation consistently
increased holding power in cadaveric and synthetic materials. However
the results of the porcine testing do not agree with this finding, with the
augmentation of the fixation being no better, and in some cases worse,
than the original screw alone. This finding tends to indicate more that
cement augmentation is inappropriate in healthy bone; rather than
commenting on its usefulness in osteoporotic bone.
If a better model of osteoporotic human bone could be found for a testing
material, then the modified screw with bone cement augmentation may yet
Design of a Hip Screw for Delivery of Bone Cement
Section 7: Conclusions 132
prove better, or at least more convenient, than standard augmentation
method.
Design of a Hip Screw for Delivery of Bone Cement
Section 8: References 133
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ASTM (2001b) F2118-01a - Test Method for Constant amplitude of force controlled fatigue testing of Acrylic Bone cement materials, ASTM, West Conshohocken.
ASTM (2002) F543-02 - Standard Specification and Test Methods for Metallic Medical Bone Screws, ASTM, West Conshohocken.
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experimental study of the failure modes of the Gamma Locking Nail and AO Dynamic Hip Screw under static loading: a cadaveric study", Med Eng Phys, 19, 446-53.
Haynes, R. C., Poll, R. G., Miles, A. W. and Weston, R. B. (1997b) "Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the gamma locking nail and AO dynamic hip screw", Injury, 28, 337-41.
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Witschger, P. M., Gerhart, T. N., Goldman, J. B., Edsberg, L. E. and Hayes, W. C. (1991) "Biomechanical evaluation of a biodegradable composite as an adjunct to internal fixation of proximal femur fractures", J Orthop Res, 9, 48-53.
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Yamamoto, H. (1998) "Mechanical strength of calcium phosphate cement in vivo and in vitro", Biomaterials, 19, 1587-1591.
Yetkinler, D., Goodman, S. B., Reindel, E. S., Carter, D. and Poser, R. (1998) In 11th Conference of the ESBToulouse, France, pp. 5.
Yetkinler, D. N., Goodman, S. B., Reindel, E. S., Carter, D., Poser, R. D. and Constantz, B. R. (2002) "Mechanical evaluation of a carbonated apatite cement in the fixation of unstable intertrochanteric fractures", Acta Orthop Scand, 73, 157-64.
Yetkinler, D. N. and Litsky, A. S. (1998) "Viscoelastic behaviour of acrylic bone cements", Biomaterials, 19, 1551-9.
Design of a Hip Screw for Delivery of Bone Cement
Section 8: References 138
Design of a Hip Screw for Delivery of Bone Cement
Appendices 139
Appendix 1: Technical Drawings
1.1. Slotted Screw design
1.2. Final Screw design
1.3. Closed End Screw Design
1.4. Hounsfield Adaptor
1.5. 45o Alignment Jig
1.6. Pig Test Rig
1.7. Pressure Transducer T-piece Adaptor
Design of a Hip Screw for Delivery of Bone Cement
Appendices 140
1.1. Slotted Screw design
Design of a Hip Screw for Delivery of Bone Cement
Appendices 141
1.2. Final Screw design
Design of a Hip Screw for Delivery of Bone Cement
Appendices 142
1.3. Closed End Screw Design
Design of a Hip Screw for Delivery of Bone Cement
Appendices 143
1.4. Hounsfield Adaptor
Design of a Hip Screw for Delivery of Bone Cement
Appendices 144
1.5. 45o Alignment Jig
Design of a Hip Screw for Delivery of Bone Cement
Appendices 145
1.6. Pig Test Rig
Design of a Hip Screw for Delivery of Bone Cement
Appendices 146
1.7. Pressure Transducer T-piece Adaptor
Design of a Hip Screw for Delivery of Bone Cement
Appendices 147
2. Appendix 2: Ringers Foam Compression Test
2.1. Toad Ringers Solution Formula
Toad Ringers Solution mol 1 litre 4 litres 20 litres mmol One litre Weight Grams mmol Grams Grams Na 144.54NaCl 58.44 7.00 119.77 28 140 K 1.88 KCl 74.55 0.14 1.88 0.56 2.8 Ca 1.80 CaCl2 110.98 0.222 1.80 Mg ≈ 1 MgCl2 95.21 0.095 Cl 129.46NaHCO3 84.01 2.00 23.81 8 40 PO4 0.96 NaH2PO4 156.01 0.15 0.96 0.6 3 HCO3 23.81 Glucose 180.20 2.00 11.10 8 40 Glucose 11.10 Note: CaCl2 and MgCl2 will kill the solution over time Make up 1M solutions of each – add as required 1 ml of 1 M solution into 1 litre = 1mMol Final concentration of: CaCl2 = 2 mMol MgCl2 = 1 mMol
Design of a Hip Screw for Delivery of Bone Cement
Appendices 148
3. Appendix 3: Statistical analysis of Sawbones 90o Data
A power analysis of the Sawbones 90o data was used to determine the number of samples required in further testing. A slight overestimate of the average standard deviation for the groups being compared was used, with results shown in Table 1. Power is 0.8 in each instance and the significance level is 0.05. Table 1: The approximate no of subjects required in each group are:
Difference in Means 100 200 400
Modified plugged vs. Modified 37 10 4
Mod plugged vs. Original Cemented 64 17 5
Modified vs. Original Cemented 85 22 7
Results were also calculated using the maximum standard deviation for the groups being compared in each case (this is a more conservative estimate) in Table 2. Table 2: approximate no of subjects required in each group are:
Difference in Means 100 200 400
Modified plugged vs. Modified 52 14 5
Mod plugged vs. Original Cemented 125 32 9
Modified vs. Original Cemented 125 32 9
It was decided to use the results of the power analysis using the maximum standard deviations, with a difference of 400. As such it was decided to test nine samples of each method in the subsequent angled testing in the Sawbones material.
Design of a Hip Screw for Delivery of Bone Cement
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4. Appendix 4: Sawbones website details
The foam material used as a bone analogue in the initial stages of testing was purchased from Sawbones (Pacific Research laboratories, 10221 SW 188th St, Vashon, Washington, 98070, USA). At the time of initial purchase the manufacturers’ website stated that the material was a cellular rigid polyurethane foam, and gave basic material properties. Unfortunately a copy of the website was not made at this time. Several months later the company released a new Open cell, cellular rigid polyurethane foam. At this time the page relating to the original Cellular foam was updated to include the information that it is 95% Closed Cell.
“The appearance of cellular rigid polyurethane foam resembles that of cadaveric cancellous bone, however, the cell structure is 95% closed as compared to the open cell structure of cancellous bone. “