A rgon Packaged · C o m p ression Molded
®
WEAR IS THE ISSUE
Polyethylene wear continues to be the
single largest threat to the long term success of
a joint replacement. Excessive wear may lead
to gross mechanical failure such as fracture
or dislocation, and the release of particulate
wear debris may induce biological responses
that resorb bone and cause implant loosening.
T h rough common-sense engineering
and the employment of advanced technology,
Biomet has developed a systematic approach
to improving the long-term performance of
polyethylene implants.
Biomet, Inc. is pleased to present
ArCom —Argon packaged Compression
molded polyethylene. ArCom was specifically
developed to address the orthopaedic
s u rgeon’s primary polyethylene concern:
the problem of wear.
C o n t e n t s
Inside CoverIntroduction
1Overview
3Polyethylene Consolidation
6Ram Extrusion
8Sheet Compression Molding
8Direct Compression Molding
9Isostatic Molding
1 0Effect of Consolidation on Mechanical Properties
13Packaging and Sterilization
14Sterilization, Chain Scission and Cross-Linking
14Sterilization and Consolidation
1 5Benefits of Gamma Irradiation and
Associated Cross-Linking to Wear Performance
1 5Post-Irradiation and Post-Implantation Oxidation
1 6Summary
ArC o m® p rocessed polyethylene is an
e v o l u t ion a r y development firmly based upon
clinical experience.
This clinical experience
dates back to the early 1980s and the introduction of the A G C®
d i rect compression molded tibial component. Knutson, et al.,1 determined that this
knee system showed the lowest revision rate in the Swedish Knee A r t h ro p l a s t y
Register —which evaluated 34,000 total knee prostheses (Figure 1).
B a n k s t o n , e t a l . , s h o w e d o v e r a 5 0 % d e c re a s e
i n i n v i v o w e a r r a t e f o r m o l d e d a c e t a b u l a r c o m p o-
n e n t s , a s c o m p a re d t o c o m p o n e n t s m a c h i n ed
f ro m e x t ru d e d b ar 2 ( F i g u re 2 ) .
R i t t e r, et al.,3 showed a >98% survival rate at 10
years with the AGC direct compression molded
tibial component. The authors believe that the
c o m p ression molded polyethylene was the pri-
mary reason for the success
of the AGC design.3
Figure 2. In Vivo Acetabular Wear.
Figure 1. Swedish Knee Study.
BI O M E T I S
COM M I T T E D TO T H E
U S E O F C O M P R E S S I O N
M O L D I N G, B A S E D O N
C L I N I C A L W E A R D ATA.
Po lye t hylene Consolidation:
T h e re a re m u l t i p l e m e t h o d s t oc o n s o l i d a t e p o l y e t h y l e n e .
The majority of c o m p o n e n t s on the m a r k e t today are m a ch i n e d
f rom ram extruded bar stock material. Other components are
either d i re c t c o m p ression molded from a resin into a finished
p roduct, or material is molded into sheets or blocks and
machined into implant shapes. In addition to the common
techniques, isostatic molding h a s
re c e n tly emerged as another
e ff e c t i v e manufacturing option that offers benefits in
i m p roved mechanical and wear properties due to
i m p roved resin fusion.
The ultimate goal is, of course, providing a
t h o roughly consolidated polyethylene material that is
t ruly wear resistant. Inherent diff e rences in manufacturing
between extrusion and molding indicate that molding
p rovides a more thoro u g h l y
consolidated material.3 , 4 , 5 , 6
WEAR-RESISTANT POLYETHYLENE COMPONENTS ARE A DIRECT RESULT
OF THOROUGHLY CONSOLIDATED POLYETHYLENE MATERIAL.
Isostatic Molding Chamber
Hip Simulator Wear Testing
Figure 3. Consolidation of Polyethylene.
F i g u re 3 shows a cross-section of components taken from a hospital shelf with varying consolidation.6
100x Magnification —Cross-section of a pressure crystallized Hylamer® extrudedbar component showing extraneous inclusions and non-consolidated areas.
100x Magnification —Cross-section of an extruded bar component showing non-consolidated areas.
100x Magnification —Cross-section of an ArCom isostatic molded acetabular liner showing highly consolidated material.
100x Magnification —Cross-section of an ArCom direct molded tibial bearing showing consolidated material.
Ram Extrusion:
Ram extrusion is a continuous process that pro d u c e s
UHMWPE bar stock of varying cross-sections (Aschematic
of a ram extruder is shown in Figure 4). Resin is fed from a
hopper into a chamber where an oscillating ram forces the
material into the die. As the powder is moved through the
open-ended die, heat is applied, causing the resin to expand
and thereby producing resistance to the ram.
The resin melts from the surface to the center of the die
forming a cone of unmelted material that extends into the
die (Figure 5). Non-consolidated regions can result in the
center of the material if the cone of unmelted material
Figure 4. Extruded Bar Processing.
Figure 5. Unmelted Resin Cone in Ram Extrusion.
Figure 6. Non-consolidated Center in Ram Extruded Material.
extends beyond the heated zone of the die, or if pre s s u re is
reduced prior to the cooling of the center of the extru d e d
material (Figure 6).
In the ram extru d e r, pre s s u re is applied to the melted
resin by the friction of the resin against the die which re s i s t s
the force being applied by the ram. It is important to note
that the applied pre s s u re is not constant but varies. These
variations are due to the oscillation of the ram (on/off
p re s s u re) as well as the diff e rences between dynamic and
static friction as the bar is extruded, and may result in
variation in the end pro d u c t .
Stearate inclusions may also affect consolidation.
Stearates are a resin additive that increases the efficiency of
the extrusion process by acting as a lubricant and corro s i o n
i n h i b i t o r. At the same time, stearates can interfere with re s i n
c o n s o l i d a t i o n .
In an attempt to improve material consistency, a
secondary processing method called Pre s s u re Crystallization
has been applied to the extruded bar prior to machining. This
p rocess involves applying further heat and extreme pre s s u re
to the already consolidated extruded bar, the outcome being
a stiffer more crystalline material. Studies have shown
Figure 7. In Vivo Acetabular Wear Chart.
questionable in vivo wear rates for components made fro m
this pre s s u re crystallized material sold under the trade
name Hylamer®.7,8,9 Linear wear rates for the Hylamer
material have been reported to be as high as. 2 7 m m / y e a r,
as compared to .11mm/year for conventionally-pro c e s s e d
acetabular liners8 ( F i g u re 7).
Sheet Compression Molding:
Sheet molding is a form of compression molding
used to manufacture large plates of material. To form these
sheets, resin is poured onto a platen and leveled with a
s t r i ke bar. Once the presses are loaded with resin, the
platens are brought together and heat is applied. After a
specified heating cycle has been completed, the pre s s u re
is increased to the desired set point and the material is
allowed to cool under pre s s u re. An exploded view of sheet
molding platens is shown in Figure 8.
The sheet that is removed from the press is usually
trimmed to remove the surface, then it is cut into bars,
annealed and sent to the end user wheret h e final shapes a re
m a c h i n e df ro mt h e bar stock. One disadvantage with sheet
molding is that there can be areas of varying density caus-
ing varying mechanical properties from the surface to the
center of the block.
Another disadvantage of sheet molding is that the
variance in bulk density of the resin can lead to inconsis-
tency in the amount of powder in one region of the plate
versus another. This will result in pre s s u re diff e rentials
during consolidation from one area of the block to another,
thus resulting in property diff e re n c e s .
Direct Compression Molding:
The direct compression molding process is used to
form a finished component from raw resin. Instead of using
flat platens, the molds have the contour of the components
being formed (Figure 9). The molds may also allow for an
insert —such as a metal backing for an acetabular liner or
tibial bearing —to be molded into the com p o n e n t .
To mold a component, the bottom plunger of the
mold is placed into the sleeve and a known weight of resin
Figure 8. Sheet Molding.
Direct Compression Molding
Figure 9. Direct Compression Mold Tool and Component.
is poured into the mold. The top plunger is then placed into
the mold. This is typically done in a controlled atmosphere .
The mold, with the resin, is then placed into a press and
cycled through a specified pre s s u re and temperature pro fi l e .
The component is then cooled under pre s s u re and is essen-
tially finished at this point.
This process has many advantages over the other
p rocessing methods. These include control over surface
ro u g h n e s s ,c o n t ro l over resin selection, o p t i m i z a t i o n of
a p p l i e d heat and pre s s u re for each component configuration,
as well as elimination of machine lines on articular surfaces.
The bearing surface finish is controlled by the roughness of
the plungers used to produce the component. Better initial
surface finishes may reduce the amount of debris generated
during the “bre a k - i n ” period for a device.
By consolidating the resin using an in-house pro c e s s ,
an orthopaedic device manufacturer is able to choose the
type and quality of the resin, as opposed to the converters
selecting resin for ram extru s i o n . Since the area of the mold
is much smaller than a sheet forming press, higher pre s s u re s
and more uniform heat can economically be applied. I n
a d d i t i o n ,t h e c o m p o n e n t s do not usually re q u i re additional
machining or annealing.
Figure 10. Cold Isostatic Pressing of UHMWPE Powder.
Isostatic Molding:
In an attempt to improve consolidation and
reduce residual oxygen in the material, isostatic m o l d i n g
was developed.
T h i st e c h n i q u ei sb a s e do ni s o s t a t i cp r essing technology
m a k i n gu s eo ft h ec o n c e p to f uniform applicationofp re s s u re
via a pressurized gas or fluid contained in a vessel.
The process consists of cold compaction of the re s i n
into a shape (Figure10), followed by vacuum sealing in a
n o n - g a sp e r m e a b l ec o n t a i n e r( F i g u re11 ) ,t h e nh o ti s o s t a t i-
cally pressing the cold compact to fuse the resin. The major
advantages of this process are uniform application of heat
and pre s s u re over irregular shapes and thickness and t h e
ability to apply a known amount of pre s s u re during the cool
down cycle to all surfaces, re g a rdless of shape.
In laboratory testing, the material has shown superior
wear characteristics as compared to material consolidated
by ram extru s i on6,1 0 ( F i g u re 12).
Another significant advantage to isostatic molding
is that the heated resin is not exposed to oxygen during
p rocessing, thereby reducing a possible source of oxidative
degradation. Material processed in oxygen may perform
d i ff e re n t l yt h a nf rom non-oxygenp ro c e s s e dp o l y e t h y l e n e.11
Since the material is formed using omnidire c t i o n a l
p re s s u re , the final product has dimensional stability and
does not risk additional i n c reased oxygen absorption that
can occur during annealing.
E f fect of Consolidation on
M e chanical Pro p e rt i e s :
F i g u re 13 reveals the tensile strength relationship to
consolidation. The results show that the more consolidated t h e
m a t e r i a l ,t h eh i g h e r the tensile s t re n g t h of t he p o l y e t h y l e ne.6
These various manufacturing processes can yield
quite diff e rent pro d u c t s . F i g u re 14 compares the diff e re n c e s
in mechanical properties of the UHMWPE processed by
various methods.
Figure 11. Hot Isostatic Press. Figure 12. Hip Simulator Wear Testing, Arcom® vs. Extruded Bar Polyethylene.7
Figure 15. AGC® Nine-Year Retrieval.8
Figure 13. Tensile Strength vs. Consolidation of UHMWPE.6
Figure 14. Mechanical Properties of UHMWPE Processed by Various Methods.6
The consolidation and excellent wear resistance
of compre s s i o n m o l d e d p a r t s c a nb e f o u n d i ne x p l a n t ed
c o m p o n e n ts. Figure 15 shows a knee component that was
implanted for nine years in a 58-year-old, active male weigh-
ing over 200lbs. The o n e - p i e c em o l d e dt i b i a l a n d m e t a l - b a c k e d
patellar components w e re direct compression molded and
show little wear.
*Elongation based on crosshead displacement
BI O M E T I S C O M M I T T E D
TO T H E U S E O F A R G O N
PA CKAG I N G A N D GA MMA
I R R A D I AT I ON S T E R I L I Z AT I ON,
B A S E D O N C L I N ICA L
W E A R D ATA.
THE SECOND MAJOR PIECE OF THE POLYETHYLENE
WEAR RESISTANCE PUZZLE FOCUSES ON PACKAGING AND STERILIZATION
Sterilization methods have been identified as one area
of concern with respect to oxidation. Today, some c o m p a n i e s
a re advocating ethylene oxide sterilization (EtO), or gas plas-
m a sterilization rather than gamma irradiation as the method
of sterilization. Polyethylene oxidation has been copiously
documented in the literature for devices irradiated in an air
e n v i ro n m e n t .11 , 1 2 , 1 3 , 1 4 H o w e v e r, existing literature also states
that irradiation in an inert environment greatly decre a s e s
oxidative degradation.11 , 1 4 , 1 5 , 1 6 , 1 7 In several of these articles,
testing of polyethylene irradiated in an inert gas enviro n m e n t
showed reduced oxidation and improved wear resistance .
The following is a summary of how ArCom’s
A rgon packaging and sterilization techniques dire c t l y
a d d ress oxidation and its relation to wear re s i s t a n c e .
Argon Packaging
S t e r i l i z a t i o n , Chain Scission and Cro s s - l i n k i n g :
Sterilization can affect the wear resistance of
UH M W P E . During gamma irradiation, free radicals are
formed (Figure 16). In material irradiated in air, the fre e
radicals may join with oxygen resulting in chain scission, or
a series of smaller chains and oxidation occurs. In A rgon, the
chains recombine, or cross-link, to form a thre e-d i m e n s i o n a l
s t ru c t u re, reducing the number of active free radicals and
i n c reasing wear re s i s t a n c e .
During EtO sterilization and other non-energetic
sterilization techniques such as gas plasma, the chains with-
i n the resin are not altered. There f o re, some manufacture r s
a re claiming that oxidation does not occur in EtO sterilized
material. But the question still remains. How does the EtO
p rocess affect the clinical wear performance of UHMWPE?
Sterilization and Consolidation:
The issues with EtO sterilization are many. First of
a l l , a change in sterilization technique does not address
the consolidation issue. If the base material is not completely
consolidated, sterilization methods cannot fully make up for
fusion defects. In a completely fused piece of material, the
a p p e a r a n c e o ft h ep o l y e t h y l e n ei sh o m o g e n e o u s( F i g u r e 1 7 a)
as compared to unfused material (Figure 17b) which clearly
show extraneous inclusions and non-consolidated are a s .
M a t e r i a lf re eo fd e f e c t s has showns u p e r i o rw e ar p e r f o r m a n c e
in total joint arthroplasty compare dt o unfused material.4,5
In addition to the elimination of the beneficial cro s s -
linking effect seen in gamma irradiated polyethylene, there
a re other drawbacks directly related to the use of EtO steril-
ization. Woolston states, “The dominance of EtO as a steril-
ization technology is in decline as concerns increase over
residue levels, CFC and toxic emissions, and operator safety
connected with its use.” Woolston goes on to state that EtO
sterilization cycles often suffer biological indicator failure s
which can leave sterilization assurance in question.1 8
Figure 16. Gamma Radiation Sterilization.
Figure 17a. Fused UHMWPE. Figure 17b. Unfused UHMWPE.
Post-Irradiation and Post-Implantation Oxidation:
The issue as to whether a component will oxidize on the shelf
or after implantation when the component is exposed to air has
arisen. Some articles mention “long-lived” free radicals during the
post-irradiation stage.11,1 7 The longevity of free radicals is dire c t l y
dependent upon the material properties, storage temperatures after
irradiation, packaging environment and type of radical formed.
Some authors note that most chain scission or cross-linking activity
takes place within two months of sterilization.11,3 1,3 2
Keep in mind that ArCom components will be shielded fro m
oxygen exposure until the component is removed from its sterile
packaging immediately prior to implantation.
Furman, et al., has presented data indicating that component
implantation alters and perhaps mediates the oxidative pro c e s s.3 3
Li, in a recent interview has stated that compression molded
components appear to be more resistant to wear and oxidation
than components made from extruded bar.3 4
Although oxidation remains a factor in the performance of
UHMWPE, wear resistance is still the most important criterion.
H ow beneficial is gamma irradiation and the associated cross-linking to UHMWPE wear perfo r m a n c e ?
*ArCom polyethylene which
was gamma irradiated expe-
rienced 42 percent less
w e a r, as compared to EtO
sterilized polye t h y l e ne 2 4
(Figure 18).
*Kurth and Eyerer showed
50 percent less wear for
UHMWPE that was gamma
irradiated, as compared to
non-irradiated material.2 5
*Stein states that cross-
linking has been found to
improve wear resistance by
as much as 30 percent over
non-cross-linked polyethyl-
e n e.2 6
*Hawkins and Gsell showed
a 30 percent decrease in
wear for an irradiated,
unoxidized specimen.27
*Streicher states that irradia-
tion in an inert environment
enhances cross-linking of
UHMWPE which is benefi-
cial to wear properties.2 8
*Sommerich, et al., showed
a 36 percent decrease in
wear for gamma irradiated
UHMWPE, as compared to
EtO sterilized material.2 9
* Wang, et al., showed that
EtO sterilized components
exhibited 141 percent more
wear than similar gamma
irradiated components.3 0
When compared to EtO sterilization, sterility assurance
levels can be maintained more reliably using the gamma
irradiation pro c e s s.1 9 In addition, EtO sterilization may leave
toxic residuals such as ethylene chlorohydrin, ethylene glycol
and ethylene oxide on sterilized components; residuals which
must dissipate before the components are safe to use.2 0,2 1,2 2,2 3
Figure 18. Hip Simulator Wear Testing — EtO vs. Gamma.
Tensile Strength Testing
IN SU M M A RY, ARCO M®
PO LY E T H Y L E N E PR O V I D E S
SU P E R I O R WE A R —
AN D TH E DATA TO PR O V E IT!
C o m p ression molded UHMWPE components
a re more uniformly consolidated and possess higher wear
resistance than extruded bar.
U H M W P Es t e r i l i z e db y g a m m a i r r a d i a t i o ni n an inert
e n v i ronment pre f e rentially undergoes molecular cro s s - l i n k-
ing which increases abrasive wear resistance.
●
●
Material manufactured in a non-oxygenated enviro n-
ment, packaged and sterilized in an inert atmosphere, re s i s t s
post-irradiation oxidation.
ArCom is manufactured and processed to produce the
highest quality components by:
1 . Direct and Isostatic moldi n g .
2 . P rocessing bar in an inert env i ro n m e n t .
3 . Gamma irradiation sterilization.
4 . Pa ck ag i n g , sterilizing and storage in an
i n e rt env i ro n m e n t .
As a result, ArCom exhibits optimal
characteristics including:
1 . I m p roved wear resistance.
2 . I m p roved resistance to oxidation degradation.
3 . I m p roved mechanical pro p e rt i e s .
T h e issues withpolyethylene are n u m e rous and
c o m p l icated. However, A r C o m® polyethylene processing is
based on common sense, proven testing, clinical results and
copious literature re f e re n c e s .
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H y l a m e r® is a Registered Trademark of DePuy-DuPont Orthopaedics.
U.S. Pat. No. 5,466,530Other patents pending.AGC and ArCom are Registered Trademarks of Biomet, Inc.
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29. Sommerich, R., T. Flynn, M.D. Schmidt, and E. Zalenski, “The Effects ofSterilization on Contact A rea and Wear Rate of UHMWPE,” 42nd A n n u a lMeeting, Orthopaedic Research Society, February 19-22, 1996, Atlanta, Georg i a .
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