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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●

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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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“The Swedish Knee A r t h roplasty Register, AN a t i o n - Wide Study of 30,003Knees 1976-1992, Department of Orthopedics, University Hospital, Lund,Sweden,” Acta Orthopaedica Scandinavica, 1994, 65 (4): 375-386.

2. Bankston, A.B., Keating, E.M., Ranawat, C., Faris, P.M., Ritter, M.A., “TheComparison of Polyethylene Wear in Machined vs. Molded Polyethylene,”Clinical Orthopaedics and Related R e s e a rc h , No. 317, pp. 37-43, August 1995.

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5. Jensen, R.E., Collier, J.P., Mayor, M.B., Surprenant, V.A., “The Role ofPolyethylene Uniformity and Patient Characteristics in the Wear of Tibial KneeComponents,” Implant Retrieval Symposium of the Society for Biomaterials, St.Charles, Illinois, September, 1992.

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“Early Failure of Hylamer Acetabular Inserts Due to Eccentric We a r.” The Journal of Arthro p l a s t y, Vol. 11, No. 3, 1996, pp. 351-353.

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14. Rose, R.M., Goldfarb, E.V., Ellis, E., Crugnola, A.N., “Radiation Sterilization andthe Wear Rate of Polyethylene,” Journal of Orthopaedic Researc h, Vol.2, No. 4, pp.393-400, 1984.

15. E y e re r, P., Ellwanger, R., Federolf, H.A., Kurth, M., Madler, H., “Polyethylene,”Concise Encyclopedia of Medical and Dental Materials, Edited by D. Williams, TheMIT Press, Massachusetts, c. 1990.

16. Azuma, K., Tsunoda, H., Hirata, T., Ishitani, T., Tanaka, Y., “Effects of theConditions for Electron Beam Irradiation on Amounts of Volatiles fro mIrradiated Polyethylene Film,” Agricultural and Biological Chemistry, Vol. 48, No.8, pp. 2009-2016, 19 8 4 .

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21. Boonk, W.J., VanKetel, W.G., “APossible Case of Delayed Hypersensitivity toEthylene Oxide,” Clinical and Experimental Dermatology, Vol. 6, No. 4, pp. 385-390, 1981.

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24. S c h ro e d e r, D.W., K.M. Pozorski, “Hip Simulator Wear Testing of IsostaticallyMolded UHMWPE, Effect of EtO and Gamma Irradiation,” 42nd A n n u a lMeeting, Orthopaedic Research Society, February 19-22, 1996, Atlanta, Georg i a .

25. Kurth, M., P. Eyere r, and D. Cui, “Effects of Radiation Sterilization on UHMW-PE,” Annual Technical Conference Society of Plastics Engineers 45th, pp. 11 9 3 -1197, 1987.

26. Stein, H., “Ultra-High Molecular Weight Polyethylene (UHMWPE),”E n g i n e e red Materials Handbook, Vol. 2, Engineering Plastics, A S MInternational, Metals Park, Ohio, 1988.

27. Hawkins, M.E., and R. Gsell, “Effects of Nitrogen on Radiation InducedOxidation to Ultra-High Molecular Weight Polyethylene,” 20th Annual Meetingof the Society for Biomaterials, April 5-9, Boston, Massachusetts.

28. S t re i c h e r, R.M., “Improving UHMWPE by Ionizing Irradiation Cro s s - L i n k i n gDuring Sterilization,” The 17th Annual Meeting of the Society for BiomaterialsMay 1-5, 1991, Scottsdale, A r i z o n a .

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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