57Wear in Conventional and Highly Cross-Linked Polyethylene 57 57

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Chapter 57 · Wear in Conventional and Highly Cross-Linked Polyethylene – M.D. Ries ³⁶¹ 57

Summary

Ultra-high-molecular-weight polyethylene (UHMWPE) has been used successfully as a bearing surface in total knee arthroplasty for over 30 years, although material failures have typically resulted from gamma irradiation- induced oxidative degradation and the high cyclic stress environment of the knee. Since conversion to non-gamma irradiation sterilization methods, the failure mechanisms that were observed with gamma irradiation in air- sterilized UHMWPE have not occurred. Highly cross- linked UHMWPE has been developed in an effort to further reduce wear in total joint arthroplasty. However, cross-linking reduces the mechanical properties of UHMWPE, including fatigue crack propagation resistance,which may limit its application in total knee arthroplasty.

Relationship Between Contact Stress and Wear Mechanisms

As a result of the different loading conditions and contact stresses in the hip and knee, the wear mechanisms that occur in total hip and total knee arthroplasties are different (⊡ Fig. 57-1).

The hip is a congruent ball-and-socket joint with a relatively large contact area at the bearing surface. The larger contact area of the hip results in lower contact stress. At low contact stress, surface wear mechanisms (abrasion and adhesion) predominate. However, the contact stresses in the knee are typically an order of magni- tude higher than in the hip [1].As a result of the lower conformity and contact area in total knee tibial components, the yield stress of UHMWPE is exceeded in most designs [2]. In a tibial insert with relatively high contact stresses and moving contact area, alternating tensile and com- pressive stresses are created which can lead to fatigue (delamination and pitting) wear mechanisms [3, 4]. Surface- wear mechanisms produce relatively small particles, typically less than 1 µm in size, while fatigue-wear mechanisms produce larger particles.The smaller particles can elicit more of an osteolytic response than the larger par-

ticles [5].Wear particles are generally smaller in total hip compared with total knee arthroplasty,and osteolysis appears to be more common in total hips than in total knees [6]. However, osteolysis does occur in total knee replacements, particularly in those which have large contact ar-

57 Wear in Conventional and

Highly Cross-Linked Polyethylene

M. D. Ries

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⊡ Fig. 57-1a, b. a A clinically retrieved total hip acetabular component which had failed as a result of wear and osteolysis. The articulating surface appears smooth, consistent with surface-wear mechanisms (abrasion and adhesion), which produce relatively small, submicron particles. b A clini- cally retrieved total knee tibial insert which failed as a result of UHMWPE wear. The surface is delaminated and fragmented, consistent with fatigue- wear mechanisms (delamination and pitting), which occur at high contact stress and usually produce particles that are larger than those retrieved form total hip components. (Reproduced with permission from [10])

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eas and low contact stresses, such as mobile-bearing designs [7].

Conventional UHMWPE

Total knee arthroplasty with the use of extruded or molded UHMWPE which is sterilized by gamma irradiation in air has been reported to have survivorship rates of 90%- 95% after 10 years [8, 9]. Failures are typically associated with UHMWPE fatigue damage and wear is associated with oxidative degradation.Oxidative degradation occurs after gamma irradiation sterilization and exposure to air [10]. Gamma irradiation causes polymer chain scission and the formation of chemically unstable free radicals.

The free radicals can react with oxygen to form a chemically stable carbonyl group. This process results in oxidative degradation. The free radicals can also remain present for long periods of time after gamma irradiation. As oxygen diffuses into the UHMWPE implant more oxidative degradation occurs, as oxygen reacts with remaining free radicals. The fatigue strength and wear resistance of UHMWPE are both reduced by oxidative degradation [11, 12]. Components which are shelf-aged and then implant- ed have a higher failure rate than those which are stored for shorter periods of time [13]. Since oxidative degradation requires both gamma irradiation sterilization and exposure to oxygen, the process can be avoided by using either non-irradiation sterilization (ethylene oxide or gas plasma) or an inert environment during and after irradiation (vacuum, argon, or nitrogen) which eliminates oxygen exposure.

Molded components, made from Himont 1900 resin, are particularly resistant to oxidative degradation even after gamma irradiation sterilization and exposure to air [14]. The reason for this resistance to oxidative degradation is not clear, but it may be related to better consolida- tion of the resin material which limits oxygen diffusion into the polymer. However, since UHMWPE total knee tibial inserts are no longer sterilized by gamma irradiation in air, the potential benefit of a material with greater resistance to gamma irradiation-induced oxidative degradation is not clear.

In the past, conventional UHMWPE could be consid- ered to be gamma-irradiated UHMWPE, which is sterilized and stored in air. However, this material is no longer manufactured for use in total knee tibial components.Cur- rent conventional UHMWPE may be defined as either non-gamma-irradiation sterilized (ethylene oxide or gas plasma) or gamma-irradiation sterilized and stored in an inert (vacuum, nitrogen, or argon gas) environment. Both of the current methods used to sterilize UHMWPE eliminate or minimize the potential for oxidative degradation compared with materials which have been used in the past and on which most long-term clinical studies are based.

Effect of Current Sterilization Methods

UHMWPE implants are currently sterilized by either a non-irradiation method (gas plasma or ethylene oxide) or gamma irradiation and storage in an inert atmosphere without oxygen. Gas sterilization methods eliminate oxidative degradation since free radicals, formed by gamma irradiation, are not present. Gamma irradiation in an inert environment reduces oxidative degradation since free radicals are still present after sterilization, but oxygen is not available in the atmosphere to react with the free radicals. However, some in vivo oxidative degradation may occur, since oxygen can be present in the joint fluid.

The mechanical properties of UHPMWPE, including fatigue strength,are not affected by gas sterilization since there is no chemical change in the polymer structure [11].

The fatigue crack propagation resistance is reduced by gamma-irradiation sterilization, although abrasive wear resistance is improved as a result of cross-linking caused by irradiation.To the author’s knowledge,the failures typically observed with gamma-irradiated in air UHMWPE have not been reported with the use of current gas, or gamma irradiation in an inert atmosphere, sterilization methods.

Highly Cross-linked UHMWPE

Highly cross-linked UHMWPE has been developed in an effort to reduce the abrasive and adhesive wear which occurs in total hip arthroplasty. In total hip simulators, volumetric wear is dramatically reduced after cross-linking [15,16].However,the mechanical properties,including fatigue crack propagation resistance, tensile strength, yield strength, and elongation, are also reduced. The reduced mechanical properties, particularly fatigue crack propagation resistance, may lead to problems of fatigue wear after cross-linking.

Fatigue crack development can be separated into phases of crack initiation and crack propagation. Crack initiation requires a defect in the material, which may be present from manufacturing flaws or may occur sponta- neously due to in vivo loading (⊡ Fig. 57-2).

Crack propagation, or the rate at which the defect en- larges, is dependent on the material properties. After cross-linking, fatigue crack propagation resistance is reduced, indicating that a crack would be expected to trav- el through highly cross-linked UHMWPE more rapidly than non-cross-linked UHMWPE [17]. If cracks are not initiated in highly cross-linked UHMWPE, then fatigue failures would not be expected to occur. However, early clinical retrieval studies of highly cross-linked UHMW- PE demonstrate a high rate of surface defects which could lead to further fatigue wear mechanisms [14].

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Despite the reduction in material properties, knee wear simulator studies demonstrate less volumetric wear of highly cross-linked than of non-cross-linked UHMWPE under clean conditions [18].Although fatigue wear mechanisms occur more commonly in total knee tibial components than in total hip acetabular components, surface wear mechanisms (abrasive and adhesive wear) do occur in total knee arthroplasty. Wear simulators which test total knee components under ideal conditions may evalu- ate only abrasive and adhesive wear mechanisms and thus can be expected to show an apparent benefit to using highly cross-linked UHMWPE. However, fatigue wear as well as wear caused by counterface roughening commonly occurs in vivo and may not be represented by wear sim- ulations under clean conditions. Roughening of a total knee femoral component in vivo may occur as a result of third-body abrasives (such as cement or bone particles) or oxidative wear of the metal [19]. In order to predict the effects of counterface roughening on wear in vivo, wear simulator studies with artificially roughened femoral

components have been performed. One method of in vitro roughening has been produced by tumbling the femoral component in alumina particles prior to wear testing [20]. The roughness of the components was similar to that of in vivo retrieved femoral components. The roughened implants were then articulated with highly cross-linked and non-cross-linked UHMWPE in a knee simulator.Wear was increased by counterface roughening for both highly cross-linked and non-cross-linked UHMWPE [21].These findings indicate that highly cross- linked UHMWPE can reduce surface wear mechanisms in a knee simulator. However, if in vivo roughening of the femoral counterface occurs, wear may be increased.

Clinical studies will be necessary to determine the safety and efficacy of highly cross-linked UHMWPE in total knee arthroplasty. However, because of the reduction in fatigue crack propagation resistance caused by cross- linking and sensitivity to counterface roughening, currently available highly cross-linked polyethylenes may not to be beneficial for use in fixed-bearing total knee arthroplasty.

Effect of Counterface Roughness

Although fatigue wear mechanisms are more common in total knee than in total hip UHMWPE components, both fatigue and abrasive wear occur in total knee tibial components.The roughness of the femoral component surface can have an effect on UHMWPE abrasive wear. A single scratch in the metallic counterface articulating with UHMPWE can significantly increase wear [22].Total knee femoral components, which are typically made from cast cobalt chrome, roughen in vivo [19]. In vivo roughening can occur from third-body abrasives such as cement,bone, or metal particles.Wear of UHMWPE in total knee arthroplasty is increased after counterface roughening [20].

However, wear is reduced by the use of a scratch-resistant femoral component under both clean and abrasive conditions [20, 23]. Use of a scratch-resistant femoral component,such as oxidized zirconium,subjected to roughening with alumina particles produces wear similar to a cobalt- chrome femoral component under clean conditions.These observations imply that the use of a scratch-resistant counterface rather than highly cross-linked UHMWPE may be more effective in reducing abrasive wear in total knee arthroplasty without compromising the mechanical properties of the UHMWPE tibial insert.

References

1. Bartel DL, Bicknell VL, Wright TM (1986) The effect of conformity, thick- ness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg [Am] 68:1041-1051 Chapter 57 · Wear in Conventional and Highly Cross-Linked Polyethylene – M.D. Ries ³⁶³ 57

⊡ Fig. 57-2a, b. a A clinically retrieved e-beam highly cross-linked tibial insert removed after only 3 months in vivo, demonstrating abrasive scratches. b Scanning electron micrograph of the highly cross-linked tib- ial plateau demonstrating abrasive sratches in line with the flexion axis and cracks in the surface perpendicular to scratches

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