Design of hard-on-hard bearing couples has traditionally been characterized by the material of the bearing couple, clearance between the bearing surfaces, sphericity of the components, surface roughness, and the radii of the components. All of these factors play a role in the lambda ratio and fluid film thickness calculations. However, the fluid film for hard on hard bearings can be interrupted by issues like the presence of 3rd body particles, intermittent walking, jogging, and subluxation. Only recently have researcher begun to simulate some of these disruptions in the fluid film for hard on hard bearings.

Recent laboratory testing has looked at the effects of utilizing different materials and methodologies to evaluate hard-on-hard bearings. Ceramic-on-metal is a unique combination of components that is currently available. Several authors have shown that this combination can reduce the amount of metal wear generated during the test by a factor of 4–100. However, an occasional anomaly has shown up in some of these tests where a wear couple in a steady state wear mode will have a several-fold increase in wear for a short duration.

For bearing couples that have a metal component, ion analysis of the serum lubricant can be utilized to monitor the amount of wear. This technique can provide real-time data on the amount of wear seen in simulator testing without removing the specimens from the machine. Further, there are some designs of metal cups that cannot be removed from the simulator without causing damage to the component. Data from a ceramic-on-metal simulator test confirmed that the short-term anomaly in gravimetric wear correlated with an increase in metal ion levels.

Distraction testing evaluates the change in wear due to the unintended subluxation of the hip. This may occur during a standard walking gait if the hip is loose, during impingement, or during deep-knee bends, squatting, or rising from a chair. Distraction testing has various effects on wear depending on the material of the bearing couple. UHMWPE is insensitive to this additional mode of simulator testing. Metal-on-metal and ceramic-on-ceramic can increase in wear by up to an order of magnitude. The utilization of Biolox-delta rather than Biolox-forte can reduce the amount of wear seen during distraction testing. Diamond-on-diamond is insensitive to this wear mode and showed immeasurable wear.

Other issues during testing of hard-on-hard bearings are still being explored. It is well known that 3rd body particles will disrupt fluid films and can increase wear. But the results from adding particles is variable. Metal-on-metal tests can have one specimen with very little increase while another specimen has an order of magnitude increase. Deformation of the shell caused by insertion during surgery has been shown to occur. Currently, this deformation has not been able to be replicated in a simulator, therefore, its effects are unknown.

The design and laboratory testing of hard-on-hard bearings has improved significantly over the past decade. Further research is still needed to evaluate designs that may potentially increase resistance to failure modes other than standard walking gait cycles.

Vitamin E stabilized highly crosslinked UHMWPE (E-Poly^™) was developed to improve upon the properties of first generation highly crosslinked UHMWPE’s. The post-crosslinking processing for E-Poly^™ maximizes the strength of the material while at the same time stabilizing residual free radicals that remain after irradiation. E-Poly^™ is crosslinked with 100 kGy gamma irradiation prior to infusion of vitamin E. The infusion process involves diffusing vitamin E into the crosslinked material at temperatures beneath the melt temperature.

Small punch testing (ASTM F2183-02) was completed to evaluate strength of E-Poly^™ compared to gamma-inert sterilized UHMWPE. The results showed that the E-Poly^™ material had equivalent or better properties before and after accelerated aging than the gamma-inert sterilized UHMWPE (96–105 N vs. 75–88 N unaged; 100–115 N vs. 42–56 N 2-week aged).

Environmental stress crack testing evaluated the resistance to oxidation while the material was subjected to fatigue testing. A constant stress beam was tested for 5 weeks at 80C. Failure was defined as the appearance of cracks or fracture of the specimen. All 4 specimens of gamma sterilized components showed evidence of cracking prior to the completion of the test. 2 of 4 sequentially crosslinked and annealed specimens fractured prior to completion. None of the E-Poly^™ specimens showed cracks during testing. An examination of the amount of oxidation induced during this testing showed that the addition of fatigue loading increased the oxidation index for UHWMPE’s that had unstabilized free radicals. The surface oxidation index for gamma sterilized UHWMPE increased from ~0.3 to 1.1 and for sequentially crosslinked UHMWPE from ~0.3 to 0.7; the oxidation index for E-Poly^™ was negligible for all test condition.

Hip simulator testing (ISO 14242-1) showed that the volumetric wear rates for E-Poly^™ were 95–99 % less than that of ArCom. For 28mm head diameters the rates were 53.3 mm3/Mc for ArCom and 0.24 mm3/Mc for E-Poly^™. Wear particle morphology analysis showed that the E-Poly^™ wear particles were similar to ArCom. Qualitatively, there appeared to be fewer E-Poly^™ particles.

Knee simulator testing (ISO14243-1) was performed on both cruciate retaining and posterior stabilized Vanguard^® knees. The E-Poly^™ tibial bearing, CR and PS, showed 86% less wear than direct compression molded UHMWPE, the current gold standard. Fatigue testing of the PS post before and after accelerated aging (ASTM F2003) loaded to 1300lbs showed no degradation or failure of the post following 3 million cycles.

Vitamin E stabilized highly crosslinked UHMWPE has demonstrated excellent material properties, wear properties, and resistance to oxidation. These properties have been optimized through the combination of cross-linking, processing below the melt temperature subsequent to crosslinking, and the stabilization effect of vitamin E. These properties provide rational support to the utilization of vitamin E stabilized highly crosslinked UHMWPE for hip and knee applications.

Concerns about reduced strength, fatigue resistance, and oxidative stability of highly crosslinked UHMWPE have limited the acceptance of these materials for TKR. It was hypothesized that a new crosslinked UHMWPE stabilized with vitamin E would substantially improve wear performance and resistance to oxidative degradation without compromising mechanical properties. The purpose of this study was to comprehensively test this hypothesis in vitro.

GUR1020 was machined from isostatic molded bar-stock, crosslinked with 100 kGy, and then doped with vitamin E. This material was compared to direct molded GUR1050 UHMWPE. Both materials were gamma irradiation sterilized as for clinical use. Small punch testing, crack growth rate fatigue testing and oxidation index measurements were performed on each material before and after accelerated aging. Knee simulator testing evaluated wear of each material for 5-million walking cycles. CR knees were tested on a 6-station AMTI knee simulator; PS knees were tested on two 4-station Instron-Stan-more knee simulators. Statistical differences in all metrics were evaluated for significance with ANOVA (p < 0.05).

After 4-week accelerated aging, the control material showed elevated oxidation, loss of small punch mechanical properties and decreased fatigue crack growth resistance. In contrast, the vitamin E stabilized material had minimal changes in these properties. Further, the vitamin E stabilized material exhibited 85% reduction in wear for both the CR and PS designs.

Highly crosslinked UHMWPE stabilized with vitamin E appears to be promising for use as a bearing surface in TKA.