Introduction and Aims: Studies have shown substantial reduction in wear rates in elevated cross-linked polyethylene (crosslinking to a higher degree than that obtained by radiation sterilisation alone). The aim of this study was to test the effect of increased crosslinking and increased head size on polyethylene wear rates.

Method: Four groups of acetabular liners from a single manufacturer were tested: 28mm nominally cross-linked, 32mm nominally cross-linked, 28mm elevated cross-linked, and 32mm elevated cross-linked. Three implants from each group were tested in a 12-station hip wear simulator using 90% bovine serum as lubricant. Liners were articulated with the appropriately sized cobalt-chrome femoral head. Additional liners from each design were subjected only to the same load without motion to serve as load-soak controls to account for any weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of 5,000,000 cycles.

Results: Nominally cross-linked liners demonstrated mean wear rates of 14.97±2.70 and 16.92±2.58 milligrams/million cycles for 28mm and 32mm head sizes, respectively. Both of the elevated cross-linked liners had significantly lower wear rates than controls with a mean of 1.51±1.08 and 2.57±1.78 milligrams/million cycles for 28mm and 32mm head sizes, respectively (p< 0.001).

Conclusion: Larger femoral head sizes reduce dislocation in total hip arthroplasty; however, they have been associated with unacceptably high wear rates. The dramatic reduction in wear rates with polyethylene crosslinking even with the larger head size may increase the potential for use of 32mm head components in total hip arthroplasty.

Polyethylene wear is a significant factor limiting survivorship of total knee arthroplasty (TKR). Crosslinking of polyethylene has been shown to significantly reduce wear in hip arthroplasty but has not been reported for TKR. This study measured wear in polyethylene cross-linked to two levels in a knee wear simulator.

Six polyethylene knee inserts were tested in a knee wear simulator. Inserts were manufactured from polyethylene crosslinked to two different levels: 2.5 Mrad (Low-X) and 10.5 Mrad (High-X). Each implant was enclosed in a closed lubricant (50% alpha fraction calf serum) recirculation chamber, maintained at 37°C and changed every 500,000 cycles. Physiologic levels of load and motion were applied at 1 Hz for a total of 6,000,000 cycles. Wear was measured by the gravimetric method before wear testing and at every 500,000 cycles. Semi-quantitative wear assessment was performed by imaging the insert surfaces at 10x magnification.

The Low-X inserts demonstrated significantly higher wear rates (mean 4.66 mg/million cycles) than the High-X inserts (mean 1.55 mg/million cycles, p < 0.001). Wear scars on the Low-X inserts were irregular and visibly deeper than those on the High-X inserts. The machining marks on the surface of the insert were also better preserved in the High-X insert wear scars. These results suggest that crosslinked PE can significantly reduce wear in TKR under physiologic conditions. This can result in reduced lysis and increased survivorship. Localized damage can cause catastrophic failure in polyethylene knee inserts. Therefore, further studies are necessary to evaluate wear under these conditions.