Ex vivo wear of conventional and cross-linked polyethylene acetabular liners

Explanted highly cross-linked polyethylene acetabular liners show third-body scratching. It was suggested that this surface damage could increase the wear rate of highly cross-linked polyethylene. We investigated the wear behavior of surgically explanted highly cross-linked and conventional acetabular liners using a hip simulator. Highly cross-linked explants showed no weight loss. The scratches and original machine marks of the highly cross-linked liners present at explantation remained during testing; the wear rates of these liners were 1.1 +/- 2.3 mg/million cycles, representing weight gain. In contrast, conventional liners showed wear rates of -12.9 +/- 1.4 mg/million cycles, representing weight loss. Wear was associated with rapid surface polishing and scratch elimination. After testing, the liners were melted to trigger shape memory and recover plastic deformation-induced surface changes. Highly cross-linked liners showed substantial recovery of original machining marks after melting, indicating that the scratches were a result of plastic deformation and not wear. None of the conventional polyethylenes showed any recovery after melting. The wear resistance of the highly cross-linked polyethylene studied was not compromised by surface changes that occurred during the first year of in vivo service.