Variation in the wear rate during the life of a total hip arthroplasty: a simulator and retrieval study

The limitation of wear is fundamental to the optimization of total hip arthroplasty longevity. The maintenance of the supersmooth femoral head surface is considered to be paramount in maximizing prosthesis life expectancy. Ex vivo studies have failed to substantiate a relationship between roughness and the clinical wear factor, however. A hip simulator wear study was undertaken to investigate this contradiction. Three explanted femoral heads were articulated for 5 million cycles against new acetabular liners. The simulator wear rate was 5 times the ex vivo value. This difference can be explained only if the explant head roughness was not that which existed for most of the joint's life. The relationship between surface roughness and wear deduced for simulator testing is substantially different from that of unidirectional wear screening methods. The multiphasic nature of wear in cementless joints has been illustrated: a wear-in period, followed by a steady-state phase, until a head-roughening event causes a rapid wear period.