High cup angle and microseparation increase the wear of hip surface replacements

Abstract

High wear rates and high patient ion levels have been associated with high (> 55 degrees) cup inclination angles for metal-on-metal surface replacements. Wear rates and patterns have been simulated for ceramic-on-ceramic bearings by applying microseparation to replicate head offset deficiency. We tested 39-mm metal-on-metal surface replacements (n = 5) in a hip simulator with (A) an increased cup inclination angle of 60 degrees and (B) an increased cup inclination angle and microseparation over 2 million cycles. (A) resulted in a ninefold increase in wear rate and (B) resulted in a 17-fold increase in wear rate compared to a standard gait condition study. Wear particles produced under microseparation conditions were larger than those produced under standard conditions but of similar shape (round to oval). The data suggest both head and cup position influence the wear of surface replacements; we believe it likely bearings with high wear either have a high cup inclination angle, an offset deficient head, or a combination of both.

Fig. 1

Volumetric wear rates of 39-mm surface replacements tested under standard, high cup angle and high cup angle combined with microseparation conditions are shown, demonstrating increased wear rates under high cup angle and microseparation conditions compared to standard. Mean ± 95% confidence limits.

Fig. 2A–B

Contact profilometer traces across wear area of (A) head and (B) cup after two million cycles of testing under high cup angle combined with microseparation conditions are shown. Stripe wear appears on the femoral component and maximum penetration at the edge of the acetabular component.

Fig. 3

The figure shows size distribution of wear particles produced under standard and microseparation conditions, showing increased size of wear particles produced under microseparation conditions. Mean ± 95% confidence limits.