High Cup Angle and Microseparation Increase the Wear of Hip Surface Replacements

TLDR

High cup inclination angles (>55°) in metal‑on‑metal hip surface replacements are linked to elevated wear rates and patient ion levels. The study simulated ceramic‑on‑ceramic wear patterns using microseparation to mimic head offset deficiency and tested 39‑mm metal‑on‑metal bearings in a hip simulator at 60° cup inclination alone and with microseparation over 2 million cycles. The 60° inclination alone increased wear ninefold, while adding microseparation raised it 17-fold; particles were larger under microseparation but similarly shaped, indicating that both cup angle and head offset affect wear.

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.