Volumetric wear assessment of failed metal-on-metal hip resurfacing prostheses

Recent advancements in hip arthroplasty have allowed the operation to boast excellent results and high survivorship. However, failures do still occur and a major cause is complications arising from wear debris. It is essential therefore that debris is minimized by reducing wear at the bearing surface. One proposed method of achieving this wear reduction is through the use of metal-on-metal articulations. One of the latest manifestations of this biomaterial combination is in designs of hip resurfacing which are aimed at younger, more active patients who might wear out a conventional metal-on-polymer hip prosthesis. However, do these metal-on-metal hip resurfacings show less wear when implanted into patients? Using a co-ordinate measuring machine and a bespoke computer program, volumetric wear measurements for retrieved Articular Surface Replacements (ASR™, DePuy) metal-on-metal hip resurfacings were undertaken. Thirty-two femoral heads and twenty-two acetabular cups were measured. Acetabular cups exhibited mean volumetric wear of 29.00 mm3 (range 1.35–109.72 mm3) and a wear rate of 11.02 mm3/year (range 0.30–63.59 mm3/year). Femoral heads exhibited mean wear of 22.41 mm3 (range 0.72–134.22 mm3) and a wear rate of 8.72 mm3/year (range 0.21–31.91 mm3/year). In the 22 cases where both head and cup from the same prosthesis were available, mean total wear rates of 21.66 mm3/year (range 0.51–95.50 mm3/year) were observed. Compared with in many vitro tests, these are significantly higher than those expected in a well functioning metal-on-metal hip resurfacing prosthesis and are of concern.