Can metal-on-metal bearings improve the longevity of total hip prostheses?

Purpose of the study In total hip arthroplasty (THA), inter-series comparative clinical results cannot be considered worthy before at least 10 years of average follow-up, as shown in the Swedish Arthroplasty Register experience (1978-1993). Last generation metal-on-metal bearings were introduced in France only nine years ago (1995). To date, using mid term information, data from the literature, and our experience, one could assume that this bearing material has the capacity to improve THA longevity. Three types of information were analyzed: 1) comparative radiographic and EBRA studies published on early migration (< 2 years) of acetabular implants; 2) preliminary comparative data on wear and osteolysis at the 8-year maximum follow-up; 3) clinical data on dislocation frequency and in vitro and in vivo observations of bearing separation and sliding. Study no 1: primary stability of these metal-on-metal acetabular cups was better than for polyethylene (PE) cups or alumina liners; study no 2: no detectable wear and less osteolysis were observed to date with metal-on-metal bearings versus their alumina-on-PE counterparts. Study no 3: less dislocation and less head sliding were measured with the metal-on-metal versus alumina-on-PR bearings possibly due to the interfacial forces provided by lubricating fluid (suction fit). Conclusion According to current knowledge, this metal-on-metal bearing still represent, with alumina-on-alumina and highly crosslinked PE, a competitive and comprehensive option to improve THA longevity. The real pending problem concerns the frequency and above all the intensity of metal delayed hypersensitivity reaction and their potential effect on implant loosening.