Failure rates of stemmed metal-on-metal hip replacements

Alison Smith and colleagues (March 31, p 1199)

compare survivorship data between stemmed hip replacements with various bearing materials. However, their conclusion that “Metal-on-metal stemmed articulations give poor implant survival compared with other options and should not be implanted” is invalid.

Smith and colleagues lump together all designs of metal-on-metal prostheses listed in the National Joint Registry of England and Wales, but they do not show that all of the metal-on-metal designs had inferior survivorship results compared with any of the other bearings. In fact, some metal-on-metal total hip replacements have been implanted for more than 20 years, with survivorship results that exceed those of most alternative bearings.

The coverage angle of the femoral head by the acetabular cup has been shown to greatly affect the potential for edge-loading wear,

and the manufacturing tolerances (clearance and roundness) affect the ability of the bearing to operate under fluid-film lubrication.

Coverage angle and tolerances differ from one design to another. The higher failure rate of metal-on-metal implants noted in this study is thus design-dependent.

Additionally, the dominant modes of failure were not reported in this study, leaving the reader with only guesses as to why a difference in survivorship rate was seen between bearing materials. Finally, the study tends to identify the bearing material and the prosthetic type that is the best fit for the overall population of patients in need of a hip arthroplasty. This information is unlikely to fully satisfy patients and surgeons in search of a more individualised prosthetic solution. In hip arthroplasty, the era of “one device fits all” has long gone because the patient population has become a lot more diverse and the indications for hip arthroplasty greatly widened.

HCA receives royalties from Wright Medical Technology. MJLD declares that he has no conflicts of interest.