Metal-on-metal hip resurfacing: is it forgiven for malorientation?

Abstract

BackgroundDespite the benefits and clinical success of modern metal-on-metal articulations, there are many concerns about the release of metal ions and their hazardous effects. The exact mechanism and risk factors are a matter of debate. The goal of this prospective study was to evaluate the effect of cup abduction angle and other independent factors (age, sex, prosthetic head size, and period of follow-up) on the levels of serum metal ions in patients after metal-on-metal hip resurfacing (MOMHR) arthroplasty.Patient and methodsBetween January 2010 and June 2013 a total of 50 patients were operated upon and followed up after a unilateral MOMHR, with an average follow-up period of 28.3 months. The patients' ages ranged between 20 and 50 years. There were 24 male and 26 female patients. Clinical and radiological evaluations were made and serum metal ion levels were measured. Correlations between serum metal ion levels and different variables were also evaluated.ResultsThe average preoperative Harris hip score was 46.24. This increased to 90.12 at the end of follow-up. There was a statistically significant increase (P < 0.05) in serum cobalt and chromium levels at the sixth postoperative month, compared with the preoperative levels. However, the condition was significantly (P < 0.05) reversed after the first year. Cup abduction angle was associated with an insignificant effect (P > 0.05) on hip function (Harris hip score) but was associated with a significant increase (P < 0.05) in serum ion levels. Prosthetic head size was associated with a statistically significant (P < 0.05) inverse relation with metal ion levels.ConclusionThe present study found that three main variables had a statistically significant effect on metal ion release: namely, cup orientation, size of the prosthetic head, and period of follow-up. Of them, the only controllable factor that can be carefully addressed during surgical procedures is the first one. Hence, cup inclination during surgery must be considered carefully in order to reduce early failures when performing large-bearing MOMHR.