Background: The effect of obesity on the outcomes of metal-on-metal resurfacing arthroplasty is not currently known. In this study, we assessed the influence of body mass index on the survival of a metal-on-metal hybrid hip resurfacing prosthesis by comparing the clinical results of patients with a body mass index of =30 with those of patients with a body mass index of <30.
Methods: We retrospectively reviewed our registry to identify all patients who had been followed for at least two years after a metal-on-metal hip resurfacing arthroplasty, and we divided those patients according to whether they had had a body mass index of =30 (the study group) or <30 (the control group) at the time of the surgery. One hundred and twenty-five patients (144 hips) with an average weight of 104.6 kg and an average body mass index of 33.4 were included in the study group, and 531 patients (626 hips) with an average weight of 78.3 kg and an average body mass index of 25.4 were included in the control group. We compared the clinical results (UCLA [University of California at Los Angeles] and Harris hip scores, SF-12 [Short Form-12] survey results, and complication rates), radiographic results, and prosthetic survival rates of the two groups.
Results: There was no significant difference postoperatively between the groups with regard to the UCLA pain or walking scores or the mental component score of the SF-12. However, the UCLA function and activity scores were lower in the study group than in the control group (9.2 compared with 9.6 points [p = 0.001] and 7.1 compared with 7.6 points [p = 0.002], respectively). The control group had a significantly higher postoperative physical component score on the SF-12 (51.4 points compared with 49.3 points in the study group, p = 0.01) and postoperative Harris hip score (93.8 compared with 90.6 points, p = 0.0003). Two hips (1.4%) were revised in the study group. In contrast, thirty-one hips (5.0%) were converted to a total hip replacement in the control group; twenty of the thirty-one were revised because of loosening of the femoral component. The five-year survivorship of the hip prostheses was 98.6% in the study group and 93.6% in the control group (p = 0.0401). When the entire cohort was divided into three groups according to whether the body mass index was <25, 25 to 29, or =30, the risk of revision was found to have decreased twofold from one group to the next as the body mass index increased (p = 0.013). No acetabular component loosened in either group. The average diameter of the femoral component was 48.3 mm in the study group and 46.8 mm in the control group (p = 0.0001). There were no revisions for any reason and no radiolucencies were observed in a subset of twenty-seven patients with a body mass index of =35.
Conclusions: Metal-on-metal resurfacing hip arthroplasty is performing well in patients with a high body mass index, although the function scores are reduced compared with those for patients with a body mass index of <30. The protective effect of a high body mass index on survivorship results may be explained by a reduced activity level and a greater component size in this patient population.
Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.