Understanding why metal-on-metal hip arthroplasties fail: a comparison between patients with well-functioning and...

A large proportion of metal-on-metal hip arthroplasty failures are due to unexplained pain. The mechanism of failure has been thought to be associated with factors that increase material loss, including specific design features and surgical positioning of components. However, recent evidence suggests that there is not a simple dose-response relationship. An analysis of failed metal-on-metal hip arthroplasties involving a single design was performed in an attempt to help resolve this issue. Our aim was to identify the clinical and component variables associated with failure of metal-on-metal hip arthroplasties, particularly in patients undergoing revision arthroplasty because of unexplained hip pain, and to clarify the role of material loss.

We prospectively recruited fifty-five patients who were undergoing revision of a metal-on-metal Birmingham Hip Resurfacing System (BHR) arthroplasty (Smith & Nephew). We collected clinical data preoperatively, intraoperatively, and following the revision arthroplasty. Data included chromium and cobalt levels in whole blood, which were measured with use of inductively coupled plasma mass spectrometry (ICPMS), and component orientation, which was typically measured with use of computed tomography (CT) scans. The wear of the retrieved components was also quantified postoperatively. All parameters were compared with those in a comparable group of patients with a well-functioning BHR arthroplasty.

Sixty-nine percent of the patients who underwent revision arthroplasty did so following a diagnosis of unexplained hip pain. When compared with patients with a well-functioning arthroplasty, patients who underwent revision arthroplasty had a significantly higher acetabular cup inclination angle (p < 0.01), a significantly smaller femoral head diameter (p < 0.01), and significantly higher blood cobalt and chromium ion levels (p < 0.01). However, almost 50% of the patients who underwent revision arthroplasty had blood metal ion levels below the clinical threshold of 7 ppb and low component wear rates of <5 μm/year.

In a large number of patients with unexplained hip pain leading to revision of a metal-on-metal hip arthroplasty, the acetabular cup orientation was satisfactory and the material loss rate was low. We suspect that patient-specific factors may have been responsible for the failure in a large proportion of these patients.