Abstract
Introduction
Studies of metal-on-metal (MoM) hip resurfacing arthroplasty (HRA) have reported high complication and failure rates due to elevated metal ion levels. These rates were shown to be especially high for the Articular Surface Replacement (ASR) HRA, possibly due to its unique design. Associations between metal ion concentrations and various biological and mechanical factors have been reported. Component positioning as measured by acetabular inclination has been shown to be of especially strong consequence in metal ion production in ASR HRA patients, but few studies have evaluated acetabular anteversion as an independent variable.
The primary objective of this study was to evaluate the associations between component orientation, quantified by acetabular inclination and anteversion, and blood metal ions. Secondly, we sought to report whether conventional safe zones apply to MoM HRA implants or if these implants require their own positioning standards.
Methods
We conducted a multi-center, prospective study of 512 unilateral ASR HRA patients enrolled from September 2012 to June 2015. At time of enrollment our patients were a mean of 7 (3–11.5) years from surgery. The mean age at surgery was 56 years and 24% were female. All subjects had complete demographic and surgical information and blood metal ions. In addition, each patient had valid AP pelvis and shoot-though lateral radiographs read by 5 validated readers measuring acetabular abduction and anteversion, and femoral offset. A multivariate logistic regression was used with high cobalt or chromium (greater than or equal to 7ppb) as the dependent variable. The independent variables were: female gender, UCLA activity score, age at surgery, femoral head size, time from surgery, femoral offset, acetabular abduction, and acetabular anteversion.
Results
The average acetabular inclination angle was found to be 44.7° (20.6°–64.5°), and the average anteversion angle was 24° (0.2°–55.3°) (Figure 1). After controlling for the possible confounding variables, the factors contributing to elevated metal ions (≥ 7 ppb) were found to be time from surgery (OR = 1.29, p = 0.011), high abduction angle (– 55°) (OR = 4.40, p = 0.001), low anteversion angle (0°–10°) (OR = 3.82, p = 0.001), and female gender (OR = 3.45, p = 0.001).
Discussion and Conclusion
We found that blood metal ion levels are affected by both acetabular inclination and anteversion (Figure 2). Furthermore, we observed that there was a high degree of variation in the positioning of these implants, and we conclude that those with high inclination and/or low anteversion angle should be most vigilantly monitored.
