Abstract
Introduction
Edge loading is a common wear mechanism in Metal-on-Metal (MOM) hip resurfacing and is associated with higher wear rates and the incidence of pseudotumour. The purpose of this study was to develop a method to investigate the contributions of patient, surgical and implant design variables on the risk of edge loading.
Method
We created a mathematical model to calculate the distance from the head-cup contact patch to the rim of the cup and used this to investigate the effect of component position, specific design features and patient activity on the risk of edge loading. We then used this method to calculate the contact patch to rim distance (CPRD) for 160 patients having undergone revision of their MOM hip resurfacing in order to identify any possible associations.
Results
We identified several variables that reduce the CPRD and increase the risk of edge loading, including; increased cup anteversion and inclination, activities involving increased hip flexion, reduced clearance, reduced hip diameter and reduced cup arc angle. We also determined the threshold cup orientation for edge loading for five resurfacing designs (Figure 1). In patients with failed MOM hip resurfacings, there was a significant correlation between CPRD and both component wear rates (Figure 2) and blood metal ion levels (all p < 0.005). The ASR was associated with increased wear, reduced CPRD, and increased prevalence of edge loading (all p < 0.05).
Conclusions
Edge loading is common and difficult to avoid in MOM hip resurfacing. Whilst some designs, such as the ASR, are more susceptible to edge loading, all are unforgiving of suboptimal cup position. Furthermore, common activities involving flexion of the hip result in edge loading even in patients with a well-positioned cup.
