Abstract
Introduction:
A disturbing prevalence of painful inflammatory reactions has been reported in metal-on-metal (MoM) hip resurfacing arthroplasty. A contributing factor is localized loading of the acetabular shell leading to “edge wear” which is often seen after precise measurement of the bearing surfaces of retrieved components. Factors contributing to edge wear include adverse cup orientation leading to proximity (<10 mm) of the hip reaction force to the edge of the acetabular component. As this phenomenon is a function of implant positioning and patient posture, this study was performed to investigate the occurrence of edge loading during different functional activities as a function of cup inclination and version.
Methods:
We developed a computer model of the hip joint through reconstruction of CT scans of a proto-typical pelvis and femur and virtually implanting a hip resurfacing prosthesis in an ideal position. Using this model, we examined the relationship between the resultant hip force vector and the edge of the acetabular shell during walking, stair ascent and descent, and getting in and out of a chair. Load data was derived from 5 THR patients implanted with instrumented hip prostheses (Bergmann et al). We calculated the distance from the edge of the shell to the point of intersection of the load vector and the bearing surface for cup orientations ranging from 40 to 70 degrees of inclination, and 0 to 40 degrees of anteversion.
Results:
Previous studies have shown that wear of MOM bearings becomes significantly elevated once the load vector comes within 10 mm of the edge of the acetabular cup. Our simulations demonstrated that normal gait, stair climbing and stair descent do not generate edge loading unless the shell was oriented in 70° of inclination and 20° of anteversion. Conversely, edge loading was predicted during sit-to-stand and stand-to-sit activities for every orientation of the implanted components, including values within the “safe zone” (Figure 1). Cup anteversion was not a consistent predictor of edge loading during gait, stair climbing or stair descent, but did affect the distance to the edge of the cup in sit-to-stand and stand-to-sit activities.
Conclusions:
We demonstrated that normal gait, stair-climbing and stair descent do not appear to explain the edge wear seen in many of the retrieved resurfacing components. Edge loading does occur during sit to stand and stand to sit activities in virtually any cup orientation and is postulated as the missing factor explaining component wear. In our work we have effectively demonstrated that, in the absence of other confounding factors, edge loading and pseudotumor formation can happen in even the “safe” acetabular orientations. We propose this as a new way to understand the forces upon the components following HRA.
