Impingement Between the Acetabular Cup and the Femoral Neck After Hip Resurfacing Arthroplasty

Loosening, femoral neck fracture, and metal ion release have been well documented after hip resurfacing arthroplasty, but impingement between the acetabular cup and the femoral neck has not. The goal of this study was to analyze radiographic findings that were presumed to represent impingement of the neck after hip resurfacing arthroplasty, and to describe the mechanism of impingement. Of the 635 hips (579 patients) that underwent hip resurfacing arthroplasty between September 1998 and May 2008, forty hips (6.3%) with positive radiographic findings were available for this study. The mean age of the patients was 34.9 years and there were thirty-five men and five women. The average follow-up was sixty-eight months (twenty-four to 132 months). Radiographic evaluation included an analysis of neck-shaft angle, inclination of the acetabular cup, head-neck ratio, lateral protrusion of the cup, anterior protrusion of the cup, and stem angle to the axis of the femoral neck. Impingement between the acetabular cup and the femoral neck occurred in 6.3% of hips and appeared at an average of seven months after surgery. There was no further change two years postoperatively. The impingement findings did not correlate with the clinical outcome, but all instances of impingement were located in the lateral or anterolateral part of the femoral neck along the arc from the center of the femoral head to the lateral edge of the socket. No significant factors were found to be related to the occurrence of impingement. Femoral-neck impingement should be differentiated from notching, narrowing, stress-shielding, or osteolysis of the femoral neck. Although we found no significant factors to explain the impingement found in the hips in our study, it appears that repetitive extreme motion of the involved hip and malposition of the implants can cause impingement after hip resurfacing arthroplasty.