Performance of the resurfaced hip. Part 2: The influence of prosthesis stem design on remodelling and fracture of the femoral neck.

Hip resurfacing is a popular treatment for osteoarthritis in young, active patients. Previous studies have shown that occasional failures--femoral neck fracture and implant loosening, possibly associated with bone adaptation--are affected by prosthesis sizing and positioning, in addition to patient and surgical factors. With the aim of improving tolerance to surgical variation, finite element modelling was used to indicate the effects of prosthesis metaphyseal stem design on bone remodelling and femoral neck fracture, with a range of implant orientations. The analysis suggested that the intact femoral neck strength in trauma could be maintained across a wider range of varus-valgus orientations for short-stemmed and stemless prostheses. Furthermore, the extent of periprosthetic bone remodelling was lower for the short-stemmed implant, with slightly reduced stress shielding and considerably reduced densification around the stem, potentially preventing further progressive proximal stress shielding. The study suggests that a short-stemmed resurfacing head offers improved tolerance to misalignment and remodelling stimulus over traditional designs. While femoral neck fracture and implant loosening are multifactorial, biomechanical factors are of clear importance to the clinical outcome, so this may reduce the risk for patients at the edge of the indications for hip resurfacing, or shorten the surgical learning curve.