Component malposition in hip resurfacing.

Abstract

Component malposition in hip resurfacing can cause early failure through the 2 main failure modes: femoral neck fracture and wear-related failures. The femoral component should be aligned in slight valgus with no notching of the superior femoral neck. Varus placement and notching are strongly correlated with early femoral neck fracture. We have developed special mushroom templates to assist with accurate femoral component sizing and positioning. Issues related to radiographic magnification, ruler measurements, and plastic overlay templates are avoided. Acetabular component malposition can result in wear-related failures. The geometry of most resurfacing sockets is such that the wall is thinner at the rim than at the apex. This is done to improve range of motion before impingement and results in the bearing having less coverage and a higher inclination than one would expect from the radiograph. Surgeons should therefore aim for a lower inclination than they would with total hip replacement. Acetabular anteversion, if excessive, can reduce the available contact area, and lead to edge wear and subluxation. Dysplasia, which is more common in women, can also result in excessive femoral anteversion. Taken together, the combined acetabular and femoral anteversion should not exceed 45°. Pseudotumors have been reported adjacent to metal-on-metal bearings. These reports point out that pseudotumors are often associated with component malposition. It is thought that edge wear due to malposition results in excessive metal debris, which is locally toxic. Proper component position is necessary for the short- and long-term success of hip resurfacing.