Does hip resurfacing require larger acetabular cups than conventional THA?

UNLABELLED: Hip resurfacing is femoral bone preserving, but there is controversy regarding the amount of bone removed at the acetabular side. We therefore compared the implanted acetabular cup sizes in primary THAs between two resurfacing devices and a conventional press-fit cup using a series of 2134 THAs (Allofit cup 1643 hips, Durom Hip Resurfacing 249 hips, and Birmingham Hip Resurfacing 242 hips). The effects of patient demographics and cup position in the horizontal plane also were assessed. After controlling for gender, patients were matched for height, weight, body mass index, and age. The mean size for Allofit cups was smaller than the sizes for Durom and Birmingham Hip Resurfacing cups in women (49.9 mm, 51.6 mm, 52.3 mm, respectively) and men (55.1 mm, 56.7 mm, 57.8 mm; respectively). Although patient height was associated with the implanted cup size, the cup position in the horizontal plane had no effect on the size used. Larger cups were used with hip resurfacing than for THA with a conventional press-fit cup. However, additional studies are needed to determine whether these small differences have any clinical implications in the long term. The association of cup size and patient height should be considered in future studies comparing component sizes among different implants. LEVEL OF EVIDENCE: Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.