Aims One goal of total hip arthroplasty is to restore normal hip anatomy. The aim of this study was to compare displacement of the centre of rotation (COR) using a standard reaming technique with a technique in which the acetabulum was reamed immediately peripherally and referenced off the rim. Patients and Methods In the first cohort the acetabulum was reamed to the floor followed by sequentially larger reamers. In the second cohort the acetabulum was only reamed peripherally, starting with a

more »

... eamer the same size as the native femoral head. Anteroposterior pelvic radiographs were analysed for acetabular floor depth and vertical and horizontal position of the COR. Results Horizontally, the mean medial displacement of the COR was 0.8 mm (standard deviation (SD) 1.4) in the peripheral remaing group and 5.0 mm (SD 3.30) in the standard reaming group (p < 0.001). Vertically, the mean superior displacement of the COR was 0.7 mm (SD 1.3) in the peripheral reaming group and 3.7 mm (SD 2.6) in the standard reaming group (p < 0.001). In the standard reaming group, there was a strong correlation between the preoperative acetabular floor depth and displacement of the COR (p < 0.001). Conclusion Reaming the acetabulum to the floor can lead to significant displacement of the COR medially and superiorly. This displacement is related to the pre-operative acetabular floor depth and cannot always be compensated by using a high offset stem. Cite this article: Bone Joint J 2016;98-B:1597-603.