Do we medialise the hip centre of rotation in total hip arthroplasty? Influence of acetabular offset and surgical technique

Acetabular offset (AO) is the distance between the centre of the femoral head and the true floor of the acetabulum. We quantified the AO in normal hips and compared the displacement of the centre of rotation of the hip (CRH) after conventional and anatomical cup implantation during THA. 100 CT-scans of normal hips were analysed before and after simulating implantation of the acetabular component. Mean AO was 30.8 mm +/- 3. The medial shift of the CRH was 1.6 mm +/- 1.2 with the anatomical and 4.8 mm +/- 1.9 with the conventional technique (p< .0001). Medialisation was greater than 5 mm in 44% of the cases when the conventional technique was used, but occurred in no case when using the anatomical technique. Differences between men and women were significant: 5.6 mm +/- 1.6 and 3.5 mm +/- 1.7 with the conventional technique; 2.0 mm +/- 1.1 and 0.9 mm +/- 0.9 with the anatomical technique (p<0.0001 for both measurements). The concept of hip offset cannot be limited to that of the femoral offset. AO widely varies and cannot be neglected. In patients with significant AO, surgeons should pay close attention to the preparation of the acetabulum. This should be done conservatively so that the acetabular cup can be placed anatomically in order to restore the native hip biomechanics.