Deviation of the centre of rotation of the femoral head in dysplastic hip joints.

Abstract

Hip instability may be based not only on some mismatch between the size of the femoral head and acetabulum but also on the deviation of the centre of rotation of the femoral head. The aim of this study was to compare the differences between the centre of rotation of the femoral head and that of the acetabulum in normal and dysplastic hips. Thirty dysplastic hips and 30 control hips without dysplastic hip were included in this study. Computed tomography data were imported into three-dimensional analysis software. The sphere radius of the acetabulum, the sphere radius of the femoral head, and the deviation of the centre of rotation of the femoral head from that of the acetabulum were analysed. The sphere radii of the acetabulum and femoral head were significantly larger in dysplastic hips. Distances between the centre of rotation of the acetabulum and that of the femoral head were significantly larger in dysplastic group than in control group. The centre of rotation of the femoral head was significantly deviated anterosuperiorly in dysplastic hips. We found significant negative correlations between centre-edge angle and the distance between the centre of rotation of the acetabulum and that of the femoral head in all subjects. In dysplastic hip joints, the centre of rotation of the femoral head was more deviated anterosuperiorly from that of the acetabulum than in normal hip joints even in static condition, which might be clinically associated with the micro-instability in dysplastic hips.