Evaluation of Automatic HJC Definition

The hip joint center (HJC) does not only define the hip specific coordinate system in most definitions, but is also one of the most relevant functional parameters of the hip joint. Sphere fit of the femoral head is the most common method for HJC definition. In terms of preoperative planning for THA, many patients show deformities on the femoral heads and/or the acetabular region. Therefore, the approximation of the HJC via sphere fit has to be questioned. In our analysis, we studied different methods for automatic HJC definition and the influence of femoral head deformities on the location of the HJC for CT images of 201 THA patients. The different methods were ellipsoid fit on the femoral head, center of mass analysis of the femoral head, sphere fit on the acetabular region and geometric analysis based on ASIS location and pelvic width, height and depth. We compared the deviations between the sphere fit center and the different methods. We found best accordance with sphere fit for ellipsoid fit, followed by center of mass, acetabular sphere fit and geometric definition. The same tendency was found for the differences between sphere-like and deformed femoral heads, with deformed heads showing higher deviations for all methods. While no dynamic data was available, it has to be questioned, whether a sphere fit on the femoral head is the suitable for definition of HJC as center of rotation, especially for patients with deformed femoral heads. Further processing that takes different femur positions into account is recommended.