Aim: In residual hip dysplasia the fovea capitis femoris lays often more cranial than in the normal hip morphology, the ligamentum capitis femoris thereby articulating with the weight-bearing area of the acetabular cartilage. The aim of this study was to quantitate this aspect with regard to its potential negative effect for the degeneration of the dysplastic acetabulum.
Method: The relation between the fovea capitis femoris and the weight-bearing area were studied using the a.p.-pelvis view in normal and dysplastic hips. The measurements were made by digital image analysis.
Results: The hypothesis that the "fovea alta" is characteristic for the dysplastic hip was confirmed. In dysplastic hips the typically wider fovea lays on average 30 degrees more cranial, touching the weight-bearing area over 11 degrees. The fovea in normal hips has on average a distance of 26 degrees to the acetabular roof. In dysplastic hips the steeper roof, the tendency for decentering of the femoral head as well as a higher CCD angle explain this phenomenon to a certain degree. In a theoretical model to correct the acetabular position over the femoral head alone one quarter of the foveae would still touch the weight-bearing area. Nevertheless, in our series after periacetabular osteotomy this was never the case due to better centering.
Conclusion: lt is our hypothesis that a "fovea alta", which further reduces the already smaller loaded cartilage surface, is one part leading to the early degeneration of the dysplastic hip. Therefore, it should be considered in biomechanical models and in the planning of corrective procedures.