Why do subchondral cysts occur at the medial aspect of the femoral head in hip dysplasia?

Introduction: Osteoarthritis in dysplastic hips should develop from the lateral side of the acetabulum and the femoral head just below. However, the existence of subchondral cysts located more on the medial side contradicts the weight-loading theory. The aim of this study was to confirm the presence of medial cysts at the femoral head and to investigate the relationship between medial cysts and injuries of the ligamentum teres in hip dysplasia.
Methods: A retrospective analysis was conducted on 257 cases of hip dysplasia. All patients had x-rays and 3-dimensional computed tomographies (3D CT) preoperatively and 123 patients had magnetic resonance arthrographies. A comparison was performed between cases with and without medial cysts according to the severity of damage to the ligamentum teres, the presence of bony spurs around the fovea capitis, and the Tönnis grade.
Results: Medial subchondral cysts around the fovea capitis were found in 100 cases. Mild osteoarthritis (Tönnis grade 0 or 1) was present in 89% of cases in the medial cyst group. A significant difference between the groups was observed in the incidence of bony spurs around the fovea capitis ( p  < 0.05) and injuries of the ligamentum teres ( p  < 0.05).
Conclusions: The formation of subchondral cysts at the medial femoral head in hip dysplasia may be related to damage in the ligamentum teres. Considering that subchondral cysts develop in early osteoarthritis, the progression of arthritis in hip dysplasia appears to correlate with damage to the ligamentum teres, as well as compressive pressure on the joint.