Subchondral insufficiency fracture (SIF) of the femoral head occurs without evidence of trauma or with only a history of minor trauma [1]. The etiology of SIF is insufficient bone quality at the subchondral area (SA) [2,3], and histopathologic studies of SIF cases have demonstrated the presence of thin disconnected bone trabeculae throughout the femoral head [4].

The stress concentration at the SA is also considered to be a factor affecting the occurrence of SIF. Patients with developmental dysplasia of the hip (DDH) are more susceptible to SIF than a healthy control group because of the increased joint contact force on the anterolateral edge [[5], [6], [7], [8]]. Stress distribution has been evaluated using finite element (FE) model analysis [[7], [8], [9]]. However, although studies have suggested the importance of concentration of contact force at the femoral head in the development of SIF, no study has evaluated the distribution of stress inside the femoral head. As SIF involves a fracture in the SA [1], the pathology of SIF suggests that the stress is most concentrated at the SA. We hypothesized that in SIF, the stress that is most concentrated at the SA has not only great contact force but also specific direction.

In the present study using FE model analysis, three directions of force, including compressive, shearing, and torsional, were applied to the femoral head, and the distribution and mean value of von Mises stress (Mises stress) were analyzed at the SA, principal compressive trabeculae (PC), and principal tensile trabeculae (PT).