Common site of subchondral insufficiency fractures of the femoral head based on three-dimensional magnetic resonance imaging

Objectives: The objective of this study was to investigate the common sites of subchondral insufficiency fractures of the femoral head (SIF) based on three-dimensional (3-D) reconstruction of MR images. Materials and methods: In 33 hips of 31 consecutive patients diagnosed with SIF, 3-D reconstruction of the bone, fracture, and acetabular edge was performed using MR images. These 3-D images were used to measure the fractured areas and clarify the positional relationship between the fracture and degree of acetabular coverage. Results: The fractured area in the anterior portion was significantly larger than in the posterior area. In 11 cases, the fractures contacted the acetabular edge and were distributed on the lateral portion. The indices of acetabular coverage (center-edge angle and acetabular head index) in these cases were less than the normal range. In the remaining 22 cases, the fractures were apart from the acetabular edge and distributed on the mediolateral centerline of the femoral head. The majority of these cases had normal acetabular coverage. Conclusions: The common site of SIF is the anterior portion. In addition, two types of SIF are proposed: (1) Lateral type: the contact stress between the acetabular edge and lateral portion of the femoral head causes SIF based on the insufficient acetabular coverage, and (2) Central type: the contact stress between the acetabular surface and the mediolateral center of the femoral head causes SIF independent from the insufficiency of acetabular coverage. These findings may be useful for considering the treatment and prevention of SIF.