Histopathologic confirmation of subchondral fracture in symptomatic pre-collapse osteonecrosis of the femoral head with bone marrow edema on magnetic resonance imaging

Objective: The presence of bone marrow edema on magnetic resonance imaging (MRI) in pre-collapse osteonecrosis of the femoral head is suggested to be a sign of occult subchondral fracture; however, to our knowledge, there are no histopathological studies verifying this. This study aimed to histopathologically verify the presence of subchondral fracture at the lateral necrotic boundary in symptomatic pre-collapse osteonecrosis of the femoral head with bone marrow edema on MRI. Materials and methods: Of 149 consecutive necrotic femoral heads resected during total hip arthroplasty at our hospital from January 2019 to June 2024, we included 13 femoral heads that did not show apparent collapse on preoperative radiographs and exhibited bone marrow edema on MRI. Continuous coronal-slice hematoxylin and eosin-stained specimens of each femoral head were examined for the presence of subchondral fracture. Bone microarchitectures around subchondral fractures were measured using micro-computed tomography (CT) images. Results: In all femoral heads, subchondral fractures were histopathologically confirmed at the lateral junction between the reparative and the necrotic zone. On micro-CT, bone volume fraction, trabecular thickness, and bone mineral density of the reparative zone adjacent to the subchondral fracture were all significantly higher than those of the necrotic zone adjacent to the subchondral fracture. Conclusion: Subchondral fracture invariably existed when bone marrow edema was present on MRI during pre-collapse osteonecrosis of the femoral head. When bone marrow edema is observed on MRI, it should be known that subchondral fracture has already occurred, even if femoral head collapse is unclear on radiographs.