Initiation of Collapse in Osteonecrosis of the Femoral Head: A Patient-Specific Finite Element Simulation Study.

Collapse in osteonecrosis of the femoral head (ONFH) is thought to result from stress concentration at the sclerotic boundary zone, but its precise initiation site has not been identified. We aimed to identify the initiation of collapse using patient-specific finite element models (FEMs) that incorporate the sclerotic boundary zone. Ten hips from 10 patients with type C ONFH who had both pre-collapse (stage 2) and post-collapse (stage 3) computed tomography (CT) images were analyzed. Actual collapse sites were identified on post-collapse CT images or, when inconclusive, determined using micro-CT and subsequent histopathological examination of resected specimens. Patient-specific FEMs were constructed from pre-collapse CT scans with incorporation of the sclerotic boundary zone. For comparison, FEMs were also generated for four femoral heads without sclerotic changes on the articular surface. Equivalent stress on the articular surface and simulated initiation sites of collapse were evaluated and compared with actual collapse sites. In all 10 femoral heads, actual collapse occurred at the interface between the lateral sclerotic boundary and the adjacent necrotic lesion. FEMs demonstrated stress concentration at the sclerotic boundary, and simulated initiation sites corresponded to actual collapse sites. In contrast, femoral heads without sclerotic changes showed no stress concentration at the articular surface, and no simulated collapse was observed on the femoral head surface. In conclusion, collapse in ONFH initiates at the interface between the lateral sclerotic boundary and the adjacent necrotic lesion, highlighting the critical biomechanical role of the sclerotic boundary zone.