Association of anterior necrotic boundary location with collapse progression in symptomatic osteonecrosis of the femoral head: analysis of ARCO 2021 type 2 lesions

OBJECTIVE: Anterior necrotic boundary location and collapse progression in symptomatic osteonecrosis of the femoral head remain incompletely understood. We investigated whether anterior necrotic boundary location is associated with collapse progression in symptomatic osteonecrosis classified as ARCO 2021 type 2. MATERIALS AND METHODS: Thirty-seven hips in 33 patients with symptomatic post-collapse osteonecrosis (ARCO 2021 type 1 or 2) followed conservatively for more than 1 year were retrospectively reviewed. The anterior necrotic boundary angle (ANBA) was measured on mid-sagittal CT as the angle between a line drawn from the femoral head center to the anterior necrotic margin and a vertical reference line. Collapse width was measured on serial biplane radiographs, and progression was defined as a ≥ 1 mm increase. Receiver operating characteristic analysis determined the optimal ANBA cut-off, and Kaplan-Meier analysis evaluated progression-free survival. RESULTS: No collapse progression occurred in two ARCO 2021 type 1 hips, whereas 25 of 35 type 2 hips (71%) demonstrated progression. ANBA was significantly greater in hips with progression (P = 0.0019). The optimal ANBA cut-off for predicting progression was 75° (area under the curve, 0.840). Hips with ANBA ≥ 75° showed significantly lower progression-free survival than those with ANBA < 75° (P = 0.0029). CONCLUSION: Anterior necrotic boundary location is associated with collapse progression in symptomatic ARCO 2021 type 2 ONFH. ANBA may serve as a CT-based imaging parameter for risk stratification after initial collapse.