Discrepancy between radiography and magnetic resonance imaging in Japanese Investigation Committee classification type C osteonecrosis of the femoral head

Purpose The Japanese Investigation Committee (JIC) classification for osteonecrosis of the femoral head (ONFH) is based on the necrotic area relative to the weight-bearing surface on anteroposterior (AP) radiographs or central coronal MRI. Discrepancies exist between these methods, potentially related to the AP necrosis area. This study evaluated these discrepancies and the extent of AP necrotic lesions. Methods We retrospectively reviewed 139 patients (188 hips) with nontraumatic ONFH, JIC type C1 or C2 on radiography, and collapse<3 mm. Cases with and without discrepancies between radiography and MRI were designated as discrepancy and consistent groups, respectively. We assessed the proportion of patients in the discrepancy group and survival rates in both groups, with femoral head collapse>3 mm as the endpoint. The cutoff value for AP necrotic regions on lateral radiographs identifying discrepancies was calculated using ROC curve analysis. Results The discrepancy group comprised 28 hips (14.9%) vs. 160 hips in the consistent group. Five-year survival rates were 73.3% vs. 31.9% (P<0.01), and AP necrotic region extent was 61.2 vs. 73.8 mm (P<0.001) in discrepancy vs. consistent groups. The cutoff value for necrotic region extent revealing discrepancies was 66.9% (AUC 0.833, sensitivity 83.8%, specificity 82.4%). Conclusion Patients with AP necrotic regions<66.9% were more likely to show discrepancies between radiography and MRI in type classification. This study can help improve accuracy in assessing ONFH severity and prognosis.