Comparison of the Progression Rate of Femoral Head Collapse in Japanese Investigation Committee Classification Types B1 and B2 of Osteonecrosis of the Femoral Head.

Abstract

The Japanese Investigation Committee classification of type B osteonecrosis of the femoral head (ONFH) should be further subdivided into types B1 and B2 based on the center of the weight-bearing area. In this study, we investigated whether there is a difference in survival rates between types B1 and B2. This study included patients diagnosed with type B ONFH at our institution between 2010 and 2021 who either progressed to collapse or did not collapse and were followed up for at least 3 years. Magnetic resonance imaging (MRI) and X-ray were used to classify the cases into types B1 and B2, and femoral head survival rates were evaluated using progression to collapse (> 3 mm) as the endpoint. The study analyzed 50 cases (62 hips) classified by MRI (25 men, mean age 38 years) and 45 cases (59 hips) classified by X-ray (22 men, mean age 37 years). Femoral head collapse progression in type B was observed in 10 hips (16.1%) on MRI and seven hips (11.8%) on X-ray. For MRI, type B1 showed one hip (3.4%) and type B2 showed nine hips (27.3%), while using X-ray, type B1 showed zero hips (0%) and type B2 showed seven hips (20.6%), with significant differences (P = 0.015 and P = 0.017, respectively). The 5-year survival rate, with collapse progression as the endpoint, was 100% for type B1 and 72.5% for type B2 on MRI, and 100% for type B1 and 79.4% for type B2 on X-ray, both showing significant differences (P = 0.005 and P = 0.02, respectively). Comparison of types B1 and B2, classified by MRI and X-ray, showed significantly different survival rates. Subdividing JIC type B ONFH into types B1 and B2 provides an effective indicator for predicting the prognosis of femoral head survival.