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

Background: 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.

Methods: 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).

Results: 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).

Conclusions: 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.