CT and MRI findings beyond the subchondral bone in osteonecrosis of the femoral head to distinguish between ARCO stages 2 and 3A

Objectives: To determine the diagnostic values of deep changes beyond the subchondral bone in osteonecrosis of the femoral head (ONFH) to distinguish between Association Research Circulation Osseous (ARCO) stages 2 and 3A. Methods: This retrospective study included 124 hips with ONFH of stages 2 (n = 49; 23 females; mean age, 50.7 years) and 3A (n = 75; 20 females; mean age, 53.2 years) from May 2017 to August 2022, who underwent CT (n = 124) and MRI (n = 85). Deep changes beyond subchondral bone were analyzed on CT (bone resorption area and cystic change) and on MRI (bone marrow edema [BME] and joint effusion). Diagnostic performance and multivariate analysis were evaluated for detecting stage 3A. Results: Stage 3A showed more frequent bone resorption area (72.0% vs. 4.1%), cystic change (52.0% vs. 0.0%), BME (93.5% vs. 43.6%), and joint effusion (76.0% vs. 24.5%) than stage 2 (p < 0.001, all). Bone resorption area and cystic change showed low sensitivities (52.0~72.0%) but high specificities (96.0~100.0%), while BME and joint effusion showed high sensitivities (76.0~93.0%) but low specificities (56.0~76.0%) for stage 3A. Predictors were in the order of bone resorption area, cystic change, and joint effusion (odds ratio: 32.952, 26.281, 9.603, respectively), and combined bone resorption area and cystic change had the best predictive value (AUC, 0.900) for stage 3A. Conclusions: Among deep changes, bone resorption area and cystic changes were highly specific and BME and joint effusion were highly sensitive for stage 3A. Combined bone resorption area and cystic change had the best predictive value for predicting ARCO stage 3A. Key Points: • The exact classification between ARCO stage 2 and 3A is essential but it is sometimes difficult to distinguish between ARCO stage 2 and 3A only by subchondral fracture, especially early post-collapse stage with preservation of femoral head contour. • The predictors of stage 3A were in the order of bone resorption area, cystic change, and joint effusion and combined bone resorption area and cystic change had the best predictive value for predicting stage 3A. • Analysis of deep changes beyond the subchondral bone may make it easier to distinguish between ARCO stage 2 and 3A.