The 2021 Association Research Circulation Osseous Classification for Early-Stage Osteonecrosis of the Femoral Head

The Association Research Circulation Osseous (ARCO) developed a novel classification system for early-stage (pre-collapse) osteonecrosis of the femoral head (ONFH). Necrotic lesions were classified into three types: Type 1 is a small-sized lesion, in which the lateral necrotic margin is medial to the apex of the femoral head; Type 2 is a medium-sized lesion, in which the lateral necrotic margin is between the apex of the femoral head and the lateral acetabular edge; and Type 3 is a large-sized lesion, which extends outside the lateral acetabular edge. Reliabilities and validities of four classification systems, namely, modified Kerboul; Japanese Investigation Committee; Steinberg; and the novel classification, were compared. The novel classification system is the most reliable and valid method of those examined. ARCO recommends using this method as a unified classification for early-stage ONFH.