Pathogenesis of idiopathic osteonecrosis: the ubiquitous crescent sign.

The repair of a large subchondral osteonecrotic lesion follows a precise sequence of biologic events. The repair of osteonecrosis becomes clinically significant with the development of a subchondral fracture manifested radiographically as the crescent sign. Once the subchondral fracture occurs, collapse and subsequent degenerative arthritis are generally inevitable. The repair process, radiologic appearance, and clinical features (pain and loss of function) are identical regardless of the site of the disease.