- group a: including Catteral groups I and II, where the crescent sign involves less than 50% of the femoral head.
- group b: including Catterall groups III and IV, where the crescent sign involves more than 50% of the femoral head.
- 1. Salter RB, Thompson G. Legg-Calvé-Perthes disease. The pronostic significance of the subchondral fracture and a two-group classification of the femoral head involvment. J Bone Joint SurgAm1984;66:479-89