‘Head at risk’ signs. Catterall introduced the concept of a ‘head at risk’. Clinical signs constituted progressive loss of movement and radiological signs included fine calcification lateral to the epiphysis, and widening of the inferomedial joint space suggesting early flattening of the cartilaginous head. Other radiological features included extensive metaphyseal involvement, Gage’s sign (a lucent area of the lateral epiphysis and the adjacent metaphysis) and a relatively horizontal growth plate reflecting a clinical flexion/adduction posture of the hip.