Explain Perthes disease to a family — OSCE

The parents of a six-year-old boy are in the orthopaedic clinic after their son developed a three-week limp that comes and goes, worse after activity. He points to his knee when asked where it hurts. The registrar has diagnosed Legg-Calve-Perthes disease on the persistent limp, the stiff hip with loss of abduction and internal rotation, and the radiograph showing a smaller, denser femoral head. The parents are anxious, they think the limp began after a fall at the playground, and they do not understand why a minor fall could lead to a bone disease that lasts years. Counsel them.