Perthes disease is a condition that affects children (often boys) between 4 and 10 years of age. There is disruption to the blood supply of the femoral head, which causes softening of the cartilage and eventually a change in the shape of the femoral head. Once the collapse occurs, the bone then heals and remains abnormally shaped. Because this can occurs at such a young age, the socket can remodel to try to match the head. While this can give the patient a functional hip for some years, once they reach their 20's or 30's, degenerative changes can set in and the hip becomes painful.
Once non-operative treatments such as pain-killers and physiotherapy are no longer effective, the only option is to replace the joint. Surgeons have attempted to use arthroscopic techniques to treat this condition but it tends to treat the damage rather then the underlying cause (which is the abnormally shaped head and socket). Inevitably, after a temporary period of improvement, the pain returns. The grossly deformed anatomy can make the surgery technically difficult. The cup can be uncovered and require either screw fixation or bone graft. The upper femoral anatomy can mean that specially designed implants are required.
However, with careful pre-operative planning and meticulous surgical technique, "normal" anatomy can be restored with almost equal leg length. Once the new hip has fully bonded, then patients can return to a full and active lifestyle.