Long-term results of this disease vary based on the age of the child at diagnosis and severity of damage to the bone. In this disease the blood supply to the top of the thigh bone (femur) is temporarily turned off. Without blood, the bone starts to die. This process is called bone necrosis.

Some doctors use a formula to predict a good, fair, or poor prognosis for Perthes' disease. Points are given based on the child's age and the appearance of their X-rays.

In the normal hip, the head of the femur is a round ball or in children, a round cap. In Perthes' disease this shape changes. The still growing cap of the femur can get dislodged or dislocated. Either of these events predicts a worse outcome.

Having a formula like this will help doctors advise families about medical treatment. Surgery may be avoided if the prognosis is good. Early treatment for children with a poor prognosis may improve the final results.