This depends on many factors; however the age of the child is perhaps the most important predictor of outcome. Generally, the older the child when first diagnosed with Perthes disease, the worse the outcome. The long term outcome of the hip after Perthes diease depends mostly on the degree of deformity remaining in the joint at the end of skeletal growth (around age 16-18 years).
Hip joints with significant irregularity in shape are predisposed to early onset osteoarthritis. Severe deformity due to Perthes disease typically includes a flattened femoral head (ball of the hip joint) and characteristic changes of the upper thigh bone resulting in a stiff hip and short leg.