Explanation

Perthes' disease is a problem with the blood supply to the head of your femur. This means that your osteoblasts don't get the supplies they need to build new bone in your hip joint, so it doesn't become hard bone.

The head of your femur becomes soft and weak and can lose its shape. It then doesn't fit properly into the 'socket' of your pelvis and your hip joint can't move in all directions as well as it used to. It will be bit stiff and won't move as much, and you may also walk with a limp. Perthes' disease only affects the head of your femur, the 'ball' part of your hip joint, not the socket part in your pelvis.

After a while, the blood supply in the head of your femur starts working properly again, and osteoblasts can start building strong new strong bone. Sometimes this happens really quickly and your hip gets better in months, but sometimes it takes ages, and it can take years before it's better again.