The capital femoral epiphysis is somewhat unique. It is one of the few epiphyses in the body that is inside the joint capsule. (The joint capsule is the tissue that surrounds the joint.) The blood vessels that go to the epiphysis run along the side of the femoral neck and are in danger of being torn or pinched off if something happens to the growth plate. This can result in a loss of the blood supply to the epiphysis.
Children who have abnormal blood clotting (a condition called thrombophilia) may also have a higher risk of developing Perthes disease. These children have blood that clots easier and quicker than normal. This may lead to blood clotting that blocks the small arteries going to the femoral head. As a result of new evidence, the certainty of thrombophilia as a cause of Perthes is now under debate. This will remain an area of study until scientists clear up the significance of thrombophilia as a possible cause of Perthes.
There is some new evidence that Perthes disease may be genetic as a result of a mutation (abnormal change) in the type II collagen (fibers that make up soft tissue structures). Previously there was no known increase in risk for children whose parent had Perthes disease as a child, but this belief may no longer be accurate.
Studies among Asian families who have many family members with this disease have been found with this mutation in the type II collagen gene. Scientists think that the mutation results in weakening of the hip joint cartilage that also affects the blood vessels within the cartilage.
Whatever the true cause of ischemia (lack of blood to the area), the result is bone death (called necrosis) of the femoral head. Without a normal blood supply, the bone loses its strength and shape. The loss of bone density and softening of the head result in a femoral head that is misshaped. With the hip supporting the weight of the body, tiny microfractures in the soft, necrotic bone fail to heal. This is another reason why normal wear and tear results in a deformity.