The direct cause is a reduction in blood flow to the joint, though what causes this is unknown. It is thought that the artery of the ligamentum teres femoris closes too early, not allowing time for the medial circumflex femoral artery to take over. Genetics does not appear to be a determining factor, though it may be involved. When the disease is genetic in origin, it typically runs along the male line. Some evidence suggests that parental smoking may be a factor, though this is not yet proven, or more recently that a deficiency of some blood factors used to disperse blood clots may lead to blockages in the vessels supplying the joint, but that, too, has not been proven.