Why is primary Osteoarthritis of the ankle uncommon?
Primary OA of the ankle is nine times less common than OA of the knee or hip and usually a result of trauma (i.e., secondary OA). Because the weighted load (five times body weight when walking) on the ankle exceeds that of the knee (four times body weight) and hip (three times body weight), it is surprising that primary OA is so uncommon in the ankle. The reason for this is not entirely clear. However, there are clear anatomical and biomechanical differences between the ankle and the knee or the hip. For instance, the ankle is mainly a rolling joint with congruent surfaces at high load, whereas the knee joint is a mixture of sliding, rolling, and rotation with less congruent surfaces resulting in more stress on the knee cartilage. There are also differences in thickness and composition of ankle cartilage, which increases its tensile strength and gives it more resistance to catabolic cytokines such as IL-1β.
