Fibula and its ligaments in load transmission and ankle joint stability.

A study was made of the role of the fibula in weightbearing and its contribution to ankle joint stability in 10 anatomic specimen lower limbs. On axial loading of the lower limb, the fibula was found to take an average of 17% of a 1500 N axial load. The proportion of the load carried by the fibula increased with the total loading. It also increased when the line of load was displaced laterally and when the ankle joint was in dorsiflexion and decreased when the line of loading shifted medially or the joint was plantar flexed. With loading, the lateral malleolus migrated distally relative to the medial malleolus, except after fibular osteotomy, when it migrated proximally. There was an approximately inverse relationship between proportional fibular loading and distal fibular migration. Cutting the inferior tibiofibular ligament reduced the proportional load in the fibula and increased its distal migration. The interosseous membrane modified the load distribution between the tibia and the fibula, with the distal fibula carrying a higher proportion of the axial load than did the proximal. Surgical repair of a ruptured inferior tibiofibular ligament, using either 1 or 2 screws, was associated with an abnormal pattern of load distribution and fibular displacement.