Distal fibular length needed for ankle stability.

BACKGROUND: The fibula is commonly used for bone grafts. Previous clinical and biomechanical studies have suggested that the length of the residual portion of the distal part of the fibula has an important effect on the long-term stability of the ankle joint. However, we cannot find clear-cut guidelines for the amount of bone that can be harvested safely.

METHODS: Using six normal fresh-frozen cadaver legs, motions of the tibia, talus and calcaneus were measured. The fibula was cut sequentially 3 cm from the proximal tip of the fibula and distally 10 cm, 6 cm, and 4 cm from the distal tip of the lateral malleolus. The angular motion of each bone was measured while a medial and lateral traction force of 19.6 N was applied to the proximal tibia. Angles of the tibia, talus, and calcaneus were measured.

RESULTS: Sequential resection of the fibula increased the inversion angles of the ankle joint. The proximal 3-cm cut increased the inversion angle from 42.1 +/- 6.2 degrees to 49.6 +/- 3.6 degrees, and the distal 4-cm cut increased the angle from 57.6 +/- 6.6 degrees to 67.4 +/- 5.9 degrees. The rotational angles were almost constant with sequential resections of the fibula; however, the distal 4-cm cut increased the rotational angle from 11.3 +/- 25.1 degrees to 78.7 +/- 37.5 degrees.

CONCLUSIONS: The whole fibula including the head is essential for the stability of the ankle joint complex, and the distal fibula is responsible for stabilizing the ankle mortise during external rotation and inversion. We recommend fixation of the syndesmosis or bracing to prevent ankle joint instability with rotation of the talus in the mortise, especially when the distal fibula is shortened 6 cm or more.