Certain motions at the syndesmosis of the ankle have been demonstrated. As dorsiflexion of the ankle takes place, the malleoli separate in fairly regular increments, the greatest distance being approximately 1.5 millimeters. The articular surfaces of the malleoli remain closely applied to the sides of the talus throughout the range of ankle motion. The tibia rotates a few degrees medially about its long axis on the talus and the syndesmosis opens anteriorly. There is a relative lateral rotation of the fibula with respect to the tibia. Other motions at the syndesmosis, such as forward or backward displacement or translation of the fibula on its long axis, may occur but they have not yet been demonstrated convincingly.
The syndesmosis permits this flexibility because of the elasticity of its ligaments. The elasticity of these ligaments allows the intermalleolar distance to change and facilitates tibial and fibular rotation which is essential to normal motion at the ankle joint. These motions contribute to the general springiness or elasticity of the system.
Progressive sectioning of the ligaments of the syndesmosis and the interosseous membrane leads to increasing backward mobility and increasing lateral rotation of the lateral malleolus. Loss of these structures, however, produces no significant widening of the medial or lateral clear spaces or increase in the intermalleolar distance. In order to produce widening of the mortise either the medial or the lateral group of ligaments, as well as the ligaments of the syndesmosis and the interosseous membrane, must be divided.
The most common fracture at the ankle is the spiral oblique fracture or lateralrotation fracture of the fibular malleolus. This is frequently associated with an increase in the medial clear space. When this increase is moderate, partial diastasis is present and there is injury to the deltoid ligament. Marked increase in the medial clear space indicates a complete tear of the deltoid ligament including both deep and superficial portions. Inability to reduce the medial clear space may mean that the deltoid ligament must be removed from the medial joint space and repaired.