A high ankle sprain, also known as a syndesmotic ankle sprain (SAS), is a sprain of the syndesmotic ligaments that connect the tibia and fibula in the lower leg, thereby creating a mortise and tenon joint for the ankle. High ankle sprains are described as high because they are located above the ankle. They comprise approximately 15% of all ankle sprains. Unlike the common lateral ankle sprains, when ligaments around the ankle are injured through an inward twisting, high ankle sprains are caused when the lower leg and foot externally rotates (twists out).
Mechanism The ankle joint consists of the talus resting within the mortise created by the tibia and fibula as previously described. Since the talus is wider anteriorly (in the front) than posteriorly (at the back), as the front of the foot is raised (dorsiflexed) reducing the angle between the foot and lower leg to less than 90°, then the mortise is confronted with an increasingly wider talus. The force is heightened when the foot is simultaneously forced into external rotation (turned outward). This chain of events may occur when the front of a hockey player's skate strikes the boards and the foot is forced outward. It may also occur in football, for example, when a player is on the ground with their leg behind them, the foot at right angles, and a rotational force is suddenly applied to the heel, as when someone falls on their foot. Overall, the most common mechanism is external rotation and may occur with sufficient rapidity that the actual mechanism is unrecognized. In this sequence of events, the most vulnerable structure is the anterior inferior tibio-fibular ligament, uniting the lower end of the tibia and fibula and playing an important role in the maintenance of the mortise. The injury to this ligament may vary from simple stretch to complete rupture. Some restraint to further injury is offered by the structures on the inside of the ankle, the medial malleolus and the medial collateral ligament. However, should these structures fail, then the force will be transmitted beyond the anterior inferior tibiofibular ligament to the strong membrane that holds the tibia and fibula together for most of their length. This force may then exit through the upper end of the fibula, creating a so-called Maisonneuve fracture.
Diagnosis Those who sustain high ankle sprains usually present with pain in the outside-front of the leg above the ankle, with increased discomfort when twisting (external rotation) is applied. In some cases, the diagnosis is only made after treatment for the more common, lateral, ankle sprain fails. Diagnosis may also be delayed because swelling is usually minor or nonexistent and the true nature of the injury unappreciated. A variety of diagnostic tests have been described such as the 'squeeze' (compressing the tibia and fibula above the midpoint of the calf), 'dorsiflexion with compression' (patient dorsiflexes the foot while the examiner compresses the internal and external malleolus), and 'external rotation' (patient sits with leg dangling and ankle at 90° and external rotation then applied to the foot) etc. None of them performs sufficiently well to allow diagnosis to be made on the basis of a single test, and is usually made by combining multiple tests supplemented with appropriate imaging when indicated. Plain radiographs, Ultrasound or MRI may be used for diagnosis. In the case of X-rays, demonstration of widening of the tibia and fibula 'mortise', a fracture of the medial malleolus, or a Maisonneuve fracture, will indicate an unstable or potentially unstable injury. However, 'normal' x-rays do not exclude significant ligament injury, and in one study, the ratio of diagnostic X-ray to known syndesmotic injury was only one in 17. By contrast, ultrasound may permit the injury to be visualized while the mortise is being stressed. Consequently, a diagnostic modality such as ultrasound or magnetic resonance imaging (MRI) that demonstrates the ligament itself may be helpful, if clinical suspicion remains.
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