An anatomic study of subtalar instability.

Subtalar instability has been recognized as a cause of ankle symptoms. The purpose of this anatomic study is to determine the ligament damage required to produce subtalar instability, and to define a radiographic technique to demonstrate it. Ten fresh adult cadaver ankles were dissected. Selective sectioning of the calcaneofibular ligament, capsule, and interosseous ligaments of the subtalar joint were done. Radiographic documentation of subtalar opening was recorded, using a lateral and Brodan's view. Sectioning of the calcaneofibular ligament alone produced a 5-mm opening of the subtalar joint. When combined with sectioning of the interosseous ligament, a 7-mm opening was produced. This was reproduced with a plantar flexion-supination load to the foot. Instability of the ankle joint (talar tilting) was produced only when loading caused additional tearing of the anterior talofibular ligament. This study suggests that surgical repairs to correct lateral ankle instability should include repair or substitution of the calcaneofibular ligament if subtalar instability is a consideration.