Anatomy and Biomechanics
The ankle joint proper, also known as the talocrural joint, is diarthrodial and includes the articulation between the talus, tibia, and fibula. The talocrural joint allows for substantial motion, approximately 70°, primarily dorsiflexion and plantarflexion from the anatomic position, which is important in bipedal ambulation. The most common ankle injury is the tear of the ATFL and is the most common ankle injury observed in the emergency department. ATFL injuries allow abnormal coronal plane tilt of the talus, valgus, with ambulation which causes pain and limits patient function. The syndesmosis is made up of a combination of static and dynamic anatomic structures working synergistically to produce a stable, yet highly dynamic relationship between the tibia and the fibula. Isolated syndesmosis injury occurs when disruption of these structures exists without associated fracture. The key static stabilizer is the incisura, which is a concave groove in the distal tibia in which the fibula rotates in the horizontal plane about a vertical axis. The syndesmosis ligamentous structures are composed of the anterior-inferior tibiofibular ligament (AITFL), the interosseous ligament (IOL), posterior-inferior tibiofibular ligament (PITFL), and the inferior transverse ligament. (Figure 1) The AITFL originates on the anterolateral tibia, also known as Chaput tubercle, and inserts on Wagstaffe tubercle on the fibula. The AITFL is the weakest of the syndesmotic ligaments and is the first ligament subjected to stress on external rotation force of the fibula around a vertical axis. The PITFL originates on the posterior tubercle of the tibia, also known as Volkmann tubercle, and inserts on the posterior aspect of the lateral malleolus, which is the strongest component of the syndesmosis and provides approximately 35% of syndesmosis stability. Xenos et al performed a cadaveric study evaluating ligamentous structures of the syndesmosis. They found a diastasis of 2.3 mm with sectioning of the AITFL alone, 5.5 mm with additional sectioning of the IOL, and 7.3 mm after additional sectioning of the PITFL. The syndesmosis functions to maintain the distal tibiofibular joint and resists axial, rotational, and translational forces. Studies have demonstrated that lateral displacement of the talus decreases the contact area of the tibiotalar articulation dramatically: lateral displacement by 1 mm could decrease the contact area by 42%, and in complete syndesmosis disruption, it could lead to a 36% increase in contact pressures.[5,6] Thus, a stable and congruent articulation is essential to restore normal ankle motion and decrease the risk of developing early degeneration of the talocrural joint.
Photograph showing the anterior anatomy—A, Anterior inferior tibiofibular ligament; B, Interosseous ligament; C, Talus; D, Fibula; E, Tibia; F, Anterior talofibular ligament; and G, Calcaneofibular ligament. Posterior anatomy—A, Posterior inferior tibiofibular ligament; B, Posterior talofibular ligament; C, Flexor Hallucis Longus; D, Calcaneus; and E, Peroneal Tendons.
J Am Acad Orthop Surg. 2020;28(13):517-527. © 2020 American Academy of Orthopaedic Surgeons