Anatomy 101: Ankle Syndesmosis - Distal Tibiofibular Joint

A syndesmosis joint is a fibrous joint where two bones are connected by strong ligaments or membrane. The distal tibiofibular syndesmosis, between the fibula and tibia, is formed by three major ligaments: the anterior inferior tibiofibular ligament (AITFL), the posterior inferior tibiofibular ligament (PITFL), and the interosseous tibiofibular ligament (ITFL). A fourth ligament, the inferior transverse tibiofibular ligament, is congruent with the PITFL, but sometimes considered a separate ligament (Hermans, Beumer, de Jong, & Kleinrensink, 2010). While technically the syndesmosis is the joint, most literature describes a syndesmosis injury as affecting the syndesmotic ligaments, so this blog will do the same.

These ligaments combine to provide exceptional stability to the ankle, with the fibula only externally rotating a maximum of 2° relative to the tibia, and the ankle mortice widening 1mm as the ankle ranges from full dorsiflexion to plantarflexion (Lin, Gross, & Weinhold, 2006).

It is reported ankle syndesmosis injury occurs in 1-11% of all ankle sprains without fractures, however 40% of patients report ankle instability six months post-ankle sprain. The instability could result from widening of the ankle mortice following stretching of the ligaments. A 1mm widening of the ankle mortice (doubling the amount of motion in asymptomatic individuals) decreases the tibiotalar joint contact area by 42%, causing instability, with resultant osteoarthritis (Hermans et al., 2010). Given 1mm change in mortice width causes clinical significance, I believe syndesmosis injuries are under-reported in the literature. Possibly with improved CT & MR imaging now available, the incidence of syndesmosis could become higher than 11%.

The syndesmosis complex is crucial in ankle stability, with poor understanding in current textbooks. To better treat injuries in this region, we need to understand the anatomy. The ligaments are described below:

Anterior Inferior Tibiofibular Ligament (AITFL)