Background:​ Rupture of the distal tibiofibular syndesmosis commonly occurs with extreme external rotation. Most studies of syndesmosis injuries have concentrated only on external rotation instability of the ankle joint and have not examined other defects.

Hypothesis:​ Syndesmosis injuries cause multidirectional ankle instability.

Study Design:​ Controlled laboratory study.

Methods:​ Ankle instability caused by distal tibiofibular syndesmosis injuries was examined using 7 normal fresh-frozen cadaveric legs. The anterior tibiofibular ligament, interosseous membrane, and posterior tibiofibular ligament, which compose the distal tibiofibular syndesmosis, were sequentially cut. Anterior, posterior, medial, and lateral traction forces, as well as internal and external rotation torque, were applied to the tibia;​ the diastasis between the tibia and fibula and the angular motion among the tibia, fibula, and talus were measured using a magnetic tracking system.

Results:​ A medial traction force with a cut anterior tibiofibular ligament significantly increased the diastasis from 1.1 to 2.0 mm (P = .001) and talar tilt angles from 9.6° to 15.2° (P < .001). External rotation torque significantly increased the diastasis from 0.5 to 1.8 mm (P = .009) with a complete cut;​ external rotation torque also significantly increased rotational angles from 7.1° to 9.4° (P = .05) with an anterior tibiofibular ligament cut.

Conclusion:​ Syndesmosis injuries caused ankle instability with medial traction force and external rotation torque to the tibia.

Clinical Relevance:​ Both physicians and athletes should be aware of inversion instability of the ankle joint caused by tibiofibular syndesmosis injuries.