Comparison of radiographic stress tests for syndesmotic instability of supination-external rotation ankle fractures: a cadaveric study.

OBJECTIVE: According to the classification of Lauge-Hansen, supination-external rotation IV (OTA 44-B) injuries should not have syndesmotic instability; yet, several studies have suggested disruption is present in up to 40% of these injuries based on stress tests. In this study, we examine various stress radiographic parameters in a cadaver model of supination-external rotation IV equivalent injury. We hypothesize that external rotation stress testing and widening of the medial clear space do not always represent syndesmotic instability. Rather, the better predictor of syndesmotic instability will be an increased tibia-fibula clear space with the lateral stress test.
METHODS: Eleven fresh frozen human lower limbs were each secured into a custom frame. External rotation stress test was performed by applying an external moment of 7.5 Nm, and lateral stress test was performed by applying 100 N lateral pull at the distal fibula. True mortise radiographs were taken of intact ankles and while performing external rotation and lateral stress tests at each stage of sequentially sectioning the ankle ligaments. The deltoid ligament was sectioned first, then anterior-inferior tibiofibular ligament, posterior-inferior tibiofibular ligament, and interosseous membrane. Tibiofibular clear space and medial clear space were measured on each radiograph.
RESULTS: External rotation stress test produced significant medial clear space widening when the deltoid ligaments were sectioned (P < 0.05). Lateral stress test produced no significant widening of the tibiofibular clear space until interosseous membranes were sectioned (P < 0.05).
CONCLUSIONS: Lateral stress test with widening of the tibiofibular clear space is the preferred indicator of syndesmotic instability. The external rotation stress is a poor indicator of syndesmotic injury in the setting of deltoid ligament injury.