The purpose of this study was to examine the mechanical necessity of using a syndesmosis screw to supplement rigid internal fixation of the fibula and medial malleolus in the treatment of pronation-external rotation fractures. The legs of thirty embalmed and five fresh cadavera were dissected and mounted through the tibia to a frame so that multiple radiographs could be made with a constant relationship between the specimen and the x-ray apparatus A standardized pronation- external rotation load was applied to the foot, and widening of the syndesmosis was studied on mortise radiographs that were made after each experimental step.

On the basis of previous investigations, we develo_ a model for pronation-external rotation injuries that included disruption of the syndesmosis and interosseous membrane up to the level of the fibular fracture. Accordingly, multiple repaired fibular fractures could be simulated at several levels in the same specimen by incremental proximal division of the interosseous membrane. Specimens were separated into two groups.

Group I consisted of thirteen specimens in which the deltoid ligament, syndesmosis, and interosseous membrane were serially sectioned in 1.5-centimeter increments. Group II (ten sections) was subjected to the same protocol, except that the deltoid ligament was kept intact until the final step. The five fresh specimens were sectioned in the same way as those in Group I. In Group I, since the simulated pronation-external rotation injury included a deltoid tear, rigid medial fixation was not possible;​ accordingly, there was rigid fibular fixation only. In this group, the mean widening of the syndesmosis increased only gradually from 0.5 to 4.5 millimeters as the level of fibular fracture rose from 1.5 to fifteen centimeters proximal to the ankle. Measurements for the five fresh specimens were consistent with those for the embalmed legs. In Group II, the pronationexternal rotation injury was simulated with a medial malleolar fracture rather than a deltoid tear. After simulated rigid fixation of both the medial malleolus and the fibula, only minimum widening of the syndesmosis (1.4 ± 03 millimeters) occurred, even when the fibular fracture was fifteen centimeters proximal to the ankle. The mean maximum widening of the syndesmosis in Group II, in which division of the deltoid ligament was the last step, was equivalent to that in Group I, validating comparison of the two groups.

Clinical Relevance:​ Considering the range of clinically acceptable widening of the syndesmosis, the critical transition zone for the level of a fibular fracture that is fixed with a plate is three to 4.5 centimeters proximal to the ankle. When the fibular fracture is proximal to this level and rigid medial fixation is not possible, the syndesmosis may have to be stabilized to supplement the fixation with the plate. However, rigid medial and lateral fixation should acceptably stabilize the syndesmosis without further additional supplementation. This study provides mechanical evidence that internal fixation of pronation-external rotation injuries that include disruption of the syndesmosis often does not need to be supplemented with trans-syndesmotic fixation.