An axially loaded model of the ankle after pronation external rotation injury.

Using a testing apparatus that allows axial loading and displacement in the sagittal, axial, and coronal planes, 6 ankles were tested under experimental conditions intended to model the Lauge-Hansen pronation external rotation injury. All specimens were rotated through a continuous range of sagittal motion with the ankle under 300 N of axial load as the coupled motion of the ankle in the coronal and axial axes was recorded. Combinations of fibular osteotomy, disruption of the syndesmosis up to 6 cm above the plafond, and deltoid transection were tested to mimic Stages I to III of the pronation external rotation ankle fracture. The effects of stabilization of the fibula and syndesmosis also were examined. Neither fracture of the fibula 4 cm above the plafond nor disruption of the syndesmosis to 6.0 cm resulted in a significant change in coupled motion of the talus. When the superficial deltoid was sectioned, the ankle had increased external rotation in plantar flexion. When the deep deltoid was sectioned, the ankle dislocated in plantar flexion unless the fibula was stabilized. This prevented dislocation but failed to restore normal talar kinematics. This study found no biomechanical support for placement of a syndesmotic screw unless the medial side cannot be stabilized anatomically.