Biomechanical Evaluation of Syndesmotic Screw Position: A Finite-Element Analysis

Objectives: To evaluate the stresses in syndesmotic screws and widening of syndesmosis under loading after placement of the screws at different levels from the ankle joint line and to determine the optimal level. Methods: From a set of computed tomographic data of an ankle, a 3-dimensional finite-element model was reconstructed. Six fixation configurations of the syndesmosis with placement of 3.5 or 4.5 mm single tricortical screws at 20-45 mm from the tibiotalar joint were performed on this model. Physiological loads approximating those during both midstance and heel-off states of stance phase of normal walking were applied to evaluate the stress in the screw and widening of the syndesmosis. Results: Among the 6 fixation configurations, the lowest von Mises stress was found in the screws placed 30-40 mm above the joint line (373.31-380.17 MPa for 3.5 mm cortical screw and 284.06-327.31 MPa for 4.5 mm cortical screw in midstance phases), whereas the least syndesmosis widening was determined when the screw was placed 30 mm above the tibial plafond (0.005 mm) for 3.5 mm cortical screw and 20, 25, and 30 mm above the tibial plafond (0.004 mm for each, respectively) for 4.5 mm cortical screw during midstance phases. Conclusions: This study showed that syndesmosis fixation at the level of 30-40 mm above the tibiotalar joint has advantages with regard to stress in screws in comparison with the other evaluated levels.