Syndesmosis Fixation Using Dual 3.5 mm and 4.5 mm Screws With Tricortical and Quadricortical Purchase: A Biomechanical Study

Background: Grade 3 syndesmosis (high ankle) sprains of the ankle are frequently treated using screws that fix the distal fibula to the tibia. We hypothesized that forces acting on the distal fibula and displacements of the distal fibula relative to the tibia recorded during simulated ankle loading tests would be significantly affected by syndesmosis screw size and the number of engaged tibial cortices. Methods: Distal fibular forces and displacements were measured after cutting the distal inferior tibiofibular ligaments and fixing the distal fibula to the distal fibula with 2 syndesmosis screws. Screws of 3.5 mm and 4.5 mm were applied with tricortical and quadricortical purchase. Results: There were no significant differences in distal fibular forces or displacements between any combination of screw size and cortical purchase tested. The highest mean fibular force recorded in the study (110.2 N) occurred when 10 N-m of external foot torque was applied to a dorsiflexed ankle loaded with 1000 N axial weight-bearing force. For ankle dorsiflexion and external foot torque tests, the distal fibula always displaced posteriorly with respect to the tibia. Mean displacements of the fibula from 1000 N applied axial weight-bearing force (maximum 0.15 mm) and from 10 N-m of forced foot dorsiflexion (maximum 0.43 mm) were considerably less than those from 10 N-m external foot torque (1.7 mm to 2.7 mm). Conclusions: Screw size and the number of engaged tibial cortices had no significant effect on mechanical stability of the distal fibula during these tests. Application of external foot torque (internal tibial torque) to a weight-bearing ankle produced the greatest bending displacements of the screws, and should be avoided during rehabilitation to reduce the possibility of screw breakage. Clinical Relevance: In terms of mechanical stability, surgeons may have considerable flexibility with regard to screw fixation of high ankle sprains.