Fixation height has a greater biomechanical effect than anterior tilt angle in syndesmotic fixation for weber B ankle fractures: a specific finite element study.

BACKGROUND

Distal tibiofibular syndesmotic instability in Weber B ankle fractures remains challenging, as fixation constructs and insertion parameters alter ankle biomechanics.

METHODS

A 3D finite element ankle model was developed from CT/MRI data of a healthy adult. Residual instability was simulated by transecting the anterior inferior tibiofibular and interosseous ligaments after anatomical reduction and fixation of all malleolar fractures. Eighteen fixation models were created using tricortical screws or suture-buttons, placed 20, 30, or 40 mm above the tibial plafond with anterior tilt angles of 12.5°, 27.5°, or 42.5°. Outcomes included peak tibiotalar contact stress, fibular displacement, and peak implant von Mises stress under axial loading and external rotation.

RESULTS

Residual instability increased peak tibiotalar contact stress by 36.8% and markedly increased fibular translation. Fixation at 20-30 mm restored near-physiologic mechanics, while 40 mm fixation showed biomechanical deterioration. At equivalent heights, tricortical screws better restrained fibular translation, whereas suture-buttons preserved more micromotion but had higher implant stress concentrations.

CONCLUSIONS

Fixation height had a larger biomechanical effect than anterior tilt angle. Fixation 20-30 mm above the tibial plafond is recommended, with construct selection balancing rigidity and physiologic syndesmotic motion.