A guide for surgeons to orientate the ideal trans-syndesmotic fixation – a novel technique based on CT

A. Patel, S. Merie, V. Kothari, A. Roche

Introduction: Correctly orientating a syndesmotic screw can be challenging particularly for inexperienced surgeons. Failures can lead to longer term morbidity therefore there is a demand for reproducible techniques to guide surgeons. Techniques reliant on leg rotation can be disorientating. We propose a technique to orientate fixation using identifiable soft tissue landmarks independent of leg rotation. This study uses cross-sectional computed tomography (CT) to validate the technique.

Methods: 40 CT scans of uninjured ankles were studied. Fixations were planned 15mm above the joint line to provide both stabilisation and ease of palpating tendon structures. Axial images were studied with entry points for the screw on the fibula extrapolated into the tibia – ideal screws bisect both tibia and fibula in the transverse plane. Entry points were measured from the lateral ridge of the fibula. Exit points were measured as both distance from the tibialis anterior tendon (D1) and tibialis posterior tendon (D2). Exit points were also calculated as a percentage of the distance from the tibialis anterior tendon to the tibialis posterior tendon using the formula (D1/(D1+D2))*100.

Results: The ideal entry point was calculated as 0.11±0.72mm posterior to the lateral ridge. The mean distance between the ideal exit point and the tibialis anterior tendon was 24.9±4.2mm. The mean distance between the ideal exit point and the tibialis posterior tendon was 26.6±4.2mm. The mean ideal exit point was calculated as 48.3±4.8% of the distance from tibialis anterior to tibialis posterior.

Discussion: This study shows via CT analysis that the ideal entry point for a syndesmosis screw is the lateral ridge of the fibula and the ideal exit point is 48.3±4.8% of the distance from the tibialis anterior tendon to the tibialis posterior tendon. This is an easily reproducible technique which is independent of leg orientation.