Abstract
Classical AO teaching recommends that a syndesmosis screw should be inserted at 25 to 30 degree angle to the coronal plane of the ankle. In practice accurately judging the 25/30 degree angle can be difficult, and there are several reports based on post operative CT scans demonstrating that a significant minority of patients have poorly operatively reduced syndesmotic injuries.
The CT scans of 200 normal ankles in one hundred individuals which had been performed as part a CT angiogram were retrospectively examined. The centroid of the fibula and tibia in the axial plane 15mm proximal to the talar dome was calculated. Since a force vector between the centroid of the fibula and the tibia in the axial plane should not displace the fibula relative to the tibia, a line connecting the two centroids was therefore postulated to be the ideal syndesmosis line, and also the optimum position in which to place a compression clamp after reducing the syndesmosis. Where this ideal line passed through the lateral border of the fibula, and through the medial malleolus was then noted.
The ideal syndesmosis line was shown to pass through the fibula with in 2mm of the lateral cortical apex of the fibula, and the anterior half of the medial malleolus in 100% of the ankles studied. The results support the concept that in the operatively reduced syndesmosis, the anterior half of the medial malleolus can be used as a reliable guide for aiming the syndesmosis drill hole, provided that the fibular entry point is at or adjacent the lateral fibular apex.
The corollary of these findings is that a screw inserted through a plate on the standard antero-lateral border of the fibula, or a plate in the anti-glide position posteriorly, cannot lie in the centroidal axis of the ankle.
