A computed tomography evaluation of two hundred normal ankles, to ascertain what anatomical landmarks to use when compressing or placing an ankle syndesmosis screw.

Classical AO teaching recommends that a syndesmosis screw should be inserted at 25-30 degrees to the coronal plane of the ankle. Accurately judging the 25/30 degree angle can be difficult, resulting in poor operative reduction of syndesmosis injuries. The CT scans of 200 normal ankles were retrospectively examined. The centroid of the fibula and tibia in the axial plane 15mm proximal to the talar dome was calculated. 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 when surfaces are parallel. Therefore, a line connecting the two centroids was postulated to be the ideal syndesmosis line. This line was shown to pass through the fibula within 2.5mm 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 to the lateral fibular apex. The screw should also remain parallel to the tibial plafond in the coronal plane.