Syndesmotic stability in ankle fractures is usually assessed by pulling on the fibula with a bone hook in the coronal plane (“hook test”). Our clinical observations have suggested that instability may be more marked in the sagittal plane.

Our aim was to compare movement at the tibio-fibular syndesmosis in the sagittal and coronal planes after sequential ligament division in a cadaver model.

Seven specimens were used. A blinded subject was asked to perform the hook test both in the sagittal and coronal planes. Movement was assessed by measuring the displacement of parallel k-wires three consecutive times. In all specimens, the anterior tibio-fibular, interosseous and posterior tibio-fibular ligaments were sequentially divided and movement tested. In three specimens the deltoid ligament was then divided and the interosseous membrane in another three.

After division of all three syndesmosis ligaments the mean displacement was 8.8mm (±3.9) in the sagittal plane and 1.5mm (±0.4) in the coronal plane. When the deltoid ligament was then divided, the displacement increased to 11.7mm (±2.4) and 3.2mm (±0.5) respectively. When the interosseous membrane was divided the measurements were 12.7mm (±4) and 3.1mm (±1.5).

We conclude that distal tibio-fibular instability should be assessed in the sagittal plane.