Instability of the tibio-fibular syndesmosis: have we been pulling in the wrong direction?

Syndesmotic stability in ankle fractures is usually assessed intraoperatively 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. The Hook Test was performed in both the sagittal and coronal planes. Movement was assessed by measuring the displacement of parallel k-wires. In all specimens, the anterior tibio-fibular, interosseous and posterior tibio-fibular ligaments were sequentially divided. 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 8.8 mm (+/-3.9) in the sagittal plane and 1.5 mm (+/-0.4) in the coronal plane. When the deltoid ligament was then divided, the displacement increased to 11.7 mm (+/-2.4) and 3.2 mm (+/-0.5), respectively. When the interosseous membrane was divided the measurements were 12.7 mm (+/-4) and 3.1 mm (+/-1.5). Movements were consistently greater in the sagittal plane than the coronal plane and we conclude that distal tibio-fibular instability should be assessed in the sagittal plane.