The role of the subtalar joint in patients with chronic hindfoot instability remains controversial We have made an attempt at quantifying subtalar instability clinically and comparing this with findings at dynamic ultrasound. As a result of this study we have been able to demonstrate and test for reliability a new ultrasound sign for calcaneofibular ligament (CFL) deficiency.

A preliminary dissection of four cadavers was undertaken to determine the role of the CFL in providing subtalar stability and the effect of sectioning this ligament. Fifteen patients with symptomatic hindfoot instability were examined by two orthopaedic surgeons and subsequently had dynamic ultrasound examination of their ankle and subtalar joints on both the affected and unaffected sides. Ten control ankles were also examined. It was found that in a subset of these, with positive clinical signs of subtalar instability, the CFL failed to elevate the overlying peroneal tendons and alter their roundness on ultrasound cross section (suggesting that the CFL was deficient) whilst in normal hindfeet and those without a positive clinical test for subtalar instability the tendons were elevated in a reproducible manner. There was perfect correlation with the findings (in terms of the presence or absence of the CFL) at surgery in 5 patients undergoing lateral stabilisation procedures.

We believe this new sign is reliable and demonstrates the integrity of the CFL in patients with chronic hind-foot instability.

Axial loading of the foot/ankle complex is an important injury mechanism in vehicular trauma, responsible for severe injuries such as calcaneus, talus and tibial pilon fractures. Axial loading may be applied to the leg externally, by the toepan and/or pedals, as well as internally by active muscle tension applied through the Achilles tendon during pre-impact bracing. In order to evaluate the effect of active muscle tension on the injury-tolerance of the foot/ankle complex, axial impact tests were performed on isolated lower legs, with and without experimentally stimulated muscle tension applied through the Achilles’ tendon. Acoustic emission was used to determine the exact time of fracture during the tests. The primary fracture mode was calcaneal fracture in both groups, but tibial pilon fractures occurred more frequently with the addition of Achilles tension. A linear regression model was developed that describes the expected axial loading injury tolerance of the foot/ankle complex in terms of specimen age, gender, mass and level of Achilles tension.