Whilst lateral ankle sprain is often considered a benign injury it represents between 3–5% of all A&E visits in the UK. The mechanical characteristics of ankle ligaments under sprain-like conditions are scarcely reported.

The lateral collateral ankle ligaments were dissected from n=6 human cadaveric specimens to produce individual bone-ligament-bone specimens. An Instron Electropuls E10000 was used to uni-axially load the ankle ligaments in tension. The ligaments were first preconditioned between 2 N and a load value corresponding to 3.5% strain for 15 cycles and then strained to failure at a rate of 100%/s.

The mean ultimate failure loads and their standard deviations for the anterior talofibular (ATFL), calcaneofibular (CFL) and posterior talofibular (PTFL) ligaments are 351.4±105.6 N, 367.8±76.1 N and 263.6±156.6 N, respectively. Whilst the standard deviation values are high they align with those previously reported for ankle ligament characterisation. The large standard deviations are partly due to the inherent variability of human cadaveric tissue but could also be due to varying previous activity levels of participants or a prior unreported ankle sprain. Although the sample size is relatively small the results were stratified to identify any potential correlations of age, BMI and weight with ultimate load. A strong Pearson correlation (r=0.919) was found between BMI and ultimate load of the CFL but a larger sample size is required to confirm a link. The ligament failure modes were observed and categorised as avulsion or intra-ligamentous failure. The ATFL avulsed from the fibula in five instances and intra-ligamentous failure occurred once. The CFL avulsed from the fibula twice and failed four times through intra-ligamentous failure. Finally, the PTFL avulsed from the fibula once, avulsed from the talus once and failed through intra-ligamentous failure in four instances.

The results identify the forces required to severely sprain the lateral collateral ankle ligaments and their failure modes.