LAUSR

Purpose Ultrasound (US) is a valuable tool for the evaluation of chronic lateral instability of the ankle; however, the feasibility of US for calcaneofibular ligament (CFL) assessment remains unknown. This study aimed to depict and compare CFL on US in various ankle positions to determine the optimal method for evaluating CFL with US and to interpret US findings using cadaveric specimens. Methods The US study included 43 ankles of 25 healthy individuals. The CFL was scanned with US in 20° plantar flexion, neutral position, 20° dorsiflexion and maximum dorsiflexion. The distances between fibula and CFL were compared. The cadaveric study included macroscopic qualitative observation of the dynamic change of CFL in 7 ankles and quantitative observation of the directions of CFL and footprints in 17 ankles. Results In the US study, the mean distance (mm) between fibula and CFL was 7.3 ± 1.3 in 20° plantar flexion, 6.7 ± 1.6 in neutral position, 4.3 ± 2.5 in 20° dorsiflexion and 3.1 ± 2.1 in maximum dorsiflexion. The more dorsiflexed the ankle was, the shorter the distance between fibula and CFL was (Jonckheere’s trend test p  < 0.001). In the cadaveric study, the CFL fibres were aligned parallel between the mid-substance and the fibular attachment in maximum dorsiflexion, whilst CFL was reflected and rotated in plantar flexion. Conclusions The whole length of the CFL, including its fibular attachment, is more likely to be visualized with US in dorsiflexion than in plantar flexion due to the direction of the CFL at the fibular attachment, which is parallel with the mid-substance in maximum dorsiflexion. Level of evidence IV.