Summary

Ankles with CAI display increased tibiotalar and subtalar laxity as compared to uninjured healthy ankles, while uninjured healthy ankles showed laxity values (average 3.25 mm for the ATT) that are greater than the 3 mm traditionally used cut-off to establish between lax and non-lax ankles.

Abstract

Background

Ankle laxity after an ankle sprain can contribute to the development of chronic ankle instability (CAI) and it is important to measure during diagnosis. Ankle laxity is commonly assessed with manual examination, but these manual tests are limited by poor inter-rater reliability. Arthrometers aim to provide more reliable measurements, but provide heterogenous results and are limited to device-related limitations such as no imaging control of bony displacements.

Purpose

The main purpose of this study was to implement a new arthrometer compatible with magnetic resonance imaging (MRI) to measure ankle laxity – the Porto Ankle Testing Device (PATD) – to evaluate whether ankles with CAI have increased ankle laxity as compared to uninjured healthy ankles.

Methods

This prospective study enrolled 55 ankles with CAI (selected according to the International Ankle Consortium statement) and compared against 33 uninjured healthy ankles without history of previous sprain. Groups were homogenous according to biometric and sociodemographic characteristics. All ankles underwent the ankle instrumented stress testing with the PATD within MRI setting. The PATD applies external stress to the ankle joint aiming to mimic the anterolateral drawer (posteroanterior translation with internal rotation) and the talar tilt manual tests within the MRI. Laxity measurement for the anterolateral drawer included the anterior talar translation (ATT) and anterior calcaneus translation (ACT). For the talar tilt test, the talar tilt (TT) angle, subtalar tilt (STT) angle, subtalar joint lateral opening distance (subtalar opening), and the internal rotation of the calcaneus under the talus (subtalar rotation) were measured. Laxity measurements were computed by the difference of talar and calcaneus bony position between the ankle resting position and after stress tests during the MRI exam. Laxity variables were compared between groups using the Mann-Whitney test. Cut-off points were computed by maximizing specificity and sensitivity coordinates in the receiver operating curve (ROC) and selected according to the highest Youden index.

Results

Ankles with CAI displayed significantly superior laxity bone displacements for the ATT in the anterior (4.59 ± 1.67 vs 3.25 ± 1.32 mm, P<0.001) and diagonal (5.74 ± 3.57 vs 4.02 ± 2.90 mm, P=0.020) directions, the TT (3.75 ± 3.63 vs 1.52 ± 1.71°, P=0.001), the STT (15.63 ± 5.73 vs 11.81 ± 6.14°, P=0.008), and the subtalar opening distance (11.94 ± 4.11 vs 10.05 ± 3.55 mm, P=0.036). The ACT and the subtalar rotation were not significantly different between ankles with CAI and healthy controls. The ATT (cut-off = 3.22 mm) was the laxity metric with the highest sensibility (82.4%) in discriminating pathological laxity between ankles with CAI and the TT (cut-off = 1.95°) was the most specific (77%) in excluding ankles without instability.

Conclusion

Ankles with CAI display increased tibiotalar and subtalar laxity as compared to uninjured healthy ankles. Ankles without previous sprains showed laxity values (average 3.25 mm for the ATT) that are greater than previously thought and superior to the 3 mm traditionally used cut-off to establish between lax and non-lax ankles. The ATT was the metric with the highest sensibility and the TT with the highest specificity.