Arthroscopic characterization of syndesmotic instability in the coronal plane: Exactly what measurement matters?

Background: Although ankle arthroscopy is increasingly used to diagnose syndesmotic instability, precisely where in the incisura one should measure potential changes in tibiofibular space or how much tibiofibular space is indicative of instability, however, remains unclear. The purpose of this study was to determine where within the incisura one should assess coronal plane syndesmotic instability and what degree of tibiofibular space correlates with instability in purely ligamentous syndesmotic injuries under condition of lateral hook stress test (LHT) assessment.

Methods: Ankle arthroscopy was performed on 22 cadaveric specimens, first with intact ankle ligaments and then after sequential sectioning of the syndesmotic and deltoid ligaments. At each step, a 100N lateral hook test was applied through a lateral incision 5 cm proximal to the ankle joint and the coronal plane tibiofibular space in the stressed and unstressed states were measured at both anterior and posterior third of the distal tibiofibular joint, using calibrated probes ranging from 0.1 to 6.0 mm, in 0.1 mm of increments. The anterior and posterior points of measurements were defined as the junction between the anterior and middle third, and junction between posterior and middle third of the incisura, respectively.

Results: Anterior third tibiofibular space measurements did not correlate significantly with the degree of syndesmotic instability after transection of the ligaments, neither before nor after applying LHT at all the three groups of different sequences of ligament transection (P range 0.085-0.237). In contrast, posterior third tibiofibular space measurements correlated significantly with the degree of syndesmotic instability after transection of the ligaments, both with and without applying stress in all the groups of different ligament transection (P range <0.001-0.015). Stressed tibiofibular space measurements of the posterior third showed higher sensitivity and specificity when compared to the stressed anterior third measurements. Using 2.7 mm as a cut off for posterior third stressed measurements has both sensitivity and specificity about 70 %.

Conclusion: Syndesmotic ligament injury results in coronal plane instability of the distal tibiofibular articulation that is readily identified arthroscopically with LHT when measured in the posterior third of the incisura.

Clinical Relevance: When applying LHT, tibiofibular space measurement for coronal plane instability along the anterior third of the incisura is less sensitive for identifying syndesmotic instability and may miss this diagnosis especially when subtle.