Effect of Sequential Sectioning of Ligaments on Syndesmotic Instability in the Coronal Plane Evaluated Arthroscopically.

BACKGROUND: Arthroscopic evaluation of the syndesmosis allows direct visualization of syndesmotic instability. The purpose of this study was to determine the minimum degree of ligamentous injury necessary to destabilize the syndesmosis in the coronal plane when assessed arthroscopically and pinpoint where such instability should be measured within the incisura.

METHODS: Fourteen cadaveric specimens were divided into 2 groups and arthroscopically assessed first with the syndesmosis intact and then following serial differential ligamentous transection. Group 1 (n = 7): anterior-inferior tibiofibular (AITFL), interosseous (IOL), posterior-inferior tibiofibular (PITFL), and deltoid (DL) ligament. Group 2 (n = 7): PITFL-IOL-AITFL-DL. At each step, a standard 100-N lateral hook test was applied and tibiofibular coronal plane diastasis measured arthroscopically at both the anterior and posterior third of the incisura. These measurements were in turn compared with those of the stressed intact ligamentous state.

RESULTS: There was no significant syndesmotic instability measured at either the anterior or posterior margin of the incisura after transection of a singular ligament (AITFL or PITFL) or after the IOL was additionally transected. Diastasis at the posterior margin was significantly increased when all syndesmotic ligaments were sectioned (group 1: P = .018; group 2: P = .008), but this was not noted along the anterior margin. Diastasis at the anterior margin reached significance only with complete transection of syndesmosis and DL (group 1: P < .001; group 2: P = .044).

CONCLUSION: Under arthroscopic evaluation, the syndesmosis becomes unstable in the coronal plane only when all syndesmotic ligaments are transected, which should preferentially be measured at the posterior margin of the incisura. Anteriorly, diastasis becomes apparent only with addition of DL disruption, although this added finding may aid in diagnosis of occult deltoid injury.

CLINICAL RELEVANCE: AITFL, IOL, and PITFL need to be injured to produce coronal plane syndesmotic instability. Arthroscopic assessment of such instability should occur along the posterior margin of the incisura. When they exist, similar findings anteriorly suggest concomitant deltoid injury.