Normal Variation and Instability Values of the Tibiofibular Joint on Weightbearing CT

CATEGORY: Ankle INTRODUCTION/PURPOSE: Prompt management of syndesmotic instability is critical for optimizing clinical outcome, but subtle injuries may be difficult to diagnose. Application of modern imaging modalities such as weight bearing CT (WBCT) may better identify such injuries by virtue of assessing the distal tibiofibular articulation under physiologic load. The aim of this study was to evaluate the distal tibiofibular articulation using WBCT among patients with known syndesmotic instability and compare these findings with their uninjured contralateral sides, and thereafter corroborate such measurement differences with patients devoid of any syndesmotic injury. METHODS: Patients with unilateral syndesmotic instability requiring surgical fixation (n=12) underwent bilateral ankle WBCT that incorporated the entire foot. A separate cohort of patients without ankle injury also underwent bilateral ankle WBCT for assessment of either a Lisfranc injury or forefoot condition (n=24). All WBCT imaging was performed preoperatively. A set of five axial plane tibiofibular joint measurements including one angular measurement were standardly assessed one cm above the tibial plafond. Values were recorded by two independent observers to assess for interobserver reliability scores. Interpretation of the intraclass correlation coefficients was carried out according to the guidelines proposed by Shrout: 0.00-0.10 virtually none, 0.11-0.40 slight, 0.41-0.60 fair, 0.61-0.80 moderate, 0.81 -1.00 substantial. RESULTS: Among the control population without ankle injury, no differences were found between bilateral measurements (p-value range 0.172 - 0.961). Among those with known unilateral syndesmotic instability, values differed between the injured and uninjured side in five of six measurements— including syndesmotic area, direct anterior-, middle-, and posterior- difference, and sagittal translation (p <0.001, <0.001, <0.001, <0.001, 0.039, respectively). Those same measurements also differed when comparing the left-right delta values between uninjured and injured patients (p <0.001, 0.002, <0.001, <0.001, and 0.042, respectively). Fibular rotation differed neither in direct nor delta comparisons (p=0.460 and 0.271 respectively). Substantial agreement was found for all measurements except for sagittal translation, which had only slight agreement. CONCLUSION: This study highlights the ability of WBCT to effectively differentiate syndesmotic diastasis and fibular translation among patients with surgically-confirmed syndesmotic instability as compared to those without syndesmotic instability. It also underscores the importance of using the contralateral, uninjured side as a valid internal control. Additional studies are necessary to better understand the role of WBCT in prospectively diagnosing more subtle cases of syndesmotic instability among patients for whom the diagnosis remains in question.