Diagnosing Syndesmotic Instability with Dynamic Ultrasound: Establishing Natural Variations in Normal Motion

CATEGORY: Ankle INTRODUCTION/PURPOSE: Syndesmotic instability, when subtle, is challenging to diagnose and often requires visualization of the syndesmosis during applied stress. Ultrasound is a dynamic, non-invasive diagnostic tool that can achieve this at the point of care, although accurate assessment requires better understanding of normal physiologic motion. The aim of this study is to assess normal distal tibiofibular motion in the sagittal plane using ultrasound during applied stress. METHODS: Patients with no history of injury to either ankle were included in this study. The distal tibiofibular joint was examined using an ultrasound probe placed anteriorly with the patient in supine position and posteriorly with the patient in prone position. Sagittal fibular translation was simulated by applying an increasing manual force to the fibula in the sagittal plane: 1) in the supine position, anterior to posterior (SAP), 2) in the prone position, posterior to anterior (PPA), and 3) in the prone position, anterior to posterior (PAP) (Figure 1). The relative position of the fibula and tibia were subsequently measured without force application and then at maximum force application, defined as the end point at which increased force did not result in additional fibular translation. Fibular displacement in cm was thereafter calculated. A linear mixed effect model was built for analysis, wherein subject, examiner, experiment and side were random factors. RESULTS: A total of 34 ankles were included. Mean age was 25.0±5 years and 71% were male. After adjusting for gender and side, the model showed that the application of force led to fibular motion in the sagittal plane for each measurement technique. SAP, coef. 0.06 [0.06-0.07], p-value < 0.001. PPA, coef. 0.04 [0.03-0.05], p-value < 0.001. PAP, coef. -0.16 [-0.17- -0.15], p-value < 0.001. The model also showed similar fibular motion values for left and right measurements, SAP p-value 0.589, PPA p-value 0.236, PAP p-value 0.104. Fibular movement values also were not influenced by gender, SAP p-value 0.348, PPA p-value 0.613, PAP p-value 0.714. CONCLUSION: Alterations of syndesmotic motion in the sagittal plane can be successfully identified using dynamic ultrasound under stressed conditions. On average, 0.6 mm of A to P translation and 0.4 mm of P to A translation were found. Equivalent translation values between the left and right side in each patient underscores that, in the injured setting, the contralateral ankle may be used as a reliable control. This establishes a normal value for ultrasound examination of the syndesmosis, allowing future studies examining the injured state.