Paper 97: Defining Normal Values for Distal Tibiofibular Syndesmotic Space with and without External Rotation Stress: A Prospective Study with Volunteer Controls

OBJECTIVES: The diagnosis and treatment of distal tibiofibular syndesmosis (DTFS) injury can be challenging, especially in cases of subtle instability that may be masked on two-dimensional conventional radiographs. Weightbearing computed tomography (WBCT) has recently emerged as a useful diagnostic tool allowing direct assessment of distal tibiofibular widening. Area measurements of the distal tibiofibular incisura assessed one centimeter proximal to the ankle joint have been proposed as an accurate diagnostic tool to differentiate stable and unstable DTFS, however the literature lacks information regarding normal DTFS area measurements in asymptomatic ankles. The purpose of the current study was to examine and report normal threshold values for DTFS area measurements in a cohort of healthy volunteers, assessing the ankles in natural weightbearing position and under external rotation stress. METHODS: In this prospective study, we enrolled twenty-five healthy volunteers without a history of DTFS injury or high ankle sprain, previous foot and ankle surgery, or current ankle pain. Study participants underwent bilateral standing non-stress and external rotation stress WBCT scans. On non-stress WBCT scans, subjects stood with feet shoulder width apart in a natural, upright position with body weight evenly distributed. For external rotation stress images, patients were instructed to position their lower extremity in maximal internal rotation while the foot was held in a fixed plantigrade position. The DTFS area (mm(2)) was semi-automatically quantified on axial-plane WBCT images one centimeter proximal to the apex of the talar dome using dedicated software (Figure 1). Syndesmosis area values were compared between non-stressed and stressed ankles, as well as left and right ankles. Statistical analysis was performed utilizing independent t-tests/Wilcoxon analysis with statistical significance defined as p<0.05. RESULTS: The study cohort consisted of 50 ankles in 25 patients (12 males, 48%) with a mean age of 28.7+9.3 years. In the unstressed ankle, the mean pooled DTFS area was determined to be 99.1+2.8 mm(2). The mean syndesmosis area of unstressed left ankles in the cohort was 96.5+4.0 mm(2), while unstressed right ankles had a mean area of 101.6 mm(2) (p=0.378). With external rotation stress, the DTFS area of left ankles (mean difference 1.2 mm(2); CI: -9.8-11.8; p=0.765), right ankles (mean difference -3.1 mm(2); CI: -16.1-6.6; p=0.453), and all ankles (mean difference -1.2 mm(2); CI: -12.4-9.3; p=0.775) remained similar. CONCLUSIONS: This is the first study to assess standing WBCT DTFS area measurements in a cohort of volunteer controls, with and without external rotation stress. We found a mean DTFS value of 99 mm² ± 18.8. We also observed that in normal ankles with expected intact ligaments, external rotation stress did not result in significant widening of the DTFS space. The results of this study can be utilized in the literature as normal threshold values for DTFS area measurements, fostering additional investigations with patients with suspected and confirmed DTFS instability.