Weight-bearing CT delineates the anatomy of the syndesmosis

B. Rose, M. Welck, L. Milnes, D. Singh, N. Cullen, A. Goldberg

Introduction: The syndesmosis is critical to ankle stability. Syndesmotic injuries are common and frequently undetected. Historically, radiographic measurements have been undertaken to assess the integrity of the syndesmosis, but have been shown to be unreliable. The advent of cross-sectional imaging has enabled better visualisation of the syndesmosis. Several previous studies have described the normal syndesmotic anatomy on CT. These, however, all describe non-weight-bearing information, and may not simulate the true functional anatomical pattern of the syndesmosis. The pedCAT standing CT scanner (Curvebeam, USA) is a novel technology that allows 3D CT imaging in the axial, sagittal and coronal planes with full weight-bearing. This study aims to assess and describe the normal anatomy of the syndesmosis during weight-bearing on standing CT scan. This has not been done before. It also reports the inter-and intra-observer reliability of the measurements taken.

Methods: Weight-Bearing CT scans were assessed in 25 randomly selected subjects (50 feet), with an equal gender split. The scans were taken for forefoot pathology unrelated to the ankle, hindfoot or syndesmosis. Measurements were taken according to the validated method described by Nault (2013). These were performed 9.45mm proximal to the tibial plafond. Six measurements and two angles were identified to assess the antero-posterior and medial-lateral translations of the fibula relative to the incisura, and the fibula rotation. Images were independently reviewed by two observers. Measurements were taken twice by each investigator at six weekly intervals.

Results: Inter- and intra-observer reliability were high, thus validating the methodology used.

Conclusions: This is the first study to assess the ankle syndesmosis on weight-bearing CT, and provides a comparison with existing non-weight bearing studies. Weight-Bearing CT may enable subtle changes at the injured syndesmosis to be identified, by comparing affected and unaffected ankles, or as a post-operative tool to assess accurate reduction of the injured syndesmosis.