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Qualitative Side-by-Side Comparison Unreliable for Evaluating Syndesmosis Reduction on CT

Key findings

  • In this study of postoperative weight-bearing computed tomography (CT) images from 20 patients, qualitative side-by-side evaluation of the syndesmotic reduction had substantially lower intra- and interobserver reliability than eight quantitative syndesmosis measurements
  • When determining the quality of syndesmotic reduction in the axial plane of CT scans, one of the most common methods is side-by-side comparison using the healthy contralateral ankle as a reference
  • Syndesmotic area calculation had the highest interobserver reliability
  • Fibular rotation was another promising measurement strategy

Increasingly, bilateral ankle computed tomography (CT) is used to determine the accuracy of syndesmotic reduction in the axial plane. A number of methods have been used to assess reduction, but one of the most common is a qualitative side-by-side comparison using the healthy contralateral ankle as a reference.

That approach seems intuitive, but it is actually the least reliable, according to a study by Noortje Hagemeijer, MD, lead researcher, Daniel Guss, MD, MBA, orthopaedic foot and ankle surgeon, and Christopher W. DiGiovanni, MD, chief of the Foot and Ankle Service at Massachusetts General Hospital, and colleagues. They published their findings—and recommendations for more accurate evaluation—in Foot & Ankle International.

Study Cohort and Imaging

The researchers recruited 20 adults in a Mass General registry who underwent fixation of unilateral, unstable syndesmotic injuries between January 2008 and December 2016. Between July 2017 and June 2018, the cohort (50% male, average age 43) underwent weight-bearing CT, which allows dynamic evaluation. An average of three years (range, 1–7) elapsed between surgery and the scanning performed for this research study.

Radiographic Measurements

Nine methods were chosen for the evaluation of syndesmosis:

  1. Side-by-side comparison: Qualitative assessment of whether or not the operative side was reduced, made by comparing the injured and uninjured ankle
  2. Anterior difference: The distance between the anterior border of tibial incisura and the most anterior point of the fibula
  3. Posterior difference: The distance between the posterior border of tibial incisura and the most posterior point of the fibula
  4. Middle difference: The distance between the middle of the incisura and the nearest point of the fibul
  5. Direct anterior difference: The distance represented by a line from the incisura and perpendicular to the anterior end of a line representing the fibular orientation
  6. Direct posterior difference: The distance represented by a line from the incisura and perpendicular to the posterior end of a line representing the fibular orientation
  7. Fibular translation: The distance between a line representing the direct anterior difference and the anterior border of tibial incisura
  8. Syndesmotic area: The space between the lateral cortex of the tibial incisura, the medial cortex of the lateral malleolus and two lines tangential to the anterior and posterior aspects of the tibia and fibula
  9. Fibular rotation: Angle between a line drawn between the anterior and posterior borders of the incisura and a line drawn in the fibula representing its orientation

Two investigators made these measurements independently and repeated them after six weeks.

Initial Results

  • Intraobserver reliability of the side-by-side method: 0.49 for Observer 1 and 0.24 for Observer 2
  • Intraobserver reliability of the eight quantitative methods: 0.80–0.93 for Observer 1 and 0.69–0.97 for Observer 2
  • Interobserver reliability of the side-by-side method: 0.26
  • Interobserver reliability of the eight quantitative methods: 0.62–0.96
  • Syndesmotic area interobserver reliability: the highest intraobserver reliability for Observer 2 (0.97), the second-highest intraobserver reliability for Observer 1 (0.92), and highest overall (0.96)
  • Fibular rotation interobserver reliability: second-highest interobserver reliability overall (0.84), and high intraobserver reliability for Observers 1 and 2 (0.91 and 0.80, respectively)

Retesting the Side-to-Side Method

Because the two observers had low inter- and intraobserver agreement scores for the side-to-side evaluation method, another 10 foot and ankle surgeons from various countries, with varying experience in the field, were asked to analyze the CT scans.

Even after their results were added to the scores of the initial observers, the interobserver reliability for the side-by-side method remained quite low at 0.25.

Interpreting the Findings

The low reliability of the side-to-side evaluation method is probably attributable to both its subjective nature and lack of a universally agreed-on cutoff value to define "successful reduction" or "malreduction." This technique may need to be abandoned for reviewing both weight-bearing and non–weight-bearing CT scans.

The poor results for the side-to-side method also call into question the use of intraoperative CT or other advanced intraoperative imaging that relies on the subjective assessment of the surgeon. It may not be effective without a preoperative contralateral ankle CT scan as a base of reference and quantitative algorithms that take the subjectivity out of intraoperative assessment.

In a data-driven world, qualitative surgical judgments will increasingly benefit from quantitative algorithms that work together to improve patient care.

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In this video, Christopher W. DiGiovanni, MD, chief of theFoot and Ankle Service at Massachusetts General Hospital, explains how syndesmosis, which is an instability and ligament problem in the ankle, is often underdiagnosed and undertreated.