Weight-bearing CT Useful for Diagnosis of Subtle Syndesmotic Instability
Key findings
- In a retrospective study, weight-bearing CT scans of 12 patients with surgically confirmed unilateral syndesmotic instability were compared with those of 24 patients who had no ankle injury
- Weight-bearing CT was able to distinguish between a stable and unstable distal syndesmosis
- Of seven distal tibiofibular joint measurements examined, the syndesmotic area had the highest interobserver agreement and showed the largest difference between normal and abnormal
- The study findings emphasize the critical importance of using a patient's contralateral, uninjured side as their own control
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Conventional imaging has drawbacks for diagnosing syndesmotic instability, especially when the instability is subtle. Weight-bearing radiographs have low sensitivity, cross-sectional magnetic resonance imaging can't distinguish injury from instability and traditional computed tomography (CT) only shows alignment without a physiologic load in the unstressed state.
Bilateral weight-bearing CT allows visualization of the distal tibiofibular articulation under physiologic load and simultaneously provides a direct comparison of the injured and uninjured sides. These advantages are important because there is enormous interindividual variability in distal tibiofibular morphology.
In Foot & Ankle International, lead researcher Noortje Hagemeijer, MD, Daniel Guss, MD, MBA, orthopaedic foot and ankle surgeon, Christopher W. DiGiovanni, MD, chief of the Foot and Ankle Service at Massachusetts General Hospital, and colleagues report that weight-bearing CT can distinguish between a stable and unstable distal syndesmosis.
Study Design
The research team retrospectively studied 12 patients with surgically confirmed unilateral syndesmotic instability and 24 patients with no ankle injury. All patients underwent two-dimensional, bilateral weight-bearing CT scans of the ankle and foot at Mass General between 2015 and 2018.
Patients with no ankle injury had weight-bearing CT to evaluate the Lisfranc joint or a more distal forefoot condition. These control patients' bilateral tibiofibular measurements were compared with the bilateral measurements of the injured group's ankles. Two observers performed all measures independently, using methods described in detail in the paper.
Weight-bearing CT Results
In patients with syndesmotic injuries, four measurements—the syndesmotic area, direct anterior difference, middle difference and direct posterior difference—differed significantly between the injured and uninjured sides. The average difference in syndesmotic area between ankles was 46 mm2.
In patients without syndesmosis injury, no differences were found based on laterality when comparing any of the seven syndesmotic measurements. The average difference in syndesmotic area was only 0.41 mm2.
Interobserver Agreement
There was a substantial agreement between the two observers for almost all measurements, with intraclass correlation coefficients ranging from 0.83 for fibular rotation to 0.97 for syndesmotic area. There was moderate agreement (coefficient of 0.80) for fibular sagittal plane position.
Clinical Implications
The researchers acknowledge that weight-bearing CT is a relatively new and costly technique, not yet available to all orthopedic surgeons. When possible, though, it will be useful to evaluate the distal tibiofibular articulation under physiologic load and to look carefully at any differences from the contralateral, uninjured side when evaluating for syndesmotic instability.
Because the syndesmotic area demonstrated the greatest differential between the abnormal and normal ankles and had the highest interobserver agreement, it is presumably the most useful measurement for detecting syndesmosis instability in the axial plane.
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