MRI for high ankle sprains with an unstable syndesmosis: posterior malleolus bone oedema is common and time to scan matters.

Purpose: Early clinical examination combined with MRI allows accurate diagnosis of syndesmosis instability after a high ankle sprain. However, patients often present late. The aims of the current study were to describe MRI characteristics associated with syndesmosis instability and to test the hypothesis that MRI patterns would differ according to time from injury.

Methods: Over a 5-year period, 164 consecutive patients who had arthroscopically proven syndesmosis instability requiring fixation were retrospectively studied. Patients with distal fibula fractures were not included. Injuries were classified as acute in 108 patients (< 6 weeks), intermediate in 32 (6-12 weeks) and chronic in 24 patients (> 12 weeks).

Results: Posterior malleolus bone oedema was noted in 65 (60.2%), and posterior malleolus fracture in 17 (15.7%) of acute patients, respectively, which did not significantly differ over time. According to MRI, reported rates of posterior syndesmosis disruption significantly differed over time, observed in 101 (93.5%), 28 (87.5%) and 13 (54.2%) of acute, intermediate and chronic patients, respectively (p < 0.001). Apparent rates of PITFL injury significantly reduced with time (p < 0.001).

Conclusions: MRI detected a posterior syndesmosis injury in 93.5% of patients acutely but became less reliable with time. The clinical relevance of this study is that posterior malleolus bone oedema may be the only marker of a complete syndesmosis injury and can help clinically identify those injuries which require arthroscopic assessment for instability. If suspicious of a high ankle sprain, we advocate early MRI assessment to help determine stable versus unstable injuries as MRI becomes less reliable after 12 weeks.

Level Of Evidence: III.