Stability assessment of the ankle mortise in supination-external rotation-type ankle fractures: lack of additional diagnostic value of MRI.

Background: This prospective diagnostic study assessed the utility of MRI (magnetic resonance imaging) findings for the deep aspect of the deltoid ligament in evaluating the stability of the ankle mortise in patients who have an SER (supination-external rotation)-type lateral malleolar fracture with no widening of the medial clear space.

Methods: Sixty-one patients with a unilateral lateral malleolar fracture resulting from an SER mechanism were enrolled. Two surgeons assessed the stability of the ankle mortise with use of an external-rotation stress test. The anterior and posterior parts of the deep deltoid ligament were investigated with 3.0-T MRI and were graded (as normal, edematous, partial tear, or complete tear) by two musculoskeletal radiologists. The medial clear space was measured and compared with the MRI findings for the deep deltoid ligament in stable and unstable injuries. Interobserver reliability was calculated for both external-rotation stress testing and MRI assessment.

Results: Thirty-three patients had a medial clear space of ≥ 5 mm in the external-rotation stress test. According to MRI, all of these patients had an injury involving the deep deltoid ligament (an edematous ligament in five, a partial tear in twenty-six, and a total tear in two). Twenty-eight patients had a medial clear space of <5 mm, and MRI indicated a deep deltoid ligament injury in all of these patients as well (an edematous ligament in nine and a partial tear in nineteen). The medial clear space increased according to the severity of the deep deltoid ligament injury as indicated by MRI (p < 0.001). The interobserver agreement of the external-rotation stress test was excellent (94% agreement; kappa = 0.87), whereas the interobserver reliability of the MRI assessments by the two musculoskeletal radiologists was fair to moderate (72% agreement for the posterior part of the deep deltoid ligament and 56% for the anterior part; kappa = 0.46 and 0.22, respectively).

Conclusions: On the basis of the study results, we do not recommend the use of MRI when choosing between operative and nonoperative treatment of an SER-type ankle fracture.