Session VIII - Basic Science
Diagnosing Deltoid Ligament Injury in Supination-External Rotation Ankle Fractures: The External Rotation Stress Test
Purpose: Supination-external rotation (SER) ankle fractures (AO 44-B) are the most common ankle fracture type. Stable fracture patterns have been shown to have excellent results when treated conservatively, while unstable patterns often require operative fixation to achieve good results. Recent evidence suggests that the key to ankle stability is the deep deltoid ligament. It is often difficult to assess stability with an isolated SER-type fibula fracture without medial clear space (MCS) widening. The external rotation stress test (ERST) is frequently used clinically to differentiate these two injury patterns and has been shown in recent work to predict success of treatment methods. This study seeks to validate the ERST as a predictor of deep deltoid competence.
Methods: 15 cadaveric ankles were mounted in a jig that recreated the ERST. Baseline radiographs ruled out previous ankle fractures. The MCS was measured on the following mortise radiographs taken sequentially: (1) stress with no injury, (2) stress with SER-type fibula osteotomy, (3) stress with fibula osteotomy and superficial deltoid sectioned, and (4) stress with a fibula osteotomy, and both the superficial and deep deltoid ligaments sectioned. Results were evaluated with a one-tailed paired t-test using Microsoft Excel 2003.
Results: The mean MCS measurements were: baseline 2.0 mm, stress 2.2 mm, fibula osteotomy 3.5 mm, superficial deltoid 4.0 mm, and deep deltoid 10.1 mm. Each measure, compared to baseline, was significantly wider (P <0.05). Importantly, the change in mean MCS between superficial deltoid and deep deltoid was also statistically significant (P <0.000001) When >5 mm was set as the threshold for deep deltoid incompetence, the sensitivity, specificity, and positive predictive value (PPV) for the ERST were 100%, 93.3%, and 93.8%, respectively.
Conclusion/Significance: There was a large and statistically significant change in MCS after sectioning the deep deltoid, which is what has been previously considered critical for ankle stability. If >5 mm is used as a positive value for the ERST, the sensitivity, specificity, and PPV are all excellent. This study validates the external rotation stress test as a good indicator of deep deltoid incompetence when MCS widens more than 5 mm.