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The diagnosis of clinically important instability following isolated supination-external rotation (SER) distal fibular fractures is a challenge. The purpose of this study was to investigate the accuracy of clinical findings including medial tenderness, swelling, and ecchymosis, combined with the gravity stress test and magnetic resonance imaging (MRI), in the assessment of ankle mortise stability in association with isolated SER-type lateral malleolar fractures. The external rotation (ER) stress test was used as the reference with which the methods of assessment were compared.Thirty-seven patients were enrolled prospectively. Using the ER stress test as a reference, we evaluated the sensitivity, specificity, likelihood ratio (LR), and post-test probability of instability when using clinical examination, the gravity stress test, and MRI for diagnosing ankle mortise instability after an isolated SER lateral malleolar fracture.The positive LR for clinical findings ranged from 1.45 to 2.54, and the negative LR ranged from 0.25 to 0.70, shifting the pre-test probability to a rarely important degree. In contrast, the positive LR for the gravity stress test was 5.71 with a 95% confidence interval (CI) of 1.52 to 21.48, a moderate shift from the pre-test probability, and the negative LR was 0.33 (95% CI = 0.16 to 0.66), indicating a small shift. In cases with a deep deltoid ligament disruption identified on MRI, the positive LR was 3.05 (95% CI = 1.03 to 9.02) and the negative LR was 0.53 (95% CI = 0.31 to 0.91), demonstrating a small but sometimes important shift.The gravity stress test is a reliable method for diagnosing ankle mortise instability in patients with an isolated SER lateral malleolar fracture. Nevertheless, this test alone would not qualify as a replacement for the ER stress test. However, when the gravity stress test result is consistent with the clinical or MRI findings, its diagnostic value is almost equivalent to that of the ER stress test.Diagnostic Level II. See Instructions for Authors for a complete description of levels of evidence.