Lack of Displacement of the Fibula Is Not a Confirmation of Ankle Stability in Supination External Pattern Ankle Fractures.

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To evaluate and compare radiographic findings in supination external (SE)2 injuries versus stress (+) SE4 injuries. Retrospective. Academic Level 1 trauma center. The study included 350 skeletally mature patients at a single Level 1 trauma center who presented with an isolated, Lauge-Hansen type supination-external rotation pattern, Weber B lateral malleolar fracture, OTA/AO 44-B. We reviewed 350 patients (185 men and 165 women), 18-95 years of age (avg 45), with isolated SE pattern lateral malleolar fractures. One hundred nine had SE4 injuries [medial clear space (MCS) = 8.3 mm]. Two hundred forty-one ankles were stressed; 164 were unstable and 77 were stable (SE2). Avg MCS at presentation and on stress radiographs was 3.59 mm for the SE2 (no widening) and 3.86 mm and 5.94 mm for the stress (+) SE4 group, respectively. The fibular displacement for the SE2, stress (+) SE4, and SE4 groups was 1.5 (0-4.5), 3.5 (0-6.6), and 4.1 (0-30.5), respectively. Sixteen of the 77 (20%) SE2 and 24 of the 164 (15%) stress (+) SE4 fractures had no displacement of the fibula on the lateral view. Similarly, 53 of the 77 (68%) SE2 and 91 of the 164 (55%) stress (+) SE4 had ≤2 mm of fibular displacement Fibular displacement of ≤2 mm on the lateral radiograph corresponded with 0.69 sensitivity and only 0.37 positive predictive value for stable ankle mortise on stress examination. Previous work indicated that patients with an isolated SE pattern fibula fracture, a normal MCS, and ≤2 mm of fibular displacement on the lateral radiograph have a high rate of ankle stability, with a positive predictive value of approximately 97%. We were unable to confirm this finding because 15% of unstable ankles had 0 mm and 55% had ≤2 mm of fibular displacement. We conclude that stability may not be inferred from a lack of fibular displacement on the lateral view in this population of patients. If stability is to be determined, it must be tested irrespective of fibular displacement on the lateral radiograph. Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.