Initial Fibular Displacement as a Predictor of Medial Clear Space Widening in Weber B Ankle Fractures.

BACKGROUND: The diagnosis of medial ankle instability in Weber B ankle fractures remains controversial. Manual stress and gravity stress radiographs as well as magnetic resonance imaging (MRI) are used, but there is no consensus gold standard. The purpose of this study was to determine the relationship between initial fibular displacement and medial clear space widening on a gravity stress radiograph as a predictor of instability.

METHODS: A retrospective review was conducted of all patients with isolated Weber B ankle fractures with both initial injury radiographs and gravity stress view from August 1, 2014, through April 1, 2016. A total of 17 patients were identified. On the mortise view of initial injury radiographs, medial clear space (MCS), superior clear space, lateral fibular displacement (LFDP), and fibular shortening (FS) were measured, and on the lateral view, anterior to posterior fibular gap (A to P FG) was measured. MCS was again measured on the gravity stress view (MCS-W). Statistical analyses identified the correlations of each displacement variable relative to MCS-W as well as the sensitivity and specificity of each parameter.

RESULTS: A cutoff point for MCS-W was set as less than 5.0 mm (n = 8) and 5.0 mm or more (n = 9). Strong significant correlations with MCS-W were found for A to P FG (0.84, P < .001), with a trend for LFDP (0.62, P = .008), but no significance with FS (0.38, P = .84). Linear regression analysis revealed significant ability to predict MCS-W for both LFDP ( P = .002) and A to P FG ( P = .001) but not FS. Receiver operating characteristic analysis for A to P FG using a threshold value of 1.0 mm yielded sensitivity and specificity of 100% in predicting an MCS-W of 5.0 mm or more.

CONCLUSION: The initial fibular displacement was a strong predictor of MCS-W in Weber B ankle fractures. On lateral radiographs, an A to P FG greater than 1.0 mm showed a sensitivity and specificity of 100% in predicting an MCS-W of 5.0 mm or more on gravity stress view.

LEVEL OF EVIDENCE: Level III case series, prognostic.