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Background: Variations in ankle mortise anatomy may be a predisposing factor to ankle instability. Hypothesis: A posteriorly positioned fibula associated with ankle instability may not be a true pathologic entity but rather the result of measuring off an internally rotated talus. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: The authors reviewed 60 ankle computed tomography scans performed on patients from their institution for reasons unrelated to ankle instability. They also reviewed ankle computed tomography scans on 21 patients surgically treated for clini- cal ankle instability. The position of the fibula in relation to the talar articular surface was calculated and expressed as the axial malleolar index, as described by Scranton et al. They also calculated the intermalleolar index, a new method that references the medial malleolus, not the talus. Results: Using the method of Scranton et al, the axial malleolar index in the control and instability patients was similar to val- ues previously described, and there was a significant difference between control and instability patients (Pc .01). However, using the intermalleolar index method referencing the medial malleolus, there was not a significant difference between control and instability patients (P = .43). Conclusion: The new method of referencing the medial malleolus assesses fibular position independent of talar rotation. The data, when referencing the medial malleolus, do not show significant variation in fibular position in patients with and without ankle instability. [ABSTRACT FROM AUTHOR]