The role of a posteriorly positioned fibula in ankle sprain.

BACKGROUND Specific anatomic variations of the ankle mortise may predispose people to ankle sprains. HYPOTHESIS There is a correlation between a higher malleolar index (posteriorly positioned fibula) and incidence of ankle sprain. STUDY DESIGN Prospective case control study. METHODS We compared the malleolar index (transverse plane of the talus) on computerized axial tomographic images of 61 patients with ankle sprain with that of 101 normal controls. A positive number for the malleolar index meant that the lateral malleolus was posterior to the plane of the medial malleolus. A negative number meant that the lateral malleolus was actually anterior to the plane of the medial malleolus. RESULTS The average malleolar index of the patients with ankle sprain was +11.5 degrees with a standard deviation of 7 degrees. Malleolar relationships varied from -6 degrees to +39 degrees, a range of 45 degrees. The average malleolar index in the control group was +5.85 degrees with a standard deviation of 4.9 degrees, which varied from -8 degrees to +16 degrees. However, there was no correlation between recurrence of sprains and malleolar index values. CONCLUSION Patients with an ankle sprain were more likely to have a posteriorly positioned fibula, possibly predisposing them to ankle sprain.