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.