Fibular position in relation to lateral ankle instability

Background: The position of the fibula within the ankle mortise may be a factor contributing to recurrent ankle instability. The current study was performed to determine whether significant differences in fibular position exist in a population of patients who underwent lateral ankle stabilization procedures. The hypothesis that a fibula positioned posteriorly within the mortise predisposes the ankle to chronic instability was evaluated. Methods: Sixty-five CT/MRI scans of patients who underwent lateral ankle stabilization procedures from 1998 to 2001 were reviewed. The position of the fibula in relation to the tibia at the ankle mortise was expressed as the axial malleolar index (AMI). A greater AMI corresponds to a more posterior fibula. The AMI was also calculated from 65 CT/MRI scans performed on control patients who had no ankle instability. Results: The average AMI in the study group was 17degrees +/- 6degrees (SD) compared with an average of 9degrees +/- 4degrees (SD) in the control group (p <.01). Therefore, the fibula was nearly twice as posterior in patients undergoing ankle reconstruction. In 42/65 (65%) study patients, the AMI was greater than 15degrees. Only 5/65 (8%) control patients had AMI values greater than 15% This corresponds to an eightfold greater likelihood of AMI greater that 15degrees in the instability group than in the control group. Conclusions: This study supports the hypothesis that a posteriorly positioned fibula predisposes to ankle instability.