The fibula and talus position difference in functional and mechanical ankle instability: MRI findings

Purpose: This study aimed to use MRI to evaluate the fibula and talus position difference in functional and mechanical ankle stability patients. Methods: 61 and 68 patients with functional and mechanical instability, and 60 healthy volunteers were involved. Based on the axial MRI images, the rotation of the talus was identified through the Malleolar Talus Index (MTI). The position relative to the talus (Axial Malleolar Index, AMI) and medial malleolus (Intermalleolar Index, IMI) were used to evaluated the displacement of the fibula. Results: Post hoc analysis showed that the values of malleolar talus index was significantly larger among mechanical instability (89.18 degrees +/- 2.31 degrees) than that in functional instability patients (86.55 degrees +/- 61.65 degrees, P < 0.001) and healthy volunteers (85.59 degrees +/- 2.42 degrees, P < 0.001). The axial malleolar index of the mechanical instability patients (11.39 degrees +/- 1.41 degrees) were significantly larger than healthy volunteers (7.91 degrees +/- 0.83 degrees) (P < 0.0001). There were no statistically significant differences in the above three indexes between the functional instability patients and healthy volunteers. Conclusion: The functional instability patients didn't have a posteriorly positioned fibula and an internally rotated talus. The malleolar talus index was significantly larger among mechanical instability patients than that in functional instability patients. Increased malleolar talus index may become a new indirect MRI sign for identifying functional and mechanical instability patients.