Functional vs Mechanical Instability Using MRI

Magnetic Resonance Imaging (MRI) can be used to identify fibula and talus position difference in functional and mechanical ankle instability, researchers say.

In the study, “The fibula and talus position difference in functional and mechanical ankle instability: MRI findings,” published online on January 11, 2021 in the Journal of Orthopaedic Surgery, researchers used MRI to evaluate the fibula and talus position difference in functional and mechanical ankle instability patients.

“In clinical practice, it is very important for surgeons to accurately identify the type of ankle instability for providing corresponding treatment strategies since the treatments for functional ankle instability and mechanical instability are quietly different. MRI is an important tool in assessing the status of the lateral ankle ligaments. However, the published sensitivity and specificity of MRI in directly diagnosing chronic anterior talofibular ligament and calcaneofibular ligament injuries are 44-75%, and 53-86%, respectively. Therefore, it is necessary to explore indirect MRI signs to better identify functional ankle instability and mechanical ankle instability,” they wrote.

The researchers found that functional instability patients didn’t have a posteriorly positional fibula and an internally rotated talus. In addition, the malleolar talus index was significantly larger among mechanical instability patients than among functional instability patients.

The study included 61 patients with functional ankle instability and 68 patients with mechanical instability as well as 60 healthy volunteers. The rotation of the talus was identified using the Malleolar Talus Index.

The researchers measured the position relative to the talus (Axial Malleolar Index) and medial malleolus (Intermalleolar Index).

They found that the values of malleolar talus index were larger among mechanical instability (89.18⁰±2.31⁰) than that in functional instability patients (86.55⁰±61.65⁰; p < 0.001) and healthy volunteers (85.59⁰±2.42⁰; p < 0.001).

The axial malleolar index of the mechanical instability patients (11.39⁰±1.41⁰) was also larger than healthy volunteers (7.91⁰±0.83⁰; p < 0.0001). Overall, there were no statistically significant differences in the three indexes between the functional instability patients and healthy volunteers.

“Increased malleolar talus index may become a new indirect MRI sign for identifying functional and mechanical instability patients,” the researchers wrote.