Morphological variability of the fibularis longus and brevis muscles on magnetic resonance imaging

The fibularis longus (FLT) and fibularis brevis (FBT) tendons form the dynamic stabilizers of the lateral ankle. While their anatomical variability has been well described in cadaveric studies, corresponding imaging data remain limited. This study aimed to characterize the morphology and positional relationships of the FLT and FBT using conventional magnetic resonance imaging (MRI) and to compare these findings with existing anatomical classifications.

A retrospective analysis of 120 ankle MRI examinations (65 females, 55 males) was performed. The morphology of the FLT and FBT and their positional relationships at the lateral malleolus were assessed on axial sections, and distal insertions were evaluated on coronal and sagittal images.

Both tendons were identified in all cases. The FBT exhibited four morphologies - flat (63.3%), flat with medial bulge (14.2%), flat with lateral bulge (10%), and oval (12.5%) - while the FLT was predominantly oval (79.2%). Four positional patterns were recorded, the most common being FBT overlapping the FLT with medial protrusion (49.2%). Distal bifurcated insertions were observed in 10% of FBT and 14.2% of FLT cases. No significant differences were found between sides, sex, or age.

The most important finding was that MRI reliably detects the morphological and positional variability of the fibularis tendons. These findings validate cadaveric classifications, highlight population-specific patterns, and emphasize the clinical importance of distinguishing normal variants from pathology in ankle imaging.