Anatomical variations of the peroneus brevis tendon on axial MRI: A validated cross-sectional classification at the lateral malleolus

PurposeThe shape of the peroneus brevis tendon may contribute to the development of split tears, but a classification of its anatomical variants on transverse section in vivo is lacking. This study aimed to validate a proposed classification system for peroneus brevis tendon morphology on the transverse cross section on magnetic resonance imaging (MRI). MethodsWe analysed 130 normal peroneus brevis tendons (power >0.8, = 0.05, effect size = 0.31) on consecutively selected ankle MRI performed at 3T. We excluded patients with fractures, ligament injuries, or peroneal tendon pathology. Seven independent raters classified tendons into four morphological categories: general flat, flat with a lateral bulge, flat with a medial bulge, and oval. Inter-rater reliability was assessed using Cohens kappa, Gwets AC1, and Fleiss kappa. We evaluated classification robustness using F1 score, receiver operating characteristic analysis, and precision-recall metrics. ResultsConsensus classification showed 37.7% of tendons were general flat, 20.8% flat with a lateral bulge, 19.2% flat with a medial bulge, and 22.3% oval. Inter-rater agreement was substantial, with Fleiss kappa of 0.69 (95% CI 0.63-0.75). Cohens kappa ranged from 0.59 to 0.87, and Gwets AC1 from 0.58 to 0.87, confirming strong pairwise agreement. Classification performance was high (area under the curve = 0.89), and majority voting further improved agreement (F1 score = 0.93). The most common misclassification occurred between oval and flat with a medial bulge. ConclusionThis proposed classification is robust and reliable, offering a validated framework for consistent tendon evaluation in both clinical and research settings.