Purpose The 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).

Methods We analysed 130 normal peroneus brevis tendons (power >0.8, α = 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 Cohen’s kappa, Gwet’s AC1, and Fleiss’ kappa. We evaluated classification robustness using F1 score, receiver operating characteristic analysis, and precision-recall metrics.

Results Consensus 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). Cohen’s kappa ranged from 0.59 to 0.87, and Gwet’s 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.

Conclusion This proposed classification is robust and reliable, offering a validated framework for consistent tendon evaluation in both clinical and research settings.

The authors have declared no competing interest.

This study was funded by Stiftelsen Tornspiran (934: 2023-12-01), Sweden and by the Swedish state under the agreement between the Swedish Government and the county councils, the ALF-agreement (ALFGBG-1006741).

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Swedish Ethical Authority gave ethical approval for this work, number 2024-07283-02.

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All data produced in the present study are available upon reasonable request to the authors.