# The fibularis longus muscle revisited: comparative anatomy, developmental perspectives, and clinical relevance

## TL;DR

This paper examines the fibularis longus tendon's anatomical variations and proposes a unified classification system to improve diagnosis and surgical planning.

## Contribution

A unified classification system for fibularis longus tendon variants that integrates fetal and adult anatomy with clinical implications.

## Key findings

- Three main distal insertion types of the fibularis longus tendon were identified: Type I, II, and III.
- Trifurcated Type III is observed only in prenatal specimens, while adult Type III results from postnatal fusion.
- Classification-aware interpretation can reduce diagnostic errors and improve surgical outcomes.

## Abstract

The fibularis longus tendon (FLT) shows substantial anatomical variability, yet its clinical and developmental implications are incompletely characterised. Classification systems derived from fetal and adult morphology may improve diagnostic interpretation and surgical planning.

To synthesise comparative, ontogenetic, radiological and surgical perspectives on the FLT into a unified, classification-aware framework.

Anatomical data from two previously published cohorts—one fetal (n = 94 lower limbs) and one adult (n = 100 lower limbs)—were reviewed to evaluate distal insertion morphotypes. Findings were correlated with imaging literature (MRI, ultrasound) and appraised for diagnostic pitfalls and practical applicability.

Across fetal and adult material, three principal distal insertion types were identified: Type I (single insertion), Type II (bifurcated) and Type III (trifurcated in fetuses or fusion variants in adults). Trifurcated Type III appears confined to prenatal specimens, whereas adult Type III reflects secondary postnatal fusion with neighbouring structures. Radiological correlation highlights recurring misinterpretations involving accessory bands and fusion patterns. Classification-aware interpretation suggests that surgical risk can differ by FLT type, particularly in tendoscopic assessment and tendon transfer planning.

A unified classification aligning fetal and adult variants provides a clinically relevant scaffold for preoperative planning, radiological reporting and anatomical research. Consistent recognition and reporting of FLT subtypes may reduce diagnostic error, inform procedural strategy and enhance anatomical education.