Prevalence of Fibularis Tertius: Insights from a Surface Anatomical Study

Background The fibularis tertius muscle, a variant muscle in the crural compartment of the leg, is thought to have evolved in humans in response to the development of bipedalism. Acting as both an ankle dorsiflexor and foot everter, it plays a crucial role in enabling efficient terrestrial locomotion, especially in mid-foot biomechanical stabilization. The origin and insertion of this muscle have been reported to exhibit significant variation. Objective The aim is to determine the prevalence of this muscle by conducting a surface anatomical examination of the foot among pre-clinical sciences students at Kathmandu University School of Medical Sciences. Method Each participant's fibularis tertius muscle (FTM) was assessed on both feet using a standardized surface palpation technique based on protocols that Tixa and Kendall had validated. To ensure accuracy, each foot was subjected to two separate evaluations by qualified evaluators that lasted 120 seconds each. During dorsiflexion and eversion, muscles were identified using sequential palpation techniques. Visibility was categorized into three graded responses (G1-G3) according to muscle activation. SPSS version 23 was used to analyze the data. While the Chi-square test evaluated sex-based associations, with statistical significance set at p < 0.05, descriptive statistics summarized prevalence. Result A total of 226 students (54.42% males, 45.58% females; mean age 20.8 ± 1.88 years) participated in the study. The fibularis tertius muscle had a prevalence of 95.58%, with a similar gender distribution. It was bilateral in 187 participants and unilateral in 29, mostly on the right foot. Multivariable logistic regression revealed no significant association between fibularis tertius presence and body mass index, with both crude and adjusted odds ratios (0.83 and 0.89, respectively) and p-values exceeding 0.05. Conclusion The fibularis tertius muscle is essential for ankle stability, reducing injury risk and aiding recovery during high-impact activities. Its absence increases instability and recurrent sprains. Understanding the anatomy of fibularis tertius muscle is crucial for surgical planning, tendon repair, and rehabilitation, influencing diagnosis, treatment, and injury prevention.