Anatomical variations of the fibularis tertius accessory tendon with insertion into the fifth digit: case reports and...

The fibularis tertius (FT) and extensor digitorum longus (EDL) muscles exhibit considerable anatomical variability, which may influence surgical approaches and clinical outcomes. This study aims to report a rare anatomical variation of the fibularis tertius accessory tendon (FTAT) and to explore its clinical and developmental significance.

Two cadaveric cases were examined. Morphological features of the FTAT, including its origin, course, insertion, and dimensions (length, thickness, and width), were documented.

In Case 1, the FTAT was observed in the right leg of a 94-year-old female cadaver. The FTAT originated from the fibularis tertius muscle, branched from the main tendon 33.4 mm distal to the musculotendinous junction, and inserted into the fifth digit. It measured 92.5 mm in length, 1.1 mm in thickness, and 6.4 mm in width. In Case 2, the FTAT was identified in the left leg of a 79-year-old female cadaver. It originated independently from the fibularis tertius muscle and coursed separately to the fifth digit. A thin connecting slip measuring 61.0 mm in length was present between the FTAT and the EDL tendon. The FTAT measured 179.0 mm in length, 1.0 mm in thickness, and 3.9 mm in width.

This variation of the FTAT, characterized by a distinct accessory tendon with a rare insertion into the fifth digit, may have clinical relevance in foot surgery. Awareness of such morphology is important for accurate diagnosis and safe surgical planning.