Tendon of fibularis tertius

The fibularis tertius muscle (Peroneus tertius) is located in the anterior compartment of the leg and is unique to humans. Its origin is most commonly from the distal half or distal third of the anterior surface of the fibula, and it may also arise from the interosseous membrane or, less frequently, from the tendon of the extensor digitorum longus. The muscle belly is typically slender and lies lateral to the extensor digitorum longus.

The muscle belly transitions into a tendon (Tendon of fibularis tertius) in the distal leg, which passes anterior to the lateral malleolus.

The most common insertion site for the tendon is the dorsal surface of the base of the fifth metatarsal. However, significant anatomical variation exists: the tendon may insert onto the shaft of the fifth metatarsal, the base of the fourth metatarsal, or even extend to the dorsal digital expansion or proximal phalanx of the fifth toe.The tendon may be single or bifurcated, with accessory slips occasionally present, and its distal attachment can be fan-shaped or band-shaped.