Fibularis Tertius Tendon: Anatomy, Function, Clinical Significance, MRI & CT Imaging

Topic

Fibularis tertius tendon

The fibularis tertius tendon is the distal tendon of the fibularis tertius muscle, located in the anterior compartment of the leg. It is a small, variable structure present in most but not all individuals (absent in about 5–10% of the population). The tendon runs along the anterior border of the lateral malleolus and inserts on the dorsum of the fifth metatarsal base.

Functionally, the fibularis tertius tendon contributes to ankle dorsiflexion and eversion of the foot, assisting in stabilization during gait. Although not as powerful as tibialis anterior or fibularis longus/brevis, its tendon provides important proprioceptive and stabilizing functions in dynamic ankle mechanics.

Synonyms

Peroneus tertius tendon
Distal tendon of fibularis tertius

Origin, Course, and Insertion

Origin (muscle): Lower third of the anterior surface of the fibula and the adjacent interosseous membrane
Course: The muscle fibers form a tendon in the distal leg, which passes deep to the superior and inferior extensor retinacula along with extensor digitorum longus tendons
Insertion: Dorsal surface of the base of the fifth metatarsal, sometimes extending to its shaft

Tendon Attachments

Distal attachment at the dorsum of the fifth metatarsal
May have accessory slips to adjacent dorsal metatarsals in anatomical variants

Relations

Superiorly: Fibularis tertius muscle belly, extensor digitorum longus tendons
Inferiorly: Dorsum of the foot, fifth metatarsal base
Medially: Extensor digitorum longus tendons
Laterally: Fibularis brevis tendon, lateral malleolus

Function

Ankle dorsiflexion: Assists tibialis anterior in raising the foot
Foot eversion: Contributes to lateral balance by everting the foot
Gait stabilization: Active during swing phase to clear toes and stabilize lateral foot border
Protective role: Prevents excessive inversion, reducing ankle sprain risk

Clinical Significance

Tendon injuries: Rare, but tears or avulsions may occur with ankle trauma
Anatomical absence: Present in ~90–95% of individuals; its absence does not significantly impair function
Overuse injuries: May present with lateral foot pain or stress at the fifth metatarsal insertion
Imaging relevance: MRI and CT useful for detecting tendon tears, tendinopathy, and variants

MRI Appearance

T1-weighted images:
- Normal tendon: low signal (dark band) running to fifth metatarsal base
- Muscle belly: intermediate signal intensity
- Partial tear: focal bright hyperintense changes within tendon fibers
- Complete tear: discontinuity with retracted fibers
T2-weighted images:
- Normal tendon: low signal, darker than on T1
- Muscle belly: intermediate signal, darker than on T1
- Tendinopathy or tear: bright hyperintense signal within tendon or at insertion site
- Peritendinous edema: hyperintense surrounding tendon sheath
STIR:
- Normal tendon: dark
- Pathology: tendon tears or peritendinous inflammation appear bright hyperintense
Proton Density Fat-Saturated (PD FS):
- Normal tendon: homogeneously dark
- Tears: linear bright signal within tendon substance or at insertion
- Best for subtle partial tears and inflammatory thickening
T1 Fat-Sat Post-Contrast:
- Normal tendon: minimal or no enhancement
- Tendinitis or peritendinitis: enhancement of sheath or peritendinous tissues
- Chronic tears: enhancement at margins due to scar tissue

CT Appearance

Non-Contrast CT:

Tendon: seen as a thin linear soft-tissue density extending to fifth metatarsal
Muscle belly: soft tissue density, blending with other anterior compartment muscles
Pathology: thickening, discontinuity, or adjacent soft tissue stranding in tendon injury

Post-Contrast CT:

Muscle belly enhances uniformly
Peritendinitis may show enhancement of soft tissues around tendon
Chronic tendinopathy may demonstrate thickened tendon and calcification at insertion

MRI image

CT image