Fibularis Tertius Muscle: Anatomy, Function, MRI & CT Imaging

Topic

Fibularis tertius muscle

The fibularis tertius (peroneus tertius) is a small, variable muscle located in the anterior compartment of the leg. It is often considered part of the extensor digitorum longus but has a distinct tendon that inserts into the dorsum of the 5th metatarsal.

Its function is to assist in ankle dorsiflexion and eversion of the foot, providing lateral stability during gait. The muscle is absent in about 5–10% of the population, and while its absence is usually asymptomatic, it has clinical importance in ankle biomechanics and surgical planning for tendon transfers.

Synonyms

Peroneus tertius muscle
Fifth metatarsal extensor

Origin, Course, and Insertion

Origin: Distal third of the anterior surface of the fibula and adjacent interosseous membrane
Course: Fibers descend along the anterior compartment and form a slender tendon
Insertion: Dorsal surface of the base and shaft of the 5th metatarsal bone

Tendon Attachments

Courses deep to the superior and inferior extensor retinacula
Inserts on the dorsum of the 5th metatarsal
Sometimes gives slips to the 4th metatarsal

Relations

Superiorly: Extensor digitorum longus muscle
Inferiorly: Dorsum of the foot near the 5th metatarsal
Medially: Extensor hallucis longus tendon
Laterally: Fibularis brevis and fibularis longus tendons
Posteriorly: Fibula and interosseous membrane

Nerve Supply

Deep fibular (peroneal) nerve (L5–S1), branch of the common fibular nerve

Arterial Supply

Anterior tibial artery and its muscular branches

Venous Drainage

Anterior tibial vein draining into the popliteal vein

Function

Dorsiflexion of ankle: Assists tibialis anterior in lifting the foot
Eversion of foot: Works with fibularis longus and brevis to stabilize the lateral foot
Gait stability: Prevents excessive inversion during walking and running
Clinical role: Absence may alter biomechanics; hypertrophy or injury may mimic lateral ankle pathology

Clinical Significance

Variable muscle: Absent in ~5–10% of individuals
Tendon injuries: May be injured with lateral ankle sprains or fractures
Overuse injuries: Can cause lateral foot pain in athletes and runners
Surgical importance: Tendon may be used in transfers for chronic ankle instability or dorsiflexion weakness
Imaging role: MRI and CT important in identifying tears, hypertrophy, or anatomical variants

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity
- Tendon: low signal (dark band)
- Pathology: partial tears show focal hyperintensity; complete ruptures show tendon discontinuity
T2-weighted images:
- Muscle: darker than on T1, but still intermediate
- Tendon: very low signal (black)
- Pathology: tears or tendinopathy appear bright hyperintense within tendon; edema shows as high signal in surrounding soft tissues
STIR:
- Normal muscle: intermediate to dark
- Pathology: bright hyperintensity in muscle belly or tendon (edema, inflammation, tears)
Proton Density Fat-Saturated (PD FS):
- Muscle: intermediate to dark
- Normal tendon: uniformly dark
- Tears or tendinitis: focal bright signals in tendon or sheath; excellent for subtle peritendinitis
T1 Fat-Sat Post-Contrast:
- Normal muscle and tendon: minimal or no enhancement
- Pathology: tendinitis, peritendinitis, or synovitis enhance brightly
- Complete ruptures show a gap with peripheral enhancement