Fibularis Tertius

The fibularis tertius, also known as peroneus tertius, is a small muscle located in the distal part of the anterior compartment of the leg. It is closely associated with the extensor digitorum longus and is often considered a partially separated portion of that muscle. Its tendon crosses the anterior aspect of the ankle and inserts onto the dorsal surface of the base of the fifth metatarsal.^[1][2]

Fibularis tertius contributes to dorsiflexion of the ankle and eversion of the foot. Although it is considerably smaller than the major dorsiflexors and evertors, its position allows it to assist both movements and contribute to dynamic control of the lateral side of the foot.

The muscle is supplied by the deep fibular nerve, distinguishing it from fibularis longus and fibularis brevis, which belong to the lateral compartment and are supplied by the superficial fibular nerve. This difference is important because the name fibularis tertius can suggest a closer functional and developmental relationship with the lateral compartment muscles than its actual compartmental anatomy indicates.

Location

Fibularis tertius lies in the anterior compartment of the leg, primarily within its distal portion. It is positioned lateral to the distal fibers and tendons of extensor digitorum longus.

The muscle arises from the fibula and adjacent interosseous membrane, descends toward the ankle, and becomes tendinous before crossing onto the dorsum of the foot.

Its tendon passes beneath the extensor retinacula and continues toward the lateral border of the foot, where it attaches to the fifth metatarsal.

General Anatomy

Feature	Anatomy
Muscle	Fibularis tertius
Alternative name	Peroneus tertius
Compartment	Anterior compartment of leg
Origin	Distal anterior fibula and adjacent interosseous membrane
Insertion	Dorsal surface of base of fifth metatarsal
Innervation	Deep fibular nerve
Root values	L5, S1
Blood supply	Anterior tibial artery
Principal actions	Dorsiflexion and eversion of foot

Origin

Fibularis tertius typically arises from the distal third of the anterior surface of the fibula and the adjacent portion of the interosseous membrane.^[1][3]

Additional fibers may arise from the anterior intermuscular septum and surrounding deep fascia. The proximal extent of the muscle varies considerably among individuals.

Its fibers are often continuous proximally with extensor digitorum longus, reflecting the close anatomical relationship between the two muscles.

Insertion

The tendon of fibularis tertius inserts onto the dorsal surface of the base of the fifth metatarsal, often extending onto the adjacent portion of the shaft.

This attachment should be distinguished from that of fibularis brevis, which inserts onto the tuberosity at the base of the fifth metatarsal from a more lateral direction.

The differing tendon courses and insertion sites reflect the different compartments and actions of the two muscles.

Course

From its origin on the distal fibula, fibularis tertius descends through the anterior compartment and becomes tendinous above the ankle.

The tendon passes anterior to the ankle joint and beneath the superior and inferior extensor retinacula. It then travels inferolaterally across the dorsum of the foot toward the fifth metatarsal.

Because the tendon crosses anterior to the ankle axis and approaches the foot from the lateral side, its line of pull produces both dorsiflexion and eversion.

Relationship to Extensor Digitorum Longus

Fibularis tertius is closely associated with the extensor digitorum longus. The two muscles may be partially continuous proximally, and fibularis tertius is frequently described as a separated distal portion of extensor digitorum longus.

Both muscles lie in the anterior compartment, receive innervation from the deep fibular nerve, and are supplied predominantly by branches of the anterior tibial artery.

Their distal attachments differ. Extensor digitorum longus continues to toes 2 through 5, while fibularis tertius terminates at the fifth metatarsal.

Relationship to Fibularis Longus and Fibularis Brevis

Despite sharing the name fibularis, fibularis tertius differs significantly from fibularis longus and fibularis brevis.

Fibularis longus and fibularis brevis occupy the lateral compartment of the leg and are supplied by the superficial fibular nerve. Fibularis tertius occupies the anterior compartment and is supplied by the deep fibular nerve.

Feature	Fibularis Tertius	Fibularis Brevis	Fibularis Longus
Compartment	Anterior	Lateral	Lateral
Innervation	Deep fibular nerve	Superficial fibular nerve	Superficial fibular nerve
Insertion	Dorsal base of fifth metatarsal	Tuberosity of fifth metatarsal	Base of first metatarsal and medial cuneiform
Ankle action	Dorsiflexion	Plantarflexion	Plantarflexion
Eversion	Assists	Strongly assists	Strongly assists

Relationship to the Fibula

Fibularis tertius arises directly from the distal anterior surface of the fibula. The fibula therefore provides the principal bony origin of the muscle.

Its position on the anterior aspect of the fibula distinguishes it from fibularis brevis and fibularis longus, which arise predominantly from more lateral surfaces of the bone.

The fibula also separates portions of the anterior and lateral muscular compartments and provides attachment for the intermuscular septa that help define these compartments.

Relationship to the Interosseous Membrane

Part of the origin of fibularis tertius arises from the interosseous membrane between the tibia and fibula.

The interosseous membrane contributes to the posterior boundary of the anterior compartment and provides attachment for several muscles, including tibialis anterior, extensor hallucis longus, and extensor digitorum longus.

The association of fibularis tertius with this membrane further reflects its anatomical membership in the anterior compartment.

Relationship to the Extensor Retinacula

The tendon of fibularis tertius crosses the anterior ankle beneath the superior and inferior extensor retinacula.

The superior extensor retinaculum spans the distal leg proximal to the ankle joint. The inferior extensor retinaculum lies across the anterior ankle and proximal dorsum of the foot.

These structures maintain the tendon close to the underlying skeleton during dorsiflexion and reduce bowstringing when fibularis tertius contracts.

Relationship to the Fifth Metatarsal

The fifth metatarsal receives attachments from both fibularis tertius and fibularis brevis, but at different locations.

Fibularis brevis inserts on the prominent tuberosity at the base of the fifth metatarsal. Fibularis tertius generally attaches to the dorsal surface of the base and may extend onto the proximal shaft.

This distinction is important when evaluating injuries around the base of the fifth metatarsal.

Anterior Compartment of the Leg

Fibularis tertius is one of four muscles commonly included within the anterior compartment:

• Tibialis anterior
• Extensor hallucis longus
• Extensor digitorum longus
• Fibularis tertius

These muscles are supplied by the deep fibular nerve and primarily perform dorsiflexion or extension-related functions.

Fibularis tertius differs from the digital extensors because it terminates on a metatarsal rather than continuing into a toe.

Innervation

Fibularis tertius is innervated by the deep fibular nerve, with root contributions commonly described as L5 and S1.

The deep fibular nerve is a terminal branch of the common fibular nerve. It enters the anterior compartment of the leg and descends with the anterior tibial vessels.

The same nerve supplies tibialis anterior, extensor hallucis longus, and extensor digitorum longus.

Deep Fibular Nerve

The deep fibular nerve passes into the anterior compartment after branching from the common fibular nerve near the proximal fibula.

It descends through the compartment with the anterior tibial artery and supplies the anterior compartment muscles through muscular branches.

Distally, the nerve crosses the ankle and enters the dorsum of the foot, where it provides motor supply to extensor digitorum brevis and extensor hallucis brevis and sensory innervation to the first dorsal web space.

Blood Supply

The principal blood supply of fibularis tertius comes from muscular branches of the anterior tibial artery.

The anterior tibial artery enters the anterior compartment through the proximal interosseous membrane and descends toward the ankle with the deep fibular nerve.

Distally, it continues onto the dorsum of the foot as the dorsalis pedis artery.

Actions

Dorsiflexion

Fibularis tertius assists dorsiflexion of the ankle. Its tendon passes anterior to the transverse axis of the ankle joint, so contraction pulls the dorsum of the foot toward the anterior leg.

Its dorsiflexion force is relatively modest compared with tibialis anterior and the larger long extensor muscles.

It nevertheless contributes to the combined action of the anterior compartment during walking and other movements requiring clearance of the foot from the ground.

Eversion

Fibularis tertius also assists eversion of the foot. Its tendon passes toward the lateral side of the foot and inserts on the fifth metatarsal.

This lateral line of pull opposes the inversion tendency of tibialis anterior during dorsiflexion.

The muscle works with fibularis longus, fibularis brevis, and extensor digitorum longus to contribute to eversion, although the lateral compartment fibular muscles are substantially stronger evertors.

Functional Balance During Dorsiflexion

Pure dorsiflexion of the ankle requires coordinated muscular activity because individual dorsiflexors also produce secondary movements.

Tibialis anterior combines dorsiflexion with inversion. Fibularis tertius and extensor digitorum longus combine dorsiflexion with an evertor component.

When these muscles contract together, their opposing inversion and eversion forces can partially cancel while their dorsiflexion forces reinforce one another.

Role During Walking

During the swing phase of gait, fibularis tertius assists the other anterior compartment muscles in dorsiflexing the ankle. This contributes to clearance of the foot as the lower limb advances.

During initial contact and loading, the dorsiflexor group can contract eccentrically to help control the lowering of the forefoot toward the ground.

The evertor component of fibularis tertius may also contribute to fine control of foot position during these transitions.

Role in Foot Stability

The attachment of fibularis tertius to the lateral side of the foot allows it to contribute to dynamic stabilization during changes in foot position.

Its force is much smaller than that of the major fibular muscles, but it can assist in controlling excessive inversion when the anterior compartment is active.

This function is particularly relevant during walking or running on uneven surfaces, when rapid adjustments in foot position are required.

Anatomical Variations

Fibularis tertius is notably variable and may be absent in some individuals. When present, its muscle belly, tendon size, and insertion can vary considerably.

The tendon may insert over a broader region of the fifth metatarsal or occasionally have additional slips to neighboring metatarsals or fascial structures.

Variation also occurs in the degree of separation between fibularis tertius and extensor digitorum longus. In some individuals, the two muscles are strongly integrated proximally.

These variations are generally compatible with normal lower-limb function because other dorsiflexors and evertors can compensate for the absence or reduced size of fibularis tertius.

Clinical Significance

Fifth Metatarsal Injuries

The insertion of fibularis tertius on the base of the fifth metatarsal makes the muscle relevant when assessing fractures and other injuries in this region.

Its attachment should be distinguished from the insertion of fibularis brevis on the fifth metatarsal tuberosity. Different fracture patterns can involve different portions of the proximal fifth metatarsal and therefore have different relationships to these tendons.

Anterior Compartment Syndrome

Because fibularis tertius lies within the anterior fascial compartment, it can be affected by anterior compartment syndrome.

Increasing pressure within the compartment can impair blood flow and compromise the deep fibular nerve, affecting multiple anterior compartment muscles simultaneously.

Weakness of dorsiflexion may result when muscle or nerve function becomes impaired.

Deep Fibular Nerve Injury

Damage to the deep fibular nerve can weaken fibularis tertius along with tibialis anterior, extensor hallucis longus, and extensor digitorum longus.

Clinically, more obvious findings usually result from loss of the larger dorsiflexors rather than isolated loss of fibularis tertius.

Depending on the level of nerve injury, sensory impairment may also occur within the first dorsal web space.

Foot Drop

Fibularis tertius contributes modestly to ankle dorsiflexion, so its function can be affected in neurological disorders producing foot drop.

Foot drop most commonly reflects dysfunction of the common fibular nerve, deep fibular nerve, relevant spinal roots, or anterior compartment musculature rather than isolated pathology of fibularis tertius.

Tendon Injury

The tendon of fibularis tertius can be affected by trauma around the anterior ankle or dorsolateral foot. Because the tendon is relatively superficial distally, it may be involved in lacerations or other direct injuries.

Isolated rupture produces relatively limited functional loss because other muscles can perform dorsiflexion and eversion.

Tendinopathy

Repetitive loading may produce pain along the course of the fibularis tertius tendon, particularly near the dorsolateral foot or its fifth metatarsal attachment.

Symptoms in this region require distinction from disorders involving fibularis brevis, extensor digitorum longus, the fifth metatarsal, and neighboring joints.

Surgical Anatomy

The tendon of fibularis tertius may be encountered during surgical approaches to the anterior ankle and dorsolateral foot.

Recognition of its insertion is useful for distinguishing it from the fibularis brevis tendon and the lateral tendons of extensor digitorum longus.

Anatomical variation should be anticipated because the muscle can be small, have accessory slips, or be absent.

Imaging

Fibularis tertius can be assessed with ultrasound and MRI. Ultrasound can demonstrate the distal tendon dynamically, while MRI provides visualization of both the muscle belly and tendon.

Imaging may identify tendon injury, muscular edema, atrophy, anatomical variation, or abnormalities involving the adjacent fifth metatarsal.

Surface Anatomy

Fibularis tertius is relatively small and its muscle belly is not usually prominent on surface examination. Its distal tendon may become visible or palpable on the dorsolateral aspect of the foot during resisted dorsiflexion and eversion.

The tendon can be followed toward its attachment on the dorsal aspect of the fifth metatarsal base.

Its position provides a useful anatomical distinction between the anterior compartment extensor tendons and the fibularis brevis tendon approaching the fifth metatarsal from the lateral side.