Extensor Digitorum Brevis

Extensor digitorum brevis is a thin intrinsic muscle situated on the dorsum of the foot. It arises from the superolateral region of the calcaneus and divides into tendinous slips that pass toward the second, third, and fourth toes. These slips join the tendons of extensor digitorum longus and assist extension of the lateral four toes, although extensor digitorum brevis itself usually has no tendon to the fifth toe.^[1][2]

The muscle occupies the lateral part of the dorsal foot beneath the long extensor tendons. Its medial portion is closely associated with extensor hallucis brevis, which extends the great toe. The two are developmentally and anatomically continuous and are often regarded as parts of a single muscular complex. In standard anatomical description, the most medial portion is named extensor hallucis brevis because its tendon has a distinct insertion into the great toe.

Extensor digitorum brevis is also an important surface and clinical landmark on the dorsum of the foot. Its muscle belly lies near branches of the dorsalis pedis artery and the deep fibular nerve, and its position is relevant in dorsal surgical approaches and interpretation of soft tissue masses in this region.

Location

Extensor digitorum brevis lies on the dorsolateral aspect of the foot. Its fleshy proximal portion occupies the region anterior to the lateral malleolus and extends across the dorsal surfaces of the tarsal bones toward the metatarsals.

The muscle is covered by the deep fascia of the dorsum of the foot and crossed superficially by the tendons of extensor digitorum longus. Its deeper surface is related to the calcaneus and other tarsal structures.

Because the muscle arises within the foot rather than the leg, it is classified as an intrinsic muscle of the foot, despite acting on the toes in conjunction with the extrinsic long extensor muscles.

Origin

Extensor digitorum brevis arises principally from the superolateral surface of the calcaneus, anterior to the groove for the fibularis brevis tendon. Additional fibers arise from adjacent connective tissues, including the inferior extensor retinaculum.^[1][3]

The origin is fleshy and relatively broad. From this proximal attachment, the muscle passes anteriorly and medially across the dorsum of the foot before separating into muscular and tendinous components directed toward the toes.

Insertion

The muscle usually divides into three tendinous slips for the second, third, and fourth toes. Each tendon approaches the corresponding tendon of extensor digitorum longus on the dorsum of the toe and joins the extensor apparatus.

The slips of extensor digitorum brevis therefore do not usually form independent distal insertions onto the phalanges. Instead, they blend with the long extensor tendons and contribute through the dorsal digital expansions.

The most medial part of the muscular complex forms extensor hallucis brevis. Unlike the slips to the lateral toes, its tendon inserts separately onto the dorsal aspect of the base of the proximal phalanx of the great toe.

Part	Insertion
Medial component	Forms extensor hallucis brevis and inserts on the proximal phalanx of the great toe
First EDB slip	Joins the extensor tendon of the second toe
Second EDB slip	Joins the extensor tendon of the third toe
Third EDB slip	Joins the extensor tendon of the fourth toe

Relations

The position of extensor digitorum brevis places it in close relationship with the tendons, nerves, and vessels crossing the dorsum of the foot.

Superficial Relations

The muscle is covered by skin, superficial fascia, and deep fascia. The tendons of extensor digitorum longus cross superficial to it as they travel toward the lateral four toes.

The superficial fibular nerve divides into cutaneous branches on the dorsum of the foot, and these branches lie within the more superficial tissues overlying the region.

Deep Relations

Deep to extensor digitorum brevis are the bones and joints of the dorsal tarsal region. Its proximal attachment is directly related to the calcaneus, while its more distal fibers cross over structures of the lateral and central midfoot.

Medial Relations

The medial part of the muscle is continuous with extensor hallucis brevis. The dorsalis pedis artery passes along the dorsum of the foot medial to much of the extensor digitorum brevis muscle belly and is closely related to its medial border.

The deep fibular nerve accompanies the dorsalis pedis artery in the dorsal foot and gives a lateral terminal branch that supplies extensor digitorum brevis.

Innervation

Extensor digitorum brevis is supplied by the deep fibular nerve, also called the deep peroneal nerve. More specifically, innervation is provided by its lateral terminal branch after the nerve crosses the ankle and enters the dorsum of the foot.^[1][2]

The predominant spinal root contribution is L5 and S1, although descriptions of segmental contribution can vary between sources.

The deep fibular nerve originates from the common fibular nerve and descends through the anterior compartment of the leg with the anterior tibial artery. At the ankle it passes deep to the extensor retinacula and continues onto the foot, where it divides into medial and lateral terminal branches.

The medial terminal branch continues toward the first interdigital space and provides cutaneous innervation there. The lateral terminal branch expands beneath extensor digitorum brevis and provides motor branches to extensor digitorum brevis and extensor hallucis brevis.

Blood Supply

The blood supply of extensor digitorum brevis is derived primarily from branches of the dorsalis pedis artery and its lateral tarsal branch.^[1][3]

The lateral tarsal artery passes laterally across the tarsal region beneath or in close association with extensor digitorum brevis. It supplies the muscle and participates in arterial communications on the dorsum and lateral side of the foot.

Additional small arterial contributions may arise from neighboring dorsal and lateral vascular networks. The precise pattern and caliber of these branches vary between individuals.

Actions

The principal action of extensor digitorum brevis is to assist extension of the second, third, and fourth toes. It acts together with extensor digitorum longus, which provides the major extrinsic extensor force to the lateral four toes.

Because the tendons of extensor digitorum brevis blend with the extensor apparatus, their force contributes to extension across the joints of the toes. The muscle's effect is supplementary to that of the long extensor tendons rather than functioning as an isolated primary mover.

Extensor digitorum brevis normally does not supply the fifth toe. Extension of the fifth toe is therefore principally produced through the extensor digitorum longus tendon and its associated extensor expansion.

Relationship to Extensor Hallucis Brevis

Extensor hallucis brevis is closely associated with extensor digitorum brevis and is commonly considered its medial component. Both arise from the calcaneal region and are supplied by the lateral terminal branch of the deep fibular nerve.

The major distinction is their distal attachment. Extensor hallucis brevis has a separate tendon that passes to the great toe and inserts on the dorsal base of the proximal phalanx. Extensor digitorum brevis sends its usual three slips toward the second through fourth toes, where they join the long extensor tendons.

Feature	Extensor Digitorum Brevis	Extensor Hallucis Brevis
Position	Lateral portion of the dorsal muscular complex	Medial portion
Digits supplied	Second to fourth toes	Great toe
Distal attachment	Joins extensor digitorum longus tendons	Base of proximal phalanx of great toe
Innervation	Deep fibular nerve	Deep fibular nerve
Primary action	Assists extension of toes 2 to 4	Extends great toe at metatarsophalangeal joint

Relationship to the Long Extensor Tendons

The tendons of extensor digitorum longus descend through the anterior compartment of the leg, cross the ankle beneath the extensor retinacula, and fan out across the dorsum of the foot toward the lateral four toes.

Extensor digitorum brevis lies beneath these tendons. Its own tendons approach the second, third, and fourth extensor digitorum longus tendons from the lateral side and blend with them as they continue into the dorsal digital expansions.

This arrangement allows the intrinsic short extensor to supplement the action of an extrinsic muscle originating in the leg.

Relationship to the Deep Fibular Nerve

The deep fibular nerve enters the foot deep to the extensor retinacula and divides into medial and lateral terminal branches. The lateral branch passes beneath extensor digitorum brevis and supplies the muscle.

This relationship gives extensor digitorum brevis clinical value when evaluating the motor function of the deep fibular nerve. Because its motor supply is derived from the nerve after it reaches the ankle region, the muscle can provide information about the integrity of this distal motor pathway.

The close association of the nerve with the muscle also means that procedures involving the dorsolateral foot must account for small motor branches entering the deep surface of extensor digitorum brevis.

Anatomical Variations

Extensor digitorum brevis varies in the number and arrangement of its muscular and tendinous slips. One or more slips may be reduced, absent, duplicated, or directed toward a different digit.

A slip to the fifth toe may occasionally occur, although the usual pattern consists of tendons to the second, third, and fourth toes. Variation can also occur in the degree of separation between extensor digitorum brevis and extensor hallucis brevis.

Accessory muscular slips on the dorsum of the foot may occasionally resemble soft tissue masses on clinical examination or imaging. Recognition of normal muscular variation can therefore prevent misinterpretation of an accessory muscle as a pathological lesion.

Clinical Significance

Deep Fibular Nerve Injury

Injury to the deep fibular nerve can weaken or paralyze extensor digitorum brevis. The clinical significance depends on the level of the lesion because more proximal injuries can also affect the anterior compartment muscles of the leg.

Loss of extensor digitorum brevis alone produces relatively limited functional impairment because extensor digitorum longus remains capable of extending the lateral toes if its innervation is intact.

Electrodiagnostic Examination

Extensor digitorum brevis is commonly used in nerve conduction studies and electromyography. Its superficial location and reliable innervation by the deep fibular nerve make it accessible for recording motor responses.

In motor nerve conduction testing of the fibular nerve, recordings may be obtained from extensor digitorum brevis while the nerve is stimulated at different points along its course. The resulting responses can contribute to assessment of conduction through the common and deep fibular nerve pathways.

Interpretation must account for anatomical variation and for the possibility that the muscle may be small or have variant innervation in some individuals.

Accessory Deep Fibular Innervation

A recognized anatomical variation is the accessory deep fibular nerve, in which fibers usually traveling with the superficial fibular nerve reach the dorsum of the foot and contribute to innervation of extensor digitorum brevis.

This variation is particularly relevant to electrodiagnostic testing because motor responses may persist despite stimulation patterns that would otherwise suggest interruption of the conventional deep fibular pathway.

Dorsal Foot Surgery

The muscle may be encountered during surgical approaches to the dorsal and lateral regions of the foot. Its relationship to the dorsalis pedis arterial system and terminal branches of the deep fibular nerve is particularly important when deeper structures are exposed.

Identification of the muscle belly can also help orient the surgeon to the local anatomy of the dorsal tarsal region.

Imaging

Extensor digitorum brevis can be identified on MRI, CT, and ultrasound as a muscle occupying the dorsolateral surface of the tarsal region. Its tendons extend toward the central toes beneath the long extensor tendons.

Imaging can demonstrate muscle atrophy associated with denervation as well as anatomical variants. Because accessory or hypertrophied muscular tissue on the dorsum of the foot can produce a focal contour, familiarity with the normal appearance of extensor digitorum brevis and its variants is useful when interpreting this region.

Extensor digitorum brevis provides a short intrinsic extensor pathway from the calcaneus to the extensor apparatus of the second through fourth toes. Its close anatomical association with extensor hallucis brevis, the deep fibular nerve, and the dorsalis pedis arterial system makes it an important landmark of the dorsal foot in addition to its role in assisting toe extension.