Flexor Hallucis Brevis

Flexor hallucis brevis is an intrinsic muscle of the sole that acts on the great toe. It lies in the third layer of plantar muscles and is characterized distally by two heads, medial and lateral, each associated with one of the sesamoid bones beneath the head of the first metatarsal. The two heads insert on opposite sides of the base of the proximal phalanx and act together to flex the great toe at the first metatarsophalangeal joint.^[1][2]

The muscle occupies the medial forefoot and lies deep to the tendon of flexor hallucis longus in much of its course. Distally, the flexor hallucis longus tendon passes between the medial and lateral heads of flexor hallucis brevis and between the two sesamoid bones. This arrangement creates an important muscular, tendinous, and sesamoid complex on the plantar surface of the first metatarsophalangeal joint.

Flexor hallucis brevis is particularly important anatomically because of its relationship to the first ray and sesamoid apparatus. Its medial head blends with structures associated with abductor hallucis, while its lateral head is closely associated with adductor hallucis. Together, these muscles help control the position of the great toe and stabilize the first metatarsophalangeal joint during weight bearing and gait.

Location

Flexor hallucis brevis lies in the medial part of the sole, extending from the distal tarsal region toward the first metatarsophalangeal joint. It is conventionally classified in the third plantar muscular layer with adductor hallucis and flexor digiti minimi brevis.

The muscle is situated deep to structures of the first and second plantar layers. Abductor hallucis lies more superficially along the medial border of the foot, while the tendon of flexor hallucis longus enters the plantar foot from behind the medial malleolus and passes in close relationship to flexor hallucis brevis.

Distally, the muscle divides clearly into medial and lateral heads that lie on either side of the flexor hallucis longus tendon.

Origin

The proximal attachment of flexor hallucis brevis is broader and more complex than its name might suggest. It arises principally from the plantar surfaces of the cuboid and lateral cuneiform, with attachment also associated with the tendon of tibialis posterior and adjacent ligamentous structures.^[1][3]

Descriptions of the precise proximal attachments vary somewhat among anatomical sources, reflecting the broad and partly tendinous nature of the origin. The important anatomical relationship is that the muscle begins in the medial and central midfoot and extends toward the plantar aspect of the first metatarsophalangeal joint.

From its proximal attachment, the muscle passes anteriorly and divides into medial and lateral heads.

Medial Head

The medial head passes along the plantar-medial side of the first metatarsophalangeal joint. Its distal tendon contains, or is closely integrated with, the medial sesamoid bone.

The tendon inserts on the medial side of the base of the proximal phalanx of the great toe. Near its insertion, it blends with the tendon of abductor hallucis, which approaches the hallux from the medial side.

This shared insertional region forms the medial component of the plantar soft tissue complex surrounding the first metatarsophalangeal joint.

Lateral Head

The lateral head passes along the plantar-lateral side of the first metatarsophalangeal joint. Its distal tendon is associated with the lateral sesamoid bone.

It inserts on the lateral side of the base of the proximal phalanx of the great toe. The tendon blends with the insertional fibers of adductor hallucis, which approaches the hallux from the lateral side.

The medial and lateral heads therefore form a paired arrangement around the flexor hallucis longus tendon and the plantar aspect of the first metatarsal head.

Feature	Medial Head	Lateral Head
Position	Plantar-medial side of first metatarsophalangeal joint	Plantar-lateral side of first metatarsophalangeal joint
Sesamoid	Medial sesamoid	Lateral sesamoid
Associated muscle	Abductor hallucis	Adductor hallucis
Insertion	Medial base of proximal phalanx of great toe	Lateral base of proximal phalanx of great toe
Primary action	Flexion of great toe at metatarsophalangeal joint	Flexion of great toe at metatarsophalangeal joint

Sesamoid Apparatus

The relationship between flexor hallucis brevis and the hallucal sesamoids is one of the defining anatomical features of the muscle. Two sesamoid bones are normally present beneath the head of the first metatarsal, one medial and one lateral.

These sesamoids are embedded within the distal tendinous portions of flexor hallucis brevis. They articulate with grooves on the plantar surface of the first metatarsal head and form part of the plantar apparatus of the first metatarsophalangeal joint.

The sesamoids help maintain the flexor tendons at an appropriate distance from the joint axis and modify the direction of muscular forces across the joint. They also participate in transmitting load beneath the first metatarsal head during weight bearing.

The medial and lateral sesamoids are linked by plantar soft tissues, and the flexor hallucis longus tendon passes between them in a distinct groove.

Relationship to Flexor Hallucis Longus

The tendon of flexor hallucis longus has a particularly important relationship to flexor hallucis brevis. After passing behind the medial malleolus and beneath the sustentaculum tali, the tendon continues into the sole toward the great toe.

At the first metatarsophalangeal region, it passes between the medial and lateral heads of flexor hallucis brevis and between their associated sesamoid bones. It then continues distally to insert on the plantar surface of the base of the distal phalanx of the great toe.

The two muscles consequently act at different levels. Flexor hallucis brevis inserts on the proximal phalanx and primarily flexes the first metatarsophalangeal joint, while flexor hallucis longus reaches the distal phalanx and can flex both the metatarsophalangeal and interphalangeal joints.

Feature	Flexor Hallucis Brevis	Flexor Hallucis Longus
Classification	Intrinsic foot muscle	Extrinsic muscle originating in leg
Distal insertion	Base of proximal phalanx	Base of distal phalanx
Sesamoid relationship	Sesamoids embedded in distal tendons	Tendon passes between sesamoids
Main joint action	First metatarsophalangeal flexion	Great-toe interphalangeal and metatarsophalangeal flexion
Innervation	Medial plantar nerve, with reported variation	Tibial nerve

Relations

Superficial Relations

Flexor hallucis brevis lies deep to the more superficial plantar structures. Abductor hallucis overlaps its medial aspect, while the long flexor tendons and associated structures of the second layer cross portions of the region.

The plantar aponeurosis and first-layer muscles are separated from flexor hallucis brevis by intervening muscular, tendinous, and neurovascular structures.

Deep Relations

Deep to the proximal muscle are bones and joints of the medial and central midfoot. Distally, the muscle is directly related to the plantar surface of the first metatarsal and the first metatarsophalangeal joint.

The sesamoid bones embedded in its distal tendons articulate with the plantar aspect of the first metatarsal head.

Medial and Lateral Relations

Medially, flexor hallucis brevis is related to abductor hallucis. Laterally, it approaches adductor hallucis. These relationships become particularly close near their insertions on the proximal phalanx of the great toe.

Innervation

Flexor hallucis brevis is supplied principally by the medial plantar nerve, a terminal branch of the tibial nerve.^[1][2] The commonly described segmental contribution is predominantly S2 and S3.

The medial plantar nerve enters the sole deep to abductor hallucis and travels anteriorly through the medial plantar region. It supplies abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, and the first lumbrical.

Innervation of flexor hallucis brevis shows anatomical variation. In particular, the lateral head may receive an additional or alternative contribution from the deep branch of the lateral plantar nerve. For this reason, descriptions of its nerve supply are not completely uniform across anatomical studies.

Blood Supply

Flexor hallucis brevis receives blood from branches of the medial plantar artery and the deeper plantar arterial network. Distal branches associated with the first plantar metatarsal region also contribute to the vascular supply around the muscle and sesamoid complex.

The medial plantar artery arises from the posterior tibial artery and travels anteriorly through the medial sole. The plantar arterial arch and its branches supply deeper structures of the forefoot and communicate with the dorsal arterial system.

The precise pattern of small muscular branches varies between individuals.

Actions

Flexion of the Great Toe

The principal action of flexor hallucis brevis is flexion of the great toe at the first metatarsophalangeal joint. Both heads pass plantar to the joint axis and pull the proximal phalanx toward the sole when they contract.

Because the muscle does not insert on the distal phalanx, it does not directly flex the interphalangeal joint of the hallux. That action is produced primarily by flexor hallucis longus.

Stabilization of the First Metatarsophalangeal Joint

The paired arrangement of the medial and lateral heads allows flexor hallucis brevis to contribute to dynamic stabilization of the first metatarsophalangeal joint. The two heads apply force through opposite sides of the sesamoid apparatus and proximal phalanx.

This function is coordinated with abductor hallucis, adductor hallucis, flexor hallucis longus, the joint capsule, collateral ligaments, and plantar plate.

Relationship to the Plantar Muscular Layers

Flexor hallucis brevis belongs to the third layer of plantar muscles.

Layer	Principal Muscles and Tendons
First layer	Abductor hallucis, flexor digitorum brevis, abductor digiti minimi
Second layer	Quadratus plantae, lumbricals, flexor digitorum longus and flexor hallucis longus tendons
Third layer	Flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis
Fourth layer	Plantar and dorsal interossei with deep tendons crossing the sole

The layered arrangement provides a useful framework for understanding the course of the long flexor tendons and plantar neurovascular structures through the sole.

Relationship to Abductor and Adductor Hallucis

The distal arrangement of flexor hallucis brevis links it anatomically with both abductor hallucis and adductor hallucis.

On the medial side, the medial head of flexor hallucis brevis and abductor hallucis converge near the medial sesamoid and medial base of the proximal phalanx. On the lateral side, the lateral head of flexor hallucis brevis is associated with the lateral sesamoid and blends with adductor hallucis near the proximal phalanx.

These relationships create a balanced muscular arrangement around the plantar and medial-lateral aspects of the first metatarsophalangeal joint.

Role During Weight Bearing and Gait

The first metatarsophalangeal joint and sesamoid apparatus are important load-bearing structures of the medial forefoot. During stance, forces are transmitted through the first metatarsal head and its associated plantar structures.

As the heel rises during late stance, the great toe extends at the metatarsophalangeal joint. Flexor hallucis brevis can contribute muscular control of the proximal phalanx and sesamoid apparatus while the first ray is loaded.

The muscle functions together with flexor hallucis longus and other intrinsic muscles rather than acting as an isolated source of propulsion. Its anatomical position is particularly suited to controlling the first metatarsophalangeal joint during the transition from weight acceptance to push-off.

Contribution to the Medial Forefoot

Flexor hallucis brevis is closely associated with the medial longitudinal arch through its location beneath the first ray, but it is more directly involved in control of the first metatarsophalangeal joint and medial forefoot than in maintaining the proximal arch.

The principal structural supports of the medial longitudinal arch include the shapes of the bones, the plantar calcaneonavicular ligament, plantar aponeurosis, and several extrinsic and intrinsic muscular structures. Flexor hallucis brevis contributes dynamically as part of this integrated system.

Anatomical Variations

Variation may occur in the proximal attachments, degree of separation of the two heads, and pattern of innervation of flexor hallucis brevis. Accessory slips may connect the muscle with adjacent intrinsic muscles or tendons.

The nerve supply of the lateral head is particularly variable. Although the medial plantar nerve is the principal nerve to flexor hallucis brevis, contributions from the deep branch of the lateral plantar nerve have been described.

Variation also occurs in the morphology of the hallucal sesamoids. A sesamoid may be bipartite, most commonly the medial sesamoid. Such a configuration represents a developmental variant and can be important when interpreting radiographs or other imaging.

Clinical Significance

Sesamoid Injuries

Because the hallucal sesamoids are embedded within the distal tendons of flexor hallucis brevis, injuries involving these bones are directly related to the muscle's insertional apparatus. The sesamoids experience substantial forces during loading of the medial forefoot, particularly when the heel is elevated.

Fracture, stress injury, or other abnormalities of a sesamoid can therefore affect the plantar complex of the first metatarsophalangeal joint. A bipartite sesamoid should be distinguished from an acute fracture using its morphology and clinical context.

Plantar Plate and Sesamoid Complex

Flexor hallucis brevis forms part of the broader plantar soft tissue apparatus surrounding the first metatarsophalangeal joint. The sesamoids, plantar plate, joint capsule, collateral structures, and muscular tendons function as an integrated complex.

Disruption of this complex can alter the relationship between the sesamoids, first metatarsal head, and proximal phalanx. Understanding the normal muscular attachments is therefore important when assessing injuries around the plantar first metatarsophalangeal joint.

Hallux Valgus

In hallux valgus, changes in alignment of the first metatarsal and proximal phalanx alter the relationship of the sesamoids and surrounding tendons to the first metatarsophalangeal joint.

As the deformity develops, the sesamoid complex may become displaced relative to the first metatarsal head, and the mechanical lines of pull of flexor hallucis brevis, abductor hallucis, and adductor hallucis may change. Hallux valgus is a multifactorial deformity and should not be attributed to dysfunction of any one of these muscles.

Surgical Anatomy

The two heads of flexor hallucis brevis and their sesamoids are important landmarks during procedures involving the first metatarsophalangeal joint. Surgical approaches must account for the flexor hallucis longus tendon passing between the sesamoids and for the attachments of abductor and adductor hallucis on either side.

Preservation or restoration of the normal relationship between the first metatarsal head, sesamoids, and flexor apparatus is important for maintaining the mechanics of the joint.

Imaging

MRI and ultrasound can demonstrate flexor hallucis brevis, its medial and lateral heads, the sesamoids, and the intervening flexor hallucis longus tendon. MRI is particularly useful for evaluating the soft tissues surrounding the first metatarsophalangeal joint and for identifying muscle, tendon, or sesamoid abnormalities.

On radiographs, the sesamoid bones provide indirect landmarks for the distal tendons of flexor hallucis brevis. Recognition of normal variants such as a bipartite medial sesamoid is important when evaluating the plantar first metatarsophalangeal region.

Flexor hallucis brevis is defined by its paired distal heads and their close integration with the hallucal sesamoids. By inserting on both sides of the proximal phalanx while the flexor hallucis longus tendon passes between them, the muscle forms a central part of the plantar apparatus of the first metatarsophalangeal joint and provides both flexion and dynamic stability to the great toe.