Flexor Hallucis Brevis Anatomy

Flexor hallucis brevis is a two-headed intrinsic muscle of the third plantar layer, running along the plantar surface of the first metatarsal to the base of the hallux proximal phalanx. Its defining feature is that each of its two tendons contains a sesamoid bone, with the flexor hallucis longus tendon passing between them in the intersesamoid groove.

That is why it is not simply a hallux flexor. It is the dynamic component of the plantar sesamoid complex, the load-bearing platform beneath the first metatarsal head, and almost every problem on the plantar side of the first metatarsophalangeal joint, sesamoiditis, sesamoid stress fracture, osteonecrosis, turf toe and the plantar contracture of hallux rigidus, is a disorder of this muscle and the bones inside it.

Origin. The muscle arises from the medial part of the plantar surface of the cuboid and the adjacent plantar surface of the lateral (third) cuneiform, with deep fibres from the long plantar ligament. It also takes origin from a prolongation of the tibialis posterior tendon as that tendon inserts into the plantar midfoot, which is the reason the muscle is functionally coupled to tibialis posterior and the anatomical basis for a small contribution to medial arch support. The origin is Y-shaped and tendinous, and lies deep in the midfoot, superficial to the peroneus longus tendon in its groove.

Course and division. The muscle runs distally along the plantar surface of the first metatarsal and divides into a medial and a lateral head at about the level of the metatarsal neck. The flexor hallucis longus tendon runs forward between the two heads in the intersesamoid groove, roofed by the intersesamoid ligament.

Insertion. The medial head inserts into the medial (plantar-medial) side of the base of the proximal phalanx of the hallux, its tendon containing the tibial (medial) sesamoid, and blends with the insertion of abductor hallucis. The lateral head inserts into the lateral (plantar-lateral) side of the base, its tendon containing the fibular (lateral) sesamoid, and blends with adductor hallucis to form the conjoined tendon. Both heads are continuous with the plantar plate of the first metatarsophalangeal joint, so the muscle and the plate are functionally inseparable.

The sesamoid complex. The plate, the sesamoids and the structures that hold them work as one unit, and every structure that fails in a turf toe is on this list:

• The two sesamoids, tibial in the medial head and fibular in the lateral head
• The intersesamoid ligament, joining the two sesamoids and roofing the flexor hallucis longus groove
• The plantar plate, the fibrocartilaginous plate anchoring the complex to the phalanx
• The medial and lateral metatarsosesamoid (suspensory) ligaments, from each sesamoid to the metatarsal neck
• The two heads of flexor hallucis brevis, with abductor hallucis inserting into the tibial sesamoid and adductor hallucis into the fibular

The two sesamoids are not twins. The tibial sesamoid bears more of the load and is the commoner site of disease; the fibular sesamoid is smaller, and a divided fibular sesamoid means something different.

Ossification. The sesamoids begin to ossify between about 7 and 10 years of age, from multiple centres, which is one reason a sesamoid can be bipartite. A sesamoid that appears divided in a child may simply be incompletely coalesced.

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Medial plantar nerve (S1, S2). The tibial nerve (L4 to S3) divides within the tarsal tunnel, deep to the flexor retinaculum, into the medial and lateral plantar nerves, usually at or just distal to the level of the medial malleolus. The medial plantar nerve, the larger division and the functional homologue of the median nerve in the hand, passes deep to abductor hallucis and forward between abductor hallucis and flexor digitorum brevis. It supplies only four muscles, and everything else in the sole is lateral plantar nerve:

• Flexor digitorum brevis
• Abductor hallucis
• Flexor hallucis brevis
• First lumbrical

Adductor hallucis is not on that list. It is a lateral plantar nerve muscle, however closely it fuses with the lateral head, and a stem that pairs the two under one nerve is the trap.

Motor entry point. Branches enter the medial (superficial) surface of the muscle in its proximal half, arising from the medial plantar nerve as it crosses the plantar midfoot. Because the nerve lies superficial and medial to the muscle, a plantar-medial approach to the tibial sesamoid risks the sensory digital branch rather than the motor supply.

Variation. The lateral head is described in some dissections as receiving a contribution from the lateral plantar nerve, consistent with its close fusion with adductor hallucis. Do not be caught out by a stem that offers this.

The sensory nerve that matters surgically. The medial plantar digital nerve to the hallux, from the medial plantar nerve, runs plantar and medial to the tibial sesamoid, immediately deep to the skin of a medial approach. It is the structure injured in a medial approach for tibial sesamoidectomy or medial sesamoid shaving, and the result, a painful neuroma or an anaesthetic medial hallux border on a weight-bearing surface, is far more troublesome than the original complaint. Identify it first, retract it plantarward, and never place a plantar incision directly over the weight-bearing sesamoid pad.

Denervation pattern. A medial plantar nerve lesion (proximal tarsal tunnel, jogger's foot, high tibial nerve injury) weakens flexor hallucis brevis along with abductor hallucis and flexor digitorum brevis, giving weak metatarsophalangeal plantarflexion and sensory loss over the medial sole and the medial three and a half toes. Because adductor hallucis is spared, an isolated medial plantar nerve lesion produces a hallux valgus tendency, the mirror image of the lateral plantar nerve pattern.

Arteries. The medial plantar artery, the smaller terminal branch of the posterior tibial artery, supplies the muscle belly and the medial head. The first plantar metatarsal artery from the deep plantar arch, and the deep plantar artery from the dorsalis pedis through the first intermetatarsal space, supply the distal muscle and the sesamoids. Lateral plantar artery contributions reach the lateral head through the deep plantar arch.

Why sesamoids die. Most hallux sesamoids have only one artery. Pretterklieber's dissections found a single sesamoid artery supplying 63 per cent of medial and 58 per cent of lateral sesamoids, two arteries in 26 and 32 per cent respectively, and three in only about 10 per cent. Single supply is the rule, not a minority variant, which is the opposite of how this is usually taught.

The dominant vessel is the proximal (plantar) pedicle entering through the FHB tendon and the plantar plate, with a variable medial or lateral capsular contribution. Because the majority have no second vessel to fall back on, interruption of that single pedicle, by a displaced fracture, a plantar plate rupture, or surgical dissection such as stripping the proximal pole during internal fixation of a sesamoid fracture, accounts for the high rate of osteonecrosis and delayed union in sesamoid injuries. The anatomy explains the clinical behaviour precisely.

Practical consequences. Preserve the plantar plate and the tendinous attachments when operating on a sesamoid, avoid stripping the proximal pole, and counsel patients with sesamoid stress fractures that union may be slow and osteonecrosis is a recognised outcome.

Pedicle position and flap relevance. The dominant pedicle enters the muscle on its medial and plantar aspect in the proximal half, from the medial plantar artery. Flexor hallucis brevis is a small, thin muscle with a short arc of rotation, so it is rarely used as a formal flap. Where local coverage is needed on the plantar-medial forefoot the abductor hallucis muscle flap, also medial plantar artery based, is the workhorse; flexor hallucis brevis can be advanced only as a small adjunct to fill dead space after debridement of a septic first metatarsophalangeal joint.

Superficial (plantar). The plantar aponeurosis and its medial band lie plantar to the muscle, with abductor hallucis medially overlying the medial head, and the medial plantar digital nerve and vessels to the hallux immediately plantar-medial to the tibial sesamoid. Beneath the first metatarsal head is the thick plantar fat pad, which migrates distally with age and in claw-toe deformity, unroofing the sesamoids.

Between the two heads. The flexor hallucis longus tendon runs in the intersesamoid groove, roofed by the intersesamoid ligament. This is a fibro-osseous tunnel and a site of tenosynovitis, and the tendon is what a blade dividing the ligament will meet; cutting it during sesamoid excision costs hallux interphalangeal flexion while metatarsophalangeal flexion is preserved.

Deep (dorsal). The plantar surface of the first metatarsal and its head, with the two sesamoid grooves separated by the crista. Proximally the peroneus longus tendon crosses in its groove to the first metatarsal and medial cuneiform bases, and the deep plantar arch and the deep branch of the lateral plantar nerve run transversely at the level of the metatarsal bases, deep to the adductor.

Lateral. Adductor hallucis, conjoined with the lateral head, then the lesser metatarsals, the deep transverse metatarsal ligaments, the lumbricals and the interossei.

Actions. Plantarflexion of the first metatarsophalangeal joint is its only direct action, and it is the primary metatarsophalangeal plantarflexor; flexor hallucis longus flexes the interphalangeal joint principally and the metatarsophalangeal joint secondarily. Flexor hallucis brevis does not flex the interphalangeal joint, and that distinction is the basis of the bedside examination. In terminal stance it depresses and stabilises the first metatarsal head, keeping the head loaded and preventing dorsal escape of the first ray, and through its continuity with the plantar plate, the plantar aponeurosis and, at its origin, tibialis posterior, it contributes to the windlass mechanism and to arch support.

Synergists and antagonists. The muscles that share the work at the first metatarsophalangeal joint, with their nerves:

Palpation. Sit the patient with the ankle in neutral and the hallux relaxed. The sesamoids are palpable through the plantar-medial fat pad beneath the first metatarsal head, and become far easier to localise if the hallux is passively dorsiflexed, which draws them distally and tightens the complex over them.

Palpate each sesamoid separately. The tibial sesamoid lies beneath the medial third of the metatarsal head, the fibular sesamoid beneath the lateral third and slightly more proximal. Distinguishing which one is tender is a genuine clinical skill and changes the diagnosis. Between them the intersesamoid groove and the flexor hallucis longus tendon within it can be palpated; crepitus or a painful snap on passive hallux flexion and extension suggests FHL tenosynovitis. The medial head of the muscle itself can be felt as a firm longitudinal band deep to abductor hallucis along the plantar-medial first metatarsal.

Isolating brevis from longus. This is the examination point that matters, and it mirrors the FDP/FDS logic in the hand: block the distal joint to take the long flexor out of the equation, then test the proximal joint. The manoeuvres, with their pitfalls:

Imaging. The views to request, and what each answers:

• Weight-bearing dorsoplantar and lateral radiographs of the foot: alignment, joint space, dorsal osteophyte, sesamoid position
• Axial (tangential) sesamoid view: the single most useful sesamoid projection, showing each sesamoid in its groove, the crista, degenerative change, fragmentation and position
• Dorsiflexion (forced dorsiflexion) lateral view of both feet: sesamoid excursion in suspected plantar plate injury; the injured side fails to translate distally
• MRI: the investigation of choice for turf toe (plate discontinuity, sesamoid marrow oedema, FHL pathology), sesamoid stress fracture and osteonecrosis
• CT: fracture configuration and fragment position, and helpful in distinguishing a fracture line from a bipartite synchondrosis when MRI is equivocal
• Bone scan or SPECT: occasionally useful in a stress reaction with normal radiographs, though MRI has largely superseded it

Related pages. The injuries of the complex this muscle contains: Turf Toe is the plantar plate disruption measured by sesamoid excursion, and Sesamoid Disorders covers sesamoiditis, stress fracture, osteonecrosis and sesamoidectomy in full.

Turf toe: plantar plate and sesamoid complex injury

Mechanism. The archetypal injury of this muscle is forced hyperdorsiflexion of the first metatarsophalangeal joint with the ankle plantarflexed and the heel raised, classically an axial load applied to the heel with the forefoot fixed on a hard surface. It was originally described on artificial turf with flexible shoes.

Pathology. The plantar plate and the sesamoid complex are attenuated or ruptured, potentially with sesamoid diastasis (separation of a bipartite sesamoid), a sesamoid fracture, a proximally migrated sesamoid, avulsion of the plate from the phalangeal base, and in severe injuries articular impaction of the dorsal metatarsal head. A complete rupture treated as a sprain produces progressive hallux valgus or varus, a cock-up hallux, loss of push-off and eventual metatarsophalangeal arthrosis.

Variants. A medial injury (medial plate and abductor hallucis) leads to progressive hallux valgus, a lateral injury to hallux varus, and a combined hyperdorsiflexion with valgus is the commonest pattern.

Do not confuse it with sand toe, a forced hyperplantarflexion injury of the first metatarsophalangeal joint, seen in beach volleyball, which injures the dorsal capsule rather than the plantar plate.

Sesamoiditis, stress fracture and osteonecrosis

Sesamoiditis is an umbrella clinical term for painful sesamoid overload: focal plantar pain under one sesamoid, worse in terminal stance, in heeled shoes and on inclines. It is predisposed by a plantarflexed first ray, cavus foot, fat pad atrophy, hallux rigidus and repetitive push-off loading (dance, running, gymnastics), and the tibial sesamoid is affected more often because it bears more load.

Stress fracture presents identically. Radiographs may be normal early; MRI shows marrow oedema and a fracture line. Union is slow because the proximal pedicle is often the only meaningful blood supply.

Osteonecrosis shows fragmentation, sclerosis then collapse, with low T1 and high T2 signal early. It follows displaced fracture, plate rupture or surgical devascularisation, and explains why so many sesamoid injuries fail to unite.

Management. Non-operative treatment is the default and is usually successful: offloading with a sesamoid-relief (dancer's) pad or a cut-out orthosis, a stiff-soled shoe or carbon-fibre plate to limit metatarsophalangeal dorsiflexion, activity modification, and footwear change. A period of immobilisation is appropriate for a confirmed stress fracture. When this fails the operative options are bone grafting or internal fixation for a viable non-union, partial sesamoidectomy (shaving) for a plantar prominence, or complete sesamoidectomy as a last resort with full counselling about the alignment consequences.

Bipartite versus fractured sesamoid

This is a classic single-best-answer question, and the discriminators are morphological:

Hallux rigidus

What it is. Degenerative arthrosis of the first metatarsophalangeal joint, characterised by dorsal osteophyte formation, loss of dorsiflexion and dorsal impingement pain, with the plantar joint often relatively preserved.

Why this muscle matters to it. The relevance is twofold. First, the plantar sesamoid complex becomes adherent to the plantar metatarsal head, losing its excursion and contributing directly to the loss of dorsiflexion. Second, because the plantar cartilage is often preserved, procedures that preserve the plantar joint (cheilectomy, dorsal closing wedge osteotomy of the proximal phalanx) can succeed even in moderately advanced disease. The cheilectomy itself, and the plantar release it must include, is described under Surgical Relevance.

Other presentations

• Flexor hallucis longus tenosynovitis in the intersesamoid groove, with pain and crepitus on hallux motion, distinct from the more proximal posteromedial ankle FHL entrapment of dancers
• First metatarsophalangeal joint sepsis in the diabetic foot tracks along the FHB tendons and the plantar plate; adequate debridement usually means excising the sesamoids and accepting the alignment consequence
• Sesamoid osteochondral injury with a loose body, from a hyperdorsiflexion impaction injury

Medial approach to the tibial sesamoid

Position and incision. Supine, ankle in neutral, with the hallux held in slight dorsiflexion to draw the sesamoid distally and make it palpable. The incision is a medial, mid-axial one over the first metatarsophalangeal joint, deliberately dorsal to the weight-bearing plantar skin; a plantar incision over the sesamoid creates a painful scar on a loading surface, so the approach is medial or plantar-medial, never directly over the pad. The structures are met in this order:

1. Skin
2. The medial plantar digital nerve to the hallux, running plantar-medial: identify and retract it plantarward before going deeper
3. The abductor hallucis insertion
4. The medial capsule and the tibial sesamoid within the medial FHB tendon

Shaving or excision. Sesamoid shaving (partial excision) removes the plantar prominence only, preserving the articular dorsal surface and the tendon continuity, with lower morbidity than complete excision. For complete excision, shell the sesamoid out of the tendon without dividing the tendon transversely, then repair the resulting longitudinal defect side to side so the medial head keeps its insertion. Do not divide the intersesamoid ligament unnecessarily: the flexor hallucis longus tendon lies immediately beneath it.

Approaches to the fibular sesamoid

The fibular sesamoid is the harder of the two to reach, and three routes are described:

• Plantar, through the first web space, longitudinally between the first and second metatarsal heads: direct access, but the incision is on or near a loading surface and the plantar digital nerves to the first web space are at risk
• Dorsal first web space: avoids plantar skin entirely; the sesamoid is delivered from between the metatarsal heads after retracting the adductor, at the cost of risk to the dorsal digital branches of the deep peroneal nerve
• Medial transarticular: through the same medial approach used for a bunion, working across the distracted joint; the least direct route, but it avoids both plantar skin and the dorsal web

Whichever route is used, preserve the conjoined tendon of adductor hallucis and the lateral head and repair it, or hallux varus follows.

Operative repair of a grade 3 turf toe

Approach. A medial (mid-axial) approach gives access to the plate and the tibial sesamoid; add a plantar-lateral or dorsal web space incision if the fibular sesamoid must be addressed. A combined medial-and-plantar-lateral approach preserves the central weight-bearing skin. The steps:

1. Identify and protect the medial plantar digital nerve to the hallux
2. Expose the plantar plate and define the tear, typically transverse, just proximal to its phalangeal insertion, or a sesamoid diastasis
3. Repair a diastasis of a bipartite sesamoid or a transverse sesamoid fracture directly with suture through drill holes, or excise a small non-viable fragment and repair the tendon to preserve continuity
4. Repair the plate end to end, or reattach it to the phalangeal base with suture anchors or bone tunnels
5. Restore the abductor and adductor attachments and correct any transverse-plane malalignment
6. Confirm restored sesamoid excursion and a negative vertical Lachman on the table

Rehabilitation. Non-weight-bearing or heel-weight-bearing in a splint holding the hallux in slight plantarflexion for around 4 weeks, with protected motion from about 1 week to avoid adhesion of the complex, then progressive weight-bearing in a stiff-soled shoe or carbon plate. Return to sport is typically at 3 to 4 months, depending on the repair.

Cheilectomy for hallux rigidus

Approach. A dorsal or dorsomedial longitudinal incision over the first metatarsophalangeal joint. Protect the dorsomedial cutaneous branch of the medial dorsal cutaneous nerve (from the superficial peroneal nerve) crossing the joint: it is the commonest nerve injured in this operation, and a numb or painfully hypersensitive dorsomedial hallux is the commonest cause of a dissatisfied patient after a technically satisfactory cheilectomy. The steps:

1. Capsulotomy and synovectomy; remove loose bodies
2. Resect the dorsal osteophyte and the dorsal 25 to 30% of the metatarsal head, angled to leave a smooth transition into the remaining articular surface
3. Resect dorsal phalangeal osteophytes
4. Mobilise the plantar sesamoid complex, freeing adhesions between the sesamoids and the plantar metatarsal head and releasing any capsular contracture, to restore distal excursion
5. Confirm 70 to 90 degrees of passive dorsiflexion on the table
6. Consider a dorsal closing wedge (Moberg) osteotomy of the proximal phalanx if dorsiflexion remains inadequate, accepting that it trades plantarflexion for dorsiflexion

Getting the resection right. Less than the dorsal 25 to 30% and the impingement persists; beyond about a third of the head the joint destabilises, with a risk of dorsal subluxation, and the plantar contact area is compromised. Persistent stiffness after an adequate bony resection usually means the plantar structures were not mobilised, or that grade 4 disease was not recognised.

Afterwards. Early motion after cheilectomy is important, because the complex re-adheres if immobilised. Where the mid-range arc is painful (grade 4), cheilectomy will not work and arthrodesis is the operation.

Distances and safe zones

• Medial plantar digital nerve to the hallux: plantar-medial to the tibial sesamoid, typically within 5 mm of the plantar-medial corner of the sesamoid and immediately deep to the skin. Place the medial incision at the mid-axial line or slightly dorsal to it
• Flexor hallucis longus in the intersesamoid groove: immediately deep to the intersesamoid ligament, between the sesamoids. Assume it is within 2 to 3 mm of any blade dividing that ligament
• Dorsomedial cutaneous nerve to the hallux: crosses the dorsomedial aspect of the first metatarsophalangeal joint subcutaneously, within a few millimetres of the skin, in every dorsal or dorsomedial approach
• Dorsal digital branches of the deep peroneal nerve: subcutaneous in the dorsal first web space, within about 3 to 5 mm of the skin, if that route is used for the fibular sesamoid

Sesamoids in bunion and arthrodesis surgery

Bunion lateral release. Dividing the lateral metatarsosesamoid (suspensory) ligament and the conjoined tendon is necessary for the sesamoids to reduce beneath the metatarsal head, but it unavoidably weakens the lateral head of flexor hallucis brevis. The lateral suspensory ligament is necessarily divided there; losing both is destabilising. Preserving the fibular sesamoid itself is what prevents hallux varus.

First metatarsophalangeal arthrodesis. The sesamoids do not need to be excised. They should be mobilised enough to allow joint preparation and positioning and are usually left in situ; adherent sesamoids that block positioning may be shelled out of the tendon.

Replacement and interposition arthroplasty. The plantar complex must be preserved to retain any push-off function, which is one reason these procedures perform less predictably than arthrodesis in a high-demand foot.

Anatomical Variation Across Populations

• The two-headed pattern with two sesamoids is essentially constant. Absence of a hallux sesamoid is a genuine rarity and, when seen, should prompt a search for prior surgery or trauma.
• Bipartition is reported in a minority of feet, most often affecting the tibial sesamoid, and is frequently bilateral. Reported prevalence varies widely between series because definitions differ; the clinically useful points are the strong tibial predominance and the frequency of bilaterality.
• Multipartite (three or more part) sesamoids are described but uncommon.
• Reported prevalence of a lateral plantar nerve contribution to the lateral head varies between dissection series, consistent with its fusion with adductor hallucis.
• Hallux rigidus prevalence rises with age and is a common cause of forefoot disability in adults over 50; sesamoid disorders are disproportionately seen in dancers, runners, gymnasts and field-sport athletes rather than in the general population.

Side-by-Side Guidance and Practice Differences

Areas of Genuine Uncertainty

• Operative versus non-operative Grade 3 turf toe: no randomised evidence exists and there never will be in elite sport. Practice is driven by expert series and the consequences of chronic instability.
• Sesamoid non-union: fixation, grafting or excision? Series are small and heterogeneous; the decision rests on fragment viability, size and the patient's demands.
• Partial (shaving) versus complete sesamoidectomy: partial excision preserves the tendon insertion and alignment but may not relieve pain if the whole bone is diseased.
• Return-to-play timing after turf toe repair: commonly quoted as 3 to 4 months, but based on small series and highly dependent on sport and position.
• First metatarsophalangeal implant arthroplasty and interposition versus arthrodesis: arthrodesis remains the benchmark for pain relief and durability; motion-preserving options continue to be studied without displacing it.

High- versus Limited-Resource Practice

• Well-resourced settings: MRI for turf toe and sesamoid pathology, comparative dynamic radiographs, custom carbon-plate orthoses, suture anchors for plate repair, and access to sports rehabilitation.
• Limited-resource settings: the diagnosis of turf toe and sesamoid disorders is fundamentally clinical and radiographic. Comparative forced dorsiflexion lateral radiographs of both feet cost nothing and answer the key question. A stiff-soled shoe, a simple sesamoid relief pad cut from felt, activity modification and time treat the great majority. First metatarsophalangeal arthrodesis is a reliable, implant-light definitive operation for end-stage hallux rigidus.
• Everywhere: footwear counselling, offloading and clear explanation that the sesamoids are load-bearing bones inside a tendon and therefore heal slowly.