Flexor Hallucis Brevis Anatomy

Origin

• Plantar surface of the cuboid (its medial part) and the adjacent plantar surface of the lateral cuneiform.
• A prolongation of the tibialis posterior tendon as it inserts into the plantar midfoot - the reason FHB is functionally coupled to the tibialis posterior, and the anatomical basis for a small contribution to medial arch support.
• Deep fibres from the plantar aspect of the third cuneiform and the long plantar ligament.
• The origin is Y-shaped and tendinous; it 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

• Medial head: into the medial (plantar-medial) side of the base of the proximal phalanx of the hallux, its tendon containing the TIBIAL (medial) sesamoid. Blends with the insertion of abductor hallucis.
• Lateral head: into the lateral (plantar-lateral) side of the base of the proximal phalanx, its tendon containing the FIBULAR (lateral) sesamoid. Blends with adductor hallucis to form the CONJOINED TENDON.
• Both heads are continuous with the plantar plate of the first metatarsophalangeal joint, so FHB and the plate are functionally inseparable.

The Sesamoids Themselves

• Ossification begins between about 7 and 10 years of age, from multiple centres, which is one reason bipartition is common. A sesamoid that appears divided in a child may simply be incompletely coalesced.

Nerve Supply

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, 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: flexor digitorum brevis, abductor hallucis, flexor hallucis brevis, and the first lumbrical. Everything else in the sole is lateral plantar nerve.
• 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.
• This is the structure injured in a medial approach for tibial sesamoidectomy or medial sesamoid shaving, producing a painful neuroma or an anaesthetic medial hallux border on a weight-bearing surface.
• 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 FHB along with abductor hallucis and flexor digitorum brevis, and gives sensory loss over the medial sole and the medial three and a half toes.
• Because adductor hallucis is spared (lateral plantar nerve), an isolated medial plantar nerve lesion produces a hallux valgus tendency - the mirror image of the lateral plantar nerve pattern.

Arterial Supply

• Medial plantar artery (the smaller terminal branch of the posterior tibial artery) supplies the muscle belly and the medial head.
• 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.

Sesamoid Vascularity - Why Osteonecrosis Happens

• 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 proximal pedicle - by a displaced fracture, a plantar plate rupture, or surgical dissection - 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.
• FHB 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; FHB can be advanced only as a small adjunct to fill dead space after debridement of a septic first metatarsophalangeal joint.

Superficial (plantar) to the muscle

• The plantar aponeurosis and its medial band.
• Abductor hallucis medially, overlying the medial head.
• The medial plantar digital nerve and vessels to the hallux, immediately plantar-medial to the tibial sesamoid.
• The thick plantar fat pad beneath the first metatarsal head, which migrates distally with age and in claw-toe deformity, unroofing the sesamoids.

Between the two heads

• Flexor hallucis longus tendon in the intersesamoid groove, roofed by the intersesamoid ligament. This is a fibro-osseous tunnel and a site of tenosynovitis.

Deep (dorsal) to the muscle

• The plantar surface of the first metatarsal and its head with the two sesamoid grooves separated by the crista.
• Proximally, the peroneus longus tendon in its groove crossing to the first metatarsal and medial cuneiform bases.
• 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.

Structures at Risk

1. Turf Toe (Plantar Plate and Sesamoid Complex Injury)

The archetypal injury of this muscle.

• Mechanism: 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. Originally described on artificial turf with flexible shoes.
• Pathology: attenuation or rupture of the plantar plate and the sesamoid complex, 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.
• Variants: a medial injury (medial plate and abductor hallucis) leads to progressive hallux valgus; a lateral injury leads to hallux varus; 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.

2. 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. Predisposed by a plantarflexed first ray, cavus foot, fat pad atrophy, hallux rigidus and repetitive push-off loading (dance, running, gymnastics). 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.
• Non-operative management 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.
• Operative options when this fails: 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.

3. Bipartite versus Fractured Sesamoid

This is a classic single-best-answer question.

4. Hallux Rigidus

• 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.
• The FHB 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.

• Cheilectomy technique points that involve FHB: resect roughly the dorsal 25 to 30% of the metatarsal head, remove dorsal phalangeal osteophytes and loose bodies, then mobilise the plantar sesamoid complex by freeing adhesions between the sesamoids and the plantar metatarsal head. Confirm 70 to 90 degrees of passive dorsiflexion intraoperatively before closing.
• Do not over-resect. Beyond about a third of the head the joint destabilises and the plantar contact area is compromised.
• Persistent stiffness after an adequate resection usually reflects an unmobilised plantar complex or unrecognised grade 4 disease.

5. Other Presentations

• Flexor hallucis longus tenosynovitis in the intersesamoid groove - pain and crepitus with 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.
• Post-traumatic cock-up hallux after both sesamoids have been excised or after an unrepaired grade 3 turf toe.
• Sesamoid osteochondral injury with a loose body from a hyperdorsiflexion impaction injury.

Medial Approach to the Tibial Sesamoid

• Position: supine, ankle in neutral, hallux held in slight dorsiflexion to draw the sesamoid distally and make it palpable.
• Incision: a medial, mid-axial incision over the first metatarsophalangeal joint, deliberately dorsal to the weight-bearing plantar skin. Never place the incision directly over the plantar sesamoid pad.
• Order encountered: skin; then the medial plantar digital nerve to the hallux running plantar-medial - identify and retract it plantarward before going deeper; then the abductor hallucis insertion; then the medial capsule and the tibial sesamoid within the medial FHB tendon.
• Sesamoid shaving (partial excision): remove the plantar prominence only, preserving the articular dorsal surface and the tendon continuity. Lower morbidity than complete excision.
• Complete excision: shell the sesamoid out of the tendon without dividing the tendon transversely. Repair the resulting longitudinal tendon defect side to side to restore the medial head insertion and prevent progressive hallux valgus.
• 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. 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 fibular sesamoid is delivered from between the metatarsal heads after retracting the adductor. Risk to the dorsal digital branches of the deep peroneal nerve, which are subcutaneous within about 3 to 5 mm of the skin.
• Medial transarticular: through the same medial approach used for a bunion, working across the distracted joint. Least direct but avoids both plantar skin and the dorsal web.
• Whichever route, preserve the conjoined tendon of adductor hallucis and the lateral head of FHB 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.
• 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.
• Do not simply excise both sesamoids - that converts a repairable injury into a permanent cock-up hallux.
• Rehabilitation: non-weight-bearing or heel-weight-bearing in a splint holding the hallux in slight plantarflexion for around 4 weeks, protected motion from about 1 week to avoid adhesion of the complex, progressive weight-bearing in a stiff-soled shoe or carbon plate, and return to sport typically at 3 to 4 months depending on the repair.

Cheilectomy for Hallux Rigidus

• Approach: 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 - the commonest nerve injured in this operation, giving a numb or painfully hypersensitive dorsomedial hallux.
• 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 - free adhesions between the sesamoids and the plantar metatarsal head, restoring 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.
• Early motion after cheilectomy is important - the complex re-adheres if immobilised.
• Where the mid-range arc is painful (grade 4), cheilectomy will not work and arthrodesis is the operation.