Flexor Hallucis Longus Anatomy

Origin

• Distal two-thirds of the posterior surface of the fibula - it is the only deep posterior compartment muscle arising principally from the fibula.
• Interosseous membrane (lower part), the posterior intermuscular septum, and the fascia over tibialis posterior.
• Bipennate, with an unusually low-lying muscle belly whose muscular fibres extend distally almost to the level of the ankle joint, and in some individuals beyond it into the fibro-osseous tunnel.

The three fibro-osseous tunnels

1. Posterior talar tunnel: between the medial and lateral tubercles of the posterior process of the talus, roofed by a fibrous retinacular sling continuous with the deep component of the flexor retinaculum. The lateral tubercle is the Stieda process when elongated, or an os trigonum when separate.
2. Sustentacular tunnel: beneath the sustentaculum tali on the medial calcaneus, where the tendon runs in a groove on its inferior surface.
3. Sesamoid tunnel: between the medial and lateral hallux sesamoids within the plantar plate of the first metatarsophalangeal joint, deep to the intersesamoid ligament.

Course through the foot

• After the sustentacular tunnel the tendon runs forward in the sole, crossing superficial (plantar) to the flexor digitorum longus tendon at the knot of Henry, beneath the navicular and medial cuneiform.
• It then passes between the two heads of flexor hallucis brevis and through the sesamoid tunnel.

Insertion

• Plantar base of the distal phalanx of the hallux.
• It is the only long flexor of the great toe, so its loss abolishes active interphalangeal flexion entirely.

The knot of Henry

• At the knot of Henry (master knot), plantar to the navicular and medial cuneiform, FHL crosses plantar to FDL, and the two tendons are bound by a fibrous sling and exchange tendinous slips.
• The interconnection is usually from FHL to FDL - a slip from the great toe flexor contributing to the lesser toe tendons, found in about 83 per cent of 166 legs in the largest series. In roughly one foot in seven there is no interconnection at all, and the slip's position is not fixed: it lay distal to the knot in about half of those legs, by a mean of 9 to 11 mm.
• This is the anatomical basis of two surgical facts, and both are stated wrongly in most sources:
  • Harvesting FDL for a flatfoot reconstruction. The lesser toes keep working because the FHL slip drives the distal FDL stump - which requires the cut to be PROXIMAL to where that slip joins, not "distal to the knot". O'Sullivan's own conclusion is transection proximal to the knot. Because the slip is commonly distal to the knot, a cut placed "just distal to the knot" removes it with the graft in about half of feet. Find the slip, pull FHL, watch the toes, then cut proximal to what you have seen.
  • Harvesting FHL through a distal incision is the dangerous one. Mulier's 24 cadaver feet show why: because of the cross-attachments, retraction failed at the first attempt in every single specimen, forcing extensive dissection next to the posterior tibial nerve - and 33 per cent sustained a nerve lesion, including two complete ruptures of the medial plantar nerve. The extra tendon length is rarely worth that. Prefer a short posterior ankle harvest with interference screw fixation, and if a distal harvest is genuinely required, expose the interconnection under direct vision rather than pulling blindly.

Nerve supply

• Tibial nerve, root values S1 and S2 (with an L5 contribution described in some sources).
• Motor branches arise from the tibial nerve within the deep posterior compartment and enter the medial (deep) surface of the muscle in its proximal to middle third, generally within the proximal half of the leg.
• The tibial nerve enters the deep compartment beneath the tendinous soleal arch; a lesion at that level denervates FHL along with tibialis posterior and flexor digitorum longus.

Root value logic

• S1 is tested by plantarflexion, eversion and the ankle jerk; S2 contributes to toe flexion and the intrinsics.
• Weak great toe flexion with weak inversion and weak lesser toe flexion localises to the tibial nerve or the deep posterior compartment.
• Weak great toe extension localises to L5 and the deep peroneal nerve - a completely different problem, and the two are easily confused in a hurried examination.

Clinical translation

• Deep posterior compartment syndrome infarcts FHL along with FDL and tibialis posterior. The late sequel is a fixed clawing of the toes with the hallux in flexion, sensory loss on the sole, and a cavovarus foot.
• Tarsal tunnel syndrome does not weaken FHL, because the motor supply arises well proximal to the retinaculum, but crowding of the tunnel by an FHL tenosynovitis is a recognised cause of tarsal tunnel symptoms.

Arterial supply

• The muscle belly is supplied by the peroneal artery (dominant) and by the posterior tibial artery, both giving segmental muscular branches.
• The tendon receives supply through a mesotendon and vincular system from the posterior tibial and peroneal systems, and distally from the medial plantar artery.

Watershed and relevance

• The most constrained and highest-friction segment is where the tendon rounds the posterior talus, and this corresponds to the site of stenosing tenosynovitis and of the nodule that produces hallux saltans.
• The tendon is relatively well vascularised compared with the Achilles, which is one of the arguments for using it, with its adjacent muscle belly, to augment a chronically degenerate Achilles.

Flap relevance

• FHL is not used as a muscle flap in the conventional sense. However, in FHL transfer for Achilles reconstruction the low-lying muscle belly is deliberately brought down into the repair site as a vascularised biological augment, which is a genuine and underappreciated part of the operation's rationale.

In the leg

• Most lateral and most posterior of the deep posterior compartment muscles. It lies on the posterior surface of the fibula, with tibialis posterior medial and deep and flexor digitorum longus medial.
• Posterior: the deep transverse fascia and, beyond it, the soleus.
• The posterior tibial neurovascular bundle and the tibial nerve lie medial to FHL in the distal leg, between it and flexor digitorum longus. The peroneal artery lies deep and lateral, on the fibula.
• The muscle belly forms the posterolateral wall of the deep compartment and is the muscle that must be retracted or split in a posterolateral approach to the distal tibia.

At the ankle - the tarsal tunnel and the posterior ankle

• Most posterior and most lateral of the tarsal tunnel contents, in its own groove on the posterior talus.
• Medial to FHL, in order moving anteriorly: the tibial nerve, the posterior tibial artery and veins, flexor digitorum longus and tibialis posterior.
• This ordering generates the single most important safety rule in posterior ankle arthroscopy: the FHL tendon marks the medial limit of safe dissection, because the tibial nerve and posterior tibial artery lie immediately medial to it.
• Kager fat pad lies between the FHL, the Achilles and the calcaneus. Its deep (FHL) wedge moves with the tendon and is the fat that is debrided to expose the posterior ankle at arthroscopy.

In the foot

• Crosses plantar to flexor digitorum longus at the knot of Henry, beneath the navicular and medial cuneiform.
• The medial plantar nerve and artery lie immediately adjacent to the knot of Henry, on its plantar and medial side, and are the structures at risk in a distal FHL or FDL harvest.
• Passes between the two heads of flexor hallucis brevis and beneath the intersesamoid ligament to the distal phalanx.

The mechanism

• In maximal plantarflexion - the demi-pointe and en pointe positions in ballet, the plantarflexed foot of a footballer striking a ball, the downhill kick of a swimmer - the posterior structures of the ankle are compressed between the posterior tibial plafond and the calcaneus.
• The structure caught in between is the posterior process of the talus, and specifically its lateral tubercle.
• Bony causes: an os trigonum (a separate ossicle representing an unfused secondary ossification centre of the lateral tubercle, present in roughly 7 to 14 per cent of ankles), a Stieda process (an elongated but attached lateral tubercle), a Shepherd fracture (an acute fracture of the lateral tubercle), a prominent posterior calcaneal process, or a downsloping posterior tibial lip.
• Soft-tissue causes: posterior capsular and synovial hypertrophy, a thickened posterior intermalleolar ligament, a hypertrophied FHL muscle belly extending into the tunnel, and FHL tenosynovitis.
• Frequently both coexist in the same patient - the os trigonum and the FHL tenosynovitis are two halves of one problem, because the tendon runs immediately medial to the ossicle in the same fibro-osseous space.

Who gets it

• Ballet dancers above all, because of repetitive extreme plantarflexion in en pointe and demi-pointe.
• Footballers, gymnasts, downhill runners, cricket fast bowlers and swimmers (particularly with a strong flutter kick).
• It also occurs after an acute forced plantarflexion injury, which can fracture a Stieda process or disrupt an os trigonum synchondrosis.

Clinical

• Deep posterior ankle pain, often described as "behind the ankle" and hard for the patient to localise, worse on plantarflexion and on rising en pointe.
• Posterior impingement test: rapid forced passive plantarflexion reproduces the pain. Repeat it in slight inversion and slight eversion, since a lateral component suggests bony impingement and a medial component suggests FHL involvement.
• Examine the FHL separately with passive hallux motion and palpation posteromedially. Pain and crepitus there means the tendon is involved and both problems will need addressing.
• Exclude the differentials: Achilles and retrocalcaneal pathology, posterior tibiotalar or subtalar arthritis, an osteochondral lesion of the talus, a posterior process fracture, a talar or calcaneal stress fracture, and in a dancer, a posteromedial ankle problem from the tibialis posterior or FDL.

Imaging

• Lateral radiograph in plantarflexion first.
• MRI for marrow oedema across the synchondrosis, sheath fluid and soft-tissue hypertrophy.
• CT for bony detail and to plan excision.
• Diagnostic injection where the picture is mixed.

Non-operative

• Activity modification, technique review with the dance teacher or coach, physiotherapy addressing calf and intrinsic strength and ankle range, and a period of relative rest.
• Image-guided corticosteroid injection into the posterior ankle or the FHL sheath can be both diagnostic and therapeutic, though it must be used cautiously in a load-bearing tendon and never repeatedly.
• A high proportion of dancers improve with non-operative management, and surgery is reserved for those who fail an adequate trial.

Operative

• Posterior ankle arthroscopy (two-portal hindfoot endoscopy) is now the standard approach: prone position, posterolateral and posteromedial portals at the level of the tip of the lateral malleolus either side of the Achilles, instruments directed toward the first or second web space, fat debrided until the FHL is identified, then excision of the os trigonum or Stieda process and release of the FHL sheath.
• Advantages over open surgery: less soft tissue disruption, faster return to dance, and the ability to address the os trigonum and the FHL tendon through the same portals.
• Open posteromedial approach gives direct access to the FHL and allows tenosynovectomy and nodule excision, but risks the tibial nerve and its branches. Open posterolateral approach gives access to the ossicle but not the tendon, and risks the sural nerve.
• The rule at arthroscopy: never dissect medial to the FHL tendon, because the tibial nerve and posterior tibial artery are immediately beyond.

When to transfer

• Chronic Achilles rupture with a gap - conventionally quoted as greater than about 5 cm after debridement, though the decision is made intra-operatively on tissue quality and achievable apposition rather than on a number alone.
• Chronic Achilles rupture with poor-quality tendon even where the gap is small - a degenerate, attenuated tendon will not hold a repair.
• Failed previous Achilles repair.
• Severe insertional Achilles tendinopathy where debridement removes more than about 50 per cent of the tendon insertion, leaving inadequate residual attachment.
• Achilles reconstruction in the older or lower-demand patient, where the donor deficit is well tolerated.

When to choose something else

• Gap of less than about 2 to 3 cm: direct repair, sometimes with a V-Y advancement.
• Gap of 2 to 5 cm: V-Y lengthening or a gastrocnemius turn-down flap.
• Young athlete, dancer or sprinter: loss of hallux push-off matters. Consider a turn-down, a free graft, or an FDL transfer instead.
• Very large defect with poor soft tissue: free tissue transfer with tendon reconstruction, in conjunction with plastic surgery.

Three described harvest levels

Choosing the harvest level is the technical decision examiners focus on, because it trades tendon length against the risk to the medial plantar nerve.

1. Short harvest (posterior ankle, through the Achilles incision)

• The tendon is divided at the level of the posterior ankle, just distal to where it enters the fibro-osseous tunnel behind the talus, through the same posteromedial incision used for the Achilles.
• Length obtained: short - typically enough to reach the calcaneus and be fixed with an interference screw, but not enough for a long weave.
• Advantage: a single incision, no separate plantar dissection, and no risk to the medial plantar nerve.
• Disadvantage: limited tendon length; requires an interference screw or a bone tunnel with the tendon just reaching.
• This is the technique most units now use as standard.

2. Intermediate harvest (medial midfoot, proximal to the knot of Henry)

• A second incision on the medial midfoot over the navicular allows the tendon to be followed and divided proximal to the knot of Henry.
• Length obtained: intermediate.
• Risk: the medial plantar nerve lies close to the field.

3. Long harvest (plantar, distal to the knot of Henry)

• A plantar or plantar-medial incision allows harvest distal to the knot of Henry, yielding the maximum tendon length for a long weave through the Achilles stump.
• Length obtained: longest.
• Risk: highest, and now quantified. The medial plantar nerve and artery lie immediately adjacent to the knot of Henry. In Mulier's 24 cadaver feet harvested this way, retraction failed at the first attempt in EVERY specimen because of the FHL-to-FDL cross-attachments, forcing a more extensive dissection - and 33 per cent sustained a nerve lesion, including two complete ruptures of the medial plantar nerve. A plantar scar is also poorly tolerated.
• Additional step required: the distal FHL stump should be tenodesed to flexor digitorum longus to preserve hallux interphalangeal position and prevent a cock-up deformity.

The evidence-based conclusion

Mulier's cadaveric study is the one to quote: the more distal the harvest, the greater the tendon length and the greater the risk, and the risk is not small - a one-in-three nerve lesion rate, with the failed first retraction in every specimen showing that the hazardous dissection is the expected course of a distal harvest rather than an occasional misadventure. Against that, the extra length is usually unnecessary, because a short harvest fixed with an interference screw into the calcaneus performs well clinically. Most contemporary practice therefore uses the short single-incision harvest. Two honest caveats: Mulier's lesions were found by dissection in cadavers and their clinical significance is, in the authors' own words, undetermined; and no trial has compared harvest levels by patient outcome.

Standard single-incision technique

1. Position: prone, tourniquet, both feet visible for tension comparison.
2. Incision: posteromedial, medial to the Achilles, avoiding the midline directly over the tendon where wound problems are commonest, and protecting the sural nerve laterally and the medial calcaneal branches medially.
3. Expose and debride the Achilles. Excise all degenerate tissue and measure the true gap with the ankle in the intended resting position.
4. Open the deep fascia medial to the Achilles to enter the deep posterior compartment. Identify the FHL muscle belly, confirming it by pulling on it and watching the great toe flex. The neurovascular bundle lies medial to it - stay lateral.
5. Divide the FHL tendon as far distally as safe access allows through this incision, with the ankle plantarflexed and the great toe maximally flexed to deliver the maximum length of tendon.
6. Prepare a bone tunnel in the calcaneus from dorsal to plantar, or from posterior to anterior just anterior to the Achilles insertion, sized to the tendon.
7. Tension and fix. With the ankle in about 10 to 20 degrees of plantarflexion, matching the resting tension of the contralateral limb, pass the tendon into the tunnel and secure with a biotenodesis interference screw, or pass it through a tunnel and suture back to itself.
8. Bring the muscle belly down and suture it to the Achilles and to the surrounding tissue - the vascularised muscle is a deliberate biological augment.
9. Complete the Achilles repair or reconstruction over the transfer where possible.
10. Closure and rehabilitation: cast in plantarflexion for around 2 weeks, then a boot with progressive reduction of heel wedges, protected weightbearing from around 2 to 6 weeks depending on the construct, and progressive strengthening from around 8 to 12 weeks.

Danger list

• Medial plantar nerve: immediately adjacent to the knot of Henry. This is the single most important structure at risk in a distal harvest, and the main argument for a proximal one.
• Tibial nerve and posterior tibial artery: lie medial to the FHL in the distal leg and at the ankle. In the deep compartment dissection, stay lateral to the tendon and the bundle stays safe.
• Sural nerve: lateral to the Achilles, crossing the lateral hindfoot roughly 1.5 cm posterior and inferior to the fibular tip. At risk with a lateral or midline extensile incision. This centimetre figure is a conventional approximation that does not scale with foot size; cadaveric work on the calcaneal osteotomy incision expresses the safe zone as a proportion instead, placing the incision at or beyond one third of the distance from the fibular tip to the posteroinferior calcaneal margin.
• Medial calcaneal branches of the tibial nerve: cross the medial hindfoot and are at risk in the distal end of a posteromedial incision.
• Achilles wound: the posterior midline has a poor blood supply. Use a posteromedial incision, handle the skin minimally, and close without tension.
• Calcaneal tunnel placement: too posterior risks blowing out the posterior cortex and losing fixation; too anterior risks the subtalar joint. Aim for the body of the calcaneus just anterior to the Achilles footprint, and confirm with imaging if uncertain.