Flexor Digitorum Longus Anatomy

Flexor digitorum longus (FDL) is the most medial of the three muscles of the deep posterior compartment. It arises from the posterior surface of the tibia below the soleal line - the only one of the three deep flexors to arise solely from the tibia - and its tendon runs behind the medial malleolus, crosses the sole, and divides into four slips for the lateral four toes.

Two features dominate its surgical importance. The first is that it is relatively expendable: because of its tendinous interconnections with flexor hallucis longus at the knot of Henry, FDL can be harvested and the lesser toes usually retain active flexion, driven onward by FHL through the interconnection. That underpins its role as the standard donor for flexor digitorum longus transfer in stage II posterior tibial tendon dysfunction, one of the highest-yield operations in foot and ankle surgery. But relatively is the load-bearing word, and the page returns to it below: simulated harvest still costs about 23% of lesser-toe push-off force, and in roughly one foot in seven the interconnection is absent altogether. The anatomy preserves movement rather than strength, and it is not guaranteed to be present in the foot in front of you.

The second is that it is a deforming force. Once the intrinsic muscles of the foot fail - in neuropathy, in cavovarus, in a compartment syndrome, in the diabetic foot - the unopposed long flexors and long extensors produce the claw toe, and the FDL is the muscle pulling the interphalangeal joints into flexion.

Actions

• Flexion of the distal interphalangeal joints of the second to fifth toes - its unique and irreplaceable action, since flexor digitorum brevis reaches only the middle phalanges.
• Flexion of the proximal interphalangeal and metatarsophalangeal joints of the lesser toes, assisted by brevis and the intrinsics.
• Plantarflexion of the ankle - weak, with a small moment arm.
• Inversion of the hindfoot and dynamic support of the medial longitudinal arch, because it passes beneath the sustentaculum tali and along the medial column, synergistic with tibialis posterior.

Role in gait

• Active from mid-stance to terminal stance, peaking around heel rise.
• It presses the toes to the floor during push-off, broadening the weightbearing base of the forefoot, stabilising the toes against the ground and increasing the effective lever arm of the foot.
• Toe purchase matters: a foot in which the toes do not touch the ground at push-off loads the metatarsal heads more heavily. This is one mechanism of metatarsalgia in a claw toe foot.
• It is in phase with tibialis posterior, which is why it can be transferred to substitute for it without any need for phase re-education.

Strength and excursion - the numbers that drive transfer surgery

The conclusion an examiner wants: the FDL transfer in flatfoot reconstruction works as a tenodesis, a checkrein and a proprioceptive restraint rather than as a strength substitute. The medial displacement calcaneal osteotomy performs the mechanical correction; the tendon transfer holds it and adds some dynamic support. Anyone who claims the transfer restores tibialis posterior power has misunderstood the biomechanics.

What happens when FDL fails

• Loss of active distal interphalangeal flexion of the lesser toes - detectable on examination but rarely a functional complaint in isolation.
• Loss of toe purchase at push-off, transferring load to the metatarsal heads.
• In combination with intrinsic failure, the picture is very different: the FDL becomes a deforming force, and clawing results.

Palpation and isolation

• The tendon can be palpated immediately posterior to the tibialis posterior tendon behind the medial malleolus, with the toes actively flexed and relaxed.
• Isolate FDL by stabilising the middle phalanx of a lesser toe and asking the patient to flex the distal interphalangeal joint. Flexor digitorum brevis inserts on the middle phalanx and cannot flex the DIP joint, so DIP flexion is FDL alone. This is directly analogous to isolating flexor digitorum profundus in the hand.
• Isolate flexor digitorum brevis by holding all the other toes in extension (which tenodeses the common FDL muscle belly) and asking for PIP flexion of the tested toe.

Named tests and signs

Assessing a lesser toe deformity properly

The vocabulary matters and examiners test it:

• Claw toe: metatarsophalangeal joint extended, proximal interphalangeal flexed, distal interphalangeal flexed. Usually multiple toes, and usually neurological - intrinsic failure with unopposed long flexors and extensors.
• Hammer toe: metatarsophalangeal joint neutral or extended, proximal interphalangeal flexed, distal interphalangeal neutral or extended. Usually a single toe, often the second, and usually mechanical - footwear, a long second ray, or hallux valgus crowding.
• Mallet toe: distal interphalangeal flexed alone, with the MTP and PIP normal. This is specifically an FDL problem, and it is the deformity most reliably corrected by a simple flexor tenotomy.

Always determine flexibility with the push-up (Kelikian) test: push the metatarsal head dorsally to simulate weightbearing. If the toe straightens, the deformity is flexible and a soft-tissue procedure will hold; if it does not, a bony procedure is required.

Always look for the cause. A claw toe pattern in multiple toes demands a neurological examination - diabetes, Charcot-Marie-Tooth disease, a previous compartment syndrome, a spinal lesion. Treating the toes without diagnosing the neuropathy is a missed diagnosis.

Related pages. The tendon this one replaces: Tibialis Posterior Anatomy is the failing motor, and Flatfoot Deformity - Adult is the deformity the transfer is performed for, with Flexible Flatfoot and Rigid Flatfoot separating the feet that can be reconstructed from those that need fusion. Spring Ligament Insufficiency is the ligament failure a tendon transfer cannot hold against on its own.

FDL transfer for stage II posterior tibial tendon dysfunction

Lesser toe deformity and flexor surgery

• The pathomechanics: the intrinsic muscles (interossei and lumbricals) flex the metatarsophalangeal joints and extend the interphalangeal joints. When they fail - neuropathy, cavovarus, previous compartment syndrome - the long extensors extend the MTP unopposed and the long flexors flex the interphalangeal joints unopposed. The result is the claw toe.
• The MTP joint is the key. Once the MTP hyperextends, the plantar plate is drawn distally, the metatarsal head is driven plantarwards, the fat pad migrates distally and no longer cushions the head, and metatarsalgia and plantar keratosis follow. Correcting the toe alone without addressing the MTP joint will not relieve the metatarsalgia.
• Flexible deformity (positive push-up test):
  • Flexor tenotomy - simple division of the flexor tendon, percutaneous or open.
  • Flexor-to-extensor transfer (Girdlestone-Taylor) - the FDL tendon is split longitudinally and both limbs are passed dorsally either side of the proximal phalanx and sutured to the extensor hood, converting the deforming flexor into an MTP flexor and IP extensor and reproducing the lost intrinsic function. It requires a flexible deformity and is combined with an MTP release where the joint is contracted.
• Fixed deformity (negative push-up test):
  • PIP resection arthroplasty or arthrodesis for the interphalangeal component.
  • MTP release, extensor lengthening, and a shortening metatarsal osteotomy (Weil) for the MTP component.
  • Soft-tissue procedures alone will recur.

Percutaneous flexor tenotomy in the diabetic foot

• Indication: a neuropathic ulcer at the tip of a clawed or hammered toe in a patient with adequate perfusion. The deformity drives the tip of the distal phalanx into the ground, and no dressing or offloading device will heal an ulcer that is being re-traumatised at every step.
• Procedure: a percutaneous division of the flexor tendon at the plantar aspect of the toe through a needle or a small blade stab incision, usually under local anaesthetic in the clinic, taking seconds per toe.
• Effect: the toe straightens, the tip lifts off the ground, and the ulcer offloads. Reported healing rates are high and recurrence rates low, with a very low complication rate. It is a genuinely high-value, low-cost, low-risk intervention.
• Prerequisites: adequate arterial perfusion, no active spreading infection or osteomyelitis requiring other treatment, and a flexible deformity - a rigidly fixed toe needs a bony procedure.
• Counselling: the toe will float slightly and lose active flexion. In a neuropathic foot this is a trivial price for ulcer healing, and patients should be told it will happen.

Other FDL pathology

• Tenosynovitis of the FDL occurs alongside tibialis posterior tenosynovitis in the medial hindfoot, and can contribute to tarsal tunnel symptoms by crowding the tunnel.
• Laceration of the FDL in the sole or behind the medial malleolus is uncommon and, in isolation, well tolerated. It should be looked for specifically in any medial ankle or plantar laceration, alongside the tibial nerve.
• Flexor digitorum accessorius longus is an accessory muscle arising in the deep posterior compartment and inserting into the FDL or quadratus plantae. It is present in a minority of feet and is a recognised cause of tarsal tunnel syndrome by occupying space in the tunnel; it is also a recognised incidental MRI finding that should not be mistaken for a mass.
• Deep posterior compartment syndrome infarcts FDL along with FHL and tibialis posterior. The late sequel is fixed clawing of the toes with plantar sensory loss and a cavovarus foot.

The medial hindfoot approach - the anatomy in operative order

• Incision: curved, 1 cm posterior to the medial malleolus, curving distally toward the navicular tuberosity.
• Layer 1: skin and subcutaneous tissue. Protect the saphenous vein and nerve anteriorly and the medial calcaneal branches of the tibial nerve crossing the distal wound.
• Layer 2: the flexor retinaculum (laciniate ligament), incised in line with the incision.
• Layer 3, anterior to posterior: tibialis posterior (in the malleolar groove, thick, inserting on the navicular), then flexor digitorum longus (thinner, dividing into four slips), then the posterior tibial artery and veins, then the tibial nerve, then flexor hallucis longus most posteriorly.
• The safe plane for FDL harvest is between tibialis posterior and FDL, strictly anterior to the neurovascular bundle.
• Extending the incision distally exposes the spring ligament (deep to the tendons, supporting the talar head) and the knot of Henry with the medial plantar neurovascular structures.

Distances and safe zones

• Medial plantar nerve at the knot of Henry - immediately adjacent; work under direct vision.
• Sural nerve in a calcaneal osteotomy - conventionally quoted at roughly 1.5 to 2 cm posterior and inferior to the tip of the lateral malleolus, but more reliably planned as a proportion: keep the incision at or beyond one third of the distance from the fibular tip to the posteroinferior calcaneal margin, and about 1 to 1.5 cm posterior to the peroneal tendons.
• Medial calcaneal wall in a medial displacement osteotomy - translation of 8 to 12 mm is standard; beyond about 10 mm the tibial nerve and its branches are progressively tented and the risk rises.
• Navicular drill hole - directed plantar to dorsal, entering at the plantar-medial aspect of the tuberosity, avoiding the talonavicular and naviculocuneiform joint surfaces.
• Flexor tenotomy in the toe - the flexor tendon lies immediately deep to the plantar skin at the level of the proximal or middle phalanx, with the digital neurovascular bundles at the plantar-medial and plantar-lateral corners. Aim strictly midline plantar and pass the blade transversely against the bone.

Other operations involving FDL

• Posteromedial approach to the tibia: the FDL is elevated from the posterior tibia, with the neurovascular bundle retracted, giving access to the posterior tibial cortex for plating a posterior malleolar fragment or grafting a nonunion.
• Posteromedial fasciotomy: the incision lies 1 to 2 cm posterior to the posteromedial tibial border. The superficial posterior compartment is opened first, then the deep transverse fascia must be deliberately divided to release the deep compartment containing FDL. Failing to do so is the classic incomplete fasciotomy, and its late sequel is a clawed, insensate foot.
• Tarsal tunnel release: the FDL sheath is opened as part of the release, and an accessory flexor digitorum accessorius longus is excised if present.
• FDL transfer to the fifth metatarsal base: used to restore eversion when both peroneal tendons are irreparable.
• FDL transfer for Achilles reconstruction: an alternative to FHL, weaker but preserving hallux function; occasionally preferred in a dancer or sprinter.
• Calcaneal fracture fixation: a screw directed from lateral to medial into the sustentacular fragment can transfix the FDL and the neurovascular bundle if it over-penetrates the medial cortex.

FDL as a donor - the general principles it illustrates

Flexor digitorum longus is the textbook example of a good transfer donor, and the reasons generalise:

1. Expendable - its function is duplicated or compensated (here, by the FHL interconnection at the knot of Henry).
2. Adequate excursion for the intended job - 2 to 3 cm.
3. In phase with the recipient, so no re-education is needed.
4. A straight line of pull through a smooth gliding bed, without an acute turn.
5. Adequate power - though here the power is inadequate to substitute fully, which is why the transfer is combined with a bony procedure. That candid limitation is precisely the point examiners want acknowledged.
6. A supple, passively correctable recipient joint - stage II, not stage III.

Anatomical variation across populations

• Knot of Henry interconnections are consistently present but variable in pattern. The commonest arrangement transmits drive from FHL to FDL, but reciprocal slips and more complex patterns occur, and the slips vary in number and thickness. The functional consequence of a harvest therefore cannot be assumed to be identical in every foot, and the surgeon should inspect the interconnection directly.
• Quadratus plantae varies in the presence and size of its two heads; the lateral head is absent in a minority of feet, which theoretically alters the vector correction.
• Flexor digitorum accessorius longus is an accessory muscle present in a minority of feet, arising in the deep posterior compartment and inserting into the FDL or quadratus plantae. It is a recognised cause of tarsal tunnel syndrome and an incidental MRI finding that should not be mistaken for a mass.
• Lumbrical number and configuration occasionally varies, with absence of the fourth lumbrical described.

Side-by-side guidance

Resource-dependent practice

• Well-resourced settings: weightbearing CT, MRI, interference screw fixation, spring ligament internal bracing, arthroereisis implants.
• Limited-resource settings: the diagnosis of stage II posterior tibial tendon dysfunction is entirely clinical - too-many-toes, single heel raise, passive correctability - and weightbearing radiographs suffice for planning. The FDL transfer with medial displacement calcaneal osteotomy requires only a saw, two screws, suture and basic instruments, and is fully deliverable at district hospital level. A well-executed triple arthrodesis remains a reliable operation for a rigid deformity where implants are limited.
• Percutaneous flexor tenotomy is one of the highest-value interventions in global diabetic foot care: it requires a blade, local anaesthetic and a few minutes, and it converts a chronically non-healing tip ulcer into a healing one. In settings where amputation is the default endpoint for a chronic toe ulcer, its availability changes outcomes substantially.

Registry and outcome signals

• There is no dedicated registry for flatfoot reconstruction or lesser toe surgery. Institutional cohorts consistently show durable improvement after combined FDL transfer and bony realignment for flexible stage II deformity, with the main causes of failure being under-correction, an unaddressed gastrocnemius contracture, and unrecognised fixed forefoot varus.
• Diabetic foot series consistently report high ulcer healing rates and low recurrence at the treated toe after percutaneous flexor tenotomy, with the principal caveats being adequate perfusion and a flexible deformity.
• Major trauma and open fracture audits consistently link delayed or incomplete fasciotomy - most often failure to release the deep posterior compartment - to poor functional outcomes including fixed toe clawing.