Medically reviewed by Dr. Tom Biernacki, DPM
Board-certified podiatric surgeon | Balance Foot & Ankle, Howell & Bloomfield Hills, MI
Last reviewed: May 2026
The flexor digitorum longus plays a surprisingly large role in lesser toe deformities — but the specific tendon transfer technique that reverses hammer toe depends on one biomechanical factor your podiatrist measures at the first visit. Call (810) 206-1402 — expert podiatric care across Michigan.
The flexor digitorum longus (FDL) is a deep posterior compartment muscle of the leg that originates from the posterior tibial shaft and descends behind the medial malleolus to cross the plantar foot at the knot of Henry (where it crosses over the flexor hallucis longus), dividing into four slips that insert into the distal phalanges of the lesser toes (second through fifth). FDL contracts to curl the lesser toes — a critical function for toe purchase during push-off and for stabilizing the toes against the ground during the stance phase of gait. In podiatric surgery, the FDL tendon is important as the most commonly used tendon transfer source for posterior tibial tendon dysfunction reconstruction: the FDL is expendable because the quadratus plantae and FHB provide sufficient residual toe flexion, and its size, location, and strength make it ideal for replacing the function of the torn tibialis posterior tendon. FDL pathology ranges from isolated tenosynovitis to tendinopathy to complete rupture, with clinical presentation that can mimic other posteromedial ankle and foot conditions.
FDL Tendon Conditions: Anatomy, Pathology, and Clinical Differentiation
| Condition | Mechanism | Symptoms | Exam Findings | Treatment |
|---|---|---|---|---|
| FDL tenosynovitis | Overuse from running, hiking, or dance; hyperpronation causing increased FDL load; inflammatory arthritis (RA, seronegative spondyloarthropathy); ill-fitting footwear compressing medial ankle | Posteromedial ankle pain behind and below medial malleolus; worse with walking and stairs; swelling along tendon course; crepitus with toe flexion | Tenderness along FDL tendon behind medial malleolus; crepitus (snowball crepitus) with resisted toe flexion; swelling in tendon sheath; full strength; no atrophy | CAM boot 4-6 weeks; NSAIDs; PT; ultrasound-guided corticosteroid injection in sheath (not intratendinous); tendon sheath surgical release if recalcitrant (>3-6 months) |
| FDL tendinopathy (chronic) | Cumulative load failure with mucoid degeneration; chronic hyperpronation; associated with flatfoot and PTTD; midsubstance tendon degeneration at knot of Henry or medial malleolar groove | Chronic posteromedial ankle aching; less swelling than tenosynovitis; activity-related medial arch pain; pain at knot of Henry with push-off | Tenderness at knot of Henry or along medial arch; mild thickening of tendon on palpation; FDL strength preserved but painful; ultrasound shows tendon thickening | Eccentric loading program; orthotics for pronation correction; PRP injection for chronic tendinopathy; surgical debridement for severe intratendinous degeneration |
| FDL rupture | Rare; direct laceration; closed rupture with forced toe extension; often in context of existing tendinopathy; may occur during FDL transfer harvest if excessive tension applied | Acute medial arch or plantar foot pain; inability to flex lesser toes; toe hyperextension posture; gap in tendon palpable in acute cases | Absent FDL function (unable to flex distal phalanx of lesser toes against resistance); toe extensors may dominate, creating clawing posture; MRI shows tendon discontinuity | Surgical repair for acute isolated rupture in young active patients; FDL absence generally tolerated as FDB and lumbricals maintain toe purchase; chronic rupture: rarely requires surgery; quadratus plantae partially compensates |
| FDL transfer (surgical use) | Elective harvest for PTTD reconstruction; FDL rerouted to navicular to replace torn posterior tibial tendon; combined with calcaneal osteotomy (Stage II PTTD standard of care) | Post-operative: mild loss of lesser toe flexion strength (usually not functionally significant); patients rarely notice toe flexion deficit after recovery; donor site complications uncommon | Post-transfer: reduced ability to flex distal phalanges of 2nd-5th toes (proximal interphalangeal flexion preserved via FDB, lumbricals); arch elevation; improved single heel rise | Standard of care for Stage II PTTD; 85-90% good-excellent outcomes; FDL expendable because quadratus plantae partially maintains FDL function through the knot of Henry |
FDL vs. FHL vs. Posterior Tibial Tendon: Posteromedial Ankle Anatomy and Differential
| Structure | Position Behind Medial Malleolus | Function | Pathology Clue | Clinical Test |
|---|---|---|---|---|
| Posterior tibial tendon (PTT) | Most anterior (Tom mnemonic: “T” = tibialis, first position) | Primary arch support; subtalar inversion; first MTP plantarflexion; critical for heel rise | PTTD: medial ankle pain, progressive flatfoot, failed single heel rise; “too many toes” sign | Single heel rise test (most important); resisted inversion in plantarflexion; navicular drop test |
| Flexor digitorum longus (FDL) | Middle position (“D” = digitorum, second) | Flexes distal phalanges of toes 2-5; toe purchase during push-off; arch dynamic support | Tenosynovitis: medial ankle swelling with crepitus; tendinopathy: medial arch ache; rarely ruptures in isolation | Resisted lesser toe DIP flexion against resistance; FDL moves all four lesser toe distal phalanges simultaneously |
| Flexor hallucis longus (FHL) | Most posterior (“A” = arteria = posterior, third — behind both) | Flexes hallux; major push-off power; arch support; stabilizes medial column at push-off | FHL tenosynovitis / stenosis: hallux trigger, posterior ankle pain; os trigonum; ballet dancers | Resisted hallux IP flexion; FHL locking (hallux saltans / trigger toe); Strayer test for os trigonum impingement |
| Mnemonic | “Tom, Dick, And Very Nervous Harry” — Tibialis posterior, Digitorum longus, And (artery) = posterior tibial artery, Vein, Nerve (posterior tibial nerve), Hallucis longus — positions anterior to posterior in tarsal tunnel | |||
At Balance Foot & Ankle in Howell and Bloomfield Hills, posteromedial ankle pain with crepitus on toe flexion suggests FDL tenosynovitis — distinguished from posterior tibial tendon dysfunction by the crepitus sign and preserved single heel rise — and responds well to sheath injection and boot immobilization before the inflammatory process leads to adhesive tenosynovitis requiring surgical release. Call (810) 206-1402.
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FDL tendinopathy under-diagnosed in active adults
The flexor digitorum longus tendon runs along the inside of the ankle and powers toe-curl push-off. Tendinopathy of FDL produces medial ankle pain with toe-curl resistance and is commonly mistaken for posterior tibial tendinopathy. Treatment relies on focused PT, a short period of immobilization, and selective injection. Surgical decompression is rare and reserved for refractory cases.
Balance Foot & Ankle — Howell & Bloomfield Hills, MI: board-certified podiatrists, same-week appointments, most insurance accepted.
Book a Medial-Ankle Tendon Evaluation → or call (810) 206-1402
Related reading: posterior tibial tendonitis · tarsal tunnel syndrome · best shoes for flat feet
Doctor Answer
What is the flexor digitorum longus and what conditions affect it in the foot?
The flexor digitorum longus (FDL) is a deep posterior leg muscle whose tendon passes behind the medial malleolus and divides to flex the lesser toes. It can become inflamed (FDL tendonitis), torn, or entrapped at the master knot of Henry near the navicular, causing medial arch pain. Dr. Tom Biernacki at Balance Foot & Ankle diagnoses FDL tendon pathology with ultrasound and MRI, providing targeted conservative and surgical treatment to restore function.
