Flexor Digitorum Longus Tendinitis: Symptoms, Causes & Treatment

Flexor digitorum longus (FDL) tendinitis is inflammation of the tendon that runs behind the medial ankle and along the inner arch to flex the four lesser toes. It produces pain and tenderness along the medial ankle and arch with activity, often mistaken for posterior tibial tendon dysfunction (PTTD) or tarsal tunnel syndrome. An accurate anatomical diagnosis determines whether treatment targets the FDL or an adjacent structure.

At Balance Foot & Ankle in Howell and Bloomfield Hills, MI, we differentiate FDL tendinitis from other medial ankle and arch pathologies using clinical examination, resisted toe flexion testing, and ultrasound imaging that directly visualizes the FDL tendon sheath.

FDL Tendinitis vs. Similar Medial Ankle Conditions

Condition	Pain Location	Key Clinical Sign	Imaging Finding
FDL tendinitis	Behind medial malleolus; along arch to toes	Pain with resisted flexion of toes 2–5; tender along FDL course	Ultrasound: FDL tendon sheath thickening, peritendinous fluid
Posterior tibial tendon dysfunction (PTTD)	Behind medial malleolus; navicular insertion	Unable to perform single-leg heel rise; progressive flatfoot	MRI: PTT tear or degeneration; flatfoot deformity on weight-bearing X-ray
Tarsal tunnel syndrome	Behind medial malleolus; burning into heel/arch/toes	Tinel’s sign at tarsal tunnel; sensory deficit on plantar foot	MRI: space-occupying lesion; EMG/NCV: posterior tibial nerve slowing
FHL tendinitis (flexor hallucis longus)	Behind medial malleolus; under first metatarsal	Pain with resisted hallux flexion; hallux trigger (stenosing tenosynovitis)	Ultrasound: FHL sheath effusion behind ankle; MRI: signal change in FHL
Medial ankle stress fracture	Medial malleolus; may radiate proximally	Point tenderness on bone; pain with tuning fork vibration	MRI: stress reaction; X-ray often normal early

Anatomy: Why the FDL Is Vulnerable

The flexor digitorum longus originates from the posterior tibia, travels behind the medial malleolus through a fibrous groove alongside the posterior tibial tendon and flexor hallucis longus (the “Tom, Dick, and Harry” tendons: Tibialis posterior, flexor Digitorum longus, and flexor Hallucis longus), then crosses the arch and divides into four slips that attach to the distal phalanges of toes 2–5. The FDL makes a sharp angular turn at the master knot of Henry in the arch—a fibrocartilaginous pulley where the FDL and FHL tendons cross. This corner creates a zone of increased mechanical stress and friction, making it a common site for tenosynovitis.

Causes and Risk Factors

FDL tendinitis typically results from overuse in running, dance, and jumping sports that require repetitive toe push-off. Training errors—rapid mileage increases, hard surfaces, inadequate recovery—are common precipitants. Flexible flatfoot overloads the FDL by requiring more active toe flexor work to stabilize the arch during propulsion. Hypermobile first ray (excessive dorsiflexion of the first metatarsal) shifts propulsive load onto the lesser toes, increasing FDL demand. Poorly fitting footwear with a narrow toe box can compress the FDL at the distal course.

Treatment Protocol

Phase	Timeframe	Interventions	Goals
Acute (pain control)	Weeks 1–2	Activity modification; ice 15–20 min 3x/day; NSAIDs; offloading boot if severe	Reduce inflammation; protect tendon
Subacute (load introduction)	Weeks 3–6	Physical therapy: eccentric toe flexion, calf stretching; custom orthotic (medial arch support, metatarsal pad); ultrasound-guided corticosteroid injection if needed	Restore tendon gliding; address biomechanics
Strengthening	Weeks 6–12	Progressive intrinsic foot strengthening; single-leg balance; sport-specific loading	Load tolerance for return to activity
Return to sport	Weeks 10–16	Gradual mileage reinstatement; form correction; custom orthotic in sport shoe	Full activity without pain
Surgery (if conservative fails)	>6 months conservative	FDL tenosynovectomy; master knot of Henry release; transfer if PTTD component	Remove adhesions; restore gliding

FDL Tendinitis Treatment at Balance Foot & Ankle

We offer in-office diagnostic ultrasound to confirm FDL involvement and differentiate it from PTTD and FHL tendinitis, ultrasound-guided corticosteroid injection into the FDL tendon sheath, custom orthotic casting, and physical therapy coordination. Our Howell (4330 E Grand River Ave) and Bloomfield Hills (43494 Woodward Ave #208) offices see both acute injuries and chronic overuse cases. Call (810) 206-1402.