tarsal tunnel
Summary
The tarsal tunnel is a fibro-osseous passage on the medial ankle, posterior and inferior to the medial malleolus, bounded by the flexor retinaculum. It transmits the tibial nerve and tendons of tibialis posterior, flexor digitorum longus, and flexor hallucis longus, plus the posterior tibial artery and vein. Compression of the tibial nerve here causes tarsal tunnel syndrome: burning pain, paresthesias, and numbness of the sole and toes.
Detail
ANATOMY: The tunnel lies behind and below the medial malleolus. The floor is formed by the calcaneus, talus, and medial malleolus (the bony surfaces). The roof is the flexor retinaculum (laciniate ligament). Contents, from anterior to posterior, follow the mnemonic "Tom, Dick, And Very Nervous Harry": Tibialis posterior tendon, flexor Digitorum longus tendon, posterior tibial Artery, posterior tibial Vein, tibial Nerve, flexor Hallucis longus tendon. The tibial nerve (L4-S3) divides within or just distal to the tunnel into the medial plantar nerve, lateral plantar nerve, and medial calcaneal nerve.
TARSAL TUNNEL SYNDROME (TTS): This is an entrapment neuropathy of the tibial nerve or its branches. It is the ankle analogue of carpal tunnel syndrome.
Causes: - Space-occupying lesions: ganglion cyst, lipoma, varicose veins, tenosynovitis, osteophytes, accessory muscle (accessory soleus or abductor hallucis accessorius) - Biomechanical: flatfoot/pes planus with excessive pronation, hindfoot valgus, which stretch the nerve - Trauma: ankle sprain, fracture (calcaneus, medial malleolus), post-traumatic scarring - Systemic: diabetes, rheumatoid arthritis, hypothyroidism, edema or pregnancy, obesity - Many cases are idiopathic
Clinical features: - Burning, tingling, or shooting pain and numbness in the sole, toes, and sometimes the heel - Symptoms often worsen with standing, walking, or at night, and may improve with rest - The medial calcaneal branch may arise proximal to the tunnel, so the heel can be spared - Positive Tinel sign over the tunnel (percussion reproduces paresthesias) - Dorsiflexion-eversion test reproduces symptoms - Late: intrinsic foot muscle weakness or atrophy, and clawing of the toes
Diagnosis: - Primarily clinical - Nerve conduction studies/EMG support the diagnosis but have limited sensitivity - MRI or ultrasound to identify a mass lesion
Treatment: - Conservative: rest, NSAIDs, orthotics or arch support for pes planus, corticosteroid injection - Surgical tarsal tunnel release (decompression) if conservative therapy fails or a mass is present
BOARD DIFFERENTIALS: - Plantar fasciitis: heel pain worst with the first steps in the morning, with no paresthesias - Diabetic peripheral neuropathy: stocking-glove distribution, bilateral - S1 radiculopathy: back pain, reduced ankle reflex - Morton neuroma: pain and numbness between the 3rd and 4th toes (interdigital nerve compression) - Tibial nerve injury (higher): weak plantarflexion and inversion, loss of sensation over the entire sole
HIGH-YIELD PEARL: Tarsal tunnel syndrome affects the sole (tibial nerve branches) and not the dorsum of the foot. The deep peroneal nerve supplies the dorsal first web space, and the superficial peroneal nerve supplies most of the dorsum. Neither passes through the tarsal tunnel.
Sources
- First Aid for the USMLE Step 1 (Anatomy: Lower Limb Nerves; Musculoskeletal)
- Moore's Clinically Oriented Anatomy
- Netter's Atlas of Human Anatomy
- Gray's Anatomy for Students
- UpToDate: Tarsal tunnel syndrome
- StatPearls: Tarsal Tunnel Syndrome
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