Tibial Nerve Anatomy

The tibial nerve is the workhorse of the posterior leg and foot. It plantarflexes the ankle, flexes the toes, inverts the foot and gives the sole its sensation, so it is the nerve you assess in a knee dislocation, a posterior-leg or popliteal injury, and any patient with sole symptoms.

Its exam value clusters at two sites: the popliteal fossa, where it is at risk with the popliteal artery in a knee dislocation, and the tarsal tunnel behind the medial malleolus, where it and its plantar branches are compressed or entrapped. Hold three ideas together: the course from the sciatic nerve to the plantar nerves, the deficit (loss of plantarflexion and toe flexion with an insensate sole), and the entrapments (tarsal tunnel syndrome, Baxter's nerve, jogger's foot).

Origin. The tibial nerve is the larger terminal branch of the sciatic nerve and carries fibres from the anterior (ventral) divisions of L4-S3. The sciatic nerve arises from the sacral plexus and runs deep to gluteus maximus; in the commonest of the Beaton and Anson patterns, the tibial division passes anterior to piriformis. The sciatic nerve typically divides into the tibial and common peroneal nerves at the apex of the popliteal fossa, though a higher division is a common variant.

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The popliteal fossa. Here the tibial nerve is the most superficial structure of the neurovascular bundle: nerve, then popliteal vein, then the artery deepest. The ultrasound-guided popliteal sciatic nerve block uses this arrangement, identifying the tibial and common peroneal nerves superficial to the popliteal artery.

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The leg. The nerve passes deep to gastrocnemius and the fibrous arch of soleus into the deep posterior compartment, where it runs with the posterior tibial vessels on tibialis posterior. It gives muscular branches throughout the leg and a medial sural cutaneous nerve that helps form the sural nerve.

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The tarsal tunnel. Behind the medial malleolus the nerve enters the tarsal tunnel, deep to the flexor retinaculum, which bridges the medial malleolus and the medial calcaneal tubercle and roofs the tunnel. From anterior to posterior the tunnel holds:

• Tibialis posterior, most anterior, against the medial malleolus
• Flexor digitorum longus
• Posterior tibial artery, with its venae comitantes
• Tibial nerve
• Flexor hallucis longus, most posterior

The order is remembered as "Tom, Dick, And Very Nervous Harry".

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Terminal division. The nerve ends as the medial and lateral plantar nerves, with a medial calcaneal branch to the heel, and variations such as a high division or a trifurcation in the tunnel occur. The first branch of the lateral plantar nerve (Baxter's nerve) runs across the heel.

Motor. The tibial nerve supplies both posterior compartments of the leg: the superficial (gastrocnemius, soleus, plantaris) and the deep (tibialis posterior, flexor digitorum longus, flexor hallucis longus, popliteus). Plantarflexion by gastrocnemius-soleus ("tiptoe"), inversion by tibialis posterior and toe flexion by FDL and FHL all depend on it, and through the plantar nerves it supplies all the intrinsic plantar muscles.

The sole mirrors the hand. The medial plantar nerve is the median nerve of the foot and the lateral plantar nerve its ulnar nerve, with the same medial 3.5 and lateral 1.5 digit split.

• Medial plantar nerve: the medial sole and plantar skin of the medial 3.5 toes; motor to abductor hallucis, FDB, FHB and the first lumbrical
• Lateral plantar nerve: the lateral sole and lateral 1.5 toes; motor to most of the other intrinsics
• Medial calcaneal branch: heel-pad sensation
• Medial sural cutaneous nerve: a contribution to the sural nerve (posterolateral leg and lateral foot)

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Posterior tibial nerve block. The tibial nerve is blocked behind the medial malleolus, just posterior to the posterior tibial artery, as part of an ankle block for forefoot and sole surgery; it is the component that anaesthetises the sole. The practical landmark is the nerve's position posterior to the pulse.

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Jogger's foot. The medial plantar nerve can be entrapped at the knot of Henry, where the FDL and FHL tendons cross, or under abductor hallucis. It is classically seen in runners with a valgus or pronated foot and produces medial arch and sole burning with altered sensation of the medial sole and medial toes.

Baxter's nerve. The first branch of the lateral plantar nerve, also called the inferior calcaneal nerve, can be entrapped between abductor hallucis and quadratus plantae. It is a frequently missed cause of chronic plantar heel pain that mimics plantar fasciitis: suspect it when heel pain is refractory or associated with abductor digiti minimi wasting.

The anatomy is the diagnosis. Baxter's nerve entrapment and jogger's foot are both tibial-nerve-branch entrapments, and plain plantar-fasciitis management will fix neither.

Proximal entrapment at the soleal sling. The tarsal tunnel is the distal entrapment site. The nerve can also be compressed proximally where it passes deep to the tendinous arch of soleus (the "soleal sling") into the deep posterior compartment, a proximal or high tibial neuropathy, and a recognised (often exertional) cause of posterior-leg and sole symptoms.

Localise the Tinel sign. A soleal-sling lesion produces calf pain with plantar paraesthesia and a Tinel sign high in the popliteal or proximal-calf region rather than behind the medial malleolus, so a medial-ankle examination will miss it. Consider the soleal sling when tarsal-tunnel signs are absent.

High (proximal) injury. It occurs with knee dislocation, proximal tibial trauma or popliteal injury. The deficit is loss of plantarflexion, inversion and toe flexion (a calcaneovalgus foot), loss of the intrinsic foot muscles and, crucially, an insensate sole. The insensate plantar surface historically influenced the decision between limb salvage and amputation, though this is now applied cautiously as outcomes data have evolved.

Tarsal tunnel syndrome. Compression of the tibial nerve or its plantar branches in the tunnel behind the medial malleolus causes burning or tingling sole pain and paraesthesia, worse with activity, with a positive Tinel sign over the tunnel. The causes:

• Space-occupying lesions: ganglion, varicosities, accessory muscle
• Hindfoot valgus or planus deformity, through traction
• Trauma
• Systemic disease (diabetes)

Investigate with examination, electrodiagnostics and MRI or ultrasound. Treat the cause, with release for refractory compressive cases.

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Why the heel is often spared. The medial calcaneal branch supplies the heel pad, and where it leaves the tibial nerve is the reason heel sensation is often preserved in tarsal tunnel syndrome. It most commonly arises proximal to the other two terminal branches, above the level of compression, so the sole goes numb while the heel does not.

Use that sign in one direction only. In a branching-pattern study of eleven legs the medial calcaneal nerve arose most proximally in only 64%. In the remainder it arose at the same level as the plantar nerves or distal to them, and its number and origin are described as highly variable across every cadaveric series. So a spared heel supports tarsal tunnel syndrome, an involved heel does not exclude it, and a decompression planned on the assumption that the calcaneal branch is safely proximal will sometimes be wrong.

Release beyond the retinaculum. The tibial nerve divides into the medial and lateral plantar nerves inside the tunnel in about 82% of feet, not distal to it. A release that stops at the flexor retinaculum therefore leaves the divided branches, and their own separate fibro-osseous channels, undecompressed. That is a recognised reason for persistent symptoms after apparently adequate surgery.

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The spastic equinovarus foot. The same anatomy that explains an insensate sole after a high injury explains where tone is targeted in the spastic equinovarus foot after stroke, traumatic brain injury or cerebral palsy, a high-yield neuro-orthopaedic point separate from trauma and entrapment. The targets are the tibial nerve's motor branches to the triceps surae (gastrocnemius and soleus) and tibialis posterior.

• Diagnostic block. A local anaesthetic block of the tibial nerve or its motor branches is performed first. Dynamic spasticity relaxes after the block and a fixed contracture does not, which predicts the response to definitive treatment.
• Chemodenervation. Botulinum toxin or phenol to the gastrocsoleus and tibialis posterior motor branches gives reversible tone reduction and is often the first definitive step.
• Selective (fascicular) tibial neurotomy. Partial section of the motor branches to the spastic muscles gives a durable reduction in equinovarus tone, an alternative or adjunct to the orthopaedic options of tendo-Achilles lengthening and split tibialis anterior or posterior transfer.

Global Practice Picture

Tibial nerve anatomy underpins management of tarsal tunnel syndrome, plantar heel pain (Baxter's nerve), posterior leg compartment and popliteal injuries, and the assessment of the insensate foot. The internationally consistent principles are: know the tarsal tunnel contents and plantar division, diagnose tarsal tunnel syndrome clinically with electrodiagnostic/imaging support, and treat the underlying cause.

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