Tarsal Tunnel

The tarsal tunnel is an osteofibrous passage on the posteromedial aspect of the ankle through which important tendons, vessels, and nerves pass from the posterior compartment of the leg into the foot. Its roof is formed by the flexor retinaculum, while its floor is formed by the underlying bones and ligamentous structures of the medial ankle and hindfoot.^[1][2]

The tunnel contains the tendons of tibialis posterior, flexor digitorum longus, and flexor hallucis longus, together with the posterior tibial artery and veins and the tibial nerve. These structures follow a characteristic arrangement behind the medial malleolus before diverging toward their destinations in the foot.

The tarsal tunnel is particularly important because the tibial nerve passes through this confined region and commonly divides into its medial and lateral plantar branches within or near the tunnel. Compression of the tibial nerve or its branches in this region forms the anatomical basis of tarsal tunnel syndrome.

Location

The tarsal tunnel lies on the medial side of the ankle, posterior and inferior to the medial malleolus. It extends from the distal posteromedial leg toward the medial aspect of the hindfoot and proximal sole.

The tunnel follows a curved course around the medial ankle. Structures entering it from the posterior compartment of the leg are redirected around the medial side of the ankle before continuing into the plantar foot.

Its superficial boundary can be approximated on the surface between the medial malleolus and the medial calcaneal region. The posterior tibial pulse can usually be sought within this same general area.

Boundaries

The tarsal tunnel is created when the flexor retinaculum spans the underlying osseous and ligamentous groove on the medial side of the ankle.

Boundary	Anatomical Structure
Roof	Flexor retinaculum
Anterior and superior attachment of roof	Medial malleolus
Posterior and inferior attachment of roof	Medial surface of calcaneus
Floor	Medial surfaces of the distal tibia, talus and calcaneus with associated ligamentous structures

The floor is therefore not formed by a single bone. It is a curved osteoligamentous surface following the medial contours of the ankle and hindfoot.

Flexor Retinaculum

The flexor retinaculum, also called the laciniate ligament, forms the roof of the tarsal tunnel. It is a thickening of the deep fascia extending from the medial malleolus toward the medial calcaneus.

The retinaculum holds the deep flexor tendons close to the ankle as they change direction around the medial side of the hindfoot. It also provides the superficial boundary that converts the underlying groove into a tunnel.

Fibrous septa extend from the deep surface of the retinaculum toward the underlying skeleton. These septa help create separate osteofibrous channels for the tendons rather than leaving all of the tunnel contents within a single undivided space.

Contents

The major structures passing through the tarsal tunnel can be arranged from anterior to posterior:

1. Tibialis posterior tendon
2. Flexor digitorum longus tendon
3. Posterior tibial artery
4. Posterior tibial veins
5. Tibial nerve
6. Flexor hallucis longus tendon

This sequence is commonly remembered with the mnemonic Tom, Dick, And Very Nervous Harry: tibialis posterior, flexor digitorum longus, artery, veins, nerve, and flexor hallucis longus.

Structure	Position	Continuation
Tibialis posterior tendon	Most anterior major tendon	Passes toward navicular and additional midfoot attachments
Flexor digitorum longus tendon	Posterior to tibialis posterior	Enters sole and divides for toes 2 to 5
Posterior tibial vessels	Between long flexor tendons and tibial nerve	Artery divides into medial and lateral plantar arteries
Tibial nerve	Posterior to posterior tibial vessels	Usually divides into medial and lateral plantar nerves near the tunnel
Flexor hallucis longus tendon	Most posterior major tendon	Continues into sole toward distal phalanx of great toe

Tibialis Posterior Tendon

The tibialis posterior tendon is the most anterior of the three major tendons passing through the tarsal tunnel. It descends from the deep posterior compartment of the leg and curves behind the medial malleolus.

After leaving the tunnel, the tendon passes toward the medial midfoot. Its principal insertion is on the navicular tuberosity, with expansions extending to several additional tarsal and metatarsal structures.

The medial malleolus acts as a pulley around which the tendon changes direction. The flexor retinaculum prevents the tendon from bowstringing away from the ankle during contraction of tibialis posterior.

Flexor Digitorum Longus Tendon

The flexor digitorum longus tendon passes posterior to the tibialis posterior tendon. Like tibialis posterior, it arises from a muscle in the deep posterior compartment of the leg and curves behind the medial malleolus.

After entering the sole, the tendon passes anteriorly and laterally and eventually divides into four tendons that insert on the distal phalanges of toes 2 to 5.

Within the sole, quadratus plantae attaches to the flexor digitorum longus tendon, and the four lumbricals arise from its digital tendons.

Posterior Tibial Artery and Veins

The posterior tibial artery passes through the tarsal tunnel with accompanying veins. In the conventional anterior-to-posterior arrangement, the vascular bundle lies posterior to flexor digitorum longus and anterior to the tibial nerve.

The artery is the principal source of arterial blood entering the plantar foot. Near or distal to the tunnel, it divides into the medial and lateral plantar arteries. The lateral plantar artery subsequently contributes substantially to the plantar arterial arch.

The artery's relatively superficial position in the posteromedial ankle allows its pulse to be palpated posterior to the medial malleolus.

Tibial Nerve

The tibial nerve is the principal neural structure within the tarsal tunnel. It descends from the posterior compartment of the leg with the posterior tibial vessels and enters the tunnel posterior to the vascular bundle.

The nerve commonly divides into the medial plantar nerve and lateral plantar nerve within or near the distal part of the tunnel.^[1][3] The precise level of this division is variable.

The medial and lateral plantar nerves continue into the sole, where they provide sensory innervation to substantial portions of the plantar foot and motor innervation to the intrinsic plantar muscles.

Calcaneal sensory branches also arise in the region of the tarsal tunnel. Their origins and courses are variable, and some may branch from the tibial nerve proximal to its terminal division.

Flexor Hallucis Longus Tendon

The flexor hallucis longus tendon is the most posterior of the major structures in the conventional tarsal tunnel sequence.

The tendon descends through the posterior ankle region in a groove associated with the posterior talus and then continues beneath the sustentaculum tali. It enters the sole and passes toward the great toe, ultimately inserting on the base of the distal phalanx.

Its course beneath the sustentaculum tali provides another important anatomical landmark after the tendon has passed through the retinacular region.

Tendon Compartments and Synovial Sheaths

The three tendons within the tarsal tunnel are not freely grouped together with the neurovascular bundle. Fibrous septa associated with the flexor retinaculum create separate osteofibrous compartments for the tendons.

The tendons are surrounded by synovial sheaths that permit smooth gliding as they curve around the medial ankle. The sheaths of tibialis posterior, flexor digitorum longus, and flexor hallucis longus are distinct structures, although their exact extent varies.

The posterior tibial vessels and tibial nerve occupy the neurovascular portion of the tunnel and are not enclosed within these tendon sheaths.

Relationship to the Sustentaculum Tali

The sustentaculum tali is a medial projection of the calcaneus that supports part of the talus. It is an important landmark in the distal tarsal tunnel region.

The flexor hallucis longus tendon passes in a groove on the inferior surface of the sustentaculum tali. This relationship helps guide the tendon toward the plantar aspect of the foot.

The sustentaculum tali also contributes to the osseous anatomy underlying the distal part of the flexor retinacular region.

Relationship to the Plantar Nerves

The medial and lateral plantar nerves are terminal branches of the tibial nerve. Because the tibial nerve may divide at different levels, the exact neural contents of the distal tarsal tunnel vary among individuals.

The medial plantar nerve travels anteriorly through the medial sole and supplies several intrinsic muscles as well as cutaneous territories of the plantar foot and toes.

The lateral plantar nerve courses more laterally across the proximal sole before dividing into superficial and deep branches. It supplies most of the remaining intrinsic plantar muscles and provides sensory innervation to the lateral plantar region.

The variable branching pattern is clinically important because compression may affect the tibial nerve before division or selectively involve one of its distal branches.

Relationship to the Medial Calcaneal Branches

Medial calcaneal branches provide cutaneous innervation to the heel and may arise from the tibial nerve at variable levels. Some arise proximal to the flexor retinaculum, while others originate closer to or within the tunnel region.

This variability has practical importance when localizing tibial nerve lesions. A lesion affecting the tibial nerve proximal to the origin of a calcaneal branch may produce a different sensory pattern from a more distal lesion.

The branching pattern should therefore not be assumed to be identical in every individual.

Function of the Tunnel

The tarsal tunnel provides a protected passage for structures traveling from the posterior leg into the plantar foot. Its retinacular roof holds the flexor tendons close to the underlying skeletal surfaces and maintains their effective lines of pull around the ankle.

The tunnel also organizes the posterior tibial neurovascular bundle as it enters the foot. The artery, veins, and nerve are maintained within a predictable anatomical region while the tendons occupy adjacent osteofibrous channels.

Unlike a freely expandable soft tissue space, the tunnel is bounded by relatively rigid skeletal structures on one side and a strong fibrous retinaculum on the other. This configuration is important in understanding nerve compression within the region.

Surface Anatomy

The tarsal tunnel can be projected onto the skin of the posteromedial ankle between the medial malleolus and medial calcaneus. The tunnel follows the curved contour behind and below the medial malleolus.

The posterior tibial artery is an important surface landmark. Its pulse can usually be palpated posterior to the medial malleolus, providing a clinical indication of the location of the neurovascular portion of the tunnel.

The tibial nerve lies close to the artery, generally posterior to it, but the nerve itself is not normally palpable as a distinct structure.

Anatomical Variations

The tarsal tunnel varies in dimensions and internal organization. Differences in the thickness and attachment of the flexor retinaculum can alter the configuration of its roof.

The level of tibial nerve bifurcation is one of the most important variations. The nerve may divide into medial and lateral plantar branches proximal to, within, or distal to the tunnel.

Medial calcaneal branches also show substantial variation in number, origin, and course. These differences are particularly relevant during surgical exposure and when interpreting patterns of sensory loss.

Vascular branching and the detailed relationships of the tendon sheaths may also vary without representing pathology.

Clinical Significance

Tarsal Tunnel Syndrome

Tarsal tunnel syndrome is an entrapment neuropathy involving the tibial nerve or its branches in the tarsal tunnel region. The anatomical basis is the passage of neural structures through a confined osteofibrous space beneath the flexor retinaculum.

Processes that reduce available space or increase the volume of tunnel contents can potentially compress neural structures. Depending on the level and structures involved, sensory changes may occur in plantar territories and motor involvement may affect intrinsic muscles supplied by the plantar nerves.

Because the tibial nerve may divide at different levels, the pattern of neurological involvement can vary considerably.

Tinel Sign

Percussion over the tibial nerve in the tarsal tunnel region may produce distal paresthesia when the nerve is irritated. This is commonly described as a Tinel sign at the tarsal tunnel.

The anatomical basis for the examination is the superficial position of the tibial nerve beneath the flexor retinaculum posterior to the medial malleolus.

Posterior Tibial Pulse

The tarsal tunnel provides an important landmark for locating the posterior tibial artery. Its pulse is typically assessed posterior and inferior to the medial malleolus.

The relationship is also useful when interpreting vascular anatomy because the artery passes through the tunnel before dividing into the medial and lateral plantar arteries.

Space-Occupying Lesions

The fixed boundaries of the tunnel mean that masses or enlargement of normal structures can alter the available space around the tibial nerve. Ganglion cysts, vascular abnormalities, accessory muscles, and other localized lesions may therefore be relevant to neural compression when present in this region.

An accessory or variant structure should not automatically be considered pathological. Its significance depends on its size, position, and relationship to the nerve and other tunnel contents.

Surgical Anatomy

Surgical decompression of the tarsal tunnel requires identification of the flexor retinaculum and careful protection of the tibial nerve and its branches. The posterior tibial vessels and flexor tendons are also closely related to the operative field.

The variable level of tibial nerve division and the variable courses of calcaneal branches are especially important. A simplified expectation of a single nerve trunk throughout the tunnel may not reflect the anatomy encountered during surgery.

Imaging

MRI and ultrasound can demonstrate the structures of the tarsal tunnel. MRI provides cross-sectional visualization of the tibial nerve, tendons, vessels, flexor retinaculum, and surrounding bones and soft tissues.

Ultrasound permits dynamic assessment of the tibial nerve and flexor tendons and can demonstrate their relationships during ankle and toe movement. Both modalities can also identify anatomical variants and localized abnormalities affecting the tunnel.

The tarsal tunnel is a compact anatomical gateway linking the deep posterior compartment of the leg with the plantar foot. Its organization around the medial malleolus allows three major tendons, the posterior tibial vessels, and the tibial nerve to pass into the foot while remaining closely related to the underlying skeleton.

Understanding the tunnel requires more than memorizing its contents. The arrangement of its retinacular roof, tendon compartments, neurovascular structures, and variable tibial nerve branching explains both its normal function and its importance as a site of peripheral nerve compression.