Medical School · Year 1 · Anatomy Msk · includes a quiz and discussion video

Lecture 12: Lower Limb - Foot and Ankle

Unit 1.3: Human Gross Anatomy I - Musculoskeletal System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the bones of the foot (tarsals, metatarsals, phalanges)
2. Explain the structure and movements of the ankle (talocrural) joint
3. Describe the subtalar and transverse tarsal joints
4. Identify the compartments and intrinsic muscles of the foot
5. Describe the arches of the foot and their supporting structures
6. Explain common foot and ankle pathologies

Bones of the Foot

The foot contains 26 bones organized into three groups: the tarsals (7), the metatarsals (5), and the phalanges (14). This arrangement provides both the stability needed for weight-bearing and the flexibility required for locomotion over varied terrain.

Tarsal Bones

The seven tarsal bones form the posterior half of the foot and are arranged in two rows with the navicular medially.

The talus is the keystone of the foot, articulating with the tibia and fibula above (forming the ankle joint), the calcaneus below, and the navicular anteriorly. Its dome (trochlea) is wider anteriorly than posteriorly—a feature that affects ankle stability. Uniquely among tarsal bones, the talus has no muscular attachments; its position is controlled entirely by the bones and ligaments around it.

The calcaneus is the largest tarsal bone, forming the heel. It lies beneath the talus and projects posteriorly to form the heel prominence. The sustentaculum tali is a shelf-like projection on the medial surface that supports the talus; the flexor hallucis longus tendon passes beneath it. The calcaneal tuberosity on the posterior surface receives the Achilles tendon insertion.

The navicular is a boat-shaped bone on the medial side of the foot, lying between the talus posteriorly and the three cuneiforms anteriorly. A prominent tuberosity on its medial surface provides the primary attachment for the tibialis posterior tendon.

The cuboid is positioned on the lateral side of the foot, lying between the calcaneus posteriorly and the fourth and fifth metatarsals anteriorly. A groove on its plantar surface transmits the fibularis longus tendon.

The three cuneiforms—medial, intermediate, and lateral—articulate with the navicular posteriorly and the first three metatarsals anteriorly. The medial cuneiform is the largest and articulates with the first metatarsal. The intermediate cuneiform is the smallest, set back from the others, creating a key-in-socket arrangement with the second metatarsal base that is essential for midfoot stability.

Metatarsals

The five metatarsals form the anterior half of the foot skeleton. Each has a base (proximal end articulating with the tarsals), a shaft, and a head (distal end articulating with the phalanges). The heads collectively form the "ball of the foot" and bear weight during the push-off phase of gait.

The first metatarsal is the shortest and thickest, reflecting its importance in weight-bearing. Two sesamoid bones within the flexor hallucis brevis tendons lie beneath its head.

The second metatarsal is the longest and is "recessed" into the cuneiforms, creating a particularly stable articulation. This arrangement makes the second tarsometatarsal joint the key to midfoot stability.

The fifth metatarsal has a styloid process (tuberosity) projecting from the lateral aspect of its base. This is the insertion site for the fibularis brevis tendon and is a common site of avulsion fracture.

Phalanges

The 14 phalanges form the skeleton of the toes. The great toe (hallux) has only two phalanges: proximal and distal. Each of the remaining four toes has three: proximal, middle, and distal.

<image>Panel A: Dorsal view of tarsal bones color-coded with talus (blue, dome visible), calcaneus (green, heel with sustentaculum tali), navicular (orange, tuberosity marked), cuboid (purple, plantar groove), and cuneiforms (medial largest, intermediate smallest and recessed, lateral). Panel B: Metatarsals MT1-5 numbered with MT1 shortest and thickest, MT2 longest and recessed into cuneiforms, MT5 with styloid process for fibularis brevis insertion. Panel C: Phalanges with hallux having 2 (proximal and distal) and toes 2-5 having 3 each (proximal, middle, distal), joints between each segment indicated. Panel D: Plantar view showing sesamoid bones under MT1 head with anatomical orientation and scale bar included.</image>

The Ankle (Talocrural) Joint

The ankle joint is a hinge synovial joint that permits dorsiflexion and plantarflexion.

Articular Surfaces

The ankle joint is formed by the articulation of the talus with the tibia and fibula. The mortise is the socket formed by the tibial plafond (the horizontal articular surface of the distal tibia) and the medial and lateral malleoli. The talus fits snugly into this mortise, with the dome of the talus articulating with the tibial plafond and the sides of the talus articulating with the malleoli.

The lateral malleolus extends more distally and posteriorly than the medial malleolus, creating an asymmetric mortise that enhances stability. The dome of the talus is wider anteriorly than posteriorly. This means that in dorsiflexion (when the wider anterior part of the talus wedges into the mortise), the joint is most stable and most resistant to injury. In plantarflexion (when the narrower posterior part of the talus is in the mortise), the joint is less stable—this is why most ankle sprains occur with the foot in plantarflexion.

Joint Capsule and Synovium

The fibrous capsule is thin anteriorly and posteriorly but reinforced on each side by strong ligaments. The synovial membrane lines the capsule and may extend into the distal tibiofibular joint.

Ligaments

The medial (deltoid) ligament is a strong, triangular ligament that resists eversion stress. It attaches superiorly to the medial malleolus and fans out to attach to the navicular, talus, and calcaneus. It has four named components: the tibionavicular part (most anterior), the tibiocalcaneal part, the anterior tibiotalar part, and the posterior tibiotalar part (strongest and deepest). The deltoid ligament is so strong that forceful eversion is more likely to fracture the fibula than rupture the ligament.

The lateral ligaments are three separate structures that resist inversion stress. Because inversion injuries are much more common than eversion injuries (the mechanics of the ankle favor inversion), these ligaments are frequently injured.

The anterior talofibular ligament (ATFL) runs from the anterior fibula to the lateral talus. It is the weakest of the three lateral ligaments and is the first to be injured in an inversion sprain—the most common ligamentous injury in the body.

The calcaneofibular ligament (CFL) runs from the tip of the fibula to the lateral calcaneus. It crosses both the ankle and subtalar joints. It is stronger than the ATFL and is torn in more severe inversion injuries.

The posterior talofibular ligament (PTFL) runs from the posterior fibula to the posterior talus. It is the strongest of the lateral ligaments and is rarely injured in isolation.

<image>Panel A: Anterior view of ankle mortise formed by tibial plafond and malleoli with talus dome fitting within (wider anteriorly, stable in dorsiflexion), cross-section showing malleolar asymmetry. Panel B: Medial view of deltoid ligament as triangular structure from medial malleolus with four parts: tibionavicular, tibiocalcaneal, anterior tibiotalar, and posterior tibiotalar (strongest, deepest). Panel C: Lateral view showing three separate ligaments: ATFL (weakest, most commonly injured), CFL (crossing both ankle and subtalar joints), and PTFL (strongest, rarely injured in isolation). Panel D: Stress resistance arrows showing deltoid resisting eversion and lateral ligaments resisting inversion with sequential injury pattern from ATFL to CFL to PTFL.</image>

Movements at the Ankle

The ankle joint permits movement in only one plane: dorsiflexion and plantarflexion.

Dorsiflexion brings the dorsum of the foot toward the anterior leg, decreasing the angle between the foot and the leg. The range is approximately 20 degrees. The primary dorsiflexor is the tibialis anterior, assisted by the extensor hallucis longus, extensor digitorum longus, and fibularis tertius. Dorsiflexion is limited by the gastrocnemius-soleus complex and the posterior joint capsule.

Plantarflexion moves the foot away from the leg, as when pointing the toes. The range is approximately 50 degrees. The primary plantarflexors are the gastrocnemius and soleus (triceps surae), which together are among the most powerful muscles in the body. They are assisted by the tibialis posterior, flexor hallucis longus, flexor digitorum longus, and the fibularis muscles. Plantarflexion is limited by the anterior joint capsule and the dorsiflexor muscles.

Subtalar and Transverse Tarsal Joints

While the ankle joint permits only dorsiflexion and plantarflexion, the joints below the talus—the subtalar and transverse tarsal joints—permit inversion and eversion. These movements allow the foot to adapt to uneven ground.

Subtalar Joint

The subtalar (talocalcaneal) joint is formed by the articulation between the talus above and the calcaneus below. The joint has three facets (anterior, middle, and posterior) separated by the tarsal sinus (a space containing the interosseous talocalcaneal ligament).

The axis of the subtalar joint is oblique, running anteriorly, medially, and superiorly. Movement around this axis produces inversion (the sole turns medially) and eversion (the sole turns laterally).

Transverse Tarsal Joint

The transverse tarsal (midtarsal or Chopart's) joint is actually two joints that function together: the talonavicular joint medially and the calcaneocuboid joint laterally. Together, they form an S-shaped joint line across the foot.

The transverse tarsal joint works in concert with the subtalar joint to produce inversion and eversion. It also allows some rotational movement. Surgeons may perform amputation at this level (Chopart amputation) when necessary.

Inversion and Eversion

Inversion turns the sole of the foot to face medially. It is produced primarily by the tibialis anterior (which also dorsiflexes) and the tibialis posterior (which also plantarflexes). Inversion is the more powerful movement and has a greater range than eversion.

Eversion turns the sole of the foot to face laterally. It is produced by the fibularis longus and fibularis brevis. Eversion range is limited by the deltoid ligament and the bony configuration of the lateral ankle.

It is important to understand that inversion and eversion occur at the subtalar and transverse tarsal joints, not at the ankle joint itself. Clinical terms like "inversion ankle sprain" actually describe an injury caused by excessive inversion at these joints, which stresses the lateral ligaments of the ankle.

<image>Panel A: Lateral view of subtalar joint (talus-calcaneus) with three facets (anterior, middle, posterior) separated by tarsal sinus with interosseous ligament, and transverse tarsal joint as S-shaped Chopart's line through talonavicular and calcaneocuboid joints. Panel B: Superior view of transverse tarsal joint from above with subtalar joint axis shown as oblique line running anteriorly, medially, and superiorly. Panel C: Movement diagrams showing inversion (sole facing medially, tibialis anterior and posterior as motors) and eversion (sole facing laterally, fibularis muscles as motors) around subtalar axis. Panel D: Posterior view comparing inverted and everted calcaneus positions relative to tibia with note that these movements occur below the ankle, not at the talocrural joint.</image>

Other Foot Joints

Tarsometatarsal Joints

The tarsometatarsal joints (Lisfranc's joints) connect the distal tarsal row (cuneiforms and cuboid) to the bases of the metatarsals. These are plane synovial joints that permit limited gliding movements.

The critical structure for stability at this level is the Lisfranc ligament, a strong plantar ligament running from the medial cuneiform to the base of the second metatarsal. This ligament is key to maintaining the transverse stability of the midfoot. The second metatarsal base is recessed between the cuneiforms, creating a keystone arrangement that locks the transverse arch.

Disruption of the tarsometatarsal joints (Lisfranc fracture-dislocation) is a serious injury that is often initially missed on plain radiographs. It requires surgical stabilization to prevent chronic disability.

Metatarsophalangeal Joints

The metatarsophalangeal (MTP) joints are condylar synovial joints between the rounded metatarsal heads and the concave bases of the proximal phalanges. They permit flexion, extension, abduction, and adduction, with extension having the greatest range (important for toe-off during gait).

The first MTP joint is particularly important. It bears significant weight during the push-off phase of walking and is a common site of pathology, including hallux valgus (bunion) and hallux rigidus (degenerative arthritis).

Interphalangeal Joints

The interphalangeal joints are hinge synovial joints permitting only flexion and extension. The great toe has a single interphalangeal joint, while each of the other toes has proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.

<image>Panel A: Superior dorsal view showing tarsometatarsal (Lisfranc) joint line crossing from medial cuneiform-MT1 to cuboid-MT5, second metatarsal base recessed in keystone configuration. Panel B: Lisfranc ligament (thick red band) from medial cuneiform to MT2 base as key stabilizer with MTP joints at metatarsal heads and IP joints on toes. Panel C: Sagittal section through great toe showing first MTP joint (condylar configuration, sesamoids beneath MT1 head) and single IP joint with range of motion arrows at each level. Panel D: Clinical note showing Lisfranc injury often missed on radiographs with diastasis between MT1 and MT2 as hallmark finding.</image>

The Tarsal Tunnel

The tarsal tunnel is an osseofibrous passage behind the medial malleolus through which tendons, vessels, and nerves pass from the leg into the foot.

Boundaries

The roof of the tarsal tunnel is formed by the flexor retinaculum (laciniate ligament), a strong band of fascia extending from the medial malleolus to the calcaneus. The floor is formed by the medial surfaces of the talus and calcaneus.

Contents

The contents of the tarsal tunnel pass from the leg to the plantar foot, arranged from anterior to posterior. The mnemonic "Tom, Dick, And Nervous Harry" helps remember the order:

The tibialis posterior tendon is most anterior, lying directly behind the medial malleolus. The flexor digitorum longus tendon is next. The posterior tibial artery with its accompanying veins passes through the middle of the tunnel. The tibial nerve lies posterior to the artery and divides within the tunnel into the medial and lateral plantar nerves. The flexor hallucis longus tendon is most posterior, passing in a groove between the medial and lateral processes of the posterior talus.

Clinical Significance

Tarsal tunnel syndrome results from compression of the tibial nerve within this confined space. Patients experience burning pain, numbness, and tingling on the plantar aspect of the foot. The symptoms may be worse at night or with prolonged standing. Causes include space-occupying lesions (ganglion cysts, varicose veins), post-traumatic changes, and systemic conditions affecting nerve sensitivity. Treatment may include splinting, injections, or surgical decompression.

<image>Panel A: Cross-section behind medial malleolus showing flexor retinaculum forming roof and medial surfaces of talus and calcaneus forming floor of tarsal tunnel. Panel B: Contents arranged anterior to posterior with "Tom, Dick, And Nervous Harry" mnemonic: Tibialis posterior tendon, flexor Digitorum longus, posterior tibial Artery, tibial Nerve, flexor Hallucis longus, each color-coded. Panel C: Medial view of ankle showing retinaculum as roof with tibial nerve dividing into medial and lateral plantar nerves, distribution on plantar foot shown in inset. Panel D: Clinical box showing tarsal tunnel syndrome with nerve compression causing plantar pain and paresthesias, causes and treatment outlined.</image>

Intrinsic Muscles of the Foot

The intrinsic muscles of the foot are those that both originate and insert within the foot. They are organized into dorsal and plantar groups, with the plantar muscles arranged in four layers.

Dorsal Muscles

The dorsum of the foot contains two small muscles that assist with toe extension.

The extensor digitorum brevis originates from the dorsal surface of the calcaneus and sends tendons to the extensor expansions of digits 2-4. It extends these toes and is the only muscle that can extend the toes without simultaneously dorsiflexing the ankle.

The extensor hallucis brevis (sometimes considered the medial part of the extensor digitorum brevis) originates from the calcaneus and inserts onto the base of the proximal phalanx of the great toe. Both muscles are innervated by the deep peroneal nerve.

Plantar Muscles: First Layer

The first (most superficial) layer consists of three muscles that span the length of the sole.

The abductor hallucis runs along the medial border of the foot from the calcaneal tuberosity to the medial side of the base of the great toe's proximal phalanx. It abducts and flexes the great toe and helps support the medial longitudinal arch. It is innervated by the medial plantar nerve.

The flexor digitorum brevis occupies the central sole. It originates from the calcaneal tuberosity and sends four tendons to the middle phalanges of digits 2-5. Each tendon splits to allow passage of the flexor digitorum longus tendon before inserting. It flexes the proximal interphalangeal joints and is innervated by the medial plantar nerve.

The abductor digiti minimi lies along the lateral border of the foot from the calcaneal tuberosity to the lateral side of the base of the little toe's proximal phalanx. It abducts the little toe and is innervated by the lateral plantar nerve.

Plantar Muscles: Second Layer

The second layer contains two intrinsic muscles plus the tendons of extrinsic muscles passing through.

The quadratus plantae (flexor digitorum accessorius) originates from the calcaneus and inserts into the tendon of the flexor digitorum longus. Its function is to correct the oblique pull of the FDL, allowing the toes to flex in line with the long axis of the foot rather than diagonally. It is innervated by the lateral plantar nerve.

The four lumbricals originate from the tendons of the flexor digitorum longus and insert into the extensor expansions of digits 2-5. Like their counterparts in the hand, they flex the metatarsophalangeal joints while extending the interphalangeal joints. The first lumbrical is innervated by the medial plantar nerve; the lateral three are innervated by the lateral plantar nerve.

The tendons of the flexor digitorum longus and flexor hallucis longus pass through this layer, crossing at the "knot of Henry."

Plantar Muscles: Third Layer

The third layer contains three muscles deep in the sole.

The flexor hallucis brevis originates from the cuboid and cuneiforms and inserts onto both sides of the base of the great toe's proximal phalanx via two bellies that contain the sesamoid bones. It flexes the metatarsophalangeal joint of the great toe and is innervated by the medial plantar nerve.

The adductor hallucis has two heads. The oblique head arises from the bases of metatarsals 2-4. The transverse head arises from the plantar ligaments of the MTP joints of digits 3-5. Both heads insert onto the lateral side of the base of the great toe's proximal phalanx. The adductor hallucis adducts the great toe and helps maintain the transverse arch. It is innervated by the lateral plantar nerve.

The flexor digiti minimi brevis originates from the base of the fifth metatarsal and inserts onto the base of the little toe's proximal phalanx. It flexes the little toe's MTP joint and is innervated by the lateral plantar nerve.

Plantar Muscles: Fourth Layer

The fourth (deepest) layer contains the interossei and the tendons of tibialis posterior and fibularis longus.

The three plantar interossei originate from the medial sides of metatarsals 3-5 and insert onto the medial sides of the proximal phalanges of these toes. They adduct the toes toward the second digit (the axis of the foot) and are mnemonic "PAD" (Plantar ADduct). They also flex the MTP joints and extend the IP joints. All are innervated by the lateral plantar nerve.

The four dorsal interossei are bipennate muscles originating from the adjacent sides of all five metatarsals and inserting onto the bases of proximal phalanges (digits 2-4) and extensor expansions. They abduct the toes away from the second digit, following the mnemonic "DAB" (Dorsal ABduct). They also flex the MTP joints and extend the IP joints. All are innervated by the lateral plantar nerve.

<image>Panel A: First plantar layer with abductor hallucis (medial, pink), flexor digitorum brevis (central, blue, tendons splitting for FDL), and abductor digiti minimi (lateral, green); second layer with quadratus plantae (orange, corrects oblique pull) and four lumbricals. Panel B: Third layer with flexor hallucis brevis (two bellies with sesamoids, purple), adductor hallucis (oblique and transverse heads, red), and flexor digiti minimi brevis (teal). Panel C: Fourth layer with three plantar interossei (PAD, adduct, from MT3-5) and four dorsal interossei (DAB, abduct, from adjacent metatarsals), tibialis posterior and fibularis longus tendons at deepest level. Panel D: Nerve supply overview showing medial plantar nerve to first layer and first lumbrical and FHB versus lateral plantar nerve to most of second through fourth layers, shown in progressive dissection.</image>

Arches of the Foot

The foot is not flat; it has three arches that distribute weight, absorb shock, and provide the spring needed for efficient locomotion. These arches are maintained by the shape of the bones, strong ligaments, and muscular activity.

Medial Longitudinal Arch

The medial longitudinal arch is the highest and most important arch. It extends from the calcaneus posteriorly through the talus (its highest point), navicular, three cuneiforms, and the first three metatarsals anteriorly.

The talus sits at the apex of this arch like a keystone. It receives the body weight from the tibia and distributes it posteriorly to the calcaneus and anteriorly through the navicular to the forefoot.

The primary ligamentous support for the medial arch is the spring ligament (plantar calcaneonavicular ligament), which spans from the sustentaculum tali of the calcaneus to the navicular. This ligament supports the head of the talus and is crucial for maintaining arch height. Failure of this ligament contributes to flatfoot deformity.

Dynamic support comes from the tibialis posterior muscle, whose tendon inserts onto the navicular and spreads to the cuneiforms and metatarsals. Dysfunction of the tibialis posterior is the most common cause of adult-acquired flatfoot.

Lateral Longitudinal Arch

The lateral longitudinal arch is lower and flatter than the medial arch. It extends from the calcaneus through the cuboid to the fourth and fifth metatarsals. This arch contacts the ground in standing and provides a rigid lever for weight transmission.

Support comes from the long plantar ligament (from the calcaneus to the cuboid and metatarsals) and the short plantar ligament (from the calcaneus to the cuboid), as well as from the fibularis longus tendon, which crosses the sole to insert on the medial cuneiform and first metatarsal.

Transverse Arch

The transverse arch runs across the foot from side to side, most prominent at the level of the cuneiforms and cuboid. The cuneiforms and cuboid form a dome shape, with the intermediate cuneiform at the apex. This arch flattens at the level of the metatarsal heads, which all contact the ground.

Support comes from the fibularis longus tendon (which crosses beneath the arch), the tibialis posterior (which sends slips to multiple bones), and the transverse head of the adductor hallucis.

The Plantar Aponeurosis and Windlass Mechanism

The plantar aponeurosis (plantar fascia) is a thick band of connective tissue that extends from the calcaneal tuberosity to the bases of the proximal phalanges. It supports the longitudinal arches and plays a key role in gait through the windlass mechanism.

During toe-off, as the MTP joints extend, the plantar aponeurosis winds around the metatarsal heads like a rope around a windlass. This tightens the aponeurosis, draws the calcaneus and metatarsal heads together, raises the arch, and converts the foot into a rigid lever for propulsion. This passive mechanism increases the efficiency of walking and running.

<image>Panel A: Medial longitudinal arch from calcaneus through talus (keystone) to navicular, cuneiforms, and MT1-3, with spring ligament from sustentaculum tali to navicular and tibialis posterior tendon as dynamic support. Panel B: Lateral longitudinal arch from calcaneus through cuboid to MT4-5 (lower, contacting ground) with long and short plantar ligaments and fibularis longus crossing sole. Panel C: Transverse arch at cuneiform level showing dome shape with intermediate cuneiform at apex in anterior view. Panel D: Plantar view showing plantar aponeurosis from calcaneus to proximal phalanges with windlass mechanism inset demonstrating aponeurosis winding around MT head during toe-off to tighten and raise the arch.</image>

Neurovascular Supply of the Foot

Arterial Supply

The dorsalis pedis artery is the continuation of the anterior tibial artery onto the dorsum of the foot. It runs lateral to the extensor hallucis longus tendon and is easily palpable here—an important pulse for assessing peripheral vascular disease. Its branches include the arcuate artery (which gives off dorsal metatarsal arteries) and the deep plantar artery (which dives between the first and second metatarsals to complete the plantar arch).

The posterior tibial artery passes behind the medial malleolus through the tarsal tunnel. It divides into the medial and lateral plantar arteries.

The medial plantar artery is the smaller terminal branch, running along the medial side of the sole with the medial plantar nerve. It gives off digital branches to the medial three and a half toes.

The lateral plantar artery is larger and more important. It runs obliquely across the sole, then turns medially at the base of the fifth metatarsal to form the plantar arch (deep plantar arch). The plantar arch gives off the plantar metatarsal arteries that supply the toes. It is completed medially by the deep plantar artery from the dorsalis pedis, creating communication between dorsal and plantar circulations.

Nerve Supply

The deep peroneal nerve enters the dorsum of the foot with the dorsalis pedis artery. It provides motor innervation to the extensor digitorum brevis and extensor hallucis brevis, and sensory innervation to a small area in the first web space.

The superficial peroneal nerve becomes superficial in the distal leg and provides sensory innervation to most of the dorsum of the foot, except the first web space and the lateral border.

The tibial nerve divides within the tarsal tunnel into the medial and lateral plantar nerves.

The medial plantar nerve is analogous to the median nerve in the hand. It supplies sensation to the medial three and a half digits (plantar surface) and motor innervation to the abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, and the first lumbrical.

The lateral plantar nerve is analogous to the ulnar nerve. It supplies sensation to the lateral one and a half digits and motor innervation to all other intrinsic plantar muscles.

The sural nerve provides sensation to the lateral border of the foot and the heel.

<image>Panel A: Dorsal view with dorsalis pedis artery lateral to EHL tendon (pulse point marked) giving arcuate artery and deep plantar artery, deep peroneal nerve alongside (motor to EDB/EHB, sensory to first web space). Panel B: Superficial peroneal nerve branches sensory to most of dorsum, sural nerve on lateral foot, with dorsal sensory territories color-coded. Panel C: Plantar view with posterior tibial artery dividing into medial plantar (smaller) and lateral plantar (larger, forming plantar arch at MT5 level giving metatarsal and digital arteries), deep plantar artery completing arch. Panel D: Tibial nerve dividing in tarsal tunnel into medial plantar (medial 3.5 digits, first layer muscles) and lateral plantar (lateral 1.5 digits, most other intrinsics) with plantar sensory territories mapped.</image>

Clinical Correlations

Ankle Sprains

Ankle sprains are among the most common musculoskeletal injuries. Understanding ankle anatomy explains why inversion sprains are far more common than eversion sprains.

Inversion sprains occur when the foot is forced into plantarflexion and inversion, stressing the lateral ligaments. The ATFL is injured first (grade I sprain), followed by the CFL in more severe injuries (grade II), and finally the PTFL in severe sprains (grade III). Patients have lateral ankle pain, swelling, and difficulty bearing weight. The anterior drawer test assesses ATFL integrity; the talar tilt test assesses CFL integrity.

Eversion sprains are less common because the powerful deltoid ligament resists eversion and because the fibula provides a mechanical block. When eversion is forced, the fibula often fractures before the deltoid ligament fails completely.

Fractures

Pott's fracture refers to bimalleolar or trimalleolar fracture-dislocations of the ankle. When both malleoli are fractured, the mortise loses its integrity and the talus can displace. The "third malleolus" refers to the posterior tibial margin (posterior malleolus). These injuries require anatomical reduction to prevent post-traumatic arthritis.

Jones fracture is a transverse fracture at the base of the fifth metatarsal, specifically at the metaphyseal-diaphyseal junction. This area has a relatively poor blood supply, and Jones fractures are prone to delayed union or nonunion, often requiring prolonged immobilization or surgical fixation.

Avulsion fracture of the fifth metatarsal base occurs when the fibularis brevis tendon avulses the styloid process during an inversion injury. This more proximal fracture should be distinguished from a Jones fracture because it has a much better prognosis and heals readily with symptomatic treatment.

Lisfranc fracture-dislocation involves disruption of the tarsometatarsal joints. The hallmark finding is diastasis (widening) between the first and second metatarsal bases. This injury is often missed on initial radiographs because the bony displacement may be subtle. Missed Lisfranc injuries lead to chronic midfoot pain and arthritis. Weight-bearing radiographs and CT scanning improve detection.

Plantar Fasciitis

Plantar fasciitis is the most common cause of heel pain. It involves inflammation and degeneration of the plantar aponeurosis at its attachment to the calcaneal tuberosity.

Patients typically report pain with the first steps in the morning that improves with activity but worsens again after prolonged standing. Tenderness is localized to the medial calcaneal tubercle. Risk factors include flat feet, obesity, prolonged standing, and a tight Achilles tendon.

Treatment includes stretching exercises, supportive footwear, orthotics, night splints, and anti-inflammatory measures. Corticosteroid injections may help but carry a risk of plantar fascia rupture. Plantar fascia release is reserved for refractory cases.

Morton's Neuroma

Morton's neuroma is not a true neuroma but rather a perineural fibrosis of a common digital nerve, most often between the third and fourth metatarsal heads. Patients experience burning pain in the forefoot and adjacent toes, often exacerbated by tight shoes.

Physical examination may reveal tenderness in the affected web space and a positive Mulder's click (a palpable click when the metatarsal heads are squeezed together). Treatment includes wider shoes, metatarsal pads, injections, or surgical excision.

Hallux Valgus

Hallux valgus (bunion) is lateral deviation of the great toe at the first MTP joint, creating a prominent medial bump at the metatarsal head. It is often associated with metatarsus primus varus (medial deviation of the first metatarsal).

The condition is more common in women and is associated with narrow, pointed footwear. Progressive deformity can cause pain, difficulty with shoe wear, and dysfunction of the first ray during gait. Treatment ranges from footwear modification and orthotics to surgical correction in severe cases.

Tarsal Tunnel Syndrome

Tarsal tunnel syndrome results from compression of the tibial nerve behind the medial malleolus. Patients experience burning, tingling, and numbness on the plantar aspect of the foot. Symptoms may be worse at night or with prolonged activity.

Physical examination may reveal a positive Tinel's sign (tapping over the tarsal tunnel reproduces symptoms) and sensory loss in the plantar nerve distribution. Electrodiagnostic studies can confirm the diagnosis. Treatment includes splinting, orthotics, anti-inflammatory measures, and surgical decompression in refractory cases.

<image>Panel A: Lateral ankle sprain showing ATFL rupture with inversion mechanism and anterior drawer test; Jones fracture at MT5 metaphyseal-diaphyseal junction (poor blood supply) versus avulsion fracture at styloid process (better prognosis). Panel B: Lisfranc injury on X-ray schematic with diastasis between MT1 and MT2; plantar fasciitis showing inflammation at calcaneal attachment of plantar fascia with tender point marked. Panel C: Morton's neuroma showing enlarged common digital nerve between MT3 and MT4 heads with Mulder's test position; hallux valgus showing lateral deviation of great toe with prominent medial bump and metatarsus primus varus. Panel D: Tarsal tunnel syndrome showing compressed tibial nerve behind medial malleolus with affected plantar sensory distribution shaded on sole, key features labeled.</image>

Summary

The foot contains 26 bones: 7 tarsals (including the talus, which articulates with the leg, and the calcaneus, which forms the heel), 5 metatarsals, and 14 phalanges. The talus is unique in having no muscle attachments.

The ankle (talocrural) joint is a hinge joint for dorsiflexion and plantarflexion. The talus is wider anteriorly, making the joint more stable in dorsiflexion. The lateral ligaments (ATFL, CFL, PTFL) are commonly injured in inversion sprains; the stronger deltoid ligament resists eversion.

Inversion and eversion occur at the subtalar and transverse tarsal joints, not at the ankle. The tibialis anterior and posterior produce inversion; the fibularis muscles produce eversion.

The plantar muscles are arranged in four layers, innervated primarily by the medial and lateral plantar nerves. The medial plantar nerve (analogous to the median nerve) supplies sensation to the medial 3.5 digits and the first layer muscles plus the first lumbrical. The lateral plantar nerve (analogous to the ulnar nerve) supplies the lateral 1.5 digits and most other intrinsic muscles.

Three arches—medial longitudinal, lateral longitudinal, and transverse—distribute weight and provide spring to the foot. The spring ligament, plantar aponeurosis, and tibialis posterior are key supporting structures. The windlass mechanism tightens the plantar aponeurosis during toe-off, raising the arch and converting the foot into a rigid lever for propulsion.

Key Terms

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