Ankle and Foot

Articular surfaces: Distal tibia (with medial malleolus) and distal fibula (lateral malleolus) form a mortise, receiving the trochlea (body) of the talus. The tibial plafond articulates with the superior talar surface; the malleoli grip the talus medially and laterally.

• Capsule: Thin and lax anteriorly/posteriorly (permitting dorsiflexion/plantarflexion), reinforced medially and laterally by strong collateral ligament complexes.
• Medial ligament (deltoid ligament):
• Strong, triangular, four-part ligament from the medial malleolus fanning out to the navicular, calcaneus (sustentaculum tali), and talus (tibionavicular, tibiocalcaneal, and anterior/posterior tibiotalar parts).
• Resists eversion and limits excessive talar movement; notably stronger than the lateral ligament complex, so isolated medial ligament sprains are less common, eversion injuries more often produce a fibular fracture before the deltoid ligament fails.
• Lateral ligament complex: Three separate bands (weaker overall than the deltoid ligament):
• Anterior talofibular ligament (ATFL): lateral malleolus to talar neck; most commonly injured ligament in ankle sprains (taut in plantarflexion + inversion, the classic "rolled ankle" mechanism).
• Calcaneofibular ligament (CFL): lateral malleolus to calcaneus; second most commonly injured, often together with the ATFL in more severe sprains.
• Posterior talofibular ligament (PTFL): lateral malleolus to posterior talus; strongest of the three, rarely injured in isolation.
• Movements: Dorsiflexion and plantarflexion only (a true hinge joint), inversion/eversion occur at the subtalar and transverse tarsal joints, not at the ankle joint itself.
• Stability: Greatest in dorsiflexion, where the wider anterior part of the talar trochlea wedges snugly into the mortise; least stable in plantarflexion, where the narrower posterior trochlea allows some laxity, the anatomical basis for most ankle sprains occurring with the foot plantarflexed (e.g., landing awkwardly from a jump).
• Blood supply: Malleolar anastomoses (medial and lateral), from branches of the anterior tibial, posterior tibial, and fibular arteries.
• Innervation: Deep fibular, tibial, and superficial fibular nerve articular branches.

Extensor retinacula:

• Superior extensor retinaculum: broad band above the ankle, from tibia to fibula, binds down the anterior compartment tendons before they cross the ankle.
• Inferior extensor retinaculum: Y-shaped, over the dorsum of the foot, further stabilises the extensor tendons (tibialis anterior, EHL, EDL, fibularis tertius) and the neurovascular bundle (dorsalis pedis, deep fibular nerve) as they cross onto the foot.

Flexor retinaculum:

• Extends from the medial malleolus to the calcaneus (medial process), converting the posteromedial groove into the tarsal tunnel.
• Retains the deep posterior compartment tendons, the posterior tibial vessels, and the tibial nerve as they pass into the sole.

Fibular (peroneal) retinacula:

• Superior fibular retinaculum: from lateral malleolus to calcaneus, retains fibularis longus and brevis tendons posterior to the lateral malleolus.
• Inferior fibular retinaculum: continuous with the inferior extensor retinaculum, further stabilises the fibularis tendons over the lateral calcaneus.

• Boundaries: Medial malleolus and posteromedial talus/calcaneus (bony floor), roofed by the flexor retinaculum, extending from the medial malleolus to the calcaneus.
• Contents (anterior to posterior, "Tom, Dick, AN'd Harry"): Tibialis posterior tendon, Flexor Digitorum longus tendon, posterior tibial Artery and veins, tibial Nerve, Flexor Hallucis longus tendon.
• Tibial nerve: Divides within or just distal to the tarsal tunnel into its terminal branches, medial plantar nerve and lateral plantar nerve (and the medial calcaneal branch, given off before entering the tunnel, supplying heel skin).
• Posterior tibial vessels: Divide into medial and lateral plantar vessels alongside the nerve.
• Clinically analogous to the carpal tunnel of the wrist: a rigid osteofascial tunnel through which a major nerve passes, vulnerable to compression (see Tarsal Tunnel Syndrome below).

Dorsum:

• Thin skin, extensor tendons, dorsalis pedis vessels, and the small extensor digitorum brevis and extensor hallucis brevis muscles (deep fibular nerve), the only intrinsic muscles on the dorsum, the rest being on the sole.

Sole (plantar surface):

• Thick, specialised skin (weight-bearing); houses the plantar aponeurosis and four muscle layers.

Plantar aponeurosis:

• Thick fibrous band, from the calcaneal tuberosity, fanning distally into five digital bands to the base of each toe.
• Supports the longitudinal arches of the foot and contributes to the windlass mechanism: dorsiflexion of the toes (as in toe-off during gait) tightens the aponeurosis, raising the longitudinal arch and converting the foot into a rigid lever for efficient push-off.
• Chronic microtears/degeneration at its calcaneal origin cause plantar fasciitis.

Fascial compartments of the foot:

• Medial (containing the abductor hallucis and flexor hallucis brevis group), central (containing the flexor digitorum brevis, quadratus plantae, lumbricals, adductor pollicis, and long flexor tendons), lateral (abductor and flexor digiti minimi brevis), and interosseous compartments, analogous in principle to the hand's thenar/hypothenar/central compartments, and similarly relevant to the spread of infection or compartment syndrome of the foot.

First Layer (most superficial)

Second Layer

The long flexor tendons (FDL, FHL) pass through this layer.

Third Layer

Fourth Layer (deepest)

Note: the foot's abduction/adduction reference axis is the 2nd digit (not the 3rd/middle digit as in the hand), reflecting its different functional role in weight-bearing.

Tendons of tibialis anterior/posterior and the long flexors/extensors pass through or alongside this layer to reach their insertions.

• Medial plantar nerve: Larger terminal branch of the tibial nerve; runs with the medial plantar artery; supplies abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, and the 1st lumbrical (motor, analogous to the median nerve's territory in the hand); sensory to the medial sole and plantar surface of the medial 3½ toes.
• Lateral plantar nerve: Smaller terminal branch of the tibial nerve; runs with the lateral plantar artery; supplies quadratus plantae, abductor digiti minimi, flexor digiti minimi brevis, adductor hallucis, all interossei, and the 2nd–4th lumbricals (motor, analogous to the ulnar nerve's territory in the hand); sensory to the lateral sole and plantar surface of the lateral 1½ toes.
• This medial/lateral plantar nerve division is functionally and clinically analogous to the median/ulnar nerve split in the hand, a useful comparative framework for recall.

• Formed predominantly by the lateral plantar artery, completed medially by the deep plantar branch of the dorsalis pedis artery (which dips from the dorsum into the sole between the 1st and 2nd metatarsals), mirroring the deep palmar arch of the hand (radial artery-dominant, completed by the ulnar artery's deep branch).
• Gives rise to plantar metatarsal arteries, then plantar digital arteries supplying the toes.
• The medial plantar artery is comparatively smaller, running alongside the medial plantar nerve, supplying the medial sole directly (loosely analogous to the superficial palmar arch's territory, though the anatomical parallel is less exact than for the deep arch).