Ankle and foot anatomy

Ankle joint

• The talocrural joint is a synovial hinge: the tibia and fibula form a mortise around the body of the talus, allowing dorsiflexion and plantarflexion.
• The trochlea of the talus is wider in front than behind. In dorsiflexion the wide front wedges tightly into the mortise, so the joint is most stable. In plantarflexion the narrow posterior talus sits loosely, permitting some side-to-side movement, so most sprains happen with the foot plantarflexed (high heels, stepping off a kerb). The instability comes from bone shape, not weak ligaments.
• Inversion and eversion occur at the subtalar (talocalcaneal) and transverse tarsal (midtarsal: talonavicular and calcaneocuboid) joints, not at the ankle.

Ligaments and sprains

• A child or adult with excessive inversion tears the ATFL; if the force continues, the lower end of the fibula (lateral malleolus) fractures. Bruising appearing next day with pain on standing on tiptoe fits a lateral ligament sprain.
• The inferior tibiofibular joint is a fibrous syndesmosis.
• Ottawa ankle rules: X-ray if there is bony tenderness at the posterior edge or tip of either malleolus, or inability to bear weight for four steps.
• Weber classification of lateral malleolar fractures relative to the syndesmosis: A below (stable), B at the level (may be unstable), C above (syndesmosis torn, unstable).

Talus, calcaneus and midfoot injuries

The talus has no muscle attachments and is mostly covered by cartilage, which is why its vessels are few and vulnerable, like the scaphoid in the hand.

Arches of the foot

• The posterior pillar of both longitudinal arches is the calcaneal tuberosity; the anterior pillars are the metatarsal heads.
• Tibialis posterior is the main dynamic support of the medial arch; its tendon dysfunction is the commonest cause of acquired flat foot in adults. A candidate rejected for flat foot (pes planus) has a collapsed medial arch.
• Pes cavus (high arch) suggests neurological disease: Charcot–Marie–Tooth disease, Friedreich ataxia, spina bifida.

Remember Spring ligament and tibialis posterior hold up the talar head; peroneus longus slings the transverse arch.

Layers of the sole

• Quadratus plantae (flexor accessorius) arises by two heads from the calcaneus and inserts into the lateral border of the FDL tendon. FDL enters the sole obliquely from behind the medial malleolus; quadratus plantae straightens its oblique pull so the toes flex in line. The lumbricals arise from the FDL tendons but do not do this.
• Plantar fasciitis: heel pain worst with the first steps in the morning, tender over the medial calcaneal tubercle.

Tarsal tunnel and the sole's nerves and vessels

• Behind the medial malleolus, under the flexor retinaculum. Contents from front to back: tibialis posterior, flexor digitorum longus, posterior tibial artery with its veins, tibial nerve, flexor hallucis longus.
• Tarsal tunnel syndrome: tibial nerve compression with burning pain and numbness in the sole.
• The tibial nerve divides into medial plantar (like the median nerve in the hand: abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, first lumbrical; medial three and a half toes) and lateral plantar (like the ulnar nerve: most small muscles; lateral one and a half toes).
• The deep plantar arch is formed by the lateral plantar artery, completed by the deep plantar branch of the dorsalis pedis, which dives between the first and second metatarsals. A penetrating wound on the dorsum between the first two metatarsals damages this branch or the first dorsal metatarsal artery. The deep arch supplies the plantar metatarsal arteries to the toes.
• Behind the lateral malleolus: peroneus longus and brevis under the peroneal retinacula; the sural nerve and small saphenous vein lie superficially.
• Front of the ankle, medial to lateral: tibialis anterior, EHL, dorsalis pedis (anterior tibial) artery, deep peroneal nerve, EDL.

Skin of the foot

Lymph from the lateral foot and heel follows the small saphenous vein to the popliteal nodes; the rest follows the great saphenous vein to the superficial inguinal nodes.

Exam trap The anterior talofibular ligament is the commonest ankle ligament injured; the deltoid ligament limits eversion.

Exam trap A displaced talar neck fracture endangers the body of the talus, not the head.

How FCPS asks about this

• Why the ankle is unstable in plantarflexion: the narrow posterior talus sits loosely in the mortise.
• Inversion sprain: ATFL; continued force fractures the lower fibula. Inversion and eversion occur at the subtalar and midtarsal joints.
• Main muscle support of the medial arch: tibialis posterior, with the spring ligament under the talar head.
• Fall onto the heels with calcaneal fracture: look for a thoracolumbar spine fracture.
• Talar neck fracture complication: avascular necrosis of the body.
• Widening between the first and second metatarsal bases: Lisfranc injury.
• Muscle from the calcaneus inserting on the FDL tendon: quadratus plantae, straightening the oblique pull.