Lower Limb — Leg, Ankle, Foot & Surface Anatomy

🟢 Lite — Quick Review (1h–1d)

Compartments of Leg (Crus)

• Divided by interosseous membrane and deep fascia into 3 compartments:

Anterior compartment:

• Muscles (all dorsiflexors): tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius
• Supplied by deep peroneal nerve (L4–L5)
• Clinical: Foot drop (deep peroneal nerve injury) — cannot dorsiflex or evert foot → high-stepping gait; anterior compartment syndrome — painful passive plantarflexion

Lateral compartment:

• Muscles (evertors): peroneus longus and brevis
• Supplied by superficial peroneal nerve
• Clinical: Eversion weakness; lateral compartment syndrome

Posterior compartment (superficial):

• Muscles: gastrocnemius (2 heads), soleus (deep to gastrocnemius), plantaris (rudimentary)
• All supplied by tibial nerve
• Achilles tendon — strongest tendon in body; inserts on calcaneus; for plantarflexion
• Clinical: Achilles tendinitis; rupture — palpable gap, positive Thompson squeeze test; gastrocnemius strain (tennis leg)

Posterior Compartment (Deep):

• Muscles: tibialis posterior (most medial), flexor hallucis longus (most lateral), flexor digitorum longus (middle)
• All share the tarsal tunnel (behind medial malleolus)
• Tarsal tunnel: bounded by flexor retinaculum (roof), medial malleolus, calcaneus, navicular (floor); contains tibial nerve, posterior tibial artery, tibialis posterior tendon, flexor digitorum longus, flexor hallucis longus
• Clinical: Tarsal tunnel syndrome — tibial nerve compression → numbness/tingling in sole, weakness of toe flexion

Ankle Joint

• Hinge joint between tibia, fibula, and talus
• Articular surfaces: tibial plafond (socket), medial malleolus (talus), lateral malleolus (talus), superior talar dome
• Ligaments: lateral collateral (ATFL — anterior talofibular, CF — calcaneofibular, PTFL — posterior talofibular), deltoid ligament (medial — fan-shaped, strong)
• Clinical: Ankle sprains — most common is lateral (ATFL) from inversion injury; syndesmosis injury (high ankle sprain) — Maisonneuve fracture; deltoid ligament injury indicates significant eversion force

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🟡 Standard — Regular Study (2d–2mo)

Bones of Foot

Tarsal Bones (7):

• Talus: superior articular surface (dome), head (anterior), neck; transmits body weight; no muscle attachments
• Calcaneus: largest tarsal bone; sustentaculum tali (medial shelf), Achilles tendon insertion, peroneal tubercle (lateral)
• Navicular: medial cuneiform, intermediate cuneiform, lateral cuneiform, cuboid

Metatarsals & Phalanges:

• 5 metatarsals; 1st is shortest and thickest (bears most weight)
• Phalanges: proximal, middle, distal; hallux has 2 phalanges; others have 3
• Clinical: March fractures (stress fractures of 2nd metatarsal); Jones fracture (5th metatarsal base — avascular zone); Freiberg disease (avascular necrosis of 2nd metatarsal head)

Arches of Foot

• Medial longitudinal arch: higher; calcaneus, navicular, cuneiforms, metatarsals 1–3; maintained by tibialis posterior, flexor hallucis longus, spring ligament
• Lateral longitudinal arch: lower; calcaneus, cuboid, metatarsals 4–5; maintained by peroneus longus/brevis, plantar fascia
• Transverse arch: metatarsal heads; maintained by peroneus longus (acts as bowstring)
• Clinical: Flat feet (fallen arches — pes planus); cavus foot (high arch — pes cavus); plantar fasciitis (pain at calcaneal insertion, worst with first steps in morning)

Foot — Intrinsic Muscles

Dorsal layer: extensor digitorum brevis, extensor hallucis brevis (innervated by deep peroneal nerve)

Plantar layer (4 layers from superficial to deep):

1st layer (most superficial): abductor hallucis, flexor digitorum brevis, abductor digiti minimi

2nd layer: quadratus plantae (assists flexor digitorum longus), lumbricals (4 — flex MCP, extend IP joints), flexor digitorum longus tendon, flexor hallucis longus tendon

3rd layer: flexor hallucis brevis, adductor hallucis (oblique + transverse heads), flexor digiti minimi brevis

4th layer (deepest): plantar interossei (3 — adduct digits 3–5), dorsal interossei (4 — abduct digits 2–4)

All intrinsic foot muscles supplied by medial and lateral plantar nerves (L4–S3)

Clinical: Hammer toe (MTP flexion, PIP extension, DIP flexion); claw toe (MTP extension, PIP and DIP flexion); Morton neuroma (interdigital nerve compression between 3rd and 4th metatarsal heads → burning pain)

Ankle & Foot — Blood Supply

• Anterior tibial artery → dorsalis pedis (foot) → lateral tarsal, arcuate, dorsal metatarsal, dorsal digital
• Posterior tibial artery → medial and lateral plantar arteries (via medial and lateral plantar arteries)
• Fibular (peroneal) artery → perforating branch anastomoses with dorsalis pedis
• Clinical: Peripheral arterial disease (PAD) — absent dorsalis pedis pulse; Buerger’s disease (segmental thrombosing vasculitis in smokers); diabetic foot ulcers (neuropathy + ischemia)

Nerve Supply of Leg & Foot

• Common peroneal nerve (L4–S2): wraps around fibular neck — very superficial and vulnerable
  • Deep peroneal: anterior compartment (foot drop, sensory loss between 1st-2nd toes)
  • Superficial peroneal: lateral compartment (eversion weakness, dorsum sensory loss)
  • Clinical: Common peroneal palsy — foot drop + sensory loss over dorsum; injury at fibular neck
• Tibial nerve (L4–S3): posterior compartment → tarsal tunnel → medial and lateral plantar nerves
  • Clinical: Sensory loss on sole; weakness of toe flexion; positive Tinel’s sign at tarsal tunnel
• Sural nerve: formed from tibial + common peroneal; sensory to lateral foot and ankle
• Saphenous nerve: terminal branch of femoral; sensory to medial leg and foot

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🔴 Extended — Deep Study (3mo+)

Ankle & Foot — Surgical & Clinical Anatomy

Ankle Ligament Complex — Detailed:

Lateral ligament (ATFL, CFL, PTFL):

• Anterior talofibular ligament (ATFL): most commonly injured; resists plantarflexion and inversion; runs from anterior malleolus to talar neck
• Calcaneofibular ligament (CFL): resists inversion when foot is plantarflexed; runs from fibular tip to calcaneus
• Posterior talofibular ligament (PTFL): strongest of the three; rarely injured; runs horizontally from fibular tip to posterior talus
• Clinical grading: Grade I (stretching), Grade II (partial tear), Grade III (complete tear); talar tilt test for CFL

Medial ligament (Deltoid):

• Strong triangular ligament; superficial and deep components
• Superficial: tibionavicular, tibiocalcaneal, tibiospring, posterior tibiotalar
• Deep: anterior and posterior tibiotalar
• Clinical: Deltoid ligament injuries uncommon in isolation; associated with syndesmotic injuries and fibular fractures

Syndesmosis:

• Tibiofibular ligament complex (anterior, posterior, inferior tibiofibular ligament)
• Maintains tibial plafond relationship with talus
• Clinical: High ankle sprain — mechanism: external rotation + dorsiflexion; squeeze test (pain on squeezing tibia and fibula together); fibular fracture above ankle (Maisonneuve fracture —伤 + interosseous membrane injury)
• Imaging: X-ray — tibial clear space, medial clear space, tibiofibular overlap; CT for subtle injuries

Tendon Sheaths Around Ankle:

• Anterior: tibialis anterior, extensor hallucis longus, extensor digitorum longus (all share common sheath)
• Posteromedial: tibialis posterior (most anterior), flexor digitorum longus (middle), flexor hallucis longus (most posterior)
• Posterolateral: peroneus longus and brevis (share common sheath until divergent)
• Clinical: Tenosynovitis — stenosing (de Quervain’s in hand — abductor pollicis longus + extensor pollicis brevis); flexor hallucis longus (“trigger toe” in dancers)

Foot — Gait Cycle & Biomechanics

Stance phase (60% of cycle): heel strike → foot flat → midstance → heel off → toe off Swing phase (40%): initial swing → midswing → terminal swing

Muscle activity during gait:

• Heel strike: tibialis anterior (eccentric dorsiflexion), quadriceps
• Foot flat: gastrocnemius/soleus (controlled plantarflexion), tibialis posterior
• Midstance: gluteus medius/minimus (hip abduction)
• Heel off: plantarflexors (calf muscles) — propulsive phase
• Toe off: intrinsic foot muscles stabilize the foot

Clinical gait abnormalities:

• Foot drop: steppage gait (high-stepping); common peroneal nerve palsy
• Trendelenburg gait: hip abductor weakness; compensated by leaning to affected side
• Cavus foot gait: overcorrection at ankle; high arches
• Ankle equinus: insufficient dorsiflexion → vaulting

Heel Pain — Differential Diagnosis:

• Plantar fasciitis: most common cause; pain at calcaneal insertion of plantar fascia; worst with first morning steps; risk factors: obesity, pes planus, runners
• Calcaneal stress fracture: gradual onset; pain with repetitive loading
• Tarsal tunnel syndrome: burning, numbness in sole
• Baxter’s nerve entrapment: first branch of lateral plantar nerve; pain inferior to heel
• Retrocalcaneal bursitis: pain posterior to heel; associated with Haglund’s deformity

Important Surface Anatomy of Lower Limb

• Patella: ballotable; patellar ligament attaches to tibial tuberosity
• Tibial tuberosity: palpable 2–3 cm below patella
• Fibula head: palpable lateral to tibia just below knee joint; common peroneal nerve winds around neck of fibula here
• Medial malleolus: medial ankle end of tibia
• Lateral malleolus: lateral ankle end of fibula (more distal than medial)
• Sole of foot: plantar fascia (central thickening — important for arch support); first metatarsal head = ball of foot medially; 5th metatarsal head = ball of foot laterally
• Achilles tendon: most prominent tendon; no tendon sheath (paratendon only); rupture at 2–6 cm from calcaneal insertion
• Femoral pulse: in femoral triangle, below inguinal ligament at midpoint
• Popliteal pulse: deepest pulse to palpate; behind knee with leg slightly flexed
• Dorsalis pedis pulse: between extensor hallucis longus and extensor digitorum longus tendons; absent in ~5% of population
• Posterior tibial pulse: behind medial malleolus, midway between malleolus and Achilles tendon

Venous Drainage of Lower Limb:

• Superficial: great saphenous (medial — opens into femoral vein at saphenofemoral junction after receiving small saphenous at popliteal fossa) and small saphenous (lateral — opens into popliteal vein)
• Deep: follow arteries
• Clinical: DVT (deep vein thrombosis — Virchow’s triad: stasis, endothelial injury, hypercoagulability); saphenous vein harvest for coronary artery bypass; varicose veins (incompetent valves → reflux → tortuous dilation)

Lymphatic Drainage of Lower Limb:

• Superficial: along great saphenous (medial) and small saphenous (lateral) veins
• Deep: along deep arteries
• Drain to: inguinal lymph nodes (superficial + deep) → external iliac → lumbar (para-aortic)
• Clinical: Lymphedema (blockage → non-pitting edema); filariasis (Wuchereria bancrofti — mosquito-borne parasite → lymphatic obstruction → massive edema of scrotum/limb — elephantiasis)

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