Ch05: Lower Limb IV — Popliteal Fossa, Leg & Foot（腘窝、小腿与足）

# Ch05: Lower Limb IV — Popliteal Fossa, Leg & Foot

Preparatory Mindset

The popliteal fossa is the "cubital fossa of the lower limb" — a diamond-shaped space behind the knee with the popliteal vessels & nerves. The leg has four compartments (anterior, lateral, posterior superficial, posterior deep) with distinct neurovascular bundles. The foot & ankle are where the ankle jerk, dorsalis pedis pulse, and tarsal tunnel are clinically relevant.

Exam mindset: "Borders & contents of the popliteal fossa", "course of the small saphenous vein", "location of the superficial peroneal nerve in the lateral leg", "location of the dorsal artery (dorsalis pedis) at the ankle" — all flagged in the lecture as must/need-to-know.

Core Concepts

1. Popliteal fossa

- Superomedial: semitendinosus & semimembranosus - Superolateral: biceps femoris - Inferomedial & inferolateral: medial & lateral heads of gastrocnemius - Roof: skin, superficial fascia, deep (popliteal) fascia - Floor: popliteal surface of femur, posterior capsule of knee, popliteus muscle

• Diamond shape, behind the knee joint
• Boundaries:
• Contents (deep to superficial — actually anterior to posterior): popliteal artery (deepest), popliteal vein (superficial to artery), tibial nerve & common peroneal nerve (most superficial); plus small saphenous vein (pierces roof to drain into popliteal vein), popliteal lymph nodes, posterior femoral cutaneous nerve
• Mnemonic for popliteal contents anterior→posterior: "A-V-N" (Artery, Vein, Nerve)
• Clinical: popliteal artery is the most common site of peripheral aneurysm; popliteal pulse is often hard to feel; popliteal (Baker's) cyst; entrapment of popliteal artery

2. Leg compartments (fascial compartments of the leg)

3. Nerves & vessels of the leg

• Tibial nerve (L4-S3): descends in the posterior compartment (deep to soleus), passes posterior to the medial malleolus (with posterior tibial artery) through the tarsal tunnel, divides into medial & lateral plantar nerves
• Common peroneal (fibular) nerve (L4-S2): winds around the neck of the fibula (superficial & vulnerable) → divides into deep peroneal (anterior compartment: dorsiflexors) and superficial peroneal (lateral compartment: evertors) nerves
• Anterior tibial artery: from popliteal artery, enters the anterior compartment through the interosseous membrane; becomes dorsalis pedis artery on the dorsum of the foot (pulse felt between extensor hallucis longus & extensor digitorum longus tendons, just lateral to the EHL tendon)
• Posterior tibial artery: gives the peroneal artery (lateral side), continues behind the medial malleolus (posterior tibial pulse), divides into medial & lateral plantar arteries
• Small saphenous vein: from lateral dorsal venous arch → posterior to lateral malleolus → ascends the posterior leg → drains into popliteal vein in the popliteal fossa

4. Tarsal tunnel (ankle)

• Location: posterior & inferior to the medial malleolus, under the flexor retinaculum
• Contents (anterior→posterior): tibialis posterior tendon, FDL tendon, posterior tibial artery, tibial nerve, FHL tendon — mnemonic "Tom, Dick, And Very Nervous Harry" (T-D-A-V-N-H)
• Clinical: tarsal tunnel syndrome (tibial n. compression) → burning pain/numbness on the sole of the foot, worse at night

5. Foot

• Dorsum: extensor tendons, dorsalis pedis artery, superficial peroneal nerve (medial/lateral)
• Sole: plantar aponeurosis, medial & lateral plantar nerves/arteries (from tibial n./posterior tibial a.); plantar fasciitis is common
• Ankle jerk reflex: S1/S2 via tibial nerve

High-Yield Points

• Mnemonic popliteal contents: "A-V-N" (popliteal artery deepest → vein → tibial & common peroneal nerves most superficial)
• Small saphenous vein: posterior to lateral malleolus → popliteal vein (vs great saphenous: anterior to medial malleolus → femoral vein)
• Common peroneal nerve at the fibular neck = vulnerable → fibular neck fracture or tight plaster → foot drop (deep peroneal: loss of dorsiflexion) + loss of eversion (superficial peroneal) + sensory loss over the dorsum of foot & lateral leg
• Mnemonic tarsal tunnel: "Tom, Dick, And Very Nervous Harry" = Tibialis posterior, FDL, Artery, Vein, Nerve (tibial), FHL
• Dorsalis pedis pulse: on the dorsum, lateral to the EHL tendon; absent with anterior tibial artery occlusion
• Anterior compartment syndrome: pain with passive stretch of toes, weak dorsiflexion — emergency fasciotomy
• Achilles tendon rupture (concept 19): sudden "snap" + inability to push off; Thompson's test (no plantar flexion when calf squeezed)

Topic Summary

The popliteal fossa (diamond behind the knee; boundaries = hamstrings + gastrocnemius heads; contents = popliteal A (deepest), popliteal V, tibial & common peroneal nerves (superficial), small saphenous vein, lymph nodes) is the gateway to the leg. The leg has four compartments: anterior (deep peroneal n. + anterior tibial a. → dorsalis pedis), lateral (superficial peroneal n.), posterior superficial (gastrocnemius/soleus), posterior deep (tibial n. + posterior tibial a. → tarsal tunnel). The common peroneal nerve is vulnerable at the fibular neck (foot drop). The tibial nerve passes behind the medial malleolus (tarsal tunnel) to the sole.