Lower Limb Neurology · Anatomy · High Yield

Sciatic Nerve Branches

The sciatic nerve (L4–S3) is the largest nerve in the body. It divides in the popliteal fossa into the tibial and common peroneal nerves, each with distinct territories, clinical syndromes, and injury patterns.

✦ The Mnemonic

"Sciatic Carries Two Trunks: Deep-Superficial Peroneal; Medial-Lateral Plantar"

Two divisions → four terminal branches in the leg and foot

CP Common Peroneal Nerve Lateral division; wraps around fibular neck; supplies anterior and lateral compartments

DP Deep Peroneal Nerve Branch of common peroneal; dorsiflexion (tibialis anterior); web space sensation between hallux and 2nd toe

SP Superficial Peroneal Nerve Branch of common peroneal; foot eversion (peronei); dorsum of foot sensation

TN Tibial Nerve Medial division; supplies all posterior compartment muscles

MP Medial Plantar Nerve Branch of tibial; 'the median of the foot' — medial 3½ toes, 4 intrinsics

LP Lateral Plantar Nerve Branch of tibial; 'the ulnar of the foot' — lateral 1½ toes, most intrinsics

📚 Clinical Breakdown

The sciatic nerve exits the pelvis through the greater sciatic foramen below piriformis (in 85% of people). In 15%, the common peroneal component passes through or above piriformis — this variant predisposes to piriformis syndrome. The nerve descends between the biceps femoris and the adductor magnus before dividing at the apex of the popliteal fossa.

Common peroneal nerve injury is the most common peripheral mononeuropathy of the lower limb. It wraps tightly around the fibular neck (subcutaneous and mobile) and is vulnerable to: fibular head fractures, prolonged squatting, tight plaster casts, and prolonged leg crossing. Injury produces: foot drop (loss of dorsiflexion — deep peroneal), loss of foot eversion (superficial peroneal), and sensory loss over the dorsum of the foot and lower lateral leg.

Tarsal tunnel syndrome: compression of the tibial nerve under the flexor retinaculum at the medial malleolus. Produces burning pain and paraesthesia in the plantar surface of the foot, worsened by activity. Analogous to carpal tunnel syndrome at the wrist. Tinel's sign positive over the medial malleolus.

Foot drop Deep peroneal nerve (tibialis anterior)

Eversion loss Superficial peroneal nerve (peroneus longus + brevis)

Plantar flexion loss Tibial nerve (gastrocnemius + soleus)

Lateral sole sensation Sural nerve (L5, S1-2) — commonly tested

⭐ Clinical Pearl

L4 vs L5 vs S1 radiculopathy distinction: L4 — weak knee extension (quadriceps), reduced knee jerk, medial lower leg numbness. L5 — weak great toe extension (EHL) and foot dorsiflexion, no reflex loss, lateral lower leg/dorsum numbness. S1 — weak plantarflexion, absent ankle jerk, lateral foot/sole numbness. The ankle jerk tests S1 (not L4 as sometimes stated).

⚠ Exam Trap

Piriformis syndrome is controversial — it produces sciatic pain from compression of the sciatic nerve by a hypertrophied or inflamed piriformis muscle. Trigger points on deep buttock palpation and pain reproduced by resisted external rotation (Pace test) are suggestive. It is a diagnosis of exclusion after ruling out disc prolapse and lumbar stenosis on MRI.

← Knee Ligaments