Nerves of the Lower Limb
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Direct answer
Foot drop after a plaster cast, a burning thigh under a tight belt, and a pelvis that drops on standing — three lesions, three nerves, and all three appear in every screening paper. The common peroneal nerve winds around the neck of the fibula where casts and crossed legs paralyse dorsiflexion and eversion while sparing inversion; the lateral femoral cutaneous nerve is compressed under the inguinal ligament near the anterior superior iliac spine in meralgia paraesthetica; and the superior gluteal nerve (L4–S1) to gluteus medius, minimus and tensor fasciae latae fails in the Trendelenburg gait, where standing on the weak side drops the pelvis of the sound side. The two great plexus cords of the limb are the femoral (L2–L4, posterior divisions, quadriceps and anterior thigh) and the obturator (L2–L4, anterior divisions, adductors), with the sciatic (L4–S3) ruling the posterior compartment before dividing into tibial and common peroneal nerves at the apex of the popliteal fossa.
What you must remember
- Femoral nerve (L2–L4): supplies iliacus, pectineus, sartorius and quadriceps; sensation over the anterior thigh and, through the saphenous nerve, the medial leg and foot; injury abolishes knee extension and the knee jerk.
- Obturator nerve (L2–L4): adductor compartment and gracilis, sensation to a patch of medial thigh; injury produces a circumduction gait, while an obturator hernia irritates it with medial thigh pain worsened by thigh extension — Howship-Romberg sign.
- Sciatic nerve (L4–S3): leaves the pelvis below piriformis, supplies the hamstrings (the short head of biceps femoris via its common peroneal component) and divides at the superior angle of the popliteal fossa; gluteal injections belong in the upper outer quadrant to avoid it.
- Tibial nerve: plantar flexion through triceps surae, inversion via tibialis posterior, toe flexion, and sole sensation through medial and lateral plantar branches; compressed in the tarsal tunnel behind the medial malleolus, it produces burning soles with a positive Tinel.
- Common peroneal nerve (L4–S2): divides within peroneus longus into the superficial branch (evertors, lateral leg and dorsum sensation) and deep branch (anterior compartment, first web space sensation); injury at the fibular neck causes foot drop with preservation of inversion.
- Gluteal pair: superior gluteal (L4–S1) to abductors — Trendelenburg sign positive when standing on the affected side, the pelvis falling contralaterally; inferior gluteal (L5–S2) to gluteus maximus — failure to rise from a chair or climb stairs, the gluteal gait.
- Lateral femoral cutaneous nerve: compressed under the inguinal ligament near the ASIS in obesity, pregnancy and tight garments, giving burning paraesthesia over the anterolateral thigh with no weakness.
Narrowing down a foot drop
A man cannot dorsiflex his right foot after sleeping with his legs crossed. Localise by what is spared. If inversion is intact, tibialis posterior (tibial nerve) is working, so the lesion is distal — at the fibular neck — and the cause is compression of the common peroneal nerve. If inversion is weak as well, the tibial-innervated muscles are involved, so the problem sits upstream: an L5 radiculopathy (disc prolapse) or a proximal sciatic lesion affecting predominantly the peroneal component, which is more vulnerable within the sciatic trunk. Check the sensory geography: paraesthesia confined to the dorsum and first web space fits peroneal; dermatomal pain radiating from the back with a positive straight leg raise fits L5. Within peroneal palsy, preserved eversion with pure dorsiflexion failure localises to the deep branch alone, while eversion failure with lateral leg sensory change points to the superficial branch.
Classic vignette patterns
The screening paper's favourites are scenario-driven. The crossed-leg or cast foot drop is peroneal. The burning thigh of the obese or pregnant woman is meralgia paraesthetica, with the lateral femoral cutaneous nerve under the inguinal ligament as the answer. The Trendelenburg stem is worded as the pelvis falling toward the normal side when the patient stands on the affected limb — candidates who reverse the direction lose the mark — and the safe injection quadrant for the gluteal region is always in options. The tarsal tunnel stem describes burning soles at night, distinguished by the Tinel sign behind the medial malleolus.
Frequently asked questions
What are the root values of the sciatic nerve?
L4 to S3, derived from the sacral plexus; it leaves the pelvis through the greater sciatic foramen below piriformis.
Which nerve is injured at the neck of the fibula and what are the signs?
The common peroneal nerve, causing foot drop with loss of dorsiflexion and eversion, sensory loss over the dorsum and first web space, with inversion preserved.
What does a positive Trendelenburg sign indicate?
Weakness of the hip abductors — gluteus medius and minimus supplied by the superior gluteal nerve — such that the pelvis drops on the contralateral side when standing on the affected limb.
Which nerve is responsible for meralgia paraesthetica?
The lateral femoral cutaneous nerve, compressed under the inguinal ligament near the anterior superior iliac spine, causing burning over the anterolateral thigh without motor loss.
Which nerve is compressed in tarsal tunnel syndrome?
The tibial nerve behind the medial malleolus, producing burning pain and paraesthesia in the sole supplied by its medial and lateral plantar branches.