Sciatic Nerve
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Direct answer
Thickest nerve in the body — about as broad as a thumb at its origin — the sciatic nerve (L4, L5, S1, S2, S3) leaves the pelvis through the greater sciatic foramen below piriformis and runs down the back of the thigh to supply the hamstrings. It descends between the ischial tuberosity and the greater trochanter, lies deep to gluteus maximus and then the long head of biceps femoris on the posterior surface of adductor magnus, and usually ends at the superior angle of the popliteal fossa by dividing into the tibial and common peroneal nerves. Through these divisions it supplies every muscle below the knee — and gives no branch at all in the gluteal region.
What you must remember
- Formation and exit: formed on the anterior surface of piriformis; enters the gluteal region through the greater sciatic foramen below piriformis.
- Surface marking: a line from just medial to the midpoint between the ischial tuberosity and the greater trochanter, down the back of the thigh — the basis of the upper-outer-quadrant rule for gluteal injections.
- Direct branches: semitendinosus, semimembranosus, long head of biceps femoris and the ischial part of adductor magnus from the tibial component; the short head of biceps femoris alone from the common peroneal component.
- Termination: divides at the superior angle of the popliteal fossa, roughly at the junction of the middle and lower thirds of the thigh — though the level varies from pelvis to popliteal fossa, and the peroneal part may pierce piriformis separately.
- Important negatives: no branches in the gluteal region and no gluteal muscle supply — the glutei are supplied by the superior and inferior gluteal nerves.
- Complete section: hamstrings plus everything below the knee paralysed — foot drop, lost ankle jerk, and anaesthesia below the knee except the medial strip of the saphenous nerve (femoral, L3–L4).
- Injury causes: misplaced gluteal injections, posterior hip dislocation and penetrating wounds; isolated foot drop is usually common peroneal injury at the fibular neck.
A gluteal injection gone wrong, read anatomically
A patient develops foot drop days after an intramuscular buttock injection. Reconstruct the geometry: the needle entered the lower inner quadrant, exactly where the nerve runs lateral to the ischial tuberosity — the safe target is the upper outer quadrant. Now localise the deficit. If the peroneal component was grazed, dorsiflexion and eversion are lost with a high-stepping gait and dorsal foot anaesthesia, but the hamstrings and ankle jerk are intact, because the tibial component escaped. If the whole trunk was infiltrated, the hamstrings weaken, the ankle jerk disappears and the patient cannot stand on tiptoe; sensation is lost below the knee except the medial strip spared by the saphenous nerve, which travels with the femoral artery. The same logic solves posterior hip dislocation: the femoral head is driven against the nerve on the posterior acetabular wall, and the laterally placed, more tethered peroneal division bears the brunt — hence foot drop after hip injury or surgery is typically peroneal, which is why distal neurology is documented before operation.
Where students slip
The commonest error is giving the sciatic nerve gluteal branches — it has none; gluteus maximus belongs to the inferior gluteal nerve, a fact examiners test directly. The second is forgetting that the short head of biceps femoris is peroneal-supplied inside a hamstring group — which is why a pure tibial lesion still permits some knee flexion. Third, the termination is quoted as fixed; the level of division varies widely, with the peroneal part sometimes piercing piriformis, which also explains atypical piriformis syndrome. Finally, every foot drop is attributed to the sciatic nerve; the examiner's counter is the fibular neck, where crossed-leg palsy and tight plasters act. "Foot drop: think fibular neck first, then L5 root, then sciatic" shows judgement rather than memory.
Frequently asked questions
What is the root value of the sciatic nerve?
L4, L5, S1, S2 and S3 — the thickest nerve of the body, about 2 cm wide at its origin.
Where does the sciatic nerve divide, and into what?
Usually at the superior angle of the popliteal fossa, into the tibial and common peroneal nerves — though the level varies from the pelvis to the popliteal fossa, and the peroneal part may pierce piriformis separately.
Why does foot drop usually mean common peroneal injury rather than sciatic injury?
The common peroneal nerve winds subcutaneously around the neck of the fibula, where plasters, crossed legs or prolonged squatting compress it; the sciatic trunk is deep and protected, so isolated foot drop with intact hamstrings and ankle jerk points to the fibular neck.
What sensory area is spared after complete sciatic section?
The medial leg and foot to the big toe — the great saphenous territory of the femoral nerve, which travels with the femoral artery, not with the sciatic nerve.