sciatic nerve

**Origin and course** - Roots: L4, L5, S1, S2, S3 (ventral rami); the largest and thickest nerve in the body. - Exits the pelvis through the greater sciatic foramen, typically inferior to the piriformis, and runs deep to the gluteus maximus. It descends midway between the greater trochanter and ischial tuberosity, then travels down the posterior thigh. - Divides into two terminal branches, usually at the apex of the popliteal fossa: - Tibial nerve (L4–S3) - Common fibular (peroneal) nerve (L4–S2) - Anatomic variation: in some people the common fibular division pierces the piriformis (relevant to piriformis syndrome).

**Motor function** - Before dividing, it supplies the hamstring muscles (semitendinosus, semimembranosus, biceps femoris long and short heads) and the ischial (hamstring) part of adductor magnus. Mnemonic: the short head of biceps femoris is innervated by the common fibular division, while the others come from the tibial division. - Tibial nerve: posterior leg compartment (plantarflexion, inversion, toe flexion) and the plantar foot muscles via the medial and lateral plantar nerves. - Common fibular nerve: anterior and lateral leg compartments. The deep branch handles dorsiflexion and toe extension, and the superficial branch handles eversion.

**Sensory function** - It has no direct cutaneous branches in the thigh. Sensation comes through its branches: sural (tibial and common fibular contributions), lateral sural cutaneous, plantar nerves, superficial and deep fibular nerves. - Sensory loss therefore affects the leg and foot below the knee, sparing the medial leg and foot (saphenous nerve, from the femoral nerve).

**Clinical significance** - **Sciatica**: radicular pain radiating down the posterior or lateral leg, most commonly from L5 or S1 radiculopathy due to a herniated disc (L4–L5 or L5–S1), not a primary sciatic nerve lesion. - Straight leg raise test: positive if pain is reproduced at 30–70° (high sensitivity). The crossed straight leg raise is more specific. - L5 radiculopathy: weak dorsiflexion and great toe extension, with sensory loss on the dorsum of the foot. - S1 radiculopathy: diminished Achilles reflex, weak plantarflexion, with lateral foot sensory loss. - **Cauda equina syndrome** (surgical emergency): bilateral sciatica, saddle anesthesia, urinary retention or overflow incontinence, fecal incontinence, and decreased rectal tone. Needs urgent MRI and decompression. - **Piriformis syndrome**: buttock pain from sciatic nerve compression by the piriformis, with pain worsened by hip flexion, adduction, and internal rotation (FAIR test). - **Posterior hip dislocation or acetabular fracture**: can injure the sciatic nerve (the common fibular portion is more vulnerable), causing hamstring weakness and foot drop. - **Intragluteal injection injury**: avoid by injecting into the upper outer quadrant of the buttock (or the ventrogluteal site). - **Common fibular nerve injury** (often from fibular neck fracture or lateral knee trauma): foot drop with a steppage gait, weak dorsiflexion and eversion, and sensory loss over the dorsum of the foot and lateral leg. - **Complete sciatic nerve lesion**: weak knee flexion, loss of all function below the knee, and a flail foot. - **Other causes**: diabetic neuropathy, tumors (schwannoma), pregnancy, and prolonged sitting or immobilization.

**Key associations for boards** - Greater sciatic foramen: the piriformis, superior gluteal vessels and nerve (above), and the inferior gluteal vessels and nerve, pudendal nerve, internal pudendal vessels, sciatic nerve, and posterior femoral cutaneous nerve (below). - Superior gluteal nerve injury causes a positive Trendelenburg sign (not a sciatic nerve injury). - The sciatic nerve does not innervate the quadriceps (femoral nerve) or the adductors (obturator nerve), except the hamstring part of adductor magnus.