The sciatic nerve is the largest peripheral nerve in the body (1.5-2 cm diameter), formed from the L4-S3 nerve roots converging in the pelvis. It exits through the infrapiriform foramen of the greater sciatic notch and descends through the posterior thigh to divide into the tibial and common peroneal nerves at the popliteal fossa. It supplies the hamstrings, all muscles below the knee, and the skin of the posterior thigh, leg, and foot.
| Origin | Sacral plexus (L4, L5, S1, S2, S3) |
|---|---|
| Course | Leaves the pelvis through the greater sciatic foramen below piriformis, descends between the greater trochanter and the ischial tuberosity deep to gluteus maximus, runs down the back of the thigh on adductor magnus deep to the long head of biceps femoris, and usually divides into the tibial and common fibular nerves at the apex of the popliteal fossa |
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Sciatic nerve compression (sciatica) most commonly arises from L4-L5 or L5-S1 disc herniation compressing the nerve root in the spinal canal. True peripheral sciatic neuropathy is less common, causes include piriformis syndrome, hip surgery injury, and gluteal compartment syndrome. The straight leg raise (Lasègue sign) is positive from 30-70 degrees in disc herniation. EMG and nerve conduction studies distinguish radiculopathy from peripheral neuropathy and identify the affected level. Sciatic nerve injury at the hip level from posterior arthroplasty produces foot drop (common peroneal division more vulnerable than tibial).
Radicular pain from L4-S1 compression radiating down the posterior leg, most commonly from lumbar disc herniation.
Deep gluteal sciatic irritation reproduced by resisted hip external rotation, diagnosed by excluding lumbar pathology.
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