The sacral plexus is formed on the anterior surface of the piriformis muscle in the greater sciatic foramen by the lumbosacral trunk (L4-L5) and the anterior rami of S1-S4. Its major branches include: the sciatic nerve (L4-S3), superior gluteal nerve (L4-S1), inferior gluteal nerve (L5-S2), posterior femoral cutaneous nerve (S1-S3), pudendal nerve (S2-S4), and nerve to piriformis (S1-S2). The plexus innervates the posterior thigh, leg, foot, perineum, and pelvic floor.
Sacral plexus injuries occur from pelvic fractures, posterior hip dislocation, intraoperative traction during hip arthroplasty, and pelvic tumours. The plexus lies immediately posterior to the internal iliac vessels in the pelvis, making it vulnerable to injury during pelvic lymphadenectomy and in bleeding from pelvic fractures. Sacral plexopathy from haematoma, tumour, or radiation produces a mixed picture of sciatic distribution weakness with superimposed superior and inferior gluteal involvement. Electrophysiology distinguishes plexopathy from L4-S1 radiculopathy.
Sacral fractures and posterior pelvic ring disruptions directly injure the sacral plexus branches, producing a variable pattern of sciatic nerve deficits, gluteal weakness, and perineal numbness; the posterior femoral cutaneous nerve sensory area over the posterior thigh is a useful clinical indicator of sacral plexus level injury.
Delayed radiation injury to the sacral plexus from pelvic radiotherapy for cervical, rectal, or prostate cancer produces progressive lower limb weakness, sensory loss, and sphincter dysfunction years after treatment; distinguished from tumour recurrence by imaging and by the typically bilateral and painless nature of radiation plexopathy.
