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Sacral plexus

The sacral plexus is a large network of nerves located in the pelvis. It is formed by the lumbosacral trunk (L4–L5) and the ventral rami of the first through fourth sacral nerves (S1–S4). The plexus lies on the anterior surface of the piriformis muscle, deep within the pelvis, and gives rise to the major nerves of the lower limb, pelvis, and perineum.

It is the origin of the largest nerve in the body, the sciatic nerve, and numerous other nerves important for locomotion, pelvic stability, and pelvic organ function.

Synonyms

  • Plexus sacralis

  • Lumbosacral plexus (posterior part)

  • Sacral nerve plexus

Origin, Course, and Branches

  • Origin:

    • Formed by the lumbosacral trunk (L4–L5) and the ventral rami of S1–S4 spinal nerves

  • Course:

    • Lies anterior to the piriformis muscle

    • Positioned posterior to the internal iliac vessels and ureter

    • Nerve roots converge into large terminal branches that exit via the greater sciatic foramen

  • Branches:

    • Major branches:

      • Sciatic nerve (L4–S3) – largest nerve of the body, supplies posterior thigh, leg, and foot

      • Pudendal nerve (S2–S4) – supplies perineum, external genitalia, anal and urethral sphincters

      • Superior gluteal nerve (L4–S1) – supplies gluteus medius, minimus, and tensor fasciae latae

      • Inferior gluteal nerve (L5–S2) – supplies gluteus maximus

    • Other branches:

      • Nerve to quadratus femoris (L4–S1)

      • Nerve to obturator internus (L5–S2)

      • Posterior femoral cutaneous nerve (S1–S3)

      • Perforating cutaneous nerve (S2–S3)

      • Pelvic splanchnic nerves (S2–S4, parasympathetic fibers)

      • Small muscular branches to piriformis and other deep pelvic muscles

Relations

  • Anteriorly: Internal iliac vessels, ureter, pelvic fascia

  • Posteriorly: Piriformis muscle and posterior pelvic wall

  • Laterally: Pelvic bones (ilium, ischium)

  • Medially: Pelvic viscera (rectum, uterus/vagina in females, prostate in males)

Function

  • Provides motor innervation to:

    • Gluteal muscles

    • Posterior thigh muscles

    • Most leg and foot muscles

  • Provides sensory innervation to:

    • Skin of posterior thigh, leg, foot, and perineum

  • Provides autonomic fibers:

    • Parasympathetic fibers (pelvic splanchnic nerves) to pelvic organs (bladder, rectum, reproductive organs)

Clinical Significance

  • Injury/trauma: Pelvic fractures, sacroiliac dislocations, or tumor infiltration may damage plexus

  • Neuropathies: Sacral plexopathy can cause weakness, numbness, or pain radiating to thigh, leg, or perineum

  • Oncology: Important in staging pelvic tumors (e.g., rectal, cervical, prostate cancers)

  • Surgical relevance: Landmark in pelvic, spinal, and orthopedic surgery

  • Pain syndromes: Involved in sciatica, pudendal neuralgia, and pelvic neuropathies

MRI Appearance

T1-weighted images:

  • Sacral plexus roots and trunks appear as low-to-intermediate signal intensity structures

  • Surrounded by bright pelvic fat, aiding delineation

T2-weighted images:

  • Nerves show intermediate to mildly hyperintense signal compared to muscle

  • Pathology (inflammation, edema, tumor) shows brighter signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal nerves show low signal

  • Abnormal nerves show bright hyperintensity in cases of neuritis, edema, or infiltration

T1 Fat-Sat Post-Contrast:

  • Normal plexus shows minimal or no enhancement

  • Pathology (tumor, infection, neuritis) shows enhancement along roots or trunks

3D T2 SPACE / CISS:

  • Nerves show intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright fat and CSF, providing excellent contrast

  • Useful for mapping nerve course, detecting compression or infiltration

CT Appearance

Non-Contrast CT:

  • Sacral plexus is not directly visible, but appears as soft tissue density strands within pelvic fat

  • Bony changes or fractures may affect its course (e.g., sacral fractures)

  • Fat helps localize the nerve roots as they emerge from sacral foramina

Post-Contrast CT:

  • Normal nerves show little to no enhancement

  • Pathological infiltration (tumor, infection, hematoma) appears as soft tissue thickening or enhancement around plexus region

  • Fat stranding may be seen in inflammatory or neoplastic disease

MRI image