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Lumbosacral trunk

The lumbosacral trunk is a thick nerve bundle that forms the connection between the lumbar plexus and sacral plexus. It is composed of a large portion of the ventral ramus of L5 and a smaller descending branch from L4. These fibers unite near the pelvic brim, descend into the pelvis, and contribute significantly to the formation of the sacral plexus, which supplies the lower limb, pelvis, and perineum.

It lies close to major vascular structures at the pelvic brim, making it clinically important during pelvic surgery, trauma, and in radiologic evaluation of lumbosacral plexopathies.

Synonyms

  • L4–L5 trunk

  • Lumbosacral cord

  • Lumbosacral plexus root

Origin, Course, and Branches

  • Origin:

    • Formed by the descending part of L4 ventral ramus (lumbosacral contribution) and the entire ventral ramus of L5

  • Course:

    • Emerges from the medial border of the psoas major muscle

    • Passes over the sacral ala into the pelvis

    • Lies anterior to the sacroiliac joint and posterior to the common iliac vessels

    • Joins with the S1 ventral ramus to participate in the sacral plexus

  • Branches:

    • The trunk itself gives no direct peripheral branches; instead, it contributes fibers to the sacral plexus, including the sciatic nerve, superior gluteal nerve, and inferior gluteal nerve

Relations

  • Anteriorly: Common iliac vessels, pelvic peritoneum

  • Posteriorly: Sacral ala, sacroiliac joint

  • Laterally: Psoas major muscle

  • Medially: Sacral ventral rami (S1–S4) forming sacral plexus

Function

  • Provides major contribution to the sacral plexus

  • Supplies motor and sensory innervation to:

    • Muscles of the gluteal region (via gluteal nerves)

    • Muscles of the posterior thigh and entire leg and foot (via sciatic nerve and branches)

    • Portions of the pelvis and perineum

  • Acts as a key anatomical link uniting lumbar and sacral plexuses

Clinical Significance

  • Injury/trauma: May be affected in pelvic fractures, sacroiliac joint injuries, or retroperitoneal hemorrhage

  • Tumor compression: Pelvic masses (sarcomas, lymphomas, gynecological tumors) may compress the trunk, producing lumbosacral plexopathy

  • Surgical relevance: Important landmark in spine, vascular, and gynecological surgeries

  • Neuropathies: May cause radiating pain from the back to the leg (mimicking radiculopathy)

  • Imaging relevance: Must be differentiated from adjacent vessels and pelvic structures on MRI/CT

MRI Appearance

T1-weighted images:

  • Lumbosacral trunk appears as a low-to-intermediate signal linear/oval structure

  • Surrounded by bright retroperitoneal fat, aiding visualization

T2-weighted images:

  • Appears as intermediate to mildly hyperintense signal compared to muscle

  • Pathology (edema, tumor infiltration) appears as brighter signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal trunk shows low signal

  • Abnormal trunk (edema, neuritis, infiltration) appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal trunk: minimal or no enhancement

  • Pathological conditions: focal, diffuse, or nodular enhancement depending on etiology

3D T2 SPACE / CISS:

  • Trunk shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by very bright CSF or fat, giving excellent contrast

  • Ideal for tracing the nerve course along pelvic brim and distinguishing it from vessels

CT Appearance

Non-Contrast CT:

  • Nerve trunk is not directly seen, inferred as a soft tissue density at the pelvic brim

  • Surrounded by fat, which enhances visibility

  • Pelvic fractures may show displacement or impingement along its course

Post-Contrast CT:

  • Nerve itself does not enhance

  • Pathological processes (tumor, inflammation) may appear as enhancing soft tissue masses or infiltrations along its course

  • Fat stranding may be seen in inflammatory or infiltrative conditions

MRI image