What is the origin and extent of the psoas sheath?

Origin and Extent of the Psoas Sheath

The psoas sheath originates from the psoas fascia covering the psoas major muscle and extends from the lumbar vertebrae down to the lesser trochanter of the femur, forming a potential space that contains loose connective tissue and important neurovascular structures.

Anatomical Origin

The psoas sheath is a fascial covering that surrounds the psoas major muscle. Its key anatomical features include:

• Originates at the lumbar vertebrae (L1-L5) where the psoas muscle attaches 1
• Forms from the thoracolumbar fascia that envelops the psoas muscle
• Creates a distinct fascial compartment that separates the psoas from surrounding structures

Extent and Course

The psoas sheath follows the course of the psoas major muscle:

• Begins at the lumbar spine attachments (L1-L5 with contributions from T12) 1
• Extends inferiorly along the psoas muscle through the pelvis
• Passes deep to the inguinal ligament into the thigh
• Terminates at the lesser trochanter of the femur where the psoas muscle inserts

Important Anatomical Features

The Cleft of Psoas Major (CPM)

A significant anatomical feature within the psoas sheath is the cleft of psoas major (CPM):

• Located beneath the psoas major muscle, particularly evident from L4/5 level downward 2
• Filled with loose connective tissue and contains important neurovascular structures
• Forms a potential space that can be visualized during lateral surgical approaches 2

Neurovascular Contents

The psoas sheath contains several important neurovascular structures:

• Lumbar plexus nerve roots (L1-L4) that emerge from within the psoas muscle 3
• Femoral nerve running within the lateral portion of the psoas sheath 1
• Sympathetic trunk, ascending lumbar vein, and iliolumbar vessels 2
• Obturator nerve in the lower portion of the sheath 2

Clinical Significance

Surgical Considerations

The psoas sheath has significant implications for surgical approaches:

• Posterior lumbar plexus blocks (psoas sheath blocks) provide greater efficacy than femoral nerve blocks in total hip replacement 1
• The psoas sheath can be used as a surgical landmark during retroperitoneal approaches 1
• For extended resection of retroperitoneal liposarcomas, the psoas fascia/muscle may need to be resected 1

Pathological Considerations

The psoas sheath can be involved in various pathological processes:

• Psoas abscesses can track along the psoas sheath from the spine to the thigh 4
• Large psoas abscesses can penetrate the sheath and involve the thigh adductors 4
• Hematomas can develop within the psoas sheath, sometimes concurrently with rectus sheath hematomas 5

Imaging Considerations

For visualization of the psoas sheath and related pathology:

• MRI lumbosacral plexus without and with IV contrast is the preferred imaging modality (rated 9/9 for appropriateness) 1
• CT with IV contrast can also be used to visualize the psoas sheath and its contents (rated 6/9 for appropriateness) 1
• The psoas muscle thickness and area on imaging can be used as prognostic indicators in various conditions 1

Surgical Approaches Involving the Psoas Sheath

• During retroperitoneal approaches, the psoas fascia serves as an important landmark 1
• In the psoas hitch technique for ureteral injuries, the bladder is hitched to the psoas minor tendon 1
• When performing posterior lumbar plexus blocks, the psoas sheath is targeted for anesthetic delivery 1

Understanding the origin and extent of the psoas sheath is essential for surgeons, radiologists, and clinicians dealing with pathologies and procedures involving the retroperitoneal space and lumbar region.