Advancing the Science of Ultrasound Guided Regional Anesthesia and Pain Medicine

Psoas Compartment Block


The Lumbar Plexus

The lumbar plexus is formed by the anterior divisions of L1, L2, L3 and the greater part of L4. The L1 root often receives a branch from T12. The lumbar plexus is situated most commonly in the posterior one third of the psoas major muscle, anterior to the transverse processes of the lumbar vertebrae.

Structures usually visualized on ultrasound are:

  1. Spinous process
  2. Lamina and Facet joint
  3. Transverse process
  4. Ilium bone
  5. Multifidus and Erector Spinae muscles
  6. Psoas Major muscle

Structures not usually visualized on ultrasound are:

  1. Lumbar plexus (adult)
  2. Iliolumbar ligament

Scanning Technique

  • Place the patient in the lateral decubitus position with the side to be blocked uppermost.
  • Scan the paravertebral region at L2-3 cephalad to the iliac crest.
  • After skin and transducer preparation, place a curved transducer with the appropriate frequency range (2-5 MHz) longitudinally adjacent to the spine (midline) to capture a longitudinal view of the transverse processes.
  • Optimize machine imaging capability; select appropriate depth of field (usually > 8 cm), focus range and gain.
A curved 2-5 MHz transducer over the right paraspinal region to capture a longitudinal view of the psoas compartment; the patient is in a left lateral decubitus position

Longitudinal Paravertebral Scan at the L2-3 Level

ESM = erector spinae muscle
PsMM = psoas major muscle
TP = transverse processes
  • Turn the transducer transverse to obtain a transverse view of the psoas muscle
  • A curved 2-5 MHz transducer over the right paraspinal region to capture a transverse view of the psoas compartment; the patient is in a left lateral decubitus position.

Anatomical Correlation

Transverse Paravertebral Scan at the L2-3 Interspace

Nerve Localization

  • The adult lumbar plexus lies deep within the psoas major muscle. It is usually not visualized under ultrasound but is expected to lie within the posterior 1/3 of the muscle bulk.
  • The goal of ultrasound guided lumbar plexus block is to visually identify the transverse process and the psoas muscle and to determine the distance from skin to these 2 structures thereby allowing the operator to estimate the depth of the lumbar plexus prior to needle insertion.
  • Perform a systematic anatomical survey from medial (spinous process) to lateral (transverse process).
  • A paravertebral longitudinal scan locates several (2-3) lumbar transverse processes in the paravertebral space. Note the depth of the transverse processes.
  • Identify the paraspinal muscles (PSM, i.e., erector spinae and quadratus lumborum muscles) superficial (posterior) to the transverse processes.
  • Also identify the psoas muscle deep (anterior) to the transverse processes.
  • Move the transducer more medially to assess the facet joints (pictures below).
  • Then move the transducer laterally to assess the length of the transverse processes. The bony shadow will disappear when scanning lateral to the transverse process (picture below).

1. Longitudinal Scan Showing the Facet Joints Medially

The facet joints (F) are seen as a continuous hyperechoic line with the hypoechoic bony shadows below.
PSM = paraspinal muscle

2. Longitudinal Scan Showing the Transverse Process

The bony shadows of the transverse processes (TP) and the psoas major muscle (PsMM) in between the transverse processes are seen.
ESM = erector spinae muscle

3. Deeper Longitudinal Scan

This deeper scan shows the depth of the psoas major muscles (PsMM, approximately 7-8 cm from the skin, yellow arrows). The peritoneum (white arrows) is visualized deep to the PsMM.
ESM = erector spinae muscle
TP = transverse processes

Now place the transducer in the transverse plane at the level of the intended block between the transverse processes after the longitudinal scan. In this view, the transverse process bony shadow is not seen.

A Transverse Scan Showing The Psoas Muscle

L/F = lamina/facet
PsMM = psoas major muscle
SP = spinous process
VB = vertebral body

Needle Insertion Approach

  • Ultrasound guided psoas compartment block is considered an ADVANCED skill level block because of the depth of needle placement. Real time ultrasound guided psoas compartment block is technically challenging to perform.

In Plane Approach

  • Place the curved 2-5 MHz transducer in the transverse plane at the lumbar level (L2-4) intended for the psoas compartment block. This transverse view will show the psoas muscle without the transverse process.
  • The preliminary anatomical examination will define the depth of needle insertion to the posterior 1/3 of the psoas muscle.
  • Also, identify bowel within the peritoneum that is deep to the psoas muscle. Note the skin to peritoneum distance. This defines the maximum safe distance of needle insertion before inadvertent entry into the peritoneum.
  • Insert a 12-15 cm 22 G insulated needle in plane with the transducer perpendicular to the skin (figure below).
  • Because of the depth of needle insertion, needle and tissue movement is observed under real time ultrasound imaging but not usually the full view of the needle shaft or tip.
Aim to place the needle tip within the posterior 1/3 of the psoas muscle bulk. It is often necessary to identity the lumbar plexus by electrical stimulation. Aim to evoke quadriceps muscle contraction.

Out of Plane Approach

Coming Soon

Local Anesthetic Injection

  • Once satisfied with needle placement, inject 20-30 mL of local anesthetic for surgical anesthesia or postoperative analgesia. Observe fluid and tissue expansion within the psoas muscle bulk.
Arrowheads = local anesthetic spread within the psoas muscle
L/F = lamina/facet
PsMM = psoas major muscle
SP = spinous process
VB = vertebral body

Clinical Pearls

Nerve Localization - Body Position

It may be technically easier to scan the lumbar plexus with the patient lying prone. The ease of transducer placement and application of firm transducer pressure against the skin is superior in the prone position compared to the conventional lateral decubitus position. Pillows are placed under the abdomen to counter lumbar lordosis and widen the interspinous spaces. The disadvantage is impaired visualization of the quadriceps muscle contraction to nerve stimulation in the prone position.

Needle Insertion and Local Anesthetic Injection

1. Medial to Lateral Insertion

Needle advancement may be safer in the medial to lateral direction because the dura cuff may extend laterally beyond the neural foramina. Unintentional subarachnoid injection can result in high spinal anesthesia and has been reported during lumbar plexus block.
A slow incremental bolus local anesthetic injection through the needle is highly recommended. Frequent aspiration is performed to monitor for cerebrospinal fluid return.

2. Inadvertent Peritoneal Puncture

The peritoneum lying deep to the psoas muscle is easy to identify. It is important to note the skin to peritoneum distance and define the maximum distance of needle insertion before inadvertent entry into the peritoneum.

ESM = erector spinae muscle
PsMM = psoas muscle
TP = transverse process
Yellow arrow = skin to peritoneum distance

3. Kidney Trauma

Unintentional kidney (K) puncture and hematoma has been reported following psoas compartment block when the needle is inserted above L1-2 and too lateral. A pre-block ultrasound scan can locate the lower pole of the adjacent kidney thus defining the safe margin of needle insertion.

Catheter Insertion

Coming Soon

Image Gallery

A. A pre block longitudinal scan in the paravertebral space to assess skin to transverse process (TP) distance (distance A ~4.5 cm) and the width of the psoas muscle (distance B ~ 3 cm).
PSM = paraspinal muscles
B. A pre block transverse scan between 2 transverse processes shows the spinous process (SP) bony shadow, the lamina (L) and psoas muscle and the peritoneum.
PSM = paraspinal muscles
PsMM = psoas major muscle
C. A needle (arrows) is inserted in the lateral to medial direction into the psoas muscle. A motor response (quadriceps muscle contraction) is elicited with electrical stimulation.
SP = spinous process
PsMM = psoas major muscle
D. Local anesthetic spread (arrowheads) is observed within the psoas muscle compartment.
PsMM = psoas major muscle

Video Gallery

Selected References

  • Morimoto M, Kim JT, Popovic J, Jain S, Bekker A: Ultrasound guided lumbar plexus block for open reduction and internal fixation of hip fracture. Pain Pract 2006; 6: 124-6.
  • Awad IT, Duggan EM: Posterior lumbar plexus block: anatomy, approaches, and techniques. Reg Anesth Pain Med 2005; 30: 143-9.
  • Johr M. The right thing in the right place: lumbar plexus block in children. Anesthesiology 2005; 102: 865-866.
  • Kirchmair L, Enna B, Mitterschiffthaler G, Moriggl B, Greher M, Marhofer P, Kapral S, Gassner I. Lumbar plexus in children. A sonographic study and its relevance to pediatric regional anesthesia. Anesthesiology 2004; 101: 445-450.
  • Kirchmair L, Entner T, Kapral S, Mitterschiffthaler G. Ultrasound guidance for the psoas compartment block: an imaging study. Anesth Analg 2002; 94: 706-710.
  • Kirchmair L, Entner T, Wissel J, Moriggl B, Kapral S, Mitterschiffthaler G. A study of the paravertebral anatomy for ultrasound-guided posterior lumbar plexus block. Anesth Analg 2001; 93: 477-81.
  • Pusch F, Wildling E, Klimscha W, Weinstabl C. Sonographic measurement of needle insertion depth in paravertebral blocks in women. Br J Anaesth 2000; 85: 841-843.

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