Lumbar plexus block (LPB), ^1,2 also referred to as a psoas compartment block (PCB), ^3,4 is frequently used on its own or in combination with a sciatic nerve block for anesthesia and/or analgesia during hip or lower extremity surgery. ^1,3,5,6 During an LPB the local anesthetic is injected into a fascial plane within the posterior aspect of the psoas muscle. ⁷ This produces complete blockade of the major components of the ipsilateral lumbar plexus, namely the femoral nerve (FN), lateral femoral cutaneous nerve (LFC), and the obturator nerve (OBN). ⁸ The term PCB was originally coined by Chayen and colleagues. ⁴ They believed that branches of the lumbar plexus and parts of the sacral plexus were located close to each other in a “compartment,” between the psoas and quadratus lumborum muscle (an “intermuscular compartment”) at the level of the L4 vertebra, which could be identified using a “loss of resistance” technique. ⁴ However, recent research has demonstrated that the lumbar plexus is located within the substance of the psoas muscle. ⁷ PCB is also referred to as posterior lumbar plexus block, ¹ and several variations of this technique have been described in the literature. ^2,3 LPB is traditionally performed using peripheral nerve stimulation, ⁸ but with the recent widespread use of ultrasound guidance for regional anesthesia ultrasound-guided (USG) LPB has also been described. ^9,10 A clear understanding of the sonoanatomy of the lumbar paravertebral region ^9–11 is a prerequisite to safely performing USG LPB. ^9,10

The lumbar plexus is formed by the union of the anterior primary rami of the L1, L2, and L3 spinal nerves and the greater part of the L4 nerve (Figs. 3-1 and 12–1). The L1 nerve root may also receive contribution from the T12 spinal nerve. In the majority of individuals the lumbar plexus is located in a fascial plane or compartment within the substance of the psoas muscle (Figs. 12–2 to 12–4). ^7,11 We will henceforth refer to this intramuscular fascial compartment as the psoas compartment. Anatomically the psoas compartment is located between the fleshy anterior two-thirds of the psoas muscle and the posterior one-third of the muscle (Figs. 12–3 and 12–4). ^7,11 Therefore, the lumbar plexus is sandwiched between two portions of the psoas muscle and closely related to the lumbar transverse processes (Figs. 12–2 to 12–8). The bulkier anterior (fleshy) part of the psoas muscle originates from the anterolateral surface of the lumbar vertebral bodies and their intervertebral disc, whereas the thinner posterior (accessory) portion of the muscle originates from the anterior aspect of the lumbar transverse processes (Fig. 12–3). ⁷ Also the anterior and posterior parts of the psoas muscle fuse to form the main muscle bulk, but close to the vertebral bodies these two parts are separated by a fascia ⁷ or space within which the lumbar nerve roots, branches of the lumbar artery, and the ascending lumbar veins are located (Figs. 12–2 to 12–6). ^7,11 This wedge-shaped space close to the intervertebral foramen is the lumbar paravertebral space (LPVS) (Fig. 12–7). ¹¹

The lumbar nerve root after it exits the intervertebral foramen enters the fat-filled LPVS (Fig. 12–7). However, the lumbar nerve root, instead of entering the psoas muscle at the same vertebral level, takes a steep caudal course and enters the substance of the psoas muscle at the vertebral level below (Fig. 12–9). This explains why the L3 nerve of the lumbar plexus lies opposite the L4 intervertebral foramen and the L4 nerve root (Fig. 12–7). Also as seen in the sagittal anatomic section (Fig. 12–8), the L3 nerve of the lumbar plexus is located in an intramuscular compartment (ie, the psoas compartment) between the thick fleshy anterior and a thin posterior part of the psoas muscle. Outlines of the psoas compartment with the lumbar plexus can also be delineated in the transverse anatomic section (Fig. 12–7). Once the plexus is formed it displays a triangular shape, narrow in its superior portion and wider in its lower portion (Fig. 12–1). The nerves that originate from the plexus also exhibit a fanned-out distribution with the LFC being outermost, the OBN innermost, and the femoral nerve in between (Fig. 12–1). Being a fusiform muscle (ie, shaped like a spindle), the width of the psoas muscle is wider at its belly, close to the lower lumbar region, than at its origin and insertion. There are also gender- (male > female) ¹² and race- (black > white) ¹³ related differences in the width and cross-sectional area of the psoas muscle. The position of the lateral femoral cutaneous nerve and femoral nerve within the psoas compartment is relatively consistent, ⁷ but the position of the obturator can be variable and may even lie in a fold of the psoas muscle separate from that enclosing the other two nerves (Fig. 12–10). ⁷ The depth from the skin to the lumbar plexus also varies with gender and body mass index (BMI). Such differences in anthropometric parameters may be relevant when performing an LPB.

Computed Tomography Anatomy of the Lumbar Paravertebral Region

Figs. 12–11 and 12–12

Magnetic Resonance Imaging Anatomy of the Lumbar Paravertebral Region

Figs. 12–13 to 12–16

Ultrasound Scan Technique

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   Position:

   
   
   
   
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      Patient: The authors prefer to position the patient in the lateral position with the side to be blocked uppermost (Fig. 12–17A). ^9,10 The hips and knees of the patient are also flexed to mimic the position normally adapted during an LPB. The ultrasound scan can also be performed with the patient in the prone position, but the disadvantage is impaired visualization of the quadriceps muscle contraction during an LPB if nerve stimulation is used.

      
      
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      Operator and ultrasound machine: The operator sits or stands behind the patient, and the ultrasound machine is placed directly in front on the contralateral side.

   
   
   
   
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   Transducer selection: Because the lumbar plexus and psoas muscle are located at a depth in the abdomen and pelvis, it necessitates the use of a low-frequency ultrasound (5–2 MHz) and curved array transducer to image the lumbar paravertebral region. ^9–11 Low-frequency ultrasound provides good penetration but lacks spatial resolution at the depths (5–9 cm) at which the anatomy relevant for LPB is located. The latter often compromises the ability to locate the lumbar plexus within the psoas muscle. However, recent improvements in ultrasound technology, image processing capabilities of ultrasound machines, the availability of compound imaging and tissue harmonic imaging (THI), and the use of new scan protocols have significantly improved our ability to image the lumbar paravertebral region. Today, we are not only able to accurately delineate the lumbar plexus, but also the adjoining paravertebral anatomy. ^9–11

   
   
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   Scan technique: An ultrasound scan of the lumbar paravertebral region for USG LPB can be performed in the sagittal (Fig. 12–17B) ^9,14 or transverse (Figs. 12-15C and 12-15D) ^10,11,14 axis. The following anatomical landmarks are identified and marked on the skin of the nondependent side of the back using a skin marking pen: posterior superior iliac spine, iliac crest, lumbar spinous processes (midline, line A, Fig. 12–17A) and the intercristal line (line B, Fig. 12–17A). Thereafter a line (line C) parallel to the midline and which intersects the intercristal line (line B) at a point 4 cm lateral to the midline, corresponding to the point of needle insertion during a landmark-based LPB, is also marked (sagittal scan line) (Fig. 12–17A). The target vertebral level for the ultrasound scan (L3-L4-L5) is then identified as previously described. ^15,16 This involves visualizing the lumbosacral junction (L5–S1 gap) on a sagittal sonogram and then counting cranially to locate the lamina and transverse processes of the L3, L4, and L5 vertebrae. For a sagittal scan, the ultrasound transducer is positioned over the sagittal scan line (Fig. 12–18) with its orientation marker directed cranially. For a transverse scan the ultrasound transducer, with its orientation marker directed laterally, is positioned 4 cm lateral to the midline along the intercristal line and just above the iliac crest (Fig. 12–19). The transducer is also directed slightly medially (Fig. 12–19) so as to produce a transverse oblique view of the lumbar paravertebral region. ^10,11 During the paramedian transverse oblique scan (PMTOS), the ultrasound beam can be insonated either at the level of the transverse process (PMTOS-TP, Fig. 12–17C) or through the intertransverse space (PMTOS-ITS, Fig. 12–17D). ¹¹ Alternatively a transverse scan can be performed by placing the ultrasound transducer more anteriorly in the flank and above the iliac crest (Figs. 12–15 to 12–20) as described by Sauter and colleagues with the “shamrock technique.” ¹⁷

   
   
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   Sonoanatomy:

   
   
   
   
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      Sagittal sonoanatomy:

      
      
      
      
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        On a sagittal sonogram the transverse processes of the lumbar vertebrae (L3-L4-L5) are identified by their hyperechoic reflection and their corresponding acoustic shadow anteriorly (Fig. 12–21). ⁹ This produces a sonographic pattern that we refer to as the lumbar ultrasound trident or the “trident sign” ⁹ because of its similarity to the trident (Latin for tridens or tridentis) that is often associated with Poseidon (the god of the sea in Greek mythology) and the Trishula of the Hindu god Shiva. However one must bear in mind that because the L5 transverse process is the shortest of the lumbar transverse processes, it may be more difficult to locate and may require some degree of medial orientation of the transducer until the ultrasound trident is visible. The psoas muscle is seen in the acoustic window of the lumbar ultrasound trident (Fig. 12–21) as multiple longitudinal hyperechoic striations against a hypoechoic background typical of muscle (Fig. 12–22). ⁹ The lumbar plexus may also be visualized in the posterior aspect of the psoas muscle (Fig. 12–22). ⁹ It appears hyperechoic (Fig. 12–22), is sonographically distinct from the muscle fibers, and is more posterior in location than the intramuscular tendons of the psoas muscle. ⁹ The lumbar plexus nerves are also thicker than the muscle fibers (Fig. 12–22) and take an oblique course through the psoas muscle. ⁹ A laterally positioned ultrasound transducer will produce a suboptimal view without the ultrasound trident, but may visualize the lower pole of the kidney, which lies anterior to the quadratus lumborum muscle (QLM), and can reach the L3 to L4 vertebral level in some individuals.

      
      
      
      
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      Transverse sonoanatomy – Paramedian transverse oblique scan:

      
      
      
      
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        In a typical PMTOS–TP, the erector spinae muscle, the transverse process, the psoas muscle, quadratus lumborum muscle, and the anterolateral surface of the vertebral body are visualized (Fig. 12–23). ¹¹ The psoas muscle appears hypoechoic, but areas of hyperechogenicity are interspersed within the central part of the muscle (Fig. 12–23 to 12–25). These dots or speckles represent the intramuscular tendon fibers of the psoas muscle, and they are more pronounced below the level of the iliac crest. The IVC (on the right side, Fig. 12–24) and the aorta (on the left side) are also identified anterior to the vertebral body and are useful landmarks to look out for while performing a transverse scan. ¹¹ The lower pole of the kidney, which can extend to the L3 vertebral level, is visualized as an oval structure and moves synchronously with respiration in the retroperitoneal space (Fig. 12–25). The acoustic shadow of the transverse process obscures the posterior aspect of the psoas muscle (Fig. 12–23). ¹¹ Therefore, the lumbar nerve root or lumbar plexus are rarely visualized through the PMTOS-TP scan window. However, the dura, epidural space, and the intrathecal space may be visualized during a PMTOS-TP (Fig. 12–23). ¹¹ We believe this is because the ultrasound signal, which is medially directed, enters the spinal canal through the interlaminar space (Fig. 12–17C). Being able to visualize the neuraxial structures during a lumbar paravertebral scan may be useful in documenting epidural spread after an LPB.

        
        
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        In the PMTOS-ITS ¹¹ apart from the erector spinae, psoas, and quadratus lumborum muscles, the intervertebral foramen, articular process, and the lumbar spinal nerve root are clearly delineated (Figs. 12–24 to 12–26). ¹¹ The LPVS is also seen as a hypoechoic space adjacent to the intervertebral foramen (Figs. 12–24 to 12–26), and the lumbar spinal nerve root can be seen exiting the foramen (Figs. 12–24 and 12–25). ¹¹ The latter does not enter the psoas muscle directly opposite the intervertebral foramen from which it emerges (Figs. 12–24 and 12–25), but takes a caudal course as seen in the CT (Fig. 12–12), MRI (Figs. 12–14 to 12–16), and cadaver anatomical section (Fig. 12–9). In some individuals an additional hyperechoic structure surrounded by a hypoechoic space (Figs. 12–24 to 12–26) is seen in the posterior aspect of the psoas muscle. ¹¹ Based on our observation in the anatomical sections (Fig. 12–7) and MRI images (Fig. 12–14) we believe this represents the lumbar plexus within the psoas compartment. ¹¹ Currently there are limited data validating the transverse sonoanatomy of the lumbar paravertebral region, but it is our experience that there is good correlation between structures that are visualized in a lumbar paravertebral sonogram and that in corresponding cadaver anatomical sections, CT, and MRI images of the lumbar paravertebral region (Figs. 12–27 to 12–30). ¹¹ Because the lumbar plexus and the paravertebral anatomy are clearly delineated through the PMTOS-ITS ultrasound scan window, it is our preferred window for imaging during an ultrasound-guided LPB. ¹⁰

      
      
      
      
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      Transverse sonoanatomy – Shamrock method:

      
      
      
      
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        In a transverse sonogram produced by the shamrock method (Fig. 12–20) the psoas, erector spinae, and quadratus lumborum muscles are also clearly visualized (Figs. 12–31 and 12–32). The anatomical arrangement of the three muscles around the transverse process, that is, the psoas muscle lying anterior, the erector spinae muscle lying posterior, and the quadratus lumborum muscle lying at the apex (Fig. 12–31), produces a sonographic pattern that has been likened to a shamrock, with the muscles representing its three leaves. ¹⁷ The lumbar nerve root is visualized close to the angle between the vertebral body and the transverse process (Figs. 12–32 and 12–33), and the lumbar plexus within the posterior aspect of the psoas muscle, typically about 2 cm anterior to the transverse process (Figs. 12–31 to 12–33). ¹⁷ From this position if the transducer is gently tilted caudally, the acoustic shadow of the L4 transverse process disappears and the ultrasound beam is now insonated through the intertransverse space and at the level of the articular process of L4 vertebra, similar to that with a PMTOS-AP (Fig. 12–17D). ^10,11,17 In the resultant sonogram the psoas, erector spinae and quadratus lumborum muscles, the intervertebral foramen, and lumbar plexus are now clearly visualized (Figs. 12–34 and 12–35). In our experience, the lumbar plexus is better visualized with the shamrock method than with a PMTOS. This may be because the ultrasound beam is more at right angles to the lumbar plexus nerves during a shamrock scan.