CHAPTER 27 Psoas block
The lumbar plexus is formed by the ventral rami of the first three lumbar nerves and the greater part of the ventral ramus of the fourth, with a contribution from the twelfth thoracic nerve root in 50% of cases. It lies in front of the transverse processes of the lumbar vertebrae, deep within the psoas major muscle (Fig. 27.1). The nerve roots of the lumbar plexus lie in a ‘cleavable’ space in the psoas major muscle (Fig. 27.2). The space is limited superiorly by the insertion of psoas major on the body of the vertebrae; posteriorly by the lumbar transverse processes and peridural space; and anteriorly by the aponeurotic continuation of the fascia iliaca. The erector spinae (medial) and quadratus lumborum (lateral) muscles are superficial to and posterior to the psoas muscle.
Figure 27.1 Lumbar (L2) sagittal section illustrating anatomy relevant to the psoas block.1, Spinous process; 2, vertebral body; 3, transverse process; 4, erector spinae muscle; 5, quadratus lumborum muscle; 6, psoas muscle; 7, lumbar nerve; 8, kidney.
Figure 27.2 Coronal T1-weighted MR image of anatomy relevant to psoas block. 1: right kidney; 2: retroperitoneal space; 3: psoas muscle; 4: anterior superior iliac spine; 5: lumbar nerve roots; 6: iliacus muscle.
Anatomically, the psoas muscle is regarded as one mass and there is no ‘compartment’ as such within the muscle. Nevertheless, injectate does infiltrate the muscle (Fig. 27.3) and covers the lumbar roots and nerves running within the muscle. The roots join and form the lumbar plexus within the psoas muscle. Branches of the plexus include the femoral, obturator, and lateral cutaneous nerves of the thigh.
Important landmarks for the psoas block include the iliac crests, the posterior superior iliac spine, and the vertebral column (Fig. 27.4). The posterior superior iliac spine is the bony prominence at the posterior end of the iliac crest. It is directly below the ‘sacral dimple’ (dimple of Venus), a dimple in the skin visible above the buttock, close to the midline.
Figure 27.4 Landmarks for the psoas block. The iliac crest and posterior superior iliac spine are marked. A line is drawn joining both iliac crests. A line is drawn, parallel to the spine, which passes through the posterior superior iliac spine. Where both lines intersect is the needle insertion point. 1: iliac crest; 2: posterior superior iliac spine; 3: spinous processes; 4: needle insertion point.
A vertical line is drawn between the highest points of the iliac crests. This is called the Tuffier line and passes through the disc space of L3 and L4. A second line is drawn parallel to the spinous processes and passes through the posterior superior iliac spine on the side to be blocked. Where these two lines intersect is the needle insertion point (usually 4–5 cm from the midline).
Ultrasonographic visualization of the psoas muscle in adults requires a low frequency transducer (5–8 MHz) due to the depth of the lumbar plexus (5–8 cm). A high frequency transducer can also be used, particularly in children. For longitudinal sonograms, the transducer is placed 3 cm lateral to the spinous processes (Fig. 27.5). This allows for identification of the transverse processes. The transverse processes produce bright hyperechoic signals, with signal loss distally. The psoas muscle is seen deep to these structures (Fig. 27.6). The transducer is advanced caudally and then cranially to identify the respective lumbar interspaces. The sacrum is identified as a continuous hyperechoic line. The longitudinal sonographic pattern of the psoas muscle demonstrates a hypoechoic background interspersed with hyperechoic bands (dots on transverse view) representing fibrous structures within the muscle. Unlike the sonoanatomy in children, visualization of the lumbar plexus in adults is substantially impaired by these structures, and often is impossible to identify. The kidneys are visualized as oval shaped structures usually at the level of L2 or L3, and therefore can be avoided during ultrasound-guided psoas compartment block. The kidneys can also be seen to move with respiration. The more hyperechoic, wedge-shaped, psoas muscle lies medial and deeper to the kidneys. At the interspace of L4–L5, the transducer is rotated 90° into the transverse plane (Fig. 27.7).