CHAPTER 12: Sonoanatomy Relevant for Ultrasound-Guided Lumbar Plexus Block

INTRODUCTION

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

GROSS ANATOMY

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 ...