Central neuraxial blocks (CNBs), which include spinal, epidural, and combined spinal epidural (CSE) injections, are frequently performed in the lumbar region for anesthesia and analgesia and for managing chronic pain. ¹ Traditionally, they are performed using a combination of surface anatomic landmarks, the operator’s tactile perception of “loss of resistance” during needle advancement through the ligamentum flavum, and/or visualizing the efflux of cerebrospinal fluid. Anatomic landmarks (eg, the spinous processes) are useful but they are not always easily palpable in patients with edema, obesity, ² underlying spinal deformity, or previous back surgery. The “Tuffier’s line,” which is a line joining the highest points of the iliac crests, is another surface anatomical landmark that is widely used to estimate the location of the L4 to L5 interspace; however, the correlation is inconsistent. ³ Even in the absence of spine abnormalities, estimation of a specific intervertebral level may not be accurate in many patients ^4,5 and may result in needle placement one or two spinal levels higher than intended. ^4–7 This inaccuracy is exaggerated in the obese and in the upper spinal levels. ^4,6,8 Furthermore, using surface anatomical landmarks alone, it is not possible to predict the ease or difficulty of needle placement prior to skin puncture. Unanticipated technical difficulty, multiple attempts at needle placement, and failure of CNB are therefore prevalent in clinical practice. ^9,10

Recently, ultrasound imaging of the spine ^11–13 has emerged as a useful tool to overcome many of the shortcomings of the traditional approach to CNBs, and it has been used with great success. Ultrasound is most frequently used as a preprocedural tool, ¹¹ but can also be used to guide the epidural or spinal needle in real time during CNBs. ¹⁴ Advantages of the preprocedural scan include being able to accurately locate the midline, ¹⁵ identify a given lumbar interspace, predict the depth to the epidural space, detect any vertebral rotational defects (eg, in scoliosis), and identify patients with a potentially difficult CNB. ^11,16 In expert hands the use of ultrasound for epidural needle insertion reduces the number of puncture attempts, ^17–22 improves the success rate of epidural access on the first attempt, ¹⁸ reduces the need to puncture multiple levels, ^18–20 and improves patient comfort during the procedure. ¹⁹ This chapter briefly outlines the anatomy, the technique of ultrasound imaging, and sonoanatomy relevant for CNBs in the lumbar region.

The lumbar spine makes up the lower back and is made up of five vertebra, numbered L1 to L5 (Figs. 8–1 and 8–2). It connects with the thoracic spine above and with the sacrum below at the lumbosacral joint. L1 to L4 are typical lumbar vertebrae because they share common characteristics, but L5 is atypical because it has certain peculiarities. The lumbar vertebral body is designed to bear weight, and therefore the size of the lumbar vertebrae increases from L1 to L5. The lumbar spine also has a curvature, being slightly convex anteriorly, and this is referred to as lordosis.

Typical Lumbar Vertebra

A typical lumbar vertebra (L1–L4) is identified by its large vertebral body and the absence of costal facets on the body (Fig. 8–3). The body of a typical lumbar vertebra is wider in the transverse axis than in the anteroposterior axis (Fig. 8–3). The height of the vertebral body is also greater anteriorly than posteriorly, and this difference contributes to the forward convexity of the lumbar spine. The vertebral foramen is triangular in shape (Fig. 8–3) and larger than that in the thoracic region but smaller than that in the cervical region. The pedicles are short and strong and directed posteriorly from the upper part of the body (Figs. 8–2 and 8–3). This results in an inferior vertebral notch that is significantly deeper than the superior vertebral notch (Figs. 8–2 and 8–3). The laminae are short and thick, directed backwards and medially, and form the posterior part of the vertebral arch. The spinous process is thick, wide, and quadrilateral in shape, and directed backwards (Figs. 8–1 to 8–3). The transverse processes are thin and directed laterally and slightly backwards (Fig. 8–4). The width of the transverse process increases from L1 to L3 after which it decreases as one moves caudally. In a typical lumbar vertebra, the superior articular processes lie farther apart from each other than the inferior articular processes (Fig. 8–4). The superior articular processes face backwards and medially, whereas the inferior articular process faces laterally and forward (Figs. 8–3 and 8–4).

Fifth Lumbar Vertebra (L5)

The body of the L5 vertebra is the largest of all the lumbar vertebrae. Its anterior surface is wider than its posterior surface (Fig. 8–5), and this difference results in the sharp lumbosacral angulation (Fig. 8–1). The pedicles are short and directed backwards and laterally (Fig. 8–5). The superior articular processes face more backwards than medially, and the inferior articular process also looks more anteriorly than laterally when compared to the other lumbar vertebrae (Fig. 8–5). The distance between the inferior articular processes are also equal to or more than the distance between the superior articular processes. The transverse process of L5 is short, thick, pyramidal in shape, and attached to the entire thickness of the pedicle (Fig. 8–5). The spine of L5 is also relatively short and has a rounded tip.

The adjacent lumbar vertebrae articulate with each other at the facet joints between the superior and inferior articular processes and the intervertebral disc between the vertebral bodies (Fig. 5-7). This results in two gaps—the “interspinous space” and the “interlaminar space”—between the adjacent spinous processes and the laminae of the vertebrae, respectively (Fig. 8–4). These gaps allow the ultrasound energy to enter the spinal canal and thereby act as acoustic windows for ultrasound imaging during spinal sonography. The reader should refer to Chapter 5 for a detailed description of the anatomy of the interlaminar and interspinous spaces, major ligaments that support the lumbar vertebra (ie, ligamentum flavum, supraspinous and interspinous ligament, and the anterior and posterior longitudinal ligament), spinal canal, and the epidural space in the lumbar region.

Gross Anatomy of the Lumbar Spine

Figs. 8–6 to 8–11

Computed Tomography Anatomy of the Lumbar Spine

Figs. 8–12 to 8–18

Magnetic Resonance Imaging Anatomy of the Lumbar Spine

Figs. 8–19 to 8–26

The lumbar spine is imaged using a low-frequency (5–2 MHz) curved array transducer and in the transverse or sagittal plane. During a median transverse scan (Figs. 8–27 to 8–38) the “transverse spinous process view” (Figs. 8–27 to 8–29) and “transverse interspinous view” (Figs. 8–34 to 8–36) are acquired. During a median sagittal scan (Figs. 8–39 to 8–41) the lumbar spinous processes and the interspinous spaces are visualized. ^12,13 The lumbar spinous processes appear as crescent-shaped structures (Figs. 8–40 and 8–41) and occupy most of the median plane (ie, there is a lot of bone). Therefore, the acoustic window for imaging is relatively narrow in the midline (Fig. 8–41). Also any clinical condition that causes narrowing of the interspinous spaces (eg, in the elderly) further compromises the acoustic window. Consequently ultrasound imaging through the median plane provides a limited view of the neuraxial structures (Fig. 8–41). In contrast there is less bony obstruction in the paramedian sagittal plane, particularly at the level of the lamina, which creates a large acoustic window for imaging through the interlaminar spaces. Sonographic views of the neuraxis are also more detailed through the paramedian sagittal plane (Figs. 8–42 to 8–67). Therefore it is the preferred route for spinal sonography and for real-time ultrasound-guided CNBs. ^11–13 For a detailed ultrasound examination of the lumbar spine, it must be imaged in both the transverse and sagittal planes because the information obtained from either plane complements the other.