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Ultrasound imaging of the sacrum 1,2 and lumbosacral (L5–S1) interlaminar space 3–7 is frequently performed to identify the sonoanatomy relevant for central neuraxial blocks, that is, spinal and epidural (lumbar and caudal) injection. 1–7 Because the lumbosacral interlaminar space and sacrum are relatively superficial structures, they lend themselves well to ultrasound imaging. 3–5,7 This chapter briefly outlines the anatomy, technique of ultrasound imaging, and sonoanatomy of the sacrum and lumbosacral interlaminar space relevant for central neuraxial blocks.
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BASIC ANATOMY OF THE SACRUM
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The sacrum is a large, triangular bone formed by the fusion of the five sacral vertebrae (Figs. 9–1 and 9–2). It makes up the posterior aspect of the bony pelvis and articulates with the corresponding hip bones laterally at the sacroiliac junctions. Because it is triangular in shape it has a base, an apex, and four surfaces (right and left lateral surfaces, dorsal and ventral or pelvic surface). Anatomically the pelvic surface of the sacrum faces downwards and forward, whereas the dorsal surface faces backwards and slightly upwards. The sacrum is divided by a row of foramina on either side of the midline into a median section and a pair of lateral masses (Fig. 9–1). The median section is traversed by the sacral canal, which contains adipose tissue, cauda equina nerves (including the filum terminale), epidural space, spinal meninges (dura and arachnoid), and the thecal sac. The thecal sac ends at the level of the S2 but can vary from S1 to S3. The sacral canal also contains the epidural venous plexus, which generally ends at the level of the S4 but may extend more caudally. The lateral masses are formed by fusion of the transverse processes posteriorly and the costal elements anteriorly. The base is formed by the superior surface of the body of the S1 vertebra, which is large, lumbar in type, and articulates with the L5 vertebra at the lumbosacral junction. The vertebral foramen of the S1 vertebra is triangular in shape and continuous cranially with the lumbar spinal canal and caudally with the sacral canal. The spine of the S1 vertebra forms the first spinous tubercle. The apex of the sacrum is formed by the body of the S5 vertebra (inferior surface) that articulates with the coccyx (Figs. 9–1 and 9–2).
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The pelvic surface of the sacrum (Fig. 9–1), although not visualized during ultrasound imaging, is concave and directed downwards and forward. Four transverse ridges on the median area indicate the lines of fusion of the ...