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.

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).

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 bodies of the four sacral vertebrae (Fig. 9–1). These transverse ridges connect the four pelvic sacral foramina on either side of the midline and are continuous with the sacral canal through the intervertebral foramen. The pelvic sacral foramen decrease in size in a craniocaudal direction consistent with the decrease in size of the sacral vertebra. In contrast the dorsal surface (Fig. 9–2), which can be visualized using ultrasound, is convex, irregular in appearance, narrower than the pelvic surface, and directed backwards and slightly upwards (Fig. 9–2). The median area bears the median sacral crest with three to four spinous tubercles representing the fused spines of the upper four sacral vertebrae (Fig. 9–2). A ridge joining the articular tubercles forms the intermediate sacral crest. Four dorsal sacral foramina lie lateral to the intermediate sacral crest (Fig. 9–2) and communicate with the sacral canal through the intervertebral foramina (Fig. 9–3). The lateral sacral crest lies lateral to the dorsal sacral foramina. Below the fourth sacral tubercle there is an inverted U-shaped defect on the posterior aspect of the sacrum: the “sacral hiatus” (Fig. 9–2). This results from a failure of fusion of the laminae of the fourth and fifth sacral vertebrae. The inferior articular processes of the fifth sacral vertebra form the sacral cornua and lie lateral to the sacral hiatus (Fig. 9–2). The sacral hiatus is roofed by a firm elastic membrane, the sacrococcygeal ligament, which is an extension of the ligamentum flavum. The terminal end of the filum terminale exits through the sacral hiatus and traverses the dorsal surface of the S5 vertebra and sacrococcygeal joint to end at the coccyx. The fifth spinal nerve also exits through the sacral hiatus lying medial to the sacral cornua.

Gross Anatomy of the Sacrum

Fig. 9–4

Computed Tomography Anatomy of the Sacrum

Figs. 9–5 to 9–7

Magnetic Resonance Imaging Anatomy of the Sacrum

Figs. 9–8 and 9–9

The caudal epidural space is the continuation of the lumbar epidural space and can be accessed via the sacral hiatus. Ultrasound imaging of the sacrum and sacral hiatus can be performed in the transverse or sagittal axis (Fig. 9–10). ^1,2,5 Because the sacral hiatus is a superficial structure, it can be imaged using a high-frequency linear transducer (12–5 MHz). ^1,2,5 Ultrasound imaging of the sacrum for a caudal epidural injection produces a typical sonographic appearance of the osseous structures that are illustrated in Fig. 9–11.

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   Position:

   
   
   
   
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      Patient: The patient is positioned in the lateral decubitus position for a caudal epidural injection (Fig. 9–12). When fluoroscopy is used in conjunction with ultrasound for the caudal epidural injection, as in chronic pain medicine, then the patient may be positioned in the prone position with a pillow under the abdomen.

      
      
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      Operator and ultrasound machine: The operator stands behind the patient, and the ultrasound machine is placed directly in front of the patient.

   
   
   
   
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   Transducer selection: High-frequency linear transducer (12–5 MHz).

   
   
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   Scanning technique: Ultrasound scan for the sacral hiatus is commenced by placing the ultrasound transducer at the lower end of the sacrum and over the coccyx. Thereafter the transducer is gradually moved cranially until the sacral cornua and hiatus are visualized (Fig. 9–12).

   
   
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   Sonoanatomy: The sacral hiatus is covered by the sacrococcygeal ligament. Its lateral margins are formed by the two sacral cornua. On a transverse sonogram of the sacrum at the level of the sacral hiatus, the sacral cornua are seen as two hyperechoic reversed U-shaped structures, one on either side of the midline (Figs. 9–12 and 9–13). Connecting the two sacral cornua and deep to the skin and subcutaneous tissue is a hyperechoic band, the sacrococcygeal ligament (Figs. 9–12 and 9–13). Anterior to the sacrococcygeal ligament is another hyperechoic linear structure, which represents the dorsal surface of the sacrum (Fig. 9–12). The hypoechoic space between the sacrococcygeal ligament and the bony dorsal surface of the sacrum is the caudal epidural space (Figs. 9–12 and 9–13). The two sacral cornua and the posterior surface of the sacrum produce a sonographic pattern that we refer to as the “frog eye sign” because of its resemblance to the eyes of a frog (Figs. 9–12 and 9–13). If one moves the transducer slightly cephalad to the midsection of the sacrum, the dorsal surface of the sacrum with the median sacral crest is visualized (Fig. 9–14). On a sagittal sonogram of the sacrum at the level of the sacral cornua, the sacrococcygeal ligament, the base of sacrum, and the sacral hiatus are also clearly visualized (Figs. 9–15 and 9–16). However, due to the acoustic shadow of the posterior surface of the sacrum, only the lower part of the caudal epidural space is seen (Fig. 9–16).