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CHAPTER 6 Sonoanatomy Relevant for Ultrasound-Guided Injections of the Cervical Spine Introduction Injections of the cervical spine are frequently used for pain management in chronic pain medicine. The concentration of bony structures and nerves in the cervical spine, each of which can be a cause of pain, as well as vessels, requires an intimate knowledge of the anatomy. The relevant procedures in the cervical spine include facet joint and medial branch blocks, selective nerve root injection, third occipital nerve block, epidural steroid injection, and stellate ganglion block. In this chapter we discuss the anatomy relevant for these procedures. Basic Cervical Spine Anatomy The cervical spine (Figs. 6-1 to 6-3) is a column of seven vertebrae supporting the skull and neck structures. The atlanto-occipital and atlantoaxial joints are unique. The former is an ellipsoid joint, and the atlantoaxial joint is a rotatory joint. The atlantoaxial joint is bordered by the C2 dorsal root ganglion and vertebral artery. The cervical vertebrae are identified by the presence of the foramen transversarium (transverse foramen) for the vertebral artery. 276 FIGURE 6-1 Cervical spine – lateral view. FIGURE 6-2 Cervical spine – anterior view. VB, vertebral body. 277 FIGURE 6-3 Cervical spine – posterior view. Typical Cervical Vertebra (C3 to C6) The third to sixth cervical vertebra are considered typical cervical vertebra (Fig. 6-4), whereas the first, second, and seventh cervical vertebra are atypical with certain unique features (Figs. 6-5 and 6-6). The general characteristics of a typical cervical vertebra are described next. The upper five cervical vertebrae (C3 to C7) each have a concave superior surface and are convex on the inferior surface. They articulate with the adjacent vertebrae via uncovertebral joints (joints of Luschka). These are thought to be due to degenerative tears in the annulus of the intervertebral disc, leading to creation of the uncovertebral joint. Uncovertebral joint osteophytes can contribute to narrowing of the exit foramina. The spinal canal (vertebral canal) in the cervical spine is larger than the size of the body. It is also triangular shaped because the pedicles are directed backwards and laterally (Fig. 6-4). The superior and inferior vertebral notches are usually equal sized. The laminae are relatively long and narrow and thinner above than below. The superior and inferior articular processes form the articular pillars and project laterally at the junction of the pedicle and transverse process. The superior articular facets are directed backwards and upwards, whereas the inferior articular facets are directed forwards and downwards (Fig. 6-1). The transverse process of each vertebra is pierced by the foramen transversarium (Fig. 6-4) to allow for the passage of the vertebral arteries on their upward course to the foramen magnum (Fig. 6-7). Each transverse process has an anterior and a posterior tubercle with the groove for the spinal nerve between them (Figs. 6-1 and 6-2). The anterior tubercle of the sixth cervical vertebra is large and called the “carotid tubercle” (tubercle of Chassaignac). The posterior tubercles of C3 to C5 are located lower and laterally (Figs. 6-1 and 6-2). The spinous processes of C3 to C6 can be bifid (Figs. 6-3 and 6-8), and the two divisions can be of unequal size. The first bifid spinous process is C2, and this landmark is used to identify the remaining cervical vertebrae. The facet joints are oriented at 45 degrees to the axial plane and allow sliding of one articular facet on another (Figs. 6-9 and 6-10). 278 FIGURE 6-4 A typical cervical vertebra (C4 - fourth cervical vertebra). SAF, superior articular facet; SAP, superior articular process; VB, vertebral body; IAF, inferior articular facet; IAP, inferior articular process. 279 FIGURE 6-5 Atlas (superior, anterior, and lateral view). Note the kidney-shaped SAFs. SAF, superior articular facet; IAF, inferior articular facet. FIGURE 6-6 Axis (superior, anterior, and lateral view). SAF, superior articular facet; VB, vertebral body; IAF, inferior articular facet; AAF, anterior articular facet; IAP, inferior articular process; PAF, posterior articular facet. 280 FIGURE 6-7 Cervical spine (anterior view) showing the relationship of the cervical spinal nerves and the vertebral artery to the transverse processes of the vertebra. Note the transverse processes of the C7 vertebra lack an anterior tubercle and the relationship of the vertebral artery to the C7 spinal nerve and the transverse processes. FIGURE 6-8 Cross-sectional cadaver anatomic section through the C2 vertebral body showing the bifid spinous process of C2. This is an anatomical landmark used to identify the C2 vertebra as it is the first cervical vertebra with a bifid spinous process. The spinous process may be tilted to the right or left. Gentle left and right angulation of the probe in the longitudinal sagittal plane may be required to visualize these spinous processes. 281 FIGURE 6-9 Paramedian sagittal cadaver anatomic section through the cervical spine demonstrating the lamina of the cervical vertebrae. VB, vertebral body. FIGURE 6-10 Cross-sectional cadaver anatomic section through the cervical spine demonstrating the facet joints. Note that the facet joints are orientated at about 45 degrees to the horizontal plane in transverse section. The cervical spinal canal measures about 14 to 20 mm in the mediolateral dimension and 15 to 20 mm in the anteroposterior dimension. The spinal nerves (formed by the anterior and posterior nerve roots) exit through the neural foramina. These foramina are largest at C2 to C3 and progressively decrease in size to the C6 to C7 levels. The spinal nerve and ganglion take up about 33% of the foraminal space. The foramen is bordered anteromedially by the uncovertebral joints and posterolaterally by the facet joints. The pedicles border the exit foramina superior and inferiorly. The spinal nerves exit above their corresponding vertebral bodies. The C1 nerve exits above the C1 vertebra (atlas). The next spinal nerve is C2, exiting above the C2 vertebra (axis). Following this naming convention, the last cervical nerve root is C8, and it exits between the C7 and T1 vertebrae (Figs. 6-11 and 6-12). 282 FIGURE 6-11 Cross-sectional cadaver anatomic section through the cervical spine demonstrating the exiting C5 nerve root. The C5 nerve root exits the neural foramen and is in close relation to the vertebral artery posteriorly. Both these structures are bound by the larger anterior tubercle and the smaller posterior tubercle. TP, transverse process. FIGURE 6-12 Sagittal cadaver anatomic section of the exit neural foramina demonstrating the C5 nerve root exiting between the transverse processes (TP) of C4 superiorly (C4 TP) and C5 (C5 TP) inferiorly. The bulk of sternocleidomastoid muscle lies anteriorly and may be traversed during procedures in the cervical spine. The anterior spinal artery is located in the central sulcus of the cord, with paired posterior arteries running on the posterolateral aspect of the cord dorsally. The anterior spinal artery is an important artery: it supplies the anterior two-thirds of the cervical spinal cord. The artery 283 receives blood supply from the paired anterior spinal branches that arise from the cervicomedullary junction portion of the vertebral arteries. This anatomy is relevant for epidural steroid injections. The radicular arteries also supply the nerve roots and spinal cord. These radicular arteries arise from the aorta. In the lower cervical spine, they arise from the vertebral arteries and run in an anteromedial direction with respect to the neural foramina. In the lower cervical spine, large radiculomedullary branches contribute blood supply to the anterior spinal artery as well. Branches of the ascending and deep cervical arteries anastomose with the vertebral artery branches and contribute to the anterior spinal artery. The ascending cervical artery arises from the thyrocervical trunk or subclavian artery. The posterior subclavian artery also gives off the deep cervical artery and the superior intercostal artery. The deep cervical artery gives spinal branches from levels C7 to T1, known as the cervical radiculomedullary arteries. As mentioned earlier, these arteries can contribute supply to the anterior spinal artery. These radiculomedullary arteries are found along the length of the intervertebral foramina and can be compromised during injection, potentially leading to damage to the anterior spinal artery. The posterior third of the cervical spinal cord is supplied by small paired posterior spinal branches. Atlas (C1) The atlas is the first cervical vertebra (Fig. 6-5) and forms the joint that connects the spine to the skull (Fig. 6-13). It is ring shaped and lacks both a vertebral body and spinous process (Fig. 6-5). It also lacks a true facet joint and has two arches: anterior and posterior. The posterior arch is usually quite small. A thick anterior arch, lateral masses, and transverse processes on either side make up the rest of the atlas ring. It also has a rudimentary posterior tubercle. On each lateral mass is a facet (zygapophyseal) joint. The superior articular facets are kidney shaped (Fig. 6-5), concave, and face upwards and inwards (imagine your hands cupping water from a running tap). The inferior articular facets are flat and face downwards and outwards. The transverse processes project laterally from each lateral mass and are longer than all the others (Figs. 6-2 and 6-3). FIGURE 6-13 Median sagittal cadaveric anatomic section through the cervical spine demonstrating C1 in relation to the occiput and the rest of the cervical vertebrae. Note how 284 closely the dura and the cervical spinal cord are to the spinous processes. The vertebral bodies (VB) are labeled as anterior complex to demonstrate that sonographically, the individual components (including the posterior longitudinal ligament complex) are difficult to distinguish individually. SP, spinous process. Axis (C2) The second cervical vertebra (Fig. 6-6) is recognized by the presence of the dens (odontoid process), which is a strong toothlike process that projects upwards from the body (Fig. 6-6). The dens is believed to represent the body (centrum) of the atlas, which has fused with the body of the axis. The odontoid process articulates with the atlas to form the rotatory atlantoaxial joint. The joint is strengthened by periarticular ligaments (the apical, alar, and transverse ligaments). The axis is made up of a vertebral body, pedicles, lamina, and transverse and spinous processes. The atlas articulates with the axis (Fig. 6-2) at the superior articular facets of C2. In order to meet the inferior articular processes of C1, the C2 superior articular facets face upwards and outwards. There is an extensive and densely packed network of blood vessels around the dens. These are supplied by the paired anterior and posterior ascending arteries (which arise from the vertebral arteries at the C3 level, carotid wall vessels, and the ascending pharyngeal arteries). The transverse ligament secures the odontoid process to the posterior atlas and acts to prevent subluxation of C1 on C2. Accessory ligaments arise posterior to the transverse ligament and insert on the lateral aspects of the atlantoaxial joint. The apical ligament, part of the accessory ligaments mentioned earlier, connect the anterior lip of the foramen magnum to the tip of the dens. Paired alar ligaments also attach the tip of the dens to the anterior foramen magnum. The tectorial membrane is a cranial continuation of the posterior longitudinal ligament, attaching to the anterior lip of the foramen magnum. A broad accessory atlantoaxial ligament connects C1 and C2 and connects to the occiput. They contribute to craniocervical stability. The lack of bony borders at the atlantoaxial joint results in wider acoustic windows at this level, but this is countered by the tortuous course of the ascending vertebral arteries. Seventh Cervical Vertebra (C7) This is also known as the “vertebral prominence” because it has a long and prominent spinous process (Fig. 6-1) that is palpable from the skin surface. The spinous process is also thick, nearly horizontal, and is not bifid but ends in a tubercle. The transverse process of C7 is relatively large and lacks an anterior tubercle (Fig. 6-7). The foramen transversarium on the transverse processes of C7 are small but may be duplicated or even absent. Computed Tomography Anatomy of the Cervical Spine Figs. 6-14 to Fig. 6-21 285