Vertebral anatomy

A detailed understanding of spinal anatomy is requisite to performing spinal procedures. The spine consists of bones, ligaments, discs, blood vessels, and nerves.

Bony structures and ligaments of the spine

The bony components of the spine, or vertebrae, begin at the transition from the skull to the cervical spine, also known as the craniocervical junction . In total, 33 bones make up the entire vertebral column. Of these bones, 24 are individually separate though linked to each other through joints and ligaments to provide both support and flexibility. The lower nine bones are fused in adults and make up the sacrum and coccyx ( Fig. 1.1 ). This chapter will focus on the upper 24 vertebrae.

The upper 24 vertebrae can be divided into 7 cervical, 12 thoracic, and 5 lumbar. Accurate and consistent numbering of vertebrae is critical in guiding interventions. Therefore, it is important to note anatomical variants that may be encountered, including L5 sacralization, which is incorporation of the L5 vertebral body into the sacrum ( Fig. 1.2 ).

The vertebrae are arranged such that, when in the upright position, they make naturally occurring curves. These curves assist in the spine’s ability to distribute vertical compressive forces. When viewed from the side, the cervical and lumbar spines appear concave, known as lordosis , whereas the thoracic spine appears convex, known as kyphosis ( Fig. 1.3 ).

While there are structural variations, most of the vertebrae are composed of an anterior part and a posterior part. The anterior part of a vertebra contains the vertebral body. Externally, it is made of a hard shell of compact bone; internally, it consists of marrow-containing trabecular bone, which is innervated by the sinuvertebral and basivertebral nerves ( Fig. 1.4 ).

Each vertebral body is separated by intervertebral discs that sit between adjacent vertebrae. These discs aid in load bearing and are discussed later. The posterior part of a vertebra contains the vertebral arch, which includes the pedicle, lamina, spinous process, transverse process, and superior and inferior articular processes, which form the facet or zygapophyseal joints ( Fig. 1.5 ).

The pedicles connect the vertebral body to the transverse processes as well as the laminae, together creating the arch, which helps encase the spinal cord. Nerve roots exit inferior to each pedicle; thus, the pedicle is a key anatomical landmark to identify during needle placement for interventional procedures. With fluoroscopic imaging, pedicles appear as rounded areas of increased bone density ( Fig. 1.6 ).

The laminae connect the transverse processes to the spinous process and form the roof of the spinal canal through which the spinal cord travels. The spinous process is the point for muscle and ligament attachment, notably, those muscles used for extension of the vertebral column. The articular processes also project from the laminae and allow a vertebra to articulate with the vertebrae above and below it to form the zygapophyseal, or facet, joints ( Fig. 1.7 ).

The thicker portion of the lamina that acts as the junction connecting the spinous processes to the superior and inferior articular processes on a single vertebra is known as the pars interarticularis . It is prone to “pars defects,” or fractures known as spondylolysis , which can lead to the displacement of a vertebral body, known as spondylolisthesis ( Fig. 1.8 ). Spondylolisthesis can result in nerve root compression.

Cervical spine

The superior-most vertebra, C1, is termed the atlas . The second, C2, is termed the axis ( Fig. 1.9 ). Together, they account for most of the rotational ability of the cervical spine. C1 attaches the skull to the spine. It is unlike the other vertebrae in that it lacks a vertebral body and a spinous process. It is shaped like a ring: two lateral masses are connected by an anterior and posterior arch. The anterior arch comes to a midpoint that contains the dorsal facet, which allows for a connection with the dens on the vertebral body of C2, allowing C1 to pivot the skull. ^, The posterior arch of C1 contains the posterior tubercle at its midpoint, which serves as the origin for the rectus capitis posterior minor muscle, which assists head and neck extension. ^,

In the posterior portion of each posterior arch is a groove in which the vertebral artery and first cervical spinal nerve are found. Injecting in this region can be technically challenging. The posterior cavity of the atlas formed by the anterior and posterior arches contains the spinal cord. The lateral masses of C1 contain the superior articular processes/facets that connect it to the occiput, or skull base, and inferior articular processes that connect it to C2.

The C2 vertebra, or the axis, is composed of a body that contains a vertical pillar of bone, the dens, which articulates with C1 above. The inferior facets are located at the junction of the pedicles and laminae. The spinous process of C2 is bifid and provides a prominent bony landmark for palpation. It is also the attachment for several suboccipital muscles and the ligamentum nuchae (nuchal ligament), which extends from the external occipital protuberance to the C7 spinous process.

The lower cervical vertebrae (C3–C7) adopt the more usual vertebral structure detailed previously, but they are distinguished (except for C7) by the presence of a perforation in each transverse process, termed the foramen transversarium , that transmits the vertebral artery and sympathetic plexuses ( Fig. 1.10 ). Further, the spinous processes of C3 to C6 are bifid, whereas the spinous process of C7 is monofid. The spinal canal of the cervical vertebrae is triangular; its anterior border is the vertebral body, its lateral borders are the pedicles, and its posterior border is the laminae. The transverse process in the cervical vertebrae is unique not only because it contains the foramen transversarium but also in that it gives off two tubercles, anterior and posterior, that attach the scalene muscles used for lateral neck flexion. The anterior tubercle of C6 is notable in that it is termed the carotid tubercle and is immediately posterior to the carotid artery.

Thoracic spine

The thoracic region is the least mobile area of the spine, partly due to stability provided by the rib cage and sternum. A primary function of the thoracic spine, in connection with the ribs and sternum, is protection of thoracic organs, including the heart and lungs. The bodies of the thoracic spine vertebrae are unique in that their upper and lower lateral borders house areas, known as demi-facets , for articulation with the heads of the ribs ( Fig. 1.11 ).

The posterior arches of the thoracic vertebrae contain the same components as those seen in Fig. 1.5 : the vertebral foramen, pedicles, superior and inferior articular processes, spinous processes, and transverse processes. The spinous processes of thoracic vertebrae are more acutely slanted caudad.

There are usually 12 ribs on each side of the thoracic vertebra. Ribs 1 to 7 are true ribs in that they connect to the costal cartilage of the sternum anteriorly; ribs 8 to 12 are false ribs in that their costal cartilages are connected to the rib above. Ribs 11 and 12 are also floating ribs, which do not project anteriorly.

Lumbar spine

Five vertebrae compose the lumbar portion of the spine. The vertebral bodies are large, and the posterior arch, formed by the pedicles, laminae, and articular processes, encloses the vertebral foramen. These vertebrae are unique in their lack of costal facets and foramen transversaria ( Fig. 1.12 ). The spinous processes are near horizontal, making for large interlaminar spaces that are easily visualized using a fluoroscopic approach during interventions ( Fig. 1.13 ). ^,