Viewed as a whole, the typical adult spine has primary curvatures that are kyphotic in the thoracic, sacral, and coccygeal spine and secondary lordotic curvatures that develop after birth in the cervical and lumbar spine (Fig. 3.1). In the frontal plane there are typically no curvatures. A frontal curvature of more than 10° is scoliosis; thoracic kyphosis is typically limited to 30–35° and lumbar lordosis 50–60°.

A functional segmental unit of the spine consists of three joints that join adjacent vertebrae: anteriorly, the intervertebral disc (Fig. 3.2) and posteriorly, the two facet joints at that level [27, 28]. This functional unit was first described by Junghanns as a “mobile segment [29, 30].” The functional unit model applies to all mobile segments of the spine except at the atlantooccipital (C0–C1) and atlantoaxial (C1–C2) junctions, where there are no intervertebral discs. The muscles and nerves of the spine are intimately related to the functional unit. The following sections describe the specific anatomical structures of the spine, with an emphasis on function and pain.

The vertebral column as a whole consists of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal [31]. Vertebrae are irregular, composite bones, consisting of a ventral body and a dorsal neural arch. The exception is the atlas, which has no vertebral body. The load-bearing system of the vertebral bones can be understood as a tripod: one vertebral body interface and two (paired) facet joint interfaces [32]. For example, lumbar vertebral bodies bear 80–90 % of the weight load, and lumbar pedicles and laminae function as struts, supporting the tripod “legs.” Pedicles connect the anterior and posterior weight-bearing elements, spanning from the transverse process to the body. Transverse processes originate at the junction of the pedicle and the lamina, while the spinous process originates at the junction of right and left laminae. The functional significance of the transverse and spinous processes is to increase the moment arm of muscles attaching to these sites [17, 32]. The vertebral bodies have thin cortical bone and cancellous trabecular bone in vertical, oblique, and horizontal patterns [31]. This overlapping trabecular medullary structure provides great strength, but the relative lack of overlap in the anterior portion may predispose to compression fractures.

Cervical vertebrae (Fig. 3.3) bear the least weight, and their vertebral bodies are relatively narrow in AP dimension and have distinct uncinate processes at their superolateral borders, which articulate with the vertebral body above (uncovertebral joint). Vertebrae C1–C6 have bilateral foramen transversarium containing the vertebral artery (C7 has this foramen without the vertebral artery). As shown in the figure, the vertebral artery is very susceptible to injury in the anterolateral approach for cervical transforaminal injections (Fig. 3.4). The transverse processes have anterior and posterior projections for attachment of muscles, and the projections form a groove which contains the exiting spinal nerve. The cervical articular pillars are oblique to the horizontal plane and appear stacked as a column. The atlas, C1, has no vertebral body and has unique articular processes for occipital condyle and axis. The axis, C2, is most recognized by the odontoid process or dens. The occipital-atlantoaxial complex allows nodding and rotational movements: about 50 % of flexion-extension is at C0–C1, and 50 % of rotation is at C1–C2. C6 transverse process anterior tubercle is known as Chassaignac’s tubercle (also known as carotid tubercle, an important landmark for stellate ganglion blocks). Vertebra C7 has a larger inferior body, a large spinous process (vertebra prominens), and steeply sloping articular pillars.

The defining features of the 12 thoracic vertebrae are the costotransverse and costovertebral articulations of the ribs (Fig. 3.5). Thoracic facets are arranged in the coronal plane in a way that allows rotation and side bending.

Typical features of lumbar vertebrae are shown (Fig. 3.6). L5 is usually largest and more wedge-shaped when viewed laterally, which allows the superior aspect of L5 to be closer to horizontal [34]. The L5 disc is also wedge-shaped, allowing 16° average angle between the respective superior and inferior surfaces of S1 and L5 [35]. L5 has thick transverse processes but has the smallest lumbar spinous process. L1 has the shortest and L3 the longest transverse processes [36]. L5–S1 facets tend to be more flat, while other levels are more curved [31]. Because L5 is angled, more of its axial load is transmitted by its posterior elements, including the transverse processes and iliolumbar ligaments [37]. The lumbar vertebral bodies have indentations in the posterior aspects of superior and inferior rims, vestigial uncinate processes that may contribute to posterolateral disc protrusions [17]. The mamillary process is an attachment site for longissimus and rotator muscles. The accessory process is thought to be a vestigial costal process and is an attachment site for longissimus muscles. The mamilloaccessory ligament joins these two processes; this structure may be an obstacle in medial branch blocks or radiofrequency ablation.

The sacrum (Fig. 3.7) is formed of five fused vertebrae and vestigial costal elements. The sacrum articulates with the ilium. The sacrum has a central canal which is the terminal portion of the spinal canal. The ventral primary rami of S1–S4 exit via anterior foramina, and the posterior rami exit the posterior foramina. The center of mass is anterior to the vertebral column in 75 % of adults, typically 2 cm anterior to S2 [31]. The sacrum is tilted forward so that the most superior sacral end plate is 50–53° from horizontal [31, 38]. The coccyx is usually four vertebrae, with the first being larger and having cornua.

Mixter and Barr, respectively, from the neurosurgery and orthopedic surgery services of Massachusetts General Hospital, are often cited as the first to recognize the importance of intervertebral disc herniations, although Mixter and Barr themselves cite several prior reports [44]. Although the intervertebral disc has been compared to a diarthrodial joint, it is correctly classified as a symphysis, a major cartilaginous synarthrosis [45]. Adjacent vertebral levels are separated by an intervertebral disc (except at C0–C1 and C1–C2). The functions of the disc are to distribute force and allow movement between adjacent vertebral bodies and to hold the vertebral bodies together. The discs also comprise 20–25 % of the total length of the spine, separating the vertebrae, allowing nerve roots and vessels to travel between vertebrae [27].

The intervertebral disc is the largest avascular structure in the body [46]. Each disc contains superior and inferior cartilaginous end plates, anulus fibrosus, and nucleus pulposus. Nucleus pulposus is 70–90 % water, variable with age, and proteoglycans are the majority of the remainder. Type II collagen fibers join with proteoglycans, forming the matrix of the nucleus. Embedded in the proteoglycans, toward the end plate regions, are chondrocytes that synthesize the substance of the nucleus.

The nucleus pulposus functions to bear weight and distribute forces across vertebral segments. The healthy nucleus distributes load very evenly, in a manner as a fluid, according to Pascal’s law, a mechanism that dissipates force and prevents injury [32]. With age, the glycosaminoglycan and water content of the nucleus decrease, and the fibrocartilaginous content increases. This transfers load bearing to the anulus and other structures.

The anulus fibrosus is the external ring of the disc and is formed by 10–20 sheets of parallel collagen fibers (lamellae), each sheet having fibers oriented in alternating directions 30–70° from vertical. The anulus is mostly water (60 %). Collagen is half of the remaining portion. The anulus can be divided into three zones: the outer zone is made of fibrocartilaginous Sharpey’s fibers that attach to the vertebral body. The intermediate and inner zones are also fibrocartilage, but do not attach to the vertebral body. The posterolateral fibers of the lumbar anulus are most prone to injury and degeneration. While the posterocentral disc may be protected by attachments of posterior longitudinal ligament, there are no extrinsic ligament attachments to support the posterolateral disc [47]. The lumbar posterior anulus is thinner radially due to the eccentric location of the nucleus. It is also longitudinally thinner, due to the lordotic curve of the lumbar spine, which means that a given amount of displacement causes relatively greater strain on these shorter fibers.

Vertebral end plates are cartilaginous structures at the superior and inferior aspect of each vertebral body, between the body and the disc. They are firmly attached to the disc by the collagenous fibers of the lamellae (intermediate and inner zones), which invest the fibrocartilaginous side of the end plate. Blood vessels on the surface of the vertebral bodies contact the hyaline side of the end plate, and nutrients diffuse through the end plate to the disc, a process of imbibition [ 48].

Several studies have confirmed nerve fibers in the outer portions of the anulus fibrosus, especially the posterolateral disc, likely rendering the disc capable of becoming a pain generator. Disc innervation may arise from sinuvertebral nerve, ventral rami direct branches, and rami communicans (Table 3.5) [17, 49–52, 54–57].

Pressure within the disc is affected by body position. The natural lordotic posture of the lumbar spine reduces vertical pressure on the disc. Direct vertical pressure increases fluid pressure within the disc. This can even lead to herniation of nucleus pulposus through defects in the end plate, known as Schmorl’s nodes [first observed by Luschka [31]]. Fluid shifts in the disc also account for the 1–2 cm decrease in height that can be observed after a day of upright posture compared to the morning [58]. Disc pressure when standing is only 35 % of pressure when seated, while lifting greatly increases disc pressure and can be estimated at ten times the weight of the object lifted, as demonstrated in Nachemson’s classic study [59].

Disc herniations can be classified as protrusion, extrusion, or sequestration. A protrusion has a base wider than the greatest extent of herniation (Fig. 3.9). Nucleus material extends into the epidural space in an extrusion, and the extent of herniation may exceed the width of the base (Fig. 3.10). In a sequestration, the nucleus material has been extruded as a fragment into the epidural space and has lost continuity with the disc. For further standardized terminology for description of location and type of disc herniations, the reader is referred to “Nomenclature and Classification of Lumbar Disc Pathology. Recommendations of the Combined Task Forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology [47].”

Ligaments of the spine are detailed in Tables 3.6, 3.7, and 3.8 [17, 32]. Of great practical interest to the pain physician is the ligamentum flavum, a paired structure, running from lumbar to cervical spine, composed of highly elastic fibers. There may be a narrow gap between the right and left halves [36]. It forms the posterior boundary of the epidural space as approached between vertebral lamina and may contribute to the boundary in the intervertebral foramen. On right and left, a ligamentum flavum attaches from the superior vertebral level to the adjacent level below. Laterally, fibers of ligamentum attach to the facet articular processes and form part of the joint capsule. Lateral fibers may attach to the pedicle. The lateral fibers of ligamentum contribute to the posterior boundary of the intervertebral foramen. In the lumbar spine, ligamentum flavum may be the strongest and most elastic of spinal ligaments and resists distraction and flexion. In the lumbar spine, ligamentum flavum can be 2–3 mm thick [36]. The elasticity is important to prevent buckling of the ligament, which can compress neural structures. Buckling can occur if there is loss of intervertebral disc height. However, the ligamentum flavum is less robust in the cervical and upper thoracic spine, and its paper thinness and midline absence may affect cervical or thoracic interlaminar injections [60]. The interspinous and supraspinous ligaments are relatively weak and are often the first to be sprained [31, 61]. The iliolumbar ligament is a complex structure that begins as a muscle and becomes a ligament in the third decade and is only present in adults [31, 62].