2 Anatomy of the Spine: An Overview

The normal adult spine typically has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 coccygeal. Of these, the seven cervical vertebrae are always conserved in number, but variations in the number of thoracic, lumbar, and sacral vertebrae occur in approximately 5% of the population and are not considered pathologic.¹ The 24 rostral vertebrae permit movement to varying degrees and are limited by their shape and by muscle, ligaments, and the 23 intervertebral discs. The absence of discs between the occiput and the atlas and between the atlas and the axis contributes to the increased movement potential at these two levels. The four coccygeal vertebrae are relatively fixed and in some cases completely fused. The length of the spine is variable from one person to another, but approximately 75% of its length is due to the height of the vertebral bodies and 25% is due to the height of the discs.² With age, as the discs lose water, this proportion changes.

The human spine has four curvatures: two kyphotic and two lordotic. The kyphotic curvatures are located in the thoracic and sacral regions. They develop during embryogenesis and are known as the two primary curvatures. This physiologic kyphosis occurs because of the relatively greater height of the posterior vertebral wall compared to the anterior vertebral wall.³ The cervical and lumbar curvatures begin to develop before birth but are not accentuated until the spine bears weight axially. The cervical curvature develops when the infant begins to hold its head erect, and the lumbar curvature develops when the infant begins to stand and walk.² Consequently, these two curvatures are known as lordotic or secondary curvatures. The cervical curvature is transiently lost during flexion, whereas the other curvatures are either increased or decreased during flexion and extension but are not lost in their entirety. Because the sacrum is fused, its curvature does not change with movement.

In general, the cervical, thoracic, and lumbar vertebrae are composed of a body and a vertebral arch. The atlas is the exception; it is a ring structure without a body or pedicles. The vertebral bodies increase in size from T4 to the sacrum to accommodate the increasing load at each caudal level.⁴ Above T4, there is a general progression in size in rostral caudal fashion, but in some instances a rostral vertebra may be larger than a caudal vertebra² (refer to the Cervical Vertebrae section for a more specific description). The vertebral arch is formed as the two pedicles protrude posteriorly from the superolateral aspect of the vertebral body. Where two adjacent vertebrae articulate, the pedicles form the rostral and caudal borders of the intervertebral foramina. The nerve roots exit through these foramina, and the dorsal root ganglia reside within them.²

The laminae extend from the pedicles and triangulate to form the spinous process in the posterior midline of the spine. The enclosed space is known as the vertebral foramen, and the sum total of these foramina forms the spinal canal. The spinous and transverse processes serve as origin and insertion sites for tendons and ligaments. The cervical vertebrae are unique in that they contain a foramen transversarium in each of the transverse processes. The facets arise where the lamina and pedicle join together and their angle varies between cervical, thoracic, and lumbar regions.^2,4 Sensory innervation of the facets comes from the dorsal rami of spinal nerves exiting the intervertebral foramina.²

2.2 Development of the Spine

The vertebrae and intervertebral discs arise from the somites during embryologic development. They are initially cartilaginous structures but the vertebrae begin to ossify around the seventh week.² Ossification occurs in three primary centers within the vertebrae: one in the centrum (which will become the vertebral body) and one in each half of the vertebral arch.⁵ The sacrum and coccyx do not begin to ossify until infancy.⁵ At birth, each vertebra consists of three ossified components that are joined by hyaline cartilage. The right and left halves of the vertebral arch connect with the centrum at the neurocentral joint. This will become the pedicle.⁵ The spinous process has yet to completely form and remains the cartilaginous connection between each half of the vertebral arch posteriorly. Fusion across the arch takes place in the cervical region during the first year of life and progressively occurs throughout the rest of the spine until it is complete around age 6.⁵ The neurocentral joint fuses between 5 and 8 years of age.⁵ During puberty, five secondary ossification sites develop: one at the tip of the spinous process; one at the tip of each transverse process; and one at each of the two epiphyses (superior and inferior). By age 25, these ossification sites will disappear.⁵ Failure of ossification has been implicated in ossiculum terminale and os odontoideum.

Early during embryonic development of the spine, the spinal cord extends through the entire length of the spinal canal. Hence, the vertebrae and associated spinal cord segments reside at the same level. As development proceeds throughout life, the vertebrae lengthen at a faster rate than the spinal cord.⁵ At birth, the spinal cord resides at the L2–L3 vertebrae. Subsequent growth causes the spinal cord to typically end at the L1 vertebra. Below the cervical spine, the nerve roots must descend before exiting through the appropriate intervertebral foramina. The distance they must travel before leaving increases with each caudal level and is most evident at the cauda equina.⁵

2.3 Vertebrae and Surrounding Structures

2.3.1 Cervical Vertebrae

There are seven cervical vertebrae that differ considerably from each other. The atlas (C1) is a ring structure that has a posterior tubercle and two transverse processes that act as origin and insertion sites for muscle and ligamentous structures. The lateral masses of C1 include the transverse process, the tubercle for the transverse ligament, and the superior articular surface that articulates with the occipital condyle. The area within the anterior and posterior arches is divided into two foramina by the transverse ligament. The anterior foramen houses the dens. The posterior foramen is the vertebral foramen. Lateral to this foramen, in the posterior arch, are the grooves for the two vertebral arteries that ascend through the foramen magnum after passing through the foramen transversaria of the transverse processes ( Fig. 2.1).

The atlas has five articulating synovial surfaces. Four are typically described as facets. The superior facets of C1 are articulating surfaces that allow flexion, extension, and limited rotation of the occipital condyles on C1. The inferior facets of C1 articulate with the superior facets of the axis (C2) and also allow for flexion and extension, but rotation is less than that of the occipital condyles and occurs by translation.⁴ The greatest amount of rotation at C1–C2 is permitted by the anterior articulating surface within the ring that comes in contact with the dens, the fifth articular surface at this level.⁴ Flexion, extension, and translation of C1 on the dens are limited because of the transverse and alar ligaments that hold the dens in place around a cushion of surrounding synovium.⁴ Because C1 does not have a true body, there are no intervertebral discs adjacent to it. The first cervical disc is at the C2–C3 interspace.

Fig. 2.1 C1–C2 anatomy. Craniocervical junction demonstrating the attachments of the transverse ligament to the lateral masses of C1. Anterior to the ligament, the dens is held in place against the anterior arch of C1. The vertebral arteries run through the vertebral foramen and then run medially along the rostral aspect of C1 until entering the foramen magnum dorsolaterally.

The morphology of the remainder of the cervical spine is on a continuum, with small changes in the dimensions and surfaces of the vertebrae as they are stacked caudally.^2,4 The spinous process at C7 is the longest and consequently is known as the vertebra prominens. Conversely, cervical body height decreases from C3 to C6 before enlarging again.^2,4 This is unlike the thoracic and lumbar regions where increased axial load progressively results in larger stronger vertebrae.² In fact, C2 is the strongest of the cervical vertebrae.^2,4

The spinal canal also progressively narrows to its minimum at C5 where the cervical enlargement is at its maximal circumference. The spinal cord at C5 is typically 8 mm anteroposterior by 13 mm wide, and the spinal canal at this location is typically 14 mm anteroposterior by 24 mm wide.⁶ Of these dimensions, the anteroposterior diameter is of the most clinical relevance in cervical stenosis. The canal width is usually more than adequate unless a mass is present.⁶

There are four facets on each of the cervical vertebrae below C2, two superior and two inferior. They are at approximately 45-degree angles with respect to the vertebral body end plates and are arranged so that the superior facets are anterior to the inferior facets of the next rostral vertebra. These zygapophyseal articulations allow variable flexion and extension but minimal rotation individually. Their angulation prevents translation. The lateral masses are the bony area between the superior and inferior facets of a single vertebra ( Fig. 2.2 and Fig. 2.3).

The pedicles of the cervical spine join the lateral masses to the vertebral body. The distinction between the pars interarticularis and the pedicle of C2 is important because of the popularity of transpars and transpedicular screw techniques. The bone in between C1–C2 and C2–C3 is the pars interarticularis of C2. The bone joining this structure to the vertebral body is of the pedicle of C2. At other levels, the distinction is easier to make. Pedicle width from C3 to C7 is approximately 6 mm, and height is between 5 and 10 mm at each level.^7,8,9,10,11 Although screw placement within the cervical pedicles is possible, the small size of the pedicles and their proximity to the vertebral artery (above C6) make screw placement dangerous.

Fig. 2.2 Anatomy of the subaxial cervical vertebrae. Note the position and angle of the facet joints (shaded areas) as well as the relationship of the cervical pedicle to the spinal canal and vertebral foramen.

Fig. 2.3 Angulation of facet joints. Subaxial cervical vertebral bodies (left) have facets that are angled approximately 45 degrees in the sagittal plane. This helps prevent anteroposterior translation while allowing flexion and extension. The thoracic facets (middle) are nearly coronal in orientation, preventing any translation whatsoever. The rib cage (via the costovertebral joints) adds significant rigidity to this portion of the spine. The lumbar facets (right) are oriented in the sagittal plane, allowing for some rotation, flexion, and extension.

Transverse processes project from the lateral masses and contain a foramen through which the paired vertebral arteries can pass¹² ( Table 2.1). The exception to this is C7.¹² It contains the foramina, but the vertebral arteries usually pass anterior to the transverse processes and then enter the foramina of C6 as they ascend toward the foramen magnum ( Fig. 2.4). C6 has a prominence known as the carotid tubercle that projects anteriorly from the transverse process.

2.3.2 Thoracic Vertebrae

The thoracic vertebrae differ from the cervical and lumbar vertebrae in many ways; however, T1 to T4 have features on a continuum with the cervical vertebrae, and T9 to T12 are similarly on a continuum with the lumbar vertebrae² ( Fig. 2.5). At the superolateral aspect of the pedicle, the posteroinferolateral aspect of the vertebral body, and the inferolateral aspect of the transverse process are three additional articular surfaces not present in the other sections of the spine. The first two combine with the rib head, and the third with the rib tubercle to form the costal facets. The rib tubercles point dorsally and mark the joining of the rib neck and shaft. They provide attachment sites for the ribs bilaterally to the first 10 thoracic vertebrae ( Fig. 2.6). Ribs 11 and 12 articulate via facets on the bodies of their respective vertebrae only. They have a single facet on their heads and no neck or tubercle. T1 is somewhat divergent in its configuration in that it has a complete costal facet on the superolateral aspect of its body for the first rib and a demifacet on its inferolateral border for the second rib. The 12th rib is of particular importance anatomically because the pleura passes rostral to this rib beginning 3 to 4 cm lateral to its head and continuing along its shaft approximately 7 to 8 cm.²

Table 2.1 Percentage of vertebral arteries that enter the transverse foramina at each level

Fig. 2.4 The course of the vertebral artery and deep cervical ligaments of the spine. (a) Anterior cervical spine anatomy. (b) Anterior cervical spine anatomy with vertebral body removed.

Fig. 2.5 Bony and deep ligamentous anatomy of the thoracic spine. (a) Lateral thoracic anatomy. (b) Anterior thoracic anatomy. (c) Posterior thoracic anatomy. 1, costotransverse ligaments; 2, superior costotransverse ligaments; 3, lateral costotransverse ligaments; 4, ligamentum flavum; 5, supraspinal ligaments; 6, intertransverse ligaments; 7, costotransverse facet; 8, rib.

Pedicle dimensions vary significantly over the entire thoracic spine. Initially, the transverse diameter is around 8.2 mm at T1 but decreases to 5.5 mm at T4. The pedicle diameter then progressively grows in size again to its maximum at T12 of approximately 8.8 mm.^7,10,13 The width of the pedicle in the sagittal plane is on an increasing continuum from T1 to T11. The superior and inferior articular facets are angled primarily in the coronal plane to prevent anterior translation of a rostral vertebra on the caudal one.⁴ Orientation of the facets changes to a more lumbar orientation in the T9 to T12 region.⁴

Fig. 2.6 The thoracic vertebral body. The thoracic vertebral body is characterized by coronally oriented facets and the presence of articulations for the rib heads. Pedicle dimensions are highly variable dependent on level and individual variation. (a) This drawing was made from the T7 vertebral body. (b) Anterior view showing the anterior longitudinal ligament. (c) Posterior view after laminectomy showing the posterior longitudinal ligament.

Fig. 2.7 (a,b) The lumbar vertebral body is characterized by larger pedicles, a sagittal orientation of the articular facets (except at L5–S1), and large transverse processes for the attachment of the paraspinal muscles.

The lumbar spine is distinguished anatomically by its large vertebral bodies, strong lamina, and gaps between its spinous processes. Its vertebral bodies are kidney shaped and increase in size at each caudal level. The lumbar pedicles are wide and thick and are widely spaced, with their height typically half that of their associated vertebral body. Typical pedicle width is 9 mm at L1 and 18 mm at L5, and pedicle height is about 15 mm throughout the lumbar spine.^7,8,9,10,11 The transverse pedicle angle increases from 12 to 30 degrees from L1 to L5, and the transverse pedicle diameter increases as well from 9 to 18 mm.¹⁴ The sagittal pedicle angle decreases from about 3 degrees at L1 to 0 degrees at L5 ( Fig. 2.7).^3,14

The rostral hemifacet is a concave structure that faces dorsomedially and articulates with the convex caudal hemifacet of the next rostral vertebra. The caudal hemifacet is an extension of the lamina and faces anteromedially, complementing its caudal counterpart. The junction of these two articulating surfaces forms the root of the intervertebral foramina. The near perpendicularity (90-degree angle) of the facets in the transverse plane and variable lateral angle from 15 to 70 degrees from L1–L2 to L5–S1 significantly limit rotation and translation of the lumbar spine at each level and as a functional unit.¹⁴ In fact, rotation at any given level of the lumbar spine is limited to approximately 1 to 2 degrees.¹⁵ The first 60 degrees of flexion of the spine occurs in the lumbar region with an additional 25 degrees at the hip, and this is reversed with extension.¹⁶ Flexion/extension increases at each increasing level from between 5 and 16 degrees at L1 to between 10 and 24 degrees at S1.¹⁵ Lateral flexion varies between 3 and 12 degrees at each level except at S1, where it is between 2 and 6 degrees.

2.3.4 Sacral and Coccygeal Vertebrae

The sacrum is the only part of the spine that is entirely fused. It tapers caudally at the lateral masses because this region (the inferior half of the sacrum) is not weight-bearing. The superior half of the sacrum transmits the axial load of the spinal column to the pelvis via the sacroiliac joints.² The sacrum also acts to stabilize and strengthen the pelvic girdle.² The base of the sacrum is actually its most rostral part. Its superior articulating processes articulate with the inferior articulating processes of L5 forming the lumbosacral angle. The L5–S1 facet acts to prevent anterolisthesis of the lumbar spine on the sacrum. The anterior edge of the first sacral vertebra forms the sacral promontory and it is typically larger in males than in females.² However, the width/length proportion of the sacrum as a whole is usually larger in women.²

In approximately 5% of the population, the L5 vertebra is incorporated into the sacrum.² If it is completely fused, it is referred to as sacralization of the L5 vertebra; if it is partially fused, it is termed hemisacralization. The reverse is also true in that the S1 vertebra can sometimes be incorporated into the lumbar spine. This is known as lumbarization. If L5–S1 is fused as in a sacralized L5 vertebra, a greater strain is placed on the L4–L5 interspace with consequences to the intervertebral disc, ligaments, and facets. If the S1 vertebra is lumbarized, the strain will be placed on S1–S2.²

The pelvic surface of the sacrum is smooth and concave, and the posterior surface is rough and convex. On the anterior surface are four transverse lines that indicate where the sacral vertebrae have fused. Unlike the cervical, thoracic, and lumbar regions, which have one pair of intervertebral foramina that opens laterally, the sacrum has one pair of foramina that opens to the anterior surface and one pair that opens to the posterior surface at each level. There are four levels in total, and they are located between the adjacent fused vertebrae. This allows for the exit of the posterior and anterior sacral nerve roots. On the posterior surface are five longitudinal ridges. The two lateral ridges represent the fused sacral transverse processes, the two intermediate ridges represent the fused articular processes/pedicles, and the central ridge represents the small spinous processes of S1–S4.² Anterior to the S1–S4 laminae is the sacral canal. It contains the sacral nerve roots, connective tissue, and the caudal thecal sac.² Because S5 does not have a spinous process or lamina, a dorsal opening develops, known as the sacral hiatus. The depth of the hiatus is often dependent on the amount of S4 lamina and spinous process that is missing, as the S4 vertebra may not form a complete posterior arch.² The hiatus contains the filum terminale, the S5 and coccygeal nerves, and fatty connective tissue. Inferiorly are the sacral cornua (horns) that represent the inferior articular surfaces of S5.² They extend from the apex of the sacrum and articulate with the coccygeal cornua that represent the superior articulating surfaces of the first coccygeal vertebra forming the sacrococcygeal joint.²

The coccyx is the remnant of the embryologic tail that persists until the beginning of the eighth week of development.⁵ It typically has four vertebrae but this can vary by one vertebrae in the normal population.² The vertebrae are rudimentary and the first three consist of vertebral bodies only without arches or processes.² The first coccygeal vertebra usually does not fuse with the sacrum except in the elderly.² The remaining vertebrae fuse earlier in life, typically around age 40.² The coccyx has a more anterior curvature in males than in females and is part of the pelvic inlet.² The wider curvature in females assists in the birthing process. It also serves as an insertion site for the gluteus maximus and coccygeus muscles, and the anococcygeal ligament.

2.3.5 Intervertebral Discs

The intervertebral discs are fibrocartilage structures that are important in weight-bearing and movement. They are typically 45% of the height of the surrounding vertebral bodies.² The annulus fibrosis is a ligamentous structure that forms the outer limit of the disc, and the nucleus pulposus is the gelatinous center that gives the disc its height and cushioning effect.

The annulus fibrosis is composed of concentric lamellae of fibrocartilage that run between the smooth hyaline cartilage plates of the vertebral bodies. The lamellae are at right angles to each other.² They are fewer, thinner, and less numerous posteriorly than they are anteriorly or laterally, which may help to explain why disc protrusions typically occur at the posterior surface of the disc.^2,14 The annulus surrounds the nucleus pulposus, a structure that forms the core of the disc. It contains significantly more cartilage than fibrous tissue and is highly elastic.¹⁴ The nucleus is slightly posterior in its location within the disc complex.¹⁴ It retains a significant amount of water (88% in infancy) that is lost with age (65% in the elderly) and must get its nourishment via diffusion because it is an avascular structure.²

2.4 Ligaments

The ligaments that connect the vertebrae together are tough fibrous bands that are strategically placed to limit movement. The anterior ligaments prevent excessive hyperextension, whereas the posterior ligaments are primarily responsible for limiting flexion.¹⁵ All ligaments are involved in stabilizing the spine against translational motion.¹⁵ The atlanto-occipital and atlantoaxial regions are inherently the most mobile portions of the cervical spine. More strategically placed ligaments are necessary to restrain unwanted motion of these vertebrae with respect to the occiput and each other.

2.4.1 Anterior Ligaments

The anterior longitudinal ligament (ALL) binds the vertebral bodies beginning at the anterior surface of the sacrum and extending to the ring of C1. It continues rostrally to the basilar occipital bone as the anterior atlanto-occipital membrane. It is a strong ligament that is thickest opposite the intervertebral discs and is fixed to their anterior surface and also to the vertebral periosteum. The ALL primarily limits hyperextension and consequently is stronger in the thoracic region than in the cervical region¹⁵ (see Fig. 2.4).

2.4.2 Posterior Ligaments

The posterior longitudinal ligament (PLL) is more narrow and weaker than the ALL¹⁵ (see Fig. 2.4). Its course is within the spinal canal and it spans the posterior aspect of the vertebral bodies beginning caudally at the sacrum. Like the ALL, the PLL is also fixed to the intervertebral discs. The PLL functionally limits hyperflexion and aids in preventing disc retropulsion.¹⁵ It continues rostrally from C2 as the accessory tectorial membrane, which is the widest portion of the ligament. The accessory tectorial membrane bridges the anterior portion of the foramen magnum to the posterior aspect of the arch of C1 and the body of C2 as it traverses over the dens and cruciform ligament. It limits extension between the occiput and C2 and flexion to a lesser extent.¹⁵ This ligament, however, does not limit axial rotation.¹⁵

There is a small synovial capsule between the atlas and the dens that is surrounded posteriorly by the cruciform ligament. The cruciform ligament is composed of superior and inferior fibers, and the transverse ligament of the atlas. The superior fibers anchor the dens to the anterior portion of the foramen magnum, whereas the inferior fibers originate at the base of the dens and the body of C2. The transverse ligament is a 10-mm thick band that contains no elastic fibers and is the largest component of the cruciform ligament.¹⁷ It originates from the medial aspect of the lateral masses of C1 and wraps around the dens, thereby limiting atlantoaxial subluxation anteriorly. The anterior portion of the dens and posterior surface of the anterior arch of C1 are thus tightly bound together.

2.4.4 Odontoid Ligaments

Deep to the cruciform ligament are the odontoid ligaments, which consist of the apical and paired alar ligaments. The apical ligament is a thin band that originates at the tip of the dens and inserts on the anterior margin of the foramen magnum. It confers little stability between the occiput and C2.¹⁵ The alar ligaments are subdivided into two portions. The posterior bands anchor the dens to each of the occipital condyles. If present, the anteroinferior bands usually anchor the atlas to each of the occipital condyles.¹⁵ The alar ligaments collectively limit axial rotation.¹⁵

2.4.5 Ligamentum Flavum

The remaining cervical, thoracic, and lumbar vertebrae are linked together by their lamina externally to the next rostral vertebral lamina internally within the spinal canal by the ligamentum flavum. This extremely strong ligament is larger and thicker in the lumbar regions and progressively becomes smaller rostrally as the load-bearing of the spine decreases.² Some of its lateral fibers contribute to the posterior aspect of the intervertebral foramina before inserting on the facet joints.² The posterior atlanto-occipital membrane that anchors the occiput to the atlas is an extension of this ligament. The ligamentum flavum functions to maintain the normal curvature of the spine and helps assist the posterior musculature in straightening out the spine when returning from a flexed position² ( Fig. 2.8).

Ligamentum Nuchae

The cervical spinous processes are all anchored to each other and to the inferior nuchal line via the dense fibrous bands of the broad ligamentum nuchae. This ligament is a combination of the weaker interspinous ligaments and a strong cordlike supraspinous ligament that is present throughout the entire spine. In the thoracic and lumbar regions, the intraspinous ligaments become somewhat larger and stronger as the load increases caudally.²

Ligamentous Capsules

There are also ligamentous capsules that bind the facets at all levels. They are longer and less taut in the cervical region to promote increased mobility, particularly flexion.² They permit gliding of the articular surfaces of the facet joints between vertebrae. Small, weak intertransverse ligaments attach between the transverse processes throughout the spine but are scattered and provide minimal support except in the lumbar region where they are membranous and more developed.²

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