The cervical spine is composed of seven cervical vertebrae. The first two cervical vertebrae are unique and called the atlas and axis (Fig. 2-4). The atlas (C1) holds the head on the neck (the titan Atlas of Greek mythology held the heavens on his shoulders as punishment by Zeus). The axis (C2) is the point of articulation where the head turns on the neck, providing an “axis of rotation.”

Table 2-1 summarizes key features of the cervical vertebrae. The cervical region is a fairly mobile portion of the spine, allowing for flexion and extension as well as rotation and lateral bending.

TABLE 2-1

Key Features of the Cervical Vertebrae (C1-C7)

VERTEBRAE	DISTINGUISHING CHARACTERISTICS
Atlas (C1)	Ringlike bone; superior facet articulates with occipital bone.
	Two lateral masses with facets
	No body or spinous process
	C1 rotates on articular facets of C2.
	Vertebral artery runs in groove on posterior arch.
Axis (C2)	Dens projects superiorly.
Axis (C2)	Strongest cervical vertebra
C3 to C7	Large, triangular vertebral foramen
	Transverse foramen through which vertebral artery passes (except C7)
	Narrow intervertebral foramina
	Nerve roots at risk of compression
C3 to C5	Short, bifid spinous process
C6 to C7	Long spinous process
C7	Vertebra prominens; nonbifid

Clinical Focus 2-2 Cervical Fractures

Fractures of the axis (C2) often involve the dens and are classified as types I, II, and III. Type I fractures are usually stable, type II fractures are unstable, and type III fractures, which extend into the body, usually reunite well when immobilized. The hangman fracture, a pedicle fracture of the axis, can be stabilized, if survived, with or without spinal cord damage. A Jefferson fracture is a burst fracture of the atlas (C1), often caused by a blow to the top of the head.

The thoracic spine is composed of 12 thoracic vertebrae (Fig. 2-5 and Table 2-2). The 12 pairs of ribs articulate with the thoracic vertebrae. This region of the spine is more rigid and inflexible than the cervical region.

TABLE 2-2

Key Features of Thoracic, Lumbar, Sacral, and Coccygeal Vertebrae

VERTEBRAE	DISTINGUISHING CHARACTERISTICS
Thoracic (T1-T12)	Heart-shaped body, with facets for rib articulation
	Small circular vertebral foramen
	Long transverse processes, with facets for rib articulation in T1-T10
	Long spinous processes, which slope posteriorly and overlap next vertebra
Lumbar (L1-L5)	Kidney-shaped body, massive for support
	Midsized triangular vertebral foramen
	Facets face medial or lateral direction, which permits good flexion and extension
	Spinous process is short, strong, and horizontal.
	L5: largest vertebra with massive transverse processes
Sacrum (S1-S5)	Large, wedge-shaped bone that transmits body weight to pelvis
	Five fused vertebrae, with fusion complete by puberty
	Four pairs of sacral foramina on dorsal and ventral (pelvic) side
	Sacral hiatus, the opening of sacral vertebral foramen
Coccyx (Co1-Co4)	Co1 often is not fused.
	Co2 to Co4 are fused.
	No pedicles, laminae, or spines
	Remnant of embryonic tail

The lumbar spine is composed of five lumbar vertebrae (see Figs. 2-3 and 2-5 and Table 2-2). The lumbar vertebrae are comparatively large for bearing the weight of the trunk and are fairly mobile, but not nearly as mobile as the cervical vertebrae.

Sacrum and Coccyx

The sacrum is composed of five fused vertebrae that form a single, wedge-shaped bone (Fig. 2-5 and Table 2-2). The sacrum provides support for the pelvis. The coccyx is a remnant of the embryonic tail and usually consists of four vertebrae, with the last three often fused into a single bone. The coccyx lacks vertebral arches and has no vertebral canal.

The features and number of vertebrae can vary, and clinicians must always be aware of subtle differences, especially on radiographic imaging, that may be variants within a normal range.

Clinical Focus 2-3 Osteoarthritis

Osteoarthritis is the most common form of arthritis and often involves erosion of the articular cartilage of weight-bearing joints, such as those of the vertebral column.

Joints and Ligaments of Craniovertebral Spine

The craniovertebral joints include the atlanto-occipital (atlas and occipital bone of the skull) and atlanto-axial (atlas and axis) joints. Both are synovial joints that provide a relatively wide range of motion compared with other joints of the vertebral column. The atlanto-occipital joint permits one to nod the head up and down (flexion and extension), whereas the atlanto-axial joint is a pivot joint that permits one to rotate the head from side to side, as if to indicate “no” (Fig. 2-6 and Table 2-3).

TABLE 2-3

Key Features of Atlanto-occipital and Atlanto-axial Joints

LIGAMENT	ATTACHMENT	COMMENT
Atlanto-occipital (Biaxial Condyloid Synovial) Joint
Articular capsule	Surrounds facets and occipital condyles	Allows flexion and extension
Anterior and posterior membranes	Anterior and posterior arches of C1 to foramen magnum	Limit movement of joint
Atlanto-axial (Uniaxial Synovial) Joint
Tectorial membrane	Axis body to margin of foramen magnum	Is continuation of posterior longitudinal ligament
Apical	Dens to occipital bone	Is very small
Alar	Dens to occipital condyles	Limits rotation
Cruciate	Dens to lateral masses	Resembles a cross; allows rotation

Joints and Ligaments of Vertebral Arches and Bodies

The joints of the vertebral arches (zygapophysial joints) occur between the superior and inferior articular processes (facets) of adjacent vertebrae and allow for some gliding or sliding movement (Fig. 2-7 and Table 2-4). These joints slope inferiorly in the cervical spine (facilitate flexion and extension), are more vertically oriented in the thoracic region (limit flexion and extension but allow for rotation), and are interlocking in the lumbar spine (but do allow flexion and extension, but not to the degree present in the cervical spine). Corresponding ligaments connect the spinous processes, laminae, and bodies of adjacent vertebrae (see Tables 2-2 and 2-3). Strong anterior and posterior longitudinal ligaments run along most of the length of the vertebral column. Of these two ligaments, the anterior longitudinal ligament is stronger and prevents hyperextension (see Table 2-4).

TABLE 2-4

Features of the Zygapophysial and Intervertebral Joints

LIGAMENT	ATTACHMENT	COMMENT
Zygapophysial (Plane Synovial) Joints
Articular capsule	Surrounds facets	Allows gliding motion. C5-C6 is most mobile. L4-L5 permits most flexion.
Intervertebral (Secondary Cartilaginous [Symphyses]) Joints
Anterior longitudinal (AL)	Anterior bodies and intervertebral discs	Is strong and prevents hyperextension
Posterior longitudinal (PL)	Posterior bodies and intervertebral discs	Is weaker than AL and prevents hyperflexion
Ligamenta flava	Connect adjacent laminae of vertebrae	Limit flexion and are more elastic
Interspinous	Connect spines	Are weak
Supraspinous	Connect spinous tips	Are stronger and limit flexion
Ligamentum nuchae	C7 to occipital bone	Is cervical extension of supraspinous ligament and is strong
Intertransverse	Connect transverse processes	Are weak ligaments
Intervertebral discs	Between adjacent bodies	Are secured by AL and PL ligaments

The joints of the vertebral bodies (intervertebral joints) occur between the adjacent vertebral bodies (see Fig. 2-7 and Table 2-4). The intervertebral joints are lined by a thin layer of hyaline cartilage with an intervening intervertebral disc (except between first two cervical vertebrae). These stable, weight-bearing joints also absorb pressure because the intervertebral disc is between the bodies. Intervertebral discs are composed of a central nuclear zone of collagen and hydrated proteoglycans called the nucleus pulposus, which is surrounded by concentric lamellae of collagen fibers that compose the anulus fibrosus. The inner gelatinous nucleus pulposus (remnant of embryonic notochord) is hydrated and acts as a “shock absorber,” compressing when load bearing and relaxing when the load is removed. The outer fibrocartilaginous anulus fibrosus, arranged in concentric lamellae, is encircled by a thin ring of collagen and resists compression and shearing forces.

The lumbar are the thickest and the upper thoracic the thinnest intervertebral discs. The anterior and posterior longitudinal ligaments help to stabilize these joints (see Table 2-4).

Clinical Focus 2-4 Osteoporosis

Osteoporosis (porous bone) is the most common bone disease and results from an imbalance in bone resorption and formation, which places bones at a great risk for fracture.

Clinical Focus 2-5 Spondylolysis and Spondylolisthesis

Spondylolysis is a congenital defect or an acquired stress fracture of the lamina that presents with no slippage of adjacent articulating vertebrae (most common at L5-S1). Its radiographic appearance suggests a “Scottie dog” (terrier) with a collar (fracture site shown as red collar).

Spondylolisthesis is a bilateral defect (complete dislocation, or luxation) resulting in an anterior displacement of the L5 body and transverse process. The posterior fragment (vertebral laminae and spinous process of L5) remains in proper alignment over the sacrum (S1). This defect has the radiographic appearance of a dog with a broken neck (highlighted in yellow, with the fracture in red). Pressure on spinal nerves often leads to low back and lower limb pain.

Clinical Focus 2-6 Intervertebral Disc Herniation

The intervertebral discs are composed of a central nuclear zone of collagen and hydrated proteoglycans called the nucleus pulposus, which is surrounded by concentric lamellae of collagen fibers that compose the anulus fibrosus. The nucleus pulposus is hydrated and acts as a “shock absorber,” compressing when load bearing and relaxing when the load is removed. Over time, the repeated compression-relaxation cycle of the intervertebral discs can lead to peripheral tears of the anulus fibrosus that allow for the extrusion and herniation of the more gelatinous nucleus pulposus. This often occurs with age, and the nucleus pulposus becomes more dehydrated, thus transferring more of the compression forces to the anulus fibrosus. This added stress may cause thickening of the anulus and tears. Most disc herniations occur in a posterolateral direction because the anulus fibrosus tears often occur at the posterolateral margins of the disc (rim lesions). Moreover, the posterior longitudinal ligament reinforces the anulus such that posterior herniations are much less common; otherwise, the disc would herniate into the vertebral canal and compress the spinal cord or its nerve roots.

The most common sites for disc herniation in the cervical region are the C5-C6 and C6-C7 levels, resulting in shoulder and upper limb pain. In the lumbar region the primary sites are the L4-L5 and L5-S1 levels. Lumbar disc herniation is much more common than cervical herniation and results in pain over the sacro-iliac joint, hip, posterior thigh, and leg.

Clinical Focus 2-7 Back Pain Associated with the Zygapophysial (Facet) Joints

Although changes in the vertebral facet joints are not the most common cause of back pain (~15%), such alterations can lead to chronic pain. Although the articular surfaces of the synovial facet joints are not directly innervated, sensory nerve fibers derived from the dorsal rami of spinal nerves do supply the synovial linings of the capsules surrounding the joints. Two examples of painful conditions associated with facet joints are degeneration of the articular cartilage and osteophyte overgrowth of the articular processes.

Clinical Focus 2-8 Low Back Pain

Low back pain, the most common musculoskeletal disorder, can have various causes. Physical examination, although not always revealing a definite cause, may provide clues to the level of spinal nerve involvement and relative sensitivity to pain. The following causes are identified most often:

• Intervertebral disc rupture and herniation

• Nerve inflammation or compression

• Degenerative changes in vertebral facet joints

• Sacro-iliac joint and ligament involvement

• Metabolic bone disease

• Psychosocial factors

• Abdominal aneurysm

• Metastatic cancer

• Myofascial disorders

Movements of the Spine

The essential movements of the spine are flexion, extension, lateral flexion (lateral bending), and rotation (Fig. 2-8). The greatest freedom of movement occurs in the cervical and lumbar spine, with the neck having the greatest range of motion. Flexion is greatest in the cervical region, and extension is greatest in the lumbar region. The thoracic region is relatively stable, as is the sacrum.