Axial Skeleton: Osteology and Arthrology

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Chapter 9

Axial Skeleton

Osteology and Arthrology

CHAPTER AT A GLANCE

The skeleton as a whole is divided into the axial skeleton and the appendicular skeleton. The appendicular skeleton consists of the bones of the extremities, including the clavicle, scapula, and pelvis; the axial skeleton, in contrast, consists of the cranium, vertebral column (spine), ribs, and sternum (Figure 9-1). As indicated in Figure 9-1, A, the axial and appendicular skeletons are joined by the sternoclavicular joints superiorly and the sacroiliac joints inferiorly.

The osteology and associated arthrology presented in this chapter focus primarily on the axial skeleton. This focus includes the craniocervical region, vertebral column, and sacroiliac joints, describing how these articulations provide stability, movement, and load transfer throughout the axial skeleton. Muscles play a large role in this function and are the primary focus of Chapter 10.

Disease, trauma, overuse, and normal aging can cause a host of neuromuscular and musculoskeletal problems involving the axial skeleton. Disorders of the vertebral column are often associated with neurologic impairment, primarily because of the close anatomic relationship between neural tissue (spinal cord and nerve roots) and connective tissue (vertebrae and associated ligaments, intervertebral discs, and synovial joints). A “slipped” or herniated disc, for example, can increase pressure on the adjacent neural tissues, resulting in local inflammation and also weakness, sensory disturbances, and reduced reflexes throughout the lower limb. To further complicate matters, certain movements and habitual postures of the vertebral column increase the likelihood of connective tissues impinging on neural tissues. An understanding of the detailed osteology and arthrology of the axial skeleton is crucial to an appreciation of the associated pathomechanics, as well as the rationale for many clinical tests and interventions.

Table 9-1 summarizes the terminology used to describe the relative location or region within the axial skeleton.

TABLE 9-1.

Terminology Describing Relative Location or Region within the Axial Skeleton

Term Synonym Definition
Posterior Dorsal Back of the body
Anterior Ventral Front of the body
Medial None Midline of the body
Lateral None Away from the midline of the body
Superior Cranial Head or top of the body
Inferior Caudal (the “tail”) Tail, or the bottom of the body

The definitions assume a person is in the anatomic position.

OSTEOLOGY

Components within the Axial Skeleton

CRANIUM

The cranium encases and protects the brain and several essential sensory organs (eyes, ears, nose, and vestibular system). Of the many individual bones of the cranium, only the temporal and occipital bones are relevant to the material covered in Chapters 9 and 10.

Temporal and Occipital Bones: Each of the two temporal bones forms part of the lateral external surface of the skull, immediately surrounding and including the external acoustic meatus (Figure 9-2). The mastoid process, an easily palpable structure, is just posterior to the ear. This prominent process serves as an attachment for many muscles, such as the sternocleidomastoid.

The occipital bone forms much of the posterior base of the skull (Figure 9-3). The external occipital protuberance is a palpable midline point, serving as an attachment for the ligamentum nuchae and the medial part of the upper trapezius muscle. The superior nuchal line extends laterally from the external occipital protuberance to the base of the mastoid process of the temporal bone. This thin but distinct line marks the attachments of several extensor muscles of the head and neck, such as the trapezius and splenius capitis muscles. The inferior nuchal line marks the anterior edge of the attachment of the semispinalis capitis muscle.

The foramen magnum is a large circular hole located at the base of the occipital bone, serving as the passageway for the spinal cord. A pair of prominent occipital condyles projects from the anterior-lateral margins of the foramen magnum, forming the convex component of the atlanto-occipital joint. The basilar part of the occipital bone lies just anterior to the anterior rim of the foramen magnum.

VERTEBRAE: BUILDING BLOCKS OF THE SPINE

In addition to providing vertical stability throughout the trunk and neck, the vertebral column protects the spinal cord, ventral and dorsal nerve roots, and exiting spinal nerve roots (Figure 9-4). The relationship between the spinal cord and exiting nerve roots throughout the entire vertebral column is schematically shown in Figure III-1 in Appendix III, Part A.

The midthoracic vertebrae demonstrate many of the essential anatomic and functional characteristics of any given vertebra (Figure 9-5). As a general orientation, a given vertebra can be subdivided into three sections. Anteriorly is the large vertebral body—the primary weight-bearing component of a vertebra. Posteriorly are the transverse and spinous processes, laminae, and articular processes, collectively referred to as posterior elements (also referred to as the “vertebral arch” or “neural arch”). The pedicles, the third section, act as bridges that connect the body with the posterior elements. Thick and strong, the pedicles transfer muscle forces applied to the posterior elements forward, for dispersion across the vertebral body and intervertebral discs. Table 9-2 provides greater details on the structure and function of the components of a typical midthoracic vertebra.

TABLE 9-2.

Major Parts of a Midthoracic Vertebra

Part Description Primary Function
Body Large cylindric mass of trabecular bone lined by a thin cortex of bone. The multidirectional trabecular core is lightweight while still offering excellent resistance against compression. Primary weight-bearing structure of each vertebra.
Intervertebral disc Thick ring of fibrocartilage between vertebral bodies of C2 and below. Shock absorber and spacer throughout the vertebral column.
Interbody joint A cartilaginous joint joint formed between the superior and inferior surfaces of an intervertebral disc and adjacent vertebral bodies. Primary bond between vertebrae.
Pedicle Short, thick dorsal projection of bone from the mid-to-superior part of the vertebral body. Connects the vertebral body to the posterior elements of a vertebra.
Lamina Thin vertical plate of bone connecting the base of the spinous process to each transverse process. (The term laminae refers to both right and left laminae.) Protects the posterior aspect of the spinal cord.
Vertebral canal Central canal located just posterior to the vertebral body. The canal is surrounded by the pedicles and laminae. Houses and protects the spinal cord.
Intervertebral foramen Lateral opening between adjacent vertebrae. Passageway for spinal nerve roots exiting the vertebral canal.
Transverse process Horizontal projection of bone from the junction of a lamina and a pedicle. Attachments for muscles, ligaments, and ribs.
Costal facets (on body) Rounded impressions formed on the lateral sides of the thoracic vertebral bodies. Most thoracic vertebral bodies have partial superior and inferior facets (called demifacets). Attachment sites for the heads of ribs (costocorporeal joints).
Costal facets (on transverse process) Oval facets located at the anterior tips of most thoracic transverse processes. Attachment sites for the articular tubercle of ribs (costotransverse joints).
Spinous process Dorsal midline projection of bone from the laminae. Midline attachments for muscles and ligaments.
Superior and inferior articular processes, including articular facets and apophyseal joints Paired vertical articular processes arising from the junction of a lamina and pedicle. Each process has smooth cartilage-lined articular facets. In general, superior articular facets face posteriorly, and inferior articular facets face anteriorly. Superior and inferior articular facets form paired apophyseal joints. These synovial joints guide the direction and magnitude of intervertebral movement.

RIBS

Twelve pairs of ribs enclose the thoracic cavity, forming a protective cage for the cardiopulmonary organs. The posterior end of a typical rib has a head, a neck, and an articular tubercle (Figure 9-6). The head and tubercle articulate with a thoracic vertebra, forming two synovial joints: costocorporeal (also called costovertebral) and costotransverse, respectively (see Figure 9-5, B).188 These joints anchor the posterior end of a rib to its corresponding vertebra. A typical costocorporeal joint connects the head of a rib to a pair of costal demifacets that span two adjacent vertebrae and the intervening intervertebral disc. A costotransverse joint connects the articular tubercle of a rib with a costal facet on the transverse process of a corresponding vertebra.

The anterior end of a rib consists of flattened hyaline cartilage. Ribs 1 through 10 attach either directly or indirectly to the sternum, thereby completing the thoracic rib cage anteriorly. The cartilage of ribs 1 to 7 attaches directly to the lateral border of the sternum via seven sternocostal joints (Figure 9-7). The cartilage of ribs 8 to 10 attaches to the sternum by fusing to the cartilage of the immediately superior rib. Ribs 11 and 12 do not attach to the sternum but are anchored by lateral abdominal muscles.

STERNUM

The sternum is slightly convex and rough anteriorly, and slightly concave and smooth posteriorly. The bone has three parts: the manubrium (Latin, meaning “handle”), the body, and the xiphoid process (from the Greek, “sword”) (see Figure 9-7). Developmentally, the manubrium fuses with the body of the sternum at the manubriosternal joint, a cartilaginous (synarthrodial) articulation that often ossifies later in life.188 Just lateral to the jugular notch of the manubrium are the clavicular facets of the sternoclavicular joints. Immediately inferior to the sternoclavicular joint is a costal facet that accepts the head of the first rib at the first sternocostal joint.

The lateral edge of the body of the sternum is marked by a series of costal facets that accept the cartilages of ribs 2 to 7. The arthrology of the sternocostal joints is discussed in greater detail in Chapter 11, within the context of ventilation. The xiphoid process is attached to the inferior end of the body of the sternum by the xiphisternal joint. Like the manubriosternal joint, the xiphisternal joint is connected primarily by fibrocartilage. The xiphisternal joint often ossifies by 40 years of age.188

Vertebral Column as a Whole

The vertebral (spinal) column consists of the entire set of vertebrae. The word “trunk” is a general term that describes the body of a person, including the sternum, ribs, and pelvis but excluding the head, neck, and limbs.

The vertebral column usually consists of 33 vertebral bony segments divided into five regions. Normally there are seven cervical, twelve thoracic, five lumbar, five sacral, and four coccygeal segments. The sacral and coccygeal vertebrae are usually fused in the adult, forming individual sacral and coccygeal bones. Individual vertebrae are abbreviated alphanumerically; for example, C2 for the second cervical, T6 for the sixth thoracic, and L1 for the first lumbar. Each region of the vertebral column (e.g., cervical and lumbar) has a distinct morphology that reflects its specific function and movement potential. Vertebrae located at the cervicothoracic, thoracolumbar, and lumbosacral junctions often share characteristics that reflect the transition between major regions of the vertebral column. It is not uncommon, for example, for the transverse processes of C7 to have thoracic-like facets to accept a rib, or L5 may be “sacralized” (i.e., fused with the base of the sacrum).

NORMAL CURVATURES WITHIN THE VERTEBRAL COLUMN

The human vertebral column consists of a series of reciprocal curvatures within the sagittal plane (Figure 9-8, A). These natural curvatures contribute to “ideal” spinal posture while one is standing. The curvatures also define the neutral position of the different regions of the spine. In the neutral (anatomic) position, the cervical and lumbar regions are naturally convex anteriorly and concave posteriorly, exhibiting an alignment called lordosis, meaning to “bend backward.” The degree of lordosis is usually less in the cervical region than in the lumbar region. The thoracic and sacrococcygeal regions, in contrast, exhibit a natural kyphosis. Kyphosis describes a curve that is concave anteriorly and convex posteriorly. The anterior concavity provides space for the organs within the thoracic and pelvic cavities.

The natural curvatures within the vertebral column are not fixed but are dynamic and change shape during movements and adjustment of posture. Further extension of the vertebral column accentuates the cervical and lumbar lordosis but reduces the thoracic kyphosis (see Figure 9-8, B). In contrast, flexion of the vertebral column decreases, or flattens, the cervical and lumbar lordosis but accentuates the thoracic kyphosis (see Figure 9-8, C). In contrast, the sacrococcygeal curvature is fixed, being concave anteriorly and convex posteriorly.

The embryonic vertebral column is kyphotic throughout its length. Lordosis in the cervical and lumbar regions occurs after birth, in association with motor maturation and the assumption of a more upright posture. In the cervical spine, extensor muscles pull on the head and neck as the prone-lying infant begins to observe the surroundings. More caudally, the developing hip flexor muscles pull inferiorly on the front of the pelvis as the infant starts walking. This muscular pull rotates (or tilts) the pelvis anteriorly relative to the hips, thereby positioning the lumbar spine into a relative lordosis. Once the child stands, the natural lordosis of the lumbar spine directs the body’s line of gravity through or near the first lumbar vertebra (L1) and the base of the sacrum.

The sagittal plane curvatures within the vertebral column provide strength and resilience to the axial skeleton. A reciprocally curved vertebral column acts like an arch. Compression forces between vertebrae are partially shared by tension in stretched connective tissues and muscles located along the convex side of each curve. As is true with long bones such as the femur, the strength and stability of the vertebral column are derived, in part, from its ability to “give” slightly under a load, rather than to support large compression forces statically.

A potentially negative consequence of the natural spinal curvatures is the presence of shear forces at regions of transition between curves. Shear forces can cause premature loosening of surgical spinal fusions, especially those performed in the cervicothoracic and thoracolumbar regions.

LINE OF GRAVITY PASSING THROUGH THE BODY

Although highly variable, the line of gravity acting on a standing person with ideal posture passes near the mastoid process of the temporal bone, anterior to the second sacral vertebra, just posterior to the hip, and anterior to the knee and ankle (Figure 9-9). In the vertebral column, the line of gravity typically falls just to the concave side of the apex of each region’s curvature. Ideal posture therefore allows gravity to produce a torque that helps maintain the optimal shape of the spinal curvatures. The external torque attributed to gravity is greatest at the apex of each region: C4 and C5, T6, and L3.

The image depicted in Figure 9-9 is more ideal than real because each person’s posture is unique and transient. Factors that alter the spatial relationship between the line of gravity and the spinal curvatures include fat deposition, the specific shapes of the regional spinal curvatures, static posturing of the head and the limbs, muscle strength, connective tissue extensibility, and the position and magnitude of loads supported by the body. The particular orientation of the line of gravity relative to the axial skeleton has important biomechanical consequences on the stress placed on the region. For example, gravity passing posterior to the lumbar region produces a constant extension torque on the low back, facilitating natural lordosis. Alternatively, gravity passing anterior to the lumbar region produces a constant flexion torque. In both cases the external torque created by the line of gravity (and its associated external moment arm) must be neutralized by forces and torques produced actively by muscle and passively by connective tissues. In extreme postures, these forces may be high; if prolonged, they may lead to undesirable postural compensations as well as structural changes, often associated with pain.

Strictly anatomic factors can influence the unique shape of the spinal curves throughout the vertebral column; these include wedged-shaped intervertebral discs or vertebral bodies, spatial orientation of apophyseal (facet) joints, tension in ligaments, and the degree of natural muscle stiffness. The intervertebral discs in the cervical and lower lumbar regions are slightly thicker anteriorly, for example, thereby favoring an anterior convexity in these regions.

The normal sagittal plane alignment of the vertebral column may be altered by disease, such as ankylosing spondylosis, poliomyelitis, spinal cord injury, muscular dystrophy, or osteoporosis and muscle weakness associated with advanced age. Often, relatively minor forms of abnormal or deviated postures occur in otherwise healthy persons. As illustrated in Figure 9-10, excessive lumbar lordosis may develop as compensation for excessive thoracic kyphosis and vice versa. The “swayback” posture shown in Figure 9-10, C, for example, describes a combined exaggerated lumbar lordosis and thoracic kyphosis. Often, other unexplainable postures exist such as the “rounded back” appearance in Figure 9-10, E. This posture shows a combined excessive thoracic kyphosis with reduced lumbar lordosis. Regardless of the cause or location of the postural deviation, the associated abnormal curvatures alter the spatial relation between the line of gravity and each spinal region. When severe, abnormal vertebral curvatures increase stress on muscles, ligaments, bones, discs, apophyseal joints, and exiting spinal nerve roots. Abnormal curves also change the volume of body cavities. An exaggerated thoracic kyphosis, for example, can significantly reduce the space for the lungs to expand during deep breathing.

LIGAMENTOUS SUPPORT OF THE VERTEBRAL COLUMN

The vertebral column is supported by an extensive set of ligaments. Spinal ligaments limit motion, help maintain natural spinal curvatures, and, by stabilizing the spine, protect the delicate spinal cord and spinal nerve roots. These ligaments, described in the following paragraphs and illustrated in Figure 9-11, all possess slightly different strengths and functions depending on their locations within the vertebral column.85 The basic structure and generic function of each ligament are summarized in Table 9-3.

The ligamentum flavum originates on the anterior surface of one lamina and inserts on the posterior surface of the lamina below. Consisting of a series of paired ligaments, the ligamenta flava (plural) extend throughout the vertebral column, situated immediately posterior to the spinal cord. The ligamenta flava and adjacent laminae form the posterior wall of the vertebral canal.

Ligamentum flavum literally means “yellow ligament,” reflecting its high content of light yellow elastic connective tissue. Histologically, the ligamentum flavum consists of about 80% elastin and 20% collagen.223 The tissue’s highly elastic nature is ideal for exerting a relatively constant, although modest, resistance throughout a wide range of flexion.85 Measurements have shown that between the neutral position and full flexion, the ligamentum flavum experiences an approximately 35% increase in strain (elongation) (Figure 9-12).139 Extreme and very forceful flexion beyond this length can ultimately lead to its rupture, possibly creating damaging compressive forces on the anterior side of the intervertebral disc.4 The ligamenta flava are thickest in the lumbar region,188 where the magnitude of intervertebral flexion is the largest of any region within the vertebral column.

The highly elastic nature of the ligamentum flavum is interesting from both a functional and a structural perspective. In addition to providing gradual resistance to the full range of flexion, its inherent elasticity also exerts a small but constant compression force between vertebrae, even in the neutral position.23 The elasticity may prevent the ligament from buckling inward during full extension. Such a buckling, or in-folding, might otherwise pinch and possibly injure the adjacent spinal cord.

The interspinous ligaments fill much of the space between adjacent spinous processes. The deeper, more elastin-rich fibers blend with the ligamenta flava; the more superficial fibers contain more collagen, and blend with the supraspinous ligaments.223 The fiber direction and organization of the interspinous ligaments vary from region to region.88 The interspinous ligaments in the lumbar region, for example, fan in an oblique posterior-cranial direction (see Figure 9-11, A). Fibers in this region are drawn taut only at the more extremes of flexion.

As evident by their name, the supraspinous ligaments attach between the tips of the spinous processes. As with the interspinous ligaments, these ligaments resist separation of adjacent spinous processes, thereby resisting flexion.85 The ability to resist flexion is greatest in regions of the vertebral column where these structures are more robust and contain a greater proportion of collagen. Throughout the lumbar region, for example, the ligaments are not extensively developed; they are either sparse (especially between L4 and L5) or partially replaced by strands of thoracolumbar fascia or small musculotendinous fibers.23,86,188 Not surprisingly, therefore, the supraspinous ligaments within the lumbar region are typically the first structures to rupture in extreme flexion.5

In the cervical region the supraspinous ligaments are very well developed and extend cranially as the ligamentum nuchae. This tough membrane consists of a bilaminar strip of fibroelastic tissue that attaches between the cervical spinous processes and external occipital protuberance.188 Passive tension in a stretched ligamentum nuchae adds a small but useful means of support for the head and neck.53 The ligamentum nuchae also provides a midline attachment for muscles, such as the trapezius and splenius capitis and cervicis. A prominent ligamentum nuchae accounts for some of the difficulty often encountered in palpating the spinous process in the mid to upper cervical region (Figure 9-13).

The intertransverse ligaments are poorly defined, thin or membranous structures that extend between adjacent transverse processes.188 These tissues become taut in contralateral lateral flexion and, to a lesser degree, forward flexion.

The anterior longitudinal ligament is a long, strong, straplike structure attaching to the basilar part of the occipital bone and the entire length of the anterior surfaces of all vertebral bodies, including the sacrum. The deeper fibers blend with and reinforce the anterior sides of the intervertebral discs.188 The anterior longitudinal ligament becomes taut in extension and slack in flexion.85 In the cervical and lumbar regions, tension in the anterior longitudinal ligament helps limit the degree of natural lordosis. This ligament is narrow at its cranial end but widens as it courses caudally.

The posterior longitudinal ligament is a continuous band of connective tissue that attaches the entire length of the posterior surfaces of the vertebral bodies, between the axis (C2) and the sacrum. The posterior longitudinal ligament is located within the vertebral canal, immediately anterior to the spinal cord (see Figure 9-11, A). (It is important to note that the posterior and anterior longitudinal ligaments are named according to their relationship to the vertebral body, not the spinal cord.) Throughout its length, the deeper fibers of the posterior longitudinal ligament blend with and reinforce the posterior side of the intervertebral discs.188 Cranially, the posterior longitudinal ligament is a broad structure, narrowing as it descends toward the lumbar region. The slender lumbar portion limits its ability to restrain a posterior bulging (or herniated) disc. As with most ligaments of the vertebral column, the posterior longitudinal ligament becomes increasingly taut with flexion.85

Capsular ligaments of the apophyseal joints consist mostly of collagen fibers that attach along the rim of the facet surfaces (see Figure 9-11, A). As will be described in an upcoming section on arthrology, apophyseal joints help interconnect and stabilize the intervertebral junctions. Equally important is their unique role in guiding the specific direction of intervertebral movement. Sensory mechanoreceptors embedded within the capsule likely provide muscles information to assist with this guidance.36 The apophyseal joint capsules are reinforced by adjacent muscles (multifidus) and ligamenta flava, most notably in the lumbar region.188

The capsular ligaments of the apophyseal joints are strong, capable of supporting up to 1000 N (225 lb) of tension before failure.48 The capsular ligaments are relatively loose (lax) in the neutral position but become increasingly taut as the joint approaches the extremes of all its movements. Passive tension is greatest in motions that create the largest translation or separation between joint surfaces. These kinematics are highly specific to the particular region of the vertebral column and will be revisited in subsequent sections of this chapter.

In closing, with the possible exception of the capsular ligaments of the apophyseal joints, knowledge of a ligament’s location relative to the axis of rotation within a given intervertebral junction provides major insight into its primary functions. As will be further described in an upcoming section, the axis of rotation for intervertebral movement is near or through the region of the vertebral body. When sagittal plane movement is considered, for example, any ligament located posterior to the vertebral body is stretched during flexion. Conversely, any ligament located anterior to the vertebral body is stretched during extension. As noted by reviewing Figure 9-11, A, all ligaments except the anterior longitudinal ligament would become taut in flexion.

Regional Osteologic Features of the Vertebral Column

The adage that “function follows structure” is very applicable to the study of the vertebral column. Although all vertebrae have a common morphologic theme, each also has a specific shape that reflects its unique function. The following section, along with Table 9-4, highlights specific osteologic features of each region of the vertebral column.

CERVICAL REGION

The cervical vertebrae are the smallest and most mobile of all movable vertebrae. The high degree of mobility is essential to the large range of motion required by the head. Perhaps the most unique anatomic feature of the cervical vertebrae is the presence of transverse foramina located within the transverse processes (Figure 9-14). The important vertebral artery ascends through this foramen, coursing toward the foramen magnum to transport blood to the brain and spinal cord. In the neck, the vertebral artery is located immediately anterior to the exiting spinal nerve roots (see Figure 9-4).

The third through the sixth cervical vertebrae show nearly identical features and are therefore considered typical of this region. The upper two cervical vertebrae, the atlas (C1) and the axis (C2), and the seventh cervical vertebra (C7) are atypical for reasons described in a subsequent section.

Typical Cervical Vertebrae (C3 to C6): C3 to C6 have small rectangular bodies made of a relatively dense and strong cortical shell.199 The bodies are wider from side to side than front to back (Figures 9-14 and 9-15). The superior and inferior surfaces of the bodies are not as flat as those of most other vertebrae but are curved or notched. The superior surfaces are concave side to side, with raised lateral hooks called uncinate processes (uncus means “hook”). The inferior surfaces, in contrast, are concave anterior-posterior, with elongated anterior and posterior margins. When articulated, small, synovial-lined uncovertebral joints form between the uncinate process and adjacent part of the superior vertebra between C3 and C7. Uncovertebral joints are often called the “joints of Luschka,” named after the person who first described them.84 The exact function of uncovertebral joints is unclear, although they likely facilitate the kinematics of cervical motion. Clinically these joints become important when osteophytes form around their margins, often reducing the size of the adjacent intervertebral foramen. If large, these osteophytes may impinge on and irritate exiting cervical spinal nerve roots, thereby causing neurologic symptoms.

SPECIAL FOCUS 9-1   imageCervical Osteophytes Causing Neurologic Symptoms in the Upper Extremity: One Possible Consequence of a Degenerated Intervertebral Disc

Fully hydrated and normal intervertebral discs naturally act as “spacers” between individual vertebrae. One benefit of this function is to partially unload the nearby uncovertebral joints. Lacking substantial articular cartilage, the relatively small uncovertebral joints are not designed to support large forces. Figure 9-16 illustrates how a healthy, full disc located between C3 and C4 creates a small protective gap between the adjacent C3-C4 uncovertebral joint. Figure 9-16 also shows how a degenerated and thinned disc between C4 and C5 increases the compression force at the C4-C5 uncovertebral joint. Over time, the increased compression can stimulate growth of an osteophyte (“bone spur”). The osteophyte is shown compressing elements of the C5 spinal nerve root, which may, over time, lead to neurologic symptoms, including radiating (radicular) pain, muscle weakness, or altered sensations throughout the nerve’s peripheral distribution, typically down the lateral aspect of the arm. In an indirect way, healthy intervertebral discs protect not only the surrounding bone, but also the nerve roots.

The pedicles of C3 to C6 are short and curved posterior-lateral (see Figure 9-14). Very thin laminae extend posterior-medially from each pedicle (Figure 9-17). The triangular vertebral canal is large in the cervical region in order to accommodate the thickening of the spinal cord associated with the formation of the cervical plexus and brachial plexus.

Within the C3 to C6 region, consecutive superior and inferior articular processes form a continuous articular “pillar,” interrupted by apophyseal joints (Figure 9-18). The articular facets within each apophyseal joint are smooth and flat, with joint surfaces oriented midway between the frontal and horizontal planes. The superior articular facets face posterior and superior, whereas the inferior articular facets face anterior and inferior.

The spinous processes of C3 to C6 are short, with some processes being bifid (i.e., double) (see Figure 9-14, C3). The transverse processes are short lateral extensions that terminate as variably shaped anterior and posterior tubercles. The tubercles are unique to the cervical region, serving as attachments for muscles, such as the anterior scalene, levator scapulae, and splenius cervicis.

Atypical Cervical Vertebrae (C1, C2, and C7):

Atlas (C1): As indicated by the name, the primary function of the atlas is to support the head. Possessing no body, pedicle, lamina, or spinous process, the atlas is essentially two large lateral masses joined by anterior and posterior arches (Figure 9-19, A). The short anterior arch has an anterior tubercle for attachment of the anterior longitudinal ligament. The much larger posterior arch forms nearly half the circumference of the entire atlantal ring. A small posterior tubercle marks the midline of the posterior arch. The lateral masses support the prominent superior articular processes, which in turn support the cranium.

The large and concave superior articular facets of the atlas generally face cranially, in a position to accept the large, convex occipital condyles. The inferior articular facets are generally flat to slightly concave. These facet surfaces generally face inferiorly, with their lateral edges sloped downward, approximately 20 degrees from the horizontal plane (see Figure 9-19, B). The atlas has large, palpable transverse processes, usually the most prominent of the cervical vertebrae. These transverse processes serve as attachment points for several small but important muscles that control fine movements of the cranium.

Axis (C2): The axis has a large, tall body that serves as a base for the upwardly projecting dens (odontoid process) (Figure 9-20). Part of the elongated body is formed from remnants of the body of the atlas and the intervening disc. The dens provides a rigid vertical axis of rotation for the atlas and head (Figure 9-21). Projecting laterally from the body is a pair of superior articular processes (see Figure 9-20, A). These large processes have slightly convex superior articular facets that are oriented about 20 degrees from the horizontal plane, matching the slope of the inferior articular facets of the atlas. Projecting from the prominent superior articular processes of the axis are a pair of stout pedicles and a pair of very short transverse processes (see Figure 9-20, B). A pair of inferior articular processes projects inferiorly from the pedicles, with inferior articular facets facing anteriorly and inferiorly (see Figure 9-18). The spinous process of the axis is bifid and very broad. The palpable spinous process serves as an attachment for many muscles, such as the semispinalis cervicis.

“Vertebra Prominens” (C7): C7 is the largest of all cervical vertebrae, having many characteristics of thoracic vertebrae. C7 can have large transverse processes, as illustrated in Figure 9-15. A hypertrophic anterior tubercle on the transverse process may sprout an extra cervical rib, which may impinge on the brachial plexus. This vertebra also has a large spinous process, characteristic of other thoracic vertebrae (see Figure 9-18).

THORACIC REGION

Typical Thoracic Vertebrae (T2 to T9): The second through the ninth thoracic vertebrae usually demonstrate similar features (see T6 and T7 in Figure 9-5). Pedicles are directed posteriorly from the body, making the vertebral canal narrower than in the cervical region. The large transverse processes project posterior-laterally, each containing a costal facet that articulates with the tubercle of the corresponding rib (costotransverse joint). Short, thick laminae form a broad base for the downward-slanting spinous processes.

The superior and inferior articular facets in the thoracic region are oriented vertically with a slight forward pitch (Figure 9-22). The superior articular facets face generally posterior; the inferior articular facets face generally anterior. Once articulated, the superior and inferior facets form apophyseal joints, which are aligned relatively close to the frontal plane.

Each of the heads of ribs 2 through 9 typically articulates with a pair of costal demifacets that span one thoracic intervertebral junction (see pair of costal demifacets for the eighth rib in Figure 9-22). As described earlier, these articulations are called costocorporeal joints. A thoracic (intercostal) spinal nerve exits through a corresponding thoracic intervertebral foramen, located just anterior to the apophyseal joints.

Atypical Thoracic Vertebrae (T1 and T10 to T12): The first and usually last three thoracic vertebrae are considered atypical mainly because of the particular manner of rib attachment. T1 has a full costal facet superiorly that accepts the entire head of the first rib, and a demifacet inferiorly that accepts part of the head of the second rib (see Figure 9-18). The spinous process of T1 is especially elongated and often as prominent as the spinous process of C7. Although variable, the bodies of T10 through T12 may have a single, full costal facet for articulation with the heads of the tenth, eleventh, and twelfth ribs, respectively. T10 to T12 usually lack costotransverse joints.

LUMBAR REGION

Lumbar vertebrae have massive wide bodies, suitable for supporting the entire superimposed weight of the head, trunk, and arms (Figure 9-23). The total mass of the five lumbar vertebrae is approximately twice that of all seven cervical vertebrae.

For the most part, the lumbar vertebrae possess similar characteristics. Laminae and pedicles are short and thick, forming the posterior and lateral walls of the nearly triangular vertebral canal. Transverse processes project almost laterally; those associated with L1 to L4 are thin and tapered; however, the transverse processes of L5 are short, thick, and strong. Spinous processes are broad and rectangular, projecting horizontally from the junction of each lamina (Figure 9-24). This shape is strikingly different from the pointed, sloped spinous processes of the thoracic region. Short mammillary processes project from the posterior surfaces of each superior articular process. These structures serve as attachment sites for the multifidi muscles.

The articular facets of the lumbar vertebrae are oriented nearly vertically. The superior articular facets are moderately concave, facing medial to posterior-medial. As depicted in Figure 9-23, the superior facet surfaces in the upper lumbar region tend to be oriented closest with the sagittal plane, and the superior facet surfaces in the mid-to-lower lumbar region are oriented approximately midway between the sagittal and frontal planes. The inferior articular facets are reciprocally matched to the shape and orientation of the superior articular facets. In general, the inferior articular facets are slightly convex, facing generally lateral to anterior-lateral (see Figure 9-24).

The inferior articular facets of L5 articulate with the superior articular facets of the sacrum. The resulting L5-S1 apophyseal joints are typically oriented much closer to the frontal plane than the other lumbar articulations. The L5-S1 apophyseal joints provide an important source of anterior-posterior stability to the lumbosacral junction.

SPECIAL FOCUS 9-2   imageDevelopmental Anomalies of the Lumbar Apophyseal Joints

At birth the articular surfaces of the apophyseal joints within the lumbar spine are oriented very close to the frontal plane, similar to most thoracic apophyseal joints. Between birth and about 11 or 12 years of age, however, the orientation within all but the lower lumbar apophyseal joints gradually transforms to their final adult position biased slightly closer to the sagittal plane (Figure 9-25).23,167 The slow structural transformation is governed by different rates of ossification within the articular processes. Bogduk describes the possibility that this transformation may be influenced by the developing upright posture of the child and the demands placed on certain muscles, such as the lumbar multifidi.23 Although the apophyseal joints continue to grow throughout adolescence, their spatial orientation is essentially established before the teenage years.

Natural variations in the development of the lumbar apophyseal joints in childhood can create structural variations that persist into adulthood. Although variations can be extreme, most are relatively minor, such as a slight bilateral asymmetry between the right and left articular surfaces of the joints. (An example of this asymmetry is evident by comparing the superior articular facets of the lumbar vertebrae illustrated in Figure 9-23.) Slight bilateral asymmetries exist in about 20% to 30% of all adult lumbar vertebrae, although they are likely inconsequential.23 In more extreme cases, however, the bilateral asymmetry could create uneven stress and instability throughout the intervertebral junctions.47 Although evidence is mixed, the increased stress could potentially predispose a person to premature degeneration in the apophyseal joints or intervertebral discs.63

SACRUM

The sacrum is a triangular bone with its base facing superiorly and apex inferiorly (Figure 9-26). An important function of the sacrum is to transmit the weight of the vertebral column to the pelvis. In childhood, each of five separate sacral vertebrae is joined by a cartilaginous membrane. By adulthood, however, the sacrum has fused into a single bone, which still retains some anatomic features of generic vertebrae.

The anterior (pelvic) surface of the sacrum is smooth and concave, forming part of the posterior wall of the pelvic cavity (see Figure 9-26). Four paired ventral (pelvic) sacral foramina transmit the ventral rami of spinal nerve roots that form much of the sacral plexus. The dorsal surface of the sacrum is convex and rough due to the attachments of muscle and ligaments (Figure 9-27). Several spinal and lateral tubercles mark the remnants of fused spinous and transverse processes, respectively. Four paired dorsal sacral foramina transmit the dorsal rami of sacral spinal nerve roots.

The superior surface of the sacrum shows a clear representation of the body of the first sacral vertebra (Figure 9-28). The sharp anterior edge of the body of S1 is called the sacral promontory. The triangular sacral canal houses and protects the cauda equina. Pedicles are very thick, extending laterally as the ala (lateral wings) of the sacrum. Stout superior articular processes have superior articular facets that face generally posterior-medially. These facets articulate with the inferior facets of L5 to form L5-S1 apophyseal joints (see Figure 9-27). The large auricular surface articulates with the ilium, forming the sacroiliac joint. The sacrum narrows caudally to form its apex, a point of articulation with the coccyx.

COCCYX

The coccyx is a small triangular bone consisting of four fused vertebrae (see Figure 9-27). The base of the coccyx joins the apex of the sacrum at the sacrococcygeal joint. The joint has a fibrocartilaginous disc and is held together by several small ligaments. The sacrococcygeal joint usually fuses late in life. In youths, small intercoccygeal joints persist; however, these typically are fused in adults.188

ARTHROLOGY

Typical Intervertebral Junction

The typical intervertebral junction has three functional components: (1) the transverse and spinous processes, (2) the apophyseal joints, and (3) an interbody joint (Figure 9-29). The spinous and transverse processes provide mechanical outriggers, or levers, that increase the mechanical leverage of muscles and ligaments. Apophyseal joints are primarily responsible for guiding intervertebral motion, much as railroad tracks guide the direction of a train. As will be emphasized, the geometry, height, and spatial orientation of the articular facets within the apophyseal joints greatly influence the prevailing direction of intervertebral motion.

Interbody joints connect an intervertebral disc with a pair of vertebral bodies. The primary function of these joints is to absorb and distribute loads across the vertebral column. Normally, at least in the lumbar region, the interbody joint accepts the overwhelming majority of weight that is borne through the intervertebral junction. As indicated in Figure 9-29, flexion of the spine shifts an even greater proportion of the superimposed body weight forward to the interbody joint. In addition, the interbody joints provide the greatest source of adhesion between vertebrae,85 serve as the approximate axes of rotation, and function as deformable intervertebral spacers. As spacers, the intervertebral discs constitute about 25% of the total height of the vertebral column. The functional importance of the space created by a healthy intervertebral disc cannot be overstated. The greater the relative intervertebral space, the greater the ability of one vertebral body to “rock” forward and backward on another, for example. Without any disc space, the nearly flat bone-on-bone interface between two consecutive bodies would block rotation in the sagittal and frontal planes—allowing only tipping or translation. Finally, the space created by the intervertebral discs provides adequate passage for the exiting spinal nerve roots.

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