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.

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.

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):

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.

Structural Considerations of the Lumbar Intervertebral Discs: Most of what is known about the structure and function of intervertebral discs is based on research performed in the lumbar region.²³ The research focus reflects the region’s disproportionately high frequency of disc degeneration, especially in the lower vertebral segments.

A lumbar intervertebral disc consists of a central nucleus pulposus surrounded by an annulus fibrosus (Figure 9-33). The nucleus pulposus is a pulplike gel located in the mid-to-posterior part of the disc. In youth the nucleus pulposus within the lumbar discs consists of 70% to 90% water.¹⁸⁸ The hydrated nucleus allows the disc to function as a modified hydraulic shock absorption system, capable of continuously dissipating and transferring loads across consecutive vertebrae. The nucleus pulposus is thickened into a gel-like consistency by relatively large branching proteoglycans. Each proteoglycan is an aggregate of many water-binding glycosaminoglycans linked to core proteins (see Chapter 2).^6,66 Dispersed throughout the hydrated proteoglycan mixture are thin type II collagen fibers, elastin fibers, and other proteins. The collagen forms an infrastructure that helps support the proteoglycan network. Very small numbers of chondrocytes and fibrocytes are interspersed throughout the nucleus, ultimately responsible for the synthesis and regulation of the proteins and proteoglycans. In the very young, the nucleus pulposus contains a few chondrocytes that are remnants of the primitive notochord.^169,188

The annulus fibrosus in the lumbar discs consists primarily of 15 to 25 concentric layers, or rings, of collagen fibers.²³ Like dough surrounding jelly in a doughnut, the collagen rings encase and physically entrap the liquid-based central nucleus. The annulus fibrosus contains material and cells similar to what is found in the nucleus pulposus, differing mainly in proportion. In the annulus, collagen makes up about 50% to 60% of the dry weight, as compared with only 15% to 20% in the nucleus pulposus.²³ Abundant elastin protein is interspersed in parallel to the rings of collagen, bestowing an element of circumferential elasticity to the annulus fibrosus.²²⁵

The outermost or peripheral layers of the annulus fibrosus consist primarily of type I and type II collagen.³⁵ This arrangement provides circumferential strength and flexibility to the disc, as well as a means to bond the annulus to the anterior and posterior longitudinal ligaments and to the adjacent rim of the vertebral bodies and endplates. (The outer layers of the annulus fibrosus contain the disc’s only sensory nerves; see innervation of the disc, Chapter 10). The deeper, internal layers of the annulus contain less type I collagen and more water—gradually transforming into tissue with characteristics similar to those of the centrally located nucleus pulposus.²²⁸

Normally, compression forces acting on the disc increase the hydrostatic pressure within the water-logged nucleus pulposus. This rise in and containment of hydrostatic pressure ultimately absorb and evenly distribute loads across the entire intervertebral junction. Fully hydrated and pressurized discs protect not only the interbody joints, but also, indirectly, the apophyseal joints. A dehydrated and thinned disc places disproportionately greater compressive loads on the apophyseal joints. For this reason, some authorities claim that a degenerated disc leads to subsequent arthritis (or arthrosis) of the apophyseal joints.⁹⁷ Some authors, however, argue the opposite cause-and-effect relationship—that the loss of joint space within a degenerated apophyseal joints favors disc degeneration.⁶¹ Both arguments are indeed valid.

The intervertebral discs are very important stabilizers of the spine. This stabilizing function is primarily a result of the structural configuration of the collagen fibers within the annulus fibrosus. As shown in Figure 9-34, most fibers are oriented in a rather precise geometric pattern. In the lumbar region, collagen rings are oriented, on average, about 65 degrees from the vertical, with fibers of adjacent layers traveling in opposite directions.^23,122 This structural arrangement offers significant resistance against intervertebral distraction (vertical separation), shear (sliding), and torsion (twisting).⁸⁵ If the embedded collagen fibers ran nearly vertically, the disc would most effectively resist distraction forces, but not sliding or torsion. In contrast, if all fibers ran nearly parallel to the top of the vertebral body, the disc would most effectively resist shear and torsion, but not distraction. The 65-degree angle likely represents a geometric compromise that permits tensile forces to be applied primarily against the most natural movements of the lumbar spine. Distraction forces are an inherent component of flexion, extension, and lateral flexion, occurring as one vertebral body tips slightly and thus separates from its neighbor. Shear and torsion forces are produced during virtually all movements of the vertebral column. Because of the alternating layering of the annulus, only collagen fibers oriented in the direction of the slide or twist become taut; fibers in every other layer slacken.

In contrast to the lumbar region, the annulus fibrosus in the cervical region does not have complete concentric rings that surround the nucleus.¹³⁰ When a cervical disc is viewed from above, the annulus has a near-crescent shape, thick along the anterior rim and progressively tapering to a very thin layer at the disc’s lateral margins. Little or no annular fibers exist at the region of the uncovertebral joints. A small fissure (or cleft) typically extends horizontally inward from each uncovertebral joint, coursing to the deeper regions of the disc.¹³⁰ Although the function of the fissures is uncertain, they likely increase the freedom of movement within the cervical region. The posterior annulus is separate from the anterior and lateral regions; it is thin and oriented vertically, parallel with the adjacent posterior longitudinal ligament.¹³⁰

Vertebral Endplates: The vertebral endplates in the adult are relatively thin cartilaginous caps of connective tissue that cover most of the superior and inferior surfaces of the vertebral bodies (see Figure 9-33). At birth the endplates are very thick, accounting for about 50% of the height of each intervertebral space. During childhood the endplates function as growth plates for the vertebrae; in the adult the endplates recede and occupy only about 5% of the height of each intervertebral space.¹⁶⁹

The surface of the vertebral endplate that faces the disc is composed primarily of fibrocartilage, which binds directly and strongly to the collagen within the annulus fibrosus (Figure 9-35). This fibrocartilaginous bond forms the primary adhesion between consecutive vertebrae. In contrast, the surface of the endplate that faces the vertebral body is composed primarily of calcified cartilage that is weakly affixed to the bone. This endplate-bone interface is often described as the “weak link” within the interbody joint, often the first component of the interbody joint to fracture under high or repetitive compressive loading.¹³⁶ A perforated or fractured endplate can allow the proteoglycan gel to leak from the nucleus pulposus, causing structural disruption of the disc.^77,162 Such disruption has been shown to cause spinal instability.²²⁷

Only the outer, more peripheral rings of the annulus fibrosus contain blood vessels. For this reason, most of the disc has an inherently limited healing capacity. Essential nutrients, such as glucose and oxygen, must diffuse a great distance to reach the deeper cells that sustain the disc’s low but essential metabolism. The source of these nutrients is in the blood vessels located in the more superficial annulus and, more substantially, blood stored in the adjacent vertebral bodies.⁷⁷ Most of these nutrients must diffuse across the vertebral endplate and through the disc’s extracellular matrix, eventually reaching the cells residing deep in the disc.^62,165 These cells must receive nourishment to manufacture extracellular proteoglycans of the essential quantity and quality. Aged discs, for example, typically show reduced permeability and increased calcification of the vertebral endplates, which, in turn, reduce the flow of nutrients and oxygen into the disc.^25,169 This age-related process can inhibit cellular metabolism and synthesis of proteoglycans. Less proteoglycan content reduces the ability of the nucleus to attract and retain water, thereby limiting its ability to effectively absorb and transfer loads.⁹¹

Intervertebral Disc as a Hydrostatic Pressure Distributor: The vertebral column is the primary support structure for the trunk and upper body. While one stands upright for example, approximately 80% of the load supported by two adjoining lumbar vertebrae is carried through the interbody joint; the remaining 20% is carried by posterior elements, such as apophyseal joints and laminae.⁵

The intervertebral discs are uniquely designed as shock absorbers to protect the bone from excessive pressure that might result from body weight or strong muscle contraction. Compression forces push the endplates inward and toward the nucleus pulposus. Being filled mostly with water and therefore essentially incompressible, the young and healthy nucleus responds by deforming radially and outwardly against the annulus fibrosus (Figure 9-36, A). Radial deformation is resisted by the tension created within the stretched rings of collagen and elastin of the annulus fibrosus (see Figure 9-36, B). Pressure within the entire disc is thus uniformly elevated and transmitted evenly to the adjacent vertebra (see Figure 9-36, C). When the compressive force is removed from the endplates, the stretched elastin and collagen fibers return to their original preloaded length, ready for another compressive force. This mechanism allows compressive forces to be shared by multiple structures, thereby preventing a small spot of high pressure on any single tissue. Because it has viscoelastic properties, the intervertebral disc resists a fast or strongly applied compression more than a slow or light compression.¹⁰¹ The disc therefore can be flexible at low loads and relatively rigid at high loads.

In Vivo Pressure Measurements from the Nucleus Pulposus: In vivo studies have confirmed that pressure within the nucleus pulposus in the lumbar region is relatively low at rest in the supine position.13,140,219 Much larger disc pressures occur from activities that combine forward bending and the need for vigorous trunk muscle contraction. Intradiscal pressures can rise to surprisingly high levels and can produce transient changes in the shape of even the healthy disc. Sustained flexion in the lumbar spine, for example, can reduce the height of the disc slightly as water is slowly forced outward. Sustained and full lumbar extension, in contrast, reduces the pressure in the disc; this allows water to be reabsorbed into the disc, thus reinflating it to its natural level.

In vivo data on pressure within the disc during movement and changes in posture have greatly increased the understanding of ways to reduce injury to the disc. Data produced by two separate studies are compared in Figure 9-37.^138,219 Both studies reinforce three points: (1) disc pressures are large when one holds a load in front of the body, especially when bending forward; (2) lifting a load with knees flexed places less pressure on the lumbar disc than does lifting a load with the knees straight (the latter method typically generating more demands on the back muscles); and (3) sitting in a forward-slouched position produces greater disc pressure than sitting erect. These points serve as the theoretic basis for many educational programs designed for persons with disc degeneration, including disc herniation.

Diurnal Fluctuations in the Water Content within the Intervertebral Discs: When a healthy spine is unloaded, such as during bed rest, the pressure within the nucleus pulposus is relatively low.¹³⁸ This relatively low pressure, combined with the hydrophilic nature of the nucleus pulposus, attracts water into the disc. As a result, the disc swells slightly when one is sleeping. When one is awake and upright, however, weight bearing produces compression forces across the vertebral endplates that push water out of the disc.⁹⁴ The natural cycle of swelling and contraction of the disc produces on average a 1% diurnal variation in overall body height.²⁰¹ The daily variation has a strong inverse relation to age. Karakida and colleagues used magnetic resonance imaging (MRI) to measure the variation in water content in the discs of a group of working persons between the ages of 23 and 56 years old, with no medical history of low-back pain.¹⁰⁰ Remarkably, significant diurnal variation in water content was found only in the discs of persons younger than 35 years of age. These findings are consistent with the fact that the water-retaining capacity of intervertebral discs naturally declines with increasing age.^6,14 The relative dehydration is caused by the parallel, age-related decline in the discs’ proteoglycan content.^158,203

A relatively dehydrated nucleus pulposus exerts less hydrostatic pressure when compressed.²³ Once relatively depressurized, the disc may bulge outward when compressed, similar to a “flat tire.” The older, degenerated intervertebral disc is subsequently less able to uniformly cushion the vertebral body and endplates against compressive loads.¹⁴⁵ As a consequence, disc degeneration increases with age and affects most persons, to varying degrees, over 35 or 40 years of age.^{78,100,158,164,206} A diagnosis of disc degeneration is most effectively made using MRI, typically based on a diminished signal intensity of the T2-weighted image (indicative of reduced water content), loss of distinction between the border of the annulus fibrosus and the nucleus pulposus, nuclear bulging, and loss of disc space.^{78,100,157,164} The MRI scan in Figure 9-38 shows diminished signal intensity between L4-L5 and L5-S1 along with nuclear bulging. Furthermore, a degenerated disc may display circumferential, radial, and peripheral fissures (clefts) within the annulus.⁷⁸ According to Adams, these fissures can often be observed even in young adolescent persons.⁶ Excessive degeneration may also be associated with complete depressurization of the nucleus in conjunction with delamination of the annular fibers and microfractures of the vertebral endplates.⁷⁸ In some cases the internal disruption of the annular fibers may lead to a herniation (prolapse) of the nucleus pulposus (typically posteriorly toward or into the spinal canal). Remarkably, a significant percentage of persons with observable signs of disc degeneration on MRI remain asymptomatic, without experiencing continued mechanical deterioration or loss of function.⁹⁵ The important topic of disc degeneration, including disc herniation, is described in more detail later in this chapter.

Atlanto-occipital Joint: The atlanto-occipital joints provide independent movement of the cranium relative to the atlas. The joints are formed by the protruding convex condyles of the occipital bone fitting into the reciprocally concave superior articular facets of the atlas (Figure 9-40). The congruent convex-concave relationship provides inherent structural stability to the articulation.

Anteriorly, the capsule of each atlanto-occipital joint blends with the anterior atlanto-occipital membrane (Figure 9-41). Posteriorly, the capsule is covered by a thin, broad posterior atlanto-occipital membrane (Figure 9-42). As depicted on the right side of Figure 9-42, the vertebral artery pierces the posterior atlanto-occipital membrane to enter the foramen magnum. This crucial artery supplies blood to the brain.

The concave-convex structure of the atlanto-occipital joints permits angular rotation in two degrees of freedom. The primary motions are flexion and extension. Lateral flexion is slight. Axial rotation is severely restricted and not considered as the third degree of freedom.

Atlanto-axial Joint Complex: The atlanto-axial joint complex has two articular components: a median joint and a pair of laterally positioned apophyseal joints. The median joint is formed by the dens of the axis (C2) projecting through an osseous-ligamentous ring created by the anterior arch of the atlas and the transverse ligament (Figure 9-43). Because the dens serves as a vertical axis for horizontal plane rotation of the atlas, the atlanto-axial joint is often described as a pivot joint.

The median joint within the atlanto-axial joint complex has two synovial cavities. The smaller, anterior cavity is formed between the anterior side of the dens and the posterior border of the anterior arch of the atlas (see Figure 9-43). A small anterior facet on the anterior side of the dens marks this articulation (see Figure 9-20, A). The much larger posterior cavity separates the posterior side of the dens and a cartilage-lined section of the transverse ligament of the atlas. This strong, 2-cm long ligament is essential to the horizontal plane stability of the atlanto-axial articulation.³³ Without its restraint, the atlas (and articulated cranium) can slip anteriorly relative to the axis, possibly damaging the spinal cord.^68,172

The two apophyseal joints of the atlanto-axial joint are formed by the articulation of the inferior articular facets of the atlas with the superior facets of the axis (see exposed right joint in Figure 9-41). The surfaces of these apophyseal joints are generally flat and oriented close to the horizontal plane, a design that maximizes the freedom of axial rotation.

The atlanto-axial joint complex allows two degrees of freedom. About 50% of the total horizontal plane rotation within the craniocervical region occurs at the atlanto-axial joint complex. The second degree of freedom at this joint complex is flexion-extension. Lateral flexion is very limited and not considered a third degree of freedom.

Intracervical Apophyseal Joints (C2 to C7): The facet surfaces within apophyseal joints of C2 to C7 are orientated like shingles on a 45-degree sloped roof, approximately halfway between the frontal and horizontal planes (see Figure 9-18, C2-C3 articulation). This orientation enhances the freedom of movement in all three planes, a hallmark of cervical arthrology.

Osteokinematics of Flexion and Extension: About 120 to 130 degrees of combined flexion and extension occur across the entire craniocervical region. From the neutral position of about 30 to 35 degrees of extension (resting lordosis), the craniocervical region extends some additional 75 to 80 degrees and flexes 45 to 50 degrees (Figures 9-45 and 9-46). Extension exceeds flexion throughout the craniocervical region, on average by a margin of just over 1.5 to 1.

In addition to muscles, connective tissues limit the extremes of craniocervical motion. For example, the ligamentum nuchae and interspinous ligaments provide significant restraint to the extremes of flexion, whereas the approximation of the apophyseal joints limits the extremes of extension.¹⁶³ Flexion is also limited by compression forces from the anterior margin of the annulus fibrosus, whereas extension is limited by the compression forces from the posterior margin of the annulus fibrosus. Additional tissues that limit or restrict sagittal plane motion across the craniocervical region are listed in Table 9-6.

About 20% to 25% of the total sagittal plane motion at the craniocervical region occurs over the atlanto-occipital joint and atlanto-axial joint complex, and the remainder occurs over the apophyseal joints of C2 to C7.¹⁴⁷ The axis of rotation for flexion and extension is directed in a medial-lateral direction through each of the three joint regions: near the occipital condyles at the atlanto-occipital joint, the dens at the atlanto-axial joint complex, and the bodies or adjacent interbody joints of C2 to C7.^34,56

The volume of the cervical vertebral canal is greatest in full flexion and least in full extension.⁸⁷ For this reason, a person with stenosis (narrowing) of the vertebral canal may be more vulnerable to spinal cord injury during hyperextension activities. Repeated episodes of hyperextension-related injuries may lead to cervical myelopathy (from the Greek root myelo, denoting spinal cord, and pathos, suffering) and related neurologic deficits.

Arthrokinematics of Flexion and Extension:

Osteokinematics of Protraction and Retraction: In addition to flexion and extension in the craniocervical region, the head can also translate forward (protraction) and backward (retraction) within the sagittal plane.¹⁴⁸ As indicated in Figure 9-47, from a neutral position, full range of protraction naturally exceeds full range of retraction by about 80% (6.23 cm versus 3.34 cm in the normal adult, respectively).⁵⁸ The neutral position is about 35% forward from the fully retracted position.

Typically, protraction of the head flexes the lower-to-mid cervical spine and simultaneously extends the upper craniocervical region (see Figure 9-47, A). Retraction of the head, in contrast, extends or straightens the lower-to-mid cervical spine and simultaneously flexes the upper craniocervical region (see Figure 9-47, B). In both movements, the lower-to-mid cervical spine follows the translation of the head. Protraction and retraction of the head are physiologically normal and useful motions, often associated with enhancing vision. Prolonged periods of protraction, however, may lead to a chronic forward head posture, causing increased strain on the craniocervical extensor muscles.

Osteokinematics of Axial Rotation: Axial rotation of the head and neck is a very important function, intimately related to vision and hearing. The full range of craniocervical rotation is about 65 to 75 degrees but varies considerably with age.¹⁹⁷ Figure 9-48 shows a young person with about 80 degrees of active rotation to one side, for a total bilateral range of about 160 degrees. With an additional 160 to 170 degrees of total horizontal plane movement of the eyes, her bilateral visual field approaches 330 degrees, with little or no movement of the trunk.

About half the axial rotation of the craniocervical region occurs at the atlanto-axial joint complex, with the remaining throughout C2 to C7.²²⁶ Rotation at the atlanto-occipital joint is restricted because of the deep-seated placement of the occipital condyles within the superior articular facets of the atlas.

Arthrokinematics of Axial Rotation:

Osteokinematics of Lateral Flexion: Approximately 35 to 40 degrees of lateral flexion are available to each side throughout the craniocervical region (Figure 9-49). The extremes of this movement can be demonstrated by attempting to touch the ear to the tip of the shoulder. Most of this movement occurs at the C2 to C7 region; however, about 5 degrees may occur at the atlanto-occipital joint. Lateral flexion at the atlanto-axial joint complex is negligible.

Arthrokinematics of Lateral Flexion:

Kinematics of Flexion and Extension: Approximately 30 to 40 degrees of flexion and 20 to 25 degrees of extension are available throughout the thoracic region. These kinematics are shown in context with flexion and extension over the entire thoracolumbar region in Figures 9-52 and 9-53, respectively. The extremes of flexion are limited by tension in connective tissues located posterior to the vertebral bodies, including the capsule of the apophyseal joints and supraspinous and posterior longitudinal ligaments. The extremes of extension, on the other hand, are limited by tension in the anterior longitudinal ligament and by potential impingement between laminae or between adjacent downward-sloping spinous processes, especially in the upper and middle thoracic vertebrae. The magnitude of thoracic flexion and extension is greater in the extreme caudal regions, in great part because of the “free-floating” most caudal ribs and the shift to a more sagittal plane orientation of the apophyseal joints.^124,182

The arthrokinematics at the apophyseal joints in the thoracic spine are generally similar to those described for the C2 to C7 region. Subtle differences are related primarily to different shapes of the vertebrae, rib attachments, and different spatial orientations of the articular facets of apophyseal joints. Flexion between T5 and T6, for example, occurs by a superior and slightly anterior sliding of the inferior facet surfaces of T5 on the superior facet surfaces of T6 (see Figure 9-52, A). The moderately forward-sloped articular surfaces of the apophyseal joints naturally facilitate flexion throughout the region. Extension occurs by a reverse process (see Figure 9-53, A).

Kinematics of Axial Rotation: Approximately 30 to 35 degrees of horizontal plane (axial) rotation occur to each side throughout the thoracic region. This motion is depicted in conjunction with axial rotation across the entire thoracolumbar region in Figure 9-54. Rotation between T6 and T7, for instance, occurs as the near–frontal plane–aligned inferior articular facets of T6 slide for a short distance against the similarly aligned superior articular facets of T7 (see Figure 9-54, A). The freedom of axial rotation declines in lower regions of the thoracic spine. In this region, the apophyseal joints are slightly more vertically oriented as they shift to a more sagittal plane orientation.^124,182

Kinematics of Lateral Flexion: The predominant frontal plane orientation of the thoracic facet surfaces suggests a relative freedom of lateral flexion. This potential for movement is never fully expressed, however, because of the stabilization provided by the attachments to the ribs. Lateral flexion in the thoracic region is illustrated in context with lateral flexion over the entire thoracolumbar region in Figure 9-55. Approximately 25 to 30 degrees of lateral flexion occur to each side in the thoracic region. As depicted in Figure 9-55, A, lateral flexion of T6 on T7 occurs as the inferior facet surface of T6 slides superiorly on the side contralateral to the lateral flexion and inferiorly on the side ipsilateral to the lateral flexion. Note that the ribs drop slightly on the side of the lateral flexion and rise slightly on the side opposite the lateral flexion.

L1 to L4 Region: The facet surfaces of most lumbar apophyseal joints are oriented nearly vertically, with a moderate-to-strong sagittal plane bias (Figure 9-56). The orientation of the superior articular facet of L2, for example, is on average about 25 degrees from the sagittal plane (see Figure 9-32). This orientation favors sagittal plane motion at the expense of rotation in the horizontal plane.

L5-S1 Junction: As any typical intervertebral junction, the L5-S1 junction has an interbody joint anteriorly and a pair of apophyseal joints posteriorly. The facet surfaces of the L5-S1 apophyseal joints are usually oriented in a more frontal plane than those of other lumbar regions (see Figure 9-56).

The base (top) of the sacrum is naturally inclined anteriorly and inferiorly, forming an approximate 40-degree sacrohorizontal angle when one is standing (Figure 9-58, A).¹⁰⁶ Given this angle, the resultant force resulting from body weight (BW) creates an anterior shear force (BW_S), and a compressive force (BW_C) acting perpendicular to the superior surface of the sacrum. The magnitude of the anterior shear is equal to the product of BW times the sine of the sacrohorizontal angle. A typical sacrohorizontal angle of 40 degrees produces an anterior shear force at the L5-S1 junction equal to 64% of superimposed BW. Increasing the degree of lumbar lordosis enlarges the sacrohorizontal angle, which in effect increases the anterior shear at the L5-S1 junction. If the sacrohorizontal angle were increased to 55 degrees, for example, the anterior shear force would increase to 82% of superimposed BW. During standing or sitting, lumbar lordosis can be increased by this amount by anterior tilting of the pelvis (see Figure 9-63, A). (Tilting the pelvis is defined as a short-arc sagittal plane rotation of the pelvis relative to both femoral heads. The direction of the tilt is indicated by the direction of rotation of the iliac crests.)

Several structures resist the natural anterior shearing force produced at the L5-S1 junction. The wide and strong anterior longitudinal ligament crosses anterior to the L5-S1 junction. The iliolumbar ligament arises from the inferior aspect of the transverse processes of L4-L5 and adjacent fibers of the quadratus lumborum muscle. The ligament attaches inferiorly to the ilium, just anterior to the sacroiliac joint, and to the upper-lateral aspect of the sacrum (see Figure 9-70). Bilaterally, the iliolumbar ligaments provide a firm anchor between the naturally stout transverse processes of L5 and the underlying ilium and sacrum.^23,72,224

In addition to the aforementioned connective tissues, the wide, sturdy articular facets of the L5-S1 apophyseal joints provide bony stabilization to the L5-S1 junction. The near–frontal plane inclination of the facet surfaces is ideal for resisting the anterior shear at this region. This resistance creates a compression force within the L5-S1 apophyseal joints (see blue force vector in Figure 9-58, A, labeled JF). Without adequate stabilization, the lower end of the lumbar region can slip forward relative to the sacrum.⁷⁶ This abnormal, potentially serious condition is known as anterior spondylolisthesis.

Sagittal Plane Kinematics: Flexion and Extension: Although data vary considerably across studies and populations, about 40 to 50 degrees of flexion and 15 to 20 degrees of extension occur at the lumbar spine.144,152,153,198,213 The total 55- to 70-degree arc of sagittal plane motion is substantial, considering it occurs across only five intervertebral junctions. This predominance of sagittal plane motion is largely a result of the prevailing sagittal plane orientation of the facet surfaces of the lumbar apophyseal joints.

Many important and common activities of daily living involve flexion and extension of the midsection of the body, including the hips. Consider, for example, bending forward to touch the ground, ascending steep steps, or getting out of an automobile or a young child transitioning between crawling and sitting. All these activities involve a kinematic interaction among the trunk, lumbar spine, and the pelvis on femurs (hips). As described later in this chapter, this kinematic interaction exists as far cranially as the craniocervical region.

The following section of the chapter focuses on several subtopics within the broad topic of sagittal plane kinematics of the lumbar spine. Box 9-2 lists the order of these subtopics.

Horizontal Plane Kinematics: Axial Rotation: Only about 5 to 7 degrees of horizontal plane rotation occur to each side throughout the lumbar region.151,198 Clinical measurements often exceed this amount, likely because of extraneous motion at the hip joint (pelvis rotating on the femur) and the lower thoracic region.¹⁴⁴ The 5 to 7 degrees of rotation are shown in context with the rotation of the thoracolumbar region in Figure 9-54, B. Axial rotation between L1 and L2 to the right, for instance, occurs as the left inferior articular facet of L1 approximates or compresses against the left superior articular facet of L2. Simultaneously, the right inferior articular facet of L1 separates (distracts) slightly from the right superior articular facet of L2.

The limited amount of axial rotation permitted within the lumbar region is remarkable. Just over 1 degree of unilateral axial rotation has been measured at the L3-L4 intervertebral junction.¹⁸⁹ The relatively strong sagittal plane orientation of the lumbar apophyseal joints physically restrict axial rotation. As indicated in Figure 9-54, B, the apophyseal joints located contralateral to the side of the rotation compress (or approximate), thereby blocking further movement. Much of the actual rotation is accompanied by compression of the articular cartilage within the contralateral apophyseal joint. (Recall that the direction of rotation of any part of the axial skeleton is based on a point on the anterior side of the region, not the spinous process.) Axial rotation is also restricted by tension created in the stretched annulus fibrosus.¹⁰⁷ In theory, an axial rotation of 3 degrees at any lumbar intervertebral junction would damage the articular facet surfaces and tear the collagen fibers in the annulus fibrosus.²³ Most normal physiologic movements remain safely under this potentially damaging limit.

The natural bony resistance to axial rotation in the lumbar region provides vertical stability throughout the lower end of the vertebral column. The well-developed lumbar multifidi muscles and relatively rigid sacroiliac joints reinforce this stability.

Frontal Plane Kinematics: Lateral Flexion: About 20 degrees of lateral flexion occur to each side in the lumbar region.151,198,213 Except for differences in orientation and structure of the apophyseal joints, the arthrokinematics of lateral flexion are nearly the same in the lumbar region as in the thoracic region. Ligaments on the side opposite the lateral flexion limit the motion (see Figure 9-55, B). Normally the nucleus pulposus deforms slightly away from the direction of the movement, or, stated differently, toward the convex side of the bend.²⁰⁰

Sitting Posture and Its Effect on Alignment within the Lumbar and Craniocervical Regions: For many persons a great deal of time is spent sitting—at work, at school, at home, or in a vehicle. The posture of the pelvis during sitting can have a substantial influence on the spinal alignment throughout the vertebral column. The topic of sitting posture therefore has important therapeutic implications on the treatment and prevention of problems throughout the axial skeleton. The following discussion highlights the effects of sagittal plane posturing of the pelvis, specifically as it affects the lumbar and craniocervical regions.

Consider the classic contrast made between “poor” and “ideal” sitting postures (Figure 9-65). In the poor or slouched posture depicted in Figure 9-65, A, the pelvis is posteriorly tilted with a relatively flexed (flattened) lumbar spine. Eventually this posture may lead to adaptive shortening in connective tissues and muscles, ultimately perpetuating the undesirable posture.

A slouched sitting posture increases the external moment arm between the line of force of the upper body and lumbar vertebrae (see red line in Figure 9-65, A). This situation places greater demands on tissues that normally resist flexion of the lower trunk, including the intervertebral discs. As explained earlier in this chapter, in vivo pressure measurements typically demonstrate larger pressures within the lumbar discs in slouched sitting compared with erect sitting.²¹⁹ Even in healthy persons, the increased pressures from the slouched sitting position can deform the nucleus pulposus posteriorly slightly, especially in the L4-L5 and L5-S1 regions.⁹ A habitually slouched sitting posture may, in time, overstretch and thus weaken the posterior annulus fibrosus, reducing its ability to block a posteriorly protruding nucleus pulposus. This biomechanical scenario may be related to the pathogenesis of a significant number of cases of nonspecific low-back pain.⁹ The position of the pelvis and lumbar spine during sitting strongly influences the posture of the axial skeleton as far cranially as the craniocervical region.¹²⁸ On average, the flat posture of the low back is associated with a more protracted position of the craniocervical region (i.e., a “forward head” posture) (see Figure 9-65, A).²² Sitting with the lumbar spine flexed tips the thoracic and lower cervical regions forward slightly, toward flexion. In order to maintain a horizontal visual gaze—such as that typically required to view a computer monitor—the upper craniocervical region must compensate by extending slightly. Over time, this posture may result in adaptive shortening in the small posterior suboccipital muscles (see Chapter 10) and posterior ligaments and membranes associated with the atlanto-axial and atlanto-occipital joints. As depicted in Figure 9-65, B, the ideal sitting posture that includes the natural lordosis (and increased anterior pelvic tilt) extends the lumbar spine. The change in posture at the base (caudal aspect) of the spine has an optimizing influence on the adjacent more cranial segments. The more upright and extended thoracic spine facilitates a more retracted (extended) base of the cervical spine, yielding a more desirable “chin-in” position. Because the base of the cervical spine is more extended, the upper craniocervical region tends to flex slightly to a more neutral posture.

The ideal sitting posture depicted in Figure 9-65, B is difficult for many persons to maintain, especially for several hours at a time. Fatigue often develops in the lumbar extensor muscles. A prolonged, slouched sitting posture may thus be an unavoidable occupational hazard, at least some of the time. In addition to the possible negative effects of a chronically flexed lumbar region, the slouched sitting posture may also increase the muscular stress at the base of the cervical spine. The forward-head posture increases the external flexion torque on the cervical column as a whole, necessitating greater force production from the extensor muscles and local connective tissues. Sitting posture may be improved by a combination of awareness; strengthening and stretching the appropriate muscles; eyeglasses if needed; and ergonomically designed seating, which includes adequate lumbar support.