Applied Anatomy of the Spine

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CHAPTER 2 Applied Anatomy of the Spine

The spine is a segmental column of similar formed bones that constitutes the major subcranial part of the axial skeleton. Its individual elements are united by a series of intervertebral articulations that form a flexible, although neuroprotective, support to the trunk and limbs. The spinal column typically consists of 33 vertebrae. The mobile section of the spine comprises 7 cervical, 12 thoracic, and 5 lumbar vertebrae; 5 fused vertebrae form the inflexible sacrum that offers a relatively rigid connection to the innominate bones. Caudad to the sacrum, four or five irregular ossicles compose the coccyx.

Vertebrae

The movements of the spine involve 97 diarthroses (i.e., synovial joints, having substantial motion) and an even greater number of amphiarthroses (i.e., fibrocartilaginous joints, having less motion). The individual vertebrae bear multiple processes and surface markings that indicate the attachments of the numerous ligaments that stabilize these articulations. Despite an appreciable degree of regional variation of these characteristics, the embryologically homologous segmental origin of the spine provides a basic uniformity so that a single generalized description can be applied to the basic morphology of all but the most superior and inferior elements.

The typical vertebra consists of two major components: a roughly cylindrical ventral mass of mostly trabecularized cancellous bone, called the body, and a denser, more cortical posterior structure, called the dorsal vertebral arch. The vertebral bodies vary considerably in size and sectional contour but exhibit no salient processes or unique external features other than the facets for rib articulation in the thoracic region. In contrast, the vertebral arch has a more complex structure. It is attached to the dorsolateral aspects of the body by two stout pillars, called the pedicles. These are united dorsally by a pair of arched flat laminae that are surmounted in the midline by a dorsal projection, called the spinous process. The pedicles, laminae, and dorsum of the body form the vertebral foramen, a complete osseous ring that encloses the spinal cord.

The transverse processes and the superior and inferior articular processes are found near the junction of the pedicles and the laminae. The transverse processes extend laterally from the sides of the vertebral arches, and because all vertebrae are phyletically and ontogenetically associated with some form of costal element, they either articulate with or incorporate a rib component. In the thoracic spine, the costal process persists as a rib proper. In the cervical spine, the costal process becomes the anterior part of the transverse process that encloses the vertebral artery foramen, and in the lumbar spine it becomes the mature transverse process; the immature posterior (neural arch) component becomes the mammillary process.

The articular processes (zygapophyses) form the paired diarthrodial articulations (facet joints) between the vertebral arches. The superior processes (prezygapophyses) always bear an articulating facet, whose surface is directed dorsally to some degree, whereas the complementary inferior articulating processes (postzygapophyses) direct their articulating surfaces ventrally. Variously shaped bony prominences (mammillary processes or parapophyses) may be found lateral to the articular processes and serve in the multiple origins and insertions of the spinal muscles.

The superoinferior dimensions of the pedicles are roughly half that of their corresponding body, so that in their lateral aspect the pedicles and their articulating processes form the superior and inferior vertebral notches. Because the base of the pedicle arises superiorly from the dorsum of the body, particularly in the lumbar spine, the inferior vertebral notch appears more deeply incised. In the articulated spine, the opposing superior and inferior notches form the intervertebral foramina that transmit the neural and vascular structures between the corresponding levels of the spinal cord and their developmentally related body segments.

Pars Interarticularis

The pars interarticularis defines the parts of the arch that lie between the superior and inferior articular facets of all subatlantal movable vertebral elements (Fig. 2–1). The term pars interarticularis arose to designate that area of the arch that is most stressed by translational movement between adjacent segments, particularly in the second cervical and fifth lumbar vertebrae, which are susceptible to traumatic and stress fractures in this region (i.e., hangman’s fracture of C21 and isthmic spondylolysis of L5). In sequential alternation with the intervertebral facet joints, it roofs the lateral recesses of the spinal canal and contributes to the dorsal margins of the intervertebral foramina. In the subcervical vertebrae, it also provides the dorsal part of the base of the transverse process.

Biomechanical forces on the pars interarticularis place it in a position to receive the shearing stresses that occur when translational (spondylolisthetic) forces tend to displace, in a dorsoventral plane, the superior articular processes with respect to their inferior counterparts on the same vertebra. The usual site of failure in the pars interarticularis permits the superior articular facets, pedicles, and vertebral body to be ventrally displaced as a unit, while the inferior articular facets remain attached to the dorsal arch components. These tend to retain their articular relationships with the superior facets of the next lower vertebra.

In the case of the second cervical vertebra (axis) there is a unique anterior relationship of its superior articular facets with the more posteriorly positioned inferior processes that elongates the C2 pars interarticularis. As this offset area receives the greatest leverage between the “cervicocranium” and the lower cervical spine, the indicated line in the illustration in Figure 2–2 shows the common site of mechanical failure in hyperextension injuries to the upper cervical spine.

In the case of the lumbar vertebrae, the pars interarticularis has been subdivided further. McCulloch and Transfelt2 referred to the “lateral buttress,” which they believed offered particular structural support to the intervening structures. They described it as the bony bridge that connects the superolateral edge of the inferior facet to the junction of the transverse process and the pedicle. In a follow-up anatomic study, Weiner and colleagues3 measured the surface area of the lateral buttress in human cadaveric lumbar spines. They found the greatest areas (about 80 mm2) from L1 to L3, whereas area averaged 50 mm2 at L4 and only 15 mm2 at L5. These investigators thought that the broadness of the buttress in the upper lumbar spine can obscure or confuse landmarks for placement of pedicle screws, and its relative thinness (or nonexistence) in the lower lumbar spine can be a predisposing factor to stress fractures or iatrogenic injury to the pars interarticularis.

Regional Characteristics

Although the 24 vertebrae of the presacral spine are divided into three distinct groups (Fig. 2–3), in which the individual members may be recognized by one or two uniquely regional features, there is a gradual craniocaudal progression of morphologic changes. The vertebrae found above and below the point of regional demarcation are transitional and bear some of the characteristics of both areas.

Cervical Vertebrae

Of the seven cervical vertebrae, the first two (Fig. 2–4A to D) and the last require special notation, but the third to the sixth are fairly uniform, and a common description suffices (Fig. 2–4E and F). Because the cervical vertebrae bear the least weight, their bodies are relatively small and thin with respect to the size of the vertebral arch and vertebral foramen. In addition, their diameter is greater transversely than in the anteroposterior direction. The lateral edges of the superior surface of each body are sharply turned upward to form the uncinate processes that are characteristic of the cervical region. The most obvious diagnostic feature of the cervical vertebrae is the transverse foramina that perforate the transverse processes and transmit the vertebral arteries. The anterior part of the transverse processes represents fused costal elements that arise from the sides of the body. The lateral extremities of the transverse processes bear two projections, the anterior and posterior tubercles. The former serve as origins of anterior cervical muscles; the latter provide origins and insertions for posterior cervical muscles. A deep groove between the upper aspects of the tubercles holds the cervical spinal nerves.

The cervical pedicles connect the posterior vertebral arch to the vertebral body. Anatomic studies have shown that the cervical pedicle height ranges from 5.1 to 9.5 mm, and width ranges from 3 to 7.5 mm.4,5 The pedicle is angled medially between 90 and 110 degrees.5

The superior and inferior articular processes appear as obliquely sectioned surfaces of short cylinders of bone that, when united with the adjacent vertebrae, form two osseous shafts posterolateral to the stacked vertebral bodies. The cervical vertebrae present a tripod of flexible columns for the support of the head. As in the upper cervical spine, the combination of the articular processes and the intervening bone is often referred to as the lateral mass in the subaxial region. It is a common site for screw insertion during internal fixation of the cervical spine.6

The laminae are narrow and have a thinner superior edge. At their mid-dorsal junction, they bear a bifid spinous process that receives the insertions of the semispinalis cervicis muscles. The height of the lamina of C4 is 10 to 11 mm, whereas the lamina thickness at C5 is about 2 mm.7 The lamina is thickest at T2, where it measures an average of 5 mm.

Atlantoaxial Complex

The first two cervical vertebrae are structurally and developmentally different. Together, they form a complex articular system that permits the nutational (i.e., nodding) and rotational movements of the head. The first cervical vertebra, or atlas, is a bony ring consisting of an anterior and a posterior arch, which are connected by the two lateral masses. It has all the homologous features of a typical vertebra with the exception of the body. The lateral masses correspond to the combined pedicles and articular pillars of the lower cervical vertebrae, but the superior and inferior articular facets are concave. The superior articular surfaces face upward and internally to receive the occipital condyles of the skull, whereas the inferior articulating surfaces face downward and internally to rotate on the sloped “shoulders” of the axis. This slope helps prevent lateral translation while permitting rotation.

The posterior arch consists of modified laminae that are more round than flat in their sectional aspect and a posterior tubercle that represents an attenuated spinous process that gives origin to suboccipital muscles. Immediately behind the lateral masses on the superior surface of the posterior arch of C1, two smooth grooves house the vertebral arteries as they penetrate the posterior atlanto-occipital membrane. These arteries take a tortuous course from the transverse processes of the atlas, making an almost 90-degree turn medially as they exit the foramen and a subsequent 90-degree turn superiorly to enter the dura and subsequently the foramen magnum. This second turn occurs more than 1.5 to 2 cm lateral to the midline, risking injury to the structure with surgical dissection beyond this point.

The anterior arch forms a short bridge between the anterior aspects of the lateral masses. It bears an anterior tubercle that is the site of insertion of the longus colli muscle. On the posterior surface of the anterior arch, a semicircular depression marks the synovial articulation of the odontoid process. Internal tubercles on the adjacent lateral masses are the attachment sites of the transverse atlantal ligaments that hold the odontoid against this articular area.

The second cervical vertebra, or axis, provides a bearing surface on which the atlas may rotate. Its most distinctive characteristic is the vertically projecting odontoid process that serves as a pivotal restraint against horizontal displacements of the atlas. This bony prominence represents the phyletically purloined centrum of the first cervical vertebra. It exhibits a slight constriction at its neck and an anterior facet for its articulation with the anterior arch of the atlas. Posteriorly, a groove in the neck of the odontoid marks the position of the strong transverse atlantal ligament.

The apex of the odontoid process is slightly pointed. It is the attachment site of the apical ligament. Posterior to the apex, two lateral roughened prominences indicate the attachments of the alar ligaments. These structures and the apical ligament connect the odontoid process to the base of the skull at the basion, the anterior aspect of the foramen magnum. The superior articulating surfaces of the axis are convex and are directed laterally to receive the lateral masses of the atlas. The inferior articulating surfaces are typical of those of the cervical vertebrae and serve as the start of the articular columns. The transverse processes of the axis are directed downward. Anteriorly, the inferior aspect of the body of the axis forms a liplike process that descends over the first intervertebral disc and the body of the third cervical vertebra.

The seventh cervical vertebra is transitional. The inferior surface of its body is proportionately larger than the superior surface. It has a long, distinct spinous process that is usually easily palpable (the vertebra prominens). The superior and inferior articulating facets are more steeply inclined and presage the form of these structures in the thoracic region. Blunt transverse processes have heavy posterior struts and much lighter anterior struts that surround transverse foramina that are often bilaterally unequal and seldom pass the vertebral arteries. Frequently, one or both of the anterior struts realize their true potential as a costal element and develop into a cervical rib.

Thoracic Vertebrae

All 12 thoracic vertebrae support ribs and have facets for the diarthrodial articulations of these structures. The first and last four have specific peculiarities in the manner of costal articulations, but the second to the eighth are similar (Fig. 2–4G and H).

The body of a mid-thoracic vertebra is heart-shaped. Its length and width are roughly halfway between that of the cervical and lumbar bodies. Often a flattening of the left side of the body indicates its contact with the descending aorta. In the mid-thorax, the heads of the ribs form a joint that spans the intervertebral disc, so that the inferior lip of the body of one vertebra and the corresponding site of the superior lip of the infrajacent element share in the formation of a single articular facet for the costal capitulum. The typical thoracic vertebra bears two demifacets on each side of its body. The thoracic vertebral arch encloses a small, round vertebral foramen that would not admit the tip of an index finger, even when the specimen is from a large adult. This limited space for the spinal cord predisposes to severe spinal cord injury with minimal dimensional compromise.

Because the pedicles arise more superiorly on the dorsum of the body than they do in the cervical region, the inferior vertebral notch forms an even greater contribution to the intervertebral foramen. The pedicle height increases from T1 to T12, but the transverse pedicle width (which is more critical for transpedicular screw containment) does not follow this same craniocaudal pattern.8 Cinotti and colleagues9 found that the pedicles in the T4 to T8 region had the smallest transverse diameter. Scoles and colleagues10 documented similar findings in 50 cadaveric human spines, with the smallest diameters measured at T3 to T6. On average, the transverse pedicle diameter at T3 is 3.4 mm in women and 3.9 mm in men. At T6, it averages 3 mm in women and 3.5 mm in men. At T1, however, the mean diameter is 6.4 mm in women and 7.3 mm in men.

The superior articular facets form a stout shelflike projection from the junction of the laminae and the pedicles. Their ovoid surfaces are slightly convex, are almost vertical, and are coronal in their plane of articulation. They face dorsally and slightly superolaterally, and in bilateral combination they present the segment of an arc whose center of radius lies at the anterior edge of the vertebral body. They permit a slight rotation around the axis of this radius. The inferior articular facets are borne by the inferior edges of the laminae. The geometry of their articular surfaces is complementary to the superior processes.

On the ventral side of the tip of the strong transverse processes, another concave facet receives the tuberculum of the rib whose capitulum articulates with the superior demifacet of the same vertebra. The spinous processes of the thoracic vertebrae are long and triangular in section. The spinous processes of the upper four thoracic vertebrae are more bladelike and are directed downward at an angle of about 40 degrees from the horizontal. The middle four thoracic spinous processes are longer but directed downward at an angle of 60 degrees, so that they completely overlap the adjacent lower segment. The lower four resemble the upper four in direction and shape.

The first thoracic vertebra has a complete facet on the side of its body for the capitulum of the first rib and an inferior demifacet for the capitulum of the second rib. The costal articulations of the 9th to 12th thoracic vertebrae are confined to the sides of the bodies of their respective segments. On the last two thoracic vertebrae, transitional characteristics are evident in the diminution of the transverse processes and their failure to buttress the last two ribs. Because the ribs are disconnected from the sternum, they are frequently referred to as “floating ribs.”

Lumbar Vertebrae

The lumbar vertebrae are the lowest five vertebrae of the presacral column (see Fig. 2–4I and J). All their features are expressed in more massive proportions. They are easily distinguished from other regional elements by their lack of a transverse foramen or costal articular facets. The body is large, having a width greater than its anteroposterior diameter, and is slightly thicker anteriorly than posteriorly. All structures associated with the vertebral arch are blunt and stout. The thick pedicles are widely placed on the dorsolaterosuperior aspects of the body, and with their laminae they enclose a triangular vertebral foramen. Although the inferior vertebral notch is deeper than the superior, both make substantial contributions to the intervertebral foramen. The transverse processes are flat and winglike in the upper three lumbar segments, but in the fifth segment they are thick, rounded stumps. The fourth transverse process is usually the smallest.

Aside from their relative size, the lumbar vertebrae can be recognized by their articular processes. The superior pair arise in the usual manner from the junction of the pedicles and laminae, but their articular facets are concave and directed dorsomedially, so that they almost face each other. The inferior processes are extensions of the laminae that direct the articulating surfaces ventrolaterally and lock themselves between the superior facets of the next inferior vertebra in an almost mortise-and-tenon fashion. This arrangement restricts rotation and translation in the lumbar region. The lumbar segments also have pronounced mammillary processes, which are points of origin and insertion of the thick lower divisions of the deep paraspinal muscles.

Sacral Vertebrae

The sacrum consists of five fused vertebrae that form a single triangular complex of bone that supports the spine and forms the posterior part of the pelvis (Figs. 2–5 and 2–6). It is markedly curved and tilted backward, so that its first element articulates with the fifth lumbar vertebra at a pronounced angle (the sacrovertebral angle).

Close inspection of the flat, concave ventral surface and the rough, ridged convex dorsal surface reveals that, despite their fusion, all the homologous elements of typical vertebrae are still evident in the sacrum. The heavy, laterally projecting alae that bear the articular surfaces for articulation with the pelvis are fused anterior costal and posterior transverse processes of the first three sacral vertebrae. These lateral fusions require that separate dorsal and ventral foramina provide egress for the anterior and posterior divisions of the sacral nerves. The ventral four pairs of sacral foramina are larger than their dorsal counterparts because they must pass the thick sacral contributions to the sciatic nerve. The ventral surface of the sacrum is relatively smooth. There are four transverse ridges that mark the fusions of the vertebral bodies and enclose remnants of the intervertebral discs. Lateral to the bodies of the second, third, and fourth elements, the ridges of bone that separate the anterior sacral foramina are quite prominent and give origin to the piriformis muscle.

The dorsal aspect of the sacrum is convex, rough, and conspicuously marked by five longitudinal ridges. The central one, the middle sacral crest, is formed by the fusion of the spinous processes of the sacral vertebrae. On either side, a sacral groove separates it from the medial sacral articular crest that represents the fused articular process. The superior ends of these crests form the functional superior articular processes of the first sacral vertebra, which articulate with the inferior processes of the fifth lumbar vertebra. They are very strong, and their facets are directed dorsally to resist the tendency of the fifth lumbar vertebra to be displaced forward. Inferiorly, the articular crests terminate as the sacral cornua, two rounded projections that bracket the inferior hiatus where it gives access to the sacral vertebral canal. More laterally, the lateral crests and sacral tuberosities form uneven elevations for the attachments of the dorsal sacroiliac ligaments.

The sacrum and its posterior ligaments lie ventral to the posterior iliac spines and form a deep depression that accommodates, and gives origin to, the inferior parts of the paraspinal muscles. The grooves between the central spinous crest and the articular crests are occupied by the origins of the multifidus muscles. Dorsal and lateral to these are attached the origins of the iliocostal and iliolumbar muscles.

Arthrology of the Spine

The articulations of the spine include the three major types of joints: synarthroses, diarthroses, and amphiarthroses (Figs. 2-7 to 2-9). The synarthroses are found during development and the first decade of life. The best examples are the neurocentral joints of the immature spine, which are the two unions between the centers of ossification for the two halves of the vertebral arch and that of the centrum. Until they are obliterated during the 2nd decade, they possess a thin plate cartilage between the two apposed bony surfaces. Another example is the early union between the articular processes of the sacral vertebrae, known as ephemeral synchondroses.

The diarthroses are true synovial joints, formed mostly by the facet joints and costovertebral joints, but also include the atlantoaxial and sacroiliac articulations. All the spinal diarthroses are of the arthrodial or gliding type, with the exception of the trochoid or pivot joint of the atlantodens articulation.

The amphiarthroses are nonsynovial, slightly movable connective tissue joints. They are of two types: the symphysis, as exemplified by the fibrocartilage of the intervertebral disc, and the syndesmosis, as represented by all the ligamentous connections between the adjacent bodies and the adjacent arches.

Articulations of the Vertebral Arches

The synovial facet joints formed by the articular processes of the vertebral arches possess a true joint capsule and are capable of a limited gliding articulation. The capsules are thin and lax and are attached to the bases of the engaging superior and inferior articulating processes of opposing vertebrae. Because it is mostly the plane of articulation of these joints that determines the types of motion characteristic of the various regions of the spine, it would be expected that the fibers of the articular capsules would be longest and loosest in the cervical region and become increasingly taut in an inferior progression.

The syndesmoses between the vertebral arches are formed by the paired sets of ligamenta flava, the intertransverse ligaments, the interspinous ligaments, and the unpaired supraspinous ligament. The ligamenta flava bridge the spaces between the laminae of adjacent vertebrae from the second cervical to the lumbosacral interval. The lateral extent of each half of a paired set begins around the bases of the articulating processes and can be traced medially where they nearly join in the midline. This longitudinal central deficiency serves to transmit small vessels and facilitates the passage of a needle during lumbar punctures. The fibers of the ligamenta flava are almost vertical in their disposition, but are attached to the ventral surface of the cephalad lamina and to the superior lip of the suprajacent lamina.

This shinglelike arrangement conceals the true length of the ligaments because of the overlapping of the superior lamina. Their morphology is best appreciated from the ventral aspect as in Figure 2–9B. The yellow elastic fibers that give the ligamenta flava their name maintain their elasticity even in embalmed specimens. It has been stated in some texts that the elasticity of the ligamenta flava serves to assist in the maintenance of the erect posture. A more probable reason for this property is simply to keep the ligament taut during extension, where any laxity would permit redundancy and infolding toward the ventrally related nervous structures, as occurs in degenerative lumbar spinal stenosis.

There are two separable layers of the ligamentum flavum, one superficial and one deep, that have distinct attachments to the inferior lamina.11 The superficial component inserts at the classically described location along the posterosuperior aspect of the lamina. The deep component inserts along the anterosuperior surface of the lamina.11 This attachment can have significance during surgical removal of the ligamentum flavum for exposure of the neural elements.

The intertransverse ligaments are fibrous connections between the transverse processes. They are difficult to distinguish from extensions of the tendinous insertions of the segmental muscles and in reality may be just that in some regions. They appear as a few tough, thin fibers between the cervical transverse processes, and in the thoracic area they blend with the intercostal ligaments. Being most distinct between the lumbar transverse processes, the intertransverse ligaments may be isolated here as membranous bands.

The interspinous ligaments (see Fig. 2–9A) are membranous sets of fibers that connect adjoining spinous processes. They are situated medial to the thin pairs of interspinal muscles that bridge the apices of the spine. The fibers of the ligaments are arranged obliquely as they connect the base of the superior spine with the superior ridge and apex of the next most inferior spinous process. These midline ligaments are found in pairs with a distinct dissectible cleft between them.

The supraspinous ligament (see Fig. 2–9A) is a continuous fibrous cord that runs along the apices of the spinous processes from the seventh cervical to the end of the sacral spinous crest. Similar to the longitudinal ligaments of the vertebra, the more superficial fibers of the ligament extend over several spinal segments, whereas the deeper, shorter fibers bridge only two or three segments. In the cervical region the supraspinous ligament assumes a distinctive character and a specific name, the ligamentum nuchae. This structure is bowstrung across the cervical lordosis from the external occipital protuberance to the spine of the seventh cervical vertebra. Its anterior border forms a sagittal fibrous sheet that divides the posterior nuchal muscles and attaches to the spinous processes of all cervical vertebrae. The ligamentum nuchae contains an abundance of elastic fibers. In quadrupeds, it forms a strong truss that supports the cantilevered position of the head.

Special Articulations

The atlanto-occipital articulation consists of the diarthrosis between the lateral masses of the atlas and the occipital condyles of the skull and the syndesmoses formed by the atlanto-occipital membranes. The articular capsules around the condyles are thin and loose and permit a gliding motion between the condylar convexity and the concavity of the lateral masses. The capsules blend laterally with ligaments that connect the transverse processes of the atlas with the jugular processes of the skull. Although the lateral ligaments and the capsules are sufficiently lax to permit nodding, they do not permit rotation.

The anterior atlanto-occipital membrane is a structural extension of the anterior longitudinal ligament that connects the forward rim of the foramen magnum, also known as the basion, to the anterior arch of the atlas and blends with the joint capsules laterally. It is dense, tough, and virtually cordlike in its central portion.

The posterior atlanto-occipital membrane is homologous to the ligamenta flava and unites the posterior arch of the atlas. It is deficient laterally where it arches over the groove on the superior surface of the arch. Through this aperture, the vertebral artery enters the neural canal to penetrate the dura. Occasionally, the free edge of this membrane is ossified to form a true bony foramen (called the ponticulus posticus) around the artery.

The median atlantoaxial articulation is a pivot (trochoid) joint (Figs. 2–10 and 2–11). The essential features of the articulation are the odontoid process (dens) of the axis and the internal surface of the anterior arch of the atlas. The opposition of the two bones is maintained by the thick, straplike transverse atlantal ligament. The ligament and the arch of the atlas have true synovial cavities intervening between them and the odontoid process. Alar expansions of the transverse ligament attach to tubercles on the lateral rims of the anterior foramen magnum, and a single, unpaired cord, the apical odontoid ligament, attaches the apex of the process to the basion. The entire joint is covered posteriorly by a cranial extension of the posterior longitudinal ligament, which is named tectorial membrane in this region. Because the atlas freely glides over the superior articulating facets of C2, the atlantoaxial pivot is essential for preventing horizontal displacements between C1 and C2. Fracture of the odontoid or, less likely, rupture of the transverse ligament produces a very unstable articulation.

Articulations of the Vertebral Bodies

The vertebral bodies are connected by the two forms of amphiarthroses. Symphyses are represented by the intervertebral discs, and syndesmoses are formed by the anterior and posterior longitudinal ligaments.

Nucleus Pulposus

Typically, the nucleus pulposus occupies an eccentric position within the confines of the anulus, usually being closer to the posterior margin of the disc. Its most essential character becomes obvious in either transverse or sagittal preparations of the disc in which, as evidence of internal pressure, it bulges beyond the plane of section. Palpation of a dissected nucleus from a young adult shows that it responds as a viscid fluid under applied pressure, but it also exhibits considerable elastic rebound and assumes its original physical state on release. These properties may still be shown in the spine of a cadaver that has been embalmed for many months.

Histologic analysis provides a partial explanation for the characteristics of the nucleus. As the definitive remnant of the embryonic notochord, it is similarly composed of loose, delicate fibrous strands embedded in a gelatinous matrix. In the center of the mass, these fibers show no geometric preference in their arrangement but form a felted mesh of undulating bundles. Only the fibers that are in approximation to the vertebral chondral plates display a definite orientation. These approach the cartilage at an angle and become embedded in its substance to afford an attachment for the nucleus. Numerous cells are suspended in the fibrous network. Many of these are fusiform and resemble typical reticulocytes, but vacuolar and darkly nucleated chondrocytes are also interspersed in the matrix. Even in the absence of vascular elements, the profusion of cells should accentuate the fact that the nucleus pulposus is composed of vital tissue. There is no definite structural interface between the nucleus and the anulus. Rather, the composition of the two tissues blends imperceptibly.

Anulus Fibrosus

The anulus is a concentric series of fibrous lamellae that encase the nucleus and strongly unite the vertebral bodies (Fig. 2–12). The essential function of the nucleus is to resist and redistribute compressive forces within the spine, whereas one of the major functions of the anulus is to withstand tension, whether the tensile forces be from the horizontal extensions of the compressed nucleus, from the torsional stress of the column, or from the separation of the vertebral bodies on the convex side of a spinal flexure. Without optical aid, simple dissection and discernment reveals how well the anulus is constructed for the performance of this function.

On horizontal section, it is noted that an individual lamella encircling the disc is composed of glistening fibers that run an oblique or spiral course in relation to the axis of the vertebral column. Because the disc presents a kidney-shaped or heart-shaped horizontal section, and the nucleus is displaced posteriorly, these lamellae are thinner and more closely packed between the nucleus and the dorsal aspect of the disc. The bands are stoutest and individually more distinct in the anterior third of the disc, and here when transected they may give the impression that they are of varying composition because every other ring presents a difference in color and elevation with reference to the plane of section. Teasing and inspection at an oblique angle shows in the freed lamellae, however, that this difference is due to an abrupt change in the direction of the fibers of adjacent rings. Previous descriptions of the anulus have claimed that the alternating appearance of the banding is the result of the interposition of a chondrous layer between each fibrous ring.13 In reality, the alternations of glistening white lamellae with translucent rings result from differences in the incidence of light with regard to the direction of the fiber bundles. This repeated reversal of fiber arrangement within the anulus has implications in the biomechanics of the disc, which are discussed later.

The disposition of the lamellae on sagittal section is not consistently vertical. In the regions of the anulus approximating the nucleus pulposus, the first distinct bands curve inward, with their convexity facing the nuclear substance. As one follows the successive layers outward, a true vertical profile is assumed, but as the external laminae of the disc are approached, they may again become bowed, with their convexity facing the periphery of the disc.14,15

The attachment of the anulus to its respective vertebral bodies warrants particular mention. This attachment is best understood when a dried preparation of a thoracic or lumbar vertebra is examined first. In the adult, the articular surface of the body presents two aspects: a concave central depression that is quite porous and an elevated ring of compact bone that appears to be rolled over the edge of the vertebral body. Often a demarcating fissure falsely suggests that the ring is a true epiphysis of the body, but postnatal studies of ossification have indicated that it is a traction apophysis for the attachment of the anulus and associated longitudinal ligaments.16

In life, the depth of the central concavity is filled to the level of the marginal ring by the presence of a cribriform cartilaginous plate. In contrast to other articular surfaces, there is no closing plate of compact osseous material intervening between this cartilage and the cancellous medullary part of the bone. The trabeculations of the spongiosa blend into the internal face of the chondrous plate, whereas fibers from the nucleus and inner lamellae of the anulus penetrate its outer surface. As intimate as this union between the central disc and vertebra may appear, the outer bony ring affords the disc its firmest attachment because the stoutest external lamellar bands of fibers actually penetrate the ring as Sharpey fibers. Scraping the disc to the bone shows the concentric arrangements reflecting the different angles at which the fibers insert (see Fig. 2–12). The fibers of the outermost ring of the anulus have the most extensive range of attachment. They extend beyond the confines of the disc and blend with the vertebral periosteum and the longitudinal ligaments.

Regional Variations of the Disc

The discs in aggregate make up approximately one fourth of the length of the spinal column, exclusive of the sacrum and coccyx. Their degree of contribution is not uniform in the various regions. According to Aeby,17 the discs provide more than one fifth of the length of the cervical spine, approximately one fifth of the length of the thoracic column, and approximately one third of the length of the lumbar region.

The discs are smallest in the cervical spine. Their lateral extent is less than that of the corresponding vertebral body because of the uncinate processes (Fig. 2–13). Here, as in the lumbar region, they are wedge-shaped, the greatest width being anterior, producing lordosis. The thoracic discs are heart-shaped on section, with the nucleus pulposus being more centrally located than in the lumbar region. The thickness and the horizontal dimensions of the thoracic disc increase caudad with the corresponding increase in size of the vertebral bodies. The normal thoracic kyphosis results from a disparity between the anterior and posterior heights of the vertebral bodies because the discs are of uniform thickness. The lumbar discs are reniform and are relatively and absolutely the thickest in the spine. The progressive caudal increase in the degree of lumbar lordosis is due to the equivalent increase in the differential between the anterior and posterior thickness of the disc.

The cervical intervertebral discs have been a source of controversy because of the so-called joints of Luschka, or uncovertebral joints. These articular modifications are found on both sides of the cervical discs as oblique, cleftlike cavities between the superior surfaces of the uncinate processes and the corresponding lateral lips of the interior articular surface of the next superior vertebra. Because they initially appear in the latter part of the first decade and are not universally demonstrable in all cervical spines, or even in all subaxial discs of the same cervical spine, it is preferable to call them “accommodative joints” that have developed in response to the shearing stresses of the torsions of cervical mobility (see Fig. 2–13).

Spinal Ligaments

Anterior Longitudinal Ligament

The anterior longitudinal ligament is a strong band of fibers that extends along the ventral surface of the spine from the skull to the sacrum. It is narrowest and cordlike in the upper cervical region, where it is attached to the atlas and axis and their intervening capsular membranes. It widens as it descends the column to the extent, in the lower lumbar region, of covering most of the anterolateral surfaces of the vertebral bodies and discs before it blends into the presacral fibers. The anterior longitudinal ligament is not uniform in its composition or manner of attachment. Its deepest fibers, which span only one intervertebral level, are covered by an intermediate layer that unites two or three vertebrae and a superficial stratum that may connect four or five levels. Where the ligament is adherent to the anterior surface of the vertebra, it also forms its periosteum. It is most firmly attached to the articular lip at the end of each body. It is most readily elevated at the point of its passage over the midsection of the discs, where it is loosely attached to the connective tissue band that encircles the anulus (Fig. 2–14).

Posterior Longitudinal Ligament

The posterior longitudinal ligament differs considerably from its anterior counterpart with respect to the clinical significance of its relationships to the intervertebral disc. Similar to the anterior ligament, it extends from the skull to the sacrum, but it is within the vertebral canal. Its central fiber bundles diminish in breadth as the size of the spinal column increases. The segmental denticulate configuration of the posterior longitudinal ligament is one of its most characteristic features. Between the pedicles, particularly in the lower thoracic and lumbar regions, it forms a thick band of connective tissue that is not adherent to the posterior surface of the vertebral body. Instead, it is bowstrung across the concavity of the dorsum of the body. The large vascular elements enter and leave the medullary sinus located beneath its fibers.

In approximating the dorsum of the disc, the posterior longitudinal ligament displays two strata of fibers. The superficial, longer strands form a distinct strong strap whose filaments bridge several vertebral elements. A second, deeper stratum spans only two vertebral articulations and forms lateral curving extensions of fibers that pass along the dorsum of the disc and out through the intervertebral foramen. These deeper intervertebral expansions of the ligament have the most significant relationship with the disc.

These fibers are most firmly fixed at the margins of their lateral expansions. This produces a central rhomboidal area of loose attachment, or in some cases an actual fascial cleft of equivalent dimensions, on the dorsolateral aspect of the disc. At dissection, this characteristic may be readily shown by inserting a blunt probe beneath the intervertebral part of the longitudinal ligament and exploring the area to define the margins of the space where the fibers are strongly inserted (Fig. 2–15). This situation is particularly pertinent to problems involving dorsal or dorsolateral prolapse of the nucleus pulposus. With a dorsocentral protrusion of a semifluid mass, the strong midline strap of posterior longitudinal fibers tends to restrain the herniation. If an easily dissectible cleft offers a space for lateral expansion, however, the mass can extend to either side, dissecting the loose attachments.

Trabeculations of connective tissue bind the dura to the dorsal surface of the posterior longitudinal ligament. This attachment is firmest along the lateral edges. Numerous venous cross connections of the epidural sinuses pass between the dura and the ligament. The venous elements are the most ubiquitous structures among the components related to the vertebral articulations.

Although not frequently included in anatomic discussions of the spine, an additional structure travels deep to the posterior longitudinal ligament, extending laterally and posteriorly to surround the dura of the cauda equina. It has been termed the peridural membrane, first by Dommissee in 197518 and later by Wiltse.19 The basivertebral veins cross the peridural membrane because it offers no obstruction to vascular communication between the intraosseous vessels of the vertebral body and the epidural space. Its possible clinical significance is that it may provide a containing membrane for herniated discs or hematomas, which may be noted on advanced imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) as a delimiting barrier to the pathology.

Relationships of the Roots of the Spinal Nerves

The dorsal and ventral nerve roots pass through the subarachnoid space and converge to form the spinal nerve at approximately the level of its respective intervertebral foramen. Owing to the ascensus spinalis—the apparent cranial migration of the distal end of the spinal cord during development that actually arises from differential growth of the lower parts of the vertebral column—the course of the nerve roots becomes longer and more obliquely directed in the lower lumbar segments. In the cervical region, the nerve root and the spinal nerve are posteriorly related to the same corresponding intervertebral disc; in other words, the nerve root exits the spinal canal at the same level it branches from the spinal cord.

In the lumbar region, a different situation prevails. The nerve roots contributing to the cauda equina travel an almost vertical course over the dorsum of one intervertebral disc to exit with the spinal nerve of the foramen one segment lower. In the cervical and lumbar regions, dorsal or dorsolateral (i.e., paracentral) protrusions of disc material affect the descending rather than exiting nerve root. When the meningeal coverings (dura) blend with the epineurium, the nerve components become extrathecal. The actual point of this transition is variable but usually occurs in relation to the distal aspect of the dorsal root ganglion.

The nerve root is intimately related to the pedicle of the vertebra. Ugur and colleagues20 found no distance between the upper cervical pedicles and their corresponding nerve roots in 20 cadaveric spines, whereas there was a slight distance in 4 of the 20 specimens in the lower cervical region. For all specimens, the distance from the nerve root to the inferior aspect of the upper pedicle ranged from 1 to 2.5 mm. The distance from the medial aspect of the pedicle to the dural sac ranged from 2.4 to 3.1 mm. A similar relationship between the thoracic nerve roots and pedicle exists.21 The distance from the pedicle to the superior nerve root in the thoracic spine ranges from 1.5 to 6.7 mm, and the distance from the pedicle to the inferior nerve root, 0.8 to 6 mm. Ebraheim and colleagues22 measured these distances in the lumbar spine, finding a mean distance of 1.5 mm from the pedicle to the inferior nerve root, 5.3 mm from the pedicle to the superior nerve root, and 1.5 mm from the medial pedicle wall to the dura.

Of particular interest is the distribution of epidural fat around and within the intervertebral foramen. This fat has a firm character and forms a mechanically supportive “bushing” for structures entering and leaving the spinal canal. A prominent extension of this fat body also follows the inferior and ventral surfaces of each lumbar nerve. It is interposed between the root and the external surfaces of the pedicle and vertebral body that define the inferior part of the intervertebral foramen. Its amelioration of the downward and ventral distraction of the nerve that accompanies the spine and lower limb motions is obvious. Histologically, it is composed of uniform cells that are contained within a fine membrane (perhaps the elusive peridural membrane).23 There is no fibrous tissue in normal epidural fat and only tenuous attachments to the dura.

Intervertebral Foramen

The intervertebral foramen is the aperture that gives exit to the segmental spinal nerves and entrance to the vessels and nerve branches that supply the bone and soft tissues of the vertebral canal. It is superiorly and inferiorly bounded by the respective pedicles of the adjacent vertebrae. Its ventral and dorsal components involve the two major intervertebral articulations. The dorsum of the intervertebral disc, covered by the lateral expansion of the posterior longitudinal ligament, provides a large part of its ventral boundary, whereas the joint capsule of the articular facets and the ligamentum flavum contribute the major parts of its dorsal limitation. Along with the root, the remaining space is filled with loose areolar tissue and fat (Fig. 2–16).

However ample the overall dimensions of the intervertebral foramen may be, its elliptical nature is responsible for many of its relational problems. In the lumbar region, the vertical diameter of the foramen ranges from 12 to 19 mm; this undoubtedly accounts for the fact that a complete collapse of the disc may produce little or no evidence of nerve compression. The sagittal diameter may be only 7 mm, however, making this dimension exquisitely sensitive to changes. Because the diameter of the fourth lumbar nerve can be just slightly less than 7 mm, the tolerance for pathologic alteration of the bony or connective tissue relationships is restricted.24

The existence of additional ligamentous elements in relation to the intervertebral foramen could limit further the space for the exiting spinal nerve. These structures, known as the transforaminal ligaments, are frequently found in the lumbar region.25,26 The transforaminal ligaments are strong, unyielding cords of fibrous tissue that pass anteriorly from various parts of the neural arch to the body of the same or the adjacent vertebra and may be 5 mm wide. Grimes and colleagues27 found these ligaments span from the nerve root itself. These investigators noted four different bands, the most significant of which spread from the nerve root to the anterior aspect of the facet capsule. Other bands spanned from the nerve root to the superior pedicle, the inferior pedicle, and the intervertebral disc anteriorly.

In the cervical spine, the space available for the exiting nerve root may be compromised by structures just lateral to the foramen. In 10 adult human cadaveric specimens, Alleyne and colleagues28 found the dorsal root ganglia of the C3 to C6 spinal nerves to be slightly compressed by the ascending vertebral artery. This compression was most pronounced at the C5 level, which the authors suggested as a possible explanation for the greater susceptibility of this nerve to iatrogenic injury during procedures such as laminoplasty.

Lumbosacral Nerve Root Variations

Numerous anatomic variations in the relationships of the lumbosacral nerve roots can exist. These variations may help explain seemingly anatomically inconsistent neurologic findings with compressive disorders such as herniated discs or lateral stenosis.

The most common variation involves atypical origins, or foraminal exits, of individual lumbosacral roots. Although myelographic studies indicated only a 4% incidence of lumbosacral root anomalies, an anatomic study by Kadish and Simmons29 reported an incidence of 14%. The L5-S1 level is the most commonly involved. Observations by these authors provided four types of variations: (1) intradural interconnections between roots at different levels, (2) anomalous levels of origin of nerve roots, (3) extradural connections between roots, and (4) extradural division of nerve roots.

A source of confusing neurologic findings may relate to the variant anatomy of the furcal nerve. The name furcal nerve has been applied to the fourth lumbar nerve because it exhibits a prominent bifurcation to contribute to the lumbar plexus (femoral and obturator nerves) and sacral plexus (lumbosacral trunk). Kikuchi and Hasue30 found that it is often indefinite in its intradural affinities, frequently exhibiting two dorsal root ganglia that have distinct root sources at the conus medullaris. They proposed that when symptoms indicate the involvement of two levels, suspicion should be directed toward four possible causes: (1) two roots compressed by a single lesion, (2) the presence of two lesions, (3) the anomalous emergence of two roots through the same foramen, or (4) the existence of the peculiarly doubled components of the furcal nerve (Fig. 2–17).

image

FIGURE 2–17 Schematic representation showing cross connection L4 and L5 nerve roots (spinal nerves) in the extraforaminal region through the furcal nerve.

(Adapted from McCulloch JA, Young PH: Essentials of Spinal Microsurgery. Philadelphia, Lippincott-Raven, 1998, p 390.)

Infrequently, variant “fixation” alters the expected sequences of nerve root exit. In a prefixed lumbosacral plexus, the furcal nerve (the division between the lumbar and sacral plexuses) exits through the third lumbar foramen, and the preceding and subsequent nerves exit one vertebral level higher than in the conventional distribution. Conversely, in the postfixed plexus, the furcal nerve exits the L5-S1 foramen, and the lumbosacral nerve sequence is all one level lower than usually described.31

Although Kadish and Simmons29 noted that the existence of anomalous interconnections between nerve root levels dispels any notion of “absolute innervation,” Parke and Watanabe32

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