Squamous part

The squamous part is an expanded plate posterosuperior to the foramen magnum, convex externally and concave internally. On its external surface the external occipital protuberance lies midway between its summit and the foramen magnum. On each side, two curved lines extend laterally from this protuberance. The upper, faintly marked and often almost imperceptible, is the highest nuchal line, and the lower is the superior nuchal line. The epicranial aponeurosis is attached to the medial part of the highest nuchal line. The median external occipital crest is often faint; it descends from the external occipital protuberance to the foramen magnum. On each side an inferior nuchal line spreads laterally from the midpoint of the crest.

The internal surface of the squamous part is divided into four deep fossae by an irregular internal occipital protuberance and by ridged sagittal and horizontal extensions from the protuberance. The two superior fossae are triangular and adapted to the occipital poles of the cerebral hemispheres; the inferior fossae are quadrilateral and shaped to accommodate the cerebellar hemispheres. A wide groove with raised banks, the superior sagittal sulcus, ascends from the protuberance to the superior angle of the squamous part. The posterior part of the falx cerebri is attached to the margins of the sulcus. A prominent internal occipital crest descends from the protuberance and bifurcates near the foramen magnum, and provides an attachment for the falx cerebelli. The occipital sinus, sometimes double, lies in this attachment. A small vermian fossa may exist at the lower end of the internal occipital crest; when present, it is occupied by part of the inferior cerebellar vermis. On each side a wide sulcus for the transverse sinus extends laterally from the internal occipital protuberance; the tentorium cerebelli is attached to the margins of these sulci. The right sulcus is usually larger than the left, and passes into the sulcus for the superior sagittal sinus, whereas the left usually receives the straight sinus. The position of the confluence of the sinuses is indicated by a depression on one side of the protuberance.

The position of the fetal posterior fontanelle coincides with the junction between the superior angle of the squamous part of the occipital bone and the occipital angle of the parietal bone on either side. The lateral angles of the squamous part are marked internally by the ends of the transverse sulci and project between the parietal and temporal bones. The lambdoid borders extend from superior to lateral angles and are serrated for articulation with the occipital borders of the parietal bones at the lambdoid suture. The mastoid borders extend from the lateral angles to the jugular processes, articulating with the mastoid parts of the temporal bones. A variety of sutural bones (ossicles) may occur at or near the lambda, e.g. the ‘interparietal’ (Inca bone or ossicle of Goethe).

Basilar part

The basilar part extends anterosuperiorly from the foramen magnum. In young skulls it presents a rough and uneven surface where it is joined to the body of the sphenoid by a growth cartilage (spheno-occipital synchondrosis). This plate has fully ossified by the 25th year, at which time the occipital and sphenoid bones are fused.

The inferior surface bears a small pharyngeal tubercle, about 1 cm in front of the foramen magnum, which gives attachment to the fibrous pharyngeal raphe. A small depression immediately anterior to the occipital condyle may occasionally be replaced by a small precondylar tubercle. The anterior atlanto-occipital membrane is attached to the anterior margin of the foramen magnum.

The superior surface has the form of a broad groove which slopes upwards and forwards from the foramen magnum, directly into the basilar part of the sphenoid: together these bones form the clivus. The lateral margins articulate below with the petrous part of the temporal bones.

Lateral (condylar) parts

The lateral (condylar) parts of the occipital bone flank the foramen magnum. On their inferior surfaces are occipital condyles for articulation with the superior articular facets of the atlas vertebra. The condyles are oval or reniform, their long axes converging anteromedially. The articular surfaces, wholly convex, face inferolaterally. They are occasionally constricted and a condyle may be in two parts (as may be the reciprocal surfaces of the atlas vertebra). A tubercle gives attachment to an alar ligament medial to each articular facet. The hypoglossal (anterior condylar) canal, which is situated anteriorly above each condyle, starts internally a little above the anterolateral part of the foramen magnum and continues anterolaterally. It may be partly or wholly divided by a spicule of bone and transmits the hypoglossal nerve and a meningeal branch of the ascending pharyngeal artery. A condylar fossa, behind each condyle, fits the posterior margin of the superior facet of the atlas vertebra in full extension of the skull. Its floor is sometimes perforated by a posterior condylar canal for a sigmoid emissary vein. A quadrilateral plate, the jugular process, projects laterally from the posterior half of each condyle, and contributes the posterior part of the jugular foramen. The jugular process is indented in front by a jugular notch, which may be partly divided by a small intrajugular process projecting anterolaterally. A paramastoid process sometimes projects downwards and may even articulate with the transverse process of the atlas vertebra. Laterally, the jugular process has a rough quadrilateral or triangular area that is joined to the jugular surface of the temporal bone by cartilage: it begins to ossify at around 25 years.

An oval jugular tubercle overlies the hypoglossal canal on the superior surface of the occipital condyle. Its posterior part often bears a shallow furrow for the glossopharyngeal, vagus and accessory nerves. A deep groove containing the end of the sigmoid sinus curves anteromedially around a hook-shaped process to end at the jugular notch. The posterior condylar canal opens into the posterior cranial fossa near the medial end of the groove.

Ossification

Above the highest nuchal lines the squamous part of the occipital bone is developed in a fibrous membrane and is ossified from two centres (one on each side) from about the second fetal month. This part of the occipital bone may remain separate as the interparietal bone. The remainder of the occipital bone is preformed in cartilage. Below the highest nuchal lines, the squamous part ossifies from two centres that appear in about the seventh week and soon unite. The two components of the squamous part unite in the third postnatal month, but the line of their union is recognizable at birth. The remainder of the cartilage of the occipital bone is ossified from five centres; two each for the lateral parts appear during the eighth week, and one for the basilar part appears around the sixth week.

At birth the occipital bone consists of four separate parts (Fig. 42.9), a basilar part, two lateral parts and a squamous part, all joined by cartilage and forming a ring around the foramen magnum. The squamous and lateral parts fuse together from the second year. The lateral parts fuse with the basilar part during years 3 and 4, but fusion may be delayed until the 7th year.

Fig. 42.9 The occipital bone of a newborn child: external surface. Parts of the chondrocranium which are still unossified are shown in blue.

Anterior aspect

The anterior aspect of the column is formed by the anterior surfaces of the vertebral bodies and of the intervertebral discs (Fig. 42.10A). It has important anatomical relations at all levels, and should be considered in continuity. It forms part of several clinically significant junctional or transitional zones, including the prevertebral/retropharyngeal zone of the neck, the thoracic inlet, the diaphragm and the pelvic inlet. The anterior aspect of the column is covered centrally by the anterior longitudinal ligament, which forms a fascial plane with the prevertebral and endothoracic fascia and with the subperitoneal areolar tissue of the posterior abdominal wall. Infection and other pathological processes may spread along this fascial plane.

Lateral aspect

The lateral aspect of the vertebral column is arbitrarily separated from the posterior by articular processes in the cervical and lumbar regions and by transverse processes in the thoracic region (Fig. 42.10C). Anteriorly it is formed by the sides of vertebral bodies and intervertebral discs. The oval intervertebral foramina, behind the bodies and between the pedicles, are smallest at the cervical and upper thoracic levels, and increase progressively in size in the thoracic and upper lumbar regions. The lumbosacral (L5/S1) intervertebral foramen is the smallest of the lumbar foramina. The foramina permit communication between the lumen of the vertebral canal and the paravertebral soft tissues (a ‘paravertebral space’ is sometimes described), which may be important in the spread of tumours and other pathological processes. The lateral aspects of the column have important anatomical relations, some of which vary considerably between the two sides.

Posterior aspect

The posterior aspect of the column is formed by the posterior surfaces of the laminae and spinous processes, their associated ligaments, and the facet joints (Fig. 42.10B). It is covered by the deep muscles of the back.

Structural defects of the posterior bony elements

Deformity and bony deficiency may occur at several sites within the posterior elements. The laminae may be wholly or partially absent, or the spinous process alone may be affected, with no abnormalities in the overlying soft tissues (spina bifida occulta). A defect may occur in the part of the lamina between the superior and inferior articular processes (pars interarticularis): this condition is spondylolysis, and may be developmental or result from acute or fatigue fracture. If such defects are bilateral, the column becomes unstable at that level, and forward displacement of that part of the column above (cranial to) the defects may occur: this is spondylolisthesis. Abnormality of the laminar bone, or degenerative changes in the facet joints, may also lead to similar displacement in the absence of pars defects. The deformity of the vertebral canal resulting from severe spondylolisthesis may lead to neural damage. Much more rarely, bony defects may occur elsewhere in the posterior elements, e.g. in the pedicles.

Detailed anatomical relations of all aspects of the vertebral column at the various levels are best appreciated by the study of horizontal (axial) sections and images (see Fig. 42.33B and Fig. 42.55).

Fig. 42.33 Lumbar vertebrae. A, Fifth lumbar vertebra, superior aspect. B, High resolution axial CT scan through the lumbar spine at the level of the fourth lumbar vertebra.

Fig. 42.55 Axial MRI of lumbar spine at the level of the L4/5 intervertebral disc.

Curvatures

Vertebral column in the elderly

In older people, age-related changes in the structure of bone lead to broadening and loss of height of the vertebral bodies. These changes are more severe in females. The bony changes in the vertebral column are accompanied by changes in the collagen content of the discs and by decline in the activity of the spinal muscles. This leads to progressive decline in vertebral column mobility, particularly in the lumbar spine. The development of a ‘dowager’s hump’ in the midthoracic region in females, caused by age-related osteoporosis, increases the thoracic kyphosis and cervical lordosis. Overall, these changes in the vertebral column lead directly to loss of total height in the individual.

In mid-lumbar vertebrae the width of the body increases with age. In men there is a relative decrease of posterior to anterior body height, while in both sexes anterior height decreases relative to width. Twomey et al (1983) observed a reduction in bone density of lumbar vertebral bodies with age, principally as a result of a reduction in transverse trabeculae (more marked in females as a result of postmenopausal osteoporosis), which was associated with increased diameter and increasing concavity in their juxtadiscal surfaces (end-plates).

Other changes affect the vertebral bodies. Osteophytes (bony spurs) may form from the compact cortical bone on the anterior and lateral surfaces of the bodies. Although individual variations occur, these changes appear in most individuals from 20 years onwards. They are most common on the anterior aspect of the body, and never involve the ring epiphysis. Osteophytic spurs are frequently asymptomatic, but may result in diminished movements within the spine.

Vascular supply and lymphatic drainage

Arteries

(See Crock 1996.)

The vertebral column, its contents and its associated soft tissues, all receive their arterial supply from derivatives of dorsal branches of the embryonic intersegmental somatic arteries (see Ch. 59). The named artery concerned depends on the level of the column. These intersegmental vessels persist in the thoracic and lumbar regions as the posterior intercostal and lumbar arteries. In the cervical and sacral regions, longitudinal anastomoses between the intersegmental vessels persist as longitudinal vessels which themselves give spinal branches to the vertebral column. In the neck the postcostal anastomosis becomes most of the vertebral artery, while the post-transverse anastomosis forms most of the deep cervical artery. The ascending cervical artery and the lateral sacral artery are persistent parts of the precostal anastomosis.

In the thorax and abdomen the primitive arterial pattern is retained by the paired branches of the descending aorta which supply the vertebral column (Fig. 42.12A). On each side, the main trunk of the artery (posterior intercostal or lumbar) passes around the vertebral body, giving off primary periosteal and equatorial branches to the body, and then a major dorsal branch. The latter gives off a spinal branch which enters the intervertebral foramen, before itself supplying the facet joints, the posterior surfaces of the laminae and the overlying muscles and skin. There is free anastomosis between these dorsal articular and soft-tissue branches, extending over several segments (Crock & Yoshizawa 1976; Boelderl et al 2002). At cervical and sacral levels the longitudinally running arteries described above have direct spinal branches. The spinal branches are the main arteries of supply to all bony elements of the vertebrae and to the dura and epidural tissues, and also contribute to the supply of the spinal cord and nerve roots via radicular branches. As they enter the vertebral canal the spinal arteries divide into postcentral, prelaminar and radicular branches. The postcentral branches, which are the main nutrient arteries to the vertebral bodies and to the periphery of the intervertebral discs, anastomose beneath the posterior longitudinal ligament with their fellows above and below as well as across the midline (Fig. 42.12A,B). This anastomosis also supplies the anterior epidural tissues and dura. The majority of the vertebral arch, the posterior epidural tissues and dura and the ligamentum flavum are supplied by the prelaminar branches and their anastomotic plexus on the posterior wall of the vertebral canal.

Fig. 42.12 Arterial supply to the vertebrae and the contents of the vertebral canal. A, Branching pattern of lumbar segmental arteries. B, Arterial anastomoses between postcentral branches of spinal arteries within the vertebral canal.

Veins

Veins of the vertebral column form intricate plexuses along the entire column, external and internal to the vertebral canal (Fig. 42.13). Both groups are devoid of valves, anastomose freely with each other, and join the intervertebral veins. Interconnections are widely established between these plexuses and longitudinal veins early in fetal life. When development is complete, the plexuses drain into the caval and azygos/ascending lumbar systems via named veins which accompany the arteries described above.

Fig. 42.13 Venous drainage of the vertebral column.

(From Sobotta 2006.)

The veins also communicate with cranial dural venous sinuses and with the deep veins of the neck and pelvis. The venous complexes associated with the vertebral column can dilate considerably, and can form alternative routes of venous return in patients with major venous obstruction in the neck, chest or abdomen. The absence of valves allows pathways for the wide and sometimes paradoxical spread of malignant disease and sepsis. Pressure changes in the body cavities are transmitted to these venous plexuses and thus to the CSF, though the cord itself may be protected from such congestion by valves in the small veins which drain from the cord into the internal vertebral plexus.

External vertebral venous plexuses

The external vertebral venous plexuses are anterior and posterior. They anastomose freely, and are most developed in the cervical region. Anterior external plexuses are anterior to the vertebral bodies, communicate with basivertebral and intervertebral veins, and receive tributaries from vertebral bodies. Posterior external plexuses lie posterior to the vertebral laminae and around spines, transverse and articular processes. They anastomose with the internal plexuses and join the vertebral, posterior intercostal and lumbar veins.

Internal vertebral venous plexuses

The internal vertebral venous plexuses occur between the dura mater and vertebrae, and receive tributaries from the bones, red bone marrow and spinal cord. (For the venous drainage of the spinal cord see p. 757 and Fig. 43.11.)

They form a denser network than the external plexuses and are arranged vertically as four interconnecting longitudinal vessels, two anterior and two posterior.

The anterior internal plexuses are large plexiform veins on the posterior surfaces of the vertebral bodies and intervertebral discs. They flank the posterior longitudinal ligament, beneath which they are connected by transverse vessels into which the large basivertebral veins open. The posterior internal plexuses, on each side in front of the vertebral arches and ligamenta flava, anastomose with the posterior external plexuses via veins which pass through and between the ligaments. The internal plexuses interconnect by venous rings near each vertebra. Around the foramen magnum they form a dense network connecting with vertebral veins, occipital and sigmoid sinuses, the basilar plexus, the venous plexus of the hypoglossal canal, and the condylar emissary veins.

Basivertebral veins

The basivertebral veins are large and tortuous channels in bone, like those in the cranial diploë. They emerge from the posterior foramina of the vertebral bodies and drain into the anterior external vertebral plexuses through small openings in the vertebral bodies. Posteriorly they form one or two short trunks that open into the transverse vessels which unite the anterior internal vertebral plexuses. The basivertebral veins enlarge in advanced age.

Intervertebral veins

The intervertebral veins accompany the spinal nerves through intervertebral foramina, draining the spinal cord and internal and external vertebral plexuses, and ending in the vertebral, posterior intercostal, lumbar and lateral sacral veins. Upper posterior intercostal veins may drain into the caval system via brachiocephalic veins, whereas the lower intercostals drain into the azygos system. Lumbar veins are joined longitudinally in front of the transverse processes by the ascending lumbar veins, in which they may terminate. Alternatively, they may proceed around the vertebral bodies to drain into the inferior vena cava. Whether the basivertebral or intervertebral veins contain effective valves is uncertain but experimental evidence strongly suggests that their blood flow can be reversed (Batson 1957). This may explain how pelvic neoplasms, e.g. carcinoma of the prostate, may metastasize in vertebral bodies: the cells spread into the internal vertebral plexuses via their connections with the pelvic veins when blood flow is temporarily reversed by raised intra-abdominal pressure or postural alterations.

Innervation

The innervation of the vertebral column and its associated soft tissues has been studied in greatest detail in the lumbar region (see below Fig. 42.44). The account given here relies particularly on the work of Bogduk, to whose textbook on the lumbosacral spine (Bogduk 2005) the interested reader is referred. See also the work of Groen et al (1990).

Fig. 42.44 The nerve supply of a lumbar intervertebral disc. Branches of the grey rami communicantes and the sinuvertebral nerves are shown entering the disc and the anterior and posterior longitudinal ligaments. Branches from the sinuvertebral nerves also supply the ventral aspect of the dural sac and the dural nerve-root sleeve.

Innervation is derived from the spinal nerves where they branch, in and just beyond the intervertebral foramina. There is an input from the sympathetic system via either grey rami communicantes or directly from thoracic sympathetic ganglia. The branches of the spinal nerve concerned are the dorsal ramus and the recurrent meningeal or sinuvertebral nerves (usually more than one at each level) (p. 753 and also Fig. 43.5). The dorsal ramus branches to supply the facet joints, periosteum of the posterior bony elements, overlying muscles and skin. The exact origin and branching pattern of the sinuvertebral nerves is controversial, but they may be best considered to be recurrent branches of the ventral rami. They receive the sympathetic input described above, then re-enter the intervertebral foramina to supply the structures that form the walls of the vertebral canal, the dura and epidural soft tissues. Their subsequent course is described on page 754.

Vertebral canal

The vertebral canal (Fig. 42.16) extends from the foramen magnum to the sacral hiatus, and follows the vertebral curves. In the cervical and lumbar regions, which exhibit free mobility, it is large and triangular, but in the thoracic region, where movement is less, it is small and circular. These differences are matched by variations in the diameter of the spinal cord and its enlargements. In the lumbar region, the vertebral canal decreases gradually in size between L1 and L5, with a greater relative width in the female.

Fig. 42.16 The vertebral canal in section: A, sagittal; B, transverse (axial); C, coronal. FM: foramen magnum.

For clinical purposes it is useful to consider the vertebral canal as having three zones. These are a central zone, between the medial margins of the facet joints, and two lateral zones, beneath the facet joints and entering the intervertebral foramina. Each lateral zone, which passes into and just beyond the intervertebral foramen, can be further subdivided into subarticular (lateral recess), foraminal and extraforaminal regions (Macnab & McCulloch 1990). The lateral zone thus described forms the canal of the spinal nerve (the radicular or ‘root’ canal). The central zone of the canal is a little narrower than the radiological interpedicular distance if the lateral recess is considered to be part of the radicular canal rather than part of the central zone.

Intervertebral foramina

Intervertebral foramina (see also p. 712) are the principal routes of entry and exit to and from the vertebral canal, and are closely related to the main intervertebral articulations. (Minor routes occur between the median, often partly fused, margins of the ligamenta flava.) The same general arrangement applies throughout the vertebral column, between the axis and sacrum, although there are some quantitative and structural regional variations. Because of their construction, contents and susceptibilities to multiple disorders, the intervertebral foramina are loci of great biomechanical, functional and clinical significance. The specializations cranial to the axis and at sacral levels are described with the individual bones and articulations.

The boundaries of a generalized intervertebral foramen (Fig. 42.17) are anteriorly, from above downwards, the posterolateral aspect of the superior vertebral body, the posterolateral aspect of the intervertebral symphysis (including the disc), and a small (variable) posterolateral part of the body of the inferior vertebra; superiorly, the compact bone of the deep arched inferior vertebral notch of the vertebra above; inferiorly, the compact bone of the shallow superior vertebral notch of the vertebra below; and posteriorly a part of the ventral aspect of the fibrous capsule of the facet synovial joint. Cervical intervertebral foramina are distinct in having superior and inferior vertebral notches of almost equal depth which, in accord with the direction of the pedicles, face anterolaterally. External to them, and oriented in the same direction, is a transverse process. The thoracic and lumbar intervertebral foramina face laterally and their transverse processes are posterior. In addition, the anteroinferior boundaries of the first to tenth thoracic foramina are formed by the articulations of the head of a rib and the capsules of double synovial joints (with the demifacets on adjacent vertebrae and the intra-articular ligament between the costocapitular ridge and the intervertebral symphysis). Lumbar foramina lie between the two principal lines of vertebral attachment of psoas major. The walls of each foramen are covered throughout by fibrous tissue which is in turn periosteal (though the presence of a true periosteum lining the vertebral canal is controversial [Newell 1999]), perichondrial, anular and capsular. The more lateral parts of the foramina may be crossed at a variable level by narrow fibrous bands, the transforaminal ligaments (for detail of these ligaments see Bogduk 2005). The true foramen is the foraminal region of the canal of the spinal nerve (the radicular or ‘root’ canal). A foramen contains a segmental mixed spinal nerve and its sheaths, from two to four recurrent meningeal (sinuvertebral) nerves, variable numbers of spinal arteries, and plexiform venous connections between the internal and external vertebral venous plexuses. These structures, particularly the nerves, may be affected by trauma or one of the many disorders which may affect tissues bordering the foramen. In particular, nerve compression and irritation may be caused by intervertebral disc prolapse, or by bony entrapment as the size of the foramen decreases. This decrease may result from facet joint osteoarthritis, osteophyte formation, disc degeneration and degenerative spondylolisthesis, all of which may lead to lateral or foraminal spinal stenosis.

Fig. 42.17 The boundaries of an intervertebral foramen.

Typical cervical vertebra

A typical cervical vertebra (Fig. 42.20, Fig. 42.21) has a small, relatively broad vertebral body. The pedicles project posterolaterally and the longer laminae posteromedially, enclosing a large, roughly triangular vertebral foramen; the vertebral canal here accommodates the cervical enlargement of the spinal cord. The pedicles attach midway between the discal surfaces of the vertebral body, so the superior and inferior vertebral notches are of similar depth. The laminae are thin and slightly curved, with a thin superior and slightly thicker inferior border. The spinous process (‘spine’) is short and bifid, with two tubercles which are often unequal in size. The junction between lamina and pedicle bulges laterally between the superior and inferior articular processes to form an articular pillar (‘lateral mass’) on each side. The transverse process is morphologically composite around the foramen transversarium. Its dorsal and ventral bars terminate laterally as corresponding tubercles. The tubercles are connected, lateral to the foramen, by the costal (or intertubercular) lamella: these three elements represent morphologically the capitellum, tubercle and neck of a cervical costal element (p. 770, Fig. 44.11). The attachment of the dorsal bar to the pediculolaminar junction represents the morphological transverse process and the attachment of the ventral bar to the ventral body represents the capitellar process. In all but the seventh cervical vertebra, the foramen transversarium normally transmits the vertebral artery and vein and a branch from the cervicothoracic ganglion (vertebral nerve).

Fig. 42.20 Fourth cervical vertebra, superior aspect. 1. Body. 2. Posterior tubercle of transverse process. 3. Pedicle. 4. Lamina. 5. Bifid spinous process. 6. Anterior tubercle of transverse process. 7. Foramen transversarium. 8. Superior articular facet. 9. Vertebral foramen.

Fig. 42.21 Fourth cervical vertebra, lateral aspect. 1. Uncinate process. 2. Body. 3. Anterior tubercle of transverse process. 4. Posterior tubercle of transverse process. 5. Superior articular process. 6. Lateral mass. 7. Lamina. 8. Spinous process. 9. Inferior articular process.

The vertebral body has a convex anterior surface. The discal margin gives attachment to the anterior longitudinal ligament. The posterior surface is flat or minimally concave, and its discal margins give attachment to the posterior longitudinal ligament. The central area displays several vascular foramina, of which two are commonly relatively larger. These are the basivertebral foramina which transmit basivertebral veins to the anterior internal vertebral veins. The superior discal surface is saddle-shaped, formed by flange-like lips which arise from most of the lateral circumference of the upper margin of the vertebral body; these are sometimes referred to as uncinate or neurocentral lips or processes. The inferior discal surface is also concave: the concavity is produced mainly by a broad projection from the anterior margin which partly overlaps the anterior surface of the intervertebral disc. The discal surfaces of cervical vertebrae are so shaped in order to restrict both lateral and anteroposterior gliding movements during articulation. The paired ligamenta flava extend from the superior border of each lamina below to the roughened inferior half of the anterior surfaces of the lamina above. The superior part of the anterior surface of each lamina is smooth, like the immediately adjacent surfaces of the pedicles, which are usually in direct contact with the dura mater and cervical root sheaths to which they may become loosely attached. The spinous process of the sixth cervical vertebra is larger, and is often not bifid.

The superior articular facets, flat and ovoid, are directed superoposteriorly, whereas the corresponding inferior facets are directed mainly anteriorly, and lie nearer the coronal plane than the superior facets. The dorsal rami of the cervical spinal nerves curve posteriorly, close to the anterolateral aspects of the lateral masses, and may actually lie in shallow grooves, especially on the third and fourth pair. The dorsal root ganglion of each cervical spinal nerve lies between the superior and inferior vertebral notches of adjacent vertebrae. The large anterior ramus passes posterior to the vertebral artery, which lies on the concave upper surface of the costal lamella: the concavity of the lamellae increases from the fourth to the sixth vertebra. The fourth to sixth anterior tubercles are elongated and rough for muscle attachment. The sixth is the longest, the carotid tubercle of Chassaignac. The carotid artery can be forcibly compressed in the groove formed by the vertebral bodies and the larger anterior tubercles, especially the sixth. The posterior tubercles are rounded and more laterally placed than the anterior, and all but the sixth are also more caudal; the sixth is at about the same level as the anterior.

C1, atlas

The atlas, the first cervical vertebra (Fig. 42.22), supports the head. It is unique in that it fails to incorporate a centrum, whose expected position is occupied by the dens, a cranial protuberance from the axis. The atlas consists of two lateral masses connected by a short anterior and a longer posterior arch. The transverse ligament retains the dens against the anterior arch.

Fig. 42.22 First cervical vertebra (atlas), superior aspect. 1. Anterior tubercle. 2. Anterior arch. 3. Outline of dens. 4. Superior articular facet, on lateral mass (bipartite facet in this specimen). 5. Outline of transverse ligament. 6. Groove for vertebral artery and C1 (beneath bony overhang from lateral mass here). 7. Posterior arch. 8. Transverse process. 9. Foramen transversarium. 10. Vertebral foramen. 11. Posterior tubercle.

The transverse ligament divides the vertebral canal into two compartments. The anterior third (approximately) of the canal is occupied by the dens. The posterior compartment is occupied by the spinal cord and its coverings, and the cord itself takes up about half of this space (i.e. the cord, like the dens, occupies one third of the canal).

The anterior arch is slightly convex anteriorly, and carries a roughened anterior tubercle to which is attached the anterior longitudinal ligament (which is cylindrical at this level). Its upper and lower borders provide attachment for the anterior atlanto-occipital membrane and diverging lateral parts of the anterior longitudinal ligament. The posterior surface of the anterior arch carries a concave, almost circular, facet for the dens.

The lateral masses are ovoid, their long axes converging anteriorly. Each bears a kidney-shaped superior articular facet for the respective occipital condyle, which is sometimes completely divided into a larger anterior and a smaller posterior part (Lang 1986). The inferior articular facet of the lateral mass is almost circular and is flat or slightly concave. It is orientated more obliquely to the transverse plane than the superior facet, and faces more medially and very slightly backwards. On the medial surface of each lateral mass is a roughened area which bears vascular foramina and a tubercle for attachment of the transverse ligament. In adults the distance between these tubercles is shorter than the transverse ligament itself, with a mean value of approximately 16 mm.

The posterior arch forms three-fifths of the circumference of the atlantal ring. The superior surface bears a wide groove for the vertebral artery and venous plexus immediately behind, and is variably overhung by the lateral mass; the first cervical nerve intervenes. The flange-like superior border gives attachment to the posterior atlanto-occipital membrane, and the flatter inferior border to the highest pair of ligamenta flava. The posterior tubercle is a rudimentary spinous process, roughened for attachment of the ligamentum nuchae.

The transverse processes are longer than those of all cervical vertebrae except the seventh (Fig. 42.18). They act as strong levers for the muscles which make fine adjustments to keep the head balanced. Maximum atlantal width varies from 74–95 mm in males and 65–76 mm in females, and this affords a useful criterion for assessing sex in human remains. The apex of the transverse process, which is usually broad, flat and palpable between the mastoid process and ramus of the mandible, is homologous with the posterior tubercle of typical cervical vertebrae: the remaining part of the transverse process consists of the costal lamella. A small anterior tubercle is sometimes visible on the anterior aspect of the lateral mass. The costal lamella is sometimes deficient, which leaves the foramen transversarium open anteriorly.

Muscle attachments

The superior oblique parts of longus colli are attached on each side of the anterior tubercle. The anterior surface of the lateral mass gives attachment to rectus capitis anterior. Rectus capitis posterior minor is attached just lateral to the posterior tubercle. Rectus capitis lateralis is attached to the transverse process superiorly, and obliquus capitis superior is located more posteriorly. Obliquus capitis inferior is attached laterally on the apex, below which are slips of levator scapulae, splenius cervicis and scalenus medius.

Ossification

The atlas is commonly ossified from three centres (Fig. 42.23). One appears in each lateral mass at about the seventh week, gradually extending into the posterior arch where they unite between the third and fourth years, usually directly but occasionally through a separate centre. At birth, the anterior arch is fibrocartilaginous, and a separate centre appears about the end of the first year. This unites with the lateral masses between the sixth and eighth year, the lines of union extending across anterior parts of the superior articular facets. Occasionally the anterior arch is formed by the extension and ultimate union of centres in the lateral masses and sometimes from two lateral centres in the arch itself.

Fig. 42.23 Ossification in the vertebral column. A, The atlas. B, The axis. C, Lumbar vertebra.

The central part of the posterior arch may be absent and replaced by fibrous tissue. Frequently bony spurs arise from the anterior and posterior margins of the groove for the vertebral artery. These are sometimes referred to as ponticles, and they occasionally convert the groove into a foramen. More often the foramen is incomplete superiorly. Rarely the atlas may be wholly or partially assimilated into (fused with) the occiput.

C2, axis

The axis, the second cervical vertebra (Fig. 42.24, Fig. 42.25), acts as an axle for rotation of the atlas and head around the strong dens (odontoid process), which projects cranially from the superior surface of the body.

Fig. 42.24 Second cervical vertebra (axis), superior aspect. 1. Dens – attachment of apical ligament. 2. Superior articular facet on lateral mass. 3. Dens – attachments of alar ligaments. 4. Foramen transversarium. 5. Pedicle. 6. Spinous process. 7. Body. 8. Transverse process. 9. Vertebral foramen. 10. Inferior articular process. 11. Lamina.

Fig. 42.25 Second cervical vertebra (axis), lateral aspect. 1. Dens – attachment of alar ligament. 2. Facet for anterior arch of atlas. 3. Groove for transverse ligament of atlas. 4. Superior articular facet. 5. Lateral mass. 6. Divergent foramen transversum. 7. Body. 8. Ventral lip of body. 9. Lamina. 10. Spinous process. 11. Inferior articular facet. 12. Transverse process.

The dens is conical in shape with a mean length of 15 mm in adults. It may be tilted a little, up to 14°, posteriorly, or, less often, anteriorly on the body of the axis: it may also tilt laterally up to 10°. The posterior surface bears a broad groove for the transverse ligament which is covered in cartilage. The apex is pointed, and from this point arises the apical ligament. The alar ligaments are attached to the somewhat flattened posterolateral surfaces above the groove for the transverse ligament. The anterior surface bears an ovoid articular facet for the anterior arch of the atlas, and the surface is pitted by many vascular foramina, which are most numerous near the apex.

The body consists of less compact bone than the dens. It is composite, and consists of the partly fused centra of the atlas and axis, and a rudimentary disc (synchondrosis) between them which usually remains detectable deep within the body of the axis throughout life. Large ovoid articular facets are present on either side of the dens at the junction of the body and neural arch: they are flat or slightly convex for articulation with the masses of the atlas. The facets lie in a plane anterior to the plane of the intercentral (Luschka) articulations, with which they are, in part, homologous. The somewhat triangular downward projecting anterior border gives attachment to the anterior longitudinal ligament. Posteriorly, the lower border receives the posterior longitudinal ligament and the membrana tectoria.

The pedicles are stout, and the superior surface carries part of the superior articular facet, which also projects laterally and downwards onto the transverse process. The anterolateral surface is deeply grooved by the vertebral artery, running beneath the thin lateral part of the inferior surface of the superior articular facet, which can become quite thin. The inferior surface of each pedicle bears a deep, smooth inferior intervertebral notch, in which the large root sheath of the third cervical nerve lies. The interarticular part of the pedicle is short and lies between the relatively small inferior posterior articular process (which is located at the pediculolaminar junction and bearing a small anteriorly facing facet) and the superior articular surface.

The transverse process is pointed, projects inferiorly and laterally, and arises from the pediculolaminar junction and the lateral aspect of the interarticular area of the pedicle. The rounded tip is homologous with the posterior tubercle of a typical cervical vertebrae. The foramen transversarium is directed laterally as the vertebral artery turns abruptly laterally under the superior articular facet. Small anterior tubercles may be present near the junction of the costal lamella with the body.

The laminae are thick, and give attachment to the ligamenta flava.

The spinous process is large, with a bifid tip and a broad base, which is concave inferiorly. The ligamentum nuchae is attached to the apical notch.

C7, seventh cervical vertebra

The seventh cervical vertebra, the vertebra prominens (Fig. 42.26), has a long spinous process which is visible at the lower end of the nuchal furrow. It ends in a prominent tubercle for the attachment of the ligamentum nuchae, and the muscles detailed below. The thick and prominent transverse processes lie behind and lateral to the foramina transversaria. The latter transmit vertebral veins, but not the vertebral artery, and each is often divided by a bony spicule. The costal lamella is relatively thin and may be partly deficient. It is grooved superiorly for the anterior ramus of the seventh cervical nerve, and usually carries a small and inconspicuous anterior tubercle. The posterior tubercle is prominent. The suprapleural membrane is attached to the anterior border of the transverse process.

Fig. 42.26 Seventh cervical vertebra, superior aspect. 1. Body. 2. Superior articular facet. 3. Inferior articular process. 4. Spinous process. 5. Uncinate process. 6. Foramen transversarium (foramina are asymmetrical in this specimen). 7. Transverse process. 8. Pedicle. 9. Vertebral foramen. 10. Lamina.

The costal lamella of the transverse process may be separate as a cervical rib. The foramina transversaria may be asymmetrical: sometimes one is absent if the costal lamella is undeveloped.

Thoracic vertebrae in general and changes with descending level

All thoracic vertebral bodies display lateral costal facets and all but the lowest two or three transverse processes also have facets (Fig. 42.27). The facets articulate with the head of the rib (costocapitular facet) and its tubercle (costotubercular facet) respectively. The first and ninth to 12th vertebrae also have atypical features, but the remainder are very similar, except for relatively minor details.

Fig. 42.27 The first, ninth, tenth, eleventh and twelfth thoracic vertebrae, lateral aspect.

The body is typically a waisted cylinder (Fig. 42.28) except where the vertebral foramen encroaches, and transverse and anteroposterior dimensions are almost equal. On each side there are two costal facets (which are really demifacets): the superior and usually larger pair at the upper border are anterior to the pedicles, while the inferior pair at the lower border are anterior to the vertebral notches. The vertebral foramen (Fig. 42.14) is small and circular, so the pedicles do not diverge as they do in cervical vertebrae: the thoracic spinal cord is smaller and more circular than the cervical cord. The laminae are short, thick and broad, and overlap from above downwards. The spinous process slants downward. The thin and almost flat superior articular processes project from the pediculolaminar junctions and face posteriorly and a little superolaterally. The inferior processes project down from the laminae and their facets are directed forwards and a little superomedially. The large, club-like transverse processes also project from the pediculolaminar junctions. Each passes posterolaterally and bears, near its tip, anterior oval facets for articulation with the tubercle of the corresponding rib.

Fig. 42.28 Fourth thoracic vertebra, lateral aspect. 1. Body. 2. Costocapitular demifacets. 3. Superior articular facet. 4. Transverse process. 5. Costotubercular facet. 6. Pedicle. 7. Inferior articular process. 8. Spinous process.

The bodies of upper thoracic vertebrae gradually change from cervical to thoracic in type, and the lower change from thoracic to lumbar. The body of the first is typically cervical, its transverse diameter being almost twice the anteroposterior; the second retains a cervical shape, but its two diameters differ less. The third body is the smallest, and has a convex anterior aspect unlike the flattened first and second thoracic vertebrae. The remaining bodies increase in size and, because of its increased anteroposterior diameter, the fourth is typically ‘heart-shaped’. The fifth to eighth increase their anteroposterior dimension but change little transversely. These four, in transverse section, are asymmetrical, their left sides being flattened by pressure of the thoracic aorta. The rest increase more rapidly in all measurements, so that the 12th body resembles that of a typical lumbar vertebra. These modifications may contribute to the greater range of flexion-extension seen at the cervical and lumbar ends of the thoracic vertebral column.

The anterior and posterior longitudinal ligaments are attached to the borders of the bodies, and around the margins of the costal facets there are the capsular and radiate ligaments of the costovertebral joints. Thoracic pedicles show a successive caudal increase in thickness. The superior vertebral notch is recognizable only in the first thoracic vertebra, whereas the inferior notch is deep in all. Ligamenta flava are attached at the upper borders and lower anterior surfaces of the laminae.

Thoracic transverse processes shorten in caudal succession. In the upper five or six vertebrae the costal facets are concave and face anterolaterally, and at lower levels the facets are flatter and face superolaterally and slightly forwards. The costotransverse ligament is attached to the anterior surface medial to the facet; the lateral costotransverse ligament is attached to its tuberculated apex and the superior costotransverse ligament is attached to its lower border.

Thoracic spines overlap from the fifth to the eighth vertebra, whose spine is the longest and most oblique. Supraspinous and interspinous ligaments are attached to the spines.

A change in orientation of articular processes from thoracic to lumbar type usually occurs at the 11th thoracic vertebra, but sometimes at the 12th or 10th. In the transitional vertebra the superior articular processes are thoracic, and face posterolaterally, while the inferior are transversely convex and face anterolaterally. The transitional vertebra marks the site of a sudden change of mobility from predominantly rotational to predominantly flexion–extension.

T1

The first thoracic vertebra (Fig. 42.27) resembles a cervical vertebra in its body, both in shape and in the distinctive posterolateral ‘lipping’ which forms the anterior border of the superior vertebral notch. There are circular superior costal facets for articulation with the whole facet on the head of the first rib. The smaller, semilunar inferior facets articulate with a demifacet on the head of the second rib. The upper costal facet is often incomplete, in which case the first rib articulates with the seventh cervical vertebra and the intervening disc. A small, deep depression often occurs below the facet. The long, thick spine is horizontal and commonly as prominent as that of the seventh cervical vertebra.

T9

The ninth thoracic vertebra (Fig. 42.27) is otherwise typical, but it often fails to articulate with the tenth ribs, in which case the inferior demifacets are absent.

T10

The 10th thoracic vertebra (Fig. 42.27) only articulates with the 10th pair of ribs, so that superior facets only appear on the body. These are usually large and semilunar, but are oval when the tenth ribs fail to articulate with the ninth vertebra and intervening disc. The transverse process may or may not bear a facet for the tubercle of the 10th rib.

T11

The 11th thoracic vertebra (Fig. 42.27) articulates only with the heads of the 11th ribs. The circular costal facets are close to the upper border of the body and extend onto the pedicles. The small transverse processes lack articular facets. The 11th and 12th thoracic spinous processes are triangular, with blunt apices, a horizontal lower and an oblique upper border.

T12

The 12th thoracic vertebra (Fig. 42.27) articulates with the heads of the 12th ribs by circular facets somewhat below the upper border, spreading on to the pedicles. The body is large and the vertebra has some lumbar features. The transverse process is replaced by three small tubercles: the superior is largest, projects upwards and corresponds to a lumbar mammillary process, though it does not lie as close to the superior articular process; the lateral tubercle is the homologue of a transverse process; the inferior is the homologue of a lumbar accessory process. The superior and inferior processes are surprisingly long in some specimens.

Lumbar vertebrae in general

The five lumbar vertebrae are distinguished by their large size and absence of costal facets and transverse foramina. The body is wider transversely (Fig. 42.29, see Fig. 42.33B). The vertebral foramen is triangular, larger than at thoracic levels but smaller than at cervical levels. The pedicles are short. The spinous process is almost horizontal, quadrangular and thickened along its posterior and inferior borders. The superior articular processes bear vertical concave articular facets facing posteromedially, with a rough mammillary process on their posterior borders (Fig. 42.30A). The inferior articular processes have vertical convex articular facets which face anterolaterally. The transverse processes are thin and long, except on the more substantial fifth pair. A small accessory process marks the posteroinferior aspect of the root of each transverse process. The accessory and mammillary processes are linked by a fine ligament, the mammillo-accessory ligament, which is sometimes ossified, and beneath which runs the medial branch of the dorsal primary ramus of the spinal nerve (Bogduk 2005).

Fig. 42.29 First lumbar vertebra, superior aspect. 1. Body. 2. Pedicle. 3. Transverse process. 4. Accessory process. 5. Mammillary process. 6. Spinous process. 7. Vertebral foramen. 8. Superior articular facet. 9. Lamina. 10. Inferior articular facet.

Fig. 42.30 Lumbar spine. A, Lumbar vertebra, lateral aspect. B, Lateral radiograph, 24-year-old male. 1. L2 vertebral body. 2. Site of L3–4 intervertebral disc. 3. S1 vertebral body. 4. Superior facet. 5. Inferior facet. 6. L3–4 facet joint. 7. L4 spinous process.

Strong paired pedicles arise posterolaterally from each body near its upper border. Superior vertebral notches are shallow and the inferior ones are deep. The laminae are broad and short, but do not overlap as much as those of the thoracic vertebrae. The fifth spine is the smallest, and its apex is often rounded and down-turned. Upper lumbar superior articular processes are further apart than inferior ones, but the difference is slight in the fourth and negligible in the fifth. The articular facets are reciprocally concave (superior) and convex (inferior), which allows flexion, extension, lateral bending and some degree of rotation. There are sex differences in the angle of inclination and depth of curvature of the articular facets. The facets are sometimes asymmetrical. Transverse processes, except the fifth, are anteroposteriorly compressed and project posterolaterally. The lower border of the fifth transverse process is angulated, passes laterally and then superolaterally to a blunt tip, and the whole process presents a greater upward inclination than the fourth. The angle on the inferior border may represent the tip of the costal element and the lateral end the tip of the true transverse process. The lumbar transverse processes increase in length from first to third and then shorten (Fig. 42.31). The fifth pair incline both upwards and posterolaterally. The costal element is incorporated in the mature transverse process.

Fig. 42.31 Anteroposterior radiograph of lumbosacral spine in a 22-year-old male. 1. L3 inferior articular process. 2. L4 superior articular process. 3. Right sacroiliac joint. 4. Ilium. 5. Psoas lateral border. 6. L3 transverse process. 7. Facet joint. 8. L4 pedicle. 9. L4 spinous process. 10. Anterior sacral foramen.

The first lumbar vertebral foramen contains the conus medullaris of the spinal cord, while lower foramina contain the cauda equina and spinal meninges (Fig. 42.11B, Fig. 42.32A). Variation occurs in the sagittal and coronal dimensions of the lumbar vertebral canal, both within and between normal populations.

Fig. 42.32 Lumbar spine. A, Median sagittal MRI. B, Paramedian sagittal MRI lumbar spine.

L5

The fifth lumbar vertebra (Fig. 42.33) has a massive transverse process which is continuous with the whole of the pedicle and encroaching on the body. The body is usually the largest and markedly deeper anteriorly, so contributing to the lumbosacral angle.

Segmentation anomalies (sacralization) are considered below with the sacrum. The costal element of the first lumbar vertebra may form a short lumbar rib, which articulates with the transverse process, but not usually with the body, of the vertebra.

Joints between the vertebral bodies

Joints between vertebral bodies are symphyses. Typical vertebral bodies are united by anterior and posterior longitudinal ligaments and by fibrocartilaginous intervertebral discs between sheets of hyaline cartilage (vertebral end-plates).

Articulating surfaces: intervertebral discs

The intervertebral discs are the chief bonds between the adjacent surfaces of vertebral bodies from C2 to the sacrum. Except at the sites of the uncovertebral (neurocentral) joints of Luschka, disc outlines correspond with the adjacent bodies. Their thickness varies in different regions and within individual discs. Each disc consists of an outer lamellated anulus fibrosus and an inner nucleus pulposus. (Fig. 42.40A1).

In cervical and lumbar regions the discs are thicker anteriorly, contributing to the anterior convexity of the vertebral column. In the thoracic region they are nearly uniform, and the anterior concavity is largely due to the vertebral bodies. Discs are thinnest in the upper thoracic region and thickest in the lumbar region. They adhere to thin layers of cartilage on the superior and inferior vertebral surfaces, the vertebral end-plates. The latter do not reach the periphery of the vertebral bodies but are encircled by ring apophyses. The end-plates contain both hyaline cartilage and fibrocartilage. The fibrocartilaginous component lies nearer to the disc, and is sometimes considered not to be part of the end-plate itself. The fibrocartilaginous components of the end-plates above and below the nucleus pulposus, together with the innermost lamellae of the anulus fibrosus, form a flattened sphere of collagen which surrounds and encloses the nucleus (Fig. 42.43). The overall proportion of fibrocartilage in the end-plate increases with age. While all discs are attached to the anterior and posterior longitudinal ligaments, discs in the thoracic region are additionally tied laterally, by intra-articular ligaments, to the heads of ribs articulating with adjacent vertebrae. Intervertebral discs form about a quarter of the length of the postaxial vertebral column: cervical and lumbar regions make a greater contribution than the thoracic and are thus more pliant.

Fig. 42.43 Structure of the vertebral end-plate: the collagen fibres of the inner two-thirds of the anulus fibrosus sweep around into the vertebral end-plate and form its fibrocartilaginous component. The peripheral fibres of the anulus are anchored into the bone of the ring apophysis.

(By permission from Bogduk N 1997 Clinical Anatomy of the Lumbar Spine and Sacrum, 3rd edn. Edinburgh: Churchill Livingstone.)

Anulus fibrosus

The anulus fibrosus has a narrow outer collagenous zone and a wider inner fibrocartilaginous zone. Its lamellae, which are convex peripherally when seen in vertical section, are incomplete collars. The internal vertical concavity of the lamellae conforms to the surface profile of the nucleus pulposus. In all quadrants of the anulus, about half the lamellae are incomplete; the proportion increases in the posterolateral region. The exact nature of the interlamellar substance remains in some doubt. Posteriorly, lamellae join in a complex manner. Fibres in the rest of each lamella are parallel and run obliquely between vertebrae at about 65° to the vertical (Fig. 42.40A2). Fibres in successive lamellae cross each other obliquely in opposite directions, thus limiting rotation. The obliquity of fibres in deeper zones varies in different lamellae. Posterior fibres may sometimes be predominantly vertical, which possibly predisposes them to herniation.

This standard description of the anulus may not apply at all spinal levels: a recent cadaveric study indicates that the anulus is usually incomplete posteriorly in adult cervical discs (Mercer & Bogduk 1999) (Fig. 42.40A3).

Nucleus pulposus

The nucleus pulposus is better developed in cervical and lumbar regions and lies between the centre of the disc and its posterior surface. At birth it is large, soft, gelatinous and composed of mucoid material. It contains a few multinucleated notochordal cells and is invaded by cells and collagen fibres from the inner zone of the adjacent anulus fibrosus. Notochordal cells disappear in the first decade, and the mucoid material is gradually replaced by fibrocartilage, derived mainly from the anulus fibrosus and the plates of hyaline cartilage adjoining the vertebral bodies. The nucleus pulposus becomes less differentiated from the remainder of the disc as age progresses, and gradually becomes less hydrated and increasingly fibrous. The type II collagen of the nucleus becomes more like the type I of the anulus as its fibril diameter increases. The quantity of aggregated proteoglycans in the nucleus decreases, while the keratan sulphate/chondroitin sulphate ratio increases. As increased cross-linking occurs between collagen and the proteoglycans the discs lose their water-binding capacity, become stiffer and more liable to injury. Contrary to what was previously thought, it has now been shown that lumbar discs do not decrease in overall height as a part of normal ageing. The anulus gradually loses height as its radial bulge increases, but the nucleus retains height and may increase in convexity as it increasingly indents the end-plate. Loss of trunk height with age results from a decrease in vertebral body height (Bogduk 2005). When the disc is not loaded, pressure in the nucleus pulposus is low at all ages.

For a review of the structure and function of the human intervertebral disc see Adams et al (2006).

Ligaments

The ligaments associated with the joints between the vertebral bodies are described on page 728.

Vascular supply

Small offshoots of spinal branches of arteries supplying the vertebral column (see p. 714) form an anastomosis on the outer surface of the anulus fibrosus and supply its most peripheral fibres. Normal discs are otherwise avascular and are dependent for their nutrition on diffusion from vertebral bone beneath adjacent end-plates and from the peripheral anulus. Vascular and avascular parts differ in their reaction to injury. Venous drainage is via the external and internal vertebral venous plexuses to the intervertebral veins and thence to the larger named veins which drain the vertebral column. Lymphatic drainage of the vertebral column is briefly considered above (see p. 715). Nothing specific is known about the lymphatic drainage of the disc.

Innervation

The nerve supply of intervertebral discs has been studied in detail in the lumbar region (Fig. 42.44). The outer third of the anulus is innervated by the sinuvertebral nerves (see p. 754): each sinuvertebral nerve supplies both the disc at the level of its spinal nerve of origin and the disc one level above. The anterior part of the anulus is supplied by the sympathetic (grey ramus communicans) rather than by the mixed nerve. In damaged and degenerate discs the nerves may penetrate more centrally into the disc substance. The sinuvertebral nerves are condensations within extensive nerve plexuses which lie on the posterior longitudinal ligament. Similar plexuses have been demonstrated anteriorly, covering the anterior longitudinal ligament, and laterally in the fetus.

Relations and ‘at risk’ structures

Posterior, lateral and anterior relations of the intervertebral disc are important in the planning of interventional investigative and therapeutic procedures ranging from discography to open disc surgery. The posterolateral surface of the disc forms the anterior boundary of the intervertebral foramen on each side, and so is closely related to the spinal nerve and its accompanying vessels. More centrally the disc is related posteriorly to the dura mater covering the spinal cord and the cauda equina. Anterior relations of the discs vary considerably with vertebral level, but important ‘at risk’ structures include the pharynx and oesophagus, the descending aorta and the inferior vena cava. Laterally, relations change with level, but the parietal pleura in the thorax, and the sympathetic trunk and psoas muscles in the lumbar region, are important examples.

Prolapsed intervertebral disc

A prolapsed intervertebral disc most commonly affects the 20–55 year age group, and is most often seen at the L4/5 and lumbosacral levels. It may also affect the cervical discs, particularly at C5/6 and C6/7. The thoracic discs are rarely affected. Acute tearing or chronic degeneration of the posterior lamellae of the anulus fibrosus allows deformation and herniation of the disc contents. The disc most often prolapses just lateral to the posterior longitudinal ligament and can compress one or two spinal nerves unilaterally (Fig. 42.45). Much less commonly, the prolapse is central, in the midline posteriorly. The compression of neural structures may then be bilateral, affecting the cord itself or the whole cauda equina. If the damaged anulus ruptures completely, some of the nuclear tissue may escape into the vertebral and ‘root’ canals. This sequestrated material may migrate within the canals and cause nerve compression at spinal levels distant from that of the disc rupture. The disc material itself may have an irritative effect on the spinal nerve.

Fig. 42.45 Posterolateral disc prolapse. A, Axial view. B, Sagittal view.

Regarding the anatomy of the vertebral canal and intervertebral foramen in relation to disc prolapse, it is important to understand that one or both of two spinal nerves and their roots may be affected by a single prolapse, depending upon the exact site of the prolapse in the horizontal plane. At the level of each disc and foramen, there are two spinal nerves (and their roots) to consider: these are the exiting nerve and the traversing nerve (Macnab & McCulloch 1990) (Fig. 42.46). The nerve usually affected at lumbar levels is the traversing nerve, which crosses the back of the disc on its way to become the exiting nerve at the level below. Thus a lumbosacral (i.e. L5/S1) disc prolapse usually compresses the S1 nerve. However, a prolapse may affect the exiting nerve at its own level. This is especially likely if the prolapse is in the extraforaminal zone of the ‘root’ canal, the so-called ‘far lateral’ prolapse. At cervical levels, because the roots and nerve leave the vertebral canal almost horizontally, the prolapse usually affects the exiting nerve. This nerve will still bear the number of the vertebra below the affected disc, because cervical nerves exit the canal above the pedicle of their numerically corresponding vertebra. Neurological presentation will include signs and symptoms of spinal nerve damage at the affected level. Thus pain and sensory loss will be dermatomal in distribution. Sensory changes usually precede motor loss.

Fig. 42.46 Exiting and traversing nerve roots. The upper root (open arrow) is the exiting root at this level: the lower (closed arrow) is the traversing root here, which becomes the exiting root at the level below. The dotted roots are traversing roots of the lower segment.

Internal disruption of a lumbar intervertebral disc is more common than disc prolapse, and is now an increasingly recognized cause of back pain. Typically, the nucleus is decompressed and the inner lamellae of the anulus appear to collapse into it.

For more detail on disc pathology and its consequences, see Adams et al (2006).

Facet (zygapophysial) joints

Joints between the vertebral articular processes (zygapophyses) are synovial and have long been called zygapophysial joints by anatomists. However, in current clinical practice they are commonly called ‘facet joints’.

Detailed description of the anatomy of these joints is to be found in Bogduk (2005), where Bogduk explains why the term ‘facet joint’ is both incorrect and essentially ambiguous, on the grounds that facets are not restricted to zygapophysial articular processes and occur, for example, in both costovertebral and costotransverse joints.

Atlanto-occipital joints

The atlas articulates with the occipital bone of the skull by a pair of synovial joints. The bones are connected by articular capsules and by the anterior and posterior atlanto-occipital membranes (Fig. 42.47, Fig. 42.48).

Fig. 42.47 Atlanto-occipital and atlanto-axial joints: anterior aspect. On each side a small cleft has been opened between the lateral part of the upper surface of the body of the third cervical vertebra and the bevelled, inferior surface of the body of the axis.

Fig. 42.48 Atlanto-occipital and atlanto-axial joints: posterior aspect.

Articulating surfaces

Each joint consists of two reciprocally curved articular surfaces, one on the occipital condyle and the other on the lateral mass of the atlas. The atlantal facets are concave and tilted medially.

Fibrous capsules

The fibrous capsules surround the occipital condyles and superior atlantal articular facets. They are thicker posteriorly and laterally, where the capsule is sometimes deficient, and may communicate with the joint cavity between the dens and the transverse ligament of the atlas.

Ligaments

The ligaments are the anterior and posterior atlanto-occipital membranes.

Anterior atlanto-ocipital membrane

The anterior atlanto-occipital membrane (Fig. 42.47, Fig. 42.49) is a broad, dense fibrous structure which connects the anterior margin of the foramen magnum to the upper border of the anterior arch of the atlas. Laterally, it blends with the joint capsule, and medially it is strengthened by a median cord, which is the anterior longitudinal ligament stretching between the basilar occipital bone and anterior atlantal tubercle.

Fig. 42.49 Median sagittal section through the occipital bone and first to third cervical vertebrae.

Posterior atlanto-occipital membrane

The posterior atlanto-occipital membrane (Fig. 42.48, Fig. 42.49) is broad, but relatively thin, and connects the posterior margin of the foramen magnum to the upper border of the posterior atlantal arch, blending laterally with the joint capsules. It arches over the grooves for the vertebral arteries, venous plexuses and first cervical nerve, and forms the ‘floor’ of the suboccipital triangle. The ligamentous border of this arch is sometimes ossified. The membrane is usually attached by a soft-tissue ‘bridge’, which may contain muscle or tendon fibres, to rectus capitis posterior minor, and is firmly attached anteriorly to the spinal dura in the same area (Zumpano et al 2006). The ligaments connecting the axis and the occipital bone are functionally involved with the posterior atlanto-occipital membrane.

Synovial membrane

The synovial cavities of one or both joints may communicate with that of the posterior component of the median atlanto-axial joint.

Vascular supply

The arterial supply of this region is derived from an anastomosis between branches of the deep cervical, occipital and vertebral arteries.

Innervation

The joints are innervated by branches of the ventral primary ramus of the first cervical spinal nerves.

Factors maintaining stability

Factors maintaining stability include the fibrous capsules, the atlanto-occipital membranes, the shape of the articular surfaces, the ligaments connecting the axis and the occipital bone, the ligamentum nuchae and the posterior neck muscles. The suboccipital muscles play an important proprioceptive and postural role.

Movements and muscles

The orientation and shape of the atlanto-occipital joints allow mainly for flexion-extension, which has a total range of about 15°. Axial rotation and lateral flexion have not been measured in living subjects, but a few degrees of motion in these directions can be produced by forceful movement in cadavers.

The following muscles produce these movements. For flexion: longus capitis and rectus capitis anterior; for extension: recti capitis posteriores major and minor, obliquus capitis superior, semispinalis capitis, splenius capitis and trapezius (cervical part); for lateral flexion: rectus capitis lateralis, semispinalis capitis, splenius capitis, sternocleidomastoid and trapezius (cervical part); and for rotation: obliquus capitis superior, rectus capitis posterior minor, splenius capitis and sternocleidomastoid.

Relations and ‘at risk’ structures

Posteriorly the joints are closely related to the vertebral arteries as they pass from the foramina transversaria into the foramen magnum. The dorsal primary ramus of the first cervical nerve and rectus capitis posterior major lie posteromedially and rectus capitis anterior lies anteriorly.

Atlanto-axial joints

The atlas articulates with the axis at three synovial joints. These are a pair between the lateral masses and a median complex between the dens of the axis and the anterior arch and transverse ligament of the atlas.

Articulating surfaces

The articular surfaces of the joints between the lateral masses are often classified as planar. The bony articular surfaces are more complex in shape and are usually reciprocally concave in the coronal plane; the medial parts are somewhat convex in the sagittal plane (especially that of the axis). The cartilaginous articular surfaces are usually less concave. The median joint is a pivot between the dens and a ring formed by the anterior arch and transverse ligament of the atlas. A vertically ovoid facet on the anterior dens articulates with a facet on the posterior aspect of the anterior atlantal arch.

Fibrous capsules

The fibrous capsules for the lateral joints are attached to the articular margins and are thin and loose. Each has a posteromedial accessory ligament attached below to the axial body near the base of its dens, and above to the lateral atlantal mass near the transverse ligament. The fibrous capsule for the median joint is also relatively weak and loose, especially superiorly.

Ligaments

Anteriorly, the vertebral bodies are connected by the anterior longitudinal ligament (Fig. 42.47, Fig. 42.49): here a strong, thickened band attaches above to the lower border of the anterior tubercle of the anterior arch of the atlas and below to the front of the axial body. Posteriorly the vertebral arches are joined by the ligamenta flava (Fig. 42.48, Fig. 42.49) which are attached to the lower border of the atlantal arch above, and to the upper borders of the axial laminae. At this level these ligaments form a thin membrane, pierced laterally by the second cervical nerves.

Transverse atlantal and cruciform ligaments

The transverse atlantal ligament (Fig. 42.50) is a broad, strong band which arches across the atlantal ring behind the dens: its length varies about a mean of 20 mm. It is attached laterally to a small but prominent tubercle on the medial side of each atlantal lateral mass, and broadens medially where it is covered anteriorly by a thin layer of articular cartilage. It consists almost entirely of collagen fibres, which, in the central part of the ligament, cross one another at an angle to form an interlacing mesh. From its upper margin a strong median longitudinal band arises which inserts into the basilar part of the occipital bone between the apical ligament of the dens and membrana tectoria, and from its inferior surface a weaker and less consistent longitudinal band passes to the posterior surface of the axis. These transverse and longitudinal components together constitute the cruciform ligament. The transverse ligament divides the ring of the atlas into unequal parts (Fig. 42.22). The posterior two-thirds surrounds the spinal cord and meninges, the anterior third contains the dens, which the transverse ligament retains in position even when all other ligaments are divided.

Fig. 42.50 Posterior aspect of the atlanto-occipital and atlanto-axial joints. The posterior part of the occipital bone and the laminae of the cervical vertebrae have been removed and the atlanto-occipital joint cavities opened.

(Adapted from Sobotta 2006.)

Ligaments connecting axis and occipital bone

Ligaments connecting the axis and the occipital bone consist of the membrana tectoria, the paired alar ligaments, the median apical ligament, and the longitudinal components of the cruciform ligament (Fig. 42.49, Fig. 42.50).

Membrana tectoria.

Inside the vertebral canal, the membrana tectoria is a broad strong band representing the upward continuation of the posterior longitudinal ligament. Its superficial and deep laminae are both attached to the posterior surface of the axial body. The superficial lamina expands as it ascends to the upper surface of the basilar occipital bone, and attaches above the foramen magnum, where it blends with the cranial dura mater. The deep lamina consists of a strong median band which ascends to the foramen magnum, and two lateral bands which pass to, and blend with, the capsules of the atlanto-occipital joints as they reach the foramen magnum. The membrane is separated from the cruciform ligament of the atlas by a thin layer of loose areolar tissue, and sometimes by a bursa.

Alar ligaments.

The alar ligaments are thick cords, about 11 mm long, which pass horizontally and laterally from the longitudinally ovoid flattenings on the posterolateral aspect of the apex of the dens to the roughened areas on the medial side of the occipital condyles. In most individuals there is also an anteroinferior band, approximately 3 mm long, which inserts into the lateral mass of the atlas in front of the transverse ligament. Fibres occasionally pass from the dens to the anterior arch of the atlas. In addition, in some 10% of cases a continuous transverse band of fibres, the transverse occipital ligament, passes between the occipital condyles immediately above the transverse ligament. The ligaments consist mainly of collagen fibres arranged in parallel. The main function of the alar ligaments is now considered to be limitation of atlanto-axial rotation, the left becoming taut on rotation to the right and vice versa. The slightly downward movement of the atlas during rotation helps permit a wider range of movement by reducing tension in the alar ligaments, and in the capsules and accessory ligaments of the lateral atlanto-occipital joint.

Apical ligament of the dens.

The apical ligament of the dens fans out from the apex of the dens into the anterior margin of the foramen magnum between the alar ligaments, and represents the cranial continuation of the notochord and its sheath. It is separated for most of its extent from the anterior atlanto-occipital membrane and cruciform ligament by pads of fatty tissue, though it blends with their attachments at the foramen magnum, and with the alar ligaments at the apex of the dens.

The ligamentum nuchae and the anterior longitudinal ligament also connect cervical vertebrae with the cranium.

Synovial membrane

The synovial membranes of the lateral joints have no special features. The median joint has two synovial cavities which sometimes communicate. The synovial cavity of the posterior component of the median joint complex is larger, lying between the horizontally orientated ovoid facet, on the posterior surface of the dens and the cartilaginous anterior surface of the transverse ligament: communication often exists with one or both of the atlanto-occipital joint cavities.

Vascular supply

The arterial supply of this region is derived from an anastomosis between branches of the deep cervical, occipital and vertebral arteries.

Innervation

The joints are innervated by branches of the ventral primary ramus of the second cervical spinal nerve.

Factors maintaining stability

The most important factors maintaining stability are the ligaments, of which the transverse atlantal ligament is the strongest. The alar ligaments are weaker. Other ligaments connecting the axis and the occipital bone, the fibrous capsules, the ligamentum nuchae and the posterior neck muscles also contribute to stability: the suboccipital muscles play an important proprioceptive and postural role.

Movements and muscles

Movement is simultaneous at all three joints, and consists almost exclusively of rotation around the axis. The shape of the articular surfaces is such that, when rotation occurs, the axis ascends slightly into the atlantal ring, limiting stretch on the lateral atlanto-axial joint capsules. Rotation is limited mainly by the alar ligaments, with a minor contribution from the accessory atlanto-axial ligament. The normal range of atlanto-axial rotation is about 40°.

The muscles that produce atlanto-axial rotation act on the cranium, transverse processes of the atlas and spinous process of the axis. They are mainly obliquus capitis inferior, rectus capitis posterior major and splenius capitis of one side, and the contralateral sternocleidomastoid.

Relations and ‘at risk’ structures

The most important ‘at risk’ relation is the spinal cord, lying posterior to the median atlanto-axial joint. Anteriorly the atlanto-axial articulations, capsules and ligaments are separated from the buccopharyngeal fascia and superior constrictor by longus capitis and longus colli, the prevertebral fascia and the retropharyngeal (potential) space.

Variants of craniovertebral joints

Anomalies of the craniovertebral joints are sometimes seen. Many are minor segmentation anomalies, and may be incidental radiological or osteological findings. More severe anomalies may lead to craniocervical instability (Piper & Traynelis 1998).

Muscles producing vertebral movements

The spinal column is moved both directly by muscles attached to it, and indirectly by muscles attached to other bones. Gravity always plays a part: movements with gravity usually involve the eccentric contraction of antagonists. Flexion is effected by longus capitis and longus colli, scaleni, sternocleidomastoid and rectus abdominis of both sides, aided in the lumbar region by the abdominal obliques; extension by the erector spinae complex and the transversospinalis group, splenius, semispinalis capitis and trapezius of both sides, together with the sub-occipital muscles; lateral flexion by ipsilateral longissimus, iliocostalis, abdominal obliques and flexors, and quadratus lumborum; rotation by sternocleidomastoid, splenius cervicis, abdominal obliques, rotatores and multifidus.

Vertebral spines

In the midline a median furrow runs from the external occipital protuberance above to the natal cleft below (Fig. 42.60, Fig. 42.61, Fig. 42.62). The furrow is most shallow in the lower cervical region and is deepest in the midlumbar zone. Inferiorly, it widens out into a flattened, triangular area, the apex of which lies at the start of the natal cleft and corresponds to the third sacral spine. Palpation of the median furrow reveals the sagittal curves of the spine: the cervical curve is convex forwards (lordosis) and extends from the first cervical to the second thoracic vertebra; the thoracic curvature is concave forwards (kyphosis) and extends from the second to the 12th thoracic vertebra; the lumbar curvature is convex forwards and extends from the 12th thoracic vertebra to the lumbosacral prominence. The external occipital protuberance is subcutaneous and can be felt and often seen (Fig. 42.61), and it can be palpated without difficulty when it is approached from below. The inion is the point situated on this protuberance in the median plane. The tips of the spines of the cervical vertebrae are obscured by the overlying ligamentum nuchae. The tubercle on the posterior arch of the atlas is impalpable; the first bony prominence which is encountered when the finger is drawn downwards in the midline from the external occipital protuberance is the spine of the second cervical vertebra. The ligamentum nuchae terminates inferiorly at the spine of the seventh cervical vertebra, which is the highest, and sometimes the only, visible projection in this region (vertebra prominens). Immediately below this the spine of the first thoracic vertebra is palpable; it is usually more prominent than the seventh cervical vertebra. The spine of the second thoracic vertebra can also often be felt. The third thoracic spinous process is level with the spine of the scapula (Fig. 42.60), and the seventh with the inferior scapular angle when the arm is by the side. The identification of the remaining thoracic spines is not easy, even in a thin subject when the trunk is fully flexed, because of the manner in which they overlap one another in the midthoracic region. In the upper and lower thoracic regions, the tips of the thoracic spines lie opposite the upper part of the body of the immediately subjacent vertebra. In the midthoracic region, they lie opposite the lower part of the vertebra below. The tip of the spine of each lumbar vertebra can usually be palpated without difficulty, especially if the trunk is flexed. Each lies opposite the inferior part of its own body. The body of the fourth lumbar vertebra is level with the summits of the iliac crests, so the fourth lumbar spine overlies the L4/5 interspace (a point useful in lumbar puncture). The second sacral spine is level with the posterior superior iliac spines. Sacral and coccygeal landmarks are described below.

Fig. 42.60 Back view of trunk. 1. Vertebra prominens (spine of C7) 2. Supracristal plane. 3. Spine of scapula. 4. Position of sacral hiatus.

Fig. 42.61 Back view of trunk, arms abducted. 1. External occipital protuberance. 2. Inferior angle of scapula. 3. Erector spinae. 4. Trapezius. 5. Latissimus dorsi. 6. Posterior superior iliac spine underlying sacral dimple. 7. Natal cleft.

Fig. 42.62 Back of trunk, oblique view. 1. Median furrow. 2. Lateral border of erector spinae. 3. Gluteus maximus. 4. Gluteal fold.

Note that when the subject lies in the lateral position (on the side), the median soft-tissue furrow may not coincide with the median plane, especially in the lumbar region in the obese. Careful palpation may be necessary to identify the spines, a point which is of particular importance during lumbar or epidural puncture in this position.

Scapular landmarks

The shape of the back in the upper thoracic region is determined largely by the scapula and the muscles which attach to it, especially trapezius. The relative prominence of the scapula and its muscles depends upon the state of contraction of the latter. Bony scapular landmarks are most evident when the arms hang by the sides.

The scapula overlies the second to seventh ribs. Its superior angle is palpable beneath trapezius, and its inferior angle is just covered by latissimus dorsi: these angles are joined by the medial border of the scapula which runs vertically (Fig. 42.60, Fig. 42.61). The scapular spine runs subcutaneously and is easily palpable from its root medially to the acromion process laterally.

The root of the spine of the scapula lies opposite the third thoracic spine and the inferior angle lies opposite the seventh thoracic spine when the arm is by the side.

Posterior pelvic and sacrococcygeal bony landmarks

At the lower part of the back the iliac crest can be palpated throughout its whole length, and can be traced backwards and upwards from the anterior superior iliac spine to its highest point, and then downwards and medially to the posterior superior iliac spine, which is overlain by a dimple in the skin (Fig. 42.61). A line joining these sacral dimples passes through the second sacral spine and the body of the second sacral vertebra, and is a useful landmark for the lower end of the adult dural sac. There may also be a less prominent pair of dimples at the level of the L4/5 intervertebral disc. The posterior superior iliac spine lies over the centre of the sacroiliac joint. The supracristal plane joins the highest points of the iliac crest on each side. It passes through the body of the fourth lumbar vertebra and has been used as a clinical landmark when performing a lumbar puncture (p. 760) though difficulty in its definition reduces its reliability in this procedure. The tip of the coccyx can be felt deeply near the centre of the natal cleft. As the examining finger passes cranially, the sacral cornua can be felt on either side: these demarcate the sacral hiatus, and form the landmark for performing a caudal anaesthetic block.