CHAPTER 42 The back

Most clinical disorders of the back present as low back pain with or without associated lower limb pain, so historically most attention has been paid to the anatomy of the lower (lumbosacral) back. In this Section, the term ‘the back’ will include the whole of the posterior aspect of the trunk and of the neck. The whole of this region has great clinical importance but its anatomy has often been neglected. Recent understanding of the detailed topography of the bony and soft-tissue elements of the lower back owes much to the work of Bogduk (2005).

The soft tissues of the back of the trunk and neck include the skin and subcutaneous fat, the underlying fascial layers and the musculature. The deep, ‘true’ or epaxial muscles lie within compartments in their own fascial ‘skeleton’. The bony framework to which the muscles and fasciae attach includes not only elements of the axial skeleton, i.e. the vertebral column and occiput, but also elements of the pectoral and pelvic girdles as well as the ribs. The occiput is described below, the scapula on page 793, the ribs on page 918 and the pelvis on page 1352.

SKIN

The skin of the back of the trunk is thick and highly protective, but has low discriminatory sensation. The superficial fascia is thick and fatty in most areas of the back. Its attachment to the deeper fascial layers is strong in the midline, especially in the neck, but becomes weaker more laterally. The skin of the back of the neck is thicker than that of the front of the neck, but thinner than that of the back of the trunk. The quantity, texture and distribution of hair vary with sex, race and the individual, though well-defined hair tracts have been delineated (Fig. 42.1).

Fig. 42.1 Hair tracts on the dorsal surface of the body.

(Redrawn by permission from Wood Jones F (ed) 1949 Buchanan’s Manual of Anatomy, 8th edn. London: Baillière Tindall and Cox.)

Lines of skin tension run horizontally in the cervical and lumbosacral regions but form segments of two adjacent circles in the thoracic region (Fig. 42.2).

Fig. 42.2 Lines of skin tension on the dorsum of the trunk and head.

(From Kraissl CL, Plast Reconstruct Surg 8: 1–28, 1951. By permission from Lippincott Williams and Wilkins.)

CUTANEOUS INNERVATION AND DERMATOMES

The skin of the back of the neck and trunk is innervated by the dorsal (posterior primary) rami of the spinal nerves (see Fig. 43.6 and pp. 755–756 where dorsal rami are covered in detail). In the cervical and upper thoracic regions (down to T6) skin is supplied by the medial branches of these rami, while in the lower thoracic, lumbar and sacral regions it is supplied by the lateral branches. The total area supplied by these dorsal rami is shown in Fig. 43.6. The spinal nerves involved include C2 to C5, T2 to L3, S2 to S4, and Co1. The pattern of their dermatomes is shown in Fig. 42.3. There is about half a segment of overlap between these cutaneous ‘strips’: the strips supplied by the dorsal rami do not correspond exactly to those served by ventral rami, and differ slightly in both width and position.

Fig. 42.3 Dermatomes on the dorsal surface of the body. The small diagram shows the regular arrangement of dermatomes in the upper and lower limbs of the embryo.

(Adapted with permission from Moffat DB 1993 Lecture Notes on Anatomy, 2nd edn. Oxford: Blackwell Scientific.)

CUTANEOUS VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

The skin of the back of the trunk receives its arterial blood supply mainly from musculocutaneous branches of posterior intercostal, lumbar and lateral sacral arteries (Fig. 42.4), which all accompany the cutaneous branches of their respective dorsal rami. In addition, there is a supply from the dominant vascular pedicles of the superficial (extrinsic) back muscles. The skin over the scapula is supplied by branches of the suprascapular, dorsal scapular and subscapular arteries. The skin of the back of the neck is supplied mainly from the occipital and deep cervical arteries. The superficial cervical or transverse cervical artery supplies the skin of the lower part of the back of the neck (see Cormack & Lamberty 1994 in the Bibliography).

Fig. 42.4 Areas of cutaneous arterial supply on the dorsum of the trunk.

(Redrawn with permission from Cormack GC, Lamberty BGH 1994 The Arterial Anatomy of Skin Flaps. Edinburgh: Churchill Livingstone.)

Veins drain the skin of the back of the neck into tributaries of the occipital and deep cervical veins. The skin of the back of the trunk drains into the azygos system, via tributaries of the posterior intercostal and lumbar veins.

Lymph from the skin of the back of the neck drains into occipital, lateral deep cervical and axillary nodes (Fig. 42.5). From the back of the trunk, drainage is to the posterior (subscapular) axillary nodes and to the lateral superficial inguinal nodes.

Fig. 42.5 Cutaneous lymphatics on the dorsum of the trunk.

(By permission from Romanes GJ (ed) 1964 Cunningham’s Textbook of Anatomy, 10th edn. London: Oxford University Press.)

FASCIAL LAYERS

The main fascial layers in the axial and paraxial regions of the trunk and neck are the thoracolumbar fascia, the deep cervical fascia and the prevertebral, endothoracic, retroperitoneal and posterior part of the pelvic fasciae (the latter four layers collectively form the continuous prevertebral plane). Other important structures with fascial components are the ligamentum nuchae and the aponeurosis of erector spinae (see below).

THORACOLUMBAR FASCIA

The thoracolumbar (lumbodorsal) fascia covers the deep muscles of the back and the trunk. Above, it passes anterior to serratus posterior superior and is continuous with the superficial lamina of the deep cervical fascia on the back of the neck. In the thoracic region the thoracolumbar fascia provides a thin fibrous covering for the extensor muscles of the vertebral column and separates them from the muscles connecting the vertebral column to the upper extremity. Medially it is attached to the spines of the thoracic vertebrae, and laterally to the angles of the ribs. In the lumbar region the thoracolumbar fascia is in three layers (Fig. 42.6, Fig. 42.7, Fig. 62.1). The posterior layer is attached to the spines of the lumbar and sacral vertebrae and to the supraspinous ligaments. The middle layer is attached medially to the tips of the lumbar transverse processes and the intertransverse ligaments, below to the iliac crest, and above to the lower border of the 12th rib and the lumbocostal ligament. The anterior layer covers quadratus lumborum and is attached medially to the anterior surfaces of the lumbar transverse processes behind the lateral part of psoas major; below, it is attached to the iliolumbar ligament and the adjoining part of the iliac crest; above, it forms the lateral arcuate ligament. The posterior and middle layers unite to form a tough raphe at the lateral margin of erector spinae, and at the lateral border of quadratus lumborum they are joined by the anterior layer to form the aponeurotic origin of transversus abdominis. At sacral levels, the posterior layer is attached to the posterior superior iliac spine and posterior iliac crest, and fuses with the underlying erector spinae aponeurosis. Bogduk (2005) describes two laminae in the posterior layer at lumbar levels, with varying orientation of the constituent collagen fibres relating to the biomechanical function of the fascia. The posterior and middle layers of the thoracolumbar fascia and the vertebral column together form an osteofascial compartment which encloses the erector spinae muscle group. The attachments of the fascia, especially those which give it continuity with the abdominal wall musculature, putatively give it an important role in lifting, though the exact details of this role remain controversial. The fascia may also play an important role in load transfer between the trunk and the limbs: its tension is affected by the actions of latissimus dorsi, gluteus maximus, and the hamstrings. An erector spinae compartment syndrome may be one cause of low back pain.

Fig. 42.6 Muscles and fasciae of the posterior abdominal wall.

Fig. 42.7 Axial CT scan of the abdomen in a 32-year-old female.

DEEP CERVICAL FASCIA

The investing layer of the deep cervical fascia forms the deep fascia of the posterior aspect of the neck (Fig. 28.4). It attaches in the midline to the external occipital protuberance, the ligamentum nuchae and the spine of the seventh cervical vertebra, and splits to enclose trapezius on each side. Inferiorly the posterior part of the investing layer attaches with trapezius to the spine and acromion of the scapula.

BONE

OCCIPITAL BONE

The occipital bone forms much of the back and base of the cranium (Fig. 42.8). It is trapezoid, internally concave and encloses the foramen magnum. It has four parts: basilar (basioccipital), which is the quadrilateral part in front of the foramen magnum; squamous, which is the expanded plate posterosuperior to the foramen; and two lateral (condylar or exoccipital), one on each side of the foramen magnum. The latter is anteromedian and wider behind, with an anteroposterior diameter greater than its transverse diameter.

Fig. 42.8 Occipital bone. A, External surface; B, internal surface; C, muscle attachments. (A and B by permission from Berkovitz BKB, Moxham BJ 1994 Colour Atlas of the Skull. London: Mosby.) A: 1. Highest nuchal line. 2. Superior nuchal line. 3. Inferior nuchal line. 4. Hypoglossal canal. 5. Occipital condyle. 6. External occipital protuberance. 7. Lambdoid margin. 8. Squamous part. 9. External occipital crest. 10. Condylar canal. 11. Foramen magnum. B: 1. Lambdoid margin. 2. Internal occipital protuberance at ‘confluence of sinuses’. 3. Lateral angle. 4. Mastoid margin. 5. Groove for sigmoid sinus. 6. Jugular process. 7. Jugular notch. 8. Groove for inferior petrosal sinus. 9. Cerebral fossa. 10. Groove for transverse sinus. 11. Internal occipital crest. 12. Cerebellar fossa. 13. Foramen magnum. 14. Jugular tubercle. 15. Margin of basilar part for articulation with body of sphenoid.

The occipital bone provides attachment for a number of the muscles of the neck and upper part of the back (longus capitis; recti capitis anterior, lateralis, posterior major and minor; superior oblique; semispinalis capitis; splenius capitis; occipitofrontalis; trapezius; sternocleidomastoid), and articulates with the first cervical vertebra at the atlanto-occipital joints.

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).

Muscle attachments

The occipital part of occipitofrontalis is attached to the lateral part of the highest nuchal line (Fig. 42.8C). Trapezius attaches to the medial third of the superior nuchal line and to the external occipital protuberance. Sternocleidomastoid attaches to the lateral half of the superior nuchal line, with splenius capitis just below the lateral third of that line. Semispinalis capitis is attached to the medial part of the area between the superior and inferior nuchal lines; obliquus capitis superior attaches to the lateral part of this area. Rectus capitis posterior major attaches to the lateral part of the inferior nuchal line and to the bone immediately below, and rectus capitis posterior minor attaches to the medial part of the inferior nuchal line and to the bone between that line and the foramen magnum.

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.

VERTEBRAL COLUMN

The vertebral column is a curved linkage of individual bones or vertebrae (Fig. 42.10). A continuous series of vertebral foramina runs through the articulated vertebrae posterior to their bodies, and collectively constitutes the vertebral canal, which transmits and protects the spinal cord and nerve roots, their coverings and vasculature (Fig. 42.11, 42.16). A series of paired lateral intervertebral foramina transmits the spinal nerves and their associated vessels between adjacent vertebrae. The linkages between the vertebrae include cartilaginous interbody joints and paired synovial facet (zygapophysial) joints, together with a complex of ligaments and overlying muscles and fasciae. The muscles directly concerned with vertebral movements and attached to the column lie mainly posteriorly. Several large muscles producing major spinal movements lie distant from the column and without direct attachment to it, e.g. the anterolateral abdominal wall musculature. Movements of the column and the muscles concerned are described on page 743. The column as a whole receives its vascular supply and innervation according to general anatomical principles which are considered below.

Fig. 42.10 The vertebral column: A, anterior aspect; B, posterior aspect; C, lateral aspect.

(From Sobotta 2006.)

Fig. 42.11 Sagittal MRI of the cervical (A) and lumbar (B) spine.

Vertebral column morphology is influenced externally by mechanical and environmental factors and internally by genetic, metabolic and hormonal factors. These all affect its ability to react to the dynamic forces of everyday life, such as compression, traction and shear. These dynamic forces can vary in magnitude and are much influenced by occupation, locomotion and posture.

The adult vertebral column usually consists of 33 vertebral segments. Each presacral segment (except the first two cervical) is separated from its neighbour by a fibrocartilaginous intervertebral disc. The functions of the column are to support the trunk, to protect the spinal cord and nerves, and to provide attachments for muscles. It is also an important site of haemopoiesis throughout life. Its total length in males is about 70 cm and in females about 60 cm. The intervertebral discs contribute about a quarter of this length in young adults, though there is some diurnal variation in this contribution (p. 744). Approximately 8% of overall body length is accounted for by the cervical spine, 20% by the thoracic, 12% by the lumbar and 8% by the sacrococcygeal regions. Although the usual number of vertebrae is 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal, this total is subject to frequent variability, and there have been reports of variation between 32 and 35 bones. The demarcation of groups by their morphological characteristics may be blurred: thus there may be thoracic costal facets on the seventh cervical vertebra, giving it the appearance of an extra thoracic vertebra; lumbar-like articular processes may be found on the lowest thoracic vertebra; the fifth lumbar vertebra may be wholly or partially incorporated into the sacrum. As a result of these changes in transition between vertebral types, there may be 23–25 mobile presacral vertebrae.

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

Embryonic and fetal curvatures

The embryonic body appears flexed. It has primary thoracic and pelvic curves which are convex dorsally. Functional muscle development leads to the early appearance of secondary cervical and lumbar spinal curvatures in the sagittal plane. The cervical curvature appears at the end of the embryonic period, and reflects the development of function in the muscles responsible for head extension, an important component of the ‘grasp reflex’. Radiographic examination of human fetuses aged from 8 to 23 weeks shows that the secondary cervical curvature is almost always present. Lumbar flattening has also been identified as early as the eighth week. Ultrasound investigations support the role of movement in the development of these curvatures. The early appearance of the secondary curves is probably accentuated by postnatal muscular and nervous system development at a time when the vertebral column is highly flexible and is capable of assuming almost any curvature.

Neonatal curvatures

In the neonate the vertebral column has no fixed curvatures. It is particularly flexible and if dissected free from the body it can easily be bent (flexed or extended) into a perfect half circle. A slight sacral curvature can be seen which develops as the sacral vertebrae ossify and fuse. The thoracic part of the column is the first to develop a relatively fixed curvature, which is concave anteriorly. An infant can usually support its head at 3 or 4 months, sit upright at around 9 months, and will commence walking between 12 and 15 months. These functional changes exert a major influence on the development of the secondary curvatures in the vertebral column and changes in the proportional size of the vertebrae, in particular in the lumbar region. The secondary lumbar curvature becomes important in maintaining the centre of gravity of the trunk over the legs when walking starts, and thus changes in body proportions exert a major influence on the subsequent shape of curvatures in the vertebral column.

Adult curvatures

In adults, the cervical curve is a lordosis (convex forwards), and the least marked. It extends from the atlas to the second thoracic vertebra, with its apex between the fourth and fifth cervical vertebrae. Sexual dimorphism has been described in the cervical curvatures. The thoracic curve is a kyphosis (convex dorsally). It extends between the second and the 11th and 12th thoracic vertebrae, and its apex lies between the sixth and ninth thoracic vertebrae. This curvature is caused by the increased posterior height of the thoracic vertebral bodies. The lumbar curve is also a lordosis. It has a greater magnitude in females and extends from the 12th thoracic vertebra to the lumbosacral angle: there is an increased convexity of the last three segments as a result of the greater anterior height of the intervertebral discs and some posterior wedging of the vertebral bodies. Its apex is at the level of the third lumbar vertebra. The pelvic curve is concave anteroinferiorly and involves the sacrum and coccygeal vertebrae. It extends from the lumbosacral junction to the apex of the coccyx.

The presence of these curvatures means that the cross-sectional profile of the trunk changes with spinal level. The anteroposterior diameter of the thorax is much greater than that of the lower abdomen. In the normal vertebral column there are well-marked curvatures in the sagittal plane and no lateral curvatures other than in the upper thoracic region, where there is often a slight lateral curvature, convex to the right in right-handed persons, and to the left in the left-handed. Compensatory lateral curvature may also develop to cope with pelvic obliquity such as that imposed by inequality of leg length. The sagittal curvatures are present in the cervical, thoracic, lumbar and pelvic regions (Fig. 42.10). These curvatures have developed with rounding of the thorax and pelvis as an adaptation to bipedal gait.

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.

Lymphatic drainage

Little is known in detail about the lymphatic drainage of the vertebral column and its associated soft tissues. In general, deep lymphatic vessels tend to follow the arteries. The cervical vertebral column drains to deep cervical nodes, the thoracic to (posterior) intercostal nodes, and the lumbar column to lateral aortic and retro-aortic nodes. The pelvic part of the column drains to lateral sacral and internal iliac nodes.

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.

VERTEBRAE: GENERAL FEATURES

A typical vertebra has a ventral body, a dorsal vertebral (neural) arch, extended by lever-like processes, and a vertebral foramen, which is occupied in life by the spinal cord, meninges and their vessels (Fig. 42.14).

Fig. 42.14 Fourth thoracic vertebra, superior aspect. 1. Bone derived from anular epiphysis. 2. Vertebral body – bone derived from centrum. 3. Pedicle. 4. Superior articular facet. 5. Transverse process. 6. Spinous process. 7. Vertebral body – bone derived from neural arch. 8. Vertebral foramen. 9. Costal facet. 10. Lamina.

Opposed surfaces of adjacent bodies are bound together by intervertebral discs of fibrocartilage. The complete column of bodies and discs forms the strong but flexible central axis of the body and supports the full weight of the head and trunk. It also transmits even greater forces generated by muscles attached to it directly or indirectly. The foraminae form a vertebral canal for the spinal cord, and between adjoining neural arches, near their junctions with vertebral bodies, intervertebral foraminae transmit mixed spinal nerves, smaller recurrent nerves and blood and lymphatic vessels.

The cylindroid vertebral body varies in size, shape and proportions in different regions of the vertebral column. Its superior and inferior (discal) surfaces vary in shape from approximately flat (but not parallel) to sellar, with a raised peripheral smooth zone, formed from an ‘anular’ epiphysial disc, within which the surface is rough. These differences in texture reflect variations in the early structure of intervertebral discs. In the horizontal plane the profiles of most bodies are convex anteriorly, but concave posteriorly where they complete the vertebral foramen. Most sagittal profiles are concave anteriorly but flat posteriorly. Small vascular foramina appear on the front and sides, and posteriorly there are small arterial foramina and a large irregular orifice (sometimes double) for the exit of basivertebral veins (Fig. 42.15). The adult vertebral body is not coextensive with the developmental centrum but includes parts of the neural arch posterolaterally.

Fig. 42.15 Median sagittal section through a lumbar vertebra.

(From Sobotta 2006.)

Viewed anteriorly there is a cephalocaudal increase in vertebral body width from the second cervical to the third lumbar vertebra, which is associated with an increased load-bearing function. The increase is linear in the neck but not in the thoracic and lumbar regions. There is some variation in size of the last two lumbar bodies, but thereafter width diminishes rapidly to the coccygeal apex. In the two lowest lumbar vertebrae there is an inverse relation between the areas of the upper and lower surfaces of the bodies and the size of the pedicles and transverse processes.

On each side the vertebral arch has a vertically narrower ventral part, the pedicle, and a broader lamina dorsally. Paired transverse, superior and inferior articular processes project from their junctions. There is a median dorsal spinous process.

Pedicles are short, thick, rounded dorsal projections from the superior part of the body at the junction of its lateral and dorsal surfaces: the concavity formed by the curved superior border of the pedicle is shallower than the inferior one (Fig. 42.15). When vertebrae articulate by the intervertebral disc and facet joints, these adjacent vertebral notches contribute to an intervertebral foramen. The complete perimeter of an intervertebral foramen consists of the notches, the dorsolateral aspects of parts of adjacent vertebral bodies and the intervening disc, and the capsule of the synovial facet joint.

The laminae are directly continuous with the pedicles. They are vertically flattened and curve dorsomedially.

Lateral to the spinous processes, vertebral grooves contain the deep dorsal muscles. At cervical and lumbar levels these grooves are shallow and mainly formed by laminae. In the thoracic region they are deeper, broader and formed by the laminae and transverse processes. The laminae are broad for the first thoracic vertebra, narrow for the second to seventh, broaden again from the eighth to 11th, but become narrow thereafter down to the third lumbar vertebra.

The spinous process (vertebral spine) projects dorsally and often caudally from the junction of the laminae. Spines vary considerably in size, shape and direction. They lie approximately in the median plane and project posteriorly, although in some individuals a minor deflection of the processes to one side may be seen. The spines act as levers for muscles which control posture and active movements (flexion/extension, lateral flexion and rotation) of the vertebral column.

The paired superior and inferior articular processes (zygapophyses) arise from the vertebral arch at the pediculolaminar junctions. The superior processes project cranially, bearing dorsal facets which may also have a lateral or medial inclination, depending on level. Inferior processes run caudally with articular facets directed ventrally, again with a medial or lateral inclination which depends on vertebral level. Articular processes of adjoining vertebrae thus contribute to the synovial zygapophysial or facet joints, and form part of the posterior boundaries of the intervertebral foramina. These joints permit limited movement between vertebrae: mobility varies considerably with vertebral level.

Transverse processes project laterally from the pediculolaminar junctions as levers for muscles and ligaments, particularly those concerned in rotation and lateral flexion. In the cervical region, the transverse processes are anterior to the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region, they are posterior to the pedicles, considerably behind those of the cervical and lumbar processes. In the lumbar region, the transverse processes are anterior to the articular processes, but posterior to the intervertebral foramina. There is considerable regional variation in the structure and length of the transverse processes. In the cervical region, the transverse process of the atlas is long and broad, which allows the rotator muscles maximum mechanical advantage. Breadth varies little from the second to the sixth cervical vertebra, but increases in the seventh. In thoracic vertebrae, the first is widest, and breadth decreases to the 12th, where the transverse elements are usually vestigial. The transverse processes become broader in the upper three lumbar vertebrae, and diminish in the fourth and fifth. The transverse process of the fifth lumbar vertebra is the most robust. It arises directly from the body and pedicle to allow for force transmission to the pelvis through the iliolumbar ligament.

The thoracic transverse processes articulate with ribs, but at other levels the mature transverse process is a composite of ‘true’ transverse process and an incorporated costal element. Costal elements develop as basic parts of neural arches in mammalian embryos, but become independent only as thoracic ribs. Elsewhere they remain less developed and fuse with the ‘transverse process’ of descriptive anatomy (Fig. 44.11).

Vertebrae are internally trabecular, and have an external shell of compact bone perforated by vascular foramina (Fig. 42.15). The shell is thin on the superior and inferior body surfaces but thicker in the arch and its processes. The trabecular interior contains red bone marrow and one or two large ventrodorsal canals which contain the basivertebral veins.

Sexual dimorphism in vertebrae has received little attention, but Taylor & Twomey (1984) have described radiological differences in adolescent humans and have reported that female vertebral bodies have a lower ratio of width to depth. Vertebral body diameter has also been used as a basis for sex prediction in the analysis of skeletal material (MacLaughlin & Oldale 1992).

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.

Spinal stenosis

Narrowing (stenosis) of the vertebral canal may occur at single or multiple spinal levels, and mainly affects the lumbar and cervical regions. Stenosis may affect the central canal and the ‘root canals’ either together or separately. There is a developmental form of the condition which mainly affects the central canal, but more commonly the stenosis is degenerative, and results from intervertebral disc narrowing and osteoarthritic changes in the facet joints. This latter combination is more likely to narrow the intervertebral foramen and the ‘root canal’, even though the sectional profile of the vertebral canal in affected lumbar vertebrae typically changes from the shape of a bell to that of a trefoil. The lumbosacral intervertebral foramen, which is normally the smallest in the region, is particularly liable to such stenosis. Severe spinal stenosis may compress the spinal cord and compromise its arterial supply. More localized ‘root canal’ stenosis will present with the clinical features of spinal nerve compression, but without the tension signs that characterize the stretching of nerve roots over a prolapsed disc. Ischaemia of the nerves and roots may provoke more damage than the actual physical compression of the neural tissue.

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.

CERVICAL VERTEBRAE

The cervical vertebrae (Fig. 42.18, Fig. 42.19) are the smallest of the moveable vertebrae, and are characterized by a foramen in each transverse process. The first, second and seventh have special features and will be considered separately. The third, fourth and fifth cervical are almost identical, and the sixth, while typical in its general features, has minor distinguishing differences.

Fig. 42.18 The cervical vertebrae (anterior aspect).

Fig. 42.19 Lateral radiograph of the cervical spine. 1. Anterior tubercle of atlas. 2. Dens of C2. 3. Soft palate. 4. Body of C2. 5. Pharyngeal part of tongue. 6. C3 characteristic cervical body. 7. Body of hyoid. 8. Epiglottis. 9. C6/7 intervertebral disc. 10. Air in trachea. 11. Air in oesophagus. 12. Posterior arch of atlas. 13. Spinous process of axis. 14. C5/6 facet joint. 15. Body of T1.

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.

Muscle attachments

The ligamentum nuchae and numerous deep extensors, including semispinalis thoracis and cervicis, multifidus, spinales and interspinales, are all attached to the spinous processes. Tendinous slips of scalenus anterior, longus capitis and longus colli are attached to the fourth to sixth anterior tubercles. Splenius, longissimus and iliocostalis cervicis, levator scapulae and scalenus posterior and medius are all attached to the posterior tubercles. Shallow anterolateral depressions on the anterior surface of the body lodge the vertical parts of the longus colli.

Ossification

Cervical vertebrae ossify according to the standard vertebral pattern described on page 768. Incomplete segmentation (‘block vertebra’) is common in the cervical spine and most commonly involves the axis and third cervical vertebra.

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.

Muscle attachments

The anterior surface of the body carries a deep depression on each side for the attachment of the vertical part of longus colli. Levator scapulae, scalenus medius and splenius cervicis are all attached to the tips of the transverse processes and the intertransverse muscles are attached to their upper and lower surfaces. The lateral surfaces of the spinous process give origin to obliquus capitis inferior, and rectus posterior major is attached a little more posteriorly. The inferior concavity of the process receives semispinalis and spinalis cervicis, multifidus more deeply, and the interspinales near the apex.

Arterial supply

Small branches arise mainly from the vertebral artery at the level of the intervertebral foramen for the third cervical nerve and form paired anterior and posterior longitudinal channels, branches of which enter the dens near the base and near the apex. The anterior channel also receives numerous twigs from nearby branches of the external carotid artery via branches to longus colli and the ligaments of the apex, hence avascular necrosis does not occur after fracture of the base of the dens.

Ossification

The axis is ossified from five primary and two secondary centres (Fig. 42.23). The vertebral arch has two primary centres and the centrum one, as in a typical vertebra. The former appear about the seventh or eighth week, and that for the centrum about the fourth or fifth month. The dens is largely ossified from bilateral centres, appearing about the sixth month and joining before birth to form a conical mass, deeply cleft above by cartilage. This cuneiform cartilage forms the apex of the odontoid process. A centre appears in it which shows considerable individual variation in both time of appearance and time of fusion to the rest of the dens: it most often appears between five and eight years, but sometimes even later, fusing with the main mass about the 12th year. The cartilage was thought to be part of the cranial sclerotomal half of the first cervical segment or pro-atlas. It has also been suggested that the apical centre for the dens is itself derived from the pro-atlas, which may also contribute to lateral atlantal masses. The dens is separated from the body by a cartilaginous disc, the circumference of which ossifies while its centre remains cartilaginous until old age; possible rudiments of adjacent epiphyses of atlas and axis may occur in the disc. A thin epiphysial plate is formed inferior to the body around puberty.

Ossification may sometimes be incomplete. Thus the apical cuneiform centre may fail to fuse with the dens, or the dens itself may fail to fuse with the body, instead forming an os odontoideum. Some believe that this results from old unrecognized trauma rather than ossification failure. Interposition of the transverse ligament may prevent union of fractures through the base of the dens. Hypoplasia of the dens is usually accompanied by atlanto-occipital assimilation and basilar invagination. Abnormalities of the dens are common, and can result in atlanto-axial subluxation. In some skeletal dysplasias there is abnormal ossification in which the dens ossifies separately and much later than the atlantal centrum. This is probably a result of abnormal mobility in the cartilaginous anlage, and normal ossification may be restored if motion is prevented by surgical fusion.

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.