Vertebral column

Chapter 3

The spinal c o l u mn and its c o n t e n ts

CHAPTER CONTENTS

Vertebral column 85 Joints of the vertebral column 91 Ligaments of the vertebral column 92 Intervertebral discs 93 Blood supply of the vertebral column 94 Spinal cord 94 Spinal meninges 95 Blood supply of the spinal cord 96 Relevant MRI anatomy - cervical spine 98 Relevant MRI anatomy - dorsolumbar spine 101

THE VERTEBRAL COLUMN

The vertebral column has 33 vertebrae - 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused) and 4 coccygeal (fused) vertebrae.

The spine of the fetus is flexed in a smooth C shape. This is referred to as the 'primary curvature' and is retained in the adult in the thoracic and sacrococcygeal areas. Secondary extension results in lordosis - known as the 'secondary curvature' - of the cervical and lumbar spine.

A typical vertebra (Fig. 3. 1; see also Figs 3. 4 and 3. 5) A typical vertebra has a vertebral body anteriorly and a neural arch posteriorly. The neural arch consists of pedicles laterally and of laminae posteriorly.

The pedicles are notched superiorly and inferiorly so that adjoining pedicles are separated by an intervertebral foramen, which transmits the segmental nerves. There are 31 segmental spinal nerves - 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The first seven cervical nerves emerge above the correspondingly named vertebra; the others emerge below.

A transverse process arises at the junction of the pedicle and the lamina and extends laterally on each side. The laminae fuse posteriorly as the spinous process.

Articular processes project superiorly and inferiorly from each lamina. Articular facets on these processes face posteriorly on the superior facet and anteriorly on the inferior facet. The part of the lamina between the superior and inferior articular facets on each side is called the pars interarticularis.

The cervical vertebrae

Typical cervical vertebra (Fig. 3. 1) The most distinctive feature is the presence of the foramen tranversarium in the transverse process. This transmits the vertebral artery (except C7) and its accompanying veins and sympathetic nerves.

Small lips are seen on the posterolateral side of the superior surface of the C3-C7 vertebral bodies, w i th corresponding bevels on the inferior surface. Small joints, called neurocentral joints (of Luschka) or uncovertebral joints, are formed between adjacent cervical vertebral bodies at these sites. These are not true synovial joints although often called so, but are due to degenerative changes in the disc.

The cervical vertebral canal is triangular in cross-section. The spinous processes are small and bifid, whereas the articular facets are relatively horizontal.

The a t l a s - C1 (Fig. 3. 2) The atlas has no body as it is fused w i th that of the axis to become the odontoid process. A lateral mass on each side has a superior articular facet for articulation, w i th the occipital condyles in the atlanto-occipital joint, also an inferior articular facet for articulation w i th the axis in the atlantoaxial joint.

The anterior arch of the atlas has a tubercle on its anterior surface and a facet posteriorly for articulation w i th the odontoid process.

The posterior arch is grooved behind the lateral mass by the vertebral artery as it ascends into the foramen magnum.

The axis - C2 (Fig. 3. 3) The odontoid process, which represents the body of the atlas, bears no weight. Like the atlas, the axis has a large lateral mass on each side that transmits the weight of the skull to the vertebral bodies of the remainder of the spinal cord. Sloping articular facets on each side of the dens are for articulation in the atlantoaxial joint.

Vertebra prominens - C7 This name is derived from its long, easily felt, non-bifid spine. Its foramen tranversarium is small or absent and usually transmits only vertebral veins.

The thoracic vertebrae (Fig. 3. 4) These have articular facets on the lateral aspects of the vertebral bodies for articulation w i th the ribs. A demi-facet is found on the upper and lower aspect on each side on T2-T10 vertebrae. T1 has a complete facet superiorly and a

Fig. 3. 4 Typical thoracic vertebra: (a) lateral view; (b) superior view.

demi-facet inferiorly, whereas a single complete facet is seen at mid-level on T11 and T12. Articular facets are also found on the anterior surface of the transverse processes for the costotransverse articulations.

The spinous processes of the thoracic vertebrae are long and slope downwards.

The facets on the articular processes are relatively vertical.

The lumbar vertebrae (Fig. 3. 5) These have larger vertebral bodies and strong, square, horizontal spinous processes. The articular facets face each other in a sagittal plane.

The transverse processes of the upper four lumbar vertebrae are spatulate and increase in size from above downwards. The transverse process of the fifth lumbar vertebra is shorter but strong and pyramidal and, in contrast to those of the other vertebrae, does not arise from

Fig. 3. 5 Typical lumbar vertebra: (a) lateral view; (b) superior view.

the junction of the pedicle and lamina but from the lateral aspect of the pedicle and the vertebral body itself.

The sacrum (Figs 3. 6 and 3. 7) This is composed of five fused vertebrae. It is triangular in shape and concave anteriorly. A central mass is formed on the pelvic surface by the fused vertebral bodies. The superior border of the central mass is the most anterior part of the sacrum and is called the sacral promontory. Four anterior sacral foramina on each side transmit the sacral anterior primary rami. Lateral to these is the lateral mass of the sacrum, the upper anterior surface of which is called the ala of the sacrum.

On the posterior surface the laminae are also seen to be fused. The fusion of the spinous processes forms a median sacral crest. A sacral hiatus of variable extent inferiorly is caused by non-fusion of the laminae of S5 and often S4 in the midline. The transverse processes are rudimentary. Four posterior sacral foramina transmit the posterior primary rami. The sacral hiatus transmits the fifth sacral nerve.

Laterally there is a large articular facet, called the auricular surface, for articulation with the pelvis in the sacroiliac joint.

Differences in the sacrum between male and female include: • The width of the body of the first sacral vertebra is less than that of the ala in the female and wider in the male. • The articular surface of the S1 joint occupies two vertebrae in the female and two-and-a-half vertebrae in the male. • The anterior surface of the female sacrum is flat superiorly and curves forward inferiorly, whereas that of the male is uniformly concave.

The coccyx

This comprises four vertebrae that are fused into a triangular bone which forms part of the floor of the pelvis.

Radiological features of the vertebrae

Radiographs of the vertebral column (Figs 3. 8 and 3. 9) The component parts of the vertebrae - the body, pedicles, laminae, and the transverse, articular and spinous processes - can be seen. Oblique views, particularly of the lumbar spine, are used for better visualization of the neural foramina and the pars interarticularis.

The point of exit of the basivertebral veins can be seen on lateral views as a defect in the cortex of the posterior surface of the vertebral body.

The anteroposterior (AP) width of the spinal canal is, measured on a lateral film from the posterior cortex of the vertebral body to the base of the spinous process. The lower limit of normal is taken as 13 mm in the cervical spine at C5 level and 15 mm in the lumbar spine at L2.

The interpedicular distance is measured on AP views (and more easily on AP tomography) and reflects the width of the spinal cord. It is maximum in the cervical spine at C5/C6 and in the thoracic spine at T12, as these are the sites of expansion of the cord for the limb plexuses. It (interpedicular distance) increases from L1 to L5.

Transitional vertebrae w i th features intermediate between the two types of typical vertebrae are developmental anomalies. These occur at the atlanto-occipital junction, where the atlas may be assimilated into the occipital bone or where an extra bone may occur - known as the occipital vertebra. Transitional vertebrae may be found at the cervicothoracic junction at the level of C7; these may have long, pointed transverse processes w i th or without true rib (cervical rib) formation. Similarly, at the thoracolumbar junction vestigial ribs may be seen on T12 or L1 vertebrae. At the lumbosacral junction, the last lumbar vertebra may be partly or completely fused w i th the sacrum - known as sacralization - or the first sacral segment may be separated from the remainder of the sacrum known as lumbarization. In these cases it is important to avoid confusion in level identification, particularly prior to surgery.

Radiographs of the cervical spine The alignment of the cervical spine w i th respect to the foramen magnum can be assessed radiographically. The following lines have been described: • Chamberlain's line (on a lateral view): from the posterior tip of the hard palate to the posterior lip of the foramen magnum. Less than 2 mm of odontoid is normally above this; • McGregor's line (on a lateral view): from the posterior tip of the hard palate to the base of the occiput (often easier to identify than the lip of the foramen magnum). Less than 5 mm of odontoid should lie above this; and • Digastric line (on an OF view of the skull): the atlantooccipital joints should be below a line between the digastric notches of both mastoid processes.

In basilar invagination these relationships are abnormal.

1. Trachea 2. Spinous process of C7 3. Left transverse process of C7 4. Tubercle of first rib, articulating with transverse process of T1 5. Medial end of clavicle 6. Superior margin of manubrium sterni 7. Lateral margin of manubrium sterni 8. Left pedicle of T5 9. T5/T6 intervertebral disc space 10. Tubercle of seventh rib, articulating with transverse process of T7 11. Head of eighth rib 12. Neck of eighth rib 13. Shaft of eighth rib 14. Left paraspinal line 15. Left transverse process of L1 16. Dome of left hemidiaphragm

Radiographs of the thoracic spine (see Fig. 3. 8) In AP views of the thoracic spine paraspinal lines are seen between the paravertebral soft-tissue shadows where these are covered w i th pleura and the air in the lungs. Only lymph nodes, intercostal vessels, sympathetic nerves and fat normally lie between the vertebrae and pleura, so that displacement of the paraspinal lines on a radiograph is a marker of pathology in these structures or in the vertebrae (see also mediastinal lines).

Radiographs of the lumbar spine Anterior wedging of the L5 vertebra is a normal finding on lateral radiographs, as is narrowing of the L5/S1 disc space, and should not be taken as a sign of disease.

Oblique views of the lumbar spine (see Fig. 3. 9) are used to visualize the intervertebral foramina and the pars interarticularis.

Ossification of the vertebrae Typical vertebrae have three primary ossification centres: the body and on each side of the neural arch. These appear in the eighth fetal week. Secondary centres appear for the upper and lower parts of the body, for each transverse process, and for the spinous process at 16 years, and fuse by the 25th year. Posterior neural arch fusion starts in the lumbar region at 1-2 years and proceeds cephalad to the cervical region up to 7 years of age. Fusion in the sacral region occurs last. Neural arch to the centrum fusion starts in the cervical region at 3 years and proceeds caudad to the lumbar region by 6 years. Fusion in the sacral region again occurs last.

The atlas has only three primary ossification centres, one for the anterior arch and one each for each lateral mass. These unite at 6 years of age. Failure to fuse posteriorly results in a posterior spina bifida. Occasionally there are two centres for the anterior arch that fuse at 8 years of age. Failure of fusion of these centres causes an anterior spina bifida.

The axis has two extra centres - those for the odontoid process. The odontoid process in adulthood may remain separated from the axis by a cartilaginous disc and simulate an ununited fracture.

The seventh cervical vertebra also has two extra centres, for the attachment of costal processes on each side.

Lumbar vertebrae have extra primary centres for the mamillary processes on each side. In the lumbosacral spine, defects in ossification of the neural arch give rise to a spina bifida defect in 5-10% of the population. Other defects, such as persistent epiphysis in the transverse process, are also common and should not be mistaken for a fracture line.

Axial computed tomography (Figs 3. 10 and 3. 11) The vertebral body anteriorly and the pedicles, laminae and spinous process posteriorly are seen as a bony ring around the spinal canal. Transverse processes are seen lateral to this, and because they are not truly horizontal they appear separate from the remainder of the vertebra in several cuts.

Where the slice passes through the intervertebral foramen, it is seen as a gap between the body and the posterior vertebral elements. The intervertebral foramen, being oval in shape, appears narrower in cuts through its upper and lower ends.

The dimensions of the spinal canal can be measured directly. Lower limits of normal for the midsagittal distance