The spinal column and its contents

A typical vertebra (Figs 3.1, 3.4, 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 eighth cervical nerve emerges below C7 and the others emerge below the correspondingly named vertebra.

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

The thoracic vertebrae (Fig. 3.4)

These have articular facets on the lateral aspects of the vertebral bodies for articulation with 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 demi-facet inferiorly, whereas a single complete facet is seen at midlevel 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. A prominence on the posterior aspect of each superior articular process is the mamillary process.

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 the junction of the pedicle and lamina but from the lateral aspect of the pedicle and the vertebral body itself (Fig. 3.5C).

The sacrum (Fig. 3.6)

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 3.7–3.9)

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.

Figure 3.7 • Lateral radiograph of the cervical spine.

1 Anterior arch of atlas

2 Dens

3 Body of axis

4 Posterior arch of atlas

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 width of the spinal canal is measured as the interpedicular distance on AP views (and more easily on coronal CT reconstruction) 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. The interpedicular distance increases from L₁ to L₅.

Transitional vertebrae with 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 with 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 with the sacrum – known as sacralization – or the first sacral segment may be separated from the remainder of the sacrum – known as lumbarization.

Radiology pearl

The possibility of transitional vertebrae must be borne in mind when numbering vertebrae especially prior to a surgical procedure.

Radiographs of the cervical spine

The alignment of the cervical spine with 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.

• Digastric line (on an OF view of the skull): the atlanto-occipital joints should be below a line between the digastric notches of both mastoid processes.

In basilar invagination these relationships are abnormal.

Radiology pearl

Cervical ribs are distinguished from thoracic ribs by the orientation of the transverse process. Cervical transverse processes are orientated inferiorly, thoracic transverse processes point upwards.

Radiographs of the thoracic spine (Fig. 3.8)

In AP views of the thoracic spine paraspinal lines are seen between the paravertebral soft-tissue shadows 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.

Computed tomography (Figs 3.1–3.6 and 3.10)

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 are taken as 12–15 mm and 20 mm for the interpedicular distance. Other factors in spinal stenosis can also be determined, for example the soft-tissue elements such as the ligamenta flava, the thickness of the lamina (14 mm is the upper limit of normal) and the development of bony spurs at the disc margins and at the facet joints.

Compact bone and cancellous bone components of the vertebrae can easily be distinguished on CT. In order to maintain the axial load imposed by weight bearing, the vertebral body is composed of both a thick outer cortical shell of compact bone and an inner supporting network of vertically oriented trabeculae or cancellous bone. The posterior element, having less of a role in weight bearing and functioning primarily to stabilize and prevent subluxation, is composed of compact cortical bone alone.

Radiology pearl

In adulthood up to 50% of the vertebral body is cancellous bone, whereas in the femoral neck, the cancellous bone constitutes only 30% by volume. This explains why changes of osteoporosis occur earlier in the vertebral body than in the femoral neck, and explains why screening for osteoporosis is aimed primarily at the vertebral body.

Magnetic resonance imaging (Fig. 3.11)

MRI is widely used to visualize the spinal column and its contents. T₁-and T₂-weighted images give information about morphology and integrity of discs and vertebrae, the intervertebral foramina and facet joints, and an outline of the spinal cord. The vertebrae have a low-signal outer rim surrounding the high-signal cancellous bone. The signal intensity derived from the vertebral body in the spine is dependent on the quantity of yellow marrow relative to haemopoietic red marrow. In adulthood, yellow marrow predominates and results in signal hyperintensity throughout the vertebral body on both T₁-weighted and fast spin echo T₂-weighted scans. The presence of small quantities of red marrow produces some signal heterogeneity and signal suppression.

Radiology Key

Fastest Radiology Insight Engine

The spinal column and its contents

The vertebral column

A typical vertebra (Figs 3.1, 3.4, 3.5)

The cervical vertebrae

Typical cervical vertebra (Fig. 3.1)

The atlas – C₁ (Fig. 3.2)

The axis – C₂ (Fig. 3.3)

Vertebra prominens – C₇

The thoracic vertebrae (Fig. 3.4)

The lumbar vertebrae (Fig. 3.5)

The sacrum (Fig. 3.6)

The coccyx

Radiological features of the vertebrae

Radiographs of the vertebral column (Figs 3.7–3.9)

Radiographs of the cervical spine

Radiographs of the thoracic spine (Fig. 3.8)

Radiographs of the lumbar spine

Computed tomography (Figs 3.1–3.6 and 3.10)

Magnetic resonance imaging (Fig. 3.11)

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Fastest Radiology Insight Engine

The spinal column and its contents

A typical vertebra (Figs 3.1, 3.4, 3.5)

Typical cervical vertebra (Fig. 3.1)

The atlas – C1 (Fig. 3.2)

The axis – C2 (Fig. 3.3)

The thoracic vertebrae (Fig. 3.4)

The lumbar vertebrae (Fig. 3.5)

The sacrum (Fig. 3.6)

Radiographs of the vertebral column (Figs 3.7–3.9)

Radiographs of the thoracic spine (Fig. 3.8)

Computed tomography (Figs 3.1–3.6 and 3.10)

Magnetic resonance imaging (Fig. 3.11)

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The atlas – C₁ (Fig. 3.2)

The axis – C₂ (Fig. 3.3)