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
A typical vertebra has a vertebral body anteriorly and a neural arch posteriorly. The neural arch consists of pedicles laterally and 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
Typical Cervical Vertebra ( Fig. 3.1 )
The most distinctive feature is the presence of the foramen transversarium in the transverse process. This transmits the vertebral artery (except C7) and its accompanying veins and sympathetic nerves. The transverse processes have anterior and posterior tubercles. Small lips are seen on the posterolateral side of the superior surface of the C3–C7 vertebral bodies, the uncinate processes , with 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 they are often called so, but are due to degenerative changes in the disc.
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Body
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Foramen transversarium
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Transverse process with anterior and posterior tubercles
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Superior articular facet
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Lamina
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Bifid spinous process
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Body
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Anterior tubercle
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Transverse process
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Posterior tubercle
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Lamina
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Spinous process
The cervical vertebral canal is triangular in cross-section. The spinous processes are small and bifid, whereas the articular facets are relatively horizontal.
The Atlas – C1 ( Fig. 3.2 )
The atlas has no body as it is fused with that of the axis to become the odontoid process. A lateral mass on each side has a superior articular facet for articulation, with the occipital condyles in the atlanto-occipital joint, also an inferior articular facet for articulation with the axis in the atlantoaxial joint.
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Anterior tubercle
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Transverse process
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Foramen transversarium
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Superior articular facet
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Groove for vertebral artery
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Posterior arch
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Posterior tubercle
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Dens
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Lateral mass of atlas
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Vertebral artery
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Spinal cord
The anterior arch of the atlas has a tubercle on its anterior surface and a facet posteriorly for articulation with 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.
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Occipital condyle
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Atlas
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Anterior arch of atlas
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Dens
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Atlantoaxial joint
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Body of axis
Vertebra Prominens – C7
This name is derived from its long, easily felt, nonbifid spine. Its foramen transversarium is small or absent and usually transmits only vertebral veins. The anterior tubercle of the transverse process is smaller than other 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.
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Vertebral body
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Intervertebral disc
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Pedicle
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Facet joint
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Rib
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.
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Vertebral body
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Intervertebral disc
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Transverse process
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Facet joint
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Pars interarticularis
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Spinous 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 (see 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.
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Sacral promontory
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Ala
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Pelvic sacral foramina
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Superior articular process
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Upper end of sacral canal
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Depressions for sacroiliac ligament
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Spinous tubercle
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Dorsal sacral foramen
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Lower end of sacral canal
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Spinous tubercle
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Superior articular facet
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Sacral promontory
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Depressions for sacroiliac ligament
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Auricular surface
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 nonfusion 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 (SI) joint. Differences in the sacrum between males and females include:
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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.
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The articular surface of the SI joint occupies two vertebrae in the female and two-and-a-half vertebrae in the male.
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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.
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Anterior arch of atlas
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Dens
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Body of axis
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Posterior arch of atlas
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Pedicle
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Lamina
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Spinous process
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Articular facets
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Trachea
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Spinous process of C7
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Left transverse process of C7
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Tubercle of first rib, articulating with transverse process of T1
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Medial end of clavicle
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Superior margin of manubrium sterni
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Lateral margin of manubrium sterni
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Left pedicle of T5
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T5/T6 intervertebral disc space
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Tubercle of seventh rib, articulating with transverse process of T7
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Head of eighth rib
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Neck of eighth rib
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Shaft of eighth rib
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Left paraspinal line
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Left transverse process of L1
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Dome of left hemidiaphragm
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Vertebral body of L2
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Superior articular process of L3
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Pedicle of L3
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Pars interarticularis
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Transverse process of L3
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Transverse process of L3
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Inferior articular process of L3
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Spinous process of L2
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 [CT] (and more easily on coronal computed tomography (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.
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:
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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.
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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.
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Digastric line (on an occipitofrontal [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 (a congenital developmental anomaly where the cervical spine is displaced upward into the foramen magnum) these relationships are abnormal.
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 (see 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/S1s 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 (see Figs. 3.1–3.6, 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 on many images.
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Body of L4
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Right lamina of L4
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Spinous process of L4
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Thecal sac containing cauda equina
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Epidural veins
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Dorsal root ganglia of L4 nerve
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Intervertebral foramen of L4/L5
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Psoas muscles
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Erector spinae muscles
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Disc of L4/L5
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Inferior articular process of L4
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Superior articular process of L5
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Right lamina of L5
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Spinous process of L5
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Ligamentum flavum
Where the CT 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 orientated trabeculae or cancellous bone. The posterior element of the vertebral body has less of a role in weight bearing and functions primarily to stabilize and prevent subluxation. It is composed of compact cortical bone alone.
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 )
Magnetic resonance imaging (MRI) is widely used to visualize the spinal column and its contents. T1- and T2-weighted images give information about the 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 and fast-spin echo T2 images. The presence of small quantities of red marrow produces some signal heterogeneity and signal suppression.
