In the foetus, the column lies flexed like a “C”, which is called the primary curvature of the column, and is retained by the thoracic, sacral, and coccygeal parts. Secondary extension of the spine produces the secondary curvatures in the neck and lumbar region
GENERAL CHARACTERISTICS
Consists of a ventral body, a dorsal neural arch which encloses the vertebral foramen. (N.b vertebral canal is a series of vertebral foramen strung together in a column). The part of the neural arch between the spinous process and transverse process is the lamina, and between the transverse process and body is the pedicle. The intervertebral foramen lies between the pedicles of adjacent vertebra, and allows passage of the nerve. At the junction of the lamina and pedicle (at the root of the transverse process) is the articular processes, superior and inferior.
The vertebrae ossify in three parts; the centrum, and the right and left halves of the neural arch. In the thoracic region, the costal elements develop separately as the ribs, in other parts of the spine the costal elements become fused to the neural arches and are incorporated as morphological parts of the vertebrae ( N.b the centrum is not the body, as part of the neural arches are incorporated into the body, with the neurocentrum junction lying anterior to the costal facets on the body). Thus the morphological neural arch is different to the anatomical neural arch as it includes part of the body.
To contrast the vertebrae, the cervical has a foramen in the transverse process for the artery, and has no costal facets. The thoracic has costal facets, one on the tip of the transverse process, and a demifacet on the body of the vertebrae at their upper end lower margins. The lumbar vertebra has neither a costal facet nor a foramen in the transverse process.
CERVICAL VERTEBRA
Typical cervical vertebra ( C3-6)
The broad kidney shaped body is smaller than or the same size as the vertebral foramen. On each side the body projects up to a posterolateral lip, the uncus, with the lower margin having a reciprocal bevel (part of the morphological neural arch). The basivertebral foramen for the veins are wider apart on the posterior body, than in the thoracic and lumbar vertebrae, with the posterior longitudinal ligament between. The anterior surface is concave from top to bottom, with the anterior longitudinal ligament attaching to this surface, with the longus colli muscle overlying.
The pedicles attach below the upper body, thus each intervertebral foramina have both vertebral bodies and disc in front. Attached to the pedicle and body is the lateral projection of the transverse process, perforated by it’s foramen. The first part of the posterior arch behind the foramen is the true transverse element, which ends in the posterior tubercle, part of the costal element. The anterior part of the arch of the foramen ends in the anterior tubercle. The largest of the anterior tubercles is C6, called the carotid tubercle of Chassignac, where the artery can be compressed. The anterior and posterior tubercles are joined by the intertubercular lamella, which with the anterior bar and tubercle, and posterior tubercle, are part of the costal element. The vertebral artery lies in this foramen, with the posterior root ganglion of the nerve lying behind it on the lamella. The anterior tubercle gives attachment to the longus capitus, scalenus anterior and longus colli. The posterior tubercle gives attachment to the scalenus medius. Levator scapulae arises from the posterior tubercles down to C4, and scalenus posterior from C5,6,7.
The laminae enclose triangular vertebral foramen. They are grooved for the ligamentum flavum, the same as the other vertebrae. The superior and inferior articular processes form a cylindrical column, with the upper facet facing obliquely up and back, and the lower facing down and forwards.
The spinous process is usually bifid, excavated inferiorly by a pair of concavities for semispinalis cervicis.
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Atypical cervical vertebrae
The vertebra prominens (C7), is atypical in that it’s long spinous process is not bifid, and the foramen in the transverse process does not transmit a vertebral artery. This canal may contain the posterior vein when the vertebral vein is double.
The atlas (C1), lacks a centrum, which is fused to the centrum of the axis, forming the odontoid process. The vertebral arch has been modified to form a thick lateral mass on each side, joined in front by a short anterior arch, with a larger longer posterior arch at the back. The anterior arch has a concave cylindrical facet for the synovial joint with the dens, and a tubercle in front for attachment of the anterior longitudinal ligament and longus colli. The upper boarder carries the anterior atlanto-occipital membrane. The upper surface of the posterior arch has a groove at it’s root for the vertebral artery and C1 nerve. The upper boarder of the posterior arch gives attachment to the posterior atlanto-occipital membrane, while the lower boarder receives the ligamentum flavum from C2. The posterior surface gives origin to the rectus capitus posterior minor muscles. The lateral mass carries the articular facets. The upper articular facet is kidney shaped, and concave for articulation with the occipital condyle, while the lower is round and flat for the lateral atlanoaxial joint. Theses joints are in line with the uncal vertebral joints of the other cervical vertebrae, not those of the neural arches. Thus C1 and C2 nerve roots exit behind, not in front of these joints. The hilum of the kidney shaped joints gives attachment to the transverse ligament for the dens. The lateral mass is projected into the transverse process, perforated by the foramen, but does not have anterior and posterior tubercles. The internal jugular vein, crossed by the accessory nerve, lies on the transverse process. Posteriorly the process gives attachment to the obliquus capitus superior and inferior, splenius cervicis, and levator scapulae
The axis (C2), is the pivot around which the head and atlas rotate. This is characterized by the dens and a large spinous process. The odontoid is the centrum of the atlas fused to the centrum of the axis, and has an anterior cylindrical facet for the synovial joint with the atlas A bursa exists at the posterior surface between bone and transverse ligament. The apex of the dens has the apical ligament, with the sides carrying the alar ligaments. It bears no weight, with the head weight transferred through the lateral mass of the atlas. The lower articulations are the same as the typical vertebrae, with intervening disc and articular facets on the neural arch, facing downwards and forwards. From here weight is carried by the vertebral bodies. The anterior longitudinal ligament attaches to the front of the body, with longus colli and prevertebral fascia overlying it. The posterior body gives attachment to the longitudinal limb of the cruciform ligament ,the tectorial membrane and posterior longitudinal ligament. The upper surface has the large facets lateral to the dens. The costal elements slope down from the body to end in a prominent transverse process, with the foramen directed upwards and outwards to give a lateral bend to the vertebral artery. Scalenus medius, splenius cervicis and levator scapulae all attach to the transverse process. The laminae are thick, and form the massive spinous process, with a grooved lower boarder ending in a bifid spinous process. The strong semispinalis cervicis muscles insert into the concavity, with rectus capitus posterior major and inferior oblique diverging widely from the outer surface. The posterior laminae gives attachment to the multifidus and longissimus attachments.
THORACIC VERTEBRAE
This area has a kyphosis, which results in the upper vertebrae leaning forwards, so their transverse processes tilt up a little. This tilts the rib neck as well, such that the 1st rib slopes upwards from the head to the tubercle. The facets on the upper transverse processes are tilted downwards a little, and those on the lower transverse processes tilt upwards a little. The spinous processes slope downwards at the top, becoming most steep at T7, then gradually straightening, becoming almost horizontal at T12. The tips the spinous processes of the upper 4 and lowest 4 lie opposite the rib tubercle of one rib lower in the series. The middle spinous processes are opposite the rib tubercle of two ribs lower in the series. The bodies increase in size from above down, and the transverse processes are the same length except T11,T12 which are shorter.
Typical vertebrae
The essential feature is the presence of costal facets. On the side of each body is a pair of demifacets, covered with hyaline cartilage, making separate synovial joints with the rib. The upper is at the junction of the upper boarder and the pedicle, and is a semicircular shape lying vertically. The lower is smaller, facing downwards. Morphologically they lie on the neural arch.
Around the circumference of the body it is concave from above down, perforated by numerous vessels. From T4 down the anterior convexity has a sharper curve, caused by the descending Aorta. The posterior body is concave from side to side, forming an almost round vertebral foramen with the laminae. Two large foramina here are for the basivertebral veins, spanned by the posterior longitudinal ligament. The upper and lower surfaces have a heart shaped ring of cortical bone, with a central cancellous area which reaches the surface. Theses surfaces are then covered by a thin layer of hyaline cartilage, to which the fibres of the annulus fibrosis attach.
The pedicle has it’s upper boarder level with the upper surface of the body, and curves upwards in a slight concavity, the superior vertebral notch, to the superior articular process. The lower surface also curves down to the inferior articular process, also forming a concave inferior vertebral notch. (forms the upper boarder of intervertebral foramen).
The upper boarders of the laminae slope down and form a deep notch. The lamina is groove on it’s external surface for the attachment of the ligamentum flavum which travels to the vetebra above. The lower part of the lamina is grooved on it’s internal surface between the inferior articular processes for the attachment of the ligamentum flavum from below. Some small veins run out between adjacent ligamentum flava to join the internal and external venous plexuses.
The superior articular process carries an oval articular facet, facing backwards and slightly laterally. The inferior facets face forwards and medially. They both form synovial joints.
The spinous process slopes down from a broad base to a tip that is expanded, and have multiple attachments. The weak interspinous ligaments and muscles connect adjacent sharp boarders, and thick strong supraspinous ligaments attach the tips. The thoracolumbar fascia is attached here, whilst the base receives the rotator muscles from the vertebra below, and between the multifidus, semispinalis and multifidus attach.
The transverse process projects slightly backwards, and expands at the anterior tip to form the costal facet. The upper 6 facets are concave, and the lower 6 are flat, and all covered with hyaline cartilage. The tip gives attachment to the lateral costotransverse ligament, and the root gives origin to the rotator muscle. Upper and lower boarders carry weak intertransverse ligaments and muscles, , the lower margin gives origin to a levator costae muscle and receives the superior costotransverse ligament from the neck of the rib below. The posterior surface has errector spinae attachments, semispinalis and longissimus.
Special vertebrae
The 1st thoracic vertebra has a large round upper facet for the single attachment of the 1st rib, and has the longus colli muscle attaching anteriorly over the anterior longitudinal ligament. The pedicle is attached below the upper margin of the body, like the cervical vertebra, and the body thus takes part in the intervertebral foramen.
The 11th thoracic vertebra has only a single costal facet for the 11th rib, located on the upper part of the broad pedicle. The transverse process is often stunted.
The 12th vertebra has a single facet for it’s rib near it’s lower margin. The transverse process is stunted, and it’s base projects upwards into a rounded mamillary process, behind the articular facet, and downwards into a sharp accessory tubercle. The inferior articular facet is lumbar in type, a vertical cylinder facing laterally.
LUMBAR VERTEBRA
The lordosis of the lumbar spine is produced by the wedged shaped discs, not the vertebral bodies. The vertebra increase in width from above down, such that the distance between the articular processes increases as well.
The body is concave from above down, has it’s pedicle attached to the upper half, and is perforated by the basivertebral veins posteriorly. It differs from the thoracic in that it is kidney shaped, not heart shaped, and the posterior surface is flatter. This makes the vertebral canal triangular, not circular. Psoas major attaches to the upper and lower margins of the body, but not to the concave surface. The anterior and posterior ligaments attach as in the thoracic region.
The laminae do not slope down as much as the thoracic, but the upper external and lower internal boarders are still grooved for the ligamentum flavum attachment.
The spinous process is horizontal, with a straight upper boarder, and a lower boarder which curves down, giving the hatchet shape. The interspinous ligaments attach, with the supraspinous to the thickened posterior boarder. Interspinales and multifidus also attach to the spines.
The transverse processes are variable, but the 4th is usually the longest. These are the fused costal elements. The true transverse elements are two elevations with a groove between them made by the medial branch of the posterior ramus of the overlying lumbar nerve. The mamillary process is a breast shaped convexity projecting back from the superior articular process, with the accessory tubercle below this , at the root of the transverse process. These two represent the stunted transverse process of the thoracic vertebra. The transverse process of the 5th is quite different, it is short, massive, pyramidal and it’s base is attached from the pedicle well forwards, onto the lateral side of the body itself. The upper boarders have a ridge on the anterior surface for the attachment of the psoas fascia, and anterior layer of the lumbar fascia. The medial and lateral arcuate ligaments attach to the ridge on the L1 process. The tip of the process receives the middle layer of the lumbar fascia. Adjacent boarders carry intertransverse ligaments and muscles, and the posterior surface receives the errector spinae.
The articular processes are characteristic, with the upper facet surfaces facing medially (L4,5 face a little posteriorly as well), and being cylindrical facets, concave from front to back. The lower facets carry a reciprocal convex surface
The 5th lumbar vertebra
The characteristic transverse process carries the strong iliolumbar ligament and quadratus lumborum origin. The anterior and middle layers of the lumbar fascia also attach to the process. The inferior articular facet faces forwards, which locks against the backwards facing sacral facets, preventing L5 slipping down the slope of S1. The strongly bound disc also prevents this slide, as this joint takes the full body weight. This joint however may fuse to the sacrum, call sacralization.
SACRUM
Formed from 5 progressively smaller vertebrae and their costal elements, and is triangular in outline. It forms the sacroiliac joints, which are not weightbearing, as the sacrum is slung on ligaments above and behind the joint, which carry the body weight. The superior articular facet for L5 are cylindrical, curved from side to side, and flat from above down, and lies vertically. The upper surface of the 1st sacral body is directed down at 30º.
Pelvic surface
In the midline the 5 bodies are fused, with 4 ridges persisting to mark the lines of ossification, which represent the intervertebral discs. On each side is 4 anterior sacral foramina, with rounded bars of bone between adjacent foramina represent the heads and necks of the ribs. Thus the medial side of the foramina is formed from the body, and the rest from the costal elements. The rounded bar of bone above the 1st sacral foramen continues from the arcuate line of the ilium to form the posterior part of the pelvic brim. This continues medially to the sacral promontory, the prominent anterior lip of the 1st sacral body.
The lateral mass, is a mass of bone lateral to the foramina, formed by fusion of the costal elements. It is grooved by the anterior rami of the 1st four nerves, as they pass laterally from the anterior foramen. Piriformis arises from the three ridges that separate the foramen, as well as the adjacent lateral mass. Below the promontory the peritoneum is draped over the upper two bodies, with the retroperitoneal rectum lying against the lower three. Between the sacrum and the peritoneum/rectum, the fascia of Waldayer and superior rectal vessels lie, with the fascia attaching below the promontory. The midline of the sacral hollow contains the median sacral artery, veins and nodes, with the sacral sympathetic trunk alongside, medial to the foramen. Lateral to the foramen is the sacral plexus on the piriformis, covered by the thick parietal pelvic fascia, on which the lateral sacral artery and vein lies.
Dorsal surface
In the midline it is closed due to the fusion of adjacent laminae, with the gap above the 1st lamina closed by the ligamentum flavum, attached to L5 lamina. The gap below is the failed fusion of S4,5 lamina, closed by fibrous tissue, the superficial sacrococcygeal ligament. Adjacent spinous processes fuse to form the median sacral crest. Below the cylindrical facet of S1, (for L5), is a line of irregular tubercles, representing the fused articular processes of the sacral vertebrae. This low ridge forms the intermediate sacral crest, and projects below alongside the sacral hiatus, to end in the rounded sacral cornu, for articulation with the coccyx. Lateral to the superior articular process of S1, is a prominent boss of bone, which is the transverse process of S1. Below this, the transverse processes are fused with each other, making a ridge, the lateral sacral crest. This lies lateral to the posterior foramina. These foramen are thus enclosed entirely by the fused sacral vertebra, with the fused costal elements lying lateral to the crest. The groove between the median and lateral sacral crests is filled by errector spinae, and the posterior layer of lumbar fascia overlying it is attached to both crests. Between the lateral crest and articular surface is several deep fossae, to which the weightbearing sacroiliac ligaments attach. The lateral crest meets the lateral boarder of the sacrum at the level of S4. This point is the apex of a small triangle below the articular surface, with part of the sacrotuberous ligament with it’s attached gluteus maximus arising from here. Below this the lateral margin gives attachment to the sacrospinous ligament and coccygeus muscle. The apex of the sacrum is attached by disc to the coccyx, and the cornu sometimes make small synovial joints with the cornu of the coccyx.
The sacral canal is triangular, curves with the sacrum, and is closed in front and behind. It contains the fibrum terminale, the dura/arachnoid and CSF down to S2, with the sacral and coccygeal nerves roots. The dura is prolonged as tubular sheaths around the nerve roots. The fibrum terminale pierces the dura and blends with the periosteum on the back of the coccyx. The space around these is filled with loose fat and the internal vertebral venous plexus.
Four intervertebral foramina are formed between the pedicles projecting back from each body. The nerve root exits are however blocked by the lateral mass, thus the nerves exit through the anterior and posterior foramina. The 5th foramen is formed behind the cornu of the sacrum and coccyx, with the S5 and coccygeal nerves emerging here.
Ala of the sacrum
This projects laterally from the upper surface of S1 vertebra, it’s margin forms the brim of the pelvis. Laterally it gives attachment to the weak ventral sacroiliac ligament, with the iliacus arising from the ligament over the ala
Articular surface
This is broad above and narrow below, with the articular cartilage having an irregular appearance, due to depressions and tubercles in the bone itself.
Sex Differences
The body of S1 is wider than the ala of the sacrum in the male, narrower in the female. In the male the anterior surface is gently concave, in the female it is flatter initially, then turns more prominently below. The articular surface in the male occupies 2 ½ vertebrae in the male, and 2 in the potentially more mobile female.
COCCYX
This is four pieces of bone fused into one triangular bone, forming a symphysis, with a fibrocartilagenous disc to the sacrum. Ventral and deep dorsal sacrococcygeal ligaments stabilize this, with lateral sacrococcygeal ligaments joining the transverse process of the coccyx with the inferolateral angle of the sacrum. These lateral ligaments complete the foramen for the S5 nerve root. The tip of the coccyx gives attachment to the anococcygeal raphe and the superficial part of the external anal sphincter. The boarders give attachment to the coccygeus muscle and sacrospinous ligament, and to the overlapping fibres of the iliococcygeus and pubococcygeus.
OSSIFICATION OF THE VERTEBRAE
The vertebra develop in the embryonic period as mesenchymal condensations around the notochord. Chondrification centres then soon appear, and a cartilaginous vertebra soon forms, with the centre for the centrum being double, which rapidly fuses. (failure of one half results in hemivertebrae). The centrum and two halves of the neural arch develop primary ossification centres by the eighth week of life. At birth the inferior sacral vertebra and coccyx are all cartilaginous.
The cartilage between the two neural arches (i.e between the laminae), begins to ossify after birth, (cervical) complete at 2-6 years.(lumbar) The neurocentral junctions then begin to ossify, complete at about 7 years.
Five secondary centres appear from 12-16 years. The two ring epiphysis for the body appear at the upper and lower vertebra body, with fusion occurring in the early 20’s. Secondary centres appear in the tip of the spinous process (double in bifid spines), and at the tips of the transverse processes and the mamillary process of T12. These fuse in the early 20’s. Costal elements of the cervical and lumbar spine do not have separate centres. (if they do a cervical rib or lumbar rib may form). However the weightbearing costal elements of the sacrum have centres, appearing at 6/40, and fusing at 5 years with the neural arches, and 20 years with each other and the sacral bodies.
The atlas ossifies in the 7/40, by a centre in each lateral mass, uniting posteriorly at 4 years. A centre appears in the anterior arch at 1 year, with it’s junction at the lateral mass passing through the articular surface. This epiphysis fuses at 7 years.
The axis ossifies in the centrum and neural arch as the others, but the dens has only it’s lower part ossified at birth, fusing with the centrum at 4 years. The apex begins to ossify then, fusing at 12 years.