Anatomy and development

The spine enables humans to stand erect and to walk. Three important functions are provided by the spine: protection for the spinal canal, weight bearing, and motion. The spinal canal protects the spinal cord from damage and provides security for the control of the brain over the body. The function of weight bearing is provided by the vertebral bodies, which therefore increase in size downwards. The intervertebral discs, the facet joints and at the cervical spine, the uncovertebral joints supply mobility to the spine. The mobility is greatest at the cervical spine thus allowing fast movements of the head towards the direction of interest and it is least at the thoracic spine where the ribs decrease mobility and increase stability, thus protecting the lungs and the heart.

In newborns, only kyphosis of the thoracic spine is present. The cervical lordosis forms when the baby learns to hold the head while lumbal lordosis results from straightening the legs. Slight scoliosis convex to the right can be normal in the upper thoracic spine.

Development

The development of the spine includes evolution of the vertebral column and of the spinal cord. Stages in formation of the cord are neurolation for the main cranial portions and retrogressive differentiation for the caudal parts and myelination. The vertebral column develops by membrane formation, chondrification and ossification.

In all vertebrates first a notochord develops when cells grow from the Hensen's node between the ectoderm and endoderm in a cranial direction. The notochord induces the formation of the neural plate in the ectoderm. Folding of the neural plate creates a neural groove. Further dorsal closure forms a neural tube which separates from the ectoderm and migrates into the centre of the body. Early segmentation of the laterally located mesoderm develops the somites which appear as para-axial protuberances. The caudal end of the neuroaxis develops by canalisation and caudal regression, forming the filum terminale and ventriculus terminalis which later becomes the conus medullaris.

At the end of the third fetal month, the cord extends throughout the whole length of the vertebral canal. The relatively faster growth of the vertebral spine causes the conus medullaris to ascend. It is located at the L2-3 level at birth and by 3 months of age at the adult level of L1-2.

F or the first segmentation, the mesoderm cells on both sides of the notochord orientate themselves as symmetrical aggregations called somites separated by the intersegmental (metameric) arteries. On the 24th day resegmentation of the membranous segments into cartilaginous vertebrae with chondrification centres on both sides of the notochord occurs. The notochord is an elastic column preventing compression of the soft vertebral blastemes of the embryo, which runs through the developing discs and vertebral bodies. At this time, segmentation of the notochord occurs.

The uniform tube of the notochord degenerates to a mucoid streak opposite the vertebral bodies and expands at the disc levels to form the nucleus pulposus later. If the notochord fails to re-expand completely, small defects in the middle to the dorsal third of the endplates may be visible in the adult. These must be differentiated from Schmorl's nodes which are true herniations of cartilage into the endplates.

Ossification starts at the end of the second month of gestation in the lower thoracic area. At the previous location of the notochord ventral and dorsal to the mucoid streak, two ossification centres develop, rapidly fusing to a single ossification centre. Paired perichondral ossification centres appear at this time in both of the neural arches. Therefore, x-rays will visualize three ossification centres in each vertebra. The atlas and the axis show a different type of ossification. It is generally accepted that the lost body of the atlas forms the odontoid process. The atlas develops two lateral ossification centres in the neural arches, while the axis has two ossification centres, one at the base and one butterfly-shape d for the odontoid process.

Between 8 to 15 years of age, ossification of the ring apophysis occurs. The ring apophysis is a cartilaginous ring in the periphery of the vertebral endplates and anchors the annulus fibrosus to the vertebrae. Additional secondary ossification centres appear in the transverse processes, the mamillary processes, the spinous processes and the tips of the articular processes of the apophyseal joints.

Vertebrae

Humans have 24 vertebrae, 7 cervical like all other mammals, but fewer vertebrae in the thoracic and lumbar spine reflecting the tendency to rebuild the ribs, which becomes evident in the different lengths of the 12th rib. Except for the atlas, which has lost its vertebral body, and the axis with its cephalad directed dens (odontoid process), the vertebrae exhibit a uniform appearance, consisting of a vertebral body and a neural arch formed by the pedicles and the laminae. The superior and inferior articular processes create the apophyseal joints on each side.

In the cervical spine, the relatively small vertebral bodies have a squared appearance when looked at from above. The uncinate processes, which are developed from parts of the neural arches, are exclusively found in the cervical spine. They are orientated in a cephalad direction and prevent the vertebral body from sliding sidewards, therefore allowing the apophyseal joints to be orientated in a coronal plane. The spinous processes are directed caudally with a bifurcated bump at the second to sixth cervical vertebrae. The spinous process of the seventh vertebra lacks bifurcation, is most prominent and can be easily palpated. The transverse processes are formed in their anterior part by the rudimentary ribs creating the anterior tubercle. Each transverse process forms a foramen containing the vertebral artery. The thoracic vertebral bodies articulate at the level of the end-plates and at the transverse processes with the ribs. The lumbar vertebrae exhibit a kidney-like appearance. The fifth lumbar vertebra is slightly sickle-shaped and is higher in its ventral portion. The reinforced superior articular processes have a small bump, the mamillary process.

Apophyseal joints

The apophyseal joints (facet joints) are formed by the superior and inferior articular processes of adjacent vertebrae and form the dorso-lateral part of the neural foramina. In the cervical spine, they are arranged parallel with an angulation of 45° from the coronal plane to the axial plane. In the thoracic spine, the facets rotate externally and more vertically to be nearly in the sagittal plane in the upper lumbar spine. In the lower lumbar spine, the facets rotate again inwards to form an angle of about 45° on transverse sections and the plane is orientated between coronal and sagittal. Especially in the cervical spine, where loading forces to the upper and lower parts of the facets are applied by forced extension and flexion, meniscoid synovial folds are found in the upper and lower parts of the apophyseal joints.

Discs

Each disc from C2 to the lumbosacral region has both a nucleus pulposus in its centre, developing from the remnants of the notochord and an annulus fibrosus, composed from reversed helical fibres in the outerparts. In children the discs are vascularized, while from the age of 5 years they are free of vessels.

Aging is normal in discs. In young adults degeneration starts with the development of mucoid degenerations leading to circular fissures in the annulus fibrosus. These fissures may communicate and progress to radial fissures. Reaching the very peripheral and vascularized parts of the annulus, a secondary vascularisation of the disc can be induced. Vascularised fibrous granulation tissue grows into the disc in an attempt to repair the tears. However, this repair will usually not be successful. Degeneration will progress with dehydration of the nucleus and loss of disc height. Vascularization of the disc can be visualized with Gadopentate-enhanced MRI. The discs show mostly linear increases in signal parallel to the end-plates. A decrease in disc height results in a loss of stability and initiates spondylosis.

In the cervical spine of children up to 8 years of age, the uncinate processes are still not erect and no fissures are present at the uncovertebral junctions. In the second decade of life under the influence of motion fissures in the lateral parts of the cervical annulus fibrosus develop and uncovertebral joints are formed. The tears progress medially and can dissect the whole cervical disc in a transverse direction.

Ligaments

The vertebral bodies are connected by the annulus fibrosus as well as the anterior and posterior longitudinal ligament. In caudad direction the anterior longitudinal ligament becomes broader and stronger. The deep layers bridge adjacent vertebrae without connection to the annulus fibrosus, the superficial layers cross 4-5 vertebra. The posterior longitudinal ligament is smaller and weaker than the anterior longitudinal ligament. In the cervical spine it is broader than in the thoracic and lumbar spine, where only a small part of the dorsal annulus is covered. In the thoracic and lumbar spine the posterior longitudinal ligament is connected mainly to the discs and bridges the dorsal surfaces of the vertebral bodies. The neural arches are connected by the symmetrical ligamenta flava between the laminae and by the interspinous ligaments between the spinous processes.

Stig Holtås, Maximilian F. Reiser and Axel Stäbler