spinal stenosis
To find an effective back pain treatment is important to understand how the pain is produced in the body. The spinal chord is the portion of the central nerve system that connects the brain to almost the whole body and most sensory nerves. It is traditionally divided in 4 levels; each of the 4 levels controls the function of a particular region of the body toward the spinal nerve roots that enters and exits particular openings in the vertebra.
There are 31 pairs of spinal nerve roots arisen from the spinal chord divided in to anterior and posterior roots: 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral and 1 coccygeal nerve.
There are 24 separated vertebrae: 7 cervical, 12 thoracic, 5 lumbar and 5 are fused together to form the sacrum. Then another 4, the smallest vertebrae, are fused to form the coccyx. The largest are the lumbar vertebrae, which bear the most weight. It is the fifth lumbar vertebra that carries the weight of the whole upper body and transfers it to the sacrum. The main cause of back pain will occur at the fourth or fifth lumbar vertebrae.
Twenty three intervertebral discs unite the vertebral bodies from the second cervical to the first sacral vertebrae. No intervertebral disc is present between the cranium, the first and the second cervical vertebrae. These discs are specialized connective tissue structures evolved to absorb and redistribute forces applied to the spine.
The intervertebral disk is a complex structure consisting of 4 distinct tissues: the nucleus pulposus, the annulus fibrosus, the cartilaginous end plates and the adjacent vertebral bodies. The nucleus pulposus is in the center region of the disk and is comprised of a gelatinous mixture of water, collagen and proteoglycans. The nucleus pulposus is able to hold fluid pressure largely because of the presence of negatively charge groups of glycosaminoglicans chains. For the other side the annulus fibrosus is comprised of collagen fibers arranged in layers and the cartilaginous end plates are composed predominantly of hyaline cartilage.
Since the Intervertebral Disk lacks the blood supply which is essential for any normal reparative process in the body another kind of reparative mechanism is necessary to occur. Although the disk is the largest avascular structure in the human body there is considerable chemical interchange and activity there. The repaired of the injured disk is as a consequence slow because the avascular disk is always completely dependent on diffusion to adjacent vertebrae for nutrition. The cellular elements of the disc cannot receive blood nutrients thorough the mediation of the synovial fluid but must rely on a diffusion system that provides a metabolic exchange with the vessels that lie within the vertebral bodies. The cells of the disc must therefore derive their nutrients and dispose their waste metabolic products by diffusion from and to blood vessels at the disc margins.
The nucleus pulposus because of its polar properties has a great capacity to absorb and bind water. It is the nucleus, the most highly hydrated part of the disc, which contributes to the majority of the internal pressure needed to balance the applied pressure.
The intervertebral disk is a hydraulic system composed of a fibroelastic envelope containing a gel in its center. The hydrodynamics of the disk depends of the nucleus pulposus possessing a gel that contains cartilaginous cells, fibroblasts, collagen, mucopolysaccharides and proteins. The nucleus pulposus is located in an area composed of fine fiber strands that lie in a mucoprotein gel containing water, collagen and proteoglycans.
The fluid contained within the annulus is a colloidal gel that behaves as a hydraulic system. When there is an increase in pressure the fluid inside the disk moves out and when the pressure is decreased the fluid returns into the disk by absorption. Compression of the disk takes place in the annulus; if the vertebral disk is subjected to great compression the fluid leaves the nucleus. The disk has the ability to absorb large amounts of water, and the main compound involved in this process is a gel that can adhere almost many times its own volume of water. The water is attracted to the ground substance because it contains glycosaminoglycans to high osmotic pressure which can support a load just as the pressure of air supports the weight of a car. The nucleus in this way can balance the average compression forces when fully hydrated.
With aging the nucleus pulposus undergoes degeneration and dehydration, and ultimately is transformed into fibro cartilage and becomes indistinct from the inner layers of the annulus fibrosus. The disk becomes dehydrated, loses part of its mucopolysaccharides content, and shows and increase in collagen. As the nucleus pulposus degenerates is unable to distribute pressure equally over the annulus and the vertebrae. When degeneration of the nucleus is produced a major change occurs in the transmission forces along the vertical axis of the spine.
The most consist chemical modification related with aging include lost of proteoglycans and water. Disk bulge can occur when loss of water causes the disk to flatten bulge beyond its normal margin and place pressure on the nerves exiting from the spine. Cell function is damage by extreme water content either to high or too low induced by fluctuation of disk compression. Tissue volume lost from dehydration produce an increase in disk bulging and a decrease in disk height. The normal disk height is greatly reduced because of loss of proteoglycans and water content from the nucleus.
When degenerative change occurs in the disk the nucleus pulposus tends to move out usually to the weakest area of the annulus producing compression of the nerve roots and spinal canal. Degeneration leads to tissue dehydration and the breakdown of the nucleus structure resulting in a reduction of osmotic properties because the fluid exchange in the disk is control by osmotic pressure.
The effects of disk degeneration include loss of cellularity, disorganization of the extracellular matrix, morphological changes and alteration in biomechanical properties. Dehydration is also correlated with decrease in disk cellularity, disorganization of the annular layers and alteration in the density of adjacent vertebra. Damage to cells produce at the same time disruption of blood vessels that deprives nerve cells for oxygen and expose them to toxic substances.
Spinal loading can alter tissue, water content and tissue shape in the nucleus and annulus producing change in cell metabolism. Change in water content produce modification in tissue permeability, density, oxygen tension and cell shape. As a consequence the disk loses its ability to attract and retain water.
Disk stress distribution is dependent on the type of loading such as compression, flexion, lateral bending or torsion and the degree of loading. Tissue stress developed through to the spinal loading can influence its biology. Within vertebrae strength can stimulate cells to develop more bone in areas of high stress or remove bone in areas of low stress. This process is the body mechanism to optimize the density and shape of bone for a particular mechanical exposure.
Weight is transmitted to the nucleus through the hyaline cartilage plate. This structure is ideally located to this function because it is avascular. If weight were transmitted through a vascularized structure, such as bone, the local pressure would block the blood supply and progressive areas of bone would die. The intervertebral discs have a blood supply at birth but later their nutrition is taken place by diffusion from tissue fluids. The fluid shift is in both directions from the vertebral body to the disk and from the disk to the vertebral body. This ability to transfer fluid from the discs to the adjacent vertebral bodies minimizes the rise in disk pressure on sudden compression loading.
During the early years of life the nucleus has sufficient moisture to act like a gelatinous mass. With advance degeneration of the nucleus pulposus the distribution forces to the annulus are completely lost since the nucleus now has a solid structure rather than a liquid state. The forces of degeneration makes the disk less elastic and this implies that as the disk degenerates, it loses the capacity to alternate shocks and distribute the load uniformly over the entire end plate.
The distribution of forces in the abnormal and normal disk can be explained when the disk function normally as in the early decades of life the nucleus distribute the forces of compression and tension equally to all parts of the annulus, when degeneration occurs the nucleus no longer function as a perfect gel and the forces transmitted to the annulus are unequal.
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