Intervertebral disc is kind of cushion between two vertebrae. The discs absorb the forces across the vertebra and provide cushion during motion of vertebrae, thus preventing grinding of vertebrae against each other.
Discs are avascular structures, and in fact largest structure of the body without blood supply.
They receive nutrition via osmosis from surrounding structures. Discs are attached to vertebral bodies by hyaline cartilage.
On x-ray, the disc is not visible but a gross idea about its height can be made by looking at intervertebral space, which is space between two vertebrae.
Structure of Intervertebral Disc
Each disc is made up of two parts: the annulus fibrosus and the nucleus pulposus.
Annulus or anulus is the word derived from the Latin word anus which means ring. The annulus is a ring-like tough structure that surrounds and encases the other structure of the intervertebral disc, called nucleus pulposus.
The annulus provides rotational stability and resists compressive stress.
The annulus is made of water and type I collagen fibers which are oriented at different angles horizontally to create the strength. Water and proteoglycans are other constituents.
Multiple layers or lamellae of collagen fibers arranged in a unique circumferential orientation along the disc periphery.
Each lamella is oriented at a 30-degree angle to the horizontal axis of the disc, and this pattern alternates in successive lamellae. The outer collagen fibers attach to the ring apophysis, and the inner layers attach to the end plate surround the nucleus.
High collagen/ low proteoglycan ratio makes the disc flexible enough.
Fibroblast-like cells are responsible for producing type I collagen and proteoglycans.
It is gel-like elastic substance within the annulus fibrous and is composed of the same material as annulus fibrosis – water, collagen, and proteoglycans but in different concentration and structural arrangements.
nucleus pulposus is composed of type II collagen, water, and proteoglycans. Nucleus pulposus contains approximately 88% water.
This hydrophilic matrix provides the height of the intervertebral disc and makes it compressible enough.
Viscoelastic matrix makes annulus fibrous elastic and enables distribution of the forces smoothly to the annulus and the endplates.
In contrast to annulus fibrous nucleus pulposus as low collagen / high proteoglycan ratio.
It contains chondrocyte-like cells which are responsible for producing type II collagen and proteoglycans and allows to tolerate hypoxic conditions.
Blood Supply of Intervertebral disc
The intervertebral disc is avascular because the capillaries terminate at the vertebral endplates. The nutrition reaches nucleus pulposus through diffusion through pores in the endplates
Nerve Supply of Intervertebral Disc
Sinuvertebral nerve originates from the dorsal root ganglion gives rise to the innervates the superficial fibers of the annulus.
The disc is not innervated beyond the superficial fibers
Vertebral End Plate
The vertebral endplates that are in direct contact with the disc, consisting of cortical bone in the periphery, which in adolescence is referred to as the ring apophysis, and compressed cancellous bone in the central disc area, which covers nearly 70% of the disk.
The outer 30% consists of dense cortical margin and is the strongest area of the end plate.
Nutrition of Intervertebral Disc
The nutrition of these cells is derived from diffusion across the hyaline cartilage and subcortical end plates.
The cells in the central part of the disc lie closest to the end plate. By virtue of their position they lie nearest to vascular supply and are responsible primarily for nutrient and waste product exchange. The ultrastructure of the bony endplate comprises numerous sinusoids with specialized loops of capillaries that come into close contact with the underlying cartilaginous layer.
Simple diffusion of molecules across the cartilage occur depending on the size of small nutrients. As there is anaerobic metabolism due to anaerobic avascular nature of the disc, the latent pH within the disc under normal conditions is acidic.
Microscopic Constitution of Disc
Collagen provides strength to the intervertebral disc and is most abundant in the outer annulus, making almost 70% of dry weight of annulus. [In contrast, collagen forms only 20% of the dry weight of nucleus pulposus]
The type of collagen is type I collagen, which is highly cross-linked for increased strength, and small amounts of types II, III, V, VI, and XI collagen.
Fibroblasts that diminish in a number closer to the disc center are interspersed.
The inner annulus contains a relatively greater proportion of chondrocytes and varying amounts of type II collagen fibrils in a loose, nonorganized manner. The central nucleus is a gelatinous core containing chondrocyte-like cells and nearly 85% of type II collagen.
Proteoglycans consist of a protein core covalently attached to a glycosaminoglycan subunit. The glycosaminoglycan units are hydrophilic in nature and retain water in its normal state.
Chondroitin sulfate and keratan sulfate are the most common glycosaminoglycans found in the disc, the former more prevalent in the normal. Multiple proteoglycan subunits are attached to a central hyaluronate filament by a link protein, a kind of glycoprotein.
The entire structure forms aggregate molecules, of which aggrecan is the largest found in the annulus, Others are versican, decorin, biglycan, and fibromodulin. Fibroblasts and chondrocytes are suspected within this mesh framework along with remnants of notochordal-like cells.
The gradient of the cellular and extracellular matrix components from the fibrous well-organized periphery to the randomly organized gelatinous center.
The gradient progresses through four less distinct zones of the disc
- Outer annulus
- Inner annulus
- Transition zone
- Central nucleus
Production and Remodeling of Disc
Chondrocytes and fibroblastic cells are within an extensive, intricate extracellular matrix.
These cells are responsible for homeostasis of the extracellular matrix, including matrix formation, maintenance, and remodeling.
The cells are responsible for production and constant remodeling of the principal components of the extracellular matrix collagen and proteoglycans.
Biomechanics of Intervertebral Disc
Highly hydrated amorphous central disc structure that in the normal state is constantly under expansible stress.
The nucleus retains water and expands, providing stiffness and resistance to compressive forces. This mechanism effectively transfers compressive load on the nucleus to a radially directed force with tensile stress to the annulus.
Constant load on the disc applies pressure to the nucleus, reducing the ability of the proteoglycans and, thus, the nucleus to hold water, causing creep of the disc.
After pressure is relieved, the time-dependent viscoelastic properties of the disc nucleus allow it to retain water.
The annular fiber orientation within the lamellae allows for resistance to tension. Because lamellae are oblique, it results in tension or relaxation of the fibers in different areas within the disk and with different forces. The phenomenon is called anisotropy.
In the anterior or posterior translation of a vertebral body and disc along the horizontal axis, all of the fibers of the annulus are stretched in the direction of the applied force.
However, fibers aligned in the direction of the force will undergo stain while the remaining fibers actually will be brought closer to one another and will relax.
Annulus fibrosus has highest tensile stresses and nucleus pulposus has highest compressive stress.
This alternating tension and relaxation with translation and axial rotation maintain the stability of the intervertebral segment.
In axial compression, the nucleus expands radially toward the inner aspect of the annulus fibers; the annulus fibers also expand circumferentially.
Anterior flexion of sagittal plane rotation causes the anterior annulus compression. The nucleus also is compressed eccentrically over its anterior aspect, after which it deforms and migrates posteriorly. The posterior annular fibers expand radially the shifting nucleus, and the fibers stretched in line with these forces resist further motion.
As rotation occurs, the weight of the upper body and trunk lead to shear strain forces at the disc and slight translation, which is resisted by the active and passive constraints of the posterior column, may occur.
Lumbar range of motion varies between vertebral levels and individuals.
The instantaneous axis of rotation continually changes throughout the range of motion and typically is located posterior toe the midvertebral body and below the superior endplate of the inferior vertebral body except at L5-S1, where it is located posterior to the disc space.
Function of Intervertebral Disc
- It allows spinal motion and provides stability
- It serves as a link to attach adjacent vertebral bodies together
- The disc is responsible for 25% of spinal column height
Changes in Intervertebral Disc with Aging and Disease
Aging of disc leads to an overall loss of water content and there is conversion to fibrocartilage.
There is a decrease in nutritional transport along with a decrease in water content. The pH becomes acidic and there is a reduction in the number of viable cells and proteoglycans.
Along with that, there is an increase in keratin sulfate to chondroitin sulfate ratio, lactate. the activity of enzymes causing degradation decreases.
In disc herniation, there is increased production of osteoprotegerin, interleukin-1 beta, receptor activator of nuclear factor-kB ligand (RANKL) and parathyroid hormone.
Clinical Significance of Intervertebral Disc
Also called disc herniation or slipped disc. In this condition, the nucleus pulposus along with protrudes out from a weakness in the annulus fibrosus and indents on the nerve root in the vicinity.
This condition can lead to radiating pain in the lower limb, commonly known as sciatica.
The bulge is usually posterior, especially in the lateral area.
Schmorl nodes are another kind of herniation, where the nucleus pulposus, herniates on vertebral endplates.
This is referred to as vertical disc herniation.
Degeneration of Disc
The disc degenerates with age. Before age 40 approximately 25% of people show evidence of disc degeneration at one or more levels. Beyond age 40, more than 60% of people have it.
In degeneration, the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases. This limits the ability of the disc to absorb shock. The annulus fibrosus also becomes weaker with age and has an increased risk of tearing.
Sclerosis of the bones underlying the end plates also occur.