Intervertebral disc

    The spine is a non-homogeneous complex-shape construction of 24 vertebrae, separated by intervertebral discs with numerous muscles and ligaments attached to them. Intervertebral discs act as a kind of cushion to soften the impacts caused by the movement of body. The intervertebral discs make up about one fourth of entire length of the vertebral column. The discs absorb the stress and strain transmitted to the vertebral column.

    The intervertebral disc is a structure composed of the anulus fibrosus, the nucleus pulposus and the end plates.
The anulus fibrosus is a collagen-fiber composite structure that surrounds the nucleus pulposus. It resists hoop stresses due to compressive loads and the bending and torsional stresses produced by everyday activities of bending and rotation. The fibers of the anulus form lamellae, or individual layers of parallel collagen fibers that attach to the superior and inferior end plates.
Nucleus pulposus is the inner gel-like (proteoglycan-laden gel), highly hydrated core. The gel-like nature of the nucleus pulposus constrained by the anulus ensures its high water content and cushioning properties. The nucleus pulposus is located slightly posterior from the center of the intervertebral disc. The cartilaginous end plate of the spine is a thin layer of hyaline cartilage, which lines the interior, and superior surface of the vertebral body. It consists primarily of collagen, proteoglycan and water. The end plate is centrally situated in the vertebral body, adjacent to the nucleus pulposus, and it has intimate attachments to the annulus fibrosus.

     The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when anulus fibers are weakened or thorn and the inner tissue of the nucleus becomes permanently bulged, distended or extruded out of its normal, internal anular confines. The mass of the herniated or "slipped" nucleus can compress a spinal nerve resulting in pain, loss of muscle control or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular leaks may occur, potentially resulting in persistent and disabling back pain.

     Whenever the nuclear tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate pain from degenerated or herniated discs, the nucleus or the disc as a whole is removed and the two adjacent vertebrae surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stresses on the discs adjacent to fused segment as they compensate for lack of motion, perhaps leading to premature degeneration of those adjacent discs.


 

     A more desirable solution would involve replacing in part or as a hole the damaged disc with a suitable prosthesis having the ability to complement the height and motion of a disc. Therefore a substantial need exists for an easily implantable, prosthetic spinal disc of loading bearing ability and pumping action simulating the natural disc physiology. Hydrophylic polymer systems exhibiting a water swelling ability, called hydrogels, seem to satisfy the major demands made on such an implant and thus the possibility of use of hydrogels in the spinal disc prosthesis construction has been investigated for a couple of years.
However, no invention has been reported to prove its functionality and become widely applied commercially available product.
Major problem immerging here is high tensile strength and durability that hydrogel systems usually lack and that should come together with good viscoelasticity and high water content.

Our efforts have aimed at the developme of such a material out of the range of synthetic polymers well know for their excellent biocompatibility, such as poly(vinyl pyrrolidone), poly(vinyl alcohol), poly(2-hydroxyethyl methacrylate), poly(methyl methacrylate) and others. To achieve best results various synthetic methods, post-synthesis treatment (chemical, thermal and with use of ionizing radiation) and mechanical testing procedures have been successfully employed.

Results we have been getting so far look interesting and render our new synthetic pathways very promising for future development of a fully functional, hydrogel based prosthetic intervertebral disc.