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Prosthetic Disc Nucleus Implants

This project began 15 years ago with study of the known and expected functions of the spinal intervertebral disc, including its developmental anatomy and structural biology under normal and degenerating conditions, particularly addressing the role of the intradiscal nucleus. At the same time, and of equal magnitude was the review of clinical correlates associated with the known, progressive cascade of disc degeneration and frequent association with discogenic pain. In actual clinical practice, the patients' histories and physical examinations, radiographs, magnetic resonance imaging and provocative discographic studies were key in appreciating the relationship between structural changes, loss of segmental function and disabling back pain. Additionally, related experience in several methods of spinal decompression and fusion provided further insight into the tissue-structure-function-clinical effect complex of spinal disability and pain. The long-term goal has been the development of a disc prosthesis that would serve to replace functions lost through degeneration while relieving the disabling pain. Focus on replacing the nucleus came after realizing that this structure is the "engine" that drives the functions of the disc. That is, the disc provides the segmental flexibility needed by vertebrate animals for the spinal rhythm, needed for their unique ability to propel themselves in space and adapt to a broad spectrum of environments. This spinal mobility is essential, together with associated muscle and ligament interaction, to being able to crawl, swim, walk, run or fly. The disc provides that flexibility along with certain inherent constraints or mechanical limits of motion for reasons of protection of the segmental unit and adjacent nerve elements. Broadly speaking, the disc as a whole is built rather like, and functions rather like, a belted radial automotive tire, which is a flexible shock absorbing member interposed between two rigid bodies, namely, the car and the road. In the case of the spine, this member lies between adjacent, stacked, rigid bony blocks. The annulus is constructed of multiple (up to 20 in the lumbar annulus) circumferential plies or laminations of flexible but inelastic fibers (collagen Type I) interposed with additional circumferential plies of a compliant, somewhat elastic polysaccharide hydrogel (glycosaminoglycan). The entrapped nucleus is primarily composed of the same gel, which is powerfully hygroscopic. The gel therefore acts rather like the air in the tire, where the swelling keeps the annular ring pressurized, thus stretching the fibers of the annulus and stabilizing the segment. The annular collagen fibers are the same as those of ligaments of the body and must have static and dynamic stretch in order to remain healthy and for its fibers to replicate normally when injured or while undergoing the degenerative effects associated with aging. It is clear that the pressure exerted by the swelling of this gel provides sufficient internal expansion of the annulus to resist the forces of