BACKGROUND OF THE INVENTION
This invention relates to orthopedic surgery, more particularly to a prosthetic intervertebral disc, and stent used in the construction thereof, and a surgical procedure for implanting the construct in the intervertebral disc space.
The normal intervertebral disc has an outer fibrous ring, constituted mainly of collagen fibers, which strongly binds the vertebral elements together. This fibrous outer layer, or annulus, encircles a soft gel-like matrix, or nucleus, which serves both as a cushion and as a mobile and compressible element that allows motion to occur between the vertebral bodies above and below the intervertebral disc. This gel matrix is 95% water. The types of motion that can occur at the level of the intervertebral disc include flexion, extension, lateral bending and varying degrees of torsion or rotation.
In the course of a day, the normal intervertebral disc may encounter various combinations of these bending or twisting motions several thousand times. As a consequence of such repetitive motion, natural discs deteriorate over time, much as the padded cushion on a well-used chair might do.
The effect of this deterioration is a loss of water content of the gel matrix of the nucleus and a concomitant compacting of its fibers with a resultant loss of disc space height which in turn causes a loosening of the surrounding support ligaments of the spine and the development of what is termed degenerative instability. This instability results in a pathologic excess of movement at the intervertebral disc space that further accentuates the degeneration of both the nucleus and the annulus of the disc. With continued deterioration, the annulus of the disc can bulge or even develop radial tears that allow the inner nuclear material to protrude or even extrude from the disc space. This bulging of the annulus or protrusion of the nucleus can compress nerves and cause disabling sciatic pain. Distension or bulging of the annulus alone is frequently sufficient to produce disabling back pain because or compression or inflammation of free nerve endings present in the outer annulus of the disc.
The time-honored method of addressing degenerative lumbar instability resulting from severely damaged intervertebral discs has been to remove the damaged disc and fuse the two adjacent vertebral bones to eliminate pathological motion. While this approach does well at eliminating pathological motion, it also prevents any natural motion at that segment. The consequence of eliminating natural motion at a single segment generally is that greater degrees of stress occur above or below that segment. This in turn accelerates degeneration of the neighboring intervertebral spaces, often necessitating additional fusion surgeries.
It would be desirable, therefore, to preserve natural motion at every disc space and thus eliminate the degenerative domino effect that disectomy and fusion seems to produce. Since the earliest pathologic change evident in a degenerative disc is loss of water content with conco-mitant loss of disc space height, maintenance of disc space height seems critical for maintaining the way opposing vertebral surfaces alter position with each other during bending and twisting. Indeed, loss of disc space height seems to be the most crucial early feature of degenerative instability. With degenerative instability the ligaments may ultimately become so lax that buckling of the ligaments occurs, or even pathologic slippage of the spine (spondylolithesis). Preserving disc space height is therefore important in preventing secondary degenerative changes that occur as a consequence of loss of disc space height from mechanical damage or dessication due to aging.
An intervertebral disc endoprosthesis ought, ideally, to restore and preserve disc space height while permitting sufficient natural motion (flexion, extension, lateral bending, and rotation) to prevent excessive stresses on spinal segments above and below the prosthesis. Natural motion may also play a role in the health of the annulus and surrounding ligaments, much as natural stresses play a role in the maintenance of strength and density of normal living bone.
Many synthetic structures have been used as intervertebral disc implants, but few materials are durable enough to withstand the tremendous and repetitive forces a natural disc must withstand. In addition, the majority of intervertebral implants fail to restore and maintain sufficient disc space height to keep spinal support ligaments taut. Many constructs designed to address natural motion at the disc space have either been to complex to achieve commercial success or too challenging to implant, from a surgical perspective.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an intervertebral disc endoprosthesis that simultaneously restores sufficient disc space height and mobility to provide a semblance of functional normalcy and to reduce or eliminate abnormal stresses on adjacent intervertebral segments.
Another object is to provide an intervertebral disc endoprosthesis which is simple to implant surgically and easy to manufacture from common materials.
To achieve these objectives, a stent according to this invention includes at least two telescoping elements, each having a head adapted to engage an endplate of each of the adjacent vertebral bodies, and means for changing the distance between said heads.
The stent is placed between adjacent vertebral bodies in a collapsed state, after the nucleus of the disc has been removed, through a small disectomy incision. The stent is then expanded in the disc space until the paraspinal ligaments and annular structures are taut and disc space height is restored. The end surfaces of the stent contacting the vertebral body endplate is smooth and convex, mirroring the normally concave surface of the vertebral endplate. The convex surfaces are in turn held apart by a connecting rod that allows expansion but not contraction when in use. The convex surface also has a spike or spur that digs into the endplate in its center portion to resist movement or shifting of the stent during distraction. This spike or spur also serves to prevent extrusion of the construct assembly in its final state. Once satisfactory distraction of the disc space has occurred, and the annulus and surrounding ligaments are deemed to be taut, the empty space formerly occupied by the damaged nucleus is replaced by synthetic material such as a resinous polymer or plastic that conforms to the shape of the nuclear envelope formed by the annulus and surrounding support ligaments. As the resin polymerizes, it hardens to form a stable endoprosthetic construct having a superior and inferior smooth metallic surface in contact with the vertebral endplates, and a surrounding ring of resinous material, that serves both to stabilize the stent and to replicate the gross anatomy of the enucleated segment of the disc. This construct serves to restore disc space height and annular ligamentous tension while simultaneously allowing the duplication of natural motion by virtue of its anatomic conformity to the preexisting disc nucleus.
The stent may be made of titanium or steel, or any other readily available biocompatible material already in use for human implantation. Any of currently available bone cements may be used for the polymer component of the construct, because they have been demonstrated to be biocompatible and have been used extensively in orthopedic joint implants. Bone cement is ideally suited as an endoprosthesis for disc replacement because it exhibits far greater strengths under compressive loads than under tensile forces. The forces which normally act on an intervertebral endoprosthesis are compressive in nature. Alternatively, any of a variety of synthetic materials that can be injected in a liquid or semisolid form and then allowed to harden could be used. Acrylics and carbonates, or other plastic materials, might also be used.
Because the endoprosthesis is in effect assembled within the disc space, the size of the stent in its collapsed state and its convex endplate surfaces are the only limiting factor regarding the size of the disectomy needed to allow implantation of the endoprosthesis, since bone cement or other synthetic polymers or plastics can be injected through relatively small ports. This potentially allows for endoprosthesis implantation through endoscopic or other similar minimally invasive surgical techniques. Indeed, because of the minimal exposure necessary for insertion of such a component construct, surgical implantation in an outpatient setting should be technically feasible. In addition, multiple disc replacements in a single sitting may be possible because of the ease of implantation.
Disc replacements done according to this invention would seem best suited to early disc degenerative conditions or so-call “black discs”, where the envelope of annular and longitudinal support ligaments are largely intact. A simple endoprosthesis in this setting can serve either as a permanent disc replacement, or as a means of temporizing and preserving functional mobility for the longest period of time possible prior to surgical fusion of the damaged segment.