Artificial intervertebral disc assembly

The present invention relates to an artificial intervertebral disc assembly which is implanted in the vacated space after a diseased or damaged intervertebral disc has been removed. It is important that such assemblies remain parallel to and fixed against the end plates of the vertebral body when subjected to off-axis loading so as to mimic the loading characteristics of a natural disc and provide the patient with substantially the same range of motion as would be provided by a healthy natural disc. It is also important for the comfort of the patient if the stiffness of the disc assembly can be set depending on the size of the patient. The assembly of the present invention obtains these objectives.

Prior art document <patcit id="pcit0001" dnum="FR2723841"><text>FR 2 723 841</text></patcit> discloses the preamble of claim 1.

The disc assembly of the present invention is generally oval-shaped, provided in different sizes to cover the end plates of the vertebral bodies of differently sized patients and employs a novel bladder and piston dampening assembly that axially biases the end plates of the disc assembly outwardly against the end plates of the vertebral body, allows for variations in the stiffness of the assembly for differently sized patients as well as pivotal movement and shock absorption under a wide range of physiological loading conditions while continually maintaining each of the assembly end plates in a parallel abutting relationship with the adjacent end plate of the vertebral body. The end plates of the disc assembly are each provided with outwardly projecting fixation features to decrease the chance of the implanted disc assembly migrating after implantation. The result of these features is a substantial improvement in artificial intervertebral disc replacement devices.

The present invention is directed to an artificial intervertebral disc assembly as defined in the appended claims.

Following is a description by way of example only with reference to the accompanying drawings of embodiments of the present invention. In the drawings: - <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> is a perspective view of the artificial intervertebral disc assembly of the present invention.</li><li><figref idref="f0001">Figure 2</figref> is a side view of the artificial intervertebral disc assembly of the present invention with the inflatable bladder being shown as transparent so as to show the positioning thereof and not to obscure the interior of the disc assembly.</li><li><figref idref="f0002">Figure 3</figref> is a perspective view of the upper end plate of the disc assembly of the present invention.</li><li><figref idref="f0002">Figure 4</figref> is a perspective view of the lower end plate of the disc assembly of the present invention.</li><li><figref idref="f0003">Figure 5</figref> is a side view of the artificial intervertebral disc assembly of the present invention.</li><li><figref idref="f0003">Figure 6</figref> is a sectional view of the intervertebral disc assembly of the present invention taken along the line 6-6 in <figref idref="f0003">Figure 5</figref>.</li></ul>

Referring now in detail to the drawings, the artificial intervertebral disc assembly 10 of the present invention comprises a pair of end plates 12 and 14, a piston 16 and a bladder 18. End plates 12 and 14 are preferably of an oval configuration with medial-lateral and anterior-poster dimensions in the same ratio as natural vertebral bodies and are preferably constructed of a titanium alloy, Cobalt-Chromium-Molybdenum alloys and the like or a radiolucent material such as polyetherketoneketone or polyetheretherketone (PEKK/PEEK), although they could be formed of other biologically acceptable inert materials that would provide the plates 12 and 14 with a rigid structure.

The end plates 12 and 14 each define some form of fixation feature on their outer surfaces for engaging the end plates of the vertebral body (not shown) so as to decrease the chance the implant migration after implantation and to promote bone in-growth. In the embodiment of the invention illustrated in the drawings, two or more dove-tailed projections 20 are employed. Such dove-tailed projections engage mating surfaces formed by the surgeon in the end plates of the vertebral body. Alternatively, a plurality of outwardly projecting spikes, ribs or ridges or other convex surfaces that mate with the natural vertebral end plates could be employed to secure the end plates of assembly 10 in place against the natural vertebral end plates. Long-term stabilization of the implant may occur using a porous coating such as beads, an hydroxyappetite coating or plasma spray to promote bone in-growth.

The inwardly facing surface 12' of the first or upper end plate 12, as shown in the drawings, defines a cylindrical projection 24 terminating in a convex end face 24' and a groove 26 extending about the perimeter of inner surface 12' (see <figref idref="f0002">Fig. 3</figref>). The inner surface 14' of the second or lower end plate 14 defines a corresponding opposed groove 28 extending about the perimeter thereof and a cylinder 30 projecting upwardly from the central portion of surface 14'. Cylinder 30 defines a plurality of radially extending channels 32 extending therethrough and is sized to receive reciprocating piston 16 therein. The upper end surface of cylinder 30 is preferably beveled at 30' as illustrated in <figref idref="f0002">Figure 4</figref>. The upper surface 16' of piston 16 is of a concave configuration so as to mate with the convex lower end face 24' on the depending projection 24 on the first end plate 12 (see <figref idref="f0003">Fig. 6</figref>). The piston 16 is reciprocally moveable within cylinder 30 and is provided with a height or axial length sufficient to project from cylinder 30 as shown in the drawings and as will be further discussed.

Bladder 18 extends between end plates 12 and 14 and is held at its upper and lower end portions in perimeter grooves 26 and 28 so as to circumscribe and enclose the space between the end plates 12 and 14 disposed about piston 16 and cylinder 30 as shown in <figref idref="f0001">Figures 1 and 2</figref>. Bladder 18 is preferably formed of a flexible polymer material and is secured in place by a suitable epoxy adhesive or, alternatively, by a mechanical connection such as a pair of locking rings (not shown) carried by the two end plates 12 and 14 or another suitable mechanical attachment device.

The space disposed interiorly of the end plates 12 and 14 and bladder 18, including the interior of cylinder 30 is filled with a biocompatible fluid 31 such as saline or hydrogel. Piston 16 actually floats in the fluid 31 within cylinder 30.

Accordingly, when an axially directed compressive force is exerted on the end plates 12 or 14 or both, the piston 16 is forced downwardly in the cylinder, causing fluid 31 in the area between the piston and the bottom of the cylinder 30 to pass outwardly through the radial channels 32 in the cylinder wall, bulging the bladder 18 outwardly. Upon a relaxation of the axial pressure against the end plates, the resiliency in the bladder will cause fluid to move inwardly through channels 32, returning the assembly 10 to its pre-stressed configuration.

By sizing the channels 32, and varying the elasticity of bladder 18, the stiffness of the disc assembly, <i>i.e.,</i> the axial force necessary to effect the outward bulging or deformation of the bladder and thus relative movement of the end plates can be regulated to accommodate individuals of varying size and enable the disc assembly 10 to mimic the physiological characteristics of a natural healthy intervertebral disc and provide such individuals with a relatively full range of motion.

As seen in the drawings, the mating curvatures of the axially aligned upper surface 16' of piston 16 and convex end face 24' on projection 24 and the positioning of the interface of those surfaces outside the confines of the cylinder 30 allows relative rotational movement of the end plates about a plurality of axes while maintaining the abutment of projection 24 against piston 16. By providing rotation about a plurality of axes, the two end plates 12 and 14 are allowed to move into non-parallel orientations whereby the assembly 10 accommodates off-axial loading and mimics the distortion of a natural disc without adversely affecting the support function between the two end plates 12 and 14 that is provided by the piston 16 and cylinder 30.

Various changes and modifications may be made in carrying out the present invention without departing from and scope thereof. Insofar as these changes and modifications are within the purview of the appended claims, they are to be considered as part of the present invention.