WO2012047279A1 - Prosthetic intervertebral disc having an inhomogeneous annular sheath - Google Patents

Abstract

Described here is a medical device and specifically a prosthetic intervertebral disc having an inhomogeneous annular sheath. The sheath may be at least partially radiopaque. The device is an implant useful as a replacement for a disc situated between two vertebrae in a spine.

Description

PROSTHETIC INTERVERTEBRAL DISC

HAVING AN INHOMOGENEOUS ANNULAR SHEATH

RELATED APPLICATIONS

[0001] This application claims benefit under 35 USC 119 from U.S. Provisional No.

61/404,254, filed on September 30, 2010, entitled "Prosthetic Intervertebral Disc Having An Inhomogeneous Annular Sheath" and from U.S. Provisional No. 61/517,912, filed on April 26, 2011, "Prosthetic Intervertebral Disc Having A Multilayer Inhomogeneous Annular Sheath" the entirety of which are incorporated by reference for all purposes.

FIELD

[0002] Described here is a medical device and specifically a prosthetic intervertebral disc having an inhomogeneous annular sheath useful as a replacement for a disc situated between two vertebrae in a spine.

BACKGROUND

[0003] The intervertebral disc is an anatomically and functionally complex joint The intervertebral disc is composed of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral endplates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.

[0004] The spinal disc may be displaced or damaged due to trauma or a disease process. If displacement or damage occurs, the nucleus pulposus may herniate and protrude through the annulus fibrosus into the vertebral canal or into intervertebral foramen. Such deformation is known as a herniated or "slipped" disc. A herniated or slipped disc may press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution.

[0005] One manner of alleviating this condition is to surgically remove the involved disc and to fuse the two adjacent vertebrae. This procedure involves inserting a spacer in the place originally occupied by the disc and securing it between the neighboring vertebrae by screws and plates/rods attached to the vertebra. Despite the excellent short-term results of such a "spinal fusion" for traumatic and degenerative spinal disorders, long-term studies have shown that alteration of the biomechanical environment by fusing the adjacent vertebrae leads to degenerative changes at adjacent mobile segments. The adjacent discs have increased motion and stress due to the increased stiffness of the fused segment In the long term, this change in the mechanics of the motion of the spine causes these adjacent discs to degenerate.

[0006] To circumvent this problem, artificial intervertebral disc replacements have been proposed as an alternative approach to spinal fusion. Various types of artificial intervertebral discs have been developed in attempts to restore the normal kinematics and load-sharing properties of the natural intervertebral discs. These disc types can be grouped into two categories, i.e., ball and socket joint type discs and elastic rubber type discs.

[0007] Artificial discs of the ball and socket type are usually made up of a pair of metal plates, one to be attached to the upper vertebra and die other to be attached to the lower vertebra, and a polyethylene core working as a ball. The metal plates have concave areas to house the polyethylene core. The ball and socket type allows free rotation between the vertebrae between which the artificial disc is installed. Also, since this style of artificial disc lacks any significant ability to resist bending motions, these discs often cause adjacent discs to take up some extra loading often resulting in the eventual degeneration of those adjacent discs. Artificial discs of this type have a very high stiffness in the vertical direction and do not replicate the normal compressive stiffness of the natural disc, and consequently often cause greater loads to adjacent levels.

[0008] In elastic rubber type artificial discs, an elastomeric polymer is embedded between two metal plates and these metal plates are fixed to the upper and the lower vertebrae. The elastomeric polymer is bonded roughened porous surfaces on the interface of the metal plates This type of disc can absorb shocks in the vertical direction and has a load bearing capability. However, this structure has a problem at the interface between the elastomeric polymer and the metal plates. Even though the interface surfaces of the metal plates may be treated for better bonding, polymeric debris may nonetheless be generated after long term usage. Furthermore, the elastomer tends to rupture after a long usage because of its insufficient shear-fatigue strength. [0009] Because of the above described disadvantages associated with either the ball/socket or elastic rubber type discs, there is a continued need for the development of new prosthetic devices.

SUMMARY

[00010] Prosthetic intervertebral discs and methods for using such discs are described. Our prosthetic discs may include an upper endplate, a lower endplate, a compressible core member located between the two endplates and usually made up of a compressible polymeric core and a fiber element, e.g., one or more filaments, connecting those two endplates, and an inhomogeneous annular sheath situated between the endplates. The sheath typically will be anchored in the endplates and surround both the compressible polymeric core and the fiber element.

[00011] Our prosthetic discs may include top and bottom endplates separated by a compressible core member. Typically, the disc includes a nuclear region and an annular region surrounding that nuclear region. The two plates are held together by at least one fiber or filament wound around at least one region of the top endplate and at least one region of the bottom endplate. The disc nuclear region may include the compressible polymer core or may contain only the compressible polymer core. The regions may include a plurality of openings, e.g., slots, through which the fibers may pass. We have found that radially- extending slots, closed at both ends, situated in the annular region of the disc provide good results when wound with fibers in the manner discussed below. The compressible polymeric core may be in the nuclear region of the disc. It provides resilience, shock-absorbing capabilities, and otherwise allows normal flexing and rotational movements. Our discs may include separate vertebral body fixation elements or they may include integrated vertebral body fixation elements.

[00012] Our prosthetic disc may comprise an inhomogeneous annular sheath situated between the endplates and surrounding the compressible core member. The inhomogeneous sheath may comprise one or more continuous or non-continuous, similar or dissimilar layers and each of those layers may, in turn, independently comprise matrix materials such as one or more elastic or inelastic polymers and further comprise inclusions in the matrix such as metallic, semi-metallic, ceramic, organic materials, and their mixtures. The added materials may cause the sheath to be wholly or partially radiopaque or possess other physical attributes, such as echogenicity, reflectance, inductance, thermal or electrical conductivity, etc. The added materials and inclusions may be included in the matrix polymer, e.g., by compounding one or more of such added materials into the matrix polymer. The added materials may be affixed to the sheath by bonding solids such as particulates or spheres or solid structures such as foils or bands onto the sheath. Such bonding may be effected by gluing, RF bonding, etc. The added materials or inclusions may be placed on the sheath by vapor or electrostatic deposition of appropriate materials onto the sheath. The added materials may be included in the sheath structure during fabrication, e.g., by including wires, rings, flakes as integral members of the structure or as random inclusions in the material. The added materials may be introduced to the exterior or interior of the sheath by inks or paints containing the added materials.

[00013] When the sheath comprises multiple layers, added materials may be incorporated in one or more layers making up the sheath body to cause the layer— and, therefore, the sheath— to have specific physical attributes. For instance, the sheath may comprise two or more layers located concentrically, one or more about the other(s). At least one of the layers may comprise added materials or inclusions such as discussed just above, which inclusions optionally may be radiopaque. The remaining layer or layers may be radiolucent or may contain the same or different inclusions, again, which inclusions optionally may be radiopaque. Each of the remaining layers may comprise a matrix polymer or polymers or may comprise one or more non-polymeric materials separating other layers found in the sheath, e.g., the sheath may comprise three layers, one layer comprising a matrix polymer and a radiopaque powder, one layer comprising a foil or radiopaque powder, and a third layer comprising a mixture of polymers. The layers may be formed by convenient methods, e.g., by sequentially spraying fluids or particulate mixtures containing the constituent materials of a sheath layer onto a receiving form, by sequentially dipping a receiving form into a series of fluid or particulate mixtures containing the constituent materials of a sheath layer, by sequentially placing such layers on a receiving form using electrostatic techniques, etc.

[00014] Another particularly suitable method for production of the sheath precursor or pre- form involves forming one or more of the various layers on a rotating mandrel or other receiving form using a troweling applicator spaced apart from the mandrel. The troweling applicator may present a.) a substantially straight surface parallel to a substantially straight mandrel surface— forming a tubing wall having a constant thickness, b.) a substantially straight surface not parallel to the mandrel surface— forming, for instance, a sheath pre-form tubing member having a tapering wall thickness, c.) a non-straight surface parallel to a mandrel having a matching surface— forming, for instance, a bellow-shaped sheath preform with a constant thickness, d.) a non-straight surface not parallel to a mandrel having a desired surface— forming, for instance, a bellow-shaped sheath pre-form with a non- constant thickness, perhaps with a portion of a bellows having a greater or smaller wall thickness than an adjacent wall portion, e.) other desired relationships. Placement of the constituent polymeric material on the mandrel is, to a large extent, dependent upon the viscosity of that applied fluid. Consequently, the user must adjust the concentration of the polymer and any included particulates in the solvent to provide a material mixture suitable for such a troweling operation. Similarly, physical "damming" limiters preventing flow of the applied polymeric solution axially from the desired site, e.g., in the form of walls extending from the mandrel to the troweling surface (or extending from the troweling surface to the to the mandrel) may be used to assure that the resulting tubing pre-form matches the opening(s) between the troweling surface and the mandrel surface.

[00015] Placement of layers made from low viscosity materials, e.g., low

concentrations of polymer and/or included particulates, may be made in conjunction with "troweled" layers by direct application of those materials, specifically, for example, using brushes, physical sprayers, electrostatic sprayers, and the like.

[00016] The compressible polymeric core may be formed of a relatively compliant material, such as a thermoplastic elastomer, (particularly a polycarbonate-polyurethane thermoplastic elastomer, and particularly a commercially available TPE from DSM as BIONATE), polyurethane, or silicone, and may be fabricated by injection molding or compression molding or a combination of the two. The core polymeric core and the compressible core member may comprise a combination of these materials, such as a fiber- reinforced polyurethane or silicone.

[00017] The disc structures may be held together by at least one fiber or filament wound around (or through) at least one region of the upper endplate and at least one region of the lower endplate. The fibers are generally high tenacity fibers with a high modulus of elasticity. The composition of the fibers, the elastic properties of the fibers, as well as factors such as the number of fibers used, the thickness of the fibers, the number of layers of fiber windings, the tension applied to each layer, the interaction of the fibers with the slots (when present), and the crossing pattern of the fiber windings enable the prosthetic disc structure to mimic the functional characteristics and biomechanics of a normal-functioning, natural disc.

[00018] Other and additional devices, apparatus, structures, and methods are described by reference to the drawings and detailed descriptions below.

BRIEF DESCRIPTION OF THE DRAWINGS

[00019] The Figures contained herein are not necessarily drawn to scale, with some components and features being exaggerated for clarity.

[00020] FIGS. 1 and 2 show, respectively, a perspective view and a cross-sectional view of our prosthetic discs.

[00021] FIG. 3 is a perspective view of the bottom endplate from the prosthetic disc shown in FIGS. 1 and 2.

[00022] FIG. 4 is another cross-sectional view of a variation of our prosthetic disc showing the fiber component.

[00023] FIG. 5 is a cross-sectional view of an endplate showing passage of the fiber component through the slots in the endplate.

[00024] FIG. 6 shows a perspective view of one variation of an inhomogeneous annular sheath.

[00025] FIGS. 7A-7C, 8A-8G, 8H1-8H3, 8I-8N, 8P-8Z, and 9A-9C show variations of the inhomogeneous annular sheath.

[00026] FIGS. 10A1-10A4 show variations of the inhomogeneous annular sheath containing active members within the sheath wall.

[00027] FIGS. 11 A-l IE show examples of junctions between the inhomogeneous annular sheath and the endplates.

DESCRDTTON [00028] Before the present invention is described, it is to be understood that this invention is not limited to the particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[00029] Where a range of values is provided, it is understood that each intervening value, taken to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[00030] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

[00031] It must be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

[00032] The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.

[00033] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions.

[00034] Prosthetic intervertebral discs, methods of using such discs, apparatus for implanting such discs, and methods for implanting such discs are described herein.

[00035] It is to be understood that the prosthetic intervertebral discs, implantation apparatus, and methods are not limited to the particular embodiments described, as these may, of course, vary.

[00036] Our invention is directed to a prosthetic intervertebral disc having an inhomogeneous sheath.

[00037] Our prosthetic discs may include an upper endplate, a lower endplate, a compressible core member located between the two endplates and usually made up of a compressible polymeric core and one or more filaments connecting those two endplates, and an inhomogeneous annular sheath situated between the endplates and surrounding the compressible core member.

[00038] We use the term "inhomogeneous" to express the concept that the composition of the annular sheath is not a neat polymer or well-mixed polymer mixture throughout the sheath, but includes other non-polymeric materials, e.g., one or more solids or does not consist of, or consist essentially of, a neat polymer or well-mixed polymer mixture.

Additionally, mixtures of polymer compositions where one of the polymer compositions is not miscible in the other, e.g., forming islands or volumes of one polymeric composition in the matrix of the other, are considered "inhomogeneous." The non-polymeric materials may comprise, for instance, organic non-polymeric materials, metals, alloys, ceramics, inorganic materials that may be insoluble or soluble in a suitable polymer, metal oxides, metal nitrides, metal oxynitrides, and other materials that lend a particular physical attribute— e.g., radiopacity to the sheath.

[00039] By "prosthetic intervertebral disc," we mean an artificial or manmade device that is configured or shaped so that it can be employed as a replacement for an intervertebral disc in the spine of a vertebrate organism, e.g., a mammal, such as a human. Our prosthetic intervertebral disc typically has dimensions that permit it to substantially occupy the space between two adjacent vertebral bodies that is present when the naturally occurring disc between the two adjacent bodies is removed, i.e.. a void disc space. By ^::subsianuai¾- occupy" we mean that it occupies at least about 75% by volume of such a void space, such as at least about 80%. Our discs may have a generally oval, perhaps a roughly bean shaped structure, generally analogous to the shape of the naturally occurring intervertebral discs that they are designed to replace.

[00040] The dimensions of our disc in the anterior/posterior direction typically fall in the range of about 10 mm to about 55 mm, such as from about 18 mm to about 46 mm, or about 12 mm to about 40 mm. The lateral dimension of our disc typically falls in the range of about 12 mm to about 75 mm, such as from about 12 mm to about 40 mm. The vertical dimension of the disc typically falls in the range from about 3 mm to about 15 mm, such as from about 5 mm to about 12 mm. These dimensions may, of course, be varied as appropriate for the size or activity of particular patients, location in the spine, and conditions.

[00041] FIG. 1 shows a perspective view of one variation of our disc (100). Our discs may include both an upper (or top) endplate (102) and lower (or bottom) endplate (104), where the upper and lower endplates are separated from each other by a compressible core member (106 in FIG.2).

[00042] Top and bottom endplates (102, 104) have dimensions in accord with the dimensions of the disc, e.g., dimensions in the anterior/posterior direction typically in the range of about 10 mm to about 55 mm, such as from about 18 mm to about 46 mm, or about 12 mm to about 40 mm and lateral dimensions typically in the range of about 12 mm to about 75 mm, such as from about 12 mm to about 40 mm. The thickness of each plate is typically in the range of about 0.5 mm to about 4 mm, such as from about 1 mm to about 3 mm. The sizes of the upper and lower endplates are selected primarily based upon the size of the void between adjacent vertebral bodies to be occupied by the prosthetic disc. Accordingly, although endplate lengths and widths outside of the ranges listed above are possible, they are not typical.

[00043] The endplates (102, 104) may be single-piece or comprise a number of components, such as may be seen in FIG. 2, joined together by welding or other joining procedure. The joint between various of the disc components may be used to secure the sheath in position in the disc as will be discussed in more detail below.

[00044] The top and bottom endplates (102, 104) may be fabricated from a

physiologically acceptable material having the requisite mechanical properties, where representative materials include: titanium, titanium alloys, stainless steel, cobalt/chromium, etc.; polymeric materials such as polyethylene with ultra high molar mass (molecular weight) (UHMW-PE), polyether ether ketone (PEEK), etc.; ceramics; graphite; etc.

[00045] The disc (100) variation shown in FIG. 1 includes annular inhomogeneous sheath (108) extending between the upper endplate (102) and the lower endplate (104). The annular sheath serves to separate the compressible core element, comprising the compressible polymeric core (106) and the fibrous member (not shown in FIG.2), from the spinal environment.

[00046] In general, the mechanical functions of the sheath are: a.) to separate the core, the fibrous member and the compressible polymeric core, from surrounding bodily fluids and b.) to isolate any solids produced by the core from passage to the surrounding spinal region. The sheath may be configured so that it functions only as a separator. However, it may also be configured to provide additional functions including acting as an adjunct to the core, e.g., by providing additional or distributed, elastic or spring-like functionality to the core assembly or by re-directing forces applied to the implant to other various regions of the implant or by providing limits to the movement of the implant during application of force on the implant. The sheath may also be configured to provide shock- or force-absorbing functions to the implant

[00047] The sheath may be located or configured so that it is always separated radially from the fibrous member without regard to the movement of the endplates. The sheath may instead be located or configured so that it is separated from the fibrous member in all but the most unlikely movement of the endplates. The sheath may be located or configured so that it is in continuous or in occasional contact with the fibrous member during movement of the endplates. *

[00048] As discussed elsewhere, the annular sheath (108) is inhomogeneous and may comprise a flexible membrane comprising a single layer or one or more continuous or non- continuous, similar or dissimilar layers, the one or more than one layers, each layer including matrix materials such as one or more elastic or inelastic polymers and further optionally comprising inclusions such as metallic, semi-metallic, ceramic, organic materials, and their mixtures. The added materials may cause the sheath to be radiopaque or to possess other physical attributes, such as being echogenic or possessing enhanced wear resistance or having enhanced resistance to failure caused by flexing.

[00049] Our disc may be further characterized by an annular region (110) defining an annulus, which is the region, domain, or area that extends around the periphery of the disc and surrounds the nuclear region (1 13) which is the region, domain, or area in the center of the disc.

[00050] FIG. 2 shows a cross-sectional view of one variation of our disc (100) showing the annular sheath (108) extending between the upper endplate (102) and the lower endplate (104). The fibrous member is not shown in FIG. 2 to allow explanation of the structure of the assembled device, but is discussed just below. Openings (112), here depicted as slots that are closed at each end, passing through the lower endplate (104) may be seen in FIG. 2, but are not visible in this view of the upper endplate (102).

[00051 ] FIG. 2 also provides visualization of a variation of our disc ( 100) in which the endplates are assemblies of inner and outer endplate components that are welded— or otherwise assembled— together to form a unitary endplate. In this variation, the outer and inner endplate components are also used to secure the ends of the annular sheath (108). Specifically, upper endplate (102) is made up of outer endplate component (114) and inner endplate component (116). Lower endplate (104) is made up of outer endplate component (118) and inner endplate component (120).

[00052] FIGS. 1 and 2 also show fixation elements (124, 126) that may be used to secure the disc in the opening between the vertebral bones. The figures show fins or keels, although other fixation element designs, e.g., pyramids, cones, hooks, etc., are also suitable.

[00053] FIG. 3 shows a perspective view of lower endplate (104). The slot openings

(112) for passage of the fiber member (not shown) and the circular opening (128) for securing the annular sheath (108 in FIGS. 1 and 2). Also seen is the annular region (110) defining an annulus mat contains the slot openings (112) and surrounds the nuclear region

(113) which is the region, domain, or area in the center of the disc.

[00054] FIG. 4 shows a cross-sectional view of another variation of our disc (100) having an upper endplate (102), a lower endplate (104), and a compressible polymeric core

(106) retained between the upper endplate (102) and the lower endplate (104). One or more fibers (i 30) are wound around the upper and lower endpiates to attach the endplates (102, 104) to one another. The fibers (130) are preferably wound to allow a degree of axial rotation, bending, flexion, and extension by and between the endplates. The compressible polymeric core (106) may be provided in an uncompressed or a pre-compressed state. An

inhomogeneous annular sheath (108) is provided in the space between the upper and lower endplates, surrounding the compressible polymeric core (106) and the fibers (130).

[00055] The upper endplate (102) and lower endplate (104) are generally fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating the upper endplate (102) and lower endplate (104) include titanium, titanium alloys, stainless steel, cobalt chromium, etc., which are

manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Optionally, the endplates may be coated with hydroxyapatite, titanium plasma spray, or other coatings to enhance bony ingrowth, osseointegration, or ingrowth.

[00056] The upper surface of the upper endplate (102) and the lower surface of the lower endplate (104) are each shown to have several anchoring fins or keels (124, 126) for securing the endplate to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be installed. The anchoring fins or keels (124, 126) are intended to engage mating grooves that are formed on the surfaces of the upper and lower vertebral bodies to thereby secure the endplate to its respective vertebral body. The anchoring fins or keels (124, 126) extend generally perpendicularly from the generally planar external surface of the upper endplate (102), i.e., upward from the upper side of the endplate as shown in FIG.4. In the FIG. 4 variation, the upper endplate (102) includes three anchoring fins or keels (124, 126) , although only two are shown in the cross-sectional view. Each of the anchoring fins or keels (124, 126) has a plurality of serrations (132) located on the top edge of the anchoring fin. The serrations (132) are intended to enhance the ability of the anchoring fin or keel to engage the vertebral body and to thereby secure the respective endplate to the spine.

[00057] Similarly, the lower surface of the lower endplate (104) also includes a plurality of anchoring fins or keels (124, 126). The anchoring fins or keels (124, 126) on the lower surface of the lower endplate (104) are identical in structure and function to the anchoring fins or keels (124, 126) on the upper surface of the upper endplate (104), with the exception of their location on the prosthetic disc (100).

[00058] FIG. 5 shows a cross-section of lower endplate ( 104) through the opening slots (112) showing the passage of filaments (130) from one side of lower endplate (104) to the other and the repassage of the filaments (130) back to the original side. As shown in FIG. 4, the filaments (130) men pass to the upper endplate (102) for weaving similar to that shown in FIG. 5.

[00059] Although the endplates may include a single region around which a fiber is wound in order to hold the plates together, the endplates may have a plurality of such regions. The endplates may include a plurality of slot openings through which fibers, e.g., of the fibrous compressible element, may be passed through or wound, as shown. The number of different slots present in the periphery region of the device may range from about 4 to about 36, perhaps from about 5 to about 25.

[00060] The fibrous element may be fabricated from any suitable material, where representative materials of interest include, but are not limited to: polyester (e.g., Dacron), polyethylene (particularly, ultrahigh molecular weight polyethylene (UHMWPE)), polyaramid, carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.

[00061 ] The fibrous elements made up of one or more fibers wound around one or more regions of the top or bottom plates may make up a variety of different configurations. For example, the fibers may be wound in a pattern that has an oblique orientation to simulate the annulus of intact disc. The number of layers of fiber winding may be varied to achieve similar mechanical properties to an intact disc.

[00062] The compressible core element may have a fibrous component limited to the annular region of the disc, e.g., to the region along the periphery of the disc.

[00063] The fibrous component may be limited solely to the annular region of the disc and include both oblique and horizontal windings. The fiber windings of the various layers of fiber may be at varying angles from each other where the particular angle for each layer may be selected to provide a configuration that best mimics the natural disc. Additionally, the tension placed on the fibers of each layer may be the same or vanea. [00064] Returning to FIG. 4, the upper endplate ( 102) contains a plurality of slots (112) through which the fibers (130) may be passed through or wound, as shown. The actual number of slots (112) contained on the endplate is variable. Increasing the number of slots will result in an increase in the circumferential density of the fibers holding the endplates together. In addition, the shape of the slots may be selected so as to provide a variable width along the length of the slot For example, the width of the slots may taper from a wider inner end to a narrow outer end, or vice-versa. Additionally, the fibers may be wound multiple times within the same slot, thereby increasing the radial density of the fibers. In each case, this improves the wear resistance and increases the torsional and flexural stiffness of the prosthetic disc, thereby further approximating natural disc stiffness. In addition, the fibers (130) may be passed through or wound on each slot, or only on selected slots, as needed. The fibers (130) may be wound in a uni-directional manner, which closely mimics natural annular fibers found in a natural disc. Other winding patterns, variously single, bi-directional, or multi-directional, may be employed.

[00065] As described above, the purpose of the fibers (130) is to hold the upper endplate (102) and lower endplate (104) together and to provide progressive resistance to motion thereby cooperating with the other disc components to mimic the motion of a natural disc. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. The fibers (130) are generally elongate fibers having a diameter that ranges from about 100 um to about 500 μτη, and preferably about 200 um to about 400 um. The length of each individual fiber making up the fibrous component may range from about 0.5 m to about 5 m, such as from about 0.5 m to about 2 m.

[00066] Optionally, the fibers may be injection molded or extruded with an elastomeric covering to encapsulate the fibers, thereby providing protection from tissue ingrowth and improving torsional and flexural stiffness, or the fibers may be coated with one or more other materials to improve fiber stiffness and wear. Additionally, the core may be injected with a wetting agent such as saline to wet the fibers and facilitate the mimicking of the viscoelastic properties of a natural disc.

[00067] The fibers (130) may be terminated on an endplate by tying a knot in the fiber on a surface of an endplate or by tying two fiber ends together. Alternatively, the fibers (130) may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure hself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint

[00068] As seen in FIG.4, the compressible core member (106) is intended to provide support to and to maintain the relative spacing between the upper endplate (102) and lower endplate (104). The compressible polymeric core (106) may be made of a relatively compliant material, for example, polyurethane or silicone or TPE's, and may be fabricated, for instance, by injection molding. As noted elsewhere, we have had excellent results with compressible polymeric cores comprising a compression molded TPE, in particular, a TPE available from Royal DSM N.V. as BIONATE.

[00069] Another variation of compressible core member (106) includes a nucleus formed of a hydrogel and an elastomer reinforced fiber annulus. For example, the nucleus, the central portion of the compressible core member (106), may comprise a hydrogel material such as a water absorbing polyurethane, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyacrylamide, silicone, or PEO based polyurethane. The annulus may comprise an elastomer, such as silicone, polyurethane or polyester (e.g., Hytrel®), reinforced with a fiber, such as polyethylene (e.g., ultra high molecular weight polyethylene, UHM PE), polyethylene terephthalate, or poly-paraphenylene

terephthalamide (e.g., Kevlar®).

[00070] The shape of the compressible core member (106) may be generally cylindrical, spherical, barrel-shaped, egg-shaped, or bean-shaped or the like, although the shape (as well as the materials making up the core member and the core member size) may be varied to obtain desired physical or performance properties. For example, the compressible core member (106) shape, size, and materials will directly affect the degree of flexion,

.. ZSZSXXL laierai bending, and axial rotation of the prosthetic disc. [00071] Also shown in FIG. 4 is an inhomogeneous annular sheath (108) extending from upper endplate (102) to lower endplate (104). The variation shown is generally cylindrical. Functions of the annular sheath include acting as a barrier keeping the disc materials (e.g., fiber strands) within the body of the disc and keeping natural in-growth outside the disc.

[00072] The combination structure involving the endplates, compressible core element, and the fiber element provides a prosthetic disc that functionally closely mimics a natural disc. The top and bottom endplates are held together by at least one fiber, e.g., of the fibrous compressible element, wound around at least one portion of each of the top and bottom endplates, often through a plurality of slots in each endplate. The two endplates may be held to each other by one or more fibers that are wrapped around (or pass through) at least one domain/portion/areas of the upper endplate and lower endplate such that the plates are joined to each other.

[00073] The sheath is inhomogeneous and may be made up of a composite flexible membrane comprising one or more continuous or non-continuous, similar or dissimilar layers, the one or more layers including matrix materials such as one or more elastic or inelastic polymers and further optionally comprising inclusions or materials such as metallic, semi-metallic, ceramic, organic materials, and their mixtures. The added materials may cause the sheath to be radiopaque or to possess other selected physical attributes, such as being echogenic or possessing enhanced wear resistance.

[00074] Our single-layer sheaths may be formed in a variety of ways, however we have found that extruding a mixture of a matrix polymer and one or more included materials into a preform tubing followed by blow-molding the pre-form tubing into a properly shaped mold providing the final shape provides, for some included materials, a sheath having a durability under accelerated flexing testing mat is quite good. In particular, we have found that a sheath comprising particulate tantalum in a BIONATE-A polymer matrix formed by the above extrusion/blow-molding procedure has a durability in the accelerated flexing test procedure that equals or exceeds sheaths made up of the polymer alone.

[00075] Although sheaths comprising included materials of titanium metal appear to be suitable for use in a prosthetic disc depending upon the manner in which the sheath is produced, high temperature extrusion procedures involving titanium metal and BIONATE-A appear to create inclusions in the resulting sheath perhaps weakening that sheath.

[00076] Our multilayer sheaths may be formed of more than one layers comprising the noted added materials or inclusions, e.g., radiopaque materials, in a polymeric matrix, e.g., by extruding, dipping, molding, blow-molding, troweling, brushing, electrostatic application, or other acceptable forming procedure or combinations of such procedures, from a mixture of a suitable polymer or polymers (for the polymer matrix) and suitably sized additive particles.

[00077] In either version of the inhomogeneous sheath, the particles may be randomly shaped or regularly shaped, e.g., spherical particles. Blends or formulations of matrix polymers may include additive materials such as radiopacifiers formulated with the polymer in percentages ranging from about 2% to about 50% additive materials, e.g., about 7% to about 25%, 8% to 15%, depending upon the additive material chosen.

[00078] Additionally, additive particles— variously in the form of a powder, a suspension, or an ink— may be applied to interior or exterior regions of a partially or completely formed sheath and made to adhere to the selected surface, e.g., by using an appropriate solvent or adhesive. The particles may be applied to completely cover either the interior or exterior surface or both, if so desired, and covered with a polymeric layer comprising the same polymer or a different one. When the radiopaque materials are applied to a surface of the sheath, ink-jet printer technology may be utilized to apply the materials in a desired pattern. The particle-coated sheath may be heat treated, optionally in a mold or other forming device, to effectively embed the particles in the finished sheath body.

[00079] The radiopacifier may be applied in such a way that it serves one or more specific functions. For instance, the radiopacifier may be applied to the sheath so that it remains on the sheath surface for only a short period of time, e.g., long enough that it may be used to assure mat the implant has been properly positioned upon implantation. The radiopacifier may be applied to allow assessment of sheath condition at a later time, e.g_, at six months or ten years after implantation.

[00080] Where the materials added to a polymer matrix are to render the sheath wholly or partially radiopaque, acceptable radiopaque metals and alloys comprise gold, platinum, platinum-iridium alloys, iridium, ruthenium, rhodium, osmium, tantalum, niobium, palladium, titanium, tungsten, titanium-tungsten alloys, various stainless steels, nickel - titanium alloys such as nitinol, cobalt-nickel alloys, bismuth, barium, and combinations thereof, including alloys and mixtures.

[00081] Other radiopaque inorganic materials include barium oxide and barium salts such as barium sulfate and barium fluoride, bismuth sulfate; metal oxides, such as titanium dioxide, zirconium oxide, chromium oxide, bismuth oxide, lanthanum oxide, tantalum oxide, tin oxide, yttrium oxide, ytterbium oxide, barium oxide, strontium oxide, zinc oxide, and combinations thereof; bismuth glasses, and nanocrystalline forms of hydroxyapatite and tricalcium phosphate.

[00082] Certain organic, biocompatible materials, e.g., brominated or chlorinated paraffins and organometallic materials, may be chosen as radiopaque materials.

[00083] In some variations of our device, the implant may include a radiopaque fluid situated within the volume enclosed by the sheath. In the event the sheath is breached, the radiopaque fluid escapes and the resulting differences may be observed by fluoroscopy. An alternative to an implant structure containing radiopaque fluids within the interior of the sheath is one having concentric sheaths having a volume between the sheaths, where the volume contains a radiopaque fluid.

[00084] In certain variations of ou device, the radiopacifier may be chosen to be reactive with a sterilizing agent, such as ethylene oxide, and applied in such a way that sterilization of the implant is assured. An ancillary radio frequency identification (RFID) tag that is readable with a sterilizing agent may be placed interior to the sheath to indicate that the implant has been sterilized. These RFID tags may be placed, for instance, upon the inner surface of the sheath.

[0008S] RFID tags may be situated upon, adjacent to, or embedded in the sheath and configured to show axial compression of the implant, translation of the endplates, near or actual contact of the endplates on a continuous, intermittent, or single instance basis.

[00086] Any of the variations discussed here, e.g., wire, foil, radiopaque-paiticle regions, may be formed upon a neat polymeric matrix or may be formed upon a polymeric matrix containing one or more radiopacifiers.

[00087] The polymeric matrix may comprise one or more biocompatible polymeric materials that at least do not interfere with the action of the prosthetic disc. Certain variations of the sheath may have passive motion-modifying components, e.g., those having spring functions or shock-absorbing functions, included in the body of the sheath.

[00088] Examples of polymeric materials suitable for the sheath include polyurethanes, Silicones, various polyolefins, e.g., polyethylene (PE), high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene, their mixtures and blends, polyolefin blends (TPOs), polyesters, thermoplastic elastomers (TPE's) such as those comprising polystyrene and polybutadiene block copolymers, styrene- butadiene-styrene block copolymers ("SBS" e.g., Kraton), poly(styrene-b-isobutylene-b- styrene) triblock copolymers ("SIBS"), polyester-polyether copolymers (e.g., Hytrel), polyamide-polyether copolymers (e.g., Pebax), polycarbonate-polyurethane copolymers ("PCU's" such as BIONATE and CHRONOFLEX) , silicone polycarbonate-polyurethane copolymers (e.g., CarboSil), styrenic block copolymers, elastomeric alloys, thermoplastic polyurethanes ('TPU's" such as Pellethane), thermoplastic copolyesters and thermoplastic polyamides and the like.

[00089] Other TPE's include polysiloxane TPU's, TPE's based on alkyl acrylates and/or alkyl methacrylates such as poly(methyl methacrylate-b-n-butyl acrylate-b-methyl methacrylate), and TPE's that comprise polyethylene terephthalate segments and fluorinated segments such as PTFE, ETFE, and hexafluoropropylethylene (HFP) segments, among many others.

[00090] One at least partially radiopaque structure includes a sheath body having an amount of foil effective to provide radiopacity. An effectively radiopaque layer of metallic foil typically is between 8 μιη and about 30 μηι thick, depending upon the material. Such foil may be patterned or continuous. Patterns may be marker spots having circular, oval, square, triangular, rectangular, pentagonal, hexagonal, or combinations thereof. The marker spots may be placed within the sheath wall or attached to the inner or outer surface of the sheath wall. Radiopaque foils may be cut or patterned by procedures such as laser cutting, wire EDM, die cutting, or the like.

[00091] Bonding between the foil and the previously formed sheath wall may take place by addition of an adhesive, by the application of heat, by the application of ultrasonic energy, by use of a laser, by the application of radio frequency energy, by the application of pressure or by combinations thereof. A material may be added to the interface that enhances the effectiveness of these bonding techniques.

[00092] Another layer comprising one or more polymeric materials and optionally one or more radiopaque materials may be placed exterior to the foil layer.

[00093] Another sheath structure having radiopacity comprises a polymeric matrix containing radiopaque wire or cord. The wire may be chopped, formed, or included as a structure such as a woven cloth or a non-woven fabric. Wire used for radiopaque markers may be fabricated from fully annealed metals or alloys, preferably that are in a soft or malleable state.

[00094] One variation of the sheath wall construction comprises an inner layer of polymer, an outer layer of polymer, and a reinforcing layer between them. The reinforcing layer may comprise a coil or braid of wire or ribbon of the metallic materials discussed above. Preferably, the wire or ribbon is malleable, with little or no spring properties, and does not exhibit any elastomeric tendencies. The flat wire may have a thickness of between about 0.0001 and 0.010 inches, perhaps about 0.002 to 0.005 inches. The width of the flat wire, for instance, may range from about 0.005 to 0.050 inches and perhaps from about 0.008 to 0.025 inches. The spacing between turns of a coil may, for example, range from substantially no spacing to approximately five times the width of the coil wire. The coil spacing may be more than zero to permit bonding of the outer and the inner layers of polymer.

[00095] The inner and outer polymeric layers may be fabricated from the same or different polymeric materials selected from those polymers such as listed above.

[00096] Wire used to fabricate the reinforcing layer may be wholly produced from the radiopaque materials discussed above, or may comprise threads, cords, or other structures formed from polymers or non-metallic materials such as polyethyleneterephthalate (PET), polyamides (e.g., various Nylons), polycarbonates, glass-filled polycarbonates, carbon fibers, or the like coated with materials that have increased radiopacity to provide visibility under fluoroscopy. Suitable radiopaque coatings may comprise gold, platinum, tantalum, platinum- iridium, and the like.

[00097] Further, our inhomogeneous sheath may comprise two or more layers, at least one of which is radiopaque. The layers may be general iy concentric to each other, i^&u, layer may be continuous or non-continuous, subject to the resulting sheath forming a substantially continuous member between the two endplates. The layers may each be of the same thickness or of differing thicknesses. The thickness of each layer may vary.

[00098] Sheath layers comprising a matrix polymer or polymers and one or more particulates, e.g., providing radiopacification, may beneficially be formed adjacent a layer or layers comprising a matrix polymer or polymers and not containing particulates.

[00099] Our inhomogeneous sheath may comprise a fabric woven from polymeric or metallic fibers, wires, or ribbons into tubing or into a sheet. If woven into a sheet, the sheet may be rolled into a tube and the ends of the sheet joined where they meet. In either case, the tubing is then covered with a polymeric coating, interior, exterior, or on both surfaces. The fabric may comprise one or more biocompatible, metallic or polymeric materials selected from the lists provided here, although polymeric materials such as, e.g., PET and polyethylene are particularly suitable as are metallic materials such as nitinol and titanium.

[000100] In addition to the structural materials and the radiopacifiers discussed above, other materials may be added to our inhomogeneous sheath for a variety of purposes. For instance, interior or exterior coatings providing long-term or temporary hydrophilic or lubricious characteristics comprise a polymeric blend compatible with the polymeric component of the polymer matrix and containing water soluble hydrophilic polymers such as polyethylene oxide, polyacrylamide, polyvinyl pyrrolidone, sulfonated polystyrene, nonhydrophilic polymers such as silicones, siloxanes and other lubricious polymers.

Hydrophilic lubricants useful as coatings include polyalkylene glycols, alkoxy polyalkylene glycols, copolymers of methylvinyl ether and maleic acid, poly(vinylpyrrolidone), poly(acrylamide) including poly(N-alkylacrylamide), poly(acrylic acid), poly(saccharide), poly(vinyl alcohol), poly(ethyleneimine), polyamides, methyl cellulose,

carboxymethylcellulose, polyvinylsulfonic acid, heparin, dextran, modified dextran, chondroitin sulphate, lecithin, and so forth.

[000101] Additionally, widely known biocompatible coatings such as poly-para- xylylene ("Paralene-C") or low temperature isotropic carbon ("LTI") may be later applied to the sheath by vapor deposition or sputtering. Lubricious materials such as silicone oils are also suitable in some instances. [000102] Biologies, drugs, analgesics, or other compounds suitable for providing desired or preventing undesired physiologic responses, e.g., preventing heterotopic ossification by addition of 1-hydroxyethane 1,1-diphosphonic acid (HEDP) or preventing inflammation or pain by addition of nonsteroidal anti-inflammatory drugs (NSAIDs) or preventing infection by addition of antimicrobials.

[000103] FIG. 6 shows a basic variation of the inhomogeneous annular sheath (108), having an upper end (140), a lower end (142), and a sheath body (144). The sheath body shown in FIG. 6 is substantially cylindrical. The inhomogeneous annular sheath wall or body (144) may comprise radiopaque particulate materials in a polymeric matrix, as discussed above. Such a structure may be formed by extruding, dipping, molding, blow-molding, troweling, brushing, electrostatic application, or other suitable procedure or combinations of procedures into a desired shape from a mixture of the desired polymer and radiopaque particulates.

[000104] One combination of procedural steps that we have found to be suitable for producing the multi-layer version of the inhomogeneous sheath involves sequentially dipping a mold suitable for forming a tubing member into a polymer/solvent bath, perhaps drying that first layer, dipping the mold into a bath containing a polymer/solvent radiopaque particle mixture, perhaps drying the second layer, dipping the mold into a polymer/solvent bath to form a third layer, and finally drying the resulting precursor sheath tubing in a modestly heated vacuum. After removing the precursor sheath tubing from the first mold, the tubing is subjected to a blow-molding step in a female mold substantially providing the final shape of the sheath. Some trimming may be required, of course.

[000105] Another especially suitable method for producing a layered sheath involves forming a sheath precursor or pre-form using the troweling procedure discussed above. A mandrel or other receiving form (or mold) spaced apart from a troweling element is rotated beneath that troweling element. A solution of a selected polymer or polymers, particulates for adding, e.g., radiopacification, to the resulting sheath, and a volatile solvent is applied to the rotating mandrel under conditions suitable for volatilizing the solvent and for forming or troweling a layer of the precursor on the surface of the mandrel. The solution or mixture must be of a viscosity suitable for such a troweling step. [000106] Inhomogeneous annular sheaths having walls or bodies ( 144) comprising included radiopaque wire segments or other radiopaque artifacts may also be formed using these procedures.

[000107] FIGS. 7A, 7B, and 7C show, respectively, a perspective view, cross-section perspective view, and a cross section side view of one variation of pur inhomogeneous sheath (150) that is generally cylindrical, but has a pair of bellows (152) in the sheath wall (153). The upper circular edge (154) and the lower circular edge (156) each includes a lip (158) suitable for forming a seal with a respective endplate. Shown at various positions in the sheath wall (153) are a number of openings (159) that are used in this configuration as passageways for gaseous sterilizing media such as ethylene oxide. Other variations of the inhomogeneous sheath (150) may include no bellows, one bellow, or three or more bellows.

[000108] FIGS. 8A and 8B show, respectively, partial cross-sections of an

inhomogeneous sheath having a cylindrical shape (166) and an inhomogeneous sheath (168) having a having two outwardly extending bellows (170) of substantially the same size. In the FIG. 8B variation, the bellows (170) are rounded. FIG. 8C shows a partial cross-section of an inhomogeneous sheath (171) having a single outwardly extending bellow (173) and two inwardly extending bellows (175, 177). The bellows in each of the depicted variations need not be of substantially the same size.

[000109] The variation shown in FIG. 8C has an outwardly extending bellow (173) that is larger than the two inwardly extending bellows (175, 177).

[000110] FIG. 8D shows a variation of our inhomogeneous sheath (179) having an outwardly extending central bellow (181) and two inwardly extending bellows (183). This variation is similar to that shown in FIG. 8C but the central bellow (181) is smaller than the central bellow (173) in that variation.

[000111] FIG. 8E shows a partial cross-section of an inhomogeneous sheath (172) including three bellows (174) that are not rounded.

[000112] FIG. 8F shows a partial cross-section of an inhomogeneous sheath (178) including a single bellows (180) and including a radiopaque ring (182). In addition to any radiopaque material that may be included in the wall (184) of the sheath (178), the ring (182) may be radiopaque as well. The ring (182) may be fastened to the inner surface of the wall (184) or included within the waii (ί δ-ΐ . [000113] FIG. 8G shows a partial cross-section of an inhomogeneous sheath (190) having a sheath wall (192) including artifacts (194) that are at least partially radiopaque and are fastened to the exterior of the sheath wall (1 2). The artifacts may be fastened to the interior wall or situated within the sheath wall (192).

[000114] FIGS. 8H1 to 8H3 show cross-sections of a portion of an inhomogeneous sheath wall where the various layers comprise polymeric materials. Each layer may independently comprise a single polymeric material, a mixture of polymeric materials, and one or more optional radiopaque artifacts. In this variation of the inhomogeneous sheath wall, the materials comprising the radiopaque artifacts are preferably particulate and selected from the metallic materials discussed elsewhere. Further, at least one layer forming a component of an inhomogeneous sheath wall is at least partially radiopaque. The various layers making up an inhomogeneous sheath wall may have different levels of radiopacity.

[000115] Although the sheath walls discussed with respect to FIGS. 8H1 to 8H3 may be formed by extruding one or more molten polymeric materials through annular die(s), we have observed that with some combinations of polymeric materials and radiopaque particulates, e.g., sheaths made from extruded sheath walls comprising TPE and titanium particles, have a shorter functional life than do sheaths made by forming the substituent walls with the lower temperature dipping or troweling techniques discussed herein. Although there are various theories why the sheaths having extruded sheath walls have a shorter functional life than do those made using the dipping techniques, e.g., the radiopaque particles may have catalytic properties in the polymeric materials at the elevated temperatures needed for extrusion and form micro bubbles or that the radiopaque particles provide mechanical stress points at the surface of the sheath wall, we do not wish to be bound by those theories.

[000116] FIG. 8H1 shows a side view of a sheath wall (181) cross section made up of a central layer (183) comprising one or more polymeric materials that may be selected from the list of suitable polymeric materials (discussed elsewhere herein) and one or more radiopaque materials and inner and outer layers (185, 186) comprising the same or different polymeric materials as comprising the central layer (183) but typically substantially without radiopaque particles. The central layer (183) in this variation is shown to be substantially thicker than the inner and outer layers (185, 186) and although such a relationship is desirable in many instances, it is not a requirement. The central layer (183) may comprise a single layer or may

..--.Vrsnse a piurauiy of thinner iayers oi^" the same or different composition. In this depicted variation, the central layer (183) typically bears most of the task of providing at least partial radiopacity to the sheath wall (181).

[000117] In this variation, the central layer may be from about 0.005 inch to about 0.050 inch in thickness, more typically from about 0.008 inch to about 0.030 inch in thickness. The inner and outer layer layers (185, 186), when made from neat polymer material(s) may be from about 0.0005 inch to about 0.015 inch, more typically from 0.001 inch to about 0.005 inch in thickness.

[000118] The thicknesses of the various layers may be controlled in a variety of ways, e.g., by control of the concentration of the polymeric material (and solvent) in the baths used for dipping the forming mandrel (or mold) or by selection of the polymeric material's molecular weight or by selection of the concentration of radiopaque particulates in the dipping baths or by control or selection each of those parameters.

[000119] As mentioned elsewhere, the sheath walls shown with regard to FIGS. 8H1 to 8H3 may be formed into other shapes, such as shown in the other Figures, by known procedures including blow-molding to achieve a desired final sheath shape.

[000120] FIG. 8H2 shows another variation of a sheath wall ( 187) having multiple layers. In this variation, the sheath wall (187) includes a central layer (189) comprising one or more polymeric materials that may be selected from the list of suitable polymeric materials (discussed elsewhere herein) and one or more radiopaque materials and inner and outer intermediate layers (191,193) comprising the same or different polymeric materials as comprising the central layer (189) but typically comprising some amount of radiopaque particles. The inner and outer intermediate layers (191,193) may comprise the same concentration of radiopaque materials as does the central layer, but such is not a requirement. Typically, the inner and outer intermediate layers (191,193) comprise a lower concentration of radiopacifiers than does the central layer (189) but need not do so. Finally, innermost and outermost layers (195, 197) typically comprise the same or different polymeric materials as comprising the central layer (183) but typically substantially without radiopaque particles.

[000121] FIG. 8H3 shows another variation of a sheath wall (213) having multiple layers. In this variation, the sheath wall (213) includes layers (199, 205, 207) comprising one or more polymeric materials that may be selected from the list of suitable polymeric materials (discussed elsewhere herein) and one or more suitable radiopaque materials and intermediate separator layers (201, 203) and outer layers (209, 211) typically comprising the same or different polymeric materials but typically not comprising radiopaque particles.

[000122] Other variations of multilayered sheath walls are suitable for producing inhomogeneous sheaths such as those comprising one or more continuous or non-continuous, similar or dissimilar layers. Combinations of the variations described elsewhere herein with the layered sheath walls discussed with regard to FIGS. 8H1 to 8H3 are also contemplated.

[000123] FIG. 81 shows a partial cross-section of an inhomogeneous sheath (198) having a sheath wall (200) and a radiopaque band (202) situated at least partially within the sheath wall (200). The band (200) may be situated in the wall (200) by known techniques, e.g., by casting a wall with a hollow and fixing a radiopaque band (202) in the hollow or by co-extruding the band (202) with the wall (200).

[000124] FIGS. 8 J, 8K, and 8L show partial side views of inhomogeneous sheaths having various artifacts placed in or on the respective sheath wall. In FIG. 8J, the radiopaque artifact (210) is made up of letters and numbers denoting a trademark or brand name. In FIG. 8K, the radiopaque artifact (212) is made up of a stripe. In FIG. 8L, the radiopaque artifact (214) is a target allowing the user to identify position of the sheath.

[000125] These artifacts may be used for a variety of purposes, including assessing the condition of the sheath, placement of a serial number, etc.

[000126] FIG. 8M shows a partial side-view, cross-section of an inhomogeneous sheath wall (220) having an inner wall component (222) and an outer sheath wall component (224). One or the other or both of the inner wall component (222) and the outer sheath wall component (224) may comprise one or more of the radiopacifiers discussed above and below. The inhomogeneous sheath wall (220) may be formed by a variety of methods, e.g., by simultaneous coextrusion of the materials making up the contiguous wall components (222, 224), simultaneous coextrusion of the materials with blow-molding, by extrusion of the material chosen for the outer wall component (224) onto a previously extruded (or otherwise formed) inner wall component (222), by expanding a parison comprising the inner wall component (222) material into a previously formed outer wall component (224) - e.g., by extrusion blow-molding, by shrink-wrapping an outer wall component (224) onto a previously formed inner wall component, etc. By appropriate choice of the production method, the materials making up the inner and outer wall components (222, 224) may be selected to achieve specific specifications or design features. For instance, the radiopacity of the two layers may be varied, e.g., the outer layer may comprise a radiopaque material and the inner layer may be radiolucent or vice-versa. The inner layer may be comparatively non- elastic (e.g., ePTFE or PET) and the outer layer elastic or vice-versa.

[000127] FIG. 8N shows a partial side-view, cross-section of an inhomogeneous sheath wall (230) comprising a polymeric foam, a polymeric matrix having gaseous inclusions. Suitable biocompatible polymers, e.g., various polycarbonates. In addition to the gaseous inclusions, the polymer matrix may contain one or more of the radiopaque materials discussed elsewhere herein.

[000128] FIG. 8P shows a partial side-view, cross-section of an inhomogeneous sheath (234) having a sheath wall component (236) and upper and lower foot components (238). The materials comprising the wall component (236) and those comprising the upper and lower foot components (238) may independently comprise different polymeric materials and may independently contain radiopaque inclusions or materials. The upper and lower foot components (238) may be used, for instance, to affix the sheath (234) to the endplates (shown elsewhere).

[000129] FIG. 8Q shows a partial side-view, cross-section of an inhomogeneous sheath (240) having a sheath wall component (242) and an exterior intermediate band (244) formed, for instance, by interrupted coextrusion.

[000130] FIGS. 8R1 and 8R2 show, respectively, a partial side-view, cross-section and a partial top-view, cross-section of an inhomogeneous sheath wall (250) having an inner wall component (252), an outer wall component (254), and longitudinal ribbons (256) situated between the inner and outer wall components (252, 254). The inner wall component (252) and the outer wall component (254) typically will comprise polymeric compositions. The two polymeric compositions may be the same or different; the two polymeric compositions may each independently comprise radiopacifiers or radiopaque artifacts. The longitudinal ribbons may comprise metallic or polymeric materials and may be radiopaque or radiolucent. Typically, the sheath wall (250) may be coextruded with addition of the ribbons during the extrusion procedure.

[000131] FIG. 8S shows a partial side-view, cross-section of an inhomogeneous sheath (260) having a sheath wall component (262) and upper and lower foot components (264). This version of our inhomogeneous sheath (260) is substantially similar to the variation shown in FIG. 8P. The major difference is the inclusion of the wires (266) in the foot components (262, 264). As with the other variation, the materials comprising the wall component (262) and those comprising the upper and lower foot components (264) may independently comprise different polymeric materials and may independently contain radiopaque inclusions or materials.

[000132] FIGS. 8T1 and 8T2 show, respectively, a partial side-view, cross-section and a partial top-view, cross-section of an inhomogeneous sheath wall (270) having an interior polymeric wall component (272), an outer wall component (274), and longitudinal wires (276) situated between the inner and outer wall components (272, 274). The inner wall component (272) and the outer wall component (274) typically will comprise polymeric compositions. The two polymeric compositions may be the same or different; the two polymeric compositions may each independently comprise radiopacifiers or radiopaque artifacts. The longitudinal wires may comprise metallic or polymeric materials and may be radiopaque or radiolucent. Typically, the sheath wall (270) may be coextruded with addition of the wires during the extrusion procedure.

[000133] FIGS. 8U1, 8U2, and 8U3 show, respectively, a perspective view, a front view, and a partial, cross section of an inhomogeneous sheath (316) having a bellowed sheath wall (318) and a pair of markers (320) located, in this instance, on the middle bellow in the approximate middle (top-to-bottom) and located on the anterior most and posterior most positions on the sheath, as the sheath will be positioned after installation on the disc assembly. This positioning of the markers allows visualization of the sheath position after implantation of the prosthetic disc.

[000134] In the variation shown in FIGS. 8U1-8U3, the radiopacifier particles were dispersed in a solvent-polymer mixture, the mixture located on the sites on the sheath, coated with a layer of the polymer, and placed in a mold to form the sheath (316).

[000135] FIGS. 8V1 and 8V2 show, respectively, a perspective view and a front view of another variation of our sheath (322) having a sheath wall (324) and a longitudinal stripe (326). The stripe (326) may be produced during extrusion of the tubing.

[000136] FIGS. 8W1 and 8W2 show, respectively, a perspective view and a front view of still another variation of our sheath (328) having a sheath wall (330) and a pair of markers (332), e.g., "knee markers," located, as was the case in the sheath shown in FIGS. 8U1-8U3, on the middle bellow in the approximate middle (top-to-bottom) and located on the anterior most and posterior most positions on the sheath. In this variation, the sheath wall (330) contains sufficient radiopaque material to render it at least partially radiopaque. The knee markers (332) should contain sufficient radiopaque material to allow contrast with the sheath wall (330).

[000137] FIGS. 8X1 and 8X2 show, respectively, a perspective view and a front view of another variation of our sheath (334) having a sheath wall (336) and a longitudinal stripe (338). Again, the sheath wall (336) contains sufficient radiopaque material to render it at least partially radiopaque and the marker stripe (338) should contrast with the sheath wall (336).

[000138] FIGS. 8Y1 and 8Y2 show, respectively, a perspective view and a front view of a variation of our sheath (340) having a sheath wall (342), a longitudinal stripe (344) that is at least partially radiopaque, and knee markers (346) that are also at least partially radiopaque. The amounts of contained radiopaque material in the two marker types may be selected to result in the same level of radiopacity in the stripe (344) and the knee markers (346) or to result in the different levels of radiopacity in the stripe (344) and in the knee markers (346).

[000139] FIGS. 8Z1 and 8Z2 show, respectively, a perspective view and a front view of a variation of our sheath (347) having a sheath wall (348), a longitudinal stripe (349) that is at least partially radiopaque, and knee markers (351) that are also at least partially radiopaque. The difference between the variation shown in FIGS. 8Z1 and 8Z2 and the variation shown in FIGS. 8V1 and 8V2 is that the sheath wall (348) is also radiopaque.

[000140] FIG. 9 A shows a partial cutaway side-view of a variation of our sheath (353) having radiopaque grommets (355) situated in the ports or openings (159) in the sheath wall (357) used for access of sterilizing media to the interior of the implant.

[000141] The grommets (355) may be metallic, polymeric, or other suitable material. The grommets (355) may be radiopaque, if so desired. Additionally, the grommets (355) may be connected by wires (359).

[000142] FIG. 9B shows a partial cutaway side-view of a variation of our sheath (361) having radiopaque rivets (363) situated in the ports or openings (159) in the sheath wall (365) used for access of sterilizing media ιο ihe inierior of ϋ¾ iirn¾^;-i. [000143] The rivets (363) may be metallic, polymeric, or other suitable material. The rivets (363) may be radiopaque, if so desired.

[000144] FIG. 9C shows a partial cutaway side-view of a variation of our sheath (367) having bioabsorbable plugs (369) situated in the ports or openings (159) in the sheath wall (371) used for access of sterilizing media to the interior of the implant. The plugs (369) are bioerodible or bioabsorbable; may be metallic, polymeric, or other suitable material; and are optionally radiopaque, if so desired.

[000145] FIG. 10A1 shows a side-view of an inhomogeneous sheath wall (280) having an exteriorly located active wall member (282) and an inner polymeric wall component (284). Fig.lOA2 shows a side-view, partial cross-section of the inhomogeneous sheath wall (280) depicted in FIG. 10A1.

[000146] The term "active" here is meant to indicate that the component referred to has a function in the implant in addition to providing radiopacity to the sheath or to providing separation between the core components and the spinal fluids. In particular, an active component may provide force-absorption functionality or force-redirecting functionality or the like to the sheath.

[000147] The active wall member (282) shown in FIGS. 10A1, 10A2, 10A3, and 10A4 is comprised of a material, e.g., nitinol, selected to have elastic properties significant to the operation of the implanted disc. The shape of the active wall member (282) includes a central section (286) configured to allow bending upon application of force and rebound upon release of the force. The active wall member (282) also includes upper and lower bands (288) supporting the central section (286).

[000148] FIG. 10A3 shows a side-view, partial cross-section of an inhomogeneous sheath wall (290) having an active wall member (292) located between an inner polymeric wall component (294) and an outer polymeric wall component (296).

[000149] FIG. 10A4 shows a side-view, partial cross-section of an inhomogeneous sheath wall (300) having an active wall member (302) located interior to an exterior polymeric wall component (304).

[000150] Our inhomogeneous annular sheaths may be attached to the implant endplates in a variety of ways. FIGS. 1 lA-1 IE show examples of such attachments. [000151] FIG. 11A shows a side-view, cross section of a variation of our prosthetic disc (400) having an inhomogeneous sheath (402). Also shown is the resilient core member (404). The fiber member has been omitted from the drawing to allow clarity relating to the rest of the structure. A lower endplate assembly (406) and an upper endplate assembly (408) is also visible here. The endplate assemblies (406, 408) are shown to have a plurality of anchoring features (410), here depicted as fins or keels, although the structure of the anchors is not important to the invention here. Other anchoring components, integrated into the endplates or not, may be utilized as desired.

[000152] The endplates (406, 408) are assemblies, respectively, of inner endplates (412, 414) and outer endplates (416, 418) that may be joined, e.g., by welding, into the endplate assemblies (406, 408). In the variation shown in FIG. 11 A, the inner endplate (412) is welded to the outer endplate (416) at the junction (420). Similarly, lower endplate assembly (406) is formed by welding outer endplate (418) to inner endplate (414) at junction (422). These endplate assembly configurations provide for a gap (430, 432) into which the sheath (402) may be secured.

[000153] FIG. 11B1 shows a side-view, partial cross-section of a variation of our prosthetic disc and particularly shows a design for affixing the annular sheath (440) to an endplate (442). In this variation, the endplate (442) is an assembly of an outer endplate or portion (444) and an inner endplate or portion (446). The two portions (444, 446) of the depicted upper endplate (442) are mated together, e.g., by laser welding or some other similar process, to form the integrated upper endplate (442). Obviously, the lower endplate may have the same structure, if so desired. The depicted structure has the ability to retain the annular sheath (440) without the need for a separate retaining ring. For example, the upper edge (448) of the annular sheath (440) may be captured and retained between the outer portion (444) and inner portion (446) of the upper endplate (442) when they are attached to one another.

[000154] The inner portion (446) is shown to have a peripheral groove (450) that cooperates with the bottom surface of the outer portion (444) of the upper endplate (442) to create an annular space (452). A similar structure, not shown in the drawings, may be provided on the lower endplate. [000155] The annular sheath (440) may include a bead (456) in its upper end to assist in this fixing. The annular sheath (440) is held in place in the annular space (452) between the upper and lower endplates by the compression forces between inner portion (446) and outer endplate portion (412). The cooperation of the annular space (452) with the bead (456) formed on annular sheath (440) creates a stronger and more secure retaining force for retaining the upper and lower edge of annular sheath (440) by the upper and lower endplates.

[000156] A reinforcing wire (458) is shown located in the bead (456) to strengthen and stabilize the overall bead structure (456).

[000157] FIG. 11B2 shows the same view of a prosthetic disc (468) having the same general design excepting that the bead (470) does not include a reinforcing wire.

[000158] FIG. 11C shows a side-view, partial cross-section of another variation of our prosthetic disc (480) and, in particular, shows another design for affixing the annular sheath (482) to an endplate assembly (484). In this variation, the endplate assembly (484) comprises an assembly of an outer endplate or portion (486) and an inner endplate or portion (488). An exterior hoop or band (490) compresses the annular sheath (482) against a landing on the inner endplate portion (488) to retain the sheath (482) in place.

[000159] FIG. 1 ID shows a side-view, partial cross-section of another variation of our prosthetic disc (500) and specifically shows another design for affixing the annular sheath (502) to an endplate (504). In this variation, an exterior hoop or band (506) compresses the annular sheath (502) into a groove in the periphery of the endplate (504) to retain the sheath (502) in place.

[000160] FIG. 1 IE shows a simple adhesive connection between an annular sheath (508) and an endplate (510).

EXAMPLE 1

[000161] We tested a version of our inhomogeneous sheath made up of particulate titanium and a thermoplastic elastomer for robustness by comparing it to a sheath formed from the TPE without the Ti particulates.

[000162] We first formed a mixture of 10% particulate Titanium (-325 mesh) and 90% BIONATE 80A and extruded the mixture through an annular die into a tubular form. The so- formed tube was then introduced into a female mold having a shape producing a sheath form generally as shown in FIG. 8D . The tubing was then blow-molded into the noted shape to form the sheath and removed from the mold. The Ti/polymer mixture was observed to be much less sticky than the BIONATE polymer by itself. Consequently, no mold-release material was needed, simplifying production. The sheath was visible under fluoroscopy.

[000163] The sheath had a wall thickness of about 0.012", an ID of about 0.432", and a height of about 0.160".

[000164] We assembled several prosthetic discs using the Ti/BIONATE 80A sheath and a similar number of discs as controls using identical (but Ti-free) sheaths. We subjected the assembled discs to an accelerated flexing fatigue assessment procedure. The procedure involved placing each of the discs into deionized water baths held at approximately 37°C in mechanized testing devices. The testing devices were used to flex the prosthetic discs at 7.5Hz for 10,000,000 cycles in flexion/extension followed by 10,000,000 cycles in combined lateral bending and torsion. The imposed movement in flexion/extension was ±7.5° with a tolerance of -0.57+1° in each mode. The imposed movement in lateral bend/torsion was ±6.0° with a tolerance of -0.57+1° in each mode. A preload nominally of 100N was applied to each disc during testing. Finally, the movements imposed on the discs by the testing devices were applied using a center-of-rotation passing through the geometric center of the disc.

[000165] At the completion of each 1,000,000 cycles, we paused the testing, collected any wear debris, and measured the height of each disc. We also visually evaluated each sheath for wear-through. We swapped the discs top-for-bottom in reinstalling the discs in the testing device for further cycling.

[000166] At the conclusion of the 20,000,000 cycles, we again collected wear debris from each disc, measured the height of each disc, and visually evaluated each sheath for wearthrough and other defects.

[000167] For the six control discs, three of the discs had one defect in the sheath, one disc had two defects, and two had no defects. The six control discs had an average weight loss of 2.76%. The data from one Ti-containing disc was discarded because the testing machine malfunctioned by imposing an improper movement on the disc for 400,000 cycles. For the remaining five discs having Ti-containing sheaths, one disc had three defects in the sheath, one disc had two defects, and one discs had one defect. The five discs having Ti- containing sheaths had an average weight loss of 4.70%. Applying various statistical criteria to these data showed the Ti-containing sheaths not to be as robust as the sheaths on the control discs.

EXAMPLE 2

[000168] We formed a multi-layered annular sheath by dipping a generally cylindrical mandrel into a series of polymer and polymer/Ti particulate baths to form a tubular member. The first bath contained 30% BIONATE 80A dissolved in a dimethylacetamide (DMAc) solvent. The thus-coated mandrel was then dipped twice into a solution having 20%

BIONATE A, about 3.3% of Ti particles (having a mean diameter of 20 microns), and DMAc solvent followed by a final dip into a bath contained 30% BIONATE A in DMAc solvent. After drying, the layer contained about 10%Ti by weight. The so-produced tubing sheath precursor was then introduced into a mold having a shape producing a sheath form generally as shown in FIG. 8BB (8C) using blow-molding techniques. The final sheath was then removed from the mold. No mold-release material was used. The sheath was visible under fluoroscopy.

EXAMPLE 3

[000169] We tested another version of our inhomogeneous sheath made up of particulate tantalum and a thermoplastic elastomer for robustness by comparing it to the control sheaths described in Example 1.

[000170] We formed an annular sheath having the shape shown in FIG. 8D, i.e., the cross-section showing a bellowed configuration with a single narrow outwardly extending bellow and two inwardly extending bellows. The upper and lower edges included a small inwardly extending foot for anchoring in the upper and lower endplates.

[000171] We first formed a mixture of 10% particulate Ta (-325 mesh)and 90%

BIONATE 80A and extruded the mixture through an annular die into a tubular form. The so- formed tube was then introduced into a female mold having a shape producing a sheath form generally as shown in FIGS. 8BB. The tubing was then blow-molded into the noted shape to form the sheath and removed from the mold. The Ta/polymer mixture was observed to be much less sticky than the BIONATE polymer by itself. Consequently, no mold-release material was needed, simplifying production. The sheath was visible under fluoroscopy.

[000172] The sheath had a wall thickness of about 0.012", an ID of about 0.432", and a height of about 0.160".

[000173] We assembled several prosthetic discs using Ta/BIONATE 80A sheaths. We subjected the assembled discs to an accelerated flexing fatigue assessment procedure. The procedure involved placing each of the discs into deionized water baths held at approximately 37°C in mechanized testing devices. The testing devices were used to flex the prosthetic discs at 7.5Hz for 10,000,000 cycles in flexion/extension followed by 10,000,000 cycles in combined lateral bending and torsion. The imposed movement in flexion/extension was ±7.5° with a tolerance of -0.5°/+l° in each mode. The imposed movement in lateral bend/torsion was ±6.0° with a tolerance of -0.5°/+ 1° in each mode. A preload nominally of 100N was applied to each disc during testing. Finally, the movements imposed on the discs by the testing devices were applied using a center-of-rotation passing through the geometric center of the disc.

[000174] At the completion of each 1,000,000 cycles, we paused the testing, collected any wear debris, and measured the height of each disc. We also visually evaluated each sheath for wear-through. We swapped the discs top-for-bottom in reinstalling the discs in the testing device for further cycling.

[000175] At the conclusion of the 20,000,000 cycles, we again collected wear debris from each disc, measured the height of each disc, and visually evaluated each sheath for wearthrough and other defects.

[000176] As noted in Example 1, for the six control discs, three of the discs had one defect in the sheath, one disc had two defects, and two had no defects. The six control discs had an average weight loss of 2.76%. For the six discs having our Ta-containing sheaths, one disc had two defects in the sheath, one disc had one defect, and four discs had no defects. The six discs having Ta-containing sheaths had an average weight loss of 1.27%. Applying various statistical criteria to these data showed our Ta-containing sheaths to be as robust or more robust than the polymeric sheaths. Typical Implantation Procedure

[000177] Our discs may be implanted using a variety of known surgical procedures.

[000178] In one illustrative procedure, a first involves exposing the two adjacent vertebrae to be treated by conventional surgical procedures and removing the natural disc. Once the natural disc has been removed, a spacer is introduced between the two adjacent vertebrae to separate them. After the vertebrae are adequately separated, the spacer is withdrawn.

[000179] A two-sided chisel ~ having cutters extending from its sides adapted to cut a pair of grooves in the opposed vertebral faces— is then advanced and its head portion is placed between the vertebral bodies. Because of the size of the chisel relative to the space between the vertebrae, the wedge-shaped cutters engage the inward-facing surfaces of the vertebrae, simultaneously creating grooves on those surfaces. After the grooves are formed as needed, the two-sided chisel is withdrawn.

[000180] Our prosthetic disc having an inhomogeneous sheath is then installed on the distal end of a disc holder. The disc holder retains the prosthetic disc and hold it in place during the implantation step. The prosthetic disc is then advanced by the holder into the space between the two vertebrae. Anchoring fins or keels on the external surfaces of the prosthetic disc are aligned with the grooves formed in the upper and lower vertebrae as the disc is implanted. Once the disc has been satisfactorily located, the holder is withdrawn, leaving the disc in place.

GENERAL

[000181] The description is intended to illustrate the apparatus and methods of making and implanting the device but is not intended to be limiting.

[000182] It is to be understood that the inventions that are the subject of this patent application are not limited to the particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[000183] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which these inventions belong. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present inventions, the preferred methods and materials are herein described.

[000184] All patents, patent applications, and other publications mentioned herein are hereby incorporated herein by reference in their entireties. The patents, applications, and publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.

[000185] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions.

[000186] The preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims.

Claims

We claim as our invention:

1. A prosthetic intervertebral disc comprising:

a first endplate,

a second endplate,

a compressible core member located between the first endplate and the second

endplate,

a fiber member connecting said first and second endplates, and

an inhomogeneous annular sheath surrounding the compressible core member and the fiber member and extending between said first and second endplates.

2. The prosthetic intervertebral disc of claim 1 wherein the inhomogeneous annular sheath is at least partially radiopaque.

3. The prosthetic intervertebral disc of claim 1 wherein at least a portion of the

inhomogeneous annular sheath is at least partially radiopaque.

4. The prosthetic intervertebral disc of claim 1 wherein the inhomogeneous annular sheath comprises one or more polymers.

5. The prosthetic intervertebral disc of claim 4 wherein the one or more polymers comprises a thermoplastic elastomer.

6. The prosthetic intervertebral disc of claim 4 wherein the thermoplastic elastomer comprises a polyurethane-polycarbonate elastomeric composition.

7. The prosthetic intervertebral disc of claim 2 wherein the inhomogeneous annular sheath comprises a thermoplastic elastomer.

8. The prosthetic intervertebral disc of claim 7 wherein the thermoplastic elastomer comprises a polyurethane-polycarbonate elastomeric composition.

9. The prosthetic intervertebral disc of claim 7 wherein the inhomogeneous annular sheath further comprises particulate tantalum.

10. The prosthetic intervertebral disc of claim 7 wherein the inhomogeneous annular sheath further comprises particulate titanium.

11. A prosthetic intervertebral disc comprising:

a first endplate,

a second endplate,

a polymeric, compressible core member located between the first endplate and the second endplate,

a fiber member connecting said first and second endplates, and

an inhomogeneous annular sheath comprising at least one thermoplastic elastomer and containing tantalum particulates, the sheath surrounding the compressible core member and the fiber member and extending between said first and second endplates.

12. The prosthetic intervertebral disc of claim 11 wherein the inhomogeneous annular sheath is at least partially radiopaque.

13. The prosthetic intervertebral disc of claim 11 wherein the thermoplastic elastomer comprises a polyurethane-polycarbonate elastomeric composition.

14. The prosthetic intervertebral disc of claim 12 wherein the thermoplastic elastomer comprises a polyurethane-polycarbonate elastomeric composition.

15. The prosthetic intervertebral disc of claim 12 further comprising a fixation member for securing said first endplate to a vertebral body and a fixation member for securing said second endplate to a vertebral body.