An intervertebral prosthesis for insertion between adjacent vertebrae, in one embodiment, includes upper and lower prosthesis plates and a movable core. The prosthesis plates and optionally the core are formed of polyaryletherketone (PAEK) for improved imaging properties. A metallic insert is provided on each of the PAEK prosthesis plates providing a bone ongrowth surface.

BACKGROUND OF THE INVENTION

The present invention relates to medical devices and methods. More specifically, the present invention relates to intervertebral disc prostheses.

Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.

Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without fusion of the two vertebrae adjacent to the disc. Fusion of the two vertebrae is achieved by inserting bone graft material between the two vertebrae such that the two vertebrae and the graft material grow together. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and graft material in place while they permanently fuse together. Although fusion often treats the back pain, it reduces the patient's ability to move, because the back cannot bend or twist at the fused area. In addition, fusion increases stresses at adjacent levels of the spine, potentially accelerating degeneration of these discs.

In an attempt to treat disc related pain without fusion, an alternative approach has been developed, in which a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) is inserted between two vertebrae. A number of different artificial intervertebral discs are currently being developed. For example, U.S. Patent Publication Nos. 2005/0021146, 2005/0021145, and 2006/0025862, which are hereby incorporated by reference in their entirety, describe artificial intervertebral discs. This type of intervertebral disc has upper and lower plates positioned against the vertebrae and a mobile core positioned between the two plates to allow articulating, lateral and rotational motion between the vertebrae.

Another example of an intervertebral disc prostheses having a movable core is the CHARITE artificial disc (provided by DePuy Spine, Inc.) and described in U.S. Pat. No. 5,401,269. Other examples of intervertebral disc prostheses include MOBIDISK™ disc prosthesis (provided by LDR Medical), the BRYAN™ cervical disc prosthesis (provided by Medtronic Sofamor Danek, Inc.), and the PRODISC™ disc prosthesis (from Synthes Stratec, Inc.) and described in U.S. Pat. No. 6,936,071. Some of these intervertebral discs are mobile core discs while others have a ball and socket type two piece design. Although existing disc prostheses provide advantages over traditional treatment methods, improvements are ongoing.

The known artificial intervertebral discs generally include upper and lower plates which locate against and engage the adjacent vertebral bodies, and a core for providing motion between the plates. The core may be movable or fixed, metallic, ceramic or polymer and generally has at least one convex outer surface which mates with a concave recess on one of the plates in a fixed core device or both of the plates for a movable core device.

The known disc materials each have advantages and disadvantages. For example, ceramic and polymer materials generally cause less artifacts in medical imaging, such as an X-ray, CT or MRI image than metals. Metals may have better bone attachment properties than polymers and better wear characteristics than polymers and ceramics. However, on MRI metals can create artifacts that may obscure adjacent tissue and make visualization at the site of the artificial disc nearly impossible. The continuing challenge in forming artificial discs is to find the right combination of materials and design to use the benefits of the best materials available.

Therefore, a need exists for an improved artificial intervertebral disc with improved visibility in medical imaging, such as X-ray, MRI and CT imaging, and with an improved surface for bone ongrowth.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided an intervertebral prosthesis for insertion between adjacent vertebrae, in one embodiment, the prosthesis comprising upper and lower prosthesis plates and a movable core. The prosthesis plates and optionally the core are formed of polyaryletherketone (PAEK) for improved imaging properties. A metallic insert is provided on each of the PAEK prosthesis plates providing a bone ongrowth surface.

According to another aspect of the invention an intervertebral prosthesis includes upper and lower prosthesis plates of PAEK configured to articulate with respect to one another by sliding motion of at least two bearing surfaces of the plates.

In accordance with one aspect of the present invention, an intervertebral disc includes an upper plate having an upper vertebra contacting surface and a lower bearing surface, wherein the upper plate is formed of polyaryletherketone (PAEK) with the upper surface formed at least in part from a metallic insert having a plurality of projections formed thereon for improving bone attachment; a lower plate having a lower vertebra contacting surface and an upper bearing surface, wherein the lower plate is formed of PAEK with the lower surface formed at least in part from a metallic insert having a plurality of projections formed thereon for improving bone attachment; and a core positioned between the upper and lower plates. The core has upper and lower surfaces configured to mate with the bearing surfaces of the upper and lower plates.

In accordance with another aspect of the invention, an intervertebral disc includes an upper plate, a lower plate, and a core positioned between the upper and lower plates. The upper plate has an upper vertebra contacting surface and a lower bearing surface and the upper plate is formed of polyaryletherketone (PAEK) with the upper surface formed at least in part from a metallic insert having a thickness of at least 0.3 mm. The lower plate has a lower vertebra contacting surface and an upper bearing surface and the lower plate is formed of PAEK with the lower surface formed at least in part from a metallic insert having a thickness of at least 0.3 mm. The core has upper and lower surfaces configured to mate with the bearing surfaces of the upper and lower plates.

In accordance with a further aspect of the invention an intervertebral disc includes an upper plate having an upper vertebra contacting surface and a lower bearing surface and a lower plate having a lower vertebra contacting surface and an upper bearing surface, wherein the upper and lower bearing surfaces are configured to allow articulation between the upper vertebra contacting surface and the lower vertebra contacting surface. The upper and lower plates are formed of polyaryletherketone (PAEK) with the vertebra contacting surfaces formed at least in part from a metallic insert having a plurality of projections formed thereon for improving bone attachment.

In accordance with an additional aspect of the invention an intervertebral disc includes an upper plate formed of polyaryletherketone (PAEK) with a metallic insert fixed to the PAEK and configured to contact a first vertebra and a lower plate formed of PAEK with a metallic insert fixed to the PAEK and configured to contact a second vertebra adjacent to the first vertebra. The upper and lower plates are arranged to articulate in a anterior-posterior direction and in a lateral direction with respect to one another and to rotate with respect to one another.

Other features of the invention are set forth in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates an intervertebral disc having an upper plate10, a lower plate12, and a core14. The upper and lower plates10,12are formed of a durable and imaging friendly material such a polyaryletherketone (PAEK), one example of which is neat poly(aryl-ether-ether-ketone) (PEEK). The PEEK portion of the upper and lower plates includes an inner bearing surface for contacting the core14and one or more fins16. The upper and lower plates10,12also include one or more metallic inserts20formed of a material which serves as a bone integration surface. The inserts20may include one or more bone integration enhancing features such a serrations or teeth to ensure bone integration. As shown in the embodiment ofFIG. 1, the bone integration enhancing features include serrations18. The metallic inserts20may be formed in a variety of shapes and with a variety of bone integration features, however, the metallic inserts cover a substantial portion of the bone contacting surfaces of the plates10,12.

The metallic inserts20shown inFIG. 1are in the form of screens formed of titanium or other metal by stamping, machining or the like. The screens20can be securely or loosely fixed to the outer surfaces of the plates10,12. Titanium screens20form surfaces which provide both immediate fixation by way of the serrations18and optional teeth and provides a bone ongrowth surface for long term stability. In addition to providing fixation, the inserts or screens20also can serve as a radiographic marker. Since PEEK is radiolucent or nearly invisible under medical imaging, the inserts20serve as markers to identify the limits of the disc and evaluate the performance of the disc under X-ray, fluoroscopy, MRI or CT scan.

PEEK is part of the family of polyaryletherketones (PAEKs), also called polyketones, which have been increasingly employed as implantable materials for orthopedic implants. PAEK is a family of inherently strong and biocompatible high temperature thermoplastic polymers, consisting of an aromatic backbone molecular chain, interconnected by ketone and ether functional groups. The PAEK family includes poly(aryl-ether-ether-ketone) (PEEK), poly(aryl-ether-ketone-ether-ketone-ketone) (PEKEKK), and poly(ether-ketone-ketone) (PEKK) and was originally developed in the aircraft industry for its stability at high temperatures and high strength.

The upper and lower plates10,12can be fabricated from a number of different PAEK materials including neat (unfilled) PEEK, PEEK-OPTIMA available from Invibio, Inc., fiber reinforced PEEK, such as PEEK-CFR (carbon fiber reinforced) from Invibio, Inc., glass fiber reinforced PEEK, ceramic filled PEEK, Teflon filled PEEK, barium sulfate filled PEEK or other reinforced or filled PAEK materials. These PAEK materials are stable, bio-inert, and strong making them ideally suited for the base material for an articulating joint. However, other materials which are invisible or near invisible under radiographic imaging, are bio-inert and have high strength can also be used. Although neat PEEK has an elastic modulus of 3-4 GPa, fiber reinforcing the PEEK can bring the modulus up to match cortical bone (18 GPa) or to match titanium (105-120 GPa).

As shown inFIG. 1, the fin16is surrounded by a raised portion or rim22which is received within an opening24in the screen20. The rim22and surrounding opening24serve to locate the screen on the surface of the plate10. The rim22and opening24can provide a snap lock feature for holding the screen20in place. Alternate fixation means for the screen20include insert molding, peripheral locking features, or adhesives. In one embodiment, the screen20and the PEEK plate are not fixed together. In the unfixed embodiment, the rim22and opening24prevent sliding movement of the screen over the surface of the plate while the screen is prevented from lifting off of the PEEK by the natural anatomy once the disc has been implanted.

The screen20is preferably a thin screen having a thickness of about 0.1 mm to about 1.0 mm preferably about 0.3-0.7 mm not including a height of any serrations or teeth. The screen20preferably covers a significant portion of the bone contacting surface of the disc, such as at least 50% of the bone contacting surface (not including any fins) and preferably at least 75% of the bone contacting surface.

In one embodiment, the screen20ends before the posterior edge of the plate10to allow improved imaging of the spinal column by moving the metallic portion of the disc further from the posterior edge of the plate. In another embodiment, the bone contacting surface is partially, i.e. 50%, covered by the screen20and a remainder of the bone contacting surface and optionally the fin is covered with a titanium plasma spray coating for improved bone ongrowth. Since the plasma spray coating can be formed thinner than the screen20, the imaging can be further enhanced by the reduced metal provided by a combination of a screen and coating.

The fin16can be an elongate fin pierced by one or more transverse holes26. The disc can be inserted posteriorly into the patient from an anterior access, such that an angled posterior end28of fin16can enter a groove in one of the vertebrae as a posterior side of the intervertebral disc enters the intervertebral space followed by an anterior side of the intervertebral disc.

On opposite surfaces of the plates10,12from the titanium screens20the plates are formed with recesses30which serve as bearing surfaces for the core14. Although the bearing surfaces are shown as PEEK bearing surfaces, metal bearing surface inserts, such as cobalt chromium alloy bearing surface inserts may also be used.

The core14can be formed as a circular disc shaped member with upper and lower bearing surfaces36which match the curvature of the recesses or bearing surfaces30of the plates10,12. The core14also has one or more annular rims32which cooperate with a retention feature34on at least one of the discs to retain the core between the plates when the intervertebral disc is implanted between the vertebrae of a patient. The core14is moveable with respect to both the upper and lower discs to allow articulation, translation and rotation of the upper and lower plates with respect to one another. The spherically curved outer surfaces36of the core14and bearing surfaces30of the plates10,12have the same radius of curvature which may vary depending on the size of the intervertebral disc.

Although the bearing surfaces have been shown as spherically curved surfaces, other shaped surfaces may also be used. For example, one flat bearing surface and one spherical surface may be used. Alternatively, asymmetrical bearing surfaces on the plates and the core may be used to limit rotational motion of the disc, such as oval or kidney bean shaped bearing surfaces.

In one embodiment of the invention, the core14has a radius of curvature which is slightly smaller than a radius of curvature of the corresponding bearing surface30of the plate10,12. The slight difference in radius of curvature is on the order of a 0.5 to 5 percent reduction in radius of curvature for the core. The slight difference in curvature can improve articulation by reducing any possible initial sticking of the bearing surfaces, and is particularly useful for a combination of a PEEK core and PEEK bearing surfaces.

In the embodiment shown inFIG. 1a single central fin16is provided on each of the plates10,12extending in an anterior posterior direction with an angled posterior edge for aiding in insertion. This embodiment is particularly useful for insertion from an anterior side of the intervertebral disc space. Alternatively, two or more fins16can also be provided on each plate. In one example, a single fin can be provided on one plate while a double fin can be provided on the other plate to achieve a staggered arrangement particularly useful for multi-level disc implant procedures. This staggered arrangement prevents the rare occurrence of vertebral body splitting by avoiding cuts to the vertebral body in the same plane for multi-level implants. The orientation of the fin(s)16can also be modified depending on the insertion direction for the intervertebral disc10. In alternative embodiments, the fins16may be rotated away from the anterior-posterior axis, such as in a lateral-lateral orientation, a posterolateral-anterolateral orientation, or the like.

In one two fin embodiment of a plate, the two fins are formed from the metal as a part of the screen. In this embodiment, two fin shaped members are cut into the flat screen and folded upwards to form the two fins. This leaves a gap between the fins that may be left as PEEK surface or may be plasma spray coated with titanium.

The fins16are configured to be placed in slots cut in the vertebral bodies. In one embodiment, the fins16are pierced by transverse holes26for bone ongrowth. The transverse holes26may be formed in any shape and may extend partially or all the way through the fins16. Preferably, the fins16each have a height greater than a width and have a length greater than the height.

The fins16provide improved attachment to the bone and prevent rotation of the plates in the bone. In some embodiments, the fins16may extend from the surface of the plates10,12at an angle other than 90°. For example on one or more of the plates10,12where multiple fins16are attached to the surface the fins may be canted away from one another with the bases slightly closer together than their edges at an angle such as about 80-88 degrees. The fins16may have any other suitable configuration including various numbers angles and curvatures, in various embodiments. In some embodiments, the fins16may be omitted altogether.

In addition to the fins16, the bone integration may be improved by providing the metallic inserts or screens20with a plurality of projections formed thereon for improving bone attachment. InFIG. 1, the projections are in the form of pyramid shaped serrations18arranged in a plurality of rows on either side of the opening24.

The projections may also include one or more finlets, teeth, or the like. The projections can be positioned in varying numbers and arrangements depending on the size and shape of the plate used. In one example 4-6 wedge shaped teeth are provided on each metallic insert20for cervical applications. Other teeth shapes may also be used, for example pyramidal, conical, rectangular and/or cylindrical teeth. The teeth and/or finlets can have varying heights which can be about 0.7-3 mm, preferably about 1-2 mm. The serrations can have heights varying from about 0.3-1 mm. With passage of time, firm connection between the screens20and the vertebrae will be achieved as bone tissue grows over the serrated finish, teeth and/or finlets. Bone tissue growth will also take place about the fins16and through the holes26therein, further enhancing the connection which is achieved.

Other geometries of bone integration structures may also be used including teeth, grooves, ridges, pins, barbs or the like. When the bone integration structures are ridges, teeth, barbs or similar structures, they may be angled to ease insertion and prevent migration. These bone integration structures can be used to precisely cut the bone during implantation to cause bleeding bone and encourage bone integration. Additionally, the outer surfaces of the plates10,12may be provided with a rough microfinish formed by blasting with aluminum oxide microparticles or the like to improve bone integration. In some embodiments, the outer surface may also be titanium plasma sprayed or HA coated to further enhance attachment of the outer surface to vertebral bone.

The screens20are shown inFIG. 1as machined flat plates with a plurality of protrusions. As shown inFIG. 2, the screens may also take the form of a thin metal plate which has been stamped with a pattern of holes forming bone engaging teeth in a vertebra contacting direction and/or a pattern of holes forming securing teeth for securing the screen to the PEEK plates.FIG. 2shows an upper plate110of an alternative intervertebral disc. The upper plate includes a PEEK portion112and a metallic screen120on the vertebral body contacting surface of the PEEK portion. The metallic screen120includes a plurality of punched teeth122arranged to function in the manner of the serrations18ofFIG. 1to provide improved fixation. The metallic screen120can also include a plurality of teeth124arranged to secure the screen to the PEEK portion112. In one example, the PEEK portion112can be insert molded around the teeth124.

FIG. 2also illustrates a locking feature130for locking the screen portion120to the PEEK or polymer portion112. The locking feature130may include a snap lock feature, an insert molded feature, or other mechanical connection. The locking feature130may be provided on two or more sides of the upper plate110and may be discrete or continuous. The same or a different locking feature may be used on the corresponding lower plate. The above described features ofFIG. 2can be combined with many of the structures shown inFIG. 1.

The core14according to the embodiment ofFIG. 1can be retained in the lower plate12by retention feature34comprising a retention ring that protrudes inwardly from an edge of the lower plate12. Although a circumferential core retaining feature is shown, other core retaining features may also be used including at least those shown in U.S. Patent Publication Nos. 2005/0251262, 2005/0021146, and 2005/0021145, which are incorporated herein by reference in their entirety.

Although the core14has been shown as circular in cross section with spherically shaped bearing surfaces36, other shapes may be used including oval, elliptical, or kidney bean shaped. These non-circular shaped cores can be used to limit rotational motion between the upper and lower plates10,12. Although the core14and plates10,12have been shown as solid members, the core and plates may be made in multiple parts and/or of multiple materials. The core can be made of low friction materials, such as titanium, titanium nitrides, other titanium based alloys, tantalum, nickel titanium alloys, stainless steel, cobalt chrome alloys, ceramics, or biologically compatible polymer materials including PEEK, UHMWPE, PLA or fiber reinforced polymers. High friction coating materials can also be used.

When the core14is formed of a polymer such as PEEK which is invisible under radiographic imaging, it may be desirable to have a radiographic marker imbedded within the core. For example, a single titanium pin may be positioned axially through a center of the core so that the PEEK core is visible in a post-operative X-ray examination. Other arrangements of pins, such as one or more radial pins, can also serve as radiographic markers and enable the position of the core14to be ascertained during such examination.

Alternatively, a PEEK core may be made more visible on radiographic examination by selection of the particular PEEK material or reinforcing material in the event of a reinforced PEEK material. In one embodiment, the PEEK core14is formed of a PEEK material with a different density (greater visibility) than that of the plates10,12to allow the core to be distinguished from the plates in X-ray. One PEEK material which may be used to form a visible core is PEEK loaded with barium sulfate. The barium sulfate loaded PEEK may also be used to improve lubricity of the core and improve sliding of the bearing surfaces over the core.

As an alternative to a PEEK core, a metallic core may be used. The metallic core, if of relatively small size, can be used with minimal distortion of an MRI or CT scan image because the core is positioned away from an area of interest for imaging, while the PEEK plates are located closest to the area of interest. A metal coated PEEK core can provide the combined benefits of the two materials. The metallic core provides the combined benefits of improved lubricity and decrease wear from metal on PEEK bearing surfaces. Alternately, the PEEK plates may be formed with a metallic bearing surface by providing a thin cup shaped bearing surface insert on the PEEK plates. The bearing surface inserts can be on the order of 1 mm or less in thickness and formed of titanium or cobalt chromium alloy. The PEEK plates with metallic bearing surface inserts can minimizes the amount of metal for improved imaging and be used in combination with a PEEK core.

The intervertebral disc according to the present invention provides articulation in two directions as well as rotation. The degree of articulation and rotation can be limited depending on the application or for a particular patient.

The plates10,12are provided with grooves34A at their lateral edges for use in grasping the disc by an instrument to facilitate holding and manipulation of the disc for insertion or removal of the disc. The grooves34A allow the plates10,12to be grasped and inserted simultaneously in a locked orientation. Other alternate grasping configurations including annular grooves or blind bores can also be used.

The upper and lower plates10,12are preferably formed from PEEK or other high strength biocompatible polymer. Portions of the upper and lower plates10,12, such as the screens20may also be formed from titanium, titanium nitrides, other titanium based alloys, tantalum, nickel titanium alloys, stainless steel, cobalt chrome alloys, ceramics, or biologically compatible polymer materials including UHMWPE, PLA or fiber reinforced polymers. The bearing surfaces30can have a hard coating such as a titanium nitride finish.

Portions of the plates10,12may be treated with a titanium plasma spray to improve bone integration. For example, the surfaces of the fins16may be titanium plasma spray coated. In another example, the fin16and screen20may be titanium plasma sprayed together. Other materials and coatings can also be used such as HA (hydroxylapatite) coating, micro HA coating, blasting procedures for surface roughing, and/or other bone integration promoting coatings. Any suitable technique may be used to couple materials together, such as snap fitting, slip fitting, lamination, interference fitting, use of adhesives, welding and/or the like.

The intervertebral disc described herein is surgically implanted between adjacent spinal vertebrae in place of a damaged disc. Those skilled in the art will understand the procedure of preparing the disc space and implanting the disc which is summarized herein. In a typical artificial disc procedure, the damaged disc is partially or totally removed and the adjacent vertebrae are forcibly separated from one another or distracted to provide the necessary space for insertion of the disc. One or more slots are cut into the vertebrae to accommodate the fins16if any. The plates10,12are slipped into place between the vertebrae with their fins16entering slots cut in the opposing vertebral surfaces to receive them. The plates may be inserted simultaneously or sequentially and with or without the core. After partial insertion of the disc, the individual plates10,12can be further advanced independently or together to a final position. Once the disc has been inserted, the vertebra move together to hold the assembled disc in place.

The vertebral contacting surfaces of the plates10,12including the serrations18and the fins16locate against the opposing vertebrae and, with passage of time, firm connection between the plates and the vertebrae will be achieved as bone tissue grows over the serrated finish and through and around the fin.

The disc and surrounding anatomy can be visualized post operatively by X-ray, fluoroscopy, CT scan, MRI, or other medical imaging techniques. In the event of excessive wear of the bearing surfaces of the core14, the core can be removed and replaced in an additional surgical procedure.

FIGS. 7,8and9are images showing two of the intervertebral discs ofFIG. 1implanted in a spine at two adjacent cervical disc levels. The images are taken by X-ray (FIG. 7), MRI (FIG. 8) and CT scan (FIG. 9). The intervertebral disc shown at A in the images has serrations18on the screens20as shown inFIG. 1. The intervertebral disc shown at B in the images has no serrations. Both of the discs are formed with neat PEEK plates and cores and titanium screens. As can be seen in the X-ray image, the titanium screens20are clearly visible, while the PEEK portion of the plates10,12and the PEEK core are completely invisible under X-ray. With some adjustment of the contrast of the X-ray image, the PEEK portion of the plates can be visualized slightly. The MRI and CT images clearly show the vertebrae and surrounding tissues with very minimal distortion caused by the discs10. This is a significant improvement over the conventional metal discs which cause major distortion under MRI or CT imaging and tend to obliterate the surrounding structures by creation of artifacts that obliterate portions of the image.

With conventional metallic discs, the MRI and CT images are of little use in viewing the area surrounding the disc. Physicians are eager to have a MRI and CT scan friendly disc, such as those shown in the present application to allow them to diagnose continued pain which may or may not involve the disc. However, with conventional metallic discs it is often impossible to diagnose continued problems by available medical imaging techniques because of poor imaging.

One advantage of the two part PEEK plates10,12with the metallic inserts is that the PEEK portion of the plates can be made to be removable without removal of the metallic insert. For example, in the event of excessive wear on the bearing surfaces of the plates10,12, the PEEK portion of the plates can be removed and replaced while leaving the metallic inserts20in place. Alternately, the PEEK portion of the plates10,12can be removed while the metallic inserts remain and are incorporated in a subsequent fusion or other fixation procedure.

FIGS. 3-6illustrate an alternative embodiment of a combination PEEK and metal disc having a two part metallic screen design. The disc (shown assembled inFIG. 5) includes an upper plate110, a lower plate112, and a core114. The upper and lower plates110,112are formed of a durable and imaging friendly material such a PAEK (PEEK) with an inner bearing surface for contacting the core114and one or more metallic inserts or screens120A,120B formed of a material which serves as a bone integration surface. As in the embodiment ofFIG. 1, the inserts120A,120B may include one or more bone integration enhancing features such a serrations118and/or teeth or finlets119to ensure retention and bone integration. This disc construction differs from that ofFIG. 1in that the metallic screen120A,120B forms not only the bone integration surface having the serrations118, but also includes a metallic fin16for better bone attachment to the fin.

The metallic screens are in the form of two part screens120A and120B formed of titanium by stamping, machining, or the like and secured together down a centerline by welding or other attachment. The two parts of the titanium screens120A,120B each include one half of a fin member116and one half of the opening124in the screens which accommodate a corresponding inner rim122of the PEEK plates110,112.

FIGS. 4 and 6illustrate the attachment of the two parts120A and120B of the metallic screen to the PEEK portion of the plate110by providing a peripheral protrusion128which surrounds and engages an outer rim132of the PEEK portion. This outer rim132has an angled outer surface which creates a locking fit when the two parts of the screen120A,120B are secured together. As in the embodiment ofFIG. 1, the plate110includes a bearing surface130and can include a retention feature for retaining the core, such as the retaining ring134. The screen120A,120B in this embodiment is preferably a thin screen having a thickness of about 0.5 mm to about 1.5 mm preferably about 0.5-0.1 mm not including a height of any serrations or teeth or the height of the peripheral protrusion128.

FIGS. 10-12are images showing the intervertebral discs ofFIG. 5implanted in a cervical spine. The images are taken by X-ray (FIG. 10), MRI (FIG. 11) and CT scan (FIG. 12). The intervertebral disc is shown at C in the images and the metallic fins are visible on the plates. As can be seen particularly in the top view CT scan ofFIG. 12, the spinal column is clearly visible without interference from the nearby disc.

In one embodiment of the invention, a PEEK core can incorporate one or more spring elements. The spring element can be formed of a metal material without concern of interaction of dissimilar metals. For example, a spring element formed of a nickel titanium alloy can be used between two PEEK end caps to form a compliant core in the manner described in U.S. patent application Ser. No. 12/358,716 filed Jan. 23, 2009, which is incorporated herein by reference in its entirety.

The combination PEEK and metal discs described herein can be used with many artificial disc designs and with different approaches to the intervertebral disc space including anterior, lateral, posterior and posterior lateral approaches. Although various embodiments of such an artificial disc are shown in the figures and described further herein, the general principles of these embodiments, namely providing a PEEK disc with a metallic insert for bone integration, may be applied to any of a number of other disc prostheses, such as but not limited to the LINK® SB CHARITE disc (provided by DePuy Spine, Inc.) MOBIDISK® (provided by LDR Medical (www.ldrmedical.fr)), the BRYAN Cervical Disc and MAVERICK Lumbar Disc (provided by Medtronic Sofamor Danek, Inc.), the PRODISC® or PRODISC-C® (from Synthes Stratec, Inc.), and the PCM disc (provided by Cervitech, Inc.).

In one alternative embodiment, the PEEK with metal screen disc is formed in a ball and socket design. In this embodiment the lower plate includes a lower surface with a titanium bone integration screen and an upper surface with a PEEK bearing surface in the form of a convex spherical surface. The upper plate includes an upper surface with a titanium bone integration screen and a lower surface with a PEEK concave bearing surface with mates with the concave bearing surface of the upper plate. This two piece PEEK and titanium disc can also take on other configurations with different shaped bearing surfaces, coated bearing surfaces and/or metallic bearing surface inserts.

Although the intervertebral discs described herein have been described primarily as including the combination of PEEK and titanium, it is understood that the disclosure of PEEK is intended to include other PAEK polymers and the disclosure of titanium is intended to include other biocompatible metals with good bone ongrowth properties.

While the exemplary embodiments have been described in some detail, by way of example and for clarity of understanding, those of skill in the art will recognize that a variety of modifications, adaptations, and changes may be employed. Hence, the scope of the present invention should be limited solely by the appended claims.