# Patent

## Title
Polyaryletherketone artificial intervertebral disc

## Abstract

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
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application is a Continuation of U.S. Ser. No. 12/435,572 filed May 5, 2009 which application claims priority to U.S. Provisional Application Nos. 61/050,455 filed May 5, 2008 and 61/082,012 filed Jul. 18, 2008; the full disclosures of which are incorporated herein by reference.


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 MOBIDISKTM disc prosthesis (provided by LDR Medical), the BRYANTM cervical disc prosthesis (provided by Medtronic Sofamor Danek, Inc.), and the PRODISCTM 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.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of an intervertebral disc according to one embodiment of the present invention;
FIG. 2 is a side cross sectional view of a portion of an upper plate for an intervertebral disc according to another embodiment of the present invention;
FIG. 3 is a perspective view of a plate for an intervertebral disc according to an alternative embodiment of the present invention;
FIG. 4 is a cross sectional view of the plate of FIG. 3 taken along the line 4-4;
FIG. 5 is a cross sectional view of the assembled intervertebral disc including the plate of FIG. 4;
FIG. 6 is an exploded perspective view of the plate of FIG. 3;
FIG. 7 is an X-ray image of two intervertebral discs according to the embodiment of FIG. 1 implanted in a spine;
FIG. 8 is an MRI image of the two intervertebral discs shown in the X-ray of FIG. 7;
FIG. 9 is a CT scan image of the two intervertebral discs shown in the X-ray of FIG. 7;
FIG. 10 is an X-ray image of one intervertebral disc according to the embodiment of FIG. 5 implanted in a spine;
FIG. 11 is an MRI image of the intervertebral disc shown in the X-ray of FIG. 10; and
FIG. 12 is a CT image of the intervertebral disc shown in the X-ray of FIG. 10.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates an intervertebral disc having an upper plate 10, a lower plate 12, and a core 14. The upper and lower plates 10, 12 are 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 core 14 and one or more fins 16. The upper and lower plates 10, 12 also include one or more metallic inserts 20 formed of a material which serves as a bone integration surface. The inserts 20 may include one or more bone integration enhancing features such a serrations or teeth to ensure bone integration. As shown in the embodiment of FIG. 1, the bone integration enhancing features include serrations 18. The metallic inserts 20 may 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 plates 10, 12.
The metallic inserts 20 shown in FIG. 1 are in the form of screens formed of titanium or other metal by stamping, machining or the like. The screens 20 can be securely or loosely fixed to the outer surfaces of the plates 10, 12. Titanium screens 20 form surfaces which provide both immediate fixation by way of the serrations 18 and optional teeth and provides a bone ongrowth surface for long term stability. In addition to providing fixation, the inserts or screens 20 also can serve as a radiographic marker. Since PEEK is radiolucent or nearly invisible under medical imaging, the inserts 20 serve 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 plates 10, 12 can 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 in FIG. 1, the fin 16 is surrounded by a raised portion or rim 22 which is received within an opening 24 in the screen 20. The rim 22 and surrounding opening 24 serve to locate the screen on the surface of the plate 10. The rim 22 and opening 24 can provide a snap lock feature for holding the screen 20 in place. Alternate fixation means for the screen 20 include insert molding, peripheral locking features, or adhesives. In one embodiment, the screen 20 and the PEEK plate are not fixed together. In the unfixed embodiment, the rim 22 and opening 24 prevent 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 screen 20 is 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 screen 20 preferably 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 screen 20 ends before the posterior edge of the plate 10 to 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 screen 20 and 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 screen 20, the imaging can be further enhanced by the reduced metal provided by a combination of a screen and coating.
The fin 16 can be an elongate fin pierced by one or more transverse holes 26. The disc can be inserted posteriorly into the patient from an anterior access, such that an angled posterior end 28 of fin 16 can 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 plates 10, 12 from the titanium screens 20 the plates are formed with recesses 30 which serve as bearing surfaces for the core 14. 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 core 14 can be formed as a circular disc shaped member with upper and lower bearing surfaces 36 which match the curvature of the recesses or bearing surfaces 30 of the plates 10, 12. The core 14 also has one or more annular rims 32 which cooperate with a retention feature 34 on 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 core 14 is 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 surfaces 36 of the core 14 and bearing surfaces 30 of the plates 10, 12 have 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 core 14 has a radius of curvature which is slightly smaller than a radius of curvature of the corresponding bearing surface 30 of the plate 10, 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 in FIG. 1 a single central fin 16 is provided on each of the plates 10, 12 extending 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 fins 16 can 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) 16 can also be modified depending on the insertion direction for the intervertebral disc 10. In alternative embodiments, the fins 16 may 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 fins 16 are configured to be placed in slots cut in the vertebral bodies. In one embodiment, the fins 16 are pierced by transverse holes 26 for bone ongrowth. The transverse holes 26 may be formed in any shape and may extend partially or all the way through the fins 16. Preferably, the fins 16 each have a height greater than a width and have a length greater than the height.
The fins 16 provide improved attachment to the bone and prevent rotation of the plates in the bone. In some embodiments, the fins 16 may extend from the surface of the plates 10, 12 at an angle other than 90°. For example on one or more of the plates 10, 12 where multiple fins 16 are 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 fins 16 may have any other suitable configuration including various numbers angles and curvatures, in various embodiments. In some embodiments, the fins 16 may be omitted altogether.
In addition to the fins 16, the bone integration may be improved by providing the metallic inserts or screens 20 with a plurality of projections formed thereon for improving bone attachment. In FIG. 1, the projections are in the form of pyramid shaped serrations 18 arranged in a plurality of rows on either side of the opening 24.
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 insert 20 for 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 screens 20 and 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 fins 16 and through the holes 26 therein, 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 plates 10, 12 may 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 screens 20 are shown in FIG. 1 as machined flat plates with a plurality of protrusions. As shown in FIG. 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. 2 shows an upper plate 110 of an alternative intervertebral disc. The upper plate includes a PEEK portion 112 and a metallic screen 120 on the vertebral body contacting surface of the PEEK portion. The metallic screen 120 includes a plurality of punched teeth 122 arranged to function in the manner of the serrations 18 of FIG. 1 to provide improved fixation. The metallic screen 120 can also include a plurality of teeth 124 arranged to secure the screen to the PEEK portion 112. In one example, the PEEK portion 112 can be insert molded around the teeth 124.
FIG. 2 also illustrates a locking feature 130 for locking the screen portion 120 to the PEEK or polymer portion 112. The locking feature 130 may include a snap lock feature, an insert molded feature, or other mechanical connection. The locking feature 130 may be provided on two or more sides of the upper plate 110 and may be discrete or continuous. The same or a different locking feature may be used on the corresponding lower plate. The above described features of FIG. 2 can be combined with many of the structures shown in FIG. 1.
The core 14 according to the embodiment of FIG. 1 can be retained in the lower plate 12 by retention feature 34 comprising a retention ring that protrudes inwardly from an edge of the lower plate 12. 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 core 14 has been shown as circular in cross section with spherically shaped bearing surfaces 36, 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 plates 10, 12. Although the core 14 and plates 10, 12 have 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 core 14 is 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 core 14 to 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 core 14 is formed of a PEEK material with a different density (greater visibility) than that of the plates 10, 12 to 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 plates 10, 12 are provided with grooves 34A 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 grooves 34A allow the plates 10, 12 to 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 plates 10, 12 are preferably formed from PEEK or other high strength biocompatible polymer. Portions of the upper and lower plates 10, 12, such as the screens 20 may 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 surfaces 30 can have a hard coating such as a titanium nitride finish.
Portions of the plates 10, 12 may be treated with a titanium plasma spray to improve bone integration. For example, the surfaces of the fins 16 may be titanium plasma spray coated. In another example, the fin 16 and screen 20 may 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 fins 16 if any. The plates 10, 12 are slipped into place between the vertebrae with their fins 16 entering 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 plates 10, 12 can 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 plates 10, 12 including the serrations 18 and the fins 16 locate 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 core 14, the core can be removed and replaced in an additional surgical procedure.
FIGS. 7, 8 and 9 are images showing two of the intervertebral discs of FIG. 1 implanted 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 serrations 18 on the screens 20 as shown in FIG. 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 screens 20 are clearly visible, while the PEEK portion of the plates 10, 12 and 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 discs 10. 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 plates 10, 12 with 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 plates 10, 12, the PEEK portion of the plates can be removed and replaced while leaving the metallic inserts 20 in place. Alternately, the PEEK portion of the plates 10, 12 can be removed while the metallic inserts remain and are incorporated in a subsequent fusion or other fixation procedure.
FIGS. 3-6 illustrate an alternative embodiment of a combination PEEK and metal disc having a two part metallic screen design. The disc (shown assembled in FIG. 5) includes an upper plate 110, a lower plate 112, and a core 114. The upper and lower plates 110, 112 are formed of a durable and imaging friendly material such a PAEK (PEEK) with an inner bearing surface for contacting the core 114 and one or more metallic inserts or screens 120A, 120B formed of a material which serves as a bone integration surface. As in the embodiment of FIG. 1, the inserts 120A, 120B may include one or more bone integration enhancing features such a serrations 118 and/or teeth or finlets 119 to ensure retention and bone integration. This disc construction differs from that of FIG. 1 in that the metallic screen 120A, 120B forms not only the bone integration surface having the serrations 118, but also includes a metallic fin 16 for better bone attachment to the fin.
The metallic screens are in the form of two part screens 120A and 120B 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 screens 120A, 120B each include one half of a fin member 116 and one half of the opening 124 in the screens which accommodate a corresponding inner rim 122 of the PEEK plates 110, 112.
FIGS. 4 and 6 illustrate the attachment of the two parts 120A and 120B of the metallic screen to the PEEK portion of the plate 110 by providing a peripheral protrusion 128 which surrounds and engages an outer rim 132 of the PEEK portion. This outer rim 132 has an angled outer surface which creates a locking fit when the two parts of the screen 120A, 120B are secured together. As in the embodiment of FIG. 1, the plate 110 includes a bearing surface 130 and can include a retention feature for retaining the core, such as the retaining ring 134. The screen 120A, 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 protrusion 128.
FIGS. 10-12 are images showing the intervertebral discs of FIG. 5 implanted 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 of FIG. 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.

## Claims

1. A method for relieving back pain with an intervertebral disc comprising:
implanting an upper plate comprising a body having an upper vertebra contacting surface and a lower bearing surface and a metal screen covering the upper vertebra contacting surface, wherein the body of the upper plate consists of polyaryletherketone (PAEK) and the metal screen is attached to the upper surface of the plate by insert molding, peripheral locking features, or adhesives and the metal screen provides improved bone attachment;
implanting a lower plate comprising a body having a lower vertebra contacting surface and an upper bearing surface and a metal screen covering the lower vertebra contacting surface, wherein the body of the lower plate is formed of PAEK and the metal screen is attached to the upper surface of the plate by insert molding, peripheral locking features, or adhesives and the metal screen provides improved bone attachment; and
positioning a ceramic core between the upper and lower plates with the ceramic core having upper and lower bearing surfaces in contact with the PAEK of the upper and lower plates; and
allowing the upper and lower plates to articulate with respect to one another and with respect to the core.

2. The method of claim 1, wherein each metal screen has a plurality of projections formed thereon and a plurality of holes formed therethrough for improving bone attachment.

3. A method for relieving back pain with an intervertebral disc comprising:
implanting an upper plate comprising a polyaryletherketone (PAEK) body and a metallic insert covering an upper surface of the body and having a plurality of projections formed thereon against an upper vertebra, wherein the metallic insert is attached to the upper surface of the plate by insert molding, peripheral locking features, or adhesives and improves bone ingrowth and bone attachment;
implanting a lower plate comprising a polyaryletherketone (PAEK) body and a metallic insert covering a lower surface of the body and having a plurality of projections formed thereon against a lower vertebra, wherein the metallic insert is attached to the upper surface of the plate by insert molding, peripheral locking features, or adhesives and improves bone ingrowth and bone attachment;
positioning a core between the upper and lower plates; and
allowing the upper and lower plates and the core to articulate with respect to one another in an anterior-posterior direction and in a lateral direction and to rotate with respect to one another.

4. The method of claim 3, wherein the core is movable with respect to the upper and lower plates.

5. The method of claim 3, further comprising proyiding anchoring elements in the form of fins extending from the upper surface of the upper plate and from the lower surface of the lower plate.

6. The method of claim 5, wherein the fins are formed of PAEK.

7. The method of claim 5, wherein the fin is formed is formed of PAEK and is covered with a metallic material.

8. The method of claim 5, wherein the fin is formed as a part of the metallic insert.

9. The method of claim 3, wherein the metallic inserts have a thickness of about 0.1 to about 1.0 mm.

10. The method of claim 3, wherein the core is formed of PAEK.

11. The method of claim 3, wherein the core is formed of ceramic.

12. The method of claim 3, wherein the core is metallic.

13. The method of claim 3, wherein the metallic inserts have a thickness of at least 0.3 mm.

14. The method of claim 3, wherein each metallic insert comprises a metal screen having the plurality of projections formed thereon and a plurality of holes formed therethrough.