Disc prosthesis

[0002] This invention relates to artificial biocompatible synthetic vertebral prostheses, and more particularly to prosthetic metal intervertebral discs.

[0003] Many types of vertebral prostheses have been proposed and patented for implantation in the vertebral disc space after surgical removal of a diseased or damaged disc. Such devices fall into three broad categories. One category of prostheses includes the use of pliable synthetic materials in an attempt to mimic the compressibility of the human spinal disc. For example, U.S. Pat. No. 5,171,281 (Parsons) discusses a disc spacer which purports to possess mechanical properties akin to those of the normal disc by varying the hardness of the elastomeric material in its nucleus and annulus. U.S. Pat. No. 5,192,326 (Bao) illustrates a prosthetic disc formed from a multiplicity of hydrogel beads having a water content of at least 30%. According to the patent, a semi-permeable membrane covers the beads and is said to permit fluids to flow in and out of the prosthetic nucleus. U.S. Pat. No. 5,071,437 (Steffee) proposes another approach to a pliable implant. That approach appears to involve upper and lower flat rigid plates sandwiching an elastomeric core made from a polyolefin rubber. U.S. Pat. No. 5,002,576 (Fuhrmann) also discusses a variation on the foregoing approach.

[0004] Another type of prosthesis attempts to mimic the shape of a natural spinal disc. For example, U.S. Pat. No. 4,714,469 (Kenna) discusses a spinal implant adapted to replace a disc between adjacent vertebrae using a predetermined thickness and profile to match the space between the vertebrae. The implant includes a porous coating on its surface. U.S. Pat. No. 4,759,766 (Buettner-Janz) illustrates a metallic disc endoprosthesis which has two symmetrical, concave end plates with an intermediate spacing piece. Similarly, U.S. Pat. No. 5,314,477 (Oka) discusses a disc prosthesis including two plates separated by a joint composed of a spherical cap and cylindrical base which attempts to ensure connection between the vertebrae.

[0005] More recently, the present inventor has patented (U.S. Pat. No. 5,865,845, Thalgott) a prosthetic intervertebral disc which takes into consideration the anatomy and the geometry of the intervertebral space sought to be filled by the prosthetic device. That device, while an improvement over prior available devices, does not work for all applications or geometries. Further, continuing experience with its use has increased understanding of areas where its design could be improved to mesh more precisely with the surrounding anatomy in the spine. Accordingly, a need continues to exist for an improved intervertebral implant which better addresses the need for a disc replacement which improves over prior spinal implants.

[0006] The foregoing objects are achieved and the disadvantages of the prior art are overcome by providing a spinal disc implant comprising an implant of a biocompatible metallic body having a front wall, a rear wall, and two side walls extending therebetween to define an interior space. Each side wall defines a substantially arcuate curve joining one end of the front wall and one end of the rear wall. Further, each wall contains at least one opening therein, and the upper and lower surfaces of the two side walls having a plurality of teeth extending therefrom for engaging adjacent vertebral bodies. The side walls of the implant taper in height from the rear wall to the front wall; and the front wall and the rear wall each lack teeth, and have a height substantially similar to that of a portion of the side wall adjacent the front and rear walls. The implant is made of a biocompatible metal such as titanium or an alloy thereof, and the first and second sides taper from the junction between the back wall and the side wall to the junction between the front wall and the side wall. Preferably, the interior space includes a porous hydroxyapatite block shaped and press fit, cemented or screwed in to fill the interior space. The porous hydroxyapatite substance helps the prosthesis integrate into the vertebral structure by allowing bone to grow into pores.

[0007] Other features and advantages of the invention will be apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings as follows:

[0008] FIG. 1 shows a perspective view of one embodiment of the spinal disc implant of the present invention;

[0009] FIG. 2 is a top view of the spinal disc implant of FIG. 1;

[0010] FIG. 3 is a side view of a representative side of the spinal disc implant of FIG. 1;

[0011] FIG. 4 is a front view of the spinal disc implant of FIG. 1;

[0012] FIG. 5 is a schematic cross-sectional view of the teeth of the implant shown in FIG. 1;

[0013] FIG. 6 is a top view of a portion of the teeth on an upper surface of the implant; and

[0014] FIG. 7 is a cross-sectional view of the implant of FIG. 1, taken along line 7—7 (before boring the central opening or machining the teeth).

[0015] FIG. 1 illustrates a spinal disc implant of the present invention, generally indicated by the reference numeral 10. The implant 10 has a generally D-shaped body 12 including a central opening 14. The implant further includes upper and lower faces 16, 18 having a plurality of teeth 19 or other gripping means included on each face. The implant 10 has a front wall 20, a back wall 22, and first and second side walls 24, 26, which are all joined. The back wall 22 is relatively straight, while the front wall 20 and side walls 24 and 26 are curved. The openings or holes 28 in the walls are preferably of an elliptical shape, although they may be square, circular, or rectangular. The embodiment of FIG. 1 has a hole or opening 28 in the center of the back wall 18 and in the center of the front wall 20. There is an opening or hole 28 at the junction between each side wall 24, 26 and the front wall 20, as well as in the approximate center of each side wall 24, 26 for a total of six openings or holes 28 in the implant 10. Teeth 19 appear on the upper or lower surfaces 16, 18 of the side walls 24, 26, but not on the upper or lower surfaces of the front and rear walls 20, 22. The upper and lower surfaces 30, 32 of the rear wall 22 includes notches 34, 36 to provide a structure to engage a gripping tool (not shown) to aid in placement of the implant 10 in the intervertebral space. The space defined by the central opening 14 (best viewed in FIG. 2) openings are preferrably filled with hydroxyapatite combined with any biological factor or composition which helps to induce growth of bone and cartilage against the surface of the implant and in the spaces defined by the teeth 19.

[0016] The teeth 19 (shown in FIGS. 1 and 5) are preferably steeply sloped four-sided pyramids of approximately 0.08 inches in height. They are preferably arranged in straight rows across and down the upper and lower faces 16, 18 of the side walls 24, 26 of the implant 10, as shown in FIG. 2. The pyramidal faces of each tooth 19 preferably form a 45° angle with the vertical. They can be formed by machining the implant 10, or as part of the casting process.

[0017] An important aspect of the present invention is its geometric compatibility with its environment. Referring to FIGS. 1 and 3, the implant 10 slopes from the rear wall 22 to the front wall 20. This shape enables the implant 10 to fit between adjacent vertebral bodies (not shown) when the spine is in an upright position. The exact angle formed at the vertex defined by the top and bottom faces varies depending on which disc is being replaced. In the lumbar region of the spine, for example, the opposed faces adjacent vertebral bodies define an angle ranging from about 0 to about 20 degrees. Similarly, since the vertebral body, which engages the first and second faces 16, 18 of the implant 10 from above, has a defined curvature, the implant 10 has a curvature which mimics that of the intervertebral space to allow it to conform to the domed shape of the vertebral body surface.

[0018] The implant 10 is preferably made from pure titanium or an alloy thereof, preferably anodized to increase its biocompatibility by making it more inert. The implant 10 may be made from bar stock, or tubing or by molding, or from titanium powder using powder metallurgy techniques. The dimensions of the implant 10 vary depending on where in the spine the implant will be inserted. The vertebral bodies in the lumbar area of spine, for example, are larger than the vertebral bodies in the thoracic area. Therefore, an implant intended for the thoracic region would be smaller than one for the lumbar region. Likewise, lower lumbar disc replacements would be larger than upper ones. By way of example, an implant sized for implantation between the third and fourth lumbar vertebrae may have approximate dimensions of 2.7 cm long, 2.5 cm wide, about 2 cm high anteriorly, and would slope down to about 1.3 cm high posteriorly. The slope from rear to front in a typical implant increases by about 3 mm. A person of ordinary skill could adapt the basic dimensions of the implant 10 to make the implant 10 suitable for the space formerly occupied by the particular vertebral disc which needs replacement. The present invention therefore includes implants having varying angles and dimensions to allow implantation at different levels of the spine.

[0019] The shape and curvature of the implant 10 provide several advantages. In the lumbar region of the spine, the discs and vertebral bodies are held at an angle creating a lordosis or curvature of the lumbar spine. To have the implant 10 parallel or coplanar would be physiologically and anatomically unacceptable. The natural discs in the lumbar spine are wider anteriorly than they are posteriorly. The disc replacement implant 10 of the present invention is therefore also wider posteriorly than it is anteriorly. This recreates the natural anatomic curvature of the spine.

[0020] Further, the implant 10 of the present invention takes into consideration the anatomy of the undersurface of the vertebral body or end plate of the vertebra on which the lower face of the implant 10 rests. The end plate is made of very compact bone circumferentially, but as the bone centralizes towards the middle, it becomes thinner. The thinner portion is dome shaped, and is responsible for the hydraulic stress middle of the end plate. This shape is mimicked by the secondary curvature in the disc implant of the present invention. The secondary arc which corresponds to the dome in the vertebral body provides a mechanism to lock the cage in place and prevent slippage or extrusion. The teeth 19 disposed on portions of the top and bottom faces of the side walls 24, 26 of the implant 10 grip the vertebral body and cause a mechanical interface between the prosthesis and the end plate of the vertebral body.

[0021] Preferably, the implant 10 optionally includes an insert of synthetic bone material, such as porous hydroxyapatite or other equivalent substance. Preferably, the synthetic bone material is Interpore ProOsteon 500 brand of porous coralline hydroxyapatite, available from Interpore Cross International, Irvine, Calif. The porous synthetic bone material is held in place by press fit (friction), or by set screws on the sides of the implant 10 (not shown). The porous synthetic bone allows independent placement of the implant 10 into the intervertebral disc space without use of a bone graft. This will help reduce morbidity and complications associated with harvesting a bone graft from the patient, reported to be as high as 21%. It will also obviate the need for use of an allograft, which carries the risk of disease transmission and added expense.

[0022] The implant 10 provides a non-articulating disc prosthesis which can be provided in multiple sizes depending on the size needed for the specific lumbar region, and can be furnished in smaller sizes for the cervical and thoracic spine, as miniature cages for placement using endoscopic techniques for minimally invasive spine surgery.

[0023] During implantation surgery, the surgeon exposes the herniated or damaged disc, and removes it. A spinal disc implant 10 (optionally including a central core of porous synthetic bone), is inserted with a tool which grips the implant 10 to enable the surgeon to lift and insert the implant 10 in the intervertebral space defined by adjacent vertebral bodies from which the damaged or diseases disc was removed. The implant 10 is positioned on the vertebral body so that its transverse curvature conforms to the dome shape of the vertebral body. At the same time, the implant 10 is positioned so that its anterior to posterior position will create the proper angulation between vertebrae to help to restore the natural anatomic curvature of the human spine. The implant 10, once implanted, encourages osseointegration in two distinct ways. The teeth 19 form an irregular surface which grip the vertebral body and allow bone tissue to grow in and around the teeth 20. Also, the synthetic porous bone segment, if present, allows bone tissue to grow into the pores, to help anchor the implant 10 in place without resorting to bone grafts or allografts.

[0024] The advantages to the present implant 10 include the following: (1) the pattern of teeth is different than previous discs; (2) the front of the device fits within the end plates of the vertebral bodies in a much more anatomic fashion as compared with previous discs; and (3) the implant provided herein can be used in the lumbar spine. However, smaller versions for use in the cervical and thoracic spine are intended to be part of the invention.

[0025] Various modifications will be apparent to those skilled in the art. Such modifications and changes are intended to be included within the scope of the invention, which is defined by the following claims.