Abstract:
The invention provides a spinal disc implant comprising two side walls which are opposed and substantially parallel spaced apart by a front wall and a back wall to define an interior space. The side walls define substantially elliptical curves joining the front and back walls. The front wall is also curved and the back wall may be either curved or straight. The walls may have openings or holes which may be circular or elliptical in shape. The upper and lower edges of the implant have a plurality of teeth extending therefrom for engaging adjacent vertebrae. The implant is made of a biocompatible metal such as titanium or an alloy thereof, and the first and second sides tapering from the second end to the first end. The interior space has a porous hydroxyapatite block shaped to fill the interior space. The porous hydroxyapatite substance helps the prosthesis integrate into the vertebral structure by allowing into the pores.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/646,169 filed on Sep. 14, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to artificial biocompatible synthetic vertebral prostheses, and more particularly to prosthetic metal intervertebral discs.  
         BACKGROUND OF THE INVENTION  
         [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  
         SUMMARY OF THE INVENTION  
         [0004]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    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:  
         [0006]    [0006]FIG. 1 shows a perspective view of one embodiment of the spinal disc implant of the present invention;  
         [0007]    [0007]FIG. 2 is a top view of the spinal disc implant of FIG. 1;  
         [0008]    [0008]FIG. 3 is a side view of a representative side of the spinal disc implant of FIG. 1;  
         [0009]    [0009]FIG. 4 is a front view of the spinal disc implant of FIG. 1;  
         [0010]    [0010]FIG. 5 is a schematic cross-sectional view of the teeth of the implant shown in FIG. 1;  
         [0011]    [0011]FIG. 6 is a top view of a portion of the teeth on an upper surface of the implant; and  
         [0012]    [0012]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). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    [0013]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 .  
         [0014]    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.  
         [0015]    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.  
         [0016]    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.  
         [0017]    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.  
         [0018]    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.  
         [0019]    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.  
         [0020]    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.  
         [0021]    In addition to titanium or a biocompatible alloy thereof, or other biocompatible metal or alloy known to those of skill in the art, the implant of the present invention may be made of other biomaterials including biocompatible metals, metal alloys, ceramics, and polymers or combinations thereof, having suitable hardness and strength characteristics. For example, the implant may be made of a synthetic biocompatible material, as disclosed in U.S. Pat. Nos. 5,306,309 (Wagner); 5,192,327 (Brantigan); 5,171,281 (Parsons); 4,911,718 (Lee); and 4,655,777 (Dunn). The entire contents of the foregoing references is incorporated herein by reference. Of particular interest, in addition to titanium or an alloy thereof, is a fiber reinforced synthetic material, such as a carbon fiber reinforced polymer. The implants may have ceramic portions, facings, or coatings, and may also include or be coated with one or more bone growth inducing factors or compositions. Optionally, the implant may include an elastomeric layer or portion, to add compressibility to the implant.  
         [0022]    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 orallografts. 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.  
         [0023]    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.