Patent Abstract:
A porous intervertebral spacer comprises a plurality of strands of fused, tortuous wire of a biologically inert material, the porosity of the spacer being sufficient to facilitate tissue ingrowth and bony fusion. The spacer also can comprise a mixture of such strands and biologically inert beads. A method of fusing adjacent vertebrae of the spine includes the steps of excising a portion of an intervertebral disc separating adjacent vertebrae and portions of the adjacent vertebrae to define a graft bed, and inserting into the graft bed at least one porous intervertebral spacer according to the invention.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to an intervertebral spacer and method for spacing and fusing adjacent vertebrae and, more particularly, to a porous, strong, intervertebral spacer formed of a biologically inert material. 
     2. Description of the Prior Art 
     Techniques and devices for fusing two or more vertebrae of the spine together are well known. Such techniques are commonly performed to correct problems, such as chronic back pain, which result from degenerated intervertebral discs. One technique for fusing together two or more vertebrae of the lumbar spine includes excising a portion of the disc extending between adjacent vertebrae and grafting one or more portions of bone of a desired shape, known as an intervertebral spacer, between the adjacent vertebrae. The intervertebral spacer may be inserted by either an anterior or posterior approach to the spinal column depending on a number of factors, including the number of vertebrae to be fused and past operative procedures. Upon healing, the vertebrae are desirably fused together through the intervertebral spacer. 
     Conventionally, intervertebral spacers have been autogenic bone harvested from other areas of the body, such as the pelvis, allogenic bone taken from cadavers or xenogenic bone, such as bovine bone sections. However, the use of bone grafts can add complications to the fusion procedure. For example, when using an autogenic bone graft, a second incision must be made in the patient to harvest the additional bone to be used in the graft, thus increasing the pain and blood loss to the patient. When allogenic or xenogenic bone grafts are used there is a potential for the transmission of disease from the cadaver or other graft source to the patient. 
     The use of non-biological implants, such as carbon fiber spacers, also has been attempted in the past, but these spacers tend to lack sufficient porosity and tissue ingrowth characteristics to function adequately. 
     It would be desirable to provide a non-biological spacer which is non-reactive in the body and which has the strength and tissue ingrowth characteristics of a bone graft spacer. 
     SUMMARY OF THE INVENTION 
     The present invention provides a porous intervertebral spacer which can be used in the same manner as a bone graft spacer to fuse vertebrae together. The inventive spacer preferably is composed biologically inert strands, or a mixture of such strands and biologically inert beads sintered in a mold of a desired shape and size. The spacer is made of metals such as titanium, and thus is non-biologically reactive and provides for tissue ingrowth to facilitate fusion with adjacent vertebrae. 
     In accordance with one aspect of the invention, a porous intervertebral spacer is formed in a variety of shapes such as a prism (for example, a rectangular prism), a cylinder, and a plate. In each instance, the spacer is made of a plurality of fused, tortuous strands or a mixture of tortuous strands and beads of a biologically inert material such as titanium or a titanium alloy. 
     In accordance with another aspect of the invention a method of fusing adjacent vertebrae of the spine includes the steps of excising a portion of an intervertebral disc separating adjacent vertebra and portions of the adjacent vertebrae to define a graft bed, and inserting into the graft bed at least one porous intervertebral spacer formed from a plurality of fused, tortuous strands or a mixture of tortuous strands and beads of a biologically inert material such as titanium or a titanium alloy. 
    
    
     In general, the invention comprises the foregoing and other features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail a certain illustrated embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the annexed drawings: 
     FIG. 1 is a perspective view of an intervertebral spacer similar to the spacer of the present invention, the spacer being in the form of a rectangular prism; 
     FIG. 2 is an elevational view of the anterior of a portion of the lumbar spine and sacrum illustrating a graft bed; 
     FIG. 3 is a view similar to FIG. 2 showing spacers according to the invention implanted in the graft bed; 
     FIG. 4 is a side elevational view of two representative lumbar vertebrae illustrating the location of a posterior-formed graft bed; 
     FIG. 5 is an elevational view of the posterior of representative lumbar vertebrae illustrating the locations of separate posteriorly formed graft beds; 
     FIG. 6 is a view similar to FIG. 5 showing two spacers according to the invention implanted in the graft beds; 
     FIG. 7 is an enlarged view of the surface of the spacer of FIG. 1; 
     FIG. 8 is a view similar to FIG. 7 showing the surface enlarged to an even greater extent; 
     FIG. 9A is a perspective view similar to FIG. 1, showing a spacer provided with a plurality of parallel apertures-opening through the top and bottom faces of the spacer; 
     FIG. 9B is a perspective view of an intervertebral spacer in accordance with the invention, the spacer being in the form of a cylinder and including a plurality of apertures that are disposed parallel to the end faces of the cylinder; 
     FIG. 9C is a view similar to FIG. 9A, showing the use of external teeth, or ribs; 
     FIG. 9D is a view similar to FIG. 9B in which a large cylindrical opening extends longitudinally through the center of the spacer; 
     FIG. 9E is a view similar to FIG. 9A in which the top and bottom faces of the spacer are rounded; 
     FIG. 9F is a perspective view of a intervertebral spacer according to the invention, the spacer being in the form of a hexagonal prism with apertures opening through the top and bottom faces; 
     FIG. 9G is a view similar to FIG. 9F in which the spacer is in the form of an octagonal prism with apertures opening through the top and bottom faces; 
     FIG. 9H is a view similar to FIG. 9F in which the end faces are rhombuses; 
     FIG. 9I is a perspective view of a plate-like intervertebral spacer which, when viewed from above, is generally C-shaped; 
     FIG. 9J is a-view similar to FIG. 9I in which ribs have been added to the upper and lower faces of the spacer; 
     FIG. 9K is a view similar to FIG. 9I showing an elliptical spacer with an elliptical opening at its center; 
     FIG. 9L is a view similar to FIG. 9K in which ribs have been added to the upper and lower faces of the spacer; 
     FIG. 9M is a view similar to FIG. 9I showing a kidney-shaped spacer; and 
     FIG. 9N is a view similar to FIG. 9M in which ribs have been added to the upper and lower faces of the spacer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings and initially to FIGS. 1,  7 , and  8 , there is shown an intervertebral spacer  10  similar to the present invention. The embodiment of FIG. 1 is disclosed and claimed in U.S. Pat. No. 5,961,554, the disclosure of which is incorporated herein by reference. 
     The spacer  10  is in the form of a porous biologically inert block in the form of a rectangular prism. The corners and edges of the spacer  10  may be formed with a small radius if desired. One or more such rectangular, block-shaped spacers  10  are sized to fit within an opening or graft bed formed between adjacent vertebrae by the surgical excision of a portion of the intervertebral disc and confronting portions of the adjacent vertebral bodies. The particular size of the spacer  10  will be determined by the particular vertebrae to be fused, and condition of the vertebrae. Advantageously, because the spacers are not made of a biological material, they are easily stored and can be manufactured in a variety of shapes and sizes to accommodate anticipated situations. A typical spacer  10  for fusing vertebrae of the lumbar spine may be from  10  to 13 millimeters in width, 12 to 18 millimeters in height, and 25 to 30 millimeters in length. 
     It will be appreciated that while the specific example of the intervertebral spacer described herein is with reference to a spacer for fusing vertebrae of the lumbar spine together or to the sacrum, the invention applies also to spacers for fusing vertebrae of the cervical or thoracic spine as well. The particular shape of the spacer is also a function of the application. While a generally rectangular spacer is well suited to fusing lumbar vertebrae, in other instances other shapes for the spacer, such as cylindrical, may be desirable. Moreover, it will be recognized that the spacers of the invention may also be used in other areas of the body to fuse bone together where necessary. 
     The spacer  10  is preferably composed of biologically inert spheres or beads  94  having a diameter such as to yield, when fused, a spacer with the fused beads  94  occupying a range of generally 40 to 70 percent of the volume of the spacer. This density provides a spacer  10  which is sufficiently porous throughout to allow for the flow of bodily fluids through the spacer and to promote tissue ingrowth and bony fusion with adjacent vertebrae. The beads  94  also result in porous surfaces  12  over the spacer  10  which when implanted develop a high friction interface with the contacting vertebral bodies to facilitate maintaining the spacer in place. The beads  94  are preferably composed of titanium or a titanium alloy (such as Ti- 6 AI-4V) which is nonreactive within the body. Since the early 1970&#39;s, titanium and titanium alloys have been approved by the United States Food and Drug Administration for use in knee, shoulder, and hip implants to promote bone ingrowth. Other suitable materials include cobaltchromium alloys, tantalum, tantalum alloys, niobium, niobium alloys, and stainless steel, or any other metal having adequate strength and biocompatibility properties. 
     It has been found that beads of a certain size range are preferred. Suitable small beads will have a mesh size of −45 +60 (0.009 inch to 0.011 inch). Suitable medium beads will have a mesh size of −25 +30 (0.016 inch to 0.027 inch). Suitable large beads will have a mesh size of −18 +30 (0.032 to 0.046 inch). The size of the beads determines the porosity of the finished spacer  10 . The larger the beads, the greater the porosity. In certain applications, it may be desirable to mix beads of various sizes to obtain a finished spacer  10  having a variable porosity. 
     The invention involves the discovery that it is possible to intermix strips, or strands, of wire mesh with beads to form a spacer  10  having variable qualities of strength and porosity. In general, the use of wire mesh results in a stronger, less porous spacer  10 . It also is possible to form the spacer  10  entirely of wire mesh. Such mesh presently is used as a porous coating for knee, shoulder, and hip implants. Such mesh sometimes is referred to a spaghetti mesh, and is commercially available from the Zimmer Company of Warsaw, Ind. The types of metals suitable for the strands of wire mesh and the beads are the same as those set forth above for the beads  94 . Reference is made to U.S. Pat. No. 3,906,550; 4,693,721; and 5,665,119, the disclosures of which are incorporated herein by reference, for a discussion of the use of metal fiber as a porous bone structure material. 
     One suitable method of fusing titanium beads, titanium mesh, or a mixture of titanium beads and mesh to form the spacer  10  includes placing the beads and/or strands into a cavity within a substantially purified graphite mold. The mold is preferably a three piece mold forming a cavity of the finished dimensions of the spacer  10 . The mold is then heated to a high temperature, for example, 2000 degrees F. or higher until the sintering is complete, around 24 hours. Other conventional methods for fusing titanium strands or beads which provide a sufficiently strong spacer  10  also may be acceptable. When titanium spaghetti mesh is used to form the spacer  10 , the strands of mesh are placed in the mold in a tangled, tortuous mass. Sintering produces strong inter-strand bonds with variably sized openings to form a spacer  10  of suitable strength and porosity. 
     The procedure for fusing two or more vertebrae together using the spacer  10  of the invention is substantially the same as the procedure for fusing vertebrae using a bone graft, but without many of the complications due to obtaining a suitable bone graft and the possibility of transmitting disease from the bone graft donor. One anterior procedure for implanting a bone graft to fuse vertebra of the lumbar spine is discussed in Collis et al., “Anterior Total Disc Replacement:, A Modified Anterior Lumbar Interbody Fusion,” Lumbar Interbody Fusion, ed. Robert Watkins, Chapter 13, pp. 149-152, Aspen Publications (1989), the disclosure of which is incorporated herein by reference. 
     Referring to FIGS. 2 and 3, there is shown an anterior elevation view of the lumbar spine  14  including the fourth and fifth lumbar vertebrae  16 ,  18 , respectively, and the sacrum  20  with the sacral vessels  22  ligated and both iliac vessels  24  retracted outwardly to expose the vertebral disc  26  between the fifth lumbar vertebra  18  and the sacrum  20 . As an example, to fuse the fifth lumbar vertebra  18  to the sacrum  20 , using an anterior approach, a graft bed  28  is prepared by surgically exposing the affected area and excising portions of the vertebral body of the vertebra  18  and the sacrum  20  and the section of the disc  24  located therebetween, as shown in FIG.  2 . An appropriate number of spacers  10 , in this example, three, are then implanted into the graft bed  28 . Over time bony tissue ingrowth will desirably fuse the vertebral bodies of the vertebra  18  and the sacrum  20  to the spacers  10  and thus fuse the vertebra to the sacrum through the spacers. The number of spacers  10  employed will be a function of a number of factors, including the particular vertebrae to be fused and the deterioration of the vertebral disc and of the vertebral bodies themselves. 
     The intervertebral spacers  10  may also be implanted through known posterior approaches. In a typical procedure using a posterior approach in which two spacers are implanted, such as is shown in FIGS. 4 through 6 which represent side and rear elevations of, two representative lumbar vertebrae  30 ,  32 , the posterior portion of the subject area of the lumbar spine is surgically exposed. Graft beds  34  are then formed by excising the required portions of adjacent vertebral bodies  36 ,  38  of the vertebrae  30 ,  32 , respectively, and a section of the disc located therebetween. The graft beds  34  may be formed using a cutting tool  40 , such as is shown in FIG. 4 (FIG. 4 omits the Canda Equina and the disc for clarity), wherein portions of the lamina  41  and/or spinous process  42  of one or both of the vertebrae are removed to open a passage  44  through which the tool may be inserted to reach the vertebral bodies. To implant the spacers  10  once the graft beds  34  have been formed, the Canda Equina and protective dura  46  are first retracted to one side to expose a graft bed and a spacer is inserted into the exposed graft bed (see FIG.  5 ), and then the Canda Equina and dura are retracted to the other side to insert a spacer into the exposed other graft bed. 
     Referring now to FIGS. 9A-9N, the spacer according to the invention is shown in a variety of configurations. In all of these configurations, the spacer is formed by sintering titanium or titanium alloy beads or spaghetti mesh within a suitably configured mold. In particular, FIG. 9A shows the spacer  10  provided with a plurality of parallel, equidistantly spaced apertures  46 . The apertures  46  open through the top and bottom faces of the spacer  10 . It also is possible to provide a longitudinally extending opening (not shown) that opens through the end faces of the spacer  10 . 
     The spacer  10  can be provided in various sizes. A typical size is 10 mm wide, 27 mm long, and a variable height of 8, 10, 12, 14, 16 or 18 mm. The spacer  10  can be provided in shorter lengths of 24 mm, or longer lengths of 30 mm. For those spacers  10  having a width of 10 mm, the apertures  46  should have a diameter of about 0.1875 inch. 
     The spacer  10  also can be provided in the different widths, for example, 13-40 mm. With a width of 13 mm, variable lengths of 24, 27 or 30 can be provided. The height also can be selected among 8, 10, 12, 14, 16 or 18 mm. For spacers  10  having a width of 13 mm, the apertures  46  should have a diameter of 0.2188 inch. 
     Referring now to FIG. 9B, a spacer  50  in the form of a cylinder is shown. The spacer  50  is provided in various diameters and lengths, for example, 10 mm, 12 mm, 14 mm and 16 mm diameter, and lengths of 24, 27 and 30 mm. As with the spacer  10 , three equidistantly spaced apertures  46  are provided for the spacer  50 . For spacers  50  having diameters of 10 or 12 mm, the apertures  46  have a diameter of about 0.1875 inch, while for spacers  50  having a diameter of 14 or 16 mm, the apertures  46  have a diameter of about 0.2188 inch. 
     Referring now to FIG. 9C, the spacer  10  is provided with laterally extending teeth or ribs  52 . In cross section, the ribs  52  are triangular with a vertex angle of  60  degrees and a height of 2 mm. The ribs  52  prevent undesired movement of the spacer  10  within the patient after the spacer  10  has been implanted in the graft bed  28 . 
     Referring to FIG. 9D, the spacer  50  is shown with two spaced-apart apertures  46 . The spacer  50  also is provided with a longitudinally extending aperture  54  that opens through the end faces of the spacer  50 . The diameter of the aperture  54  is selected such that the wall thickness of the spacer  50  is approximately 3 mm. 
     Referring now to FIG. 9E, the spacer  56  is similar to the spacer  10 , but includes flat, parallel end faces and sidewalls, and rounded top and bottom faces  58 . As with the spacer  10 , a plurality of apertures  46  are provided for the spacer  56 . The dimensions for the width, length, and height of the spacer  56  are the same as those described previously for the spacer  10 . The radius for the top and bottom faces  58  should be approximately 9 mm. 
     Referring now to FIGS. 9F,  9 G and  9 H, spacers  60 ,  62  and  64  are illustrated. The spacer  60  is a hexagonal prism, the spacer  62  is an octagonal prism, and the spacer  64  is a, rhomboidal prism. The spacers  60 ,  62 , as with the spacer  10 , are provided with a plurality of parallel, equidistantly spaced apertures  46 . If desired, the spacers  60 ,  62  and  64  could be provided with longitudinally extending openings such as the opening  54  included as part of the spacer  50 . In general, the spacer according to the invention can be provided in a variety of geometric configurations. Virtually any polyhedron prism will provide satisfactory results. 
     Referring to FIGS. 9I-9N, a variety of plate-like spacers are shown. The spacers are provided in a variety of lengths, widths, and depths to fit all male and female vertebral bodies. In FIG. 9I, a spacer  66  includes flat, parallel upper and lower faces  68 ,  70  with a rounded exterior surface  72  and a cut-out portion  74 . The spacer  66  generally is C-shaped. In FIG. 9J, the spacer  66  is provided with a plurality of ribs  76  that are similar in size and shape to the ribs  52  and which perform the same function. 
     In FIG. 9K, a spacer  78  includes an elliptical body portion  80  with an elliptical opening  82  at its center. In FIG. 9L, the spacer  78  is provided with ribs  84  of the same size and shape as the ribs  52 . 
     Referring to  9 M, a spacer  86  includes a kidney-shaped body portion  88  having a small cut-out portion  90 . In FIG. 9N the spacer  86  is provided with ribs  92  that are the same size and shape as the ribs  52 . 
     It is expected that the spacers  66 ,  78 ,  86  will be provided in sizes large enough to perform the function of two or three spacers  10  or  50 . It is expected that a single, large graft bed  28  will be formed such that the spacer  68 ,  78 ,  86  will fill the graft bed  28  entirely. 
     Although the invention has been shown and described with respect to a certain preferred embodiment, it is apparent that equivalent alterations and modifications will occur to others skilled in the art upon reading and understanding this specification. The present invention includes all such equivalent alterations and modifications and is limited only by the scope of the following claims.

Technology Classification (CPC): 0