Abstract:
The present invention is directed to a variety of interbody spinal fusion implants having at least a partially frusto-conical configuration. An external thread is employed to increase implant stability and implant surface area, and for the purpose of advancing the spinal fusion implant into the fusion site. The spinal fusion implants of the present invention may be relatively solid or hollow and may have surface roughenings to promote bone ingrowth and stability. The spinal fusion implants of the present invention may have wells extending into the material of the implant from the surface for the purpose of holding fusion promoting materials and to provide for areas of bone ingrowth fixation.

Description:
[0001]    The present application is a continuation of application Ser. No. 12/454,393, filed May 18, 2009, now U.S. Pat. No. 7,828,800; which is a continuation of application Ser. No. 08/480,908, filed Jun. 7, 1995, now U.S. Pat. No. 7,534,254; which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to interbody spinal fusion implants, and in particular to spinal fusion implants configured to restore and maintain two adjacent vertebrae of the spine in anatomical lordosis. 
         [0004]    2. Description of the Prior Art 
         [0005]    Interbody spinal fusion refers to the method of achieving bony bridging between adjacent vertebrae through the disc space, the space between adjacent vertebrae normally occupied by a spinal disc. Numerous implants to facilitate such a fusion have been described by Cloward, Brantigan, and others, and are known to those skilled in the art. Generally, cylindrical implants offer the advantage of conforming to an easily prepared recipient bore spanning the disc space and penetrating into each of the adjacent vertebrae. Such a bore may be created by use of a drill. It is an anatomical fact that both the cervical spine and the lumbar spine are normally lordotic, that is convex forward. Such alignment is important to the proper functioning of the spine. Commonly, those conditions which require treatment by spinal fusion are associated with a loss of lordosis. 
         [0006]    Therefore, there exists a need for spinal fusion implants that permit for the restoration of anatomical lordosis. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a variety of interbody spinal fusion implants having at least a partially frusto-conical configuration. In the preferred embodiment, the spinal fusion implants of the present invention have a body that is partially or fully frusto-conical shape substantially along the portion of the implant in contact with the adjacent vertebrae of the spine. The spinal fusion implants of the present invention have an external thread for engaging the adjacent vertebrae of the spine and have an insertion end and a trailing end. The external thread may have a variable or constant thread radius and/or a constant or variable thread height measured from the body of the implant. 
         [0008]    The spinal fusion implants of the present invention may be further modified so that while the upper and lower surfaces are portions of a frusto-cone, at least one side portion may be truncated to form a planar surface that is parallel to the central longitudinal axis of the implant to form straight walls. These implants may have a more tapered aspect at the insertion end of the implant to facilitate insertion. The spinal fusion implants of the present invention may be relatively solid and/or porous and/or hollow, and may have surface roughenings to promote bone ingrowth and stability. 
         [0009]    The spinal fusion implants of the present invention may have wells extending into the material of the implant from the surface for the purpose of holding fusion promoting materials and to provide for areas of bone ingrowth fixation. These wells, or holes, may pass either into or through the implant and may or may not intersect. The spinal fusion implants of the present invention may have at least one chamber which may be in communication through at least one opening to the surface of the implant. Said chamber may have at least one access opening for loading the chamber with fusion promoting substances. The access opening may be capable of being closed with a cap or similar means. 
         [0010]    The spinal fusion implants of the present invention offer significant advantages over the prior art implants:
       1. Because the spinal fusion implants of the present invention are at least partially frusto-conical in shape, those that taper from the leading edge to the trailing edge are easy to introduce and easy to fully insert into the spinal segment to be fused. In another embodiment, where the trailing edge of the implant is larger than the leading edge, the implant utilizes a tapered forward portion and an increasing thread height relative to the body from the leading edge to the trailing edge to facilitate insertion.   2. The shape of the implants of the present invention is consistent with the shape of the disc, which the implants at least in part replace, wherein the front of the disc is normally taller than the back of the disc, which allows for normal lordosis. The implants of the present invention are similarly taller anteriorly than they are posteriorly.   3. The spinal fusion implants of the present invention conform to a geometric shape, which shape is readily producible at the site of fusion, to receive said spinal fusion implants.       
 
         [0014]    The spinal fusion implants of the present invention can be made of any material appropriate for human implantation and having the mechanical properties sufficient to be utilized for the intended purpose of spinal fusion, including various metals such as cobalt chrome, stainless steel or titanium including its alloys, various plastics including those which are bio-absorbable, and various ceramics or combination sufficient for the intended purpose. Further, the spinal fusion implants of the present invention may be made of a solid material, a mesh-like material, a porous material and may comprise, wholly or in part, materials capable of directly participating in the spinal fusion process, or be loaded with, composed of, treated of coated with chemical substances such as bone, morphogenic proteins, hydroxyapatite in any of its forms, and osteogenic proteins, to make them bioactive for the purpose of stimulating spinal fusion. The implants of the present invention may be wholly or in part bioabsorbable. 
       OBJECTS OF THE PRESENT INVENTION 
       [0015]    It is an object of the present invention to provide a spinal fusion implant that is easily inserted into the spine, having a tapered leading end; 
         [0016]    It is another object of the present invention to provide a spinal fusion implant that tapers in height from one end to the other consistent with the taper of a normal spinal disc; 
         [0017]    It is yet another object of the present invention to provide a spinal fusion implant that is capable of maintaining anatomic alignment and lordosis of two adjacent vertebrae during the spinal fusion process; 
         [0018]    It is still another object of the present invention to provide a spinal fusion implant that is self stabilizing within the spine; 
         [0019]    It is yet another object of the present invention to provide a spinal fusion implant that is capable of providing stability between adjacent vertebrae when inserted; 
         [0020]    It is still another object of the present invention to provide a spinal fusion implant that is capable of participating in the fusion process by containing, being composed of, or being treated with fusion promoting substances; 
         [0021]    It is further another object of the present invention to provide a spinal fusion implant that is capable of spacing apart and supporting adjacent vertebrae during the spinal fusion process; 
         [0022]    It is still further another object of the present invention to provide a spinal fusion implant that is consistent in use with the preservation of a uniform thickness of the subchondral vertebral bone; 
         [0023]    It is another object of the present invention to provide a spinal fusion implant having a shape which conforms to an easily produced complementary bore at the fusion site; and 
         [0024]    It is a further object of the present invention to provide a frusto-conical spinal fusion implant which may be placed side by side adjacent to a second identical implant across the same disc space, such that the combined width of the two implants is less than sum of the individual heights of each implant. 
         [0025]    It is a further object of the present invention to provide a frusto-conical spinal fusion implant which may be placed side by side adjacent to a second identical implant across the same disc space, such that the combined width of the two implants is less than sum of the individual lengths of each implant. 
         [0026]    These and other objects of the present invention will become apparent from a review of the accompanying drawings and the detailed description of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a side elevational view of the spinal fusion implant of the present invention having a body that is frusto-conical with an external thread having a substantially uniform radius. 
           [0028]      FIG. 1A  is an enlarged fragmentary view along line  1 A of  FIG. 1  illustrating the surface configuration of the implant of  FIG. 1 . 
           [0029]      FIG. 1B  is an enlarged fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration of the implant of the present invention made of a cancellous material. 
           [0030]      FIG. 1C  is a cross sectional view along lines  1 C- 1 C of  FIG. 1B  illustrating the alternative embodiment of the surface configuration of the implant of the present invention made of a cancellous material. 
           [0031]      FIG. 1D  is an enlarged fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration of the implant of the present invention made of a fibrous mesh-like material. 
           [0032]      FIG. 1E  is a fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration, of the implant of the present invention comprising a plurality of spaced apart posts. 
           [0033]      FIG. 1F  is an enlarged fragmentary sectional view along lines  1 F- 1 F of  FIG. 1E  illustrating the surface configuration of the implant of  FIG. 1E . 
           [0034]      FIG. 2  is an alternative embodiment of the spinal fusion implant of the present invention having a frusto-conical body with an external thread radius and thread height that are not constant. 
           [0035]      FIG. 3  is as cross sectional view along line  3 - 3  of the implant of  FIG. 2 . 
           [0036]      FIG. 4  is a side elevational view of an alternative embodiment of the spinal fusion implant of the present invention. 
           [0037]      FIG. 5  is a side elevational view and partial cut-away of a segment of the spinal column in lordosis showing the spinal fusion implant of  FIG. 4  being implanted with a driving instrument from the posterior approach to the spinal column. 
           [0038]      FIG. 6  is a side elevational view of an alternative embodiment of the spinal fusion implant of the present invention having a frusto-conical body and truncated sides. 
           [0039]      FIG. 7  is an end view along line  7 - 7  of the spinal fusion implant of  FIG. 6  shown placed beside a second identical implant shown in hidden line. 
           [0040]      FIG. 8  is a side elevational view of an alternative embodiment of the spinal fusion implant of the present invention having a body with an irregular configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Referring to  FIG. 1 , a side elevational view of the spinal fusion implant of the present invention generally referred to by numeral  20  is shown. The implant  20  has a body  22  that is frusto-conical in shape such that the body  22  has a diameter (root diameter) that is generally frusto-conical. The body  22  has an insertion end  24  and a trailing end  26 . The insertion end  24  may include a tapered portion  25  to facilitate insertion of the spinal implant  20 . In the preferred embodiment, when the implant  20  is inserted from the anterior aspect of the spine, the body  22  of the implant  20  has a maximum diameter at a point nearest to the trailing end  26  and a minimum diameter at a point nearest to the insertion end  24 . 
         [0042]    The implant  20  has an external thread  28  having a substantially uniform radius R 1  measured from the central longitudinal axis L 1  of the implant  20 . The outer locus of the external thread  28  (major diameter) has an overall configuration that is substantially parallel to the longitudinal axis L 1 . While the major diameter of the implant  20  is substantially uniform, the external thread  28  may be modified at the leading edge by having initially a reduced thread radius to facilitate insertion of the implant  20  and may also be modified to make the external thread  28  self-tapping. In the preferred embodiment, the external thread  28  has a first thread  30  of a lesser radius than the radius R 1  of the remainder of the external thread  28  to facilitate insertion of the implant  20 . The second thread  32  has a greater radius than the first thread  30 , but is still shorter than the radius R 1  of the remainder of the external thread  28  which is thereafter of constant radius. 
         [0043]    The body  22  is frusto-conical substantially along the portion of the body  22  in contact with the adjacent vertebrae of the spine which allows for creating and maintaining the adjacent vertebrae of the spine in the appropriate angular relationship to each other in order to preserve and/or restore the normal anatomic lordosis of the spine. The substantially uniform radius R 1  of the external thread  28  of the implant  20  allows engaging the bone of the adjacent vertebrae in a position that counters the forces which tend to urge the implant  20  from between the adjacent vertebrae in the direction opposite to which the implant  20  was implanted. The greater thread height measured from the body  22  near the leading end  24  of the implant  20  provides greater purchase into the vertebral bone and again enhances the stability of the implant  20 . Further, the configuration of the external thread  28  increases the surface area of the implant  20  in contact with the vertebrae to promote bone ingrowth. 
         [0044]    The implant  20  has a recessed slot  34  at its trailing end  26  for receiving and engaging insertion instrumentation for inserting the implant  20 . The recessed slot  34  has a threaded opening  36  for threadably attaching the implant  20  to instrumentation used for inserting the implant  20 . 
         [0045]    Referring to  FIG. 1A , the implant  20  has an outer surface  38  that is porous to present an irregular surface to the bone to promote bone ingrowth. The outer surface  38  is also able to hold fusion promoting materials and provides for an increased surface area to engage the bone in the fusion process and to provide further stability. The pores of the outer surfaces  38  are microscopic in size having a diameter that is less than 1 mm, in the range of 50-1000 microns, with 250-500 microns being the preferred diameter. It is appreciated that the outer surface  38 , and/or the entire implant  20 , may comprise any other porous material or roughened surface sufficient to hold fusion promoting substances and/or allow for bone ingrowth and/or engage the bone during the fusion process. The implant  20  may be further coated with bioactive fusion promoting substances including, but not limited to, hydroxyapatite compounds, osteogenic proteins and bone morphogenic proteins. The implant  20  is shown as being solid, however it is appreciated that it can be made to be substantially hollow or hollow in part. 
         [0046]    Referring to  FIG. 1B , an enlarged fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration  38  of the implant of the present invention made of a cancellous material is shown. The cancellous material  50 , similar in configuration to human cancellous bone, having interstices  52  such that the outer surface  38  has a configuration as shown in  FIGS. 1B and 1C . As the implant of the present invention may be made entirely or in part of the cancellous material  50 , the interstices  52  may be present in the outer surface  338  and/or within the entire implant to promote bone ingrowth and hold bone fusion promoting materials. 
         [0047]    Referring to  FIG. 1D , an enlarged fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration of the implant of the present invention made of a fibrous mesh-like material is shown. The mesh-like material  60  comprises strands  62  that are formed and pressed together such that interstices  64 , capable of retaining fusion promoting material and for allowing for bone ingrowth, are present between the strands in at least the outer surface  38  of implant of the present invention. 
         [0048]    Referring to  FIGS. 1E and 1F , a fragmentary view along line  1 A of  FIG. 1  illustrating an alternative embodiment of the surface configuration  38  of the implant of the present invention comprising a plurality of spaced apart posts  70  is shown. The posts  70  have a head portion  72  of a larger diameter than the remainder of the posts  70 , and each of the interstices  74  is the reverse configuration of the posts  72 , having a bottom  76  that is wider than the entrance to the interstices  74 . Such a configuration of the posts  70  and interstices  74  aids in the retention of bone material in the surface  38  of the implant and further assists in the locking of the implant into the bone fusion mass created from the bone ingrowth. As the bone ingrowth at the bottom  76  of the interstices is wider than the entrance, the bone ingrowth cannot exit from the entrance and is locked within the interstice  74 . The surface of the implant provides for an improvement in the available amount of surface area which may be still further increased by rough finishing, flocking or otherwise producing a non smooth surface. 
         [0049]    In the preferred embodiment, the posts  70  have a maximum diameter in the range of approximately 0.1-2 mm and a height of approximately 0.1-2 mm and are spaced apart a distance of approximately 0.1-2 mm such that the interstices  74  have a width in the range of approximately 0.1 to 2 mm. The post sizes, shapes, and distributions may be varied within the same implant. 
         [0050]    In the preferred embodiment, for use in the lumbar spine, the implant  20  has an overall length in the range of approximately 24 mm to 32 mm with 26 mm being the preferred length. The body  22  of the implant  20  has a root diameter at the insertion end  24  in the range of 8-20 mm, with 14-16 mm being the preferred root diameter at the insertion end, and a root diameter at the trailing end  26  in the range of 10-24 mm, with 16-18 mm being the preferred diameter at the trailing end  26 , when said implants, are used in pairs. When used singly in the lumbar spine, the preferred diameters would be larger. 
         [0051]    In the preferred embodiment, the implant  20  has a thread radius R 1  in the range of 6 mm to 12 mm, with 9-10 mm being the preferred radius R 1 . For use in the cervical spine, the implant  20  has an overall length in the range of approximately 10-22 mm, with 12-14 mm being the preferred length. The body  22  of the implant  20  has a root diameter at the insertion end  24  in the range of 8-22 mm, with 16-18 mm being the preferred root diameter at the insertion end when used singly, and 8-10 mm when used in pairs. The body  22  of the implant  20  has a root diameter at the trailing end  26  in the range of 10-24 mm, with 18-20 mm being the preferred root diameter at the trailing end  26  when used singly, and 10-12 mm when used in pairs; a thread radius, R 1  in the range of approximately 4-12 mm, with 9-10 mm being the preferred radius R 1  when inserted singularly and 5-7 mm when inserted side by side in pairs. 
         [0052]    Referring to  FIG. 2 , an alternative embodiment of implant  20  is shown and generally referred to by the numeral  120 . The implant  120  has a body  122  similar to body  122  of implant  120  and has an external thread  128  having a radius R 3  measured from the central longitudinal axis L 3  of the implant  120 . The thread radius R 3  is not constant throughout the length of the implant  120  and the external thread  128  has a thread height that is also not constant with respect to the body  122  of the implant  120 . In the preferred embodiment, the implant  120  has an external thread  128  with a radius R 3  that increases in size from the insertion end  124  to the trailing end  126  of the implant  120 . 
         [0053]    Referring to  FIG. 3 , a cross sectional view along line  3 - 3  of the implant  120  is shown. The implant  120  has an outer wall  144  surrounding an internal chamber  146 . The large and small openings  140  and  142  may pass through the outer wall  144  to communicate with the internal chamber  146 . The internal chamber  146  may be filled with bone material or any natural bone growth material or fusion promoting material such that bone growth occurs from the vertebrae through the openings  140  and  142  to the material within internal chamber  146 . While the openings  140  and  142  have been shown in the drawings as being circular, it is appreciated that the openings  140  and  142  may have any shape, size configuration or distribution, suitable for use in a spinal fusion implant without departing from the scope of the present invention. 
         [0054]    The openings  140  and  142  are macroscopic in size having a diameter that is greater than 1 mm. The large openings  140  have a diameter in the range of 206 mm, with the preferred diameter being 3.5 mm; and the small openings have a diameter in the range of 1-2 mm, with 1.5 mm being the preferred diameter. 
         [0055]    The implant  120  has a cap  148  with a thread  150  that threadably attaches to the insertion end  124  of the spinal fusion implant  120 . The cap  148  is removable to provide access to the internal chamber  146 , such that the internal chamber  146  can be filled and hold any natural or artificial osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material. Some examples of such materials are bone harvested from the patient, or bone growth inducing material such as, but not limited to, hydroxyapatite, hydroxyapatite tricalcium phosphate; or bone morphogenic protein. The cap  148  and/or the spinal fusion implant  120  may be made of any material appropriate for human implantation including metals such as cobalt chrome, stainless steel, titanium, plastics, ceramics, composites and/or may be made of, and/or filled, and/or coated with a bone ingrowth inducing material such as, but not limited to, hydroxyapatite or hydroxyapatite tricalcium phosphate or any other osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material. The cap  148  and the implant  120  may be partially or wholly bioabsorbable. 
         [0056]    Referring to  FIG. 4 , a side elevational view of an alternative embodiment of the spinal fusion implant of the present invention generally referred to by numeral  520  is shown. The implant  520  has a body  522  having a root diameter that is frusto conical in the reverse direction as that implant  20  shown in  FIG. 1 , in order to preserve and/or restore lordosis in a segment of spinal column when inserted from the posterior aspect of the spine. The body  522  has an insertion end  524  and a trailing end  526 . In the preferred embodiment, the body  522  of the implant  520  has a minimum diameter at a point nearest to the trailing end  526  and a maximum diameter at a point nearest to the insertion end  524 . The insertion end  524  may have an anterior nose cone portion  530  presenting a tapered end to facilitate insertion. 
         [0057]    The implant  520  has an external thread  528  having a substantially uniform radius R 6  measured from the central longitudinal axis L 6  of the implant  520  such that the external diameter of the external thread  528  (major diameter) has an overall configuration that is substantially parallel to the longitudinal axis L 6 . It is appreciated that the thread  528  can have a major diameter that varies with respect to the longitudinal axis L 6 , such that the major diameter may increase from the insertion end  524  to the trailing end  526  or the reverse. The external thread  528  has a thread height measured from the body  522  that increases from the insertion end  524  to the trailing end  526 . 
         [0058]    Referring to  FIG. 5 , a segment of the spinal column S is shown with the vertebrae V 1  and V 2  in lordosis and an implant  520  shown being inserted from the posterior aspect of the spinal column S with an instrument driver D. The implant  520  is inserted with the larger diameter insertion end  524  first in order to in initially distract apart the vertebrae V 1  and V 2  which then angle toward each other posteriorly as the implant  520  is fully inserted. It is appreciated that the insertion of implant  520  does not require the adjacent vertebrae V 1  and V 2  to be placed in lordosis prior to insertion, as the full insertion of the implant  520  itself is capable of creating the desired lordotic angular relationship of the two vertebrae V 1  and V 2 . 
         [0059]    In the preferred embodiment, for use in the lumbar spine, the implant  520  has an overall length in the range of approximately 24 m 30 mm, with 26 mm being the preferred length. The body  522  of the implant  520  has a root diameter at the insertion end  524  in the range of 12-22 mm, with 16 mm being the preferred root diameter at the-insertion end, and a root diameter at the trailing end  526  in the range of 10-20 mm, with 14 mm being the preferred diameter at the trailing end  526 . In the preferred embodiment, the implant  520  has a thread radius R 6  in the range of 6 mm to 12 mm, with 8 mm being the preferred radius R 6 . 
         [0060]    Referring to  FIG. 6 , an alternative embodiment of the spinal fusion implant of the present invention generally referred to by the numeral  620  and a partial fragmentary view of a second identical implant, generally referred to by the numeral  621  are shown. The implant  620  has a body  622  that is partially frusto-conical in shape similar to body  22  of implant  20  shown in  FIG. 1 , and has an insertion end  624  and a trailing end  626 . The body  622  of the implant  620  has truncated sides  670  and  672  forming planar surfaces that are parallel to the longitudinal axis L 7 . In this manner, two implants  620  and  621  may be placed side by side, with one of the sides  670  or  672  of each implant with little space between them, such that the area of contact with the bone of the adjacent vertebrae is maximized. It is appreciated that the body  622  may also be cylindrical in shape and have truncated sides  670  and  672 . 
         [0061]    The implant  620  has an external thread  628  having a radius R 6  measured from the central longitudinal axis L 7  that may be constant, such that the major diameter or outer locus-of the external thread  628  has an overall configuration that is substantially, cylindrical. It is appreciated that the external thread  628  may have a thread radius R 7  that is variable with respect to the longitudinal axis L 7  such that the major diameter or outer locus of the external thread  628  has an overall configuration that is substantially frusto-conical. 
         [0062]    Referring to  FIG. 7 , an end view of the implant  620  placed beside implant  621  is shown. The implant  620  has a thread radius that is substantially constant and has a thread height measured from the body  622  that is greater at the sides  670  and  672 . In this manner, two implants  620  and  621  can be placed beside each other with the external thread  628  of each implant interdigitated allowing for closer adjacent placement of the two implants as a result of the substantial overlap of the external thread  628  at the side  670  or  672  of the implants. 
         [0063]    Referring to  FIG. 8 , an alternative embodiment of the implant of the present invention is shown and generally referred to by the numeral  700 . The implant  700  is similar in configuration to implant  20  shown in  FIG. 1 , except that the body  722  has an irregular configuration. The configuration of the body  722  has a root diameter D which is variable in size throughout the length of the implant  700  and, as shown in this embodiment, comprises larger diameter portions  750  and smaller diameter portions  752 . It is appreciated that each of the large diameter portions  750  may be of the same or different diameter and each of the smaller diameter portions  752  may be of the same or different diameter. 
         [0064]    The outer surface of the body  722  of implant  720  may be filled with fusion promoting substances such that the smaller diameter portions  752  may hold such fusion promoting substances. If so filled, the composite of the implant  700  and the fusion promoting material could still produce an even external surface of the body  722  if so desired. 
         [0065]    While the present invention has been described in detail with regards to the preferred embodiments, it is appreciated that other variations of the present invention may be devised which do not depart from the inventive concept of the present invention. In particular, it is appreciated that the various teachings described in regards to the specific embodiments herein may be combined in a variety of ways such that the features are not limited to the specific embodiments described above. 
         [0066]    Each of the features disclosed in the various embodiments and their functional equivalents may be combined in any combination sufficient to achieve the purposes of the present invention as described herein.