Abstract:
An apparatus and method for implanting an intervertebral cage containing a bone graft and fusing adjacent vertebrae together while maintaining or correcting the angular alignment and balance of the spine. The invention provides a greatly improved method for anterior interbody fusion to form a fused bone segment having a predetermined fixed angular orientation. The apparatus has a cage unit adjustably coupled to an expansion cap, and has a wedge which may be adjusted to support the adjacent vertebrae with facing surfaces at a predetermined angle relative to each other. The cage unit is fenestrated and hollow, to receive a packed, harvested bone graft. A connecting bolt may be threaded or fixed to the rear of the cage unit. In certain embodiments, the cage unit and expansion cap are interlocking. Also in certain embodiments, especially utilizing round cages, the expansion cap may also include upper and lower horizontal bone-supporting surfaces and an anterior recess for receiving fasteners. A pair of independently adjustable cage units is fixedly intercoupled by a link.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a Division of U.S. Ser. No. 09/552,743, filed Apr. 18, 2000, having the same title, now U.S. Pat. No. ______ 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention is generally directed to an apparatus and method for implanting an anterior installed intervertebral fusion cage system which can be selectively expanded anteriorly between two adjacent vertebrae to cause them to change position relative to each other and produce a normal alignment of the spine, while promoting fusion of the vertebrae. More particularly, the invention discloses an apparatus and method for surgically positioning an implant having a fusion cage and one or more alternative expansion caps which may be intercoupled with the cage to cause expansion of the anterior portion of the cage to form an adjustable wedge for alignment of two adjacent vertebral bodies in accordance with a predetermined and desired spinal curvature.  
           [0003]    The implant of the present invention preferably presents an anterior surface which is flush or slightly recessed within the intervertebral joint, so that it does not abrade or otherwise injure surrounding tissues. In certain embodiments the device further includes structure for supporting a substantial portion of the front of the implant against a layer of harder, more compact bone at the anterior surface of the vertebrae in order to reduce the likelihood of subsidence of the device into the bone. Adjacent cages between a pair of vertebrae are preferably linked transversely to provide additional stabilization of the vertebrae.  
           [0004]    The spine is a column of stacked vertebrae, each having a rounded, anterior element, or vertebral body which is weight-bearing. The vertebral bodies are separated from each other and cushioned by a series of fibrocartilage pads or discs which impart flexibility to the spine. Aging, injury and disease, such as degenerative disc disease, may result in drying out or collapse of the discs, causing back and leg pain. In some cases the disc or vertebra is damaged beyond repair or must be removed for medical reasons.  
           [0005]    While the spinal column appears to be straight when viewed from an anterior or posterior vantage point, when viewed laterally it is apparent that it is actually comprised of four curved regions. In some congenital conditions such as scoliosis and kyphosis, excessive curvature or other displacement of the spinal vertebrae of the spine occurs.  
           [0006]    Treatment of weakness, injury or improper curvature by removal of a disc and fusion of adjacent vertebral bodies (arthrodesis) has become relatively commonplace in recent years. More than 20,000 such interbody fusions of the lumbar region alone are now performed annually in the United States. Fusion of adjacent vertebral bodies is generally accomplished by implantation of a cage-like device in the intervertebral space. The cages are apertured, and include a hollow interior chamber which is packed with live bone chips, usually harvested from the patient&#39;s hip, less frequently from the leg, spine or ribs, or bone may be obtained from a bone bank. A bone substitute may also be employed. Following implantation, bone from each of the adjacent vertebrae grows through the apertures to fuse with the bone of the other vertebrae above and below the cage, thus stabilizing the area. The fusion process may take six to twelve months and it is desirable to stabilize both the vertebrae and the cages during the fusion process.  
           [0007]    Once the fusion cage has been inserted, the angular orientation of the top and bottom surface of each cage is of importance, because this orientation determines the fixed angular alignment of the facing surfaces of the two vertebrae upon fusion. The cervical and lumbar curves each present a region of normal anterior convexity and posterior concavity or physiological lordosis. There is a need for an implant which can be adjusted in situ to conform to and maintain lordosis of the segments involved in the fusion or adjusted to correct a preexisting deformity and to restore or initiate proper angular vertebral alignment along the spine.  
           [0008]    Like most other bones, the bones of the spine and in particular, the vertebral bodies, consist of a core of spongy, cancellous tissue surrounded by a rim of harder, more compact bone. One problem associated with the implantation of intervertebral fusion cages has been eventual subsidence of the cage into the softer or spongier bone that is normally on opposite sides of a disc following implant. However, there is an anterior crescent of harder bone close to the edge of the vertebral bodies. There is a need for an implant which can be installed to provide support along the full length of the upper and lower face of the implant cage, for positioning the cage against a substantial length of the harder, outer rim of bone to provide better anterior support.  
           [0009]    Normally, a pair of fusion cage implant devices are inserted into the area previously occupied by a disc in spaced relationship to each other. In order to provide lateral stability, it is desirable to link the two cages together. There is a need for the cages to be adjustable in situ to preserve or restore coronal, axial and sagittal alignment. It is also preferable that the cages be linked by a structure which is recessed within the intervertebral joint. When the cages are inserted into the anterior portion of the intervertebral space, any structure which projects beyond the anterior surface of the vertebral body may cause irritation or damage to the surrounding tissues and vasculature, especially major arteries that are located close to the spine, or to the ligaments and muscles along the spine.  
           [0010]    The apparatus and method of the present invention are specifically designed to provide both independent intervertebral implants and transversely linked pairs of implants, which can be selectively expanded anteriorly to conform the vertebrae to a desired angle of curvature of the affected spinal region while supporting the anterior margin of the adjacent vertebral bodies and to do so without abrading or damaging the surrounding tissues subsequent to insertion.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed to an apparatus and method for implanting an intervertebral cage containing a bone graft to allow for the fusing together of adjacent vertebrae, while maintaining or correcting the angular alignment of the spine. The invention provides an improved fusion cage that allows selective adjustment between adjacent vertebrae. The apparatus includes a pair of cage units that have tops and bottoms and are each adjustably coupled to an expansion cap, such that the top and bottom form a wedge which may be adjusted to support the adjacent vertebrae at a predetermined angle. The cage is formed of a resilient material and is generally U-shaped including a pair of legs connected by a rear plate. The expansion cap is urged, normally by a bolt threaded to the rear plate to wedge between and, thus, separate the free or anterior ends of the legs to a desired angular configuration.  
           [0012]    The cage unit is fenestrated and hollow so as to receive a packed, harvested bone graft or bone substitute material. Alternatively, the connecting bolt may be fixed to the rear of the cage unit and the cap driven by rotating a nut on the bolt. The cage unit and expansion cap may be configured for self-locking engagement. The expansion cap may also include anterior upper and lower horizontal bone supporting structure and an anterior recess. A pair of adjustable cage units is fixedly intercoupled by a recessed link.  
           [0013]    A set of caps is provided with each cap producing a different expansion so that a surgeon may select the cap best suited to provide the desired angular configuration between adjacent vertebrae. The caps are also configured to provide additional end plate support along a substantial portion of the front edge of the vertebral bodies.  
         OBJECTS AND ADVANTAGES OF THE INVENTION  
         [0014]    The principal objects of the present invention are: to provide an improved method and apparatus for fusing together adjacent vertebrae; to provide such a method and apparatus for implanting an intervertebral fusion cage system for introducing a bone graft between adjacent vertebrae; to provide such a method and apparatus for implanting an intervertebral fusion cage system while maintaining or correcting the angular alignment of the vertebrae of the spine; to provide a method and apparatus for implanting an intervertebral dual cage system; to provide such a method and apparatus for adjustment of the alignment and balance of the spine in situ; to provide such a method and apparatus for especially engaging along a substantial length thereof the anterior, hard and compact bone layers of adjacent vertebral bodies; to provide such an apparatus having an intervertebral cage which is adjustable in situ; to provide such an apparatus having two such independently adjustable intervertebral cages; to provide such an apparatus having two intervertebral cages joined by a fixed link and that can be inserted non-parallel to each other (either in toe in and toe out or skew) and/or biased to provide better purchase to the overall system; to provide such an apparatus having two such intervertebral cages joined by a link which is recessed from the anterior surfaces of the adjacent vertebrae; to provide such an apparatus having a set of expansion caps that each provide a different degree of expansion to allow for variation in the angular configuration between the top and bottom of the cage or alternatively provides a cap that is adjustably coupled with the fusion cage for adjustment of the angle between facing surfaces of two vertebral bodies; to provide such an apparatus having an expansion cap and cage having structure permitting self-locking installation of the expansion cap onto the cage; to provide such an apparatus wherein the cages are round for insertion, but having caps with upper and lower generally linear support regions for engaging the anterior, more compact and hard bone layers of vertebrae; to provide such a fusion cage which includes an interior chamber for supporting a bone graft; to provide such a fusion cage having a group of modular or interchangeable caps with each cap producing a different degree of relative angulation between the top and bottom surfaces of the cage with the caps being usable sequentially and interchangeably to increase the expansion and resulting angulation until the surgeon is satisfied with the result; to provide such a fusion cage which is fenestrated to permit outgrowth of a bone graft into the surrounding vertebrae; to provide such an apparatus having an insertion tool which may be coupled with a fusion cage and uncoupled following insertion of the cage into an intervertebral region; to provide a method for using such an apparatus for implanting a cage unit between two adjacent vertebral bodies, packing the cage unit with a bone graft, coupling the cage unit with an expansion cap for forming the cage unit into a wedge having a predetermined angle associated with each cap between top and bottom surfaces thereof, and permitting the bone graft to grow and fuse the adjacent vertebral bodies together; providing such an apparatus and method which are relatively easy to use, inexpensive to produce and particularly well-suited for their intended usage.  
           [0015]    Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the invention.  
           [0016]    The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a partially exploded perspective view of an anterior expandable spinal fusion cage apparatus in accordance with the present invention, illustrating a pair of cages, a pair of expansion bolts and a linked expansion cap unit.  
         [0018]    [0018]FIG. 2 is a fragmentary front elevational view of a pair of adjacent vertebrae of a patient with the fusion cage apparatus implanted between the vertebral bodies and showing the expansion cap unit secured to the fusion cages.  
         [0019]    [0019]FIG. 3 is a cross-sectional view of one cage and expansion cap of the apparatus, prior to final assembly with one of the bolts positioned through the illustrated expansion cap preparatory to engagement with a threaded bore in a rear wall of the cage.  
         [0020]    [0020]FIG. 4 is a cross-sectional view similar to FIG. 3, illustrating the expansion cap in an expansion configuration in the fusion cage, taken along line  4 - 4  of FIG. 1.  
         [0021]    [0021]FIG. 5 is an exploded perspective view at a reduced scale showing an insertion tool aligned with a cage unit of the invention.  
         [0022]    [0022]FIG. 6 is a fragmentary perspective view showing the tool of FIG. 5 coupled with the cage unit and positioned in the intervertebral region between adjacent vertebrae during implantation of the cage unit, with portions of vertebra broken away to show detail thereof.  
         [0023]    [0023]FIG. 7 is a side elevational view of a cage unit between a pair of adjacent vertebrae at a further reduced scale and showing the cage unit of FIG. 6 in place in the intervertebral space and the insertion tool uncoupled and removed.  
         [0024]    [0024]FIG. 8 is an enlarged front elevational view of a first modified embodiment of a single implant in accordance with the invention.  
         [0025]    [0025]FIG. 9 is a cross-sectional view of the apparatus of FIG. 8, illustrating one of a set of expansion caps secured to a fusion cage, taken along line  9 - 9  of FIG. 8.  
         [0026]    [0026]FIG. 10 is an enlarged, fragmentary side elevational view of the expansion cap of FIG. 9.  
         [0027]    [0027]FIG. 11 is a cross-sectional view of the cage of FIG. 9 coupled with a second of the set of extension caps configured to provide less anterior vertical height than the cap shown in FIG. 9.  
         [0028]    [0028]FIG. 12 is a greatly enlarged, fragmentary side elevational view of the expansion cap of FIG. 11.  
         [0029]    [0029]FIG. 13 is a cross-sectional view of the cage unit of FIG. 8 coupled with a third of the set of expansion caps configured to provide less anterior vertical height than the cap shown in FIG. 11.  
         [0030]    [0030]FIG. 14 is an enlarged, fragmentary side elevational view of the expansion cap of FIG. 13.  
         [0031]    [0031]FIG. 15 is an exploded perspective view of a second modified embodiment of a fusion cage apparatus in accordance with the invention, illustrating a cylindrical fusion cage with a fixed stud, an expansion cap, a face plate and nuts.  
         [0032]    [0032]FIG. 16 is a cross-sectional view of the apparatus of FIG. 15, preparatory to final installation of the expansion cap with respect to the cage, taken along line  16 - 16  of FIG. 15.  
         [0033]    [0033]FIG. 17 is a cross-sectional view similar to FIG. 16, illustrating vertical expansion of a front of the cage produced by installation of the expansion cap.  
         [0034]    [0034]FIG. 18 is an exploded perspective view of a third modified embodiment of a fusion cage apparatus in accordance with the invention, illustrating a cage, an expansion cap and a bolt prior to installation.  
         [0035]    [0035]FIG. 19 is a front elevational view on a reduced scale of the cage of FIG. 18.  
         [0036]    [0036]FIG. 20 is a cross-sectional view of the cage of FIG. 19, taken along line  20 - 20  of FIG. 19.  
         [0037]    [0037]FIG. 21 is a rear elevational view of the expansion cap of FIG. 18.  
         [0038]    [0038]FIG. 22 is a cross-sectional view of the expansion cap, taken along line  22 - 22  of FIG. 18.  
         [0039]    [0039]FIG. 23 is a fragmentary diagrammatic view of a spinal column showing the cage of FIG. 18 implanted with the expansion cap prior to final assembly on the cage.  
         [0040]    [0040]FIG. 24 is a view similar to FIG. 23, illustrating the expansion cap assembled onto the cage to urge the top and bottom of the cage to form a wedge which engages the adjacent vertebrae and positions the vertebrae in proper physiological alignment.  
         [0041]    [0041]FIG. 25 is an enlarged exploded perspective view of a fourth modified embodiment of a fusion cage apparatus in accordance with the invention, illustrating an apparatus having a fusion cage and expansion cap configured for self-locking.  
         [0042]    [0042]FIG. 26 is a front elevational view on a reduced scale of the cage of FIG. 25.  
         [0043]    [0043]FIG. 27 is a cross-sectional view of the fusion cage of FIG. 25, taken along line  27 - 27  of FIG. 26.  
         [0044]    [0044]FIG. 28 is a rear elevational view of the expansion cap of FIG. 25.  
         [0045]    [0045]FIG. 29 is a cross-sectional view of the expansion cap of FIG. 25, taken along line  29 - 29  of FIG. 28.  
         [0046]    [0046]FIG. 30 is a fragmentary diagrammatic view of a spinal column showing the fusion cage of FIG. 25 implanted with the expansion cap prior to expansion.  
         [0047]    [0047]FIG. 31 is a view similar to FIG. 30, illustrating the expansion cap assembled on the cage and locking structures of the cage and expansion cap in mating engagement and with the cage expanded to form a wedge which supports the adjacent vertebrae in proper physiological alignment.  
         [0048]    [0048]FIG. 32 is a perspective view of a pair of the implanted cages as depicted in FIG. 31, illustrating a cage link prior to assembly.  
         [0049]    [0049]FIG. 33 is a perspective view of the cages and cage link of FIG. 32 subsequent to final assembly.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0050]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0051]    I. Dual Cage System With Fixed Link  
         [0052]    Referring now to the drawings, an anterior expandible spinal fusion cage system in accordance with the invention is generally indicated by the reference numeral  1  and is shown in FIGS.  1 - 6 . An anterior view of a human spine showing the intervertebral region  2 , which is the functional location of implantation of the fusion cage system  1 , between upper and lower adjacent vertebral bodies or vertebrae  3  and  4 , is shown in FIG. 2.  
         [0053]    The expandible fusion cage system  1  broadly includes a pair of substantially identical, anteriorly inserted and anteriorly expandable cages or implants  10  and  11  coupled with a cap unit or expansion module  12  by a pair of set screws or bolts  13  and  14 . The description “anteriorly expandable” is used to indicate that anterior ends  15  (FIG. 4) of the cages  10  and  11  are expandable rather than posterior ends  16  thereof  
         [0054]    Each of the implants  10  and  11  presents a generally truncated cylindrical overall configuration that is generally U-shaped when viewed from the side, having a horizontal central axis A extending the length thereof. An open-sided central chamber  20  is defined by a pair of spaced apart curvate top and bottom walls or legs  21  and  22 , each having an outer surface  23  and  24 . The walls  21  and  22  are apertured by a plurality of radial ports or windows  30 , which open into the central chamber  20 . The outer surfaces  23  and  24  include partial threads  31  which are interrupted by the windows  30 .  
         [0055]    The top and bottom walls  21  and  22  are coupled in spaced relationship by an enclosed rear wall, plate or web  32  having a central, threaded bore  33  and relieved corners. A front portion  34  of each of the cages  10  and  11  includes upper and lower margins  40  and  41  framing inwardly curved, upper and lower neck portions  42  and  43 , each terminating at a shoulder  44  and  45 . Each cage front  34  opens into an associated central chamber  20 .  
         [0056]    The cages  10  and  11  are designed with curvate or arcuate top and bottom walls  21  and  22  so that the cages  10  and  11  can be received in respective cylindrical grooves, which are predrilled into the inferior and superior surfaces, respectively of the pair of adjacent vertebral bodies  3  and  4 . Those skilled in the art will appreciate that the cages may also be of a more generally rectangular configuration for implantation by tapping into the intervertebral region  2 , or they may be constructed in any other geometric configuration which is suitable for implantation in an intervertebral region  2 .  
         [0057]    The expansion module  12  includes a pair of identical rectangular expansion caps or wedges  50  and  51  intercoupled in parallel alignment by a generally rectangular link  52 . The link  52  is preferably recessed a distance of from about one to about five millimeters from faces  53  of the expansion caps in order to maintain an overall flush anterior profile of the implanted cage system  1 . Those skilled in the art will appreciate that in certain forms the link  52  may also connect the caps  50  and  51  at a slightly convergent or divergent angle (that is the axis of the cages  10  and  11  may toe in or converge or toe out and diverge from the anterior side or may even be skewed relative to each other), such that when the cages  10  and  11  are installed at corresponding angles, the cages  10  and  11  will be more difficult to disturb and also preferably provide a slight loading or bias to the cages  10  and  11  during tightening of the caps  50  and  51  to further stabilize the intervertebral cage system  1 .  
         [0058]    The link  52  is sized to maintain the implants at a selected spacing, to enhance lateral stability and to permit a bone graft to grow from the chamber  20  outwardly, through the windows  30  and into the central portion of the intervertebral region  2 , to fuse the vertebral bodies  3  and  4  together.  
         [0059]    The expansion caps  50  and  51  each present a generally rectangular, planar face  53  having a central aperture  54 , which includes a conical countersink  55  to permit flush installation of the bolts  13  and  14  having correspondingly shaped heads  62  into the caps  50  and  51 . The expansion caps  50  and  51  are of unitary construction, each including a wedge  60  having a generally frustotriangular cross section coupled with a base  61  having a generally rectangular cross section. The expansion cap bases  61  are sized for insertion between the upper and lower margins  40  and  41  at the front of each of the cages  10  and  11 . A beveled geometric configuration of the wedge  60  permits sliding engagement of the wedge  60  with surfaces of the necks  42  and  43  of the cages  10  and  11 , which force the walls or legs  21  and  22  apart as the base  61  is snugged against the implant shoulders  44  and  45 , which serve as stops.  
         [0060]    The bolts  13  and  14  are sized and shaped to be received in the expansion cap apertures  54 , with a screw head  62  received against the expansion cap countersink  55 . Each screw also includes a shank  63  of reduced diameter and terminating in a threaded surface  64 , which is operably received in a respective cage matingly threaded bore  33 . Each screw head  62  also includes an opening  70  configured to receive a driving tool such as a wrench, screwdriver or the like (not shown).  
         [0061]    The cages  10  and  11 , expansion module  12 , and bolts  13  and  14  are constructed of a strong, inert material having some elasticity such as a stainless steel or titanium alloy, although carbon fiber, porous tantalum or any other biocompatible material or combination of materials may be employed.  
         [0062]    An insertion tool  71  for use in association with certain embodiments of the invention is depicted in FIGS.  5 - 7  includes a handle  72  coupled with a centrally bored shank portion  73  and a bolt  74  sized for registry within the bore of the shank  73 . The handle  72  is centrally apertured for insertion of the bolt  74  therethrough and through the bored shank  73 . The bolt  74  includes a hex type head  75  at one end and a threaded surface  76  at the opposed end. The portion of the shank  73  remote from the handle  72  is expanded to correspond to the diameter of the implant cage  10 . A pair of opposed grooves  80  are machined into the expanded shank  73 , leaving corresponding opposed lands  81  so that the shank  73  is sized and shaped to slidably but snuggly mate with the fusion cage  10 . The lands  81  include threads  82 , which correspond to the threads  31  of the top and bottom walls  21  and  22  of the cage  10 .  
         [0063]    In use, the anterior surface of a selected intervertebral region  2  of the spine of a patient is surgically exposed. The soft tissues are separated, the disc space is distracted and the disc is removed, along with any bone spurs which may be present. The spaced upper and lower vertebral bodies  3  and  4  that are to be stabilized and fused are then anteriorly drilled between to form a pair of opposed cage receiving grooves  84  having fixed spacing and alignment predetermined to match the alignment of the cages  10  and  11  and the spacing of the expansion module  12 . One set of grooves  84  is depicted in FIG. 6, receiving one of the cages  10 . Although an anterior approach is preferred, it is foreseen that a posterior, or even lateral approach could also be employed. The grooves  84  are then threaded (not shown) to correspond with the threads  31  of the cages  10  and  11 .  
         [0064]    An implant insertion tool  71  is positioned adjacent a fusion cage  10  so that the cage top and bottom walls  21  and  22  are aligned with the grooves  80  in the tool. The tool  71  and the cage  10  are urged toward each other until the cage walls  21  and  22  are received in the grooves  80  and the tool threads  83  are in registry with the implant cage threads  31 , to form a continuously threaded surface as shown in FIG. 6.  
         [0065]    The bolt  74  is then inserted through the apertured handle  72  and advanced rearward until it contacts the threaded bore  33  in the rear wall of the implant  32 . A driving tool such as a socket wrench (not shown) is employed to rotate the bolt  74  until the threaded surface  76  of the bolt is matingly received in the bore  33 .  
         [0066]    A user then grasps the handle  72  and positions the tool  71  and intercoupled cage  10  adjacent the intervertebral bore  84 . The user rotates the handle  72  to drive the tool  71  and cage  10  into the bore  84 .  
         [0067]    When the cage  10  is properly positioned, a driving tool (not shown) is employed to rotate the bolt head  75  counter clockwise, while the cage  10  is immobilized, until the threaded surface of the bolt  76  is disengaged from the threads of the implant bore  33 . The insertion tool  71  is then removed from the intervertebral bore  84  and the cage  10  remains in place. This procedure is repeated for installation of a second cage  11  at a predetermined location spaced from the first cage  10 . Although the curvate outer surfaces  21  and  22  of the cages  10  and  11  are particularly well suited for such threaded insertion into a predrilled intervertebral set of grooves  84 , it is foreseen that they may also be inserted either by tapping into a predrilled set of grooves  84  or by tapping directly into the distracted intervertebral region  2 .  
         [0068]    As best shown in FIGS. 3 and 4, the expansion module  12  is installed anteriorly, onto the cages  10  and  11  by alignment of the base  61  of each expansion cap  50  and  51  between a respective upper and lower cage margins  40  and  41 . A respective set screw or bolt  13  or  14  is inserted through the aperture  54  of each expansion cap  50  and into the threaded bore  33  in the rear wall  32  of the cages  10  and  11 .  
         [0069]    The bolts  13  and  14  are then tightened to bring the rear surfaces of the base  61  of each expansion cap  51  into sliding engagement with the upper and lower implant neck portions  42  and  43 . Continued tightening of the bolts  13  and  14  causes each base  61  to wedge the front portions of the top and bottom cage walls  20  and  21  apart, so that the cages  10  and  11  each begin to assume a generally trapezoidal shape when viewed from the side. The bolts  13  and  14  are further tightened until the rear surface of each expansion cap base  61  contacts each respective upper and lower shoulder  44  and  45 , which cooperatively serve as a stop. In this manner, the shoulders  44  and  45  serve to prevent greater distraction of the disc space or region than is desired.  
         [0070]    The expansion caps  50  and  51  are sized so that, upon coupling with the cages  10  and  11 , they form a wedge which supports the vertebral bodies  3  and  4  at the proper height as well as a desired angular alignment to achieve physiological lordosis at the intervertebral region  2 . While expansion caps  50  and  51  of a selected size are depicted in FIGS.  1 - 4 , those skilled in the art will appreciate that caps producing varying degrees of expansion may be employed to produce the desired effect.  
         [0071]    The surgeon then transplants a quantity of packed bone cells or a suitable bone substitute material or bone growth enhancer into each of the chambers  20 , as well as into the area  2  between the implant cages  10  and  11 . The bone cells may be introduced into the chambers  20  by a lateral approach through the open area between the top and bottom implant walls  21  and  22 . Alternatively, the bone cells may be introduced into the chambers  20  by an anterior approach through the implant front  34  prior to installation of the expansion module  12  or by a combination of these methods. Bone for use in the graft may be preferably harvested from the patient as live bone, from a bone bank or from a cadaver. Demineralized bone matrix, bone morphogenic protein or any other suitable material may also be employed.  
         [0072]    Following implantation, the bone grows between vertebrae  3  and  4  through the windows  30  with the bone in the chambers  30  and between and around the cages  10  and  11  to fuse the vertebral bodies  3  and  4  together.  
         [0073]    II. Alternate Fusion Cage System  
         [0074]    The structure of a first modified embodiment of an anterior expandable spinal fusion cage system in accordance with the invention is shown in FIGS.  8 - 14  and is generally represented by the reference numeral  101 . The system  101  is in many ways similar to the embodiment previously described, except the expansion caps are not joined and the cages may be fitted with expansion caps of various sizes.  
         [0075]    In particular, the fusion cage system  101  includes a cage  102  which will normally be used in pairs between adjacent vertebrae as in the present embodiment, and a set of expansion caps, here including a large expansion cap  103 , an intermediate expansion cap  104  and a small expansion cap  105 , and a set screw or bolt  106 . Although only three caps  103 ,  104  and  105  are illustrated and described in this embodiment, it is foreseen that many different caps, each producing a different degree of expansion in cage  102 , may be incorporated in the set to allow the surgeon to achieve a desired degree of expansion and consequent positioning of the vertebrae relative to each other. Expansion caps are constructed of varying sizes in order to provide an implant system  101  to allow a surgeon to first try a cap producing less expansion and then, if the surgeon finds that the expansion resulting from the first cap is insufficient to produce a desired alignment between the adjacent vertebrae, to remove the first cap and insert one producing more expansion of the cage  102 . The process is repeated until the desired alignment between the vertebrae is achieved. Normally the surgeon would start with the cap providing the least expansion and then larger caps in order of size, if the first is insufficient. Expansion caps  103 ,  104  and  105  are depicted in FIGS. 11, 12 and  14 , as representative examples of a full range of possible sizes.  
         [0076]    The cage  102  presents a generally truncated cylindrical overall configuration that is generally U-shaped when viewed from the side, including an open-sided central chamber  111 , bounded by a pair of curvate top and bottom walls  112  and  113 . The chamber  111  is further enclosed by a rear wall  114 .  
         [0077]    The front portion  121  of the cage  101  includes upper and lower margins  122  and  123  framing inwardly curved upper and lower neck portions  124  and  125 , each portion terminating at a shoulder  131  and  132 . The cage front portion  121  opens into the central chamber  111 .  
         [0078]    The large, intermediate and small expansion caps  103 ,  104  and  105  are of unitary construction, each including a wedge-shaped head  133  having a generally frustotriangular configuration when viewed from the side, coupled with a base  134  having a generally trapezoidal configuration. An angle A is formed by the junction of the head  133  and base  134 . The size of the angle A generally conforms to the angle at the cage front  121 , but the alignment varies depending upon degree of expansion of the cage  102 .  
         [0079]    The rear surface of the expansion cap head  133 , which extends from base  134 , slidingly engages the surfaces of the implant neck  124  and  125 , forcing them apart until the base  134  rests against the shoulder stops  131  and  132 .  
         [0080]    In use, the fusion cage system  101  is implanted in a manner substantially similar to the embodiment previously described. Initially, the smallest expansion cap  105  is selected for coupling with an implant  102 . The bolt  106  is then tightened until the rear surface of the expansion cap base  134  contacts the upper and lower shoulders  131  and  132  and the rear surfaces of the expansion cap head  133  rests against the upper and lower neck surfaces  124  and  125 .  
         [0081]    In the set of caps depicted, the first cap  105  produces no expansion in the anterior portion of the cage  102 , but rather simply stabilizes the cage  102  where no expansion is needed. That is, the cage  102  upper wall  112  and lower wall  113  remain parallel after insertion. The surgeon then checks the alignment of the vertebrae and, if greater expansion is required, the first cap  105  is removed and the next larger cap  104  is inserted. The cap  104  causes the cage upper wall  112  and lower wall  113  to be nonparallel and wider to the front, see FIG. 11. If the surgeon is then satisfied with the alignment of the vertebrae, the cap  104  is left in place. If greater frontward expansion is required, the cap  104  is removed and the cap  103  is inserted. The cap  103  produces greater anterior expansion of the cage  102 , see FIG. 9, providing a wedge-shaped configuration of the cage  102  and thus angularly realigning the vertebrae above the cage  102  relative to those below the cage  102  to cause normal physiological lordosis.  
         [0082]    In particular, as is best shown in FIGS. 9 and 10, upon installation, the expansion caps  103  and  104  each cause the fusion cage  102  to form a generally trapezoidal configuration when viewed from the side. When used to expand, the larger the expansion cap, the greater the distance the anterior portions of the top and bottom walls  112  and  113  are wedged apart and the greater the angle associated with the intersection of planes passing through the faces of the adjacent vertebral bodies and the larger the central chamber  111  for receiving the bone graft. Thus, either by trial or by experience, the surgeon can adjust the angle of planes passing through the facing surfaces of adjacent vertebrae in situ to achieve a desired angular alignment of vertebrae for producing a desired curvature of the spine.  
         [0083]    III. Cylindrical Fusion Cage System With Fixed Screw  
         [0084]    A second modified embodiment of an anterior expandable spinal fusion cage system in accordance with the invention is generally represented by the reference numeral  201  and is shown in FIGS.  15 - 17  to include an expandable implant or fusion cage  202 , an expansion cap assembly  203  and a cover assembly  204 . The cage  202  has a generally open-sided cylindrical configuration, having a central axis C, and upper and lower walls  210  and  211 , discontinuously circumscribing a central chamber  212 . Each of the walls  210  and  211  is apertured by a plurality of radially aligned windows  220 . The walls  210  and  211  also each include partial threads  221 , which are interspaced by the windows  220 .  
         [0085]    The cage  202  has an enclosed rear wall  222 , which is perpendicularly coupled at the center with a post or stud  223 . The implant  202  has upper and lower front ends  230  and  231  coupled with upper and lower axially convergent beveled surfaces  232  and  233 . The front ends  230  and  231  open into the central chamber  212 .  
         [0086]    The post  223  is coaxial with axis C throughout the length of central chamber  212 , and includes a shank  240 , which terminates in a threaded surface  241 .  
         [0087]    The expansion cap  203  is generally frustoconical in shape and includes an axially converging circumscribing wall  242  intercoupling a rear wall  243 , and an outer, radially expanded face  244 . The rear wall  243  has an aperture  245  to receive the post  223 . The face  244  is sized and configured for registry with the implant upper and lower front ends  230  and  231  upon installation.  
         [0088]    The cover assembly  204  includes a generally lozenge-shaped cover plate  250  and a pair of retaining nuts  251  and  252 . The cover plate  250  includes upper and lower parallel linear or planar surfaces  253  and  254  and a central, generally circular recess  255  for receiving the nut  252 . The recess  255  serves to receive the nut  252  and prevent the nut  252  from projecting into the adjacent tissues, where it might cause irritation or damage. The center of the recess  255  includes an aperture  256 , for receiving the post  223 . It is foreseen that the cap  203  and cover assembly  204  may be manufactured as a single unit.  
         [0089]    In use, the fusion cage  202  of the cage system  201  is inserted into a predrilled threaded set of grooves forming a bore-like structure in and between adjacent vertebral bodies and a bone graft is introduced in much the same manner as the embodiments previously described. As best shown in FIG. 16, once the cage  202  is implanted, the expansion cap  203  is installed over the post  223 , so that the rear surface of the wall  242  rests against the front end surfaces  230  and  231 .  
         [0090]    A first nut  251  is threaded onto the threaded surface of the post  241  and is snugged against the rear wall of the expansion cap  243 , forcing the upper and lower walls  210  and  211  apart, so that the implant cage  202  assumes the generally wedge shape depicted in FIG. 17. The nut  251  is tightened until the rearward approach of the face ring rear wall  243  is stopped by contacting the front end surfaces  230  and  231 .  
         [0091]    The cover plate  250  is installed over the expansion cap by positioning the central aperture  256  over the post  223  and threading the second nut  252  onto the threaded surface of the post  241 . The nut  252  is tightened until the rear surface of the cover plate  250  is snug against the surface of the face ring  244 .  
         [0092]    Advantageously, the fusion cage system  201  is installed to a slightly inset depth between a pair of adjacent vertebrae such as partially illustrated vertebra  246 , so that the cover plate upper and lower horizontal surfaces  253  and  254  provide continuous horizontal support for the harder, anterior bone margins of the adjacent vertebral bodies. In this manner, the system  201  minimizes subsidence of the cage  202  into the bone  246 .  
         [0093]    IV. Rectangular Fusion Cage System With Anterior Support  
         [0094]    A third modified embodiment  301  of an anterior expandable spinal fusion cage system in accordance with the invention is shown in FIGS.  18 - 24  and includes a cage implant or fusion cage  302 , coupled with an expansion cap  303  by a bolt  304 . The cage  302  is generally U-shaped when viewed from the side and presents a generally rectangular configuration overall, having upper, lower and rear walls  310 ,  311  and  312  collectively defining an open-sided central chamber  313 . The walls  310  and  311  each have an outer surface  314  and  315 , respectively, and include an elongate central slot  320 , which extends lengthwise and opens into the central chamber  313 . The outer surfaces  314  and  315  each include a series of ridges  321 , which are interrupted by the slot  320 .  
         [0095]    The rear wall  312  includes a central, threaded bore  322 . The cage  302  has upper and lower front ends  330  and  331  and upper and lower beveled or slanted surfaces  332  and  333 .  
         [0096]    The expansion cap  303  is generally rectangular when viewed from the front, and includes a front face  340  perpendicularly joined with generally horizontal top and bottom walls  341  and  342  and planar sidewalls  343 . The sidewalls  343  converge inwardly and join with a generally square shaped rear wall  344 , having a central bore  350 . The bore  350  includes a conical countersink  351  to permit installation of the bolt  304 , flush with the rear wall  344 .  
         [0097]    The bolt  304  is sized to be operably received first by the expansion cap bore  350  and then through the matingly threaded rear wall bore  322 . The bolt  304  includes a head  352  and a shank  353 , which terminates in a threaded surface  354 . The bolt head  352  includes an opening  355  configured to receive a driving tool such as an Allen wrench (not shown).  
         [0098]    In use, the fusion cage system  301  is installed into an intervertebral region  360  of the spine  361  of a patient as shown in FIGS. 23 and 24. Anterior exposure of the intervertebral joint  361 , distraction of an affected disc  362  and preparation of the space between a pair of adjacent vertebral bodies  363  is performed as previously described.  
         [0099]    Because the rectangular configuration of the implant cage  302  is best suited to installation by tapping into the interbody space it is not necessary to drill between the adjacent vertebral bodies  363 . The implant cage  302  is inserted so that the front  323  is situated at a predetermined location which is slightly posterior to the outer bone margins  364  of the adjacent vertebral bodies  363 .  
         [0100]    The expansion cap  303  is installed anteriorly, onto the cage  302  by alignment of the sidewalls  343  between the upper and lower ends  330  and  331 . The bolt  304  is aligned with and operably received in the expansion cap bore  350  as well as the fusion cage bore  322 . A driving tool (not shown) is inserted into the opening  355  and employed to rotate the bolt  304  to cause the expansion cap sidewalls  343  to slidingly engage the upper and lower beveled surfaces  332  and  333  of the fusion cage  302 . Continued tightening of the bolt  304  biases the implant upper and lower walls  310  and  311  apart into a wedge shape. The bolt  304  is tightened until the cap face  340  is snugged against the upper and lower ends  330  and  331  of the fusion cage  302 . In this configuration, the horizontal top and bottom expansion cap walls  341  and  342  engage and abut against the outer bone margins of the vertebral bodies  364 . In this manner, the top and bottom walls  341  and  342  of the expansion cap provide continuous horizontal support for the harder, anterior margin of bone  364  of the adjacent vertebral bodies  363 .  
         [0101]    It is foreseen that the cage of the present embodiment may be utilized with cages of the type shown in the previous embodiment, including a set of caps producing different expansions, caps with linear or near linear vertebra end plate support and pairs of caps that are connected together by a cross link.  
         [0102]    V. Rectangular Fusion Cage System With Cross Link  
         [0103]    FIGS.  25 - 33  illustrate a fourth modified embodiment  401  of an anterior expandable spinal fusion cage system in accordance with the invention. The structure and function of the fourth embodiment  401  is in many ways similar to that of the embodiment  301  previously described, with the major distinction being that the system incorporates a cross linking feature.  
         [0104]    [0104]FIGS. 30 and 31 depict installation of the system  401  in a spinal column  402  having an intervertebral region  403 .  
         [0105]    The system  401  includes a pair of implant cages  411  and  412  and a pair of expansion caps  413  and  414  joined by a cross link  415 . The implants  411  and  412  are similar to the implant cage  302  of the previous embodiment in that each presents a generally rectangular cross section which is best suited for installation by tapping into the intervertebral region  403 .  
         [0106]    The implant cages  411  and  412  are generally U-shaped when viewed from the side, and each includes a top wall  421 , bottom wall  422 , and rear wall  423 , defining an open-sided central chamber  424  there between. The rear wall  423  includes a central bore  425  and the walls include a plurality of windows  426 , which open into the central chamber  424 .  
         [0107]    The implants  411  and  412  include upper and lower front ends  431  and  432 , which differ from those of the embodiment previously described in that each is stepped toward a channel or groove  433  and  434  formed in the top and bottom walls  421  and  422 , respectively. The upper and lower front ends  431  and  432  are coupled with beveled surfaces  435  and  436 .  
         [0108]    The expansion caps  413  and  414  are of identical construction and are similar to the expansion caps of the previous embodiment in that they are generally rectangular when viewed from the front, include a front face  441 , horizontal top and bottom walls  442  and  443 , convergent sidewalls  444  and a rear wall  445 . The expansion caps  413  and  414  differ from those previously described in that the horizontal top and bottom walls  442  and  443  each extend rearwardly to include top and bottom flanges  451  and  452  along the length thereof.  
         [0109]    The caps  413  and  414  include in each rear wall  445  a threaded bore  453  for receiving a bolt  454 , but do not include a countersink for recessing the bolt. The cross link  415  is generally U-shaped and includes a pair of apertures  455  and  456  for receiving the bolt  454  in feet  458  thereof.  
         [0110]    The modified apparatus  401  is installed by tapping a pair of implant cages  411  and  412  into an intervertebral region  403  in a predetermined, spaced relationship. A pair of expansion caps  413  and  414  is aligned over the cages  411  and  412  in a manner similar to that of the apparatus  401  of the previous embodiment. A connector link  415  is installed in overlapping relationship between the expansion caps  413  and  414 , so that each of the apertures  455  and  456  are in alignment with one of the bores  453 . The apertures and aligned bores  453  receive a pair of bolts  454 . Tightening advances the bolts  454  rearwardly and into the aligned bores  435  in the rear walls  423  of the cages  411  and  412 . The bolts  454  are tightened until the top flanges  451  and  452  of the expansion caps  413  and  414  are received into the upper and lower implant cage channels  443  and  444 , in mating engagement. In this manner, a pair of implant cages  411  and  412  are joined in spaced relationship at a predetermined angle and locked into place.  
         [0111]    It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.