Abstract:
A method and apparatus is provided for use in spinal fusion procedures. An interbody fusion device has a first piece that is a load bearing device designed to bear the axial loading from the end plates of adjacent vertebrae. A second piece of the interbody fusion device is a retention component whose primary functions are to prevent migration of the load bearing device and loss or migration of graft material from within the load bearing device. A secondary function of the retention component is to address fixation of fasteners when the surgeon is confronted with a challenging access to adequate boney structures due to excessive curvature/angulation of the vertebrae column, minimal invasive surgery techniques, danger to surrounding vascular or neurological tissues, poor bone quality, or similar surgical complications. A tertiary function of the retention component is to provide better alignment and stabilization of misaligned vertebrae when spondylolisthesis is a significant factor. One or more fasteners secure the retention component to each of the vertebrae above and below the load bearing device. The fasteners cause the end plates of the vertebrae to compress the end plates to the load bearing device to facilitate proper fusion.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/697,795 filed Sep. 6, 2012, which is incorporated herein by reference in its entirety as if fully set forth herein. This Application is a continuation-in-part of application Ser. No. 13/135,675 filed Jul. 12, 2011 and is a continuation-in-part of application Ser. No. 13/200,911 filed Oct. 4, 2011, each of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the field of spinal fusion. In particular, this invention is drawn to spinal fusion devices and associated methods. 
       BACKGROUND OF THE INVENTION 
       [0003]    The spine can be considered to be a series of movable segments made up of vertebrae and discs. Due to trauma, disease, and/or aging, the spine may be subject to degeneration. This degeneration may destabilize the spine and cause pain and/or nerve damage. Medical procedures are often required to either ease back pain, repair damage, or to prevent future damage. 
         [0004]    One procedure that is often used to treat back pain or spinal damage is spinal fusion. Spinal fusion is a surgical technique used to combine two or more adjacent vertebrae. Supplemental bone tissue is used in conjunction with the patient&#39;s natural osteoblastic processes in a spinal fusion procedure. Spinal fusion is used primarily to eliminate back pain caused by the motion of the damaged vertebrae by immobilizing adjacent vertebrae. Conditions for which spinal fusion might be done include degenerative disc disease, treatment of a spinal tumor, a vertebral fracture, scoliosis, degeneration of the disc, spondylolisthesis, or any other condition that causes instability of the spine. 
         [0005]    One problem with prior art spinal fusion techniques relates to device migration. For example, prior to complete bone fusion, a fusion device may migrate from the desired position. In examples where bone screws are used, the insertion and tightening of the bone screws tends to cause device migration. Another problem with typical prior art fusion techniques is that fusion devices, or associated plates or fasteners, protrude excessively from the spine, causing discomfort, damage, or danger to surrounding vascular or neurological tissues. A further problem with prior art fusion techniques also involves trajectories of screws that are used in the fusion process. 
         [0006]    Yet another problem with the prior art fusion techniques is the difficulty with placing supplemental fixation (i.e., plates and screws) due to gross anatomy constraints, surgical approach, vasculature and neurologic tissues and structures, and variable bone quality of the vertebrae. 
         [0007]    There is therefore a need for spinal fusion devices and related spinal fusion procedures that adequately treats degenerative disc disease and other spinal conditions, while providing improvements over the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    An apparatus of the invention provides a spinal fusion device including a fusion bearing component configured to fit between two adjacent vertebrae, and a retention component configured to be secured to at least one of the adjacent vertebrae to prevent migration of the fusion bearing component, wherein the retention component has a minimalist profile that reduces the potential to cause discomfort, damage, or danger to surrounding vascular or neurological tissues. 
         [0009]    One embodiment of an interbody fusion device includes a first piece configured to be placed between adjacent vertebrae, a second piece configured to at least partially fit within the first piece when the first piece is inserted between adjacent vertebrae, and one or more fastening devices for securing the second piece to at least one of the adjacent vertebrae. 
         [0010]    Another embodiment of the invention provides a spinal fusion device including a fusion bearing component configured to fit between two adjacent vertebrae, a retention component configured to prevent migration of the fusion bearing component and loss or migration of bone forming matrix placed in the fusion bearing component to promote fusion, wherein the retention component has a minimalist profile that reduces the potential to cause discomfort, damage, or danger to surrounding vascular or neurological tissues, one or more fasteners coupled to the retention component to compress the two adjacent vertebrae to the fusion bearing component. 
         [0011]    Another embodiment of the invention provides a method of fusing adjacent vertebrae including providing an interbody fusion device, inserting the interbody fusion device between two adjacent vertebrae, preparing the vertebral endplates for fusion, adding a bone forming matrix to promote fusion, providing a retention component configured to attach to the interbody fusion device, attaching the retention component to the interbody fusion device, and securing the retention component to at least one of the adjacent vertebrae. 
         [0012]    Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0014]      FIG. 1  is an isometric view of one example of an interbody fusion device in accordance with an embodiment of the invention; 
           [0015]      FIG. 2  is an exploded isometric diagram of the interbody fusion device shown in  FIG. 1  without the bone screws in accordance with an embodiment of the invention; 
           [0016]      FIG. 3  is an isometric diagram of the interbody fusion device shown in 
           [0017]      FIG. 1  installed between the end plates of two adjacent vertebrae; 
           [0018]      FIGS. 4-6  are isometric views illustrating the operation of a fusion device in accordance with an embodiment of the invention; 
           [0019]      FIG. 7  is an anterior-lateral exploded isometric view of an illustrative example of a load-bearing interbody fusion device in accordance with an embodiment of the invention; 
           [0020]      FIG. 8  is an isometric view of one example of an assembled load-bearing interbody fusion device in accordance with an embodiment of the invention; 
           [0021]      FIG. 9  is a side view of the load-bearing interbody fusion device in accordance with an embodiment of the invention; 
           [0022]      FIGS. 10A-10C  are isometric diagrams of the interbody fusion device in accordance with an embodiment of the invention; and 
           [0023]      FIG. 11  is an isometric view of an illustrative kit in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0024]    The present invention relates to spinal fusion implants and related spinal fusion procedures for use in cervical and lumbar applications. One type of spinal fusion is interbody fusion. Typically, an interbody fusion procedure places a bone graft between the vertebrae in the area normally occupied by an intervertebral disc. In preparation for a spinal fusion procedure, the intervertebral disc is removed. A device, typically containing a bone promoting matrix, such as allograph bone, may be placed between the vertebra to maintain spine alignment and disc height. Fusion then occurs between the endplates of the vertebrae. In some examples, fusion is augmented by a process called fixation, meaning the placement of screws, rods and/or plates to stabilize the vertebra to facilitate bone fusion. The present invention provides an interbody fusion device that overcomes problems found in the prior art, such as the angles of the screws, rods and/or plates that are used to stablize the vertebra. 
         [0025]    Generally, the present invention provides a two-piece interbody fusion device that may be used to perform anterior lumbar interbody fusion (ALIF). In one example, a first piece of the interbody fusion device is a U-shaped load bearing device that is designed to bear the axial loading from the end plates of adjacent vertebrae. A second piece of the interbody fusion device is a retention component, which may be attached to the U-shaped load bearing device, whose function is to prevent migration of the load bearing device and to prevent loss or migration of the bone forming matrix placed therein. One or more fasteners, such as bone screws secure the retention component to the vertebrae above and below the load bearing device. The fasteners cause the end plates of the vertebrae to compress the end plates to the load bearing device to facilitate proper fusion. If desired, the fasteners may include an anti-backout mechanism to prevent their migration. 
         [0026]      FIG. 1  is an isometric view of one example of an interbody fusion device of the present invention.  FIG. 1  shows an interbody fusion device  10 . The interbody fusion device  10  includes a load bearing device  12 , a retention component  14 , two bone screws  16 , and an anti-backout mechanism  18 , each of which are described in more detail below. The interbody fusion device comprises a hollow region  32  which can be filled with a prepared material such as a bone forming matrix to help facilitate fusion of the vertebrae. 
         [0027]      FIG. 2  is an exploded view of the interbody fusion device  10 , showing the load bearing device  12 , the retention component  14 , and the anti-backout mechanism  18  separately. The load bearing device  12  is a generally U-shaped device having an open end  20 . The open end defines an opening that allows access to the vertebrae end plates when the load bearing device is installed. The leading edges of the load bearing device  12  include holes or openings  22 , which are configured to receive pins  24  extending from the retention component  14 . The pins  24  properly align the retention component  14  with the load bearing device  12  and hold the retention component  14  in a desired position, relative to the load bearing device  12 . 
         [0028]    The load bearing device  12  also includes a plurality of ridges  30  formed on the top and bottom ends of the device  12 . The ridges  30  are angled and pointed in such a way that the ridges  30  help to hold the load bearing device  12  to the end plates of the vertebrae to reduce the chance of anterior migration of the implant. If desired, one or more openings (not shown) can be formed in the load bearing device  12  to facilitate the attachment of instrumentation devices. 
         [0029]      FIG. 2  also illustrates the components of the anti-backout mechanism  18 . The anti-backout mechanism  18  includes a locking plate  40 . The plate  40  has two opposing protrusions  42  that extend outward from the plate  40 . A set screw  44  is configured to extend through an opening formed in the plate  40 , and thread into the retention component  14 . A recess  46  is formed in the retention component  14  that is adapted to receive the locking plate  40 . The set screw  44  includes a head  48  that will shear off when enough torque is applied by a driver. By shearing off the head  48 , the surgeon will know that the set screw  44  is tight enough, and it will reduce the profile of the fusion device  10 . The retention component  14 , locking plate  40 , and set screw  44  can be pre-assembled, such that a surgeon will have a single piece that is attached to the load bearing device  12 . Once the bone screws are installed, the surgeon needs only to turn the set screw  44  with a driver to lock the bone screws in place. When the head  48  shears off, it will stay attached to the driver as the surgeon removes the driver from the patient. More details of the operation of the anti-backout mechanism  18  is described below. The set screw in this example includes a driver socket for receiving a driver, which may be used by a surgeon to tighten the set screw  44 . Of course, any desired type of anti-backout device may also be used. 
         [0030]    As described above, an interbody fusion device of the present invention is intended to be installed between the end plates of two adjacent vertebrae to facilitate the fusion of the vertebrae.  FIG. 3  is an isometric diagram of the interbody fusion device  10  shown in  FIG. 1  installed between the end plates of two adjacent vertebrae  50  and  52  to facilitate the fusion of the vertebrae  50  and  52 . The interbody fusion device  10  provides load bearing support as well as the proper spacing between the vertebrae  50  and  52  while fusion of the vertebrae takes place. As described in more detail below, the interbody fusion device  10  is positioned between the end plates of the vertebrae  50  and  52  within the vertebral body in the area usually occupied by the intervertebral disc. For clarity, the disc annulus is not shown, so the position of the load supporting device  12  can be seen. 
         [0031]    Following is an example of how an interbody fusion device of the present invention may be used in an ALIF spinal fusion procedure. As described above, a window is cut in the anterior side of the disc annulus to allow an interbody fusion device to be inserted. Next, the nucleus pulposus is cleaned out to provide room for the interbody fusion device. Next, a load bearing device  12  of the desired size (e.g., having a height to get the desired spacing between the vertebrae and surface area to maximize coverage of the endplates) is inserted between the end plates of the adjacent vertebrae using the appropriate instrumentation. Once the surgeon is satisfied that the load bearing device is in the desired position, the end plates can be prepared using the appropriate instruments (e.g., burrs, gouges, curettes, etc.). Next, the space between the endplates and within the load bearing device can be filled with a material that will help to facilitate fusion. Next, the retention component  14  is coupled to the load bearing device  12 , while aligning the pegs  24  with the holes  22 . Note that, because the height of the retention component is less than the height of the load bearing device, the retention component  14  can be put in place without interfering with the relative placement of the load bearing device  12  and the end plates of the adjacent vertebrae. Also, the retention component  14  is stress shielded and is not axial loaded by the vertebrae. Once the retention component is in place, the bone screws  16  can be installed through the openings  34  and into the vertebrae. As the bone screws  16  are tightened, the vertebrae will compress vertebral bodies  50  and  52  onto the load bearing member  12 , which will help facilitate fusion. Also, since the bone screws  16  secure the retention component  14 , and do not directly secure the load bearing device  12 , the bone screws will not tend to cause the interbody fusion device  10  to migrate. Next, the anti-backout mechanism  18  is engaged to prevent the bone screws  16  from loosening. As is described in detail below, the surgeon can turn the set screw  44  with driver until the head  48  sheers off. The protrusions  42  of the locking plate  40  will then be positioned over the ends of the bone screws  16 , preventing the screws  16  from backing out. 
         [0032]    The interbody fusion device of the present invention can be made from any desired materials. In one example, the load bearing device is made from PEEK® (or a similar material), bone, metal, or any other structural substitute. If the components of the interbody fusion device are radio-lucent (such as with PEEK®), then doctors will be able to monitor the fusion process better with X-rays. If desired, one or more radio opaque markers can be embedded into the interbody fusion device, which will show up in an X-ray. Since the positions of the markers are known relative to the fusion device, a doctor can determine the position of the fusion device in an X-ray by viewing the positions of the markers. 
         [0033]    An interbody fusion device of the present invention may be configured to any desired size or shape. In one example, load bearing devices can be provided in multiple thicknesses, allowing a surgeon to select a desired size (e.g., 8.0 mm, 10.0 mm, 12.0 mm, 14 mm, etc.). In the examples shown in the figures, the load bearing device has about 5° of lordosis. Of course any desired angle could be used. 
         [0034]      FIGS. 4-6  are isometric view illustrating the operation of the anti-backout mechanism described above.  FIG. 4  shows the interbody fusion device  10  after the bone screws have been installed. Note that the position of the protrusions  42  of the locking plate  40  are such that the openings  34  are not obstructed, allowing a surgeon to install the bone screws  16 . As mentioned above, the retention component  14  can come pre-assembled with the anti-backout mechanism  18  in the position shown in  FIG. 4 . Once the bone screws are in place, the surgeon can use a driver to turn the set screw  44  (see  FIG. 6 ).  FIG. 5  shows the interbody fusion device  10  after the set screw  44  has been turned. In this example, the set screw turned about  90  degrees until the protrusions  42  obstruct the heads of the bone screws  16 . When the locking plate  40  is in this position, the bone screws cannot come out. As shown in  FIGS. 4 and 5 , the recess  46  has multiple depths. In this example, the recess has a first depth (shown at  46 A) and a second deeper depth shown at  46 B. As the locking plate  40  is turned, the locking plate will drop from the recess  46 A and seat into the deeper recess  46 B. When the locking plate  40  is seated within the deeper recess  46 B ( FIGS. 5 and 6 ), the shape of the recess  46  will tend to prevent the locking plate  40  from turning the other way. When the surgeon applies the appropriate amount of torque to the set screw  44 , the head  48  of the set screw  44  will sheer off, eliminating the need for a torque wrench. This also lessens the profile of the implant. If the implant has to be removed in the future, a surgeon can use a driver and loosen the set screw  44  until the protrusions  42  no longer obstruct the bone screws  16 . 
         [0035]    Another embodiment of this invention is depicted in  FIGS. 7-9 , which are particularly adapted for fusion of L 2 -L 5  and S 1  and in particular L 4 /L 5  and L 5 /S 1 , which create challenging access for the surgeon to place screws in the endplates. Due to sharper curve/angle of the vertebrae column, the spinal fusion device embodiment  110  shown in  FIGS. 7-9  provide for multiple, constrained and/or non-constrained angles of entry for the fasteners, e.g., a 0 degree angle through 40 degrees on a challenging access retention component  114 . This increased range of angles is provided by the lips  116 ′,  116 ″ and the various thru-bores  134  of the challenging access retention component  114 , which abut the respective vertebrae&#39;s Apophyseal Rims. That is, the Apophyseal Rim approach allows for a decreased angle of the fasteners into the bone. 
         [0036]    The embodiment of this invention  110  depicted in  FIGS. 7-9  includes a fusion bearing component  112 , a challenging access retention component  114 , and from two to six fasteners  115 . The challenging access retention component  114  has an upper lip  116 ′ and lower lip  116 ″, each having thru-bore  134 , that abut the Apophyseal Rim during use and permit the fasteners  115  access to the anterior vertebral body to secure the spinal fusion device  110  into place. The face of the challenging access retention component  114  includes from two to four additional bores that permit the fasteners  115  access to the Apophyseal Rim to secure the spinal fusion device  110  into place. The challenging access retention component  114  also includes a threaded bore  136  configured for receipt of an anti-backout locking mechanism  118 . The locking mechanism covers all screw bores in one motion when deployed. 
         [0037]      FIG. 7  is an anterior-lateral exploded isometric view of another example of an interbody fusion device of the present invention.  FIG. 7  shows an interbody fusion device  110 . The interbody fusion device  110  includes a load bearing device  112 . The load bearing device  112  also includes a plurality of ridges  130  formed on the top and bottom surfaces  121 ,  123  of the device  112 . The ridges  130  are angled and come to a point in such a way that the ridges  130  help to hold the load bearing device  112  to the end plates of the vertebrae to reduce the chance of anterior migration of the implant. The load bearing device  112 , also having recessed features  122  on the open-face ends  125  of the U-shaped body, configured to receive and capture mating attachment features  124  provided on a challenging access retention component with a minimalist anterior profile  114 , and an anti-backout mechanism  118  comprising a shaped locking plate  140  having multiple protrusion or wings  142  and locking set screw  144  with a shear-off head  148 , each of which are described in more detail below. 
         [0038]    The challenging access retention component  114  and load bearing device  112 , when put together to form an interbody fusion device  110 , form a hollow body  132 . The hollow body  132  provides a relatively large graft volume, compared to a typical ALIF allograft. After insertion of the load bearing device  112  between adjacent vertebrae, but before placement of the retention component  114 , the endplates are debrided and prepared for fusion, and the hollow body  132  can be filled with a prepared material that will help to facilitate fusion of the vertebrae. Examples of a material include allograft bone, bone marrow, bone morphogenetic protein (BMP), Autologous Stem Cells, etc., to facilitate fusion through opening  132 . Following placement of graft material, the challenging access device  114  is positioned and attached to the load-bearing fusion device. The retention component will maintain the graft material in place within the load-bearing fusion device, in addition to providing a compressive load to the load-bearing fusion device with the application of bone fasteners, such as bone screws inserted into the adjacent vertebrae. 
         [0039]    In the example shown in  FIG. 8 , at least two holes  134  are formed in the retention component  114 , and are adapted to receive fasteners, such as bone screws  115 , pegs, etc. In the example shown in  FIGS. 7 and 8 , at least one of the holes  134  is angled down, and at least one other hole  34  is angled up, such that a first fastener  115  can be secured to the vertebra  50  above the interbody fusion device  110 , and a second fastener  115  can be secured to the vertebra  52  below the interbody fusion device  110  (described in more detail below). 
         [0040]      FIG. 8  is an isometric diagram of the interbody fusion device  110  shown in  FIG. 7  installed between the end plates of two adjacent vertebrae  50  and  52  to facilitate the fusion of the vertebrae  50  and  52 . The interbody fusion device  110  provides load bearing support as well as the proper spacing between the vertebrae  50  and  52  while fusion of the vertebrae takes place. As described in more detail below, the interbody fusion device  110  is positioned between the end plates of the vertebrae  50  and  52  within the vertebral body in the area usually occupied by the intervertebral disc. The load bearing device  112  is a generally U-shaped device having an open end ( 120  in  FIG. 7 ) that is configured to receive a challenging access retention plate component  114 . In the example shown in  FIG. 7 , a single opening is formed on the anterior side of the load bearing device  112 . An implant holder can be used to insert the load bearing device  112  into a vertebral body using the openings  120 . 
         [0041]    The hollow U-shaped load bearing device  112  provides a relatively large graft volume, compared to a typical ALIF allograft. Prior to insertion of the challenging access retention component  114 , the endplates of the vertebral bodies can be prepared for fusion as described in previous examples, then filled with a prepared material that will help to facilitate fusion of the vertebrae. Examples of a material include allograft bone, bone marrow, bone morphogenetic protein (BMP), Autologous Stem Cells, allogenic bone, growth factors and synthetic bone substitutes to facilitate fusion through opening  120 . 
         [0042]    Still referring to  FIG. 8 , a challenging access retention component  114  is provided with a anterior surface  117  and superior and inferior flanges  116 ′ and  116 ″, having a minimalist extended profile, symmetrically placed in the middle of the retention component, protruding above and below and anterior to the Apophyseal Rim  153  and  154  of each vertebra, above and below the load bearing device  110 , each flange  116 ′ and  116 ″ having a thru-hole  134  to accommodate a fastener  115 , such as a screw. Additionally the medial and lateral aspects of the challenging access retention component  114 , approximately 50% of the anterior surface, are zero profile, not extending beyond the anterior plane of the vertebral bodies. Further, there are provided at least two thru-holes  134  to accommodate fasteners  115 , such as bone screws that would allow penetration of the superior and inferior vertebral endplates  150  and  152  such that they would cause the vertebral bodies to compress the load-bearing interbody fusion device  112 , thus promoting fusion. The challenging access retention component  114  is particularly useful when anatomic structures or general anatomy restrictions make it difficult for a surgeon to gain an adequate approach angle to properly deliver and tighten the fastener(s)  115  into one or more endplate(s) locations of the vertebrae. Hence providing alternate fixation locations such as the Apophyseal Rim  153  and  154 , or anterior surface of the vertebra, just above the Apophyseal Rim. Clearance provided by any of the thru-bores  134  would be adequate to allow for constrained and/or non-constrained angulations of the screws  115 . This would be desired to assure that the screws could be placed at any reasonable angle allowed by the constrained and/or non-constraining thru-bores  134 , into the vertebral body  50 ,  52  or endplate(s)  150 ,  152 , and also allow for eventual compressive settling of the vertebrae during fusion bone formation, without placing strain on the challenging access retention component  114  itself. 
         [0043]    Still referring to  FIG. 8 , the challenging access retention component  114  is provided with an anti-backout mechanism  118  comprising a shaped locking plate  140  having multiple protrusion or wings  142  and locking set screw  144  with a shear-off head  148 , configured to simultaneously cover all fastener thru-holes  134 , wherein said shear-off head  148  is designed to shear off of the locking set screw  144 , near flush to the surface of the challenging access retention component  114 , when a pre-determined tightening torque is applied to the screw. The set screw in this example includes a driver socket for receiving a driver, which may be used by a surgeon to tighten the set screw  144 . Of course, any desired type of anti-backout device may also be used. 
         [0044]      FIG. 9  illustrates a side (lateral) view of the interbody fusion device  110 , with the challenging access retention component  114 , fasteners  115 , and anti-backout locking mechanism  118 , illustrating the minimalist dimensional features of the challenging access retention component  114  and flanges  116 ′,  116 ″, and angular insertion range of the fasteners  115  in various thru-bore positions. The dimensions shown are only illustrative and not intended to represent an upper or lower limit for sizing or angulations. 
         [0045]      FIGS. 10A-10C  are isometric diagrams of the illustrative interbody fusion device  110 , comprising a load-bearing fusion device  112 , the challenging access retention component  114  utilizing an anti-backout mechanism  118  in various stages between pre-assembly ( FIG. 10A ) and final assembly ( FIG. 10C ) (fasteners not shown, for clarity). As can be seen again in assemblies  10 B and  10 C, with the vertebral bodies removed, the superior surface of the challenging access retention component  138  (inferior surface  137 —not shown) are equal to or less than the mating anterior height of the load-bearing fusion component  121  (inferior surface  123 —not shown), at or about the location where the interbody fusion device assembly  110 , or more specifically the superior and inferior surfaces of the assembled interbody fusion device  121 ,  123 ,  138 ,  137  near the interior face  135  of the challenging access retention component  114 , would interface with the Apophyseal Rim  153 ,  154  and anterior aspects of the adjoining vertebral bodies  50 ,  52 . 
         [0046]      FIG. 11  is an isometric view of an illustrative tray or caddy  210  to hold and present a Challenging Access Kit  200  for the various sizes and configurations of the challenging access retention components  114 , various sizes and variations of anti-backout mechanisms  118 , and available sizes and lengths of fasteners  115 . (Load-bearing fusion devices  112 , having multiple sizes, heights and/or widths and lordosis would be in a separate, but similar illustrative tray or caddy). 
         [0047]    Each of the embodiments of the challenging access retention component has a low or minimal profile anteriorly. The locking mechanisms illustrated herein are not bound to any particular configuration and thus a given lock mechanism of a given embodiment in a  FIG. 7  can be used in other embodiments. 
         [0048]    Following is an example of how an interbody fusion device of the present invention may be used in an ALIF spinal fusion procedure. As described above, a window is cut in the anterior side of the disc annulus to allow an interbody fusion device to be inserted. Next, the nucleus pulposus is cleaned out to provide room for the interbody fusion device  110 . Next, a load bearing component  112  of the desired size (e.g., having a height and lordosis necessary to achieve the desired spacing between the vertebrae) is inserted between the end plates of the adjacent vertebrae using the appropriate instrumentation. During these procedures, the endplates within the confines of the load bearing component are then scraped to promote a bleeding bed. A desired bone grafting material  54  is then placed in the hollow space  132  in the center of the load bearing fusion component  112 . Once the surgeon is satisfied with placement of the graft material and that the load bearing component is in the ideal position, the challenging access retention component  114  is inserted and mated with the load bearing component  112 . Note that, because the height of the challenging access retention component  114  is less than or equal to the height of the load bearing device, the challenging access retention component  114  can slide into the anterior vertebral space created by the load bearing component  112  without interfering with the relative placement of the load bearing component  112  and the end plates of the adjacent vertebrae  50 ,  52 . Also, the challenging access retention component  114  is stress shielded and is not axial loaded by the vertebrae  50 ,  52 . Once the challenging access retention component  114  is in place, the bone screws  115  can be installed through any of the thru-bore openings  134  and into the vertebrae  50 ,  52 , preferably placing at least one fastener  115  into the cephalad vertebra  50 , and a second fastener  115  into the caudal vertebra  52 . As the bone screws  115  are tightened, they will compress vertebral bodies  50  and  52  onto the load bearing member  112 , which will help facilitate fusion. Also, since the bone screws  115  secure the challenging access retention component  114 , and do not directly secure the load bearing component  112 , the bone screws will not tend to cause the assembled interbody fusion device  110  to migrate. If desired, an anti-backout mechanism  118  (such as that described in  FIG. 7 ) can be used to prevent the bone screws  115  from loosening. 
         [0049]    The interbody fusion device of the present invention can be made from any desired materials. In one example, the load bearing device is made from PEEK® (or a similar material), bone, metal, or any other structural substitute. In one example, the retention component is made from PEEK® (or a similar material), bone, metal, or any other structural substitute. If the components of the interbody fusion device are radio-lucent (such as with PEEK®), then doctors will be able to monitor the fusion process better with X-rays. 
         [0050]    An interbody fusion device of the present invention may be configured to any desired size or shape. In one example, load bearing devices can be provided in multiple thicknesses, allowing a surgeon to select a desired size (e.g., 8.0 mm, 10.0 mm, 12.0 mm, 14.0 mm, etc.). In the examples shown in the figures, the load bearing device has about 5° of lordosis. However, any desired angle could be used. 
         [0051]    In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof and locations of use within the spine. Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.