Patent Publication Number: US-6706070-B1

Title: Multi-variable-height fusion device

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of Ser. No. 09/046,759 filed on Mar. 24, 1998 which is a continuation-in-part of Ser. No. 08/847,172 filed on May 1, 1997, now U.S. Pat. No. 6,045,579. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to methods and apparatus for promoting an intervertebral fusion, and more particularly to an apparatus for insertion into a space between adjacent vertebrae to facilitate an intervertebral fusion while maintaining a substantially natural lordosis of the human spine. 
     2. Description of the Related Art 
     Intervertebral discs that become degenerated due to various factors such as trauma or aging typically have to be partially or fully removed. Removal of an intervertebral disc can destabilize the spine, making it necessary to replace the vertebral disc to maintain the height of the spine and to restore stability. Spinal implants are often used to prevent collapse of the spine and promote fusion. U.S. Ser. No. 08/740,123 filed Oct. 24, 1996 relates to methods and apparatus for facilitating a spinal fusion and is incorporated by reference as if fully set forth herein. 
     After an intervertebral disc is removed, an implant device is typically inserted between neighboring vertebrae to maintain normal disc spacing and restore spinal stability, thereby facilitating an intervertebral fusion. A conventional implant device disposed between neighboring vertebrae is depicted in FIGS. 1 and 2. The implant device contains a pair of engaging elements  20  that typically contain threading  10  to engage the vertebrae. Prior to inserting the engaging elements, a vertebral drill is typically inserted within the surgical wound to drill into the cortical endplate and remove fibrous and nuclear material. A vertebral tap may then be used to cut threads into the ends of the neighboring vertebrae. The engaging elements tend to be relatively inflexible and substantially undeflectable. The engaging elements are typically packed with bone graft to facilitate a spinal fusion. 
     Conventional implant devices tend to not maintain the “lordosis” or natural curvature of the lower lumbar spine. As shown in FIG. 1, the implant device contains parallel engaging sides  12  and  13  to contact vertebra  15 . It is typically required that the engaging sides be parallel to prevent the fusion cage from slipping from the intervertebral space. The parallel configuration of the fusion cage tends to alter the lordosis of the spine. Such a loss of lordosis may result in an increased risk to other intervertebral discs located adjacent to the fusion level that may degenerate due to the altered force transmission in the spine. 
     FIG. 2 depicts a front view of the engaging elements  20  of the implant device. The engaging elements are substantially cylindrical and the region of contact between an engaging element and a vertebra is defined by arcuate portion  22 . The cylindrical geometry of the engaging elements tends to provide a relatively small area of contact between the fusion cage and the vertebrae. The weight of the spine creates pressure on the vertebrae that is concentrated proximate the arcuate portions. Subsidence or deformation of the cortical layer of the vertebrae tends to result. 
     U.S. Pat. No. 5,522,899 to Michelson relates to a spinal implant for placement into the spinal disc space to stabilize the spine and participate in a vertebra to vertebra bony fusion. U.S. Pat. No. 5,489,308 to Kuslich et al. relates to an implant for use in spinal stabilization that includes a cylindrical body having external threading and radially disposed openings positioned to chip bone into an interior portion of the body when the implant is installed. The above-mentioned patents are incorporated by reference as if fully set forth herein. 
     The above-mentioned prior methods and systems inadequately address, among other things, the need to maintain the natural lordosis of the spine. It is therefore desirable that an improved spinal implant be derived for facilitating an intervertebral body fusion. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a spinal implant is provided that largely eliminates or reduces the aforementioned disadvantages of conventional implant devices. An embodiment of the invention relates to a fusion device for facilitating an interbody fusion between neighboring vertebrae of a human spine. The fusion device preferably includes a pair of sides or engaging plates for engaging the vertebrae and an alignment device disposed between the engaging plates for separating the engaging plates to maintain the engaging plates in lordotic alignment. The alignment device is preferably adapted to adjust the height between the engaging plates to customize the fusion device to a particular patient. The height of the fusion device preferably varies along the length of the device such that the height proximate an anterior end of the device differs from the height proximate a posterior end of the device. 
     The engaging plates are preferably substantially planar so as to inhibit subsidence of the vertebrae. The engaging plates may contain protrusions extending from their outer faces for enhancing an engagement between the vertebra and the engaging plate. The protrusions may be adapted to extend into the vertebra. The engaging plates preferably include a plurality of openings to allow bone growth to occur through the engaging plates. The openings in the face of the engaging plates preferably have a total area that is between about 60 percent and about 80 percent of a total surface area of the face (including the area of the openings). 
     The fusion device may include a retaining plate proximate the posterior end that serves as a backing against which bone graft may be packed between the engaging plates. The fusion device may also include a removable end cap proximate the anterior end for maintaining bone graft between the engaging plates. 
     In an embodiment, the alignment device includes a first strut and a second strut that each extend between the engaging plates to define the height therebetween. The fusion device preferably includes a first side and a second side opposite the first side. The first strut preferably runs from the anterior end to the posterior end along a location proximate the first side, and the second strut preferably runs from the anterior end to the posterior end along a location proximate the second side. The engaging plates preferably include a pair of slots sized to receive ends of the struts. The slots may have a substantially dovetail-shaped cross-section that is conformed to the shape of the ends. Each slot is preferably tapered such that its width narrows in a direction from the anterior end to the posterior end whereby the width of the slot proximate the posterior end is less than the width of the end of the strut. The ends of the struts preferably have a lateral width that tapers in substantially the same manner as the slots such that a locking taper engagement is formable between the slots and the ends of the struts. 
     The height of each strut preferably varies along the length of the strut such that the height between the engaging plates differs between the anterior end and the posterior end to allow the lordosis of the spine to be maintained. The first and second struts may have differing heights to cause the height of the fusion device to vary along the device from the first side to the second side to correct for a lateral deviation in the spinal column. Each of the struts may include a hinge to allow an upper member of the strut to pivot with respect to a lower member of the strut. 
     In an alternate embodiment, the engaging plates include slots and the fusion device further includes a pair of pins disposed within the slots. Each engaging plate preferably includes a rib extending in a substantially perpendicular direction from its face. The slot for receiving the pins is preferably disposed on the rib. The pins are preferably substantially elongated and may extend in a direction from the first side to the second side. The fusion device preferably further includes a rotatable connector engaging the pins. Rotation of the connector preferably causes movement of the pins relative to one another to alter the height of the fusion device to create a desired lordotic alignment. 
     The connector is preferably adapted to move axially between the engaging plates and may contain a retaining ring for contacting an engaging plate to limit movement of the connector through the fusion device. The connector preferably moves axially between the engaging plates in a direction from the anterior end to the posterior end, thereby moving the first pin toward the anterior end and the second pin toward the posterior end to increase the height between the engaging plates. The connector may be a screw having a threaded portion. The first pin may include a threaded opening for receiving a threaded portion of the connector. The second pin may be connected to an unthreaded portion of the connector. 
     The pins preferably include a receiving section and an end. The ends of the pins are preferably sized to fit within the slots in the ribs of the engaging plates. The receiving section may have a width greater than that of the ends of the pins and preferably contains an opening for receiving the connector. 
     One engaging plate preferably includes a first slot that may terminate in an end that extends in a diverging direction from an end of another slot contained on the other engaging plate. Movement of one of the pins preferably draws the ends of the slots together to alter the amount of separation between the engaging plates. The movement of the pins relative to one another preferably alters the height proximate the anterior end at a faster rate than the height proximate the posterior end is altered to achieve a desired lordotic alignment. 
     In an alternate embodiment, the fusion device contains a load-sharing member to promote a spinal fusion. The load-sharing member may be axially disposed within the struts. The load-sharing member is preferably substantially deflectable to allow movement of one of the engaging plates when a compressive force is exerted on the engaging plates. A predetermined spacing preferably exists between the upper and lower members. Application of a compressive force onto the engaging plates preferably deflects the load-sharing member and decreases the predetermined spacing between the members, thereby decreasing the height of the strut. The deflection of the load-sharing member preferably imparts stress to bone graft proximate the engaging plates to promote the development and growth of bone in accordance with Wolff&#39;s law. 
     The load-sharing member may be a pin having a circular cross-section and preferably is disposed in a bore extending axially through the strut. The bore preferably has a greater width than that of the load-sharing member to provide space for deflection of the load-sharing member. The load-sharing member may serve as a hinge-pin about which the upper member of the strut pivots with respect to the lower member of the strut. 
     The fusion device preferably further includes a connector for engaging the load-sharing member to impart force to the load-sharing member to cause it to deflect. The strut may include a threaded opening in its end for receiving the connector. The predetermined spacing between the upper and lower members may be set to a desired length by altering the position of the connector in the opening in the end of the strut. The load-sharing member may include an indention having a substantially planar surface to provide a site for engagement with the connector. The connector preferably engages the load-sharing member at a fulcrum point located at a predetermined horizontal distance from a support location where the lower member of the strut contacts the load-sharing member. The material properties of the load-sharing member and the distance between the fulcrum point and the support location are preferably controlled such that the modulus of elasticity across the strut is substantially equal to the modulus of elasticity of bone. 
     In an alternate embodiment, the fusion device may include a bracket assembly separating the engaging plates and supporting the alignment device. The alignment device may include at least one screw coupled to at least one cam block. In the context of this description, “screw” refers generally to any elongated member having external threading. The cam block may include an opening through which the cam block is coupled to the screw. An inner surface of the opening may include threading that is complementary to the threading at an end of the screw. The threading of the screw and the threading of the cam block may form an engagement such that rotation of the screw in a first angular direction causes the cam block to move in a first lateral direction and such that rotation of the screw in an angular direction opposite the first angular direction causes the cam block to move in a lateral direction opposite the first lateral direction. 
     The inner face of each of the engaging plates may include sloped tracks. The cam block may include an upper surface and a lower surface. The surfaces of the cam block may be beveled or sloped to correspond to the slope of the tracks, such that the cam block fits into the tracks in the inner surfaces of the engaging plates. The surfaces of the cam block and the tracks in the inner faces of the engaging plates may be configured such that movement of the cam block toward the exterior of the fusion device increases the height between the engaging plates and such that movement of the cam block toward the interior of the fusion device decreases the height between the engaging plates. Alternatively, the surfaces of the cam block and the tracks in the inner faces of the engaging plates may be configured such that movement of the cam block toward the exterior of the fusion device decreases the height between the engaging plates and such that movement of the cam block toward the interior of the fusion device increases the height between the engaging plates. Alternatively, in embodiments in which the fusion device includes more than one screw, the fusion device may include cam blocks and tracks in the engaging plates incorporating each of the above mentioned design elements. 
     The alignment device may include a single screw coupled to a single cam block. Alternatively, the alignment device may include a pair of screws, each of which is coupled to a single cam block. The screws may be situated such that a first screw is substantially parallel to and substantially adjacent a first edge of the fusion device. A second screw may be situated substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge. Alternatively, the first screw may be substantially parallel to and substantially adjacent a first edge of the fusion device, and the second screw may be substantially parallel to and substantially adjacent a second edge of the fusion device adjacent to the first edge. Alternatively, the alignment device may include three screws, each of which is coupled to a single cam block, such that a first screw is substantially parallel to and substantially adjacent a first edge of the fusion device. A second screw may then be situated substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge, and a third screw may be situated substantially parallel to and substantially adjacent a third edge located substantially perpendicular to and substantially adjacent the first edge and the second edge. Alternatively, the first screw may be substantially parallel to and substantially adjacent a first edge of the fusion device, the second screw may be substantially parallel to and substantially adjacent a second edge of the fusion device, and the third screw may be substantially parallel to and substantially between the first and the second screws. 
     In an alternate embodiment, the alignment device may include at least one screw coupled to a cam block as previously described, and the inner face of each of the engaging plates may include sloped tracks as previously described. A tip of each of the screws may be substantially unthreaded. The alignment device may further include a stationary block positioned between the engaging plates. The stationary block may include openings into which the unthreaded tip of each of the screws may be inserted. The stationary block may support each of the screws and preserve the engagement between the screws and the cam blocks during use. 
     The alignment device may include two screws, each of which is coupled to a cam block. The screws may be aligned such that the screws are positioned at an angle with respect to one another. Alternatively, the screws may be aligned such that the screws share a common axis of rotation. Two screws are said to share a “common axis of rotation” if the spatial relation of the longitudinal axes of rotation of the two screws is such that the longitudinal axis about which a first screw may be rotated and the longitudinal axis about which a second screw may be rotated are defined by the same line, irrespective of the physical dimensions (e.g., the diameters) of the screws or the longitudinal separation between the screws. 
     Alternatively, the alignment device may include three screws, each of which is coupled to a cam block. A first screw may share a common axis of rotation with a second screw, and a third screw may be aligned substantially perpendicular to the first screw and the second screw. Alternatively, the first screw may be oriented substantially perpendicular to the second screw, and the third screw may be oriented substantially at a first obtuse angle to the first screw and substantially at a second obtuse angle to the second screw. Alternatively, the first screw may be oriented substantially at a first non-perpendicular angle to the second screw and substantially at a second non-perpendicular angle to the third screw. Alternatively, the first screw may be oriented substantially parallel to the second screw, with the third screw situated between and substantially parallel to the first screw and the second screw. 
     Alternatively, the alignment device may include four screws, each of which is coupled to a cam block. A first screw may share a first common axis of rotation with a second screw, and a third screw may share a second common axis of rotation with a fourth screw. The first screw and the second screw may be situated substantially parallel to and substantially adjacent a first edge of the fusion device. The third screw and the fourth screw may be aligned substantially parallel to the first screw and the second screw and substantially adjacent an edge of the fusion device opposite the first edge. Alternatively, the first screw may share a common axis of rotation with the second screw, and the third screw may share a common axis of rotation with the fourth screw. The third screw and the fourth screw may be aligned substantially perpendicular to the first screw and the second screw. Alternatively, the first screw may share a common axis of rotation with the second screw and the third screw may share a common axis of rotation with the fourth screw. The third screw and the fourth screw may be aligned at a substantially non-perpendicular angle to the first screw and the second screw. The fusion device may be configured such that its cross-section is substantially rectangular. The first screw and the second screw may then be aligned substantially along a first diagonal of the rectangle, and third screw and the fourth screw may then be aligned substantially along a second diagonal of the rectangle intersecting the first diagonal. 
     In an alternate embodiment, the alignment device may include at least one screw configured to be coupled to a pair of cam blocks configured as described above. The inner face of each of the engaging plates may include sloped tracks as previously described. The screw may include a substantially unthreaded portion having a first diameter and a substantially threaded portion having a second diameter greater than the first diameter. Alternatively, the screw may include a substantially unthreaded portion having a first diameter and a substantially threaded portion having a second diameter substantially equal to the first diameter. Each of the cam blocks may include an opening through which the cam block is coupled to the screw. The inner surface of the opening in a first cam block may be substantially unthreaded. The inner surface of the opening in a second cam block may include threading that is complementary to the threading of the screw. The bracket assembly may include projections into the interior of the fusion device and having inner surfaces having threading that is complementary to the threading of the screw. Rotation of the screw in a first angular direction may thus cause the screw to move in a first lateral direction with respect to the bracket assembly, and rotation of the screw in an angular direction opposite the first angular direction may thus cause the screw to move in a lateral direction opposite the first lateral direction. 
     The unthreaded opening in the first cam block may be substantially similar in diameter to the diameter of the unthreaded portion of the screw. The unthreaded portion of the screw may pass through the opening in the first cam block such that the screw is free to rotate within the opening in the first cam block. The screw may further include a flange coupled to the screw at a first end of the screw adjacent the unthreaded portion of the screw. The first end of the screw may include an indentation sized and shaped such that a tip of an adjusting tool may be inserted into the indentation. The adjusting tool may be a screwdriver. Preferably, the adjusting tool is an allen wrench. The adjusting tool may be used to rotate the screw. The diameter of the threaded portion of the screw and the diameter of the flange may be sufficiently larger than the diameter of the opening in the first cam block such that coupling is maintained between the first cam block and the screw as the screw is rotated (i.e., the screw remains inserted in the opening through the first cam block) and such that the first cam block is constrained to move laterally in the same direction as the screw when the screw is rotated. The threading of the screw and the threading of the second cam block may form an engagement between the second cam block and the screw such that rotation of the screw causes the second cam block to move in a lateral direction opposite the direction of lateral motion of the screw. 
     The alignment device may include a single screw coupled to a pair of cam blocks and situated substantially parallel to and substantially adjacent an edge of the fusion device. Alternatively, the alignment device may include a pair of screws that are each coupled to a pair of cam blocks. The screws may be situated such that a first screw is substantially parallel to and substantially adjacent a first edge of the fusion device. A second screw may be situated substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge. Alternatively, the first screw may be situated substantially parallel to and substantially adjacent a first edge of the fusion device and the second screw may be situated substantially parallel to and substantially adjacent a second edge of the fusion device adjacent to the first edge. Alternatively, the alignment device may include three screws, each of the screws being coupled to a pair of cam blocks, such that a first screw is substantially parallel to and substantially adjacent a first edge of the fusion device. A second screw may then be situated e substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge, and a third screw may be situated substantially parallel to and substantially adjacent a third edge of the fusion device located substantially perpendicular to the first edge and the second edge. 
     In an alternate embodiment, the screw in the alignment device may be a turnbuckle. In the context of this description, “turnbuckle” refers to a screw having external threading in a first direction at a first end and external threading in a direction opposite the first direction at a second end opposite the first end. The turnbuckle may be coupled to a pair of cam blocks via threaded openings in the cam blocks. An inner surface of each of the openings may include threading that is complementary to the threading at one of the ends of the turnbuckle. The threading of the turnbuckle and the threading of the cam blocks may form an engagement such that rotation of the turnbuckle in a first direction causes the cam blocks to move away from each other and such that rotation of the turnbuckle in a direction opposite the first direction causes the cam blocks to move toward each other. The inner face of each of the engaging plates may include sloped tracks as previously described. At least one end of the turnbuckle may include an indentation sized and shaped such that a tip of an adjusting tool may be inserted into the indentation. The adjusting tool may be a screwdriver. Preferably, the adjusting tool is an allen wrench. The adjusting tool may be used to rotate the turnbuckle. 
     The turnbuckle may include a middle portion disposed between the first end and the second end and having a thickness greater than a thickness of the first end and a thickness of the second end. The bracket assembly may include lateral projections into the interior of the fusion device. The middle portion of the turnbuckle may be configured to fit between the lateral projections from the bracket assembly. The lateral projections may include openings of a size sufficient to allow the ends of the turnbuckle to pass through without allowing the middle portion to pass through, thus maintaining the turnbuckle in the bracket assembly. 
     The alignment device may include a single turnbuckle coupled to a pair of cam blocks and situated substantially parallel to and substantially adjacent an edge,of the fusion device. Alternatively, the alignment device may include a pair of turnbuckles that are each coupled to a pair of cam blocks. The turnbuckles may be situated such that a first turnbuckle is substantially parallel to and substantially adjacent a first edge of the fusion device. A second turnbuckle may be situated substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge. Alternatively, the first turnbuckle may be substantially parallel to and substantially adjacent a first edge of the fusion device, and the second turnbuckle may be substantially parallel to and substantially adjacent a second edge of the fusion device adjacent to the first edge. Alternatively, the alignment device may include three turnbuckles that are each coupled to a pair of cam blocks, such that a first turnbuckle is substantially parallel to and substantially adjacent a first edge of the fusion device. A second turnbuckle may then be situated substantially parallel to and substantially adjacent a second edge of the fusion device opposite the first edge, and a third turnbuckle may be situated substantially parallel to and substantially adjacent a third edge of the fusion device located substantially perpendicular to the first edge and the second edge. 
     The above embodiments may be used independently or in combination. 
     An advantage of the invention relates to an intervertebral body fusion device that substantially maintains the natural lordosis of the human spine. 
     Another advantage of the invention relates to an intervertebral body fusion device adapted to correct a lateral deviation in the spinal column. 
     Anther advantage of the invention relates to an intervertebral body fusion device that substantially maintains the natural lordosis of the human spine while simultaneously being adapted to correct a lateral deviation in the spinal column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of the preferred embodiments and upon reference to the accompanying drawings in which: 
     FIG. 1 depicts a conventional intervertebral body fusion implant positioned between neighboring vertebrae; 
     FIG. 2 depicts another conventional intervertebral body fusion implant that includes a pair of cylindrical members positioned between neighboring vertebrae; 
     FIG. 3 depicts a top view of a fusion device located on a vertebral body; 
     FIG. 4 a  depicts a cross-sectional view of the fusion device of FIG. 3 taken along plane I; 
     FIG. 4 b  depicts a cross-sectional view of the fusion of FIG. 3 device taken along plane I wherein the fusion device contains bone graft and has been adjusted to maintain a substantially natural lordosis; 
     FIG. 5 depicts a front view of a fusion device; 
     FIG. 6 a  depicts a perspective view of a strut; 
     FIG. 6 b  depicts a side view of a tapered strut; 
     FIG. 7 depicts a top view of a fusion device; 
     FIG. 8 depicts a front view of a pair of engaging plates; 
     FIG. 9 depicts a front view of a fusion device having pivotable struts; 
     FIG. 10 depicts a top view of a fusion device containing a connector; 
     FIG. 11 depicts an anterior view of a fusion device having a connector and cam pins; 
     FIG. 12 depicts a cross-sectional view taken along plane III of FIG. 11 of the fusion device in a lowered position; 
     FIG. 13 depicts a cross-sectional view taken along plane III of FIG. 11 of the fusion device in a raised position; 
     FIG. 14 depicts a cross-sectional view taken along plane IV of FIG. 11 of the fusion device in a lowered position; 
     FIG. 15 depicts a cross-sectional view taken along plane IV of FIG. 11 of the fusion device in a raised position; 
     FIG. 16 depicts a side view of a fusion device disposed between vertebrae; 
     FIG. 17 depicts a top view of a strut having a tapered end; 
     FIG. 18 depicts a cross-sectional view taken along plane V of FIG. 17 of the strut in an unloaded position, 
     FIG. 19 depicts a cross-sectional view taken along plane V of FIG. 17 of the strut in a loaded position; 
     FIG. 20 depicts a top view of a fusion device located on a vertebral body; 
     FIG. 21 depicts a cross-sectional view of the fusion device taken along plane VI of FIG. 3; 
     FIG. 22 depicts a top view of a conventional fusion cage having a pair of cylindrical elements disposed on a vertebra; 
     FIG. 23 depicts a side view of one of the cylindrical elements in FIG. 22 disposed between neighboring vertebrae; 
     FIG. 24 depicts a front view of the cylindrical element in FIG. 23; 
     FIG. 25 depicts a perspective view of a fusion device in the lowered position, the fusion device including a pair of turnbuckles oriented perpendicular to the anterior and posterior edges of the fusion device; 
     FIG. 26 depicts a perspective view of the fusion device of FIG. 25 in the raised position; 
     FIG. 27 depicts an exploded view of the fusion device of FIG. 25; 
     FIG. 28 depicts a perspective view of a bracket assembly of the fusion device of FIG. 25; 
     FIG. 29 a  depicts a perspective view of the end of the bracket assembly of FIG. 25; 
     FIG. 29 b  depicts a perspective view of a cam block of the fusion device of FIG. 25; 
     FIG. 29 c  depicts a side view of the cam block of FIG. 29 b;    
     FIG. 30 a  depicts a cutaway view of the fusion device of FIG. 25 in a lowered position; 
     FIG. 30 b  depicts a cutaway view of the fusion device of FIG. 25 in a raised position; 
     FIG. 31 a  depicts a cutaway view of an alternative configuration of the fusion device of FIG. 25 in a lowered position; 
     FIG. 31 b  depicts a cutaway view of an alternative configuration of the fusion device of FIG. 25 in a raised position; 
     FIG. 32 a  depicts a top view of the alignment device of FIG. 25 in use; 
     FIG. 32 a  depicts a front view of the alignment device of FIG. 25 in use in a lowered position; 
     FIG. 32 c  depicts a front view of the alignment device of FIG. 25 in use in a raised position; 
     FIG. 33 depicts an exploded view of a fusion device including a pair of turnbuckles oriented parallel to the anterior and posterior edges of the fusion device; 
     FIG. 34 depicts a perspective view of the bracket assembly of the fusion device of FIG. 32; 
     FIG. 35 depicts an exploded view of a fusion device including a pair of screws, each of which is threaded through a pair of cam blocks, 
     FIG. 36 depicts a perspective view of the alternative bracket assembly of the fusion device of FIG. 34; 
     FIG. 37 a  depicts a cross-sectional view of the screws and cam blocks of FIGS. 35-36 in use in a first position; 
     FIG. 37 b  depicts a cross-sectional view of the screws and cam blocks of FIGS. 35-36 in use in a second position; 
     FIG. 38 depicts an exploded view of a fusion device including three turnbuckles; 
     FIG. 39 depicts a perspective view of the bracket assembly of the fusion device of FIG. 38; 
     FIG. 40 depicts an exploded view of a fusion device including three substantially parallel screws; 
     FIG. 41 depicts a perspective view of the bracket assembly of the fusion device of FIG. 40; 
     FIG. 42 depicts an exploded view of a fusion device including three non-parallel screws; 
     FIG. 43 a  depicts a perspective view of the bracket assembly of the fusion device of FIG. 42; 
     FIG. 43 b  depicts a cross-sectional view of a screw, a cam block, and the stationary block of FIG. 42; 
     FIG. 44 depicts an exploded view of a fusion device including four screws oriented as two parallel pairs; 
     FIG. 45 depicts a perspective view of the bracket assembly of the fusion device of FIG. 42; 
     FIG. 46 depicts an exploded view of a fusion device including four screws oriented in a “+” configuration; 
     FIG. 47 depicts a perspective view of the bracket assembly of the fusion device of FIG. 46; 
     FIG. 48 depicts an exploded view of a fusion device including four screws oriented in an “x” configuration; 
     FIG. 49 depicts a perspective view of the bracket assembly of the fusion device of FIG. 48; 
     FIG. 50 a  depicts a perspective view of a fusion device including one screw and at least one cam block in a lowered position; 
     FIG. 50 b  depicts a perspective view of the fusion device of FIG. 50 a  in a raised position; 
     FIG. 50 c  depicts an exploded view of the fusion device of FIG. 50 a;    
     FIG. 51 a  depicts a cutaway view of an embodiment of a bracket assembly of the fusion device FIG. 50 a;    
     FIG. 51 b  depicts a cross-sectional view of a turnbuckle of the bracket assembly of FIG. 51 a;    
     FIG. 51 c  depicts a cutaway view of an alternative embodiment of a bracket assembly of the fusion device of FIG. 50 a;    
     FIG. 51 d  depicts a cross-sectional view of a turnbuckle of the bracket assembly of FIG. 51 c;    
     FIG. 52 a  depicts a top view of the fusion device of FIG. 50 a  in use, and 
     FIG. 52 b  depicts a side view of a pair of the fusion devices of FIG. 50 a  in use. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of an interbody fusion implant device  10  for facilitating the formation of a spinal fusion is depicted in FIGS. 3-5. A top view of the fusion device is depicted in FIG.  3 . Fusion device  10  preferably includes a pair of sides or engaging plates  12  and  14  for engaging vertebral bodies  16  and  18 . The engaging plates may contain curved edges such that the outer face  15  of the plates conforms to the shape of the cross-section of the vertebral bodies as shown in FIG.  3 . The fusion device has a height  20  defined by the vertical distance between the outer faces  15  of the engaging plates  12  and  14 . The height  20  of the fusion device is preferably adjustable and may vary along the fusion device between anterior end  22  and posterior end  24  to maintain the natural lordosis of the spine. Height  20  may also vary along device  10  from first side  26  to second side  28  to correct for a lateral deviation in the spine as may occur in scoliosis. Fusion device  10  preferably further includes an alignment device for adjusting the height  20  so that the natural lordosis of the spine is substantially maintained after the fusion device is implanted. The alignment device may be used to adjust the height between the engaging plates proximate the anterior end and independently adjust the height between the engaging plates proximate the posterior end. 
     A spinal fusion is typically employed to eliminate pain caused by the motion of degenerative disk material. Upon successful fusion, fusion device  10  becomes permanently fixed within the disc space. The fusion device is preferably packed with bone graft  40  to promote the growth of bone through and around the fusion device. Such bone graft may be packed between engaging plates  12  and  14  prior to, subsequent to, or during implantation of the fusion device. Bone substitute material that is well known to those skilled in the art may be used instead of bone graft. A bone harvester kit, commercially available from Spine-Tech, Inc. located in Minneapolis, Minn., may be used to inject bone graft between the engaging plates. The pamphlet entitled “Bone Harvester: Minimally Invasive Bone Harvesting Kit” (available from Spine-Tech, Inc.) details the use of the bone harvesting kit. 
     In an embodiment of the invention, the faces  15  of engaging plates  12  and  14  contain a plurality of openings  34  disposed therein to allow bone development and growth through the engaging plates  12  and  14  and between fusion device  10  and neighboring vertebrae  16  and  18 . In an embodiment, the openings  34  have a combined area that is greater than about 50 percent of the area of face  15  (including the area of the openings  34 ), more preferably between about 60 percent and about 80 percent of the area of face  15 , and more preferably still about 70 percent or more of the area of face  15 . 
     The fusion device may contain a retaining plate  36  proximate posterior end  24  to provide a backing against which bone graft may be packed and to maintain the bone graft between the engaging plates. Retaining plate  36  may be substantially planar and may contain openings to allow bone ingrowth therethrough. A removable endcap  25  may be positioned proximate anterior end  22  to contain bone graft within the fusion device and to prevent the migration of bone graft outside the engaging plates. The endcap  25  may contain one or more openings for allowing bone ingrowth between a vertebral body and bone graft contained between the engaging plates. Endcap  25  is preferably made of a plastic material such as polyethylene that tends to be non-irritating and non-abrasive to the surrounding tissues. 
     A cross section of the fusion device taken through plane I of FIG. 3 is depicted in FIG. 4 a  and FIG. 4 b . FIG. 4 a  shows the relative position of engaging plates  12  and  14  before height  20  has been adjusted with an alignment device to achieve a substantially natural lordosis. FIG. 4 b  shows the relative position of the plates after height  20  has been adjusted and bone graft  40  has been packed between the engaging plates. FIG. 4 b  shows that height  20  is greater in the vicinity of anterior end  22  as compared to posterior end  24  to maintain the natural lordosis of the spinal column. The faces  15  of the engaging plates  12  and  14  are preferably planar to provide a relatively large contact area between the engaging plates and the neighboring vertebrae. In this manner, subsidence of the vertebrae may be prevented because the force imparted to the vertebrae from the fusion device is not concentrated across a relatively small area of the vertebrae as in some conventional implants. Alternately, the engaging plates may be non-planar. The engaging plates also preferably contain a plurality of spikes or protrusions  38  extending from the face  15  for enhancing an engagement between the vertebra and the engaging plate. The protrusions may extend into the vertebra to prevent the fusion device from moving out of the disc space. The engaging plates are preferably constructed of titanium or a titanium alloy, although it is to be understood that other materials (e.g., ceramics, metals, carbon composites) may be used. 
     A front view of the fusion implant device is depicted in FIG.  5 . In an embodiment of the invention, the alignment device includes a first strut  30  and a second strut  32  that each extend between engaging plates  12  and  13  along the length of the fusion device from anterior end  22  to posterior end  24 . As described herein, a “strut” is taken to mean any support member disposed between the engaging plates to separate the engaging plates. Strut  30  preferably extends along the fusion device proximate first side  26 . Strut  32  is preferably substantially parallel to strut  30  and may extend along the fusion device proximate second side  28 . The struts  30  and  32  serve to create a predetermined spacing between the engaging plates. The predetermined spacing is preferably such that the height  20  is approximately equal to the height of the disc material that formerly occupied the disc space between the vertebral bodies. 
     A perspective view of an embodiment of the strut is depicted in FIG. 6 a . The strut may have an “I-beam” shape and preferably includes a pair of ends  50 . The ends  50  may have a lateral width  51  that is greater than that of the sides  53 . The ends preferably have a “dovetail” shaped cross-section as shown in FIG. 6 a . The engaging plates preferably contain elongated slots  60  (shown in FIGS. 7 and 8) sized to receive ends  50  of the first and second struts. The slots  60  preferably have a complementary dovetail shape as depicted in FIG. 8 that conforms to the shape of the end  50 . The struts may be connected to the engaging plates by sliding ends  50  into the slots  60  in a direction from anterior end  22  to posterior end  24  or vice versa. 
     In an embodiment, the slots are tapered such that their width narrows in a direction from the anterior end to the posterior end as shown in FIG.  7 . The ends  50  may be tapered (as shown in FIG. 17) such that the lateral width  51  narrows along the length of the strut. The taper of the lateral width of the strut preferably matches that of slot  60 . The width of the slot proximate the anterior end is preferably sized to allow the strut end to be slid into the slot. The width of the slot proximate the posterior end is preferably less than the lateral width  51  of the narrowest portion of end  50 . The tapering of the slots preferably allows a “locking taper engagement” of the strut ends within the slots. A “locking taper engagement” is taken to mean a fixable interference fit formed between end  50  and slot  60  whereby the strut resists dislodgment when force is imparted to the fusion device from the adjacent vertebrae. In an alternate embodiment, the slots may be tapered such that the width of the slots narrows in a direction from the posterior end to the anterior end. 
     The first and second struts preferably each have a predetermined height that defines the height of the fusion device. The engaging plates  12  and  14  are preferably adapted to receive struts of various heights to allow height  20  to be varied to fit the needs of the patient. A side view of a tapered strut is depicted in FIG. 6 b . The tapered strut preferably has a height that varies along its length. In this manner, the tapered strut is positionable between the engaging plates  12  and  14  to cause height  20  to decrease in a direction from anterior end  22  to posterior end  24  whereby the natural lordosis of the human spine is maintained by the fusion device. The degree of taper of the strut corresponds to a desired lordosis and may vary depending upon the size of the patient. 
     In an embodiment, the first and second struts have differing heights to cause height  20  to vary between first end  14  and second end  16 . In this manner, the fusion device may be used to correct a lateral deviation in the spinal column as may occur in scoliosis. A front view of a fusion device containing struts having different heights is depicted in FIG.  9 . Each of the struts preferably contains a hinge pin  70  to allow an upper member  72  of the strut to pivot with respect to a lower member  74  of the strut. In this manner, the struts may be pivoted as shown in FIG. 9 such that the ends of the struts are properly aligned with the slots of the engaging plates when a height difference exists between the first and second struts. 
     To install the fusion device, a discectomy is preferably performed from an anterior approach. All cartilage and soft tissue are preferably removed from the vertebral endplate as would normally be done for placement of a femoral strut graft. Such a procedure is well within the knowledge of a skilled practitioner of the art. The engaging plates may be deployed in the disc space between the adjacent vertebrae. A distraction force may be applied to the engaging plates using a laminae spreader or similar device to force the vertebrae to a selected height and lordotic alignment. The use of a laminae spreader is well known to those skilled in the art. The proper heights for the first and second struts may be determined beforehand using x-ray techniques in which the posterior and anterior portions of the intervertebral disc space are examined. 
     Appropriately sized and tapered struts are preferably slipped into slots  60  and tapped until a locking taper engagement is achieved between the strut ends and the slots. If struts of differing heights are used to correct for a lateral deviation in the spinal column, each strut may be pivoted about hinge pin  70  prior to insertion so that ends  50  are properly aligned for placement into grooves  60 . Bone graft material is preferably inserted through the anterior end and packed between the engaging plates. Retaining plate  36  preferably prevents the bone graft material from passing through the fusion device during packing. Endcap  25  may then be placed onto the anterior end. 
     In an alternate embodiment depicted in FIGS. 10-16, the alignment device includes a connector  80  for adjusting the height  20  of the plates to achieve a desired lordotic alignment. FIG. 10 depicts a top view of the fusion device. Connector  80  is preferably a drive screw that is rotatable to adjust height  20 . Connector  80  preferably extends between engaging plates  12  and  14  and may be adapted to move axially through the fusion device in a direction from anterior end  22  to posterior end  24 . The engaging plates may contain elongated openings  82  for allowing bone growth through the faces  15  of the plates. 
     FIG. 11 depicts a front (anterior) view of the fusion device in a raised position. In an embodiment, the engaging plates include ribs  84  and  85  that may extend substantially perpendicularly from face  15 . A cross-sectional view taken along plane III of FIG. 11 is depicted in each of FIG.  12  and FIG.  13 . FIG. 12 depicts rib  84  and cam pins  86  and  88  in section with the fusion device in a “lowered position” (i.e., unadjusted for lordotic alignment). FIG. 13 depicts the rib and cam pins in section with the fusion device in the “raised position” (i.e., adjusted for lordotic alignment). As described herein, “cam pin” is taken to mean any connecting element capable of extending from the connector into the slots  90  and  92 . Each of the cam pins may be intersected by an imaginary longitudinal axis  91  axially extending through the fusion device. 
     Rib  84  preferably contains a slot  90  having a first end and a second end. The ends of slot  90  preferably terminate in a direction below axis  91 . The first end of slot  90  preferably extends downwardly substantially toward either the face of engaging plate  14  or the anterior end. The second end of slot  90  preferably extends downwardly substantially toward either the face of engaging plate  14  or the posterior end. Rib  85  preferably contains a slot  92  having a pair of ends that extend in diverging directions from the slot ends of rib  84 . The ends of slot  92  preferably terminate in a direction above axis  91 . The first end of slot  92  preferably extends upwardly substantially toward either the face of engaging plate  12  or the anterior end. The second end of slot  90  preferably extends upwardly substantially toward either the face of engaging plate  12  or the posterior end. The engaging plates are preferably connected together with cam pins  86  and  88 , which preferably have ends sized to fit within slots  90  and  92 . The cam pins preferably are disposed through the fusion device in a direction from the first side to the second side. Pins  86  and  88  preferably contain a receiving section  87  having an opening for receiving connector  80 . Receiving section  87  may have a greater width (e.g., diameter) than the ends of pins  86  and  88  disposed in slots  90  and  92 . 
     FIG.  14  and FIG. 15 each depict a cross-sectional view of the fusion device taken along plane IV of FIG.  11 . FIG. 14 depicts the connector and cam pins in section with the fusion device in the lowered position. FIG. 15 depicts the connector and the cam pins in section with the fusion device in the raised position. In an embodiment, connector  80  contains a threaded portion  94  and an unthreaded portion  96 . Pin  86  is preferably connected to the threaded portion and pin  88  is preferably connected to the unthreaded portion. 
     In an embodiment, a torque delivered to the connector is converted into a separation force between the cam pins. Rotating the connector in a counterclockwise direction preferably moves the connector in a direction from the anterior end to the posterior end. Pin  88  is preferably attached to the connector and preferably moves in the same manner as the connector. Pin  86  preferably contains an opening having complementary threading to that of the connector. Pin  86  preferably moves toward the anterior end in a direction opposite the motion of the connector to increase the separation between pin  88  and pin  86 . The ends of the pins preferably move along the angled portions of the slots  90  and  92 , causing the ends of the slots to be drawn together. In this manner, the separation between the engaging plates is increased. The connector may be rotated in a counterclockwise direction to move the connector in a direction from the posterior end to the anterior end, thereby decreasing height  20 . 
     Conventional methods of surgically implanting fusion devices tend to require that distraction instruments be inserted between the vertebrae to separate them and allow insertion of the fusion device therebetween. The surgical incision typically must be widened to accommodate the distraction instruments. In an embodiment, the fusion device in the lowered position has a height that is less than the disc space between the vertebrae. In this manner, the fusion device may be inserted between the vertebrae with minimal distraction. Connector  80  is preferably operable to separate the engaging plates (hence the vertebrae) and create a desired lordotic alignment. 
     The distance that the engaging plates are separated per unit torque applied to the connector will tend to depend upon the angle of the slots  90  and  92 . The slots are preferably angled such that the height  20  proximate the anterior end changes at a greater rate than the i height  20  proximate the posterior end when the connector is adjusted to alter the distance between the plates. In this manner, a desired lordotic alignment may be achieved. It is to be understood that the fusion device is operable in a semi-raised position that is intermediate the raised and lowered positions depicted in FIGS. 12-15. The connector is preferably rotated to a selected degree to achieve a preferred height  20  proximate the anterior and posterior ends to suit the particular patient. The angle of the slots  90  and  92  may vary among patients and is preferably selected to achieve a desired lordotic alignment. The connector may include a retaining ring  98  for contacting one or both of the engaging plates to limit the degree to which the connector can move through the fusion device. 
     FIG. 16 depicts a side view of an alternate embodiment of the fusion device installed between neighboring vertebrae. Pin  86  may be located on the unthreaded portion of the shank adjacent to the head of connector  80 . Pin  88  may be located on threaded portion  94  of the shank of connector  80 . Rib  84  preferably includes a first slot  100  that is angled diagonally upward from axis  91  in a direction substantially toward either the face of engaging plate  12  or the anterior end  22 . Rib  84  preferably also includes a second slot  102  that is angled diagonally upward from axis  91  in a direction substantially toward either the face of engaging plate  12  or the posterior end  24 . Rib  85  preferably includes a first slot  104  that is angled diagonally downward from axis  91  in a direction substantially toward either the face of engaging plate  14  or the anterior end  22 . Rib  85  preferably also includes a second slot  106  that is angled diagonally downward from axis  91  in a direction substantially toward either the face of engaging plate  14  or the posterior end  24 . To adjust the fusion device into the raised position, the connector may be rotated to cause the cam pins to be moved in a direction toward one another. Pin  86  preferably moves with the connector in a direction from the anterior end to the posterior end to increase the separation between the engaging plates proximate the anterior end. Pin  88  preferably contains a threaded opening for receiving the connector and may move in a direction toward the posterior end to increase the separation between the engaging plates proximate the posterior end. 
     In an alternate embodiment, each of the pins  86  and  88  contains a threaded opening for receiving the connector  80 . The connector may be a “double-threaded” screw having two threaded portions for complementing the threaded openings of the pins  86  and  88 . Rotation of the screw in a first direction preferably causes the pins to move toward one another to increase the separation between the engaging plates. Rotation of the screw in an opposite direction preferably causes the pins to move away from one another to reduce the separation between the engaging plates. 
     In an alternate embodiment, the alignment device includes a load-sharing member to allow the engaging plates to move in response to a compressive force of predetermined magnitude. In accordance with Wolff&#39;s law, bone growth tends to occur in the presence of stress (e.g., load), and bone tends to be absorbed in the absence of stress. The load-sharing member preferably enables the fusion device to “share” compressive forces exerted onto the spinal column with the bone graft in the vicinity of the fusion device. The load-sharing member preferably is deflected upon receiving a predetermined force to cause the engaging plates to move, thereby shifting load from the fusion device to the bone graft proximate the fusion device. It is believed that providing a selected amount of stress to the bone graft in a such a manner will tend to result in a higher fusion rate as well as a stronger fusion mass. 
     An embodiment of the load-sharing fusion device is depicted in FIGS. 17-19. A top view of a strut  30  containing a load-sharing member is depicted in FIG.  17 . FIGS. 18 and 19 depict cross-sectional views of the strut taken along plane V of FIG.  17 . Load-sharing member  110  is preferably disposed axially through the strut. The load-sharing member may be contained in a bore extending into the strut. The bore preferably has a width (e.g., diameter) that is greater than that of the load-sharing member to allow sufficient space for the load-sharing member to be deflected. The bore is preferably disposed within lower member  74 . Portion  118  of the upper member may substantially surround the bore and the load-sharing member, thereby allowing attachment of the upper and lower members. In an embodiment, the load-sharing member is a pin having a substantially circular cross-section. The pin preferably fits loosely within the bore such that its rotational freedom is maintained. The pin may be hinge pin  70  about which the upper member  72  pivots with respect to the lower member  74 . The load-sharing member preferably contains an indention  114  forming a substantially planar surface about which the load-sharing member may be deflected. 
     A connector  112  preferably extends through an opening  116  in the end  50  of the strut. The connector preferably fixes the load-sharing member to the upper member  72  and may contact the load-sharing member at fulcrum point  126 , which is preferably located on the planar surface formed by indention  114 . Connector  122  is preferably a set screw, and opening  116  preferably contains threading for engaging the set screw. FIG. 18 depicts the strut in an “unloaded” position whereby a predetermined spacing  122  exists between upper member  72  and portion  120  of lower member  74 . The predetermined spacing  122  may be adjusted by altering the location of connector  112  within opening  116 . For instance, the screw may be rotated through opening  116  to increase spacing  122 . The load-sharing member preferably remains substantially undeflected in the unloaded position. 
     Upon application of a compressive force onto the end  50  of the upper member  72 , force is preferably imparted from connector  112  to the load-sharing member at fulcrum point  126 . The compressive force is preferably sufficient to cause deflection of the load-sharing member and movement of upper member  72  toward portion  120  of the lower member such that predetermined spacing  122  is decreased. The deflection of the load-sharing member may force portion  118  of the upper member into a cavity  115  formed within the axial bore. The load-sharing member is preferably deflected in a three point bending arrangement as shown in FIG.  19 . 
     FIG. 19 depicts the strut in the “loaded” position with the load-sharing member deflected. The predetermined spacing  22  is preferably adjustable and may be adjusted to set the maximum strain that can be imparted to the load-sharing member. When the load-sharing member has been deflected a vertical distance equal to predetermined spacing  22 , the upper member  72  contacts portion  120 , thereby inhibiting further strain on the load-sharing member. In this manner, the maximum amount of strain on the load-sharing member can be limited to reduce the possibility that the member will experience fatigue failure. 
     The load-sharing member may be constructed of any of a variety of metals or alloys. In a preferred embodiment, the load-sharing member is constructed of titanium or a titanium alloy. The material properties and cross-sectional area of the load-sharing member are preferably controlled to allow a predetermined amount of stress to occur across the fusion device. The horizontal distance  124  or moment arm between fulcrum point  126  and support point  128  on the lower member is preferably selected such that the fusion device has an “effective” modulus of elasticity in the vicinity of the modulus of elasticity of bone to facilitate bone development. The a“effective” modulus of elasticity of the fusion device is taken to mean the ratio of stress to strain across the fusion device in a direction along height  20  as the device moves from the unloaded position to the loaded position upon receiving a compressive force. As described herein, “in the vicinity of the modulus of elasticity of bone” is taken to mean a Young&#39;s modulus between about 3 GPa and about 25 GPa. In an embodiment, the effective modulus of the fusion device is between about 16 GPa and about 20 GPa. The paper entitled “Variation of Young&#39;s Modulus and Hardness in Human Lumbar Vertebrae Measured by Nanoindentation” by Marcel Roy and Jae-Young Rho (Department of Biomedical Engineering, University of Memphis, Memphis, TN), and Ting Y. Tsui and George M. Pharr (Department of Materials Science, Rice University, Houston, Tex.) relates to the mechanical properties of bone and is incorporated by reference as if fully set forth herein. 
     The stresses exerted onto the spinal column are preferably shared by the fusion device and surrounding bone graft. As the spinal fusion develops, the proportion of stress experienced by the surrounding bone material preferably increases and the required load on the fusion device preferably decreases. After completion of the fusion, the fusion device preferably remains in the unloaded position during normal daily activity of the patient. 
     Fusion device  10  preferably provides a relatively large contact area between the engaging plates and the vertebral bodies defining the disc space occupied by the fusion device. FIG. 20 depicts a top view of an embodiment of a fusion device of the present invention. FIG. 21 depict a cross-sectional view of the fusion device taken along plane VI of FIG.  20 . Depicted in FIGS. 22-24 is a conventional fusion cage such as that described in U.S. Pat. No. 4,961,740 to Ray et al. This patent is incorporated by reference as if fully set forth herein. The devices in FIGS. 20-24 are sized for use in the L3-L4 disc space of an average size middle-aged female. Dimensions of the fusion devices are indicated in millimeters. 
     The “effective contact area” between an engaging plate and a vertebral body may be calculated by subtracting the fenestration area, a (i.e., the combined area of the openings  34  intended for bone ingrowth), from the total contact area, A (the area of the face  15  including the area of the openings  34 ). The total contact area and the fenestration area of the fusion device in FIGS. 20 and 21 is 581 mm 2  and 96 mm 2 , respectively. Therefore, the effective contact area between the engaging plate and the vertebra is 485 mm 2 . 
     For the fusion cage depicted in FIGS. 22-24, it is assumed that threads on the outer surface of the fusion cage penetrate into the vertebra a total of 3 mm per side as recommended by the manufacturer. It should be noted that such penetration is often difficult to achieve. In addition, the cortical layer of a vertebral body is often only 1-2 mm thick. Each of the cylindrical elements of the fusion cage has a total contact area of 283.5 mm 2  and a fenestration area of 198.5 mm 2 . Therefore, the combined effective contact area of both of the cylindrical elements is 170 mm 2 . If the threads of the fusion cage penetrate into the vertebra a distance less than 3 mm per side, the contact area will be less than that calculated above. 
     The maximum axial compressive forces in the lumbar spine resulting from everyday activity were estimated to be 3200 N in a paper entitled “The BAK™ Interbody Fusion: An Innovative Solution” by Bagby et al. and available from Spine Tech, Inc. in Minneapolis Minn. (see page 3, bottom paragraph). For a 3200 N compressive force, the stress per unit area is calculated to be 18.8 N/mm 2  for the fusion cage depicted in FIGS. 22-24 as compared to 6.6 N/mm 2  for the fusion device depicted in FIG.  20  and FIG.  21 . It is believed that such a reduction in stress per unit area will result in a significant reduction in post surgical subsidence at the interface of the fusion device and vertebral body. Typically, the loss of disc height is estimated to be about 1-3 mm at one month follow-up when conventional devices such as that depicted in FIGS. 22-24 are employed. 
     Further Improvements 
     An alternate embodiment of an interbody fusion device is depicted in FIGS. 25-27. FIG. 25 is a perspective view of the fusion device in a lowered position. FIG. 26 is a perspective view of the fusion device in a raised position. FIG. 27 is an exploded view of the fusion device. Fusion device  200  includes a pair of engaging plates  202  and  204  for engaging adjacent vertebrae. Engaging plates  202  and  204  are preferably separated by bracket assembly  206 . Engaging plates  202  and  204  and bracket assembly  206  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. For purposes of this description, “biocompatible material” is material not rejected by the body and/or not causing infection following implantation. 
     As depicted in FIG. 27, engaging plates  202  and  204  may contain a plurality of protrusions  216  from outer surfaces  203  for enhancing an engagement between the vertebrae and the engaging plates. In this manner, subsidence of the vertebrae may be substantially prevented as previously described. Outer surfaces  203  are preferably substantially planar to provide a large contact area between the engaging plates and the vertebrae; alternately, outer surfaces  203  may be non-planar. Protrusions  216  may extend into the vertebrae to prevent the fusion device from moving out of the disc space. Engaging plates  202  and  204  may include a plurality of openings  218  to allow bone development and growth through the engaging plates and between fusion device  200  and the neighboring vertebrae. In an embodiment, openings  218  have a combined area that is greater than about 50% of the total area of outer surfaces  203  (including the area of openings  218 ). More preferably, openings  218  have a total area between 60% and 80% of the total area of outer surfaces  203 . More preferably still, openings  218  have a total area of 70% or more of the total area of outer surfaces  203 . 
     Bracket assembly  206  (depicted in perspective view in FIG. 28) preferably includes an alignment device for changing a height between engaging plates  202  and  204 . In an embodiment, the alignment device includes first turnbuckle  250  and second turnbuckle  270  positioned substantially parallel to and substantially adjacent first side edge  212  and second side edge  214 , respectively, and extending between anterior edge  208  and posterior edge  210 . Bracket assembly  206  includes lateral projections  244  and  246  extending into the interior of the bracket assembly and supporting turnbuckles  250  and  270 , respectively. The turnbuckles include middle portions (e.g., middle portion  256  of turnbuckle  250 ) disposed between the ends of the turnbuckles and having a diameter greater than a diameter of the threaded portions. Lateral projection  244  is sized such that middle portion  256  is retained within lateral projection  244  while turnbuckle  250  is free to rotate within lateral projection  244 . Ends  296  of bracket assembly  206  (shown in detail in FIG. 29A) may include arcuate grooves  297  which correspond to the curvature of turnbuckles  250  and  270 . Inner surface  209  (FIG. 27) of engaging plate  204  and the inner surface of engaging plate  202  (not readily visible in FIG. 27) also may include arcuate grooves  228  and  240  (FIG.  27 ), respectively, which correspond to the curvature of turnbuckles  270  and  250 . 
     Returning to FIG. 28, first threaded portion  252  of first turnbuckle  250  is preferably threaded in a first direction and second threaded portion  254  of first turnbuckle  250  is preferably threaded in a direction opposite the first direction. First threaded portion  272  of second turnbuckle  270  is preferably threaded in a second direction and second threaded portion  274  of second turnbuckle  270  is preferably threaded in a direction opposite the second direction. First threaded portions  252  and  272  may be threaded in the same direction or in opposite directions. First turnbuckle  250  is preferably configured to be coupled to cam blocks  260  and  261 ; second turnbuckle  270  is preferably configured to be coupled to cam blocks  280  and  281 . Cam block  260  is preferably coupled to first turnbuckle  250  through opening  266  (FIG.  29 B). Opening  266  is preferably threaded complementarily to first portion  252  of first turnbuckle  250 . Cam blocks  261 ,  280 , and  281  are preferably similarly configured for coupling to turnbuckle portions  254 ,  272 , and  274 , respectively. 
     Cam block  260  preferably includes upper surface  262  having a first slope and lower surface  264  having a second slope (FIG.  29 C); cam blocks  261 ,  280 , and  281  are preferably similarly configured. The slopes of corresponding features on paired cam blocks (e.g., the slopes of upper surface  262  of cam block  260  and upper surface  263  of cam block  261 ) are preferably equivalent. Alternatively, the slopes of corresponding features on paired cam blocks may differ. In addition, the slope of upper surface  262  of cam block  260  need not be equivalent to the slope of lower surface  264  of cam block  260 . Further, the slope of upper surface  262  of cam block  260  need not be equivalent to the slope of upper surface  282  of cam block  280 , to the slope of upper surface  283  of cam block  281 , or to the slopes of the lower surfaces of cam blocks  280  and  281  (not visible in FIG.  28 ). 
     Referring to FIG. 27, inner surface  209  of engaging plate  204  preferably includes sloped tracks  220 ,  222 ,  224 , and  226 . Sloped tracks  220 ,  222 ,  224 , and  226  are preferably constructed such that the slopes of sloped tracks  220 ,  222 ,  224 , and  226  are substantially equivalent to the slopes of lower surface  264  of cam block  260  and to the lower surfaces of cam blocks  261 ,  280 , and  281 , respectively. The inner surface of engaging plate  202  also preferably includes sloped tracks  230 ,  232 ,  234 , and  236  (the ends of which are visible in FIG.  27 ). Sloped tracks  230 ,  232 ,  234 , and  236  are preferably constructed such that the slopes of sloped tracks  230 ,  232 ,  234 , and  236  are substantially equivalent to the slopes of upper surfaces  262 ,  263 ,  282 , and  283  (FIG.  28 ), respectively, of cam blocks  260 ,  261 ,  280 , and  281 . 
     Referring to FIG. 28, turnbuckles  250  and  270  may still further include indentations  258  and  278 . Indentations  258  and  278  may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. The adjusting tool may be used to rotate the turnbuckles. Rotation of first turnbuckle  250  in a first angular direction (e.g., clockwise or counterclockwise) may cause cam blocks  260  and  261  to move away from each other; rotation of first turnbuckle  250  in an angular direction opposite the first angular direction may cause cam blocks  260  and  261  to move toward each other. Rotation of second turnbuckle  270  in a second angular direction may cause cam blocks  280  and  281  to move away from each other; rotation of second turnbuckle  270  in an angular direction opposite the second angular direction may cause cam blocks  280  and  281  to move toward each other. The second angular direction may be the same as the first angular direction; alternatively, the second angular direction may be opposite the first angular direction. Lateral motion of the turnbuckles is preferably inhibited with respect to the bracket assembly. As shown in FIGS. 25-28, rotation of a turnbuckle in a first direction will cause cam blocks coupled to the turnbuckle and having threading complementary to the threading of the turnbuckle to move. Because the ends of the turnbuckle are threaded in opposite directions, the cam blocks will move laterally in opposite directions. 
     As depicted in FIGS. 25-28, the cam blocks and sloped tracks are preferably constructed such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. FIGS. 30A and 30B depict a cutaway view of interbody fusion device  200  in a lowered position and a raised position, respectively. Sloped tracks  230 ,  232 ,  220 , and  222  correspond to the slopes of cam block surfaces  262 ,  263 ,  264 , and  265 , respectively. In order for the cam blocks to move laterally toward the exterior of the fusion device, as shown in FIGS. 30A and 30B, the interior separation between engaging plates  202  and  204  must be increased to accommodate the height of the cam blocks. Increasing the interior separation in turn increases the exterior height  290  between exterior surfaces  203  of the engaging plates. Rotation of turnbuckle  250  in a first direction causes lateral motion of cam blocks  260  and  261  toward the exterior of interbody fusion device  200 . Because the slopes of surfaces  262 ,  263 ,  264 , and  265  of the cam blocks match the slopes of sloped tracks  230 ,  232 ,  220 , and  222 , the lateral motion of the cam blocks forces engaging plates  202  and  204  apart and increases height  290 . Rotation of turnbuckle  250  in a direction opposite the first direction will cause the cam blocks to move toward the interior of the fusion device, decreasing height  290 . 
     The cam blocks and sloped tracks, however, may be constructed such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase, as depicted in FIGS. 31A and 31B. Features of the interbody fusion device are labeled with the suffix “A” in FIGS. 31A and 31B to denote the alternate configuration (e.g., cam block  260 A is similar to cam block  260  except for the orientation). As depicted in FIGS. 31A and 31B, rotation of turnbuckle  250 A in a first direction causes lateral motion of cam blocks  260 A and  261 A toward the interior of interbody fusion device  200 A. Because the slopes of surfaces  262 A,  263 A,  264 A, and  265 A of the cam blocks match the slopes of sloped tracks  230 A,  232 A,  220 A, and  222 A, the lateral motion of the cam blocks forces engaging plates  202 A and  204 A apart, increasing exterior height  290 A between surfaces  203 A of the engaging plates. Rotation of turnbuckle  250 A in a direction opposite the first direction will cause the cam blocks to move toward the exterior of the fusion device, decreasing height  290 A. 
     FIG. 32A is a top view of fusion device  200  as inserted between two vertebrae (shown in phantom). Anterior edge  208 , posterior edge  210 , first side edge  212 , and second side edge  214  are indicated in FIG.  32 A. FIG. 32B is a front (anterior) view of alignment device  200  in a lowered position. Height  290  (the separation between the outer surfaces of engaging plates  202  and  204  at a location lying on first side edge  212  a specified distance from anterior edge  208 ) and height  292  (the separation between the outer surfaces of engaging plates  202  and  204  at a location lying on second side edge  214  the same specified distance from anterior edge  208 ) are substantially equivalent when alignment device  200  is in the lowered position as pictured. First turnbuckle  250  and second turnbuckle  270  may be rotated independently of one another to independently adjust heights  290  and  292  to correct a lateral deviation of the spine, as depicted in FIG.  32 C. Heights  290  and  292  may be substantially uniform between anterior edge  208  and posterior edge  210  [e.g., when the slopes of upper surfaces  262  and  263  of cam blocks  260  and  261  (FIG. 28) are substantially equivalent]. Alternatively, heights  290  and  292  may be substantially different proximate anterior edge  208  than proximate posterior edge  210  to maintain a substantially natural lordosis of the spine (e.g., when the slope of surface  262  is substantially different from the slope of surface  263 ). 
     Referring to FIG. 32C, bone graft material  294  may be packed between engaging plates  202  and  204 . A removable endcap (not shown) similar to endcap  25  (FIG. 4B) may be positioned proximate anterior edge  208  to contain bone graft material  294  within the fusion device and to prevent migration of the bone graft outside the engaging plates. The removable endcap may contain one or more openings for allowing bone ingrowth between a vertebral body and bone graft contained between the engaging plates. The removable endcap is preferably made of a plastic material such as polyethylene that tends to be non-irritating and non-abrasive to the surrounding tissues. 
     To install the fusion device, a discectomy is preferably performed from an anterior approach. All cartilage, and soft tissue are preferably removed from the vertebral endplate as would normally be done for placement of a femoral strut graft. Such a procedure is well within the knowledge of a skilled practitioner of the art. The engaging plates may be deployed in the disc space between the adjacent vertebrae. Turnbuckles  250  and  270  may be rotated to achieve the desired heights  290  and  292  between outer surfaces  203  of engaging plates  202  and  204  at first side edge  212  and second side edge  214 . The proper heights may be determined beforehand using x-ray techniques in which the side portions of the intervertebral disc space are examined. 
     FIG. 33 is an exploded view of an alternate embodiment of an interbody fusion device. Interbody fusion device  300  preferably includes engaging plates  302  and  304  and bracket assembly  306 . Engaging plates  302  and  304  and bracket assembly  306  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  302  and  304  may include protrusions  316  and openings  318 . Bracket assembly  306  is depicted in perspective view in FIG.  34 . First turnbuckle  350  and second turnbuckle  370  are preferably positioned substantially parallel to and substantially adjacent anterior edge  308  and posterior edge  310 , respectively, of interbody fusion device  300  and extend between first side edge  312  and second side edge  314 . 
     Interbody fusion device  300  as depicted is similar to interbody fusion device  200  depicted in FIGS. 25-28 except for the orientation of the adjusting mechanism. Thus, first turnbuckle  350  is preferably coupled to cam blocks  360  and  361  at first and second threaded portions  352  and  354 , respectively; second turnbuckle  370  is preferably coupled to cam blocks  380  and  381  at first and second threaded portions  372  and  374 , respectively (FIG.  34 ). Cam blocks  360  and  361  may contact engaging plate  302  in sloped tracks  330  and  332 , respectively (the ends of which are visible in FIG.  33 ); cam blocks  360  and  361  may contact engaging plate  304  in sloped track  320  and in a sloped track (not visible) similar to sloped track  320 , respectively. Cam blocks  380  and  381  may contact engaging plate  302  in sloped tracks  334  and  336 , respectively (the ends of which are visible in FIG.  33 ); cam blocks  380  and  381  may contact engaging plate  304  in sloped tracks  324  and  326 , respectively. Inner surface  309  (FIG. 33) of engaging plate  304  and the inner surface of engaging plate  302  (not readily visible in FIG. 33) also may include arcuate grooves  328  and  340 , respectively, which correspond to the curvature of turnbuckles  370  and  350 . First turnbuckle  350  may contact bracket assembly  306  at lateral projection  342  and second turnbuckle  370  may contact bracket assembly  306  at lateral projection  344  (FIG.  34 ). In addition, the turnbuckles may contact bracket assembly  306  in arcuate grooves (similar to arcuate grooves  297  in FIG. 29A) in ends  396 . 
     Adjustment of the height between engaging plates  302  and  304  by rotation of turnbuckles  350  and  370  is preferably similar to the adjustment process by rotation of turnbuckles  250  and  270  as previously described for interbody fusion device  200 . Heights between engaging plates  302  and  304  along anterior edge  308  and along posterior edge  310  may be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  302  and  304  along first side edge  312  and second side edge  314  may vary between anterior edge  308  and posterior edge  310  to maintain a substantially natural lateral alignment. 
     FIG. 35 is an exploded view of interbody fusion device  400 , which includes engaging plates  202  and  204  (see FIGS. 25-27) and bracket assembly  406 . Engaging plates  202  and  204  and bracket assembly  406  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. FIG. 36 is a perspective view of alternative bracket assembly  406  which may be placed between engaging plates  202  and  204 . Lateral projections  442  and  444  of bracket assembly  406  may support screws  450  and  470 , respectively. FIG. 37A depicts a cross-sectional view of screw  450 . Screw  450  preferably includes threaded portion  452  and unthreaded portion  454 . A diameter of threaded portion  452  may be greater than a diameter of unthreaded portion  454 . Cam block  460  may include an opening such that inner surface  461  of the opening is substantially unthreaded. Unthreaded portion  454  of screw  450  may then be free to rotate within cam block  460 . Cam block  468  may include an opening such that inner surface  469  of the opening is threaded complementarily to threaded portion  452  of screw  450 . Projection  442  of bracket assembly  406  may also include openings with inner surfaces  443  threaded complementarily to threaded portion  452  of screw  450 . 
     Screw  450  may further include flange  456 . A diameter of flange  456  may be substantially greater than a diameter of unthreaded portion  454 . Flange  456  may maintain coupling between cam block  460  and unthreaded portion  454  of screw  450 . Screw  450  may still further include indentation  458 . Indentation  458  may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Ends  498  of bracket assembly  406  may include arcuate grooves  499  to allow access of the adjusting tool to indentations  458  and  478  (shown in FIG. 36) of screws  450  and  470 , respectively. 
     Rotation of screw  450  in a first angular direction may cause cam blocks  460  and  468  to move away from each other, as depicted in FIG.  37 B. Rotation of screw  450  in an angular direction opposite the first angular direction may cause cam blocks  460  and  468  to move toward one another, as depicted in FIG.  37 A. Alternatively, rotation of screw  450  in a first angular direction may cause cam blocks  460  and  468  to move toward each other, and rotation of screw  450  in an angular direction opposite the first angular direction may cause cam blocks  460  and  468  to move away from one another. Screw  470 , cam blocks  480  and  488 , and lateral projection  444  (FIG. 36) may possess features similar to those of screw  450 , cam blocks  460  and  468 , and lateral projection  442 , respectively. 
     Heights between engaging plates  402  and  404  along first side edge  412  and along second side edge  414  may be varied substantially independently to maintain a substantially natural lateral alignment. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  402  and  404  along anterior edge  408  and posterior edge  410  may vary between first side edge  412  and second side edge  414  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device is depicted in an exploded view in FIG.  38 . Interbody fusion device  500  preferably includes engaging plates  502  and  504  and bracket assembly  506 . Engaging plates  502  and  504  and bracket assembly  506  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  502  and  504  may include protrusions  516  and openings  518 . Bracket assembly  506  (depicted in perspective view in FIG. 39) may include an alignment device for changing a height between engaging plates  502  and  504 . In an embodiment, the alignment device includes first turnbuckle  540  positioned substantially parallel to and substantially adjacent second side edge  514 , second turnbuckle  550  positioned substantially parallel to and substantially adjacent anterior edge  508 , and third turnbuckle  560  positioned substantially parallel to and substantially adjacent first side edge  512 . Bracket assembly  506  may include lateral projections  584 ,  586 , and  588  extending into the interior of the bracket assembly and supporting turnbuckles  540 ,  550 , and  560 , respectively. The turnbuckles may include middle portions (e.g., middle portions  543  and  553  of turnbuckles  540  and  550 , respectively) disposed between the ends of the turnbuckles and having a diameter greater than a diameter of the threaded portions. Lateral projections  584 ,  586 , and  588  are preferably sized such that the middle portions are retained within the lateral projections while the turnbuckles are free to rotate within the lateral projections. Inner surface  509  of engaging plate  504  (FIG. 38) and the inner surface of engaging plate  502  (not readily visible in FIG. 38) may include arcuate grooves  534  and  532 , respectively, which correspond to the curvature of turnbuckles  540 ,  550 , and  560 . 
     Returning to FIG. 39, first threaded portion  541  of first turnbuckle  540  may be threaded in a first direction and second threaded portion  542  of first turnbuckle  540  may be threaded in a direction opposite the first direction. First threaded portion  551  of second turnbuckle  550  may be threaded in a second direction and second threaded portion  552  of second turnbuckle  550  may be threaded in a direction opposite the second direction. First threaded portion  561  of third turnbuckle  560  may be threaded in a third direction and second threaded portion  562  of third turnbuckle  560  may be threaded in a direction opposite the third direction. First threaded portions  541 ,  551 , and  561  may be threaded in the same direction; alternatively, one of the first threaded portions may be threaded in a direction opposite the direction of the other two first threaded portions. 
     First turnbuckle  540  is preferably configured to be coupled to cam blocks  544  and  545 . Second turnbuckle  550  is preferably configured to be coupled to cam blocks  554  and  555 . Third turnbuckle  560  is preferably configured to be coupled to cam blocks  564  and  565 . The cam blocks are preferably coupled to the turnbuckles as depicted for cam block  260  and turnbuckle  250  (FIG.  29 B). The surfaces of cam blocks  544 ,  545 ,  554 ,  555 ,  564 , and  565  are preferably sloped as previously described for cam block  260  (FIG.  29 C). 
     Inner surface  509  of engaging plate  504  preferably includes sloped tracks (e.g.,  520 ,  521 ,  522 ,  524 , and  525  visible in FIG. 38) configured to correspond to the lower surfaces of cam blocks  544 ,  545 ,  554 ,  555 ,  564 , and  565 , respectively. The inner surface of engaging plate  502  preferably also includes sloped tracks  526 ,  527 ,  528 ,  529 ,  530 , and  531  (the ends of which are visible in FIG. 38) configured to correspond to the upper surfaces of cam blocks  544 ,  545 ,  554 ,  555 ,  564 , and  565 , respectively. 
     Turnbuckles  540 ,  550 , and  560  may still further include indentations (indentation  558  of second turnbuckle  550  is visible in FIG.  39 ). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Bracket assembly  506  preferably contains fenestrations  590  and  592  to facilitate access of the adjusting tool to the indentations. The adjusting tool may be used to rotate the turnbuckles. Rotation of first turnbuckle  540  in a first angular direction may cause cam blocks  544  and  545  to move away from each other; rotation of first turnbuckle  540  in an angular direction opposite the first angular direction may cause cam blocks  544  and  545  to move toward each other. Rotation of second turnbuckle  550  in a second angular direction may cause cam blocks  554  and  555  to move away from each other; rotation of second turnbuckle  550  in an angular direction opposite the second angular direction may cause cam blocks  554  and  555  to move toward each other. Rotation of third turnbuckle  560  in a third angular direction may cause cam blocks  564  and  565  to move away from each other; rotation of third turnbuckle  560  in an angular direction opposite the third angular direction may cause cam blocks  564  and  565  to move toward each other. The first, second, and third angular directions may be the same; alternatively, one of the angular directions may be opposite the other two angular directions. 
     As depicted in FIGS. 38-39, cam blocks  544 ,  545 ,  554 ,  555 ,  564  and  565  and sloped tracks  520 - 531  may be configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The turnbuckles in fusion device  500  may be positioned such that heights between engaging plates  502  and  504  along first side edge  512  and along second side edge  514  may be varied substantially independently to maintain a substantially natural lateral alignment. The turnbuckles in fusion device  500  may also be positioned such that heights between engaging plates  502  and  504  along anterior edge  508  and posterior edge  510  be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  502  and  504  along first side edge  512  and second side edge  514  may vary between anterior edge  508  and posterior edge  510  to maintain a substantially natural lateral alignment and such that heights between engaging plates  502  and  504  along anterior edge  508  and posterior edge  510  may vary between first side edge  512  and second side edge  514  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device is depicted in an exploded view in FIG.  40 . Interbody fusion device  600  preferably includes engaging plates  602  and  604  and bracket assembly  606 . Engaging plates  602  and  604  and bracket assembly  606  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  602  and  604  may include protrusions  616  and openings  618 . Bracket assembly  606  (depicted in perspective view in FIG. 41) preferably includes an alignment device for changing a height between engaging plates  602  and  604 . In an embodiment, the alignment device includes first screw  640  positioned substantially parallel to and substantially adjacent second side edge  614 ; second screw  650  positioned substantially parallel to and substantially centered between first side edge  612  and second side edge  614 ; and third screw  660  positioned substantially parallel to and substantially adjacent first side edge  612 . Inner surfaces  609  of engaging plate  604  (FIG. 40) and the inner surface of engaging plate  602  (not readily visible in FIG. 40) include arcuate grooves  634  and  632 , respectively, which correspond to the curvature of screws  640 ,  650 , and  660 . Bracket assembly  606  may include support portion  607  (FIG. 41) to support the ends of the screws. Support portion  607  may include arcuate grooves (not readily visible) corresponding to the curvature of screws  640 ,  650 , and  660 . 
     Threaded portion  641  of first screw  640  is preferably threaded in a first direction. Threaded portion  651  of second screw  650  is preferably threaded in a second direction. Threaded portion  661  of third screw  660  is preferably threaded in a third direction. Threaded portions  641 ,  651 , and  661  may be threaded in the same direction; alternatively, one of the threaded portions may be threaded in a direction opposite the direction of the other two threaded portions. 
     First screw  640  is preferably coupled to cam block  644 . Second screw  650  is preferably coupled to cam block  654 . Third screw  660  is preferably coupled to cam block  664 . Cam blocks  644 ,  654 , and  664  preferably differ from the cam blocks of previously described embodiments substantially only in shape. Thus, cam blocks  644 ,  654 , and  664  are preferably coupled to screws  640 ,  650 , and  6600  in a manner similar to that depicted for cam block  260  and turnbuckle  250  (FIG.  29 B). Cam blocks  644 ,  654 , and  664  preferably include sloped upper and lower surfaces similar to the sloped upper and lower surfaces as previously described for cam blocks in other embodiments. 
     Inner surface  609  of engaging plate  604  preferably includes sloped tracks  620  and  621  corresponding to the slopes of the lower surfaces of cam block  644 , sloped tracks  622  and  623  corresponding to the slopes of the lower surfaces of cam block  654 , and sloped tracks  624  and  625  corresponding to the slopes of the lower surfaces of cam block  664 . The inner surface of engaging plate  602  also preferably includes sloped tracks (the ends of which are visible in FIG.  40 ). Sloped tracks  626  and  627  preferably correspond to the slope of the upper surfaces of cam block  644 . Sloped tracks  628  and  629  preferably correspond to the slope of the upper surfaces of cam block  654 . Sloped tracks  630  and  631  preferably correspond to the slope of the upper surfaces of cam block  664 . 
     Screws  640 ,  650 , and  660  may still further include indentations  648 ,  658 , and  668  (FIG.  41 ). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Rotation of first screw  640  in a first angular direction may cause can block  644  to move toward anterior edge  608 ; rotation of first screw  640  in an angular direction opposite the first angular direction may cause cam block  644  to move toward posterior edge  610 . Rotation of second screw  650  in a second angular direction may cause cam block  654  to move toward anterior edge  608 ; rotation of second screw  650  in an angular direction opposite the second angular direction may cause cam block  654  to move toward posterior edge  610 . Rotation of third screw  660  in a third angular direction may cause cam block  664  to move toward anterior edge  608 ; rotation of third screw  660  in an angular direction opposite the third angular direction may cause cam block  664  to move toward posterior edge  610 . The first, second, and third angular directions may be the same; alternatively, one of the first, second, and third angular directions may be opposite the other two of the first, second, and third angular directions. 
     As depicted in FIGS. 40-41, the cam blocks and sloped tracks are preferably configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The screws in fusion device  600  may be positioned such that heights between engaging plates  602  and  604  along first side edge  612  and along second side edge  614  may be varied substantially independently to maintain a substantially natural lateral alignment. The screws in fusion device  600  may also be positioned such that heights between engaging plates  602  and  604  along anterior edge  608  and posterior edge  610  may be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  602  and  604  along first side edge  612  and second side edge  614  may vary between anterior edge  608  and posterior edge  610  to maintain a substantially natural lateral alignment and such that heights between engaging plates  602  and  604  along anterior edge  608  and posterior edge  610  may vary between first side edge  612  and second side edge  614  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device is depicted in an exploded view in FIG.  42 . Interbody fusion device  700  preferably includes engaging plates  702  and  704  and bracket assembly  706 . Engaging plates  702  and  704  and bracket assembly  706  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  702  and  704  may include protrusions  716  and openings  718 . Bracket assembly  706  (depicted in perspective view in FIG. 43A) preferably includes an alignment device for changing a height between engaging plates  702  and  704 . In an embodiment, the alignment device includes first screw  740 , positioned substantially parallel to and substantially centered between first side edge  712  and second side edge  714 ; second screw  750  positioned at a first angle Φ (FIG. 42) with respect to first screw  740 ; and third screw  760  positioned at a second angle Θ to first screw  740 . In the embodiment pictured in FIGS. 42 and 43A, Φ=90° and Θ=90°, such that second screw  750  and third screw  760  are preferably positioned substantially parallel to and substantially centered between anterior edge  708  and posterior edge  710  (FIG. 43A) and such that second screw  750  and third screw  760  preferably share a common axis of rotation. The alignment device further includes stationary block  790  to maintain the spatial relationship between screws  740 ,  750 , and  760 . In an alternate embodiment, Φ=90° and Θ&gt;90°. In a further alternate embodiment, Φ&gt;90° and Θ=90°. In a further alternate embodiment still, Φ&gt;90° and Θ&gt;90°. In another embodiment, first screw  740  may be positioned other than parallel to and centered between opposite edges of the fusion device, screws  750  and  760  being positioned at angles Φ and Θ, respectively, with respect to first screw  740 . 
     The inner surfaces of engaging plates  702  and  704  preferably include arcuate grooves  732  and  734  (FIG.  42 ), respectively, which correspond to the curvature of screws  740 ,  750 , and  760 . Bracket assembly  706  preferably includes support portions  707  (FIG. 43A) to support the ends of the screws. Support portions  707  preferably include arcuate grooves (not readily visible in FIGS. 42 and 43A) corresponding to the curvature of screws  740 ,  750 , and  760 . 
     Threaded portion  741  of first screw  740  preferably is threaded in a first direction. Threaded portion  751  of second screw  750  preferably is threaded in a second direction. Threaded portion  761  of third screw  760  preferably is threaded in a third direction. Threaded portions  741 ,  751 , and  761  may be threaded in the same direction; alternatively, one of the threaded portions may be threaded in a direction opposite the direction of the threading of the other two threaded portions. Screws  740 ,  750 , and  760  may also include unthreaded portions (e.g., unthreaded portion  762  visible in FIG.  43 A). FIG. 43B shows a detail of screw  740  including unthreaded portion  742 . Unthreaded portion  742  may be inserted into cavity  792  (shown in phantom) in stationary block  790 . Screw  740  thus may be free to rotate within stationary block  790 . Stationary block  790  may be configured to be similarly coupled to screws  750  and  760 . Engagement between stationary block  790  and screws  740 ,  750 , and  760  preferably maintains coupling between screws  740 ,  750 , and  760  and cam blocks  744 ,  754 , and  764 , respectively (that is, stationary block  790  preferably serves to stop motion of the cam blocks toward the interior of the fusion device such that the cam blocks are not separated from the screws). Stationary block  790  is pictured as a cube; however, stationary block  790  be of virtually any shape (e.g., cylinder, sphere, “T” shape similar to the positioning of screws  740 ,  750 , and  760 ) that will maintain coupling between the screws and the cam blocks. Cam blocks  744 ,  754 , and  764  are preferably coupled to screws  740 ,  750 , and  760  in a manner similar to that depicted for cam block  236  and turnbuckle  226  (FIG.  29 B). Cam blocks  744 ,  754 , and  764  preferably include sloped upper and lower surfaces similar to the sloped upper and lower surfaces as previously described for cam blocks in other embodiments. 
     Returning to FIG. 42, inner surface  709  of engaging plate  704  preferably includes sloped tracks  720 ,  722 , and  724  configured to correspond to the lower surfaces of cam blocks  744 ,  754 , and  764 , respectively. The inner surface of engaging plate  702  (not readily visible) also preferably includes sloped tracks  726 ,  728  and  730  (the ends of which are visible in FIG.  42 ). Sloped tracks  726 ,  728 , and  730  are preferably configured to correspond to the upper surfaces of cam blocks  744 ,  754 , and  764 , respectively. 
     Screws  740 ,  750 , and  760  may still further include indentations (e.g., indentations  748  and  758 , visible in FIG.  43 A). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Rotation of first screw  740  in a first angular direction may cause cam block  744  to move toward anterior edge  708 ; rotation of first screw  740  in an angular direction opposite the first angular direction may cause cam block  744  to move away from anterior edge  708 . Rotation of second screw  750  in a second angular direction may cause cam block  754  to move toward second side edge  714 ; rotation of second screw  750  in an angular direction opposite the second angular direction may cause cam block  754  to move away from second side edge  714 . Rotation of third screw  760  in a third angular direction may cause cam block  764  to move toward first side edge  712 ; rotation of third screw  760  in an angular direction opposite the third angular direction may cause cam block  764  to move away from first side edge  712 . The first, second, and third angular directions may be the same; alternatively, one of the first, second, and third angular directions may be opposite the other two angular directions. 
     Sloped tracks  720 ,  722 , and  724  in lower engaging plate  704  and sloped tracks  726 ,  728 , and  730  in upper engaging plate  702  are preferably configured to guide the motion of the cam blocks as the screws are rotated. As depicted in FIGS. 42 and 43A, the cam blocks and sloped tracks may be configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The screws in fusion device  700  may be positioned such that heights between engaging plates  702  and  704  along first side edge  712  and along second side edge  714  may be varied substantially independently to maintain a substantially natural lateral alignment. The screws in fusion device  700  may also be positioned such that heights between engaging plates  702  and  704  along anterior edge  708  and posterior edge  710  be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  702  and  704  along first side edge  712  and second side edge  714  may vary between anterior edge  708  and posterior edge  710  to maintain a substantially natural lateral alignment and such that heights between engaging plates  702  and  704  along anterior edge  708  and posterior edge  710  may vary between first side edge  712  and second side edge  714  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device including two parallel pairs of screws is depicted in an exploded view in FIG.  44 . Interbody fusion device  800  preferably includes engaging plates  802  and  804  and bracket assembly  806 . Engaging plates  802  and  804  and bracket assembly  806  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  802  and  804  may include protrusions  816  and openings  818 . Bracket assembly  806  (depicted in perspective view in FIG. 45) preferably includes an alignment device for changing a height between engaging plates  802  and  804 . In an embodiment, the alignment device includes first screw  850  and second screw  860  having a common axis of rotation and positioned substantially parallel to and substantially adjacent posterior edge  810  and third screw  870  and fourth screw  880  having a common axis of rotation and positioned substantially parallel to and substantially adjacent anterior edge  808 . The inner surfaces of engaging plates  802  and  804  preferably include arcuate grooves  840  and  842  (FIG.  44 ), respectively, which correspond to the curvature of screws  850 ,  860 ,  870 , and  880 . Bracket assembly  806  may include support portion  807  to support the ends of screws  860  and  880 . Support portion  807  may include arcuate grooves corresponding to the curvature of screws  860  and  880  (not readily visible in FIGS.  44  and  45 ). Bracket assembly  806  preferably further includes ends  898  including arcuate grooves (not readily visible) corresponding to the curvature of screws  850  and  870 . 
     Threaded portion  851  of first screw  850  may be threaded in a first direction. Threaded portion  861  of second screw  860  may be threaded in a second direction. Threaded portion  871  of third screw  870  may be threaded in a third direction. Threaded portion  881  of fourth screw  880  may be threaded in a fourth direction. Threaded portions  851 ,  861 ,  871 , and  881  may be threaded in the same direction. Alternatively, one of the threaded portions may be threaded in a direction opposite the direction of the other three threaded portions. Alternatively, two of the threaded portions may be threaded in a direction opposite the direction of the other two threaded portions. 
     Screws  850 ,  860 ,  870 , and  880  may include unthreaded portions (e.g., unthreaded portion  862  visible in FIG. 45) similar to unthreaded portion  742  of screw  740  (FIG.  43 A). Lateral projections  894  and  896  may include substantially unthreaded openings, similar to unthreaded openings  792  of stationary block  790  (FIG.  43 B), adapted to receive the unthreaded portions of the screws and in which the unthreaded portions of the screws are free to rotate. 
     First screw  851  is preferably configured to be coupled to cam block  854 . Second screw  860  is preferably configured to be coupled to cam block  864 . Third screw  870  is preferably configured to be coupled to cam block  874 . Fourth screw  880  is preferably configured to be coupled to cam block  884 . Cam blocks  854 ,  864 ,  874 , and  884  are preferably coupled to screws  850 ,  860 ,  870  and  880  in a manner similar to that depicted for cam block  744  and turnbuckle  740  (FIG.  43 B). Cam blocks  850 ,  860 ,  870 , and  880  preferably include sloped upper and lower surfaces similar to the sloped upper and lower surfaces as previously described for cam blocks in other embodiments. 
     Returning to FIG. 42, inner surface  809  of engaging plate  804  preferably includes sloped track  820  configured to correspond to the lower surface of cam block  850 ; a sloped track (not visible) configured to correspond to the lower surface of cam block  860 ; sloped track  824  configured to correspond to the lower surface of cam block  870 ; and sloped track  826  configured to correspond to the lower surface of cam block  880 . The inner surface of engaging plate  802  preferably includes sloped tracks  830 ,  832 ,  834 , and  836  (the ends of which are visible in FIG.  44 ). Sloped tracks  830 ,  832 ,  834 , and  836  are preferably configured to correspond to the upper surfaces of cam blocks  850 ,  860 ,  870 , and  880 , respectively. 
     Screws  850 ,  860 ,  870 , and  880  may still further include indentations (e.g., indentations  858  and  878 , visible in FIG.  45 ). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Rotation of first screw  850  in a first angular direction may cause cam block  854  to move toward second side edge  814 ; rotation of first screw  850  in an angular direction opposite the first angular direction may cause cam block  854  to move toward first side edge  812 . Rotation of second screw  860  in a second angular direction may cause cam block  864  to move toward first side edge  812 ; rotation of second screw  860  in an angular direction opposite the second angular direction may cause cam block  864  to move toward second side edge  814 . Rotation of third screw  870  in a third angular direction may cause cam block  874  to move toward second side edge  814 ; rotation of first screw  870  in an angular direction opposite the third angular direction may cause cam block  874  to move toward first side edge  812 . Rotation of fourth screw  880  in a fourth angular direction may cause cam block  884  to move toward first side edge  812 ; rotation of fourth screw  880  in an angular direction opposite the fourth angular direction may cause cam block  884  to move toward second side edge  814 . The first, second, third, and fourth angular directions may be the same. Alternatively, one of the first, second, third, and fourth angular directions may be opposite the other three of the first, second, third, and fourth angular directions. Alternatively, two of the first, second, third, and fourth angular directions may be opposite the other two of the first, second, third, and fourth angular directions. 
     As depicted in FIGS. 44-45, the cam blocks and sloped tracks are preferably configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The screws in fusion device  800  may be positioned such that heights between engaging plates  802  and  804  along first side edge  812  and along second side edge  814  may be varied substantially independently to maintain a substantially natural lateral alignment. The screws in fusion device  800  may also be positioned such that heights between engaging plates  802  and  804  along anterior edge  808  and posterior edge  810  be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  802  and  804  along first side edge  812  and second side edge  814  may vary between anterior edge  808  and posterior edge  810  to maintain a substantially natural lateral alignment and such that heights between engaging plates  802  and  804  along anterior edge  808  and posterior edge  810  may vary between first side edge  812  and second side edge  814  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device including four screws oriented in a “+” configuration is depicted in an exploded view in FIG.  46 . Interbody fusion device  900  preferably includes engaging plates  902  and  904  and bracket assembly  906 . Engaging plates  902  and  904  and bracket assembly  906  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  902  and  904  may include protrusions  916  and openings  918 . Bracket assembly  906  (depicted in perspective view in FIG. 47) preferably includes an alignment device for changing a height between engaging plates  902  and  904 . In an embodiment, the alignment device includes first screw  950  and second screw  960  having a common axis of rotation and positioned substantially parallel to and substantially centered between anterior edge  908  and posterior edge  910  and third screw  970  and fourth screw  980  having a common axis of rotation and positioned substantially parallel to and substantially centered between first side edge  912  and second side edge  914 . The inner surfaces of engaging plates  902  and  904  preferably include arcuate grooves  940  and  942  (FIG.  46 ), respectively, which correspond to the curvature of screws  950 ,  960 ,  970 , and  980 . Bracket assembly  906  preferably includes support portions  907  to support the ends of screws  950 ,  960 ,  970 , and  980 . Support portions  907  may include arcuate grooves (not readily visible in FIGS. 46-47) corresponding to the curvature of screws  950 ,  960 ,  970 , and  980 . 
     Returning to FIG. 47, threaded portion  951  of first screw  950  preferably is threaded in a first direction. Threaded portion  961  of second screw  960  preferably is threaded in a second direction. Threaded portion  971  of third screw  970  preferably is threaded in a third direction. Threaded portion  981  of fourth screw  980  preferably is threaded in a fourth direction. Threaded portions  951 ,  961 ,  971 , and  981  may be threaded in the same direction. Alternatively, one of the threaded portions may be threaded in a direction opposite the direction of the other three threaded portions. Alternatively, two of the threaded portions may be threaded in a direction opposite the direction of the other two threaded portions. 
     Screws  950 ,  960 ,  970 , and  980  may include unthreaded portions  952 ,  962 ,  972 , and  982 , similar to unthreaded portion  730  of screw  728  (FIG.  43 A). Stationary block  990  may include substantially unthreaded openings, similar to unthreaded openings  792  of stationary block  790  (FIG.  43 A), adapted to receive the unthreaded portions of the screws and in which the unthreaded portions of the screws are free to rotate. 
     First screw  951  preferably is configured to be coupled to cam block  954 . Second screw  960  preferably is configured to be coupled to cam block  964 . Third screw  970  preferably is configured to be coupled to cam block  974 . Fourth screw  980  preferably is configured to be coupled to cam block  984 . Cam blocks  954 ,  964 ,  974 , and  984  may be coupled to screws  950 ,  960 ,  970  and  980  in a manner similar to that depicted for cam block  744  and turnbuckle  740  (FIG.  43 B). Cam blocks  950 ,  960 ,  970 , and  980  preferably include sloped upper and lower surfaces similar to the sloped upper and lower surfaces as previously described for cam blocks in other embodiments. 
     Inner surface  909  of engaging plate  904  preferably includes sloped tracks  920 ,  922 ,  924 , and  926  configured to correspond to the lower surface of cam blocks  950 ,  960 ,  970 , and  980 , respectively. The inner surface of engaging plate  902  preferably includes sloped tracks  930 ,  932 ,  934 , and  936  (the ends of which are visible in FIG. 46) configured to correspond to the upper surfaces of cam blocks  950 ,  960 ,  970 , and  980 , respectively. 
     Screws  950 ,  960 ,  970 , and  980  may still further include indentations (e.g., indentations  958  and  978 , visible in FIG.  47 ). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Rotation of first screw  950  in a first angular direction may cause cam block  954  to move away from stationary block  990 ; rotation of first screw  950  in an angular direction opposite the first angular direction may cause cam block  954  to move toward stationary block  990 . Rotation of second screw  960  in a second angular direction may cause cam block  964  to move away from stationary block  990 ; rotation of second screw  960  in an angular direction opposite the second angular direction may cause cam block  964  to move toward stationary block  990 . Rotation of third screw  970  in a third angular direction may cause cam block  974  to move away from stationary block  990 ; rotation of third screw  970  in an angular direction opposite the third angular direction may cause cam block  974  to move toward stationary block  990 . Rotation of fourth screw  980  in a fourth angular direction may cause cam block  984  to move away from stationary block  990 ; rotation of second screw  980  in an angular direction opposite the fourth angular direction may cause cam block  984  to move toward stationary block  990 . The first, second, third, and fourth angular directions may be the same. Alternatively, one of the first, second, third, and fourth angular directions may be opposite the other three of the first, second, third, and fourth angular directions. Alternatively, two of the first, second, third, and fourth angular directions may be opposite the other two of the first, second, third, and fourth angular directions. 
     As depicted in FIGS. 46-47, the cam blocks and sloped tracks are preferably configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The screws in fusion device  900  may be positioned such that heights between engaging plates  902  and  904  along first side edge  912  and along second side edge  914  may be varied substantially independently to maintain a substantially natural lateral alignment. The screws in fusion device  900  may also be positioned such that heights between engaging plates  902  and  904  along anterior edge  908  and posterior edge  910  be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  902  and  904  along first side edge  912  and second side edge  914  may vary between anterior edge  908  and posterior edge  910  to maintain a substantially natural lateral alignment and such that heights between engaging plates  902  and  904  along anterior edge  908  and posterior edge  910  may vary between first side edge  912  and second side edge  914  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device including four screws oriented in an “x” configuration is depicted in an exploded view in FIG.  48 . Interbody fusion device  1000  preferably includes engaging plates  1002  and  1004  and bracket assembly  1006 . Engaging plates  1002  and  1004  and bracket assembly  1006  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  1002  and  1004  may include protrusions  1016  and openings  1018 . Bracket assembly  1006  (depicted in perspective view in FIG. 49) preferably includes an alignment device for changing a height between engaging plates  1002  and  1004 . In an embodiment, the alignment device includes first screw  1050  and second screw  1060  having a common axis of rotation and positioned substantially along a diagonal connecting corner  1001  and corner  1005  of bracket assembly  1006 . Preferably, bracket assembly  1006  further includes third screw  1070  and fourth screw  1080  having a common axis of rotation and positioned substantially along a diagonal connecting corner  1003  and corner  1007  of bracket assembly  1006 . Inner surface  1009  of engaging plate  1004  (FIG. 48) and the inner surface of engaging plate  1002  (not readily visible in FIG. 48) preferably include arcuate grooves  1042  and  1040 , respectively, which correspond to the curvature of screws  1050 ,  1060 ,  1070 , and  1080 . Bracket assembly  1006  preferably includes support portions  1098 (FIG. 49) to support the ends of screws  1050 ,  1060 ,  1070 , and  1080 . Support portions  1098  preferably include arcuate grooves (not readily visible) corresponding to the curvature of screws  1050 ,  1060 ,  1070 , and  1080 . 
     Threaded portion  1051  of first screw  1050  preferably is threaded in a first direction. Threaded portion  106   1  of second screw  1060  preferably is threaded in a second direction. Threaded portion  1071  of third screw  1070  preferably is threaded in a third direction. Threaded portion  1081  of fourth screw  1080  preferably is threaded in a fourth direction. Threaded portions  1051 ,  1061 ,  1071 , and  1081  may be threaded in the same direction. Alternatively, one of the threaded portions may be threaded in a direction opposite the direction of the other three threaded portions. Alternatively, two of the threaded portions may be threaded in a direction opposite the direction of the other two threaded portions. 
     Screws  1050 ,  1060 ,  1070 , and  1080  may include unthreaded portions  1052 ,  1062 ,  1072 , and  1082 , similar to unthreaded portion  730  of screw  728  (FIG.  43 A). Stationary block  1090  may include substantially unthreaded openings, similar to unthreaded openings  792  of stationary block  790  (FIG.  43 B), adapted to receive the unthreaded portions of the screws and in which the unthreaded portions of the screws are free to rotate. 
     First screw  1051  is preferably configured to be coupled to cam block  1054 . Second screw  1060  is preferably configured to be coupled to cam block  1064 . Third screw  1070  is preferably configured to be coupled to cam block  1074 . Fourth screw  1080  is preferably configured to be coupled to cam block  1084 . Cam blocks  1054 ,  1064 ,  1074 , and  1084  are preferably coupled to screws  1050 ,  1060 ,  1070  and  1080  in a manner similar to that depicted for cam block  744  and turnbuckle  740  (FIG.  43 B). Cam blocks  1050 ,  1060 ,  1070 , and  1080  preferably include sloped upper and lower surfaces similar to the sloped upper and lower surfaces as previously described for cam blocks in other embodiments. 
     Returning to FIG. 48, inner surface  1011  of engaging plate  1004  preferably includes sloped track  1020  configured to correspond to the lower surface of cam block  1050 ; sloped track  1022  configured to correspond to the lower surface of cam block  1060 ; sloped track  1024  configured to correspond to the lower surface of cam block  1070 ; and sloped track  1026  configured to correspond to the lower surface of cam block  1080 . The inner surface (not readily visible) of engaging plate  1002  preferably includes sloped tracks  1030 ,  1032 ,  1034 , and  1036  (the ends of which are visible in FIG. 48) configured to correspond to the upper surfaces of cam blocks  1050 ,  1060 ,  1070 , and  1080 , respectively. 
     Screws  1050 ,  1060 ,  1070 , and  1080  may still further include indentations (e.g., indentation  1058 , visible in FIG.  49 ). The indentations may be configured to receive the tip of an adjusting tool (not shown). The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. Rotation of first screw  1050  in a first angular direction may cause cam block  1054  to move away from stationary block  1090 ; rotation of first screw  1050  in an angular direction opposite the first angular direction may cause cam block  1054  to move toward stationary block  1090 . Rotation of second screw  1060  in a second angular direction may cause cam block  1064  to move away from stationary block  1090 ; rotation of second screw  1060  in an angular direction opposite the second angular direction may cause cam block  1064  to move toward stationary block  1090 . Rotation of third screw  1070  in a third angular direction may cause cam block  1074  to move away from stationary block  1090 ; rotation of third screw  1070  in an angular direction opposite the third angular direction may cause cam block  1074  to move toward stationary block  1090 . Rotation of fourth screw  1080  in a fourth angular direction may cause cam block  1084  to move away from stationary block  1090 ; rotation of second screw  1080  in an angular direction opposite the fourth angular direction may cause cam block  1084  to move toward stationary block  1090 . The first, second, third, and fourth angular directions may be the same. Alternatively, one of the first, second, third, and fourth angular directions may be opposite the other three of the first, second, third, and fourth angular directions. Alternatively, two of the first, second, third, and fourth angular directions may be opposite the other two of the first, second, third, and fourth angular directions. 
     As depicted in FIGS. 48-49, the cam blocks and sloped tracks may be configured such that motion of the cam blocks toward the edges of the engaging plates causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks away from the edges of the engaging plates causes the height between the engaging plates to increase. 
     The screws in fusion device  1000  may be positioned such that heights between engaging plates  1002  and  1004  along first side edge  1012  and along second side edge  1014  may be varied substantially independently to maintain a substantially natural lateral alignment. The screws in fusion device  1000  may also be positioned such that heights between engaging plates  1002  and  1004  along anterior edge  1008  and posterior edge  1010  be varied substantially independently to maintain a substantially natural lordosis. In addition, slopes of upper and lower surfaces of the cam blocks may be unequal such that heights between engaging plates  1002  and  1004  along first side edge  1012  and second side edge  1014  may vary between anterior edge  1008  and posterior edge  1010  to maintain a substantially natural lateral alignment and such that heights between engaging plates  1002  and  1004  along anterior edge  1008  and posterior edge  1010  may vary between first side edge  1012  and second side edge  1014  to maintain a substantially natural lordosis. 
     An alternate embodiment of an interbody fusion device is depicted in FIGS. 50-52. FIG. 50A depicts interbody fusion device  1100  in a lowered position. FIG. 50B depicts interbody fusion device  1100  in a raised position. FIG. 50C depicts interbody fusion device  1100  in an exploded view. Interbody fusion device  1100  preferably includes engaging plates  1102  and  1104  supported by bracket assembly  1106  (FIGS.  50 A- 50 C). Engaging plates  1102  and  1104  and bracket assembly  1106  may be formed of titanium, stainless steel, polymer, ceramic, composite material, or any other biocompatible material. Engaging plates  1102  and  1104  may include protrusions  1116 . Bracket assembly  1106  may include an alignment device for changing a height between engaging plates  1102  and  1104 . In an embodiment, the alignment device includes turnbuckle  1140  positioned between and substantially parallel to first elongated edge  1112  and second elongated edge  1114 . Bracket assembly  1106  may include lateral projection  1124  extending into the interior of the bracket assembly and supporting turnbuckle  1140 . Turnbuckle  1140  may include middle portion  1146  (FIG.  51 A), similar to middle portion  543  of turnbuckle  540  (FIG.  38 ), disposed between the ends of the turnbuckle and having a diameter greater than a diameter of the threaded portions. Lateral projection  1124  (FIG. 50C) is preferably sized such that middle portion  1146  of turnbuckle  1140  is retained within the lateral projection while the turnbuckle is free to rotate within the lateral projection. Inner surface  1109  of engaging plate  1104  (FIG. 50C) and the inner surface of engaging plate  1102  (not readily visible in FIG. 50C) may include arcuate grooves  1128  and  1126 , respectively, which correspond to the curvature of turnbuckle  1140 . 
     Turning to FIG. 51A, first threaded portion  1142  of turnbuckle  1140  may be threaded in a first direction and second threaded portion  1144  may be threaded in a direction opposite the first direction. Turnbuckle  1140  is preferably configured to be coupled to cam blocks  1150  and  1160 . Cam blocks  1150  and  1160  are preferably similar to cam block  260  in FIG.  29 B. The cam blocks are preferably coupled to the turnbuckle as depicted for cam block  260  and turnbuckle  250  (FIG.  29 B). The slopes of corresponding features (e.g., upper surfaces) on the cam blocks may be substantially equivalent. Alternatively, the slopes of corresponding features on the paired cam blocks may be different. Further, the slopes of the upper and lower surfaces on an individual cam block may differ. 
     In another embodiment (not shown), bracket assembly  1106  may include a turnbuckle and cam blocks similar to turnbuckle  450  and cam blocks  460  and  468  of interbody fusion device  400  (FIGS.  36 - 37 ). In still another embodiment (not shown), bracket assembly  1106  may include a screw and a cam block similar to screw  650  and cam block of  654  interbody fusion device  600  (FIG.  40 ). In yet another embodiment (not shown), bracket assembly  1106  may include a screw similar to screw  650  threaded through a cam block similar to cam block  460 . 
     Referring to FIG. 50C, inner surface  1109  of engaging plate  1104  preferably includes sloped tracks  1130  and  1132  constructed such that the slopes of the sloped tracks are substantially equivalent to the slopes of the lower surfaces of cam blocks  1150  and  1160 , respectively. The inner surface of engaging plate  1102  preferably includes sloped tracks  1134  and  1136 , the ends of which are visible in FIG. 50C, constructed such that the slopes of the sloped tracks are substantially equivalent to the slopes of the upper surfaces of cam blocks  1150  and  1160 , respectively. 
     As depicted in FIGS. 50A-50C, bracket assembly  1106  encloses openings at first narrow edge  1108  and second narrow edge  1110 . FIG. 51A is a cut-away perspective view of bracket assembly  1106 ; FIG. 51B is a cross-sectional view of bracket assembly  1106 . Openings  1120  and  1122  (FIG. 51B) may be sized such that bracket assembly  1106  substantially surrounds the ends of turnbuckle  1140  and such that turnbuckle  1140  is free to rotate within the openings. In an alternate embodiment (FIGS.  51 C and  51 D), bracket assembly  1106 A encloses openings only at one of the narrow edges. Opening  1120 A may be sized such that bracket assembly  1106 A substantially surrounds the end of turnbuckle  1140 A and such that turnbuckle  1140 A is free to rotate within opening  1120 A. 
     At least one end of turnbuckle  1140  may still further include an indentation. In an embodiment, each end of turnbuckle  1140  includes an indentation. Indentation  1148  is visible in FIG. 51A; indentations  1148  and  1149  are visible in FIG.  51 B. The indentations may be configured to receive the tip of an adjusting tool. The adjusting tool may be a screwdriver. In a preferred embodiment, the adjusting tool is an allen wrench. The adjusting tool may be used to rotate the turnbuckle. Rotation of turnbuckle  1140  in a first direction may cause the cam blocks to move away from each other; rotation of turnbuckle  1140  in a direction opposite the first direction may cause the cam blocks to move toward each other. 
     As depicted in FIGS. 50A-50C, cam blocks  1150  and  1160  and sloped tracks  1130 ,  1132 ,  1134 , and  1136  may be configured such that motion of the cam blocks toward first narrow edge  1108  and second narrow edge  1110  causes the height between the engaging plates to increase. The cam blocks and sloped tracks, however, may be configured such that motion of the cam blocks toward the first and second narrow edges causes the height between the engaging plates to decrease. 
     FIG. 52A is a top view of fusion devices  1100 L and  1100 R inserted between two vertebrae (shown in phantom). Anterior edge  1190 , posterior edge  1192 , first side edge  1194 , and second side edge  1196  of the vertebrae are as indicated. As pictured, the first narrow edge of alignment device  1100 L is oriented toward anterior edge  1190  of the vertebrae, and the second narrow edge of alignment device  1100 R is oriented toward anterior edge  1190 . Alternatively, the first narrow edge of each alignment device may be oriented toward anterior edge  1190 ; the second narrow edge of each alignment device may be oriented toward anterior edge  1190 ; or the first narrow edge of alignment device  1100 R and the second narrow edge of alignment device  1100 L may be oriented toward anterior edge  1190 . 
     To install the fusion devices, a discectomy is preferably performed from an anterior approach. All cartilage and soft tissue are preferably removed from the vertebral endplate as would normally be done for placement for a femoral strut graft. Such a procedure is well within the knowledge of a skilled practitioner of the art. The engaging plates may be deployed in the disc space between adjacent vertebrae. FIG. 52B is a front (anterior) view of alignment devices  1100 L and  1100 R installed in the intervertebral disc space. Turnbuckles  1140 L and  1140 R may be rotated to achieve the desired heights  1180 L and  1180 R between outer surfaces of the engaging plates. The proper heights may be determined beforehand using x-ray techniques in which the side portions of the intervertebral disc space are examined. Bone graft material  1188  may be packed between alignment devices  1100 L and  1100 R to facilitate fusion of the vertebrae. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.