Abstract:
A composite interbody device for use with spinal fusion surgery is described herein. The composite interbody device comprises a central body made from a radiolucent biocompatible polymer (e.g., PEEK or UHMWPE) and metallic plates, which are placed at the superior and inferior surfaces of the central body. The metallic plates are comprised of an end plate that is adjacent to a vertebral body and an intermediate plate that is adjacent to the central body. The end plates may have one or more arrays of apertures to facilitate bone growth into the end plates to secure the interbody device within the intervertebral space. The intermediate plates may also have one or more arrays of apertures to allow the central body to bond to the end plates through compression molding, injection molding, and/or heat molding. The arrays of apertures in the end plates are not aligned with the arrays of apertures in the intermediate plates so that polymer material of the central body will not penetrate into the end plate, where bone growth is encouraged, and vice versa.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to a composite interbody device adapted for insertion between two adjacent vertebrae to promote the fusion of two vertebrae. 
       BACKGROUND 
       [0002]    The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex. The complex consists of an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The over 20 bones of the spinal column are anatomically categorized as one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine which comprises the top of the spine up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine are sacral bones, including the coccyx. 
         [0003]    The spinal column of bones is highly complex in that it includes over 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. 
         [0004]    Genetic or developmental irregularities, trauma, chronic stress, tumors and disease, however, can result in spinal pathologies which either limit this range of motion or threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior or lateral implants. Lateral and anterior assemblies are coupled to the anterior portion of the spine which is in the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods (“bilateral spinal support rods”), which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through pedicles. 
         [0005]    Spinal fusion treatment is commonly used to treat spinal disc disease and/or spinal instability. The degeneration of spinal discs can create significant pain and discomfort for individuals suffering from this affliction. In many cases, this pain can be alleviated by immobilizing the vertebrae adjacent to the degenerated disc and encouraging bone growth across the immobilized area of the spine. Conventional spinal implants are designed to facilitate bone through-growth, or fusion resulting from growth of bone through holes or channels through the implants. Although effective, the bone through-growth process is slow, sometimes taking more than a year to complete. Through-growth can be further delayed if the implant area is not immobilized. Even micro-motion of the implant area can disturb and disrupt bone growth, leading to increased incidence of subsidence and pseudarthrosis. 
         [0006]    Some conventional devices attempt to improve implant stabilization by encouraging bone on-growth—a comparatively rapid, planar growth of bone upon surfaces of an adjacent implant, or upon surfaces of adjacent bone. For example, on-growth may be encouraged by coating a titanium cage with a chemical such as hydroxyapatite to encourage new-grown bone to adhere to the implant surface. However, because titanium is radioopaque, titanium implants can interfere with diagnostic assessment of bone growth, whether coated with hydroxyapatite or not. For example, implants made primarily of radio-opaque titanium may obscure visualization of bone growth (e.g., through-growth) on x-rays. Titanium may likewise cause signal artifact with MRIs or CTs, making it difficult to determine if fusion has occurred. 
         [0007]    In order to avoid the visualization problems of titanium implants, attempts have been made to mix hydroxyapatite with, or apply hydroxyapatite to, radiolucent polymer plastics (e.g., PEEK, HDPE, or other non-scattering biocompatible materials) to form a cage/implant. However, PEEK provides poorer fixation than titanium, and thus, PEEK implants often require supplemental fixation such as posterior pedicle screws and rod instrumentation. 
       SUMMARY 
       [0008]    Various embodiments of a composite interbody device for use with spinal fusion surgery are described herein. The composite interbody device may a central body made from a radiolucent biocompatible polymer (e.g., PEEK or UHMWPE) and metallic plates that are placed at the superior and inferior surfaces of the central body. The metallic plates comprise an end plate that is adjacent to a vertebral body and an intermediate plate that is adjacent to the central body. The end plates may have one or more arrays of apertures to facilitate bone growth into the end plates to secure the interbody device within the intervertebral space. The intermediate plates may also have one more arrays of apertures or linear recesses to allow the central body to bond to the end plates through compression molding, injection molding, and/or heat molding. The arrays of apertures in the end plates are not aligned with the arrays of apertures in the intermediate plates so that polymer material of the central body will not penetrate into the end plate, where bone growth is encouraged, and vice versa. 
         [0009]    According to one embodiment, a composite interbody device may comprise a first end plate comprising a biocompatible metal and having a superior surface adapted to contact an upper vertebral body, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the first end plate further comprising an array of apertures passing from the superior surface to the inferior surface; a first intermediate plate comprising a biocompatible metal and having a superior surface adapted to contact the inferior surface of the first endplate, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the first intermediate plate further comprising an array of apertures passing from the superior surface to the inferior surface; a central body comprising a biocompatible polymer and having a superior surface adapted to contact the inferior surface of the first intermediate plate, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface; a second intermediate plate comprising a biocompatible metal and having a superior surface adapted to contact the inferior surface of the central body, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the second intermediate plate further comprising an array of apertures passing from the superior surface to the inferior surface; a second end plate comprising a biocompatible metal and having a superior surface adapted to contact the inferior surface of the second intermediate plate, an inferior surface adapted to contact a lower vertebral body, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the second end plate further comprising an array of apertures passing from the superior surface to the inferior surface; wherein the array of apertures at the inferior surface of the first end plate do not overlap the array of apertures at the superior surface of the first intermediate plate when the inferior surface of the first end plate contacts the superior surface of the first intermediate plate; and wherein the array of apertures at the inferior surface of the second intermediate plate do not overlap the array of apertures at the superior surface of the second end plate when the inferior surface of the second intermediate plate contacts the superior surface of the second end plate. 
         [0010]    According to another embodiment, a composite interbody device suitable for insertion between two adjacent vertebrae is formed according to the following method: forming a first end plate comprising a biocompatible metal and having a superior surface operable to contact a upper vertebral body, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the first end plate further comprising a first array of apertures passing from the superior surface to the inferior surface, a second array of apertures passing from the interior side wall to the exterior side wall, and a third array of apertures passing from another interior side wall to another exterior side wall, wherein the first, second, and third arrays of apertures intersect each other inside the first end plate; forming a first intermediate plate comprising a biocompatible metal and having a superior surface, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the first intermediate plate further comprising a fourth array of apertures passing from the superior surface to the inferior surface, a fifth array of apertures passing from the interior side wall to the exterior side wall, and a sixth array of apertures, passing from another interior side wall to another exterior side wall, wherein the fourth, fifth, and sixth arrays of apertures intersect each other inside the first intermediate plate; connecting the first end plate to the first intermediate plate so that the first array of apertures at the inferior surface of the first end plate do not overlap the third array of apertures at the superior surface of the first intermediate plate when the inferior surface of the first end plate; forming a second intermediate plate comprising a biocompatible metal and having a superior surface, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the second intermediate plate further comprising a seventh array of apertures passing from the superior surface to the inferior surface, an eighth array of apertures passing from the interior side wall to the exterior side wall, a ninth array of apertures, passing from another interior side wall to another exterior side wall, wherein the seventh, eighth, and ninth arrays of apertures intersect each other inside the second intermediate plate; forming a second end plate comprising a biocompatible metal and having a superior surface, an inferior surface operable to contact a lower vertebral body, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface, the second end plate further comprising a tenth array of apertures passing from the superior surface to the inferior surface, an eleventh sixth array of apertures, passing from an interior side wall to an exterior side wall, and a twelfth array of apertures, passing from another interior side wall to another exterior side wall; connecting the second end plate to the second intermediate plate so that the seventh array of apertures at the inferior surface of the second intermediate plate does not overlap the tenth array of apertures at the superior surface of the second end plate; forming a central body comprising a biocompatible polymer and having a superior surface, an inferior surface, an exterior side wall connecting the superior surface to the inferior surface, and an interior side wall connecting the superior surface to the inferior surface; connecting the superior surface of the central body to the inferior surface of the first intermediate plate; and connecting the inferior surface of the central body to the superior surface of the second intermediate plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and: 
           [0012]      FIG. 1  is a perspective view of an exemplary embodiment of a composite interbody device, according to one aspect of the invention; 
           [0013]      FIG. 1A  is an exploded perspective view of an exemplary embodiment of a composite interbody device, according to one aspect of the invention; 
           [0014]      FIG. 2  is a perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0015]      FIG. 2A  is an exploded perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0016]      FIG. 3  is a perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0017]      FIG. 3A  is a cross-sectional perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0018]      FIG. 3B  is a top view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0019]      FIG. 4  is a perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0020]      FIG. 4A  is a cross-section view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0021]      FIG. 4B  is a perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0022]      FIG. 4C  is a perspective view of an exemplary embodiment of a metallic portion of a composite interbody device, according to one aspect of the invention; 
           [0023]      FIG. 5A  is a three dimensional cross-section view of a portion of a metallic plate of a composite interbody device, according to one aspect of the invention; 
           [0024]      FIG. 5B  is a three dimensional cross-section view of a portion of a metallic plate of a composite interbody device, according to one aspect of the invention; 
           [0025]      FIG. 6  is a perspective view of another exemplary embodiment of a composite interbody device; 
           [0026]      FIG. 6A  is a close-up perspective view of an exemplary embodiment of a composite interbody device; 
           [0027]      FIG. 7  is an exploded perspective view of another exemplary embodiment of a composite interbody device; 
           [0028]      FIG. 8  is a perspective view of an exemplary embodiment of a composite interbody device suitable for use in the cervical region of the spine; 
           [0029]      FIG. 9  is a perspective view of an exemplary embodiment of a composite interbody device with bone screws into adjacent vertebrae; 
           [0030]      FIG. 10  is a perspective view of another exemplary embodiment of a composite interbody device with bone screws into adjacent vertebrae; 
           [0031]      FIG. 11  is a perspective view of an exemplary embodiment of a composite interbody device in the shape of a TLIF or PLIF cage; 
           [0032]      FIG. 12  is a perspective view of an exemplary embodiment of a composite interbody device in the shape of a lateral cage; 
           [0033]      FIG. 13  is an exemplary radiograph of a composite interbody device after insertion between two adjacent vertebrae; 
           [0034]      FIG. 14A  is a perspective view of an exemplary embodiment of an installation tool for composite interbody device; 
           [0035]      FIG. 14B  is an exploded perspective view of an exemplary embodiment of an installation tool with a composite interbody device attached thereto; 
           [0036]      FIG. 14C  is a perspective view of an exemplary embodiment of an installation tool for composite interbody device; 
           [0037]      FIG. 15A  is a perspective view of an exemplary embodiment of a locking device suitable for use with a composite interbody device; and 
           [0038]      FIG. 15B  is a perspective view of an exemplary embodiment of a locking device suitable for use with a composite interbody device. 
       
    
    
       [0039]    Although similar reference numbers may be used to refer to similar elements for convenience, it can be appreciated that each of the various example embodiments may be considered to be distinct variations. 
       DETAILED DESCRIPTION 
       [0040]    Exemplary embodiments will now be described hereinafter with reference to the accompanying figures, which form a part hereof, and which illustrate examples by which the exemplary embodiments, and equivalents thereof, may be practiced. As used in the disclosures and the appended claims, the terms “embodiment,” “example embodiment” and “exemplary embodiment” do not necessarily refer to a single embodiment, although it may, and various example embodiments, and equivalents thereof, may be readily combined and interchanged, without departing from the scope or spirit of present embodiments. Furthermore, the terminology as used herein is for the purpose of describing example embodiments only and is not intended to be limitations of the embodiments. In this respect, as used herein, the term “plate” may refer to any substantially flat structure or any other three-dimensional structure, and equivalents thereof, including those structures having one or more portions that are not substantially flat along one or more axis. Furthermore, as used herein, the terms “opening,” “recess,” “aperture,” and equivalents thereof, may include any hole, space, area, indentation, channel, slot, bore, and equivalents thereof, that is substantially round, oval, square, rectangular, hexagonal, and/or of any other shape, and/or combinations thereof, and may be defined by a partial, substantial or complete surrounding of a material surface. Furthermore, as used herein, the term “in” may include “in” and “on,” and the terms “a,” “an” and “the” may include singular and plural references. Furthermore, as used herein, the term “by” may also mean “from,” depending on the context. Furthermore, as used herein, the term “if” may also mean “when” or “upon,” depending on the context. Furthermore, as used herein, the words “and/or” may refer to and encompass any and all possible combinations of one or more of the associated listed items. 
         [0041]    An embodiment consistent with one aspect of the invention is depicted in  FIG. 1 . In  FIG. 1 , a composite interbody device  100  is depicted as having multiple layers including a first end plate  105 , a first intermediate plate  110 , a central body  115 , a second intermediate plate  120 , and a second end plate  125 . Preferably, the end plates ( 105 ,  125 ) and intermediate plates ( 110 ,  120 ) are comprised of a biocompatible metal such as surgical stainless steel or titanium. The first end  105  plate comprises a superior surface  106  that is adopted to be contacted to an upper vertebral body. The first end plate  105  also comprises an inferior surface (not visible) on an opposite side of the plate from the superior surface  106 . The first end plate  105  also includes an exterior sidewall  107  that connects the superior surface  106  to the inferior surface and passes around the circumference of the inter body device  100 . In addition, the first end plate  105  may include an interior sidewall  108  that connects the superior surface  106  to the inferior surface at the interior of the first end plate  105 . 
         [0042]    Also shown in  FIG. 1  is a first intermediate plate  110  that comprises a superior surface that connects to the inferior surface of the first end plate  105 . The first intermediate plate  110  also includes an inferior surface that is located on an opposite side from the superior surface. The first intermediate plate  110  further includes an exterior sidewall  111  that connects the superior surface to the inferior surface and extends around the circumference of the first intermediate plate  110 . In addition, the first intermediate plate  110  also includes and interior sidewall  112  that extends from the superior surface of the first intermediate plate to the inferior surface of the first intermediate plate  110 . 
         [0043]    The composite inter-body device  100  also includes a central body  115  that is comprised of a biocompatible polymer such as Polyether-ether-ketone (PEEK) or Ultra High Molecular Weight Polyethylene (UHMWPE). It is preferable that the central body  115  be comprised of a material that is radiolucent so that the amount of bone on-growth and through-growth can be monitored through X-ray imaging. The central body  115  comprises a superior surface  117  that is adapted to contact these inferior surface of the first intermediate plate  110  and an inferior surface (not shown) that is adapted to contact to the superior surface  121  of the second intermediate plate  120  (see  FIG. 1A ). The central body  115  also includes an exterior side wall  116  and an interior side wall  118  that connect the superior surface of the central body  117  to the inferior surface of the central body. The central body  115  also comprises two holes  130  and  135  that are formed in the exterior side wall  116  and pass through the interior side wall  118 . Holes  130  and  135  can be used with an implantation tool to insert the interbody device  100  into the space between two adjacent vertebrae. The use of these holes  130 ,  135  will be described in more detail with reference to  FIGS. 14A-14C  below. 
         [0044]    In the embodiment depicted in  FIG. 1 , the composite interbody device  100  further comprises a second intermediate plate  120  and a second end plate  125  having similar surfaces and end walls to the first end plate  105  and first intermediate plate  110 . The inferior surface (not shown) of the second end plate  125  is adapted to contact a lower vertebral body. Thus, when the composite interbody device  100  is inserted between two adjacent vertebrae, the two end plates ( 105 ,  125 ) will maintain contact with the surfaces of the adjacent vertebrae. 
         [0045]    An exploded view of the composite inter-body device  100  is depicted in  FIG. 1A . As seen in  FIG. 1A , the central body  115  includes a superior surface  117  that contacts the inferior surface of the first intermediate plate  110 . Also shown in  FIG. 1A  is a superior surface  121  of the second intermediate plate  120  that is adopted to contact with the inferior surface of the central body  115 . 
         [0046]    Another feature of the composite interbody device  100  depicted in  FIGS. 1 and 1A  is the arrays of apertures that are found in the end plates ( 105 ,  125 ) and the intermediate plates ( 110 ,  120 ). The array of apertures in the first end plate  105  comprises a series of holes  109  that pass through the first end plate from its superior surface  106  to its inferior surface. Although the apertures  109  depicted in  FIGS. 1 and 1A  have a circular cross-section, other apertures may be utilized, including, apertures having a square, rectangular, elliptical, hexagonal, triangular, or any other cross-section that can be readily formed in the end plate according to known fabrication processes. Preferably, the apertures in the first end plate  105  must be of a sufficient size to permit bone in-growth, on-growth, and through-growth in the first end plate  105 . In addition, the array of apertures  109  formed on the superior surface of the first end plate  105  form a frictional surface that discourages relative movement of the composite interbody device  100  with respect to the adjacent vertebral body. Spikes or ridges can be incorporated into the superior surface  106  of the first end plate  105  (and on the inferior surface of the second end plate  125 ) to further restrict the relative movement of the composite interbody device  100  with respect to the adjacent vertebral body. Although not visible in  FIG. 1  or  1 A, the second end plate  125  may comprise an array of apertures from the superior surface  121  to the inferior surface of the second end plate  125 . Like the first end plate  105 , this array of apertures creates a frictional surface on the inferior surface of the second end plate  125  that discourages relative movement of the composite interbody device  100  with respect to the lower adjacent vertebral body. Further, the array of apertures in the second end plate  125  must be of a sufficient size to permit bone in-growth, on-growth, and through-growth in the second end plate  125 . Both the first intermediate plate  110  and the second intermediate plate  120  include arrays of apertures that are similar to the arrays of apertures found in the first end plate  105  and second end plate  120 . According to one embodiment, the arrays of apertures in the end plates ( 105 ,  125 ) are not aligned with the arrays in the intermediate plates ( 110 ,  120 ). Indeed, the arrays of apertures are arranged such that none of the holes in the surfaces that contact to each other will overlap. This concept is depicted in further detail in  FIGS. 2 and 2A . 
         [0047]    According to one embodiment, a composite interbody device  100  suitable for use in the lumbar region of the spine will have a width ranging from about 8 mm to about 20 mm, a height ranging from about 6 mm to about 16 mm, and a length ranging from about 25 mm to about 45 mm. According to another embodiment, a composite interbody device  100  for use in the cervical region of the spine will have a width ranging from about 12 mm to about 15 mm, a height ranging from about 6 mm to about 14 mm, and a length ranging from about 12 mm to about 15 mm. The interbody device  100  may also be provided with parallel or lordotic superior and inferior surfaces, depending upon the particularly anatomical needs of the patient. In addition, the interbody device  100  may be provided with concave or convex side walls to further suite the anatomical needs of the patient. The interbody device  100  may also be provided with a major aperture passing from the superior surface  106  of the first end plate  105 , through the device  100 , and to the inferior surface of the second end plate  125 . The major aperture is used to promote bone through-growth in the device and can be loaded with appropriate materials (e.g., biologics, hydroxyapatite, etc.) to encourage through-grown of the bone to promote fusion of the adjacent vertebrae. 
         [0048]    In  FIG. 2 , a first end plate  105  is depicted as being connected to a first intermediate plate  110 .  FIG. 2A  is an exploded view of the first end plate  105  and the first intermediate plate  110 . An array of apertures  109  in the first end plate is also depicted in  FIGS. 2 and 2A . Also shown in  FIG. 2A  is an array of apertures  112  in the first intermediate plate  110 . The non-alignment of the arrays of apertures  109  and  112  can be seen in  FIG. 2A . 
         [0049]    Yet another view of the non-alignment of the arrays of apertures is depicted in  FIGS. 3 and 3A . In  FIG. 3 , a second intermediate plate  120  is depicted as being connected to a second end plate  125 . Much like the plates shown in  FIGS. 2 and 2A , the array of apertures  122  in the second intermediate plate  120  are not aligned with the array of apertures in the second end plate  125 . This non-alignment is apparent by examining the cross-section of the composite inter-body device  100  taken along axis A and depicted in  FIG. 3A . In  FIG. 3A , the cross-section passes through four apertures in the second intermediate plate  120 , but does not pass through any of the apertures found in the second end plate  125 . A top view of the second intermediate plate  120  and its corresponding array of apertures  122  is depicted in  FIG. 3B . 
         [0050]    According to another embodiment of the invention, the end plates ( 105 ,  125 ) and the intermediate plates ( 110 ,  120 ) may further comprise a second and third array of apertures. In the embodiments depicted in  FIGS. 1 ,  2  and  3 , the first array of apertures passes from the superior surface of the plate, through the plate, to the interior surface of the plate. In  FIG. 4 , a second array of apertures is depicted as passing from the interior side walls of the plates to the exterior side walls of the plates. In particular, as shown in  FIG. 4 , a first end plate  105  comprises a first array of apertures  109  that pass from the superior surface  106  to the interior surface (not shown) of the end plate  105 . A second array of apertures  410  pass from an interior side wall  108 A of the first end plate  105  to an exterior side wall  107 A of the end plate  105 . In addition, a third array of apertures  415  may be utilized that pass from another interior side wall  108 B of the first end plate  105  to the other exterior side wall  107 B of the end plate  105 . This third array of apertures is marked in  FIG. 4  with reference numbers  415 . Also shown in  FIG. 4  is a second and third array of apertures located in the first intermediate plate  110 . The second array of apertures  420  in the first intermediate plate  110  pass from an interior side wall  112 A to an exterior side wall  111 A of the first intermediate plate  110 . Similarly, a third array of apertures  425  pass from another interior side wall  112 B to another exterior side wall  111 B of the first intermediate plate  110 . Preferably, the first, second and third array of apertures in each plate are orthogonal with respect to each other thereby forming an interconnected rectangular void of apertures in each plate. 
         [0051]      FIG. 4A  is a cross-sectional view of the combination of the first end plate  105  with the first intermediate plate  110  taken along axis A. In  FIG. 4A , the second array of apertures  410  are depicted in circular cross-section, while the first array of apertures  109  and the third array of apertures  415  are depicted in rectangular cross-section. This is due to the cross-section A being taken along the longitudinal axis of the first array of apertures  109  and the third array of apertures  415 . In addition, due to the non-alignment of the arrays of apertures between the first end plate  105  and the first intermediate plate  110 , only the second array of apertures  420  in the first intermediate plate  110  are depicted in circular cross-section. There is no overlap of the apertures between the first end plate  105  and the first intermediate plate  110 . 
         [0052]      FIG. 4B  depicts a cross-section of the combination of the first end plate  105  and the first intermediate plate  110  taken along axis B of  FIG. 4 . In  FIG. 4B , only the second array of apertures  415  is visible in the first end plate  105  since the cross-section axis B does not intersect with the first or third array of apertures in the first end plate  105 . In contrast, all three arrays of apertures are visible in the first intermediate plate  110 . The second array of apertures  420  is visible in circular cross-section, while the first array of apertures  112  and the third array of apertures  425  are depicted in rectangular cross-section. Again, this is due to the non-alignment of the arrays of apertures between the first end plate  105  and the first intermediate plate  110 . 
         [0053]      FIG. 4C  is a cross-sectional view of the combination of the first end plate  105  with the first intermediate plate  110  taken along axis C in  FIG. 4 . The arrangement of the arrays of apertures in the first end plate  105  and in the first intermediate plate  110  is identical to the arrangement shown in  FIG. 4A , thus showing the repeating pattern of the arrays of apertures. 
         [0054]    A three-dimensional cross-section of the intersection of the apertures is depicted in  FIGS. 5A and 5B . In  FIG. 5A , a three-dimensional cross-section has been taken of the first end plate  105  showing the intersection of aperture  109  with aperture  410  and aperture  415 . Depending upon the size of the aperture, a varying volume of material is removed from the interior of the end plate  105 . In  FIG. 5A , the size of apertures  109 ,  410 , and  415  has been adjusted so that approximately 40% of the volume of the cubic cross-section  500  has been removed. In  FIG. 5B , a similar cubic cross-section is depicted of a section of the first end plate  105 . In  FIG. 5B , apertures  109 ,  410 , and  415  are larger than the apertures depicted in  FIG. 5A . As a result, approximately 60% of the volume of the cubic cross-section  500  has been removed by the apertures. 
         [0055]    According to one embodiment, the end plates  105  and  125  may utilize a first, second and third array of apertures that intersect with each other such that somewhere between about 40% to about 70% of the volume of the end plates ( 105 ,  125 ) are removed. Removal of this volume has two beneficial effects. First, it creates a frictional surface on the vertebrae-facing surfaces that discourage relative movement of the interbody device  100  with respect to the adjacent vertebrae. Second, the rectangular void created by the intersection of the arrays of apertures promotes bone on-growth, in-growth, and through-growth into the end plates ( 105 ,  125 ). According to one embodiment, the size of the apertures in the end plates can range from 0.25 to 0.5 millimeters. 
         [0056]    According to another aspect, the intermediate plates  110  and  120  may also utilize first, second, and third arrays of apertures that intersect each other. The apertures in the intermediate plates ( 110 ,  120 ) may have smaller sizes such that the intermediate plates have only 30% to 60% of the volume of those plates removed by the arrays of apertures. The size of the apertures in the intermediate plates  110  and  120  can be in the range of 0.25 to 0.5 millimeters. 
         [0057]    An alternative embodiment of a composite interbody device  600  is depicted in  FIG. 6 . In  FIG. 6 , a first end plate  605  is comprised of a biocompatible metal, such as surgical stainless steel, titanium, or composites thereof. The first end plate  605  includes a superior surface  606 , an inferior surface (not visible), an exterior side wall  607 , and an interior side wall  608  wherein the interior and exterior side walls ( 607 ,  608 ) connect the superior surface  606  to the inferior surface. In  FIG. 6 , a first array of apertures  609  is depicted that pass from the superior surface of the end plate  606  to the inferior surface of the endplate. Much like the embodiments described in  FIGS. 1-4 , the first end plate  605  may also include second and third arrays of apertures that pass from the interior side walls of the end plate  608  to the exterior side walls of the end plate  607 . As described previously, these arrays of apertures can intersect and form a three-dimensional network of openings in the first end plate  605  into which bone through-growth may occur. 
         [0058]    Also depicted in  FIG. 6  is a first intermediate plate  610  that includes a superior surface (not shown) that connects to the inferior surface of the first end plate  605 . The first intermediate plate  610  may be comprised of a biocompatible metal, such as surgical stainless steel, titanium, or composites thereof. The first intermediate plate includes an exterior end wall  611  and an interior end wall  612  that connect the superior surface to the inferior surface. The inferior surface of the first intermediate plate  610  further includes an array of linear recesses  612  that pass from one exterior side wall  611  to another opposing exterior side wall  611 . A magnified view of a representative example of these linear recesses  612  is depicted in  FIG. 6A . 
         [0059]    In  FIG. 6A , each of the linear recesses has a T-shaped cross section in which the recess has a wider body portion  613  that is deeper inside the first intermediate plate  610  and a narrower neck portion  614  that is on the surface of the first intermediate plate  610 . Although this disclosed embodiment uses a linear recess  612  with a T-shaped cross section, other shapes and forms of the linear recesses can be utilized. According to another embodiment, the inferior surface of the first intermediate layer  610  may further comprise a second array of linear recesses that are arranged orthogonally to the first array of linear recesses, thereby creating an array of protrusions from the inferior surface of the first intermediate layer. According to yet another embodiment, two arrays of orthogonal linear recesses with T-shaped cross-sections can be utilized. 
         [0060]    Also depicted in  FIGS. 6 and 6A  is a central body  615  comprising a biocompatible polymer, such as PEEK, UHMWPE, or combinations thereof. The central body  615  includes a superior surface that connects to the inferior surface of first intermediate plate  610  and an inferior surface that connects to the superior surface of the second intermediate plate  620 . The superior surface of the second intermediate plate  620  may utilize an array of linear recesses  612  in the same way that as the first intermediate plate  610 . In addition, the inferior surface of the second intermediate plate  620  is connected to a second end plate  625  that may include a first, second, and third array of apertures that intersect and form a three-dimensional network of openings in the second plate  625  into which bone through-growth may occur. 
         [0061]    An exploded perspective view of the composite interbody device  700  depicted in  FIG. 6  is depicted in  FIG. 7 . In  FIG. 7 , a first end plate  705 , a first intermediate plate  710 , a central body  715 , a second intermediate plate  720 , and a second end plate  725  are depicted. An array of linear recesses  712  with T-shaped cross sections are depicted in both intermediate plates ( 710 ,  720 ) and a first array of apertures  709  are depicted in both end plates ( 705 ,  725 ). As mentioned previously, second and third arrays of apertures can be added to the end plates ( 705 ,  725 ) to form three-dimensional networks of openings in the end plates ( 705 ,  725 ) to facilitate bone through-growth. 
         [0062]    According to one embodiment, a composite interbody device can be fabricated according to the following process. End plates ( 105 ,  125 ) are formed from a biocompatible metal in a generally flat arrangement having a superior surface, an inferior surface, interior end walls that connect the superior surface to the inferior surface, and exterior end walls that connect the superior surface to the inferior surface. The thickness of the end plates may range from 0.5 mm to 1.5 mm, depending upon the anatomical placement of the implant and other surgical considerations. The end plates ( 105 ,  125 ) can be formed from a sheet of biocompatible metal, milled, or folded as needed to create the desired contours and shape. According to one embodiment, the biocompatible metal may include one or more arrays of apertures ( 109 ,  410 ,  415 ) prior to the fabrication of the end plates. According to another embodiment, the arrays of apertures ( 109 ,  410 ,  415 ) may be formed after the end plates have been fabricated. The intermediate plates ( 110 ,  120 ) can be fabricated using a similar process to the end plates ( 105 ,  125 ). However, care should be taken to ensure that the arrays of apertures in the intermediate plates ( 112 ,  420 ,  425 ) are offset from the arrays of apertures in the end plates so that none of the apertures of these two plates intersect with each other. The thickness of the intermediates plates ( 110 ,  120 ) may range from 0.25 mm to 0.8 mm, depending upon the anatomical placement of the implant and other surgical considerations. After the end plates ( 105 ,  125 ) and intermediate plates ( 110 ,  120 ) have been fabricated, the are connected to each other through metal bonding or any other appropriate joining process. 
         [0063]    The central body  115  can be formed in a variety of ways. According to one embodiment, the central body  115 , is milled from a solid piece of biocompatible polymer, such as PEEK or UHMWPE. After the central body  115  has been formed, it can be joined to the first and second end plates by heating the central body  115  to a flow temperature of the biocompatible polymer such that the polymer begins to penetrate the apertures or recesses in the intermediate plates ( 110 ,  120 ) to form a strong bond with those plates. The end plates ( 105 ,  125 ) may be compressed towards each other during this process to encourage the penetration of the biocompatible polymer into the apertures or recesses in the intermediate plates ( 110 ,  120 ). 
         [0064]    According to another embodiment, the central body  115  is formed through an injection molding process. According to this process, the end plates are placed into a appropriate mold and a biocompatible polymer is injected into the mold such that the polymer begins to penetrate the apertures or recesses in the intermediate plates ( 110 ,  120 ) to form a strong bond with those plates. Once the biocompatible polymer has set, the composite interbody device may be removed. 
         [0065]    According to another aspect, the interior side walls (e.g.,  108 ,  112 ) of the composite interbody device  100  may be formed after the end plates ( 105 ,  125 ) and intermediate plates ( 110 ,  120 ) have been joined to the central body  115 . A CNC machine or other milling device can remove an interior portion of the device  100 , thereby leaving interior side walls in place. 
         [0066]    According to yet another aspect, the screw holes ( 130 ,  135 ) can be formed by a milling process before or after the connection of the end plates ( 105 ,  110 ). The screw holes ( 130 ,  135 ) can also be formed in the injection molding process, by using a suitable mold. 
         [0067]    A exemplary embodiment of a composite interbody device suitable for insertion between cervical vertebrae is depicted in  FIG. 8 . In  FIG. 8 , a cervical composite interbody device  800  is depicted as comprising a first end plate  805 , a first intermediate plate  810 , a central body  815 , a second intermediate plate  820 , a second end plate  825 , a first hole  830 , and a second hole  835 . As described previously, the end plates ( 805 ,  825 ) and intermediate plates ( 810 ,  820 ) are comprised of biocompatible metal and may include the arrays of apertures and linear recesses to facilitate bone through-growth and bonding to the central body. The cervical composite interbody device  800  may further include a bullet-shaped face  840  that will facilitate the insertion of the device between two adjacent cervical vertebrae. 
         [0068]    Another exemplary embodiment of a composite interbody device suitable for insertion between cervical vertebrae is depicted in  FIG. 9 . In  FIG. 9 , a cervical composite interbody device  900  is depicted as comprising a first end plate  905 , a first intermediate plate  910 , a central body  915 , a second intermediate plate  920 , a second end plate  925 , a first angled screw hole  930 , and a second angled screw hole  935 . As described previously, the end plates ( 905 ,  925 ) and intermediate plates ( 910 ,  920 ) are comprised of biocompatible metal and may include the arrays of apertures and linear recesses to facilitate bone through-growth and bonding to the central body  915 . The first angled screw hole  930  is oriented such that it intersects with the superior surface of the first end plate  905  and the second angled screw hole  935  is oriented such that it intersects with the inferior surface of the second end plate  925 . First and second bone screws can be inserted into screw holes  930  and  935  to affix the interbody device  900  the adjacent vertebral bodies. The cervical composite interbody device  900  may further include a receptacle  950  for receiving a back-out plate  955  that is adjacent to angled screw holes  930  and  935 . By inserting a back-out plate  955  and locking it into place with a locking screw, the bone screws  960  and  965  will be inhibited from backing out of the upper and lower vertebral bodies. 
         [0069]      FIG. 10  is an embodiment of a composite interbody device  1000  suitable for insertion between adjacent lumbar vertebrae. In  FIG. 10 , the end plates and intermediate plates are comprised of biocompatible metal and may include the arrays of apertures and linear recesses to facilitate bone through-growth and bonding to the central body  1015 . Angled screw holes are utilized, but both bone screws  1060 ,  1065  are drilled into one adjacent lumber vertebrae. The lumbar composite interbody device  1000  may further include a receptacle  1050  for receiving a back-out plate  1055  that is adjacent to the angled screw holes. By inserting a back-out plate  1055  and locking it into place with one or more locking screws, the bone screws  1060  and  1065  will be inhibited from backing out of the lumbar vertebral body. 
         [0070]      FIG. 11  depicts yet another alternative embodiment of a composite interbody device  1100  that may act as a TLIF or PLIF cage. In  FIG. 11 , the end plates and intermediate plates are comprised of biocompatible metal and may include the arrays of apertures and linear recesses to facilitate bone through-growth and bonding to the central body  1115 , as described previously. In addition to a major aperture  1170  passing through the device  1100  that forms the interior end walls, the embodiment depicted in  FIG. 11  includes one or more lateral apertures in the central body. These lateral apertures help to facilitate the bone in-growth and through-growth process into the device  1100 . 
         [0071]      FIG. 12  depicts another alternative embodiment of a composite interbody device  1200  that may act as a lateral cage. In  FIG. 12 , the end plates and intermediate plates are comprised of biocompatible metal and may include the arrays of apertures and linear recesses to facilitate bone through-growth and bonding to the central body  1215 , as described previously. The lateral composite interbody device  1200  may further include a bullet-shaped face  1240  that will facilitate the insertion of the device between two adjacent vertebrae from a lateral approach. 
         [0072]      FIG. 13  is an exemplary radiograph of a patient&#39;s spine in which two composite interbody devices have been between adjacent vertebrae. In  FIG. 13 , a first interbody device  1300  can be seen between two adjacent vertebral bodies. Another interbody device  1305  is also depicted in  FIG. 13 . Since the central body  115  of the interbody devices ( 1300 ,  1305 ) is radiolucent, only the end plates and intermediate plates are visible in the radiograph. Accordingly, the placement of the interbody devices ( 1300 ,  1305 ) in the spine can be more readily monitored and the amount of bone on-growth, in-growth, and through-growth can also be readily monitored. 
         [0073]      FIGS. 14A-14C  depict an implantation tool  1400  that may be used to insert a interbody device  100  into the space between two adjacent vertebrae. As shown in  FIGS. 14A-14C , the tool comprises a first handle  1405 , a second handle  1410 , a pair of lockable members  1415 , and a spring  1420 . The pair of lockable members  1415  may include prongs that face towards each other. To connect the implantation tool  1400  to the interbody device, the second handle  1410  is pressed towards the first handle  1405 , thereby pressing against spring  1420  and pushing the pair of lockable members outwards from the distal end of the tool sheath. As the lockable members  1415  are pushed out of the sheath, the separate from each other, thus providing room for them to be inserted into the holes ( 130 ,  135 ) formed in an exterior side wall  116  of the central body  115 . Once the lockable members  1415  inserted into the interbody device  100 , the second handle can be released, thus causing the lockable members to retract within the sheath, thereby forming a releasable attachment to the composite interbody device  100  at end of the tool  1400 . Once the interbody device  100  has been inserted between two adjacent vertebrae, the lockable members  1415  can be disengaged from the holes ( 130 ,  135 ) by pressing the second handle  1410  towards the first handle  1405  until the lockable members can be freely disengaged from the interbody device  100  while leaving the interbody device  100  in place. 
         [0074]      FIGS. 15A and 15B  depict an driving tool  1500  that may be used to lock a back-out plate (e.g.,  955 ,  1055 ) onto the exterior side wall  116  of a interbody device  100 . Typically, this procedure is performed after one or more bone screws has been used to attach the interbody device  100  to the adjacent vertebrae, as depicted in  FIGS. 9 and 10 . Once the bone screws have been set, a back-out plate ( 955 ,  1055 ) is put into place and a locking screw  1502  is releasably attached to a distal end  1505  of the driving tool  1500 . The driving tool  1500  is then used to drive the locking screw  1502  through the back-out plate ( 955 ,  1055 ) into the central body  115 , thus locking the back-out plate ( 955 ,  1055 ) against the interbody device  100 . Once the back-out plate ( 955 ,  1055 ) has been locked to the interbody device, the locking screw  1502  can be released from the distal end  1505  of the driving tool  1500  and the driving tool can be removed from the intervertebral space. 
         [0075]    While various embodiments in accordance with the disclosed principles have been described above, it should be understood that they have been presented by way of example only, and are not limiting. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
         [0076]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings herein.