Abstract:
Implants for use in the spinal column are disclosed. The implants comprise a bone allograft coupled with a non-allogenic plate. The plate has ends that fasten to opposing spine segments, and an intermediate portion that engages the allograft using deformable fingers, or with a hollow portion sized to receive and hold part of the allograft, or with fixed tabs. Methods of using the implants are also disclosed.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a medical implant and method, and, more particularly, to an improved surgical implant and method for expanding the spinal canal to eliminate pressure on the spinal cord caused by an impinging vertebral bone.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various medical conditions may result in a reduction of the area within the vertebrae available for the spinal cord. Spinal stenosis is one such condition involving the narrowing of the canal in the center of the spine through which the spinal cord and nerve roots run. Spinal stenosis may result when the ligaments of the spine thicken and calcify (harden from deposits of calcium salts), or when bones and joints enlarge, and osteophytes (bone spurs) form. A herniated (bulging) disk may also place pressure on the spinal cord or nerve root. Furthermore, diseased bone or tumors may result in an ingrowth into the spinal cord area. This decreases the space (neural foramen) available for nerve roots leaving the spinal cord.  
           [0003]    Two surgical methods currently exist to create additional room in the spinal canal. The first is called a laminectomy, and involves removal of the lamina (roof) of one or more vertebrae. A limitation of the laminectomy procedure is that it involves removal of the supporting structures at the back of the vertebrae which align the spinal column. The result may be that a patient suffers some postural deformity. To prevent such postural problems, a graft may be installed between the ends of the removed bone to span the void and reinstate the necessary support. The second procedure is called a laminoplasty, in which the targeted vertebra is cut, spread apart and a graft is inserted to permanently enlarge the space. Unlike the laminectomy, typically no bone material is excised during the laminoplasty procedure. Two different laminoplasty procedures are currently used. The first is called the unilateral or “open door” laminoplasty in which one side (lamina) of the vertebra is cut all the way through, while the other side is cut only half way to create a hinge. The vertebral element is then rotated about the hinge, and the graft is inserted into the opening, increasing the opening of the spinal canal. The second procedure is called the bilateral or “French door” laminoplasty in which the midline of the vertebra (spinous process) is cut all the way through, and the lamina are cut half way through, creating two hinges. The vertebral element is then opened at the bisected spinous process, and a graft inserted into the opening, again increasing the opening of the spinal canal.  
           [0004]    Various materials may be used for the grafts installed during laminoplasty procedures. U.S. Pat. No. 6,080,157 to Cathro et al. and U.S. Pat. No. 5,980,572 to Kim et al. disclose the use of titanium, ceramic and nylon inserts. Further, using allografts taken from long bones such as the femur, humerus, tibia and fibula, for spinal fusion procedures is known, as disclosed by U.S. Pat. No. 5,728,159 to Stroever et al. Allografts, as such bone grafts are called, are removed from a donor and processed using known techniques to preserve the allograft until implantation. Allografts have mechanical properties which are similar to the mechanical properties of vertebrae even after processing. The benefit of such property matching is that it prevents stress shielding that occurs with metallic implants. Allografts, unlike magnetic metals, are also compatible with magnetic resonance imaging (MRI) procedures, allowing more accurate ascertainment of fusion. Furthermore, allografts are naturally osteogenic providing excellent long term fusion with the patient&#39;s own bone.  
           [0005]    Several different spacer designs have been used in laminoplasty procedures to the present. For example, the Cathro patent discloses a metal, nylon or teflon spacer for use in a unilateral laminoplasty procedure. The Cathro spacer is a rectangular plate having shouldered edges which engage the ends of the cut lamina, and is held in place by a spring mechanism. The difficulty with the Cathro spacer is that its operation relies on the continued satisfactory operation of the installed spring. Further, the Cathro device provides little available area for the packing of fusion enhancing (i.e. osteogenic) material. The Kim patent discloses a spacer for use in a bilateral laminoplasty procedure. The Kim spacer consists of inner and outer trapezoidal segments joined together by a rectangular segment. The tapered surface of the inner trapezoidal segment is designed to conform to the inner surface of the split spinous process halves, while the taper of the outer segment is designed to assume the shape of the removed spinous process tip. The Kim spacer seats on the resulting flat surface of bone. Like the Cathro device, the Kim device provides little area in which to pack osteogenic material to facilitate bone-implant fusion. Neither the Cathro nor Kim device use allograft as a spacer material, which may result in reduced propensity for fusion and the possibility for stress shielding.  
           [0006]    Accordingly, there is a need in the art to provide implants and methods for both laminectomy and unilateral and bilateral laminoplasty procedures, which provide excellent dimensional, strength and retention capability, which enhance fusion with the patient&#39;s own bone, which are easy to select, fit and install and which provide excellent compatability with post-operative imaging (MRI).  
         SUMMARY OF THE INVENTION  
         [0007]    The implants of present invention are provided for use in the spinal column. In one embodiment, the implants comprise an allograft fabricated from cancellous bone material and a member formed of non-allograft material having first and second bone engaging portions and an allograft engaging portion. The graft engaging portion may be configured to retain the allograft when the allograft contacts the graft engaging portion.  
           [0008]    The graft engaging portion may comprise at least one raised tab. Further, the implant member may have a central region between the first and second bone engaging portions and the at least one raised tab angled inward toward the central region of the member. The allograft may have first and second ends, each comprising bone engaging portions, where at least one of the bone engaging portions is comprised of partially, substantially, or fully demineralized bone. At least one of the implant member bone engaging portions may comprise a suture attachment portion configured to allow a surgeon to secure the member bone connecting portions to the first and second bone segments.  
           [0009]    In a different embodiment, an implant is provided for use in maintaining a desired distance between a first spinal bone cut end and a second spinal bone cut end, in which the implant comprises an allograft having a body and first and second ends, and a plate formed of a non-allograft material having an intermediate portion and first and second ends, where the intermediate portion has an allograft engaging portion configured to retain the allograft, and where the first and second ends of the plate have bone engaging portions which themselves have fastener receiving portions. The allograft engaging portion is configured to engage the allograft body and the bone engaging portions are configured to engage respective outer surfaces of first and second spinal bone cut ends. The allograft first and second ends are configured to contact the first and second cut bone ends. In a specific embodiment, the allograft engaging portion may comprise deformable fingers configured to engage the graft. In another specific embodiment, the allograft engaging portion may comprise a hollow portion, where the allograft has a shape complementary to the hollow portion, and where the hollow portion is configured to at least partially receive the allograft. In a further embodiment, the allograft first and second ends comprise bone engaging portions, at least one of which may comprise partially, substantially, or fully demineralized bone.  
           [0010]    A method for providing a desired distance between first and second cut bone ends of the spine is also provided. This method comprising the steps of: cutting a vertebra to produce first and second cut bone ends; separating the bone ends to define a space therebetween; providing an allograft having a body and first and second ends; providing a plate formed of a non-allograft material having an intermediate portion and first and second ends, where the intermediate portion has an allograft engaging portion configured to retain the allograft, the first and second plate ends have bone engaging portions with fastener receiving portion, and where the allograft engaging portion is configured to engage the allograft body, the bone engaging portions are adapted to engage the first and second bone outer surfaces, and the allograft first and second ends are configured to contact the first and second cut bone ends, then engaging the allograft engaging portions of the plate with the allograft; engaging the bone engaging portions with respective cut bone ends; providing at least two bone fasteners; inserting at least one fastener into the fastener receiving portion of each bone engaging portion; and engaging the at least one bone fasteners with said cut bone end. In a further embodiment, the step of cutting a vertebra may comprise cutting all the way through one lamina. In a further embodiment, the adjacent lamina further may be cut half way through.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The features and advantages of the implant and method of use will become more readily apparent from the following detailed description of the invention in which like elements are labeled similarly and in which:  
         [0012]    [0012]FIGS. 1A, 1B and  1 C are perspective, end and top views of the first embodiment of the implant, for use in a unilateral laminoplasty procedure;  
         [0013]    [0013]FIGS. 2A and 2B are side and top views of the implant of FIG. 1 installed between the cut lamina segments of a vertebra during a unilateral laminoplasty procedure;  
         [0014]    [0014]FIGS. 3A and 3B are a perspective view of a retaining plate of the present invention, and a side view of two such retaining plates installed over the implants of FIGS. 2A and 2B;  
         [0015]    [0015]FIGS. 4A and 4B are perspective and side views of a second embodiment of the implant, a unilateral implant incorporating demineralized bone flaps;  
         [0016]    [0016]FIGS. 5A, 5B and  5 C are perspective, side and end views of a third embodiment of the implant, for use in a bilateral laminoplasty procedure;  
         [0017]    [0017]FIGS. 6A and 6B are side and section views of the implant of FIG. 5 showing the incorporation of a channel to accept the corresponding arms of a set of distractor pliers used to install the implant;  
         [0018]    [0018]FIG. 7 is a detail view of the end of the implant of FIG. 5B showing a preferred embodiment of the surface projections used to facilitate retention of the implant between cut spinous process segments.  
         [0019]    [0019]FIGS. 8A, 8B and  8 C are perspective, end and side views of a fourth embodiment of the implant, for use in a bilateral laminoplasty procedure;  
         [0020]    [0020]FIGS. 9A and 9B are front and top views of the implants of FIGS. 7 and 8 installed between the cut spinous process segments of a vertebra during a bilateral laminoplasty procedure;  
         [0021]    [0021]FIGS. 10A, 10B and  10 C are perspective, end and top views of a fifth embodiment of the implant, for use in a unilateral laminoplasty procedure;  
         [0022]    [0022]FIGS. 11A, 11B and  11 C are top, side and end views of a sixth embodiment of the implant, for use in a unilateral laminoplasty procedure; and  
         [0023]    [0023]FIGS. 12A and 12B are perspective views of seventh and eighth embodiments of the implant, for use in unilateral laminoplasty procedures.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Preferred embodiments, features and aspects of an implant adapted to be used in unilateral and bilateral laminoplasty procedures are described, in which a portion of a targeted vertebra is cut, the space available for the spinal cord and associated nerves is expanded, and an implant is installed between the cut segments of bone.  
         [0025]    Referring more particularly to the drawings, FIGS. 1A, 1B and  1 C show an implant for use in a unilateral or “open door” laminoplasty. The implant  1  has a longitudinal axis “CL,” a length “L,” a wall  5  defining an outside surface  3  and an inside surface  4 , and first and second ends  6 A,  6 B. Inside surface  4  communicates with first and second ends  6 A,  6 B to define a hollow central region  7  of the implant. Outside surface  3  has an outer side region  3 A and an inner side region  3 B such that when the implant is installed between cut segments of lamina, outer side region  3 A faces outward away from the spinal canal, while inner side region  3 B faces inward toward the spinal canal. The implant  1  further has a depth “D” which is the distance between outer side region  3 A and inner side region  3 B. Implant  1  also has a width “W” which is the distance between opposing outer surfaces  3  measured along a drawn line perpendicular to a line defining the depth “D.” Length “L” preferably should be between about 11.5 millimeters (mm) to about 15.5 mm; depth “D” preferably should be between about 5.5 mm to about 6.5 mm; and width “W” preferably should be between about 8.0 mm to about 9.5 mm.  
         [0026]    The shape and size of outside surface  3  is not critical and, therefore, any implant configuration can be used preferably so long as the first and second ends  6 A,  6 B provide sufficient contact area with the lamina ends, and the implant  1  does not interfere with other anatomy, and does not intrude on the spinal cord space. In a preferred embodiment, however, the outside surface  3  is configured such that the shape of the implant, when viewed from the end, displays the form of a substantially geometric shape (e.g. ellipse, oval, circle, etc.). In this embodiment the exterior dimensions of the implant also approximate those of the outside surface of the cut lamina segments between which the implant is installed. Although implants having cross sections of greater or lesser proportion than the lamina to which they attach will function properly, for aesthetic purposes and in an attempt to minimize the amount of material introduced into a patient&#39;s body, the outer surface of the implant should preferably not extend beyond the outer surface of the adjoining bone.  
         [0027]    In a further embodiment, the inside surface  4  of the implant  1  may be machined so that the hollow central region  7  approximates the configuration and geometry of the implant exterior (i.e. form an ellipse or oval shape). The hollow central region may be designed to be packed with osteogenic material such as bone chips, etc. to facilitate fusion of the implant with the patient&#39;s lamina. Preferably, the central region may be as large as possible to enhance fusion of the implant to the patient&#39;s lamina. The thickness of wall  45  preferably should be between about 1.00 to about 1.50 mm; more preferably about 1.25 mm. Preferably the thickness of wall  5  should not be less than about 1.0 mm to ensure the implant retains sufficient strength to withstand the stresses imparted on the spine.  
         [0028]    The implant  1  may be fabricated from a biocompatable metal (e.g. stainless steel, or titanium, etc.) or polymer, or from allograft material preferably taken from a long bone (e.g. femur, tibia, fibula, humerus). Where the implant is an allograft, the inside surface  4  and hollow central region  7  may be defined by the intermedullary canal of the donor bone. The hollow center may be left as such, or the inner surface  4  may be machined, as with other implant materials, to maximize the space available for packing with osteogenic material. Again, the thickness of the implant wall  5 , preferably is not reduced to less than about 1.00 mm.  
         [0029]    During the unilateral laminoplasty procedure, the targeted lamina is cut in half and the segment attached to the spinous process is rotated or swung out to increase the area available for the spinal cord and associated nerves. Subsequent to this rotation, the lamina segments no longer reside along the same axis, but instead the ends are disposed at an angle with respect to each other. Implant  1  is substantially straight along its length, and so to accommodate this angular displacement of the lamina, first and second ends  6 A,  6 B incorporate arcuate cutouts  8 A,  8 B to grasp and retain the cut lamina segments. Viewed from the top of the implant (FIG. 1C), these arcuate cutouts  8 A,  8 B are generally concave and may be circular in shape, or they may consist of a cutout spanning an obtuse angle and converging to a small radius at the crotch of the first and second ends  6 A,  6 B. Arcuate cutouts  8 A,  8 B have a centerline  1   a  which runs parallel to the longitudinal axis of the implant  1 . The centerline  1   a  of the arcuate cutouts may be coexistent with the longitudinal axis of the implant  1 , or it may be offset with respect to that axis to further improve retention of the cut and displaced lamina ends. In a further embodiment, the centerlines  1   a  of the arcuate cutouts may each be offset on an opposite side of the implant centerline to facilitate retention of the implant in cases where the angle between the cut and spread lamina is more severe, such as when the surgeon spreads the lamina segments as wide as possible to provide maximum additional space for the spinal cord and associated nerves.  
         [0030]    In the preferred embodiment, shown in FIG. 1C, each arcuate cutout  8 A,  8 B comprises first angled faces  88 A,  89 A and second angled faces  88 B,  89 B, respectively, which meet at crotch “C” to form a face angle “A.” Preferably, face angle A is about 100 degrees. Crotch radius “R,” comprises the transition between the first and second angled faces. Crotch radius “R” is preferably about 2 mm. Each arcuate cutout further comprises first and second face depths “F1” and “F2.” The first and second face depths are a measure of the depth of the crotch relative to the inner side region  3 B and outer side region  3 A of the implant, and will be different lengths whenever the centerline  1   a  of the arcuate cutout is offset from the centerline “CL” of the implant  1 . Preferably first face depth “F1” is about 1.25 mm, and second face depth “F2” is about 1.5 mm. Each arcuate cutout  8 A,  8 B also has a centerline offset “d,” which is the degree to which the arcuate cutout  8 A,  8 B is shifted from the centerline “CL” of the implant  1 . Preferably, the centerline offset “d” is from about 0 to 2.5 mm toward the inner side region  3 B of implant  1 . The face depth “F1” of the first and  6 A of the implant  1  may be the same or different than the face depth “F1” of the second end  6 B. Likewise, the face depth “F2” of the first end  6 A may be the same or different than the face depth “F1” of the second end  6 A.  
         [0031]    In a further embodiment of the implant comprising allograft material, first and second ends  8 A,  8 B may comprise regions of partially, substantially, or fully demineralized cortical bone to further facilitate fusion of the implant to the lamina. Preferably the demineralized bone portion comprises the entire surface of each first and second end  6 A,  6 B of the implant  1 . Preferably, the depth of the demineralized portion will be up to about 2 mm.  
         [0032]    The implants further may incorporate at least one suture hole  9  in the implant wall  5  to allow the surgeon the option of suturing the implant to the cut lamina ends. These suture holes  9  may vary in number and size, with the only limitation being that they should not be so large or numerous as to compromise the strength or integrity of the implant.  
         [0033]    [0033]FIGS. 2A and 2B are side and top views of the implant of FIG. 1 installed in a patient between the cut lamina ends in a unilateral laminoplasty procedure. In FIG. 2A two different sized implants  1  are installed on the cut lamina segments  10  of adjacent vertebrae, to illustrate application of the implant design to bones of different size. FIG. 2B shows the interaction between the implant and the cut vertebra segments  10 .  
         [0034]    The design of the bone engaging ends  6 A,  6 B of the implants  1  are sufficient to ensure retention of the implants  1  between the cut ends of lamina  10 . Some surgeons, however, desire an additional measure of assurance that the implants  1  will not loosen or otherwise be expelled from between the lamina ends  10 . The implant, therefore, provides for the optional installation of a plate  12  to be secured over an installed implant in a unilateral laminoplasty procedure. FIG. 3A is a perspective view of a plate  12  which may be installed to secure the implant  1  of FIGS. 1 &amp; 2, to ensure the implant  1  is not expelled from the cut lamina ends  10 . Plate  12  has a length  13 , a thickness  14  and a body portion  15  with first and second ends  16 A,  16 B comprising bone engaging portions  17  and implant engaging portions  18 . As shown in FIG. 3A the bone engaging portions  17  and implant engaging portions  18  may consist of the holes adapted for receiving bone screws  19  or hooks  20  (not shown) capable of grasping bone screws installed in the lamina and/or implant. Each side of plate  12  may have one or more bone engaging portions  19  and one or more implant engaging portions  18 . In a further embodiment the plate  12  may be flexible to allow the surgeon to form it to the individual contour of the patient&#39;s spine, thereby achieving a tight fit between components. The plates may be fabricated from a biocompatable metal or other material known in the art that would be suitable for long term retention of an implant  1 .  
         [0035]    Instead of a single plate  12 , smaller plates without connecting body portion  15  may be utilized, each plate comprising at least one bone engaging portion  17  and one implant engaging portion  18 .  
         [0036]    [0036]FIG. 3B is a side view of the implants  1  installed in FIG. 2A, further showing the installation of optional plates  12  of FIG. 3A. Bone screws  19  are installed to secure the plates  12  to both the respective opposing lamina segment  10 , and the implant. In this embodiment, bone screws are also installed in the screw holes  18  of the implant engaging portion, to secure the plates to the implants  1 . Also in this embodiment, the plates are flexible and are bent to assume the varying contour of the lamina segments and the implant. More than one optional plate may be used to secure the implant to the lamina.  
         [0037]    [0037]FIGS. 4A and 4B show perspective and side views of an allograft implant  30  which incorporates the design features of the implants of FIG. 1, but which further includes a pair of bone flaps  31 A,  31 B disposed at first and second ends  32 A,  32 B of the implant  30 . These bone flaps are used to secure the implant  30  to the respective cut ends of lamina in a unilateral laminoplasty procedure. At least a portion of each flap comprises demineralized bone. Demineralization of the flaps, but not the implant, provides the implant with flexible attachment points which may be contoured to conform to the shape of the adjacent lamina. Bone flaps  31 A,  31 B comprise thin, flat, rectangular segments of allograft having an outer surface  34  and a bone engaging surface  35 . The outer surfaces  34  of the flaps preferably are the same width as, are contiguous with, and extend axially like wings from the outer surface  36  of the implant  30 . In a preferred embodiment, bone flaps  31 A,  31 B are machined from the same segment of donor bone as implant  30 . At least a portion of flaps  31 A,  31 B may be demineralized using any commercially acceptable process (e.g. hydrochloric acid bath, etc.) that will render the resulting flaps flexible. Flaps  31 A, B are provided with holes  36 A,  36 B suitable for receiving bone screws  37 A,  37 B which are used to secure the bone flaps  31 A,  31 B and implant  30  to the adjacent cut lamina ends.  
         [0038]    In another embodiment, these bone flaps may not be demineralized, but instead each bone flap may comprise a notch  131 A,  131 B in the respective region where the bone flaps  31 A,  31 B connect to the implant  30 . Notches  131 A,  131 B may be any type of notch or reduction in the thickness of the bone flap appropriate to provide flexibility for placing the flaps on the adjacent laminae surfaces, while retaining the requisite strength to ensure the bone flaps will not separate from the implant during installation.  
         [0039]    [0039]FIGS. 5A, 5B and  5 C show an embodiment of an implant for use in a bilateral or “french door” laminoplasty procedure, in which the spinous process of a targeted vertebra is bisected along the sagittal plane and the segments separated to enlarge the spinal canal. The implant  40  has a wall  45  having an inside surface  47  and an outside surface  48 , and first and second ends  46 A,  46 B. The outside surface  48  has an outer side region  41  having an outer side length  42  and an inner side region  43  having an inner side length  44 . Inside surface  47  communicates with first and second ends  46 A &amp;  46 B to define a hollow central region  49  of the implant. The implant  40  has a generally trapezoidal shape when viewed from the side (FIG. 5B), and inner side region forms angle “TA” with respect to the first and second ends  46 A,  46 B. This trapezoidal configuration allows the implant first and second ends  46 A,  46 B to conform to the cut, angled surfaces of the spinous process segments to which the implant will eventually fuse. Inner side length  44  preferably is from between about 6.0 mm to about 10 mm, and angle “TA” preferably is from between about 50 to about 70 degrees.  
         [0040]    The shape and size of outside surface  48  is not critical and, therefore, any implant external configuration can be used preferably so long as first and second ends  46 A,  46 B provide sufficient contact area with the cut spinous process segments, does not project out from between the bone segments so far as to interfere with other anatomy, and does not intrude on the spinal cord space For aesthetic purposes and in an attempt to minimize the amount of new material introduced into a patient, however, the outside surface  41  of the implant  40  should preferably not extend beyond the outside surface of the cut spinous process segments. In a preferred embodiment the outside surface  41  of the implant  40  is configured such that the outside surface  41 , when viewed from the end, displays the form of a substantially geometric shape (e.g. ellipse, oval, circle, etc.) (FIG. 5C).  
         [0041]    In a further embodiment, the inside surface  43  of the implant  40  may be machined so that the hollow central region  49  approximates the configuration and geometry of the implant outside surface  41  (i.e. an ellipse or oval). The hollow central area is designed to be packed with osteogenic material such as bone chips, etc. to facilitate fusion of the implant with the patient&#39;s cut spinous process segments. Preferably, this center area may be made as large as possible to facilitate the fusion process.  
         [0042]    The thickness of wall  45  preferably should be from between about 1.00 to about 1.50 mm; more preferably about 1.25 mm. Preferably the thickness of wall  45  should not be less than about 1.0 mm to ensure the implant retains sufficient strength to withstand the stresses imparted on the spine associated with daily living.  
         [0043]    The implant  40  may be fabricated from a biocompatable metal (e.g. stainless steel, or titanium, etc.) or polymer, or from allograft material preferably taken from a long bone (e.g. femur, tibia, fibula, humerus). Where the implant is fabricated from metal or polymer, it may be provided in a solid form. Preferably, however, the implant should incorporate a hollow region, and the inside surface  44 , should be formed to maximize the space available for packing with osteogenic material while maintaining adequate wall thickness. Where the implant is an allograft, the inside surface  44  and hollow center  49  may be defined by the intermedullary canal of the donor bone. The allograft may be left in this state, and the hollow central region  49  packed with osteogenic material. Preferably, however, the inside surface  44  of the allograft will be machined and the hollow central region  49  enlarged to maximize the space available for packing with osteogenic material.  
         [0044]    [0044]FIGS. 6A and 6B show first and second ends  46 A,  46 B of implant  40  each incorporating a channel  50  to accept the corresponding arms of a set of distractor pliers (not shown) which may be used to separate the bisected spinous process segments during the bilateral laminoplasty procedure. Each channel  50  has two sidewalls  51  each having a depth “CD”, a bottom surface  52  having a width “CW” and a centerline  54  which is formed by a line extending along the implant  40  from inner side surface  43  to outer side surface  41 . Preferably, each channel  50  may incorporate a radiused transition  55  between the sidewalls  51  and the bottom surface  52 . In a further preferred embodiment, the channel runs from the inner side surface  43  to the outer side surface  41  of each end  46 A,  46 B of the implant. The specific dimensions of the channels is not critical, but should be configured to accept the distractor arms used during the distraction and insertion portion of the procedure. Preferably, the channel bottom surface width “CW” is about 4 mm, and the sidewall depth “CD” is about 1 mm.  
         [0045]    [0045]FIG. 7 shows a further embodiment of bilateral laminoplasty implant  40 , in which first and second ends  46 A,  46 B comprise surface projections to improve pre-fusion retention of the implant  40  between respective cut spinous process segments. In a preferred embodiment, a plurality of saw-tooth serrations  56  having a height  58  and a tooth angle  59  are provided. Preferably the serrations are oriented to run vertically when the implant  40  is installed in the patient. Height  58  and tooth angle  59  are defined with respect to the respective planes formed by implant first and second ends  46 A,  46 B. Height  58  is measured from the trough  60  of each serration, while tooth angle is measured from the plane formed by the implant first and second ends  46 A,  46 B. Preferably, height  58  is about 0.5 mm, tooth angle  59  is about 45 degrees, and the distance between troughs  60  is about 1.2 mm. While these dimensions and profile are preferred, other suitable surface profiles (e.g. pyramidal teeth, etc.) may be used to ensure implant retention.  
         [0046]    In a further embodiment of the implant  40  comprising allograft material, first and second ends  46 A,  46 B may comprise regions of partially, substantially, or fully demineralized cortical bone to further facilitate fusion of the implant to the lamina. Preferably the partially, substantially, or fully demineralized bone portion may comprise the entire surface of each first and second ends  46 A,  46 B of the implant  40 . Preferably the depth of the demineralized portion of will be up to about 2 mm.  
         [0047]    The implant  40  may also incorporate a plurality of sutures holes  61  (see FIG. 5C) formed through the implant wall  45  to allow the surgeon to secure the implant to the cut spinous process segments. These suture holes  61  may vary in number, size and position, with the only limitation being that their size, position and number preferably should not compromise the strength and integrity of the implant.  
         [0048]    [0048]FIGS. 8A, 8B and  8 C show a further embodiment of an implant for use in a bilateral laminoplasty procedure. Implant  62  has a first and second ends  63 A,  63 B, an inner side region  68 , an outer side region  65 , and sides  66  and  67 . The implant  62 , like the implant of FIG. 5, has a generally trapezoidal shape when viewed from the side (FIG. 8C). Again, this trapezoidal configuration allows the implant first and second ends  63 A,  63 B to conform to the cut, angled surfaces of the spinous process segments to which the implant will eventually fuse. As such, inner side  68  forms angle “IA” with respect to the first and second ends  63 A,  63 B. In this embodiment, the implant  62  is an allograft, comprising “tri-cortical” bone taken from the crest of the ilium region of the pelvis. Harvesting bone from this segment of the pelvis provides an implant which naturally comprises a thin region  64  of cortical bone on outer side  65 , and sides  66  &amp;  67 . The inner side  68  of the implant, as well as the implant body portion  69  comprise cancellous bone. This combination of bone types allows the surgeon to exploit both the good strength characteristics of cortical bone, and the good osteogenic characteristics of cancellous bone in a single implant. In a further embodiment, the implant  62  comprises a cavity  70  which communicates with implant first and second ends  63 A &amp;  63 B, and which may be used for packing osteogenic material to promote fusion between the implant and the cut spinous process segments.  
         [0049]    In a preferred embodiment of the implant  62  of FIG. 8, the implant first and second ends  63 A,  63 B comprise surface projections to improve pre-fusion retention of the implant  62  between respective cut spinous process segments. Saw-tooth serrations, similar to those illustrated and described with regard to the implant of FIG. 5, may be provided. Again, other suitable surface profiles (e.g. pyramidal teeth, etc.) may also be provided to ensure implant retention.  
         [0050]    In a further embodiment of the implant  62  comprising allograft material, first and second ends  63 A,  63 B may comprise regions of partially, substantially, or fully demineralized cortical bone to further facilitate fusion of the implant to the lamina. Preferably the demineralized bone portion may comprise the entire surface of each first and second ends  63 A,  63 B of the implant  62 . Preferably, the depth of the demineralized portion of will be up to about 2 mm.  
         [0051]    In another embodiment, the implant  62  may incorporate a plurality of sutures holes (not shown) similar to those shown in FIG. 5C, to allow the surgeon to secure the implant to the cut spinous process segments. These suture holes may vary in number, size and position, with the only limitation being that their number, size and position should not compromise the strength and integrity of the implant.  
         [0052]    [0052]FIGS. 9A and 9B are front and top views of either trapezoidal implants  40 ,  62  of FIGS. 5, 8 installed in a patient. First and second ends  46 A,  46 B,  63 A,  63 B of implant  40 ,  62  contact cut spinous process segments  72  and  71  respectively. Hinge cuts  73  and  74  in lamina  75 ,  76  enable the spinous process segments to be “swung out” by the surgeon to facilitate insertion of the implant  40 ,  62  therebetween.  
         [0053]    [0053]FIGS. 10A, 10B and  10 C show a further embodiment of an implant adapted for use in a unilateral laminoplasty procedure. Implant  77  comprises first and second plate portions  78 A,  78 B for connecting to the opposing segments of cut lamina produced during a unilateral laminoplasty procedure. First and second plate portions  78 A,  78 B are connected by an intermediate portion  80 . The plate portions further comprise respective first and second bone engaging portions  79 A,  79 B which are configured to engage the opposing cut lamina segments. In a preferred embodiment, first and second bone engaging portions  79 A,  79 B comprise arcuate surfaces for engaging and cradling the respective cut lamina ends. Arcuate surfaces are particularly suited for this purpose because their concave shape can engage and retain lamina segments residing along different axes, a phenomenon which occurs during the unilateral laminoplasty procedure when a single lamina is cut and the resulting segments are swung out to enlarge the area available for the spinal cord. The swinging out process results in an angle being formed between the segments, and it is this misalignment which the arcuate surfaces of the bone engaging portions  79 A &amp;  79 B accommodate.  
         [0054]    In a further embodiment, the thickness of the intermediate portion  80  may be smaller than the height of the first and second plate portions  78 A,  78 B.  
         [0055]    Implant  77  may be fabricated from any biocompatable metal (e.g. titanium, stainless steel, etc.) or polymer, or the implant may be formed of allograft material. If allograft is used, the implant  77  preferably should be fabricated from cortical bone.  
         [0056]    In a further embodiment of the implant  77  comprising allograft material, first and second bone engaging portions  79 A,  79 B may comprise regions of partially, substantially, or fully demineralized cortical bone to further facilitate fusion of the implant to the lamina segments. Preferably the demineralized bone portion may comprise the entire surface of each first and second bone engaging portions  79 A,  79 B. Preferably, the depth of the demineralized portion will be up to about 2 mm.  
         [0057]    In another embodiment, the implant  77  may incorporate suture hole  80  to allow the surgeon to secure the implant to the cut spinous process segments. Additional suture holes (not shown) may be provided, and may vary in number, size and position, with the only limitation being that their size, position and number preferably should not compromise the strength and integrity of the implant  77 .  
         [0058]    [0058]FIGS. 11A, 11B and  11 C show a further embodiment of an implant adapted for use in a unilateral laminoplasty procedure. Implant  84  comprises a plate portion  85  having bone engaging portions  86 A,  86 B, a graft engaging portion  87 , and an allograft  91 . Bone engaging portions  86 A,  86 B further comprise a plurality of suture holes  88  configured to allow the surgeon to secure the cut lamina segments to bone engaging portions  86 A,  86 B Graft engaging portion  87  comprises a graft seating surface  89  and a graft retaining portion  90  configured to retain a correspondingly shaped allograft  91  for engaging the opposing cut lamina segment. In a preferred embodiment, graft retaining portion  90  comprises two raised tabs  92 A,  92 B, each residing along at least a portion of opposing ends of graft seating surface  89 . In a preferred embodiment, raised tabs  92 A,  92 B are angled slightly toward the center of graft seating surface  89  so as to facilitate retention of allograft  91 . Preferably the angle “A” between raised tabs  92 A,  92 B and graft seating surface  89  will be from about 70 to about 80 degrees; more preferably this angle will be about 75 degrees. Plate portion  85  further comprises a bottom surface  855 . When installed, graft  91  comprises the inner side surface of the implant (i.e. the surface which is closest to the spinal canal), while plate bottom surface  855  comprises the outer side surface of the implant (i.e. the surface which faces away from the spinal canal). In a preferred embodiment, bottom surface  855  comprises a convex shape which assumes the rounded contour of the lamina segments. Preferably, this convex surface has a radius of about 18 mm.  
         [0059]    Plate portion  85  may be fabricated from any biocompatable metal (e.g. titanium, stainless steel, etc.) or polymer, or it may be made of allograft material. If allograft is used, the plate portion  85  may be fabricated from cortical bone. Graft  91  preferably may be comprised of a cancellous type bone material to facilitate fusion of the implant to the patient&#39;s lamina.  
         [0060]    [0060]FIGS. 12A and 12B show implant embodiments comprising plates configured to attach directly to the opposing cut segments of lamina produced during a unilateral laminoplasty. These plates are further configured to capture segments of allograft and to engage these segments with the opposing cut segments of lamina to facilitate fusion between the implant and the patient&#39;s bone. Plate  93  comprises a body portion  94  having a longitudinal axis and first and second ends  95 A,  95 B, and a graft retaining portion  96 , midway between the ends  95 A,  95 B, preferably approximately midway between ends  95 A,  95 B. First and second ends  95 A,  95 B each comprise a bone engaging portion  97 . In a preferred embodiment the bone engaging portion at each first and second end comprises at least one hole suitable for receiving a bone screw  98  (not shown). The bone screws are then used to secure the plate  93  to each opposing segment of lamina. In a further embodiment the bone engaging portions may be hooks capable of grasping bone screws that are installed in the lamina segments.  
         [0061]    In the embodiment shown in FIG. 12A, the graft retaining portion  96  comprises a plurality of deformable fingers  99  which are initially arrayed flat along an axis perpendicular to the longitudinal axis of the plate  93 . These fingers  99  are capable of being deformed to cradle an allograft  100 , preferably cylindrical in shape. Allograft  100  preferably has a length sufficient to engage the cut ends of lamina upon installation, and a diameter of size sufficient to be captured by the deformed fingers  99  of the plate  93 .  
         [0062]    In the embodiment of FIG. 12B, plate  93  has a graft retaining portion  96  which comprises a hollow region  101 , preferably rectangular in shape. A correspondingly configured allograft of cancellous bone is provided having a body  102  capable of being received within the hollow region  101 , and further having shoulders  103  which extends beyond the hollow region to contact seating surface  104 . In a preferred embodiment, shoulders  103  of allograft  100  are secured to plate  93  using a bone screw  98  placed through bone engaging portion  97 .  
         [0063]    In a preferred embodiment the plate  93  may be flexible to allow the surgeon to form the body  94  to the individual contour of the patient&#39;s spine, thereby achieving a tight fit between components. The plate  93  may be fabricated from a biocompatable metal or other material known in the art that would be suitable for long term retention of an implant and graft.  
         [0064]    The current invention also provides a method of using an allograft implant according to any of the embodiments shown in FIGS. 1A, 5A,  8 A,  10 A or  11 A which further has partially, substantially, or fully demineralized end segments to promote fusion between opposing segments of lamina or spinous process produced during a unilateral or bilateral laminoplasty procedure. This method comprises the steps of cutting a targeted lamina or spinous process as required for either a unilateral or bilateral laminoplasty procedure, separating the resulting segments of bone a sufficient distance to allow for insertion of an appropriately sized allograft implant, providing an allograft implant having bone engaging surfaces which comprise partially, substantially, or fully demineralized cortical bone to a depth of up to about 2 mm, and contacting the allograft implant bone engaging surfaces with respective cut segments of lamina or spinous process. This method may be augmented, in the case of a unilateral laminoplasty, to include the additional step of installing a plate over the allograft implant to further assist retention of the implant between the bone segments. Where such a plate is provided, the additional steps of providing bone screws or other fasteners to attach the plate to the opposing segments of bone and/or to attach the plate to the implant, may further be included.  
         [0065]    A further embodiment of the above method comprises providing an allograft implant according to the above method, which implant further has partially, substantially, or fully demineralized bone flaps capable of receiving bone screws. Providing such an implant allows the surgeon to affirmatively secure the implant to the cut lamina segments, preferably without the need for a separate plate.  
         [0066]    A method of installing a tri-cortical allograft implant as part of a bilateral laminoplasty procedure is also provided. This method comprises the steps of bisecting a targeted spinous process, providing hinge cuts on both adjacent lamina sufficient to allow the spinous process segments to be spread apart, separating the spinous process segments to allow for insertion of an appropriately sized allograft implant, providing an allograft implant having first and second angled bone engaging surfaces which approximate the angle between the bisected and spread spinous process segment cut surfaces, the allograft implant comprising cancellous bone material having a thin outer layer of cortical bone surrounding the cancellous bone, and which cortical bone layer is in communication with the first and second engaging surfaces so as to support the compressive stresses imparted by the cut spinous process segments.  
         [0067]    A method of using only a screwed plate to maintain the distance between bone ends produced during a unilateral or bilateral laminoplasty procedure is also provided and described. This method comprises the steps of cutting a targeted lamina or spinous process as required for the respective laminoplasty procedure, separating the cut bone segments to increase the space available for the spinal canal and associated nerves, providing an appropriately sized plate having first and second ends, wherein each end is configured to allow engagement with the surface of the lamina opposite the surface of the spinal canal and adjacent the cut bone end, and securing first and second ends of the plate to the adjacent bone segments.  
         [0068]    In a preferred embodiment of the method, each first and second end of the plate will have at least one recess suitable for receiving a bone screw, wherein the plate is secured to the adjacent cut bone ends using bone screws. In a further embodiment, two plates may be provided to attach to the adjacent cut bone ends.  
         [0069]    Accordingly, it should be understood that the embodiments disclosed herein are merely illustrative of the principles of the invention. Various other modifications may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and the scope thereof.