Abstract:
An apparatus and method for creating a space of defined length, height, width and shape with a guided mill in preparation for receiving a spinal implant or graft of known size and configuration is disclosed.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims priority to and the benefit of, pursuant to 35 U.S.C. §119(e), U.S. provisional patent application Ser. No. 60/777,271, filed Feb. 28, 2006, entitled “Apparatus and method of shaping an intervertebral space” by Jeffrey David Gordon and John K Song and is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to a surgical device for creating a cavity between or within bones of the human body.  
       BACKGROUND OF THE INVENTION  
       [0003]     Spinal surgery is a rapidly expanding field and interbody grafts (grafts placed between two adjacent vertebrae) are an important means of supporting the space between the vertebrae for purposes of fusion or motion preservation.  
         [0004]     To implant an interbody graft, whether in the cervical, thoracic, or lumbar spine, an anterior approach to the spine is often performed. The space where the implant (e.g. cage, spacer, vertebral body replacement, bone dowel, arthoplasty device, etc.) is to be placed is most commonly prepared by hand with simple tools (such as a drill, curettes, osteotomes, etc). However, this can leave gaps between the bones and the implant resulting in sub-optimal results.  
         [0005]     Numerous methods exist for preparing an exact cavity between or within adjacent bones for accepting an implant. Cloward described a technique whereby a drill is used to create a cylindrical cavity partially within the disc space and overlapping the vertebral bodies above and below (Cloward R B, Am J Surg, 1959). This technique has been widely used throughout the world. More recently, Michaelson (REFXXXXXX) described a means of guiding a drill while distracting the disc space.  
         [0006]     We present a device which is placed into the interbody space and maintains distraction while providing a means of guiding a cutting tool. This is different from the prior art in that a guide post, not a tube, is utilized. In addition, the device will also provide for an automatic depth stop and prevent the cutting tool from penetrating too far and potentially damaging the delicate nervous structures immediately behind the disc space.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to an apparatus and method for use in spinal surgery for creating a space of selected shape and dimensions across the disc space between two adjacent vertebrae of the spine with a guided mill. The present invention comprises instrumentation and a surgical method of preparing vertebral endplates for the procedure, be it fusion or non-fusion, and specifically the creation of a space of a known shape and dimensions. The foregoing is achieved by the use of a mill which is guided by a novel guide mechanism. The instrumentation of the present invention allows for the safe, controlled and protected preparation of the disc space to the optimal depth and width. The present invention allows for the maximum stability of the graft/implant, as well as the construct, by providing for the greatest possible interface surface area and congruency between the graft/implant and each of the adjacent vertebrae.  
         [0008]     It is an object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion or in the alternative of inserting an “artificial disc implant” for the purpose of maximizing the width and optimizing the depth of the disc and the bone removed from front to back, or, back to front, from the vertebral endplates adjacent the disc space to be fused or implanted while confining such bone resection safely within the lateral, anterior (front) and posterior (back) limits of the disc space.  
         [0009]     It is another object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion or “artificial disc” implantation that provides for the rapid creation of both a known surface contour of each of the vertebral endplates adjacent to a disc space as well as a known and reproducible shape of the fusion or implantation site itself.  
         [0010]     It is another object of the present invention to provide for a surgical method and instrument means for performing interbody spinal fusion that allows for the utilization of a larger interbody spinal fusion implant(s) than was possible with the prior art, such an implant having the capacity for providing increased amounts of osteogenic material, increased surface area, increased area of contact, increased stability and the ability to provide for greater support through the fusion or bone ingrowth area.  
         [0011]     It is another object of the present invention to provide for a surgical method and instrumentation for performing the preparation of the space between adjacent vertebrae for the purpose of implanting an artificial disc or fusion implant(s) having the optimal cross sectional area of contact with said adjacent vertebrae and where said cross sectional area may be as large as possible while remaining safely within the perimeter of the endplates of the adjacent vertebrae.  
         [0012]     It is a further object of the present invention to create a counterbore for the insertion of an implant which incorporates tabs into the space between adjacent vertebrae so that the implant sits substantially flush with the front of the vertebrae.  
         [0013]     It is a further object of the present invention to describe means to create a counterbore for the insertion of an implant which incorporates tabs into the space between adjacent vertebrae with a spring loaded counterboring tool which adapts for use with multiple implant sizes.  
         [0014]     The following is a brief outline of the steps of the surgical method of the present invention describing the use of the specific instrumentation in regard to the preferred embodiment: 
        1. The appropriate area of the spine is exposed and a partial disectomy is performed, whereby a portion and preferably a large portion of the disc is removed while preserving the annulus fibrosis portion of the disc along at least one side of the disc space.     2. The interspace so created may be distracted and while not requisite, preferably to its optimal height, which height is determined by the known normal spatial relationships for that area and the adjacent soft tissue -structures. The interspace is then measured for height, depth, and width. The width of the interspace may be determined in reference to the inferior portion of the vertebral endplate of the superior vertebrae, and this determines the selection of the appropriate width for the guide mechanism. The measured depth of the interspace, that is the distance between the front and back of vertebrae, will also determine the selection of a guide mechanism. The height and depth of the interspace will determine the selection of the appropriately sized mill.     3. The guide mechanism includes a spacer portion to separate the vertebral bodies and create a space in the intervertebral region of appropriate height. The width and depth of bone resection may then be easily confirmed visually prior to any actual bone resection.     4. The properly dimensioned mill is then guided by the guiding mechanism into the disc space in the appropriate orientation.     5. The mill is inserted into the disc space and the space is then milled to remove a portion of bone from the endplates adjacent to the disc space.     6. The prepared space may be irrigated and suctioned and then the mill is removed.     7. The guide mechanism is then removed and the appropriate implant or implants are then inserted into the prepared space.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is an exploded, perspective view of the guide mechanism and insertion handle of the present invention.  
         [0023]      FIG. 2  is a perspective view of the guide mechanism and insertion handle of the present invention.  
         [0024]      FIG. 3  is an exploded, perspective view of the mill mechanism of the present invention  
         [0025]      FIG. 4  is a perspective view of the mill mechanism of the present invention.  
         [0026]      FIG. 5  is a perspective view of the guide mechanism and insertion handle assembly of the present invention showing insertion into a disc space between two vertebrae.  
         [0027]      FIG. 6  is a perspective view of the guide mechanism and insertion handle assembly of the present invention after insertion into a disc space between two vertebrae.  
         [0028]      FIG. 7  is a perspective view of the guide mechanism and insertion handle assembly of the present invention showing removal of the insertion handle after insertion of the guide mechanism into a disc space between two vertebrae.  
         [0029]      FIG. 8  is a perspective view of the guide mechanism and milling mechanism assembly of the present invention showing guided insertion into a disc space between two vertebrae.  
         [0030]      FIG. 9  is a perspective view of the guide mechanism and milling mechanism assembly of the present invention shown after insertion into a disc space between two vertebrae.  
         [0031]      FIG. 10  is a perspective view of the guide mechanism and milling mechanism assembly of the present invention showing removal of the milling mechanism from a disc space between two vertebrae.  
         [0032]      FIG. 11  is a sectioned view of the guide mechanism and milling mechanism assembly of the present invention showing guided insertion into a disc space between two vertebrae.  
         [0033]      FIG. 12  is a sectioned view of the guide mechanism and milling mechanism assembly of the present invention shown after insertion into a disc space between two vertebrae.  
         [0034]      FIG. 13  is a sectioned view of the guide mechanism and milling mechanism assembly of the present invention showing removal of the milling mechanism from a disc space between two vertebrae and the space created by the milling process.  
         [0035]      FIG. 14  is an alternative embodiment of the guide mechanism.  
         [0036]      FIG. 15   a  is a further alternative embodiment of the guide mechanism.  
         [0037]      FIG. 15   b  is a further alternative embodiment of the guide mechanism.  
         [0038]      FIG. 16   a  shows a “disc replacement prosthesis” being inserted into a cavity formed with the invention.  
         [0039]      FIG. 16   b  shows a “disc replacement prosthesis” after insertion into a cavity formed with the invention.  
         [0040]      FIG. 17   a  shows a threaded fusion cage being inserted into a cavity formed with the invention.  
         [0041]      FIG. 17   b  shows a threaded fusion cage after insertion into a cavity formed with the invention.  
         [0042]      FIG. 18   a  shows an alternative embodiment of the counterbore portion of the invention comprising a flexure.  
         [0043]      FIG. 18   b  shows a further alternative embodiment of the counterbore portion of the invention comprising a bellows.  
         [0044]      FIG. 19   a  shows an exploded perspective view of an alternative embodiment of the mill portion of the invention incorporating a removable counterbore.  
         [0045]      FIG. 19   b  shows a perspective view of an alternative embodiment of the mill portion of the invention incorporating a removable counterbore.  
         [0046]      FIG. 20  shows a perspective view of an alternative embodiment of the mill portion of the invention incorporating a non-removable counterbore. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]      FIGS. 1 &amp; 2  show a guide mechanism  200  and an insertion handle  100 . Insertion handle  100  consists of an elongated shaft  110  with a blind hole  115  and a gripping handle  105 . At the bottom of blind hole  115  is a threaded portion  120  for attachment to guide mechanism  200 . Guide mechanism  200  consists of an intervertebral spacer  210 , an elongated guide shaft  205  and a depth stop  225 . On the end of elongated guide shaft  205  is a threaded portion  240  for attachment to threaded portion  120  of insertion handle  100 . Depth stop  225  has a circular cut-out  235  creating a shoulder  230  for stopping a counterbore which will be described below. Intervertebral spacer  210  consists of sidewalls  215  and rear rim  220 .  FIG. 1  is an exploded view to show the separate parts and  FIG. 2  shows the assembly. It is anticipated that multiple sizes of guide mechanism  200  will be necessary to ensure a proper fit for a particular size of disc space  20  which will be described in the following figures.  
         [0048]      FIGS. 3 &amp; 4  show a milling mechanism assembly  380  which consists of a counterbore  500 , a compression spring  400  and a mill  300 . Mill  300  has an elongated shaft  305 , a shoulder  310 , a hex portion  315  and a cutting portion  320 . Cutting portion  320  has cutting flutes  325 , an end portion  330  and a blind hole  335 . In this preferred embodiment, end portion  330  has features for end cutting, but this may not be necessary. Spring  400  has flat ends  410  so that spring  400  rests on shoulder  310  of mill  300 . Counterbore  500  contains cutting teeth  510 , an elongated body  505 , a through hole  515 , a shoulder  525 , and a hex portion  520  which engages hex portion  315  so that counterbore  500  spins in synchronization with mill  300 .  FIG. 3  is an exploded view to show the separate parts and  FIG. 4  shows the assembly. It is anticipated that multiple sizes of milling mechanism assembly  380  will be necessary to ensure a proper fit for a particular size of disc space  20  which will be described in the following figures.  
         [0049]      FIG. 5, 6  &amp;  7  show a method of inserting guide mechanism  200  into a disc space  20  between two adjacent vertebrae  10 . Guide mechanism and insertion handle assembly  180  is inserted into disc space  20  which has been prepared by the removal of some or all of the intervertebral disc material. Typically, the annulus fibrosis will be left intact on the lateral portions of disc space  20 , but this material is not shown in the figures for clarity. Guide mechanism and insertion handle assembly  180  is pushed, tapped, vibrated or otherwise inserted into disc space  20 .  FIG. 6  shows Guide mechanism and insertion handle assembly  180  inserted into disc space  20 . When the placement and orientation of guide mechanism  200  in disc space  20  has been satisfactorily achieved, insertion handle  100  is removed by unthreading it from threaded portion  240  on guide mechanism  200  and withdrawal from the surgical site. This is shown in  FIG. 7 . After removal of insertion handle  100 , milling mechanism assembly  380  is chucked into a drill (not pictured: either a manual drill or a power drill utilizing an electric motor or pneumatic motor) and is mounted onto guide mechanism  200  by sliding elongated guide shaft  205  into blind hole  335  in mill  300  as shown in  FIG. 8 . In this way, milling mechanism assembly  380  is guided through a precise path to cut a precise circular recess  600  into adjacent vertebrae  10 . Milling mechanism assembly  380  is further inserted into disc space  20  until elongated guide shaft  205  bottoms out in blind hole  335  as shown in  FIG. 9 . During this process, counterbore  500  cuts a circular shoulder  605  into the front of adjacent vertebrae  10 . The depth of circular shoulder  605  is determined by the engagement of shoulder  525  onto depth stop  225 . Spring  400  compresses to allow full insertion of mill  300  into guide mechanism  200 . Therefore, the same milling mechanism assembly  380  can be used to make a variety of hole depths and can therefore be used with multiple sizes of guide mechanism  200 . After completing the boring operation, milling mechanism assembly  380  is withdrawn from guide mechanism  200  as shown in  FIG. 10 . The diameter of elongated guide shaft  205  will be matched with blind hole  335 , the diameters of which may be varied to correspond with different heights of intervertebral spacer  210  so as to avoid incorrect boring of circular recess  600  and circular shoulder  610  by not allowing blind hole  335  to engage incorrect sizes of elongated guide shaft  205 .  
         [0050]      FIGS. 11, 12  &amp;  13  show the same steps as described above, but in sectioned views to illustrate the creation of circular recess  600  and circular shoulder  605 .  
         [0051]      FIG. 14  shows an alternative guide mechanism embodiment  800  which incorporates an angular intervertebral portion  805  with an angle A which is meant to match a lordotic angle in disc space  20 . In addition, alternative guide mechanism  800  incorporates a tapered bore  820  to accept a tapered mill. An alternative depth stop  810  with slots  815  is meant to allow either attachment to adjacent vertebrae  10  with screws or pins or to slide over a Caspar type distractor or other distractor means.  
         [0052]      FIGS. 15   a  &amp;  15   b  show a further alternative guide mechanism embodiment  900  which incorporates two elongated guide shafts  925  and  930  for guiding mill  300  to create two circular recesses for placement of two adjacent implants. More than two elongated guide shafts can be implemented for implantation of more than two implants. The orientation of elongated guide shafts  925  and  930  may be varied to create circular recesses and/or circular shoulders with differing orientations.  FIG. 15   b  is a back view of further alternative guide mechanism embodiment  900  to illustrate attachment means  920  which may be in the form of spikes for firm attachment to adjacent vertebrae  10 .  
         [0053]      FIGS. 16   a  &amp;  16   b  illustrate implantation of a disc replacement prosthesis  1000  into disc space  20 . Disc replacement prosthesis  1000  incorporates tabs  1005  &amp;  1010  which fit into circular shoulders  605 .  FIG. 16   a  shows disc replacement prosthesis  1000  before implantation and  FIG. 16   b  shows the completed implantation.  
         [0054]      FIG. 17   a  &amp;  17   b  illustrate implantation of a fusion cage  1100  into disc space  20 . Circular shoulders  605  may or may not be necessary for this case, and counterbore  500  may therefore be eliminated from milling mechanism assembly  380  if necessary.  FIG. 17   a  shows implant  1100  before implantation and  FIG. 17   b  shows the completed implantation.  
         [0055]      FIG. 18   a  shows an alternative counterbore  1200  embodiment where spring  400  is incorporated into the counterbore by the addition of flexure slots  1210  into body  1205  to allow compliance of alternative counterbore  1200  to allow mill  300  to bore a proper hole depth. Alternative counterbore  1200  has a hex shaped bore to engage hex portion  315  on mill  300 .  FIG. 18   b  shows a further alternative counterbore  1300  embodiment which is a bellows type construction with convolutions  1305  to allow compliance of further alternative counterbore  1300  to allow mill  300  to bore a proper hole depth. A cut away view is included in the figure to illustrate convolutions  1305  and to show hex portion  1315  which engage hex portion  315  on mill  300 .  
         [0056]      FIG. 19   a  &amp;  19   b  show an alternative counterbore embodiment which does not utilize a spring. In this embodiment, counterbore  500  utilizes a set screw  1420  for attachment to mill  1400 .  FIG. 19   a  is an exploded view to show the individual pieces,  FIG. 19   b  shows the assembly.  
         [0057]      FIG. 20  shows another alternative embodiment where the mill and counterbore have been consolidated into one combination mill  1500 .