Abstract:
An anti-subsidence, anti-translocation, fusion compression cervical plate system with interface visualization, drill guide and temporary pin is provided. In one embodiment the invention provides a vertebral plate for minimizing the ability of an inter-body cage, filled with bone (or BMP-soaked sponges, etc.), to telescope into the vertebral body and to minimize failure due to cages that telescope into adjacent vertebral bodies on a delayed basis after surgery, as well as creation of reverse lordosis due to such telescoping. Another embodiment provides method to reduce the gap between the inter-body cage and the vertebral body end plate intra-operatively using compression from the plate caused by an offset between the screw bores on the plate and the drill holes, as established with an asymmetric drill guide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of U.S. Provisional Application 60/745,025, filed Apr. 18, 2006; which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of surgery, more specifically to surgery of the cervical spine. It should be understood, however, that vertebrae other than the cervical vertebrae of the human spine may be retained with the disclosed apparatus and associated methods. 
     BACKGROUND OF THE INVENTION 
     Anterior cervical discectomy with fusion is an operation that involves relieving the pressure placed on nerve roots and the spinal cord by a herniated disc or bone spurs. This condition is referred to as nerve root compression. The procedure includes removing the damaged intervertebral disc, which is usually the source of the compression and placing a bone graft between vertebrae adjacent to the space left by the removed disc. The bone graft is commonly an inter-vertebral-body cage filled with bone (or BMP-soaked sponges, etc.). Commonly, a vertebral plate is used to stabilize the spine and contain the intra-body cage while the spine heals. 
     There are currently numerous plates used for the process of internal fixation in the cervical spine. Some have locking screws and some have variable angle or axis screws and others have a mixture of both of the above. A common problem with the use of any cervical plate in conjunction with inter-body cages is that the cages telescope or subside into adjacent vertebral bodies on a delayed basis after surgery. This is due to a variety of factors including differences in biomechanical properties of the cage in relation to the vertebral body, inadvertent destruction of the end plate during end plate preparation, poor bone quality and micromotion, among others. If subsidence becomes too great, screws will break and the plate will fail. 
     A second problem is that the superior vertebral body may translocate anteriorly on a rotational basis, with the anterior inferior cage serving as the fulcrum; anterior rotational translocation. Even with locking screws in the inferior aspect of current plates, this anterior rotational translocation is not prevented. 
     A third method of cage and fusion failure is due to inadvertent creation of a gap between the interbody cage filled with bone (or BMP-soaked sponges, etc.) and the vertebral body end plate. A method to reduce this gap intraoperatively, using a (“lag screw”) compression from the plate to the vertebral body that reduces this gap, would minimize this problem. In addition, compression across a fusion mass is known to increase fusion rate, as bone fuses under compressive loads at a higher rate. 
     SUMMARY OF INVENTION 
     The invention includes an anti-subsidence, anti-translocation, fusion compression cervical plate system with interface visualization, drill guide and temporary pin. In one embodiment the invention provides a vertebral plate for minimizing the ability of an inter-body cage, filled with bone (or BMP-soaked sponges, etc.), to telescope into the vertebral body and to minimize failure due to cages that telescope into adjacent vertebral bodies on a delayed basis after surgery, as well as creation of reverse lordosis due to such telescoping. Another embodiment provides method to reduce the gap between the inter-body cage and the vertebral body end plate intra-operatively using compression from the plate caused by an offset between the screw bores on the plate and the drill holes, as established with an asymmetric drill guide. 
     In a first embodiment, the invention includes a vertebral plate which can be used with or without an inter-body cage. The plate includes a first plurality of bone screw bores disposed at the superior end of the frame. A second plurality of bone screw bores, preferably two, is disposed at the inferior end of the frame with a third, locking bone screw bore. The second plurality of bone screw bores comprises two proximal bone screw bores, relative to the inferior end of the inter-body cage, and the locking bone screw bore is a distal bone screw bore. 
     The two proximal bone screw bores are adapted to aim a bone screw in a caudal direction and the distal bone screw bore is adapted to aim a bone screw in a cephalad direction, parallel to the disc space. The distal bone screw is between the proximal bone screw bores in a preferred embodiment. 
     In another embodiment, the second plurality of bone screw bores comprises two distal bone screw bores and one proximal locking bone screw bore. In this embodiment, the two distal bone screw bores are adapted to aim a bone screw in a caudal direction and the proximal bone screw bore is adapted to aim a bone screw in a cephalad direction; here the proximal bone screw is between the proximal bone screw bores. 
     The vertebral plate can also be equipped with a window which allows direct visualization of the interface between the cage and the vertebrae. Another embodiment includes a plurality of notches on the lateral edges of the inferior end of the frame to receive a positioning device. Preferably, the notches are superior to the bone screw bores disposed in the inferior end of the frame. 
     Another embodiment of the invention includes a vertebral plating system for engaging at least two vertebrae with an inter-body cage there between. The system includes a plate having a pair of notches on the inferior end of the frame, a positioning device for placing the plate in position on at least one vertebra, the device having a body with a predetermined notch to receive the plate, and a drill guide. In alternate embodiments, the notch in the body of the positioning device is centered, thereby allowing the plate to accommodate an inter-body cage disposed along the midline of the vertebrae; or the notch in the body of the positioning device is off-center, thereby allowing the plate to accommodate an inter-body cage disposed eccentric to the midline of the vertebrae. 
     The plate of the vertebral plating system includes three bone screw bores at the inferior end of the plate, including a plurality of bone screw bores and a locking bone screw bore. The plurality of bone screw bores comprises two proximal bone screw bores adapted to aim a bone screw in a first direction and the locking bone screw bore is a distal bone screw bore adapted to aim a bone screw in a second direction. Alternatively, the plurality of bone screw bores comprises two distal bone screw bores adapted to aim a bone screw in a first direction, and the locking bone screw bore is a distal bone screw bore adapted to aim a bone screw in a second direction. 
     An alternate vertebral plating system for engaging at least two vertebrae with an inter-body cage there between includes a plate having a superior and an inferior end, a first plurality of bone screw bores disposed at the superior end of the frame, a second plurality of bone screw bores disposed at the inferior end of the frame, a locking bone screw bore disposed at the inferior end of the frame, a first drill guide having a plurality of guide tubes adapted to engage to the second plurality of bone screw bores in the plate, said first drill guide further having a guide tube adapted to engage the locking bone screw bore disposed in the plate, and a second drill guide having a guide tube with an aperture there through, said aperture adapted to be off-center by a predetermined distance whereby the holes drilled the in vertebrae using the second drill guide are asymmetric to the bone screw bores in the plate. 
     The plurality of guide tubes adapted to engage the second plurality of bone screw bores are adapted to aim a bone screw in a first direction, and the guide tube adapted to engage the locking bone screw bore is adapted to aim a bone screw in a second direction. Alternatively the second plurality of bone screw bores comprises two distal bone screw bores adapted to aim a bone screw in a first direction; and wherein the locking bone screw bore is a distal bone screw bore adapted to aim a bone screw in a second direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a front perspective view of the vertebral plate. 
         FIG. 2  is an isometric view of a 2-level vertebral plate. 
         FIG. 3  is an isometric view of a symmetric positioner. 
         FIG. 4A  is a front perspective view of the vertebral plate. 
         FIG. 4B  is an isometric view of the vertebral plate. 
         FIG. 5  is an isometric view of an asymmetric positioner. 
         FIG. 6  is an isometric view of an asymmetric drill guide. 
         FIG. 7A  is an isometric view of a drill guide for use with the inferior end of the vertebral plate. 
         FIG. 7B  is an alternate isometric view of a drill guide for use with the inferior end of the vertebral plate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
     Referring to  FIG. 1 , the cervical stabilization plate ( 10 ) of the present invention is shown. In use, plate  10  is positioned between at least two vertebrae, above and below the disc to be treated and/or removed. In the preferred embodiment, plate  10  has a substantially hour-glass shaped frame having superior end  12  and inferior end  14  spaced at a predetermined distance by arms  16 . Window  18  is defined by the open space between superior end  12 , inferior end  14  and arms  16 . Window  18  allows for direct visualization of the interface between the inferior aspect of the inter-body cage and the superior aspect of the inferior end plate. 
     Superior end  12  includes temporary pin-hole  40 , at the center position thereof, for receiving a temporary positioning screw. Superior end  12  also includes bone screw bores  22  and  24 , which are preferably located on either side of the midline of plate  10 , for fixing the superior end of plate  10  to the vertebrae with bone screws. 
     Inferior end  14  includes temporary pin-holes  42  and  44 , located on either side of the mid line of plate  10 , also for receiving temporary positioning screws. In addition, inferior end  14  includes a plurality of bone screw bores including proximal bone screw bores  32 ,  34  and distal bone screw bore  36 . In a preferred embodiment, the bone screw bores of inferior end  14  are disposed at angle to aim the bone screws in a predefined direction. In one embodiment, for example, proximal bone screw bores  32 ,  34  are disposed to aim a bone screw in a caudal direction and distal bone screw bore  36  is disposed to aim a bone screw in a cephalad direction, parallel to the disk space. 
     The arrangement of three, or more screws, defines a plane as well resisting moment the arrangement prevents rotational translocation in an anterior direction. The arrangement of  FIG. 1 , having two proximal screws and one distal screw, accentuates the anti-telescoping properties of the plate. In an alternate embodiment, there is one proximal screw and two distal screws, favoring the antitranslocation properties of the plate. 
     The screw bores disposed on the inferior end of plate  10  function as locking screws, which serve to resist rotational translocation. In order to allow some subsidence, however, the screw bores of superior end  12  are variable angle screws, forcing the superior aspect of the plate to be the dynamic side of plate  10  and inferior end  14  to be the fixed end. This configuration of inferior fixed screws and superior variable angle screws provides the safest combination of a dynamic plate combined with a prevention of telescoping and anterior rotational translocation since telescoping of the cage into the vertebral body superior to the cage is far more uncommon than telescoping of the cage into the vertebral body below the cage. 
     Cervical plate  10   a  shown in  FIG. 2  is a two-level cervical plate have intermediate bone screw bores  26   a ,  26   b ,  28   a  and  28   b  disposed between superior end  12  and inferior end  14 . Additionally, medial frame member  17  is provided between arms  16  to increase rigidity as well as provide for medial temporary pin holes  38   a  and  38   b . Medial frame member  17  also bifurcates window  18  ( FIG. 1 ) to form superior window  18   a  and inferior window  18   b.    
     In order to prevent the subsidence of the inter-body cage, the invention includes removable plate positioner  100  ( FIG. 3 ). Positioner  100  guarantees a fixed relationship between the inferior screws proximal to the inter-body cage, the vertebral body end plate and the end of the inter-body cage. Positioner  100  comprises handle  102  and substantially flat body  104 , from which tines  106  extend to form notch  110 . In use, plate  10  is place within notch  110  but inserting tines  106  into positioner notches  120  located on inferior end  14  of plate  10  ( FIGS. 4A-4B ). 
     Tines  106  have a length slightly greater than the width of plate  10 , thereby allowing tines  106  to contact the sides of the vertebral body. In the embodiment of  FIG. 4 , notch  110  is centered in body  104 . Accordingly, plate  10  is positioned in a substantially central position when tines  106  are aligned with the sides of the vertebral body. In the embodiment of  FIG. 5 , however, notch  110   a , is off-center to body  104 . Here it can be seen that tine  106  has a lesser width than tine  106   a . This arrangement allows plate  10  to be positioned eccentric to the midline of the vertebral body when tines  106  are aligned with the sides of the vertebral body. Temporary pin holes  40 ,  42  and  44  are provided for situations where the positioner cannot be used. 
     Another embodiment of the invention actively compresses across the fusion or inter-body cage by implementing a “lag effect.” In one embodiment, shown in  FIG. 6 , asymmetric drill guide  140  is provided wherein cannula  145  for the drill is located superiorly by a fixed distance off of the center of the guide tube  147 . Therefore the holes which are drilled into the vertebral bodies using asymmetric drill guide  140  are off-center (asymmetric) with the bone screw bores in plate  10 . Therefore, the process of tightening the bone screws into the vertebral body will create a lag effect, pulling the vertebral bodies tight against the end of the cage. 
     In another embodiment, compression is achieved by using a temporary pin with an asymmetric spacer to fill the screw hole. This configuration allows manipulation of the interbody-cage rostrally and caudally as the drilling is performed followed by rostral and column movement of the plate when placing the screws. Here, a standard drill guide is used in a standard hole in plate  10 . A temporary pin equipped with a spacer holds plate  10  against the vertebral body. The temporary pin is off-center within the spacer. When the spacer is rotated towards the end of the plate  10 , the plate moves away from the inter-body cage. Drilling can then be performed in the contra-lateral hole. Before placing a screw, the spacer is rotated in the opposite direction, bringing the plate in the opposite direction. The hole in the opposite end of the plate is then drilled. This allows the drill holes to be made at a defined distance further apart than the distance between the actual holes on the plate. As the screws are then placed into the vertebral body through the plate, a “lag” effect will be created. 
     In yet another embodiment, the screw bores of plate  10  are arranged in a figure-eight. The hole created by drilling and the placement of screws are thereby offset by a fixed distance allowing a “lag” effect of that distance. The drill guide is inserted into the distal hole for drilling and the screw is placed in the more proximal hole on the plate. As the screw follows the path of the drill guide, this similarly creates a “lag” effect. 
     In another embodiment, the drilling is performed through the same hole in which the screw is placed, but the angulation is increased rostral and caudal to the vertical plane in order to create a compression force across the graft in the vertical axis. For example, the upper screw is aimed rostral at a 30° direction and the inferior screw is aimed in a caudal direction by 30°. As the screws are tightened, a “lag” effect is created pulling the vertebral bodies toward the cage by the triangulation of the screws. 
     The invention also include drill guide  160 ,  FIG. 7 . Drill guide  160  includes proximal guide tubes  165   a  and  165   b , and distal guide tube  170 . Proximal guide tubes  165   a  and  165   b  are adapted to aim a screw hole in a caudal direction. Distal guide tube  170  is designed to aim a screw hole in a cephalad direction. Drill guide  160  can also be modified for embodiments, as described above, which use two distal screws and a single proximal screw. 
     It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between. Now that the invention has been described,