Abstract:
The present invention provides a cervical plate with backout protection. In particular, a bushing residing in each of a plurality of through holes has a bottom edge that aligns with a protrusion on a screw head. The bottom edge aligning with the protrusion inhibits reverse threading or backing out of the bone screw.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/632,760, filed Aug. 1, 2003, titled CERVICAL PLATE, which is a continuation in part of U.S. patent application Ser. No. 10/178,371, filed Jun. 24, 2002, titled CERVICAL PLATE, now U.S. Pat. No. 6,602,257. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to implantable devices useful in bone fusion and, more particularly, to cervical plates having backout protection. 
     BACKGROUND OF THE INVENTION 
     The vertebrae of the human spine are arranged in a column with one vertebra on top of the next. Between each vertebra exists an intervertebral disc that transmits force between adjacent vertebrae and provides a cushion between the adjacent vertebrae. 
     Sometimes, back pain is caused by degeneration or other deformity of the intervertebral disk (“diseased disk”). Conventionally, surgeons treat diseased discs by surgically removing the diseased disc and inserting an implant in the space vacated by the diseased disk, which implant may be bone or other biocompatible implants. The adjacent vertebrae are then immobilized relative to one another. Eventually, the vertebrae grow into one solid piece of bone. 
     Currently, it is difficult to insert the bone graft into the vacated space and fuse the adjacent vertebrae. The current process of inserting a bone graft and fusing the adjacent vertebrae will be explained with referring to  FIGS. 1 and 2 .  FIG. 1  shows two adjacent vertebrae  102  and  104 . Located between vertebrae  102  and  104  is an intervertebral space  106  partially filled by an implant  108 . When the implant  108  is first inserted into the intervertebral space  106 , the adjacent vertebrae  102  and  104  are manually kept apart by the surgeon using, for example, a retracting device (not shown). As shown in  FIG. 2 , once the implant  108  is placed, the surgeon releases the adjacent vertebrae  102  and  104  allowing them to squeeze the implant  108  and hold the implant  108  in place. 
     To immobilize the vertebrae  102  and  104  with the implant  108  in place, the surgeon next applies a cervical plate  202  over the adjacent vertebrae  102  and  104 . Cervical plate  202  may have a central viewing window  204  and one or more screw holes  206 , in this example four screw holes  206   a – 206   d  are shown. Four bone screws (which will be identified by reference numerals  208   a – 208   d ) would be screwed into the vertebrae using the screw holes  206  to anchor the cervical plate to the vertebrae and immobilize the vertebrae with respect to one another. 
     The bone screws  208   a – 208   d  absent a locking mechanism tend to reverse thread, which is also known as backing out. Locking mechanisms have been developed to inhibit the bone screws from backing out. Some of the devices included caps or plates that extend over the screw holes  206  to inhibit upwards movement of bone screws  208   a – 208   d . Other devices include a frictional engagement between a bushing and the bone screws  208   a – 208   d.    
     Although many devices exist that satisfactorily inhibit backout of the bone screws, it would be desirous to develop a device to inhibit the bone screws from backing out. 
     SUMMARY OF THE INVENTION 
     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a cervical plate having backout protection is provided. The cervical plate comprises a plurality at through holes. Each through hole includes a channel in which a bushing resides. The bushing has a bottom edge that can align with a protrusion on a screw head that inhibits the screw from backing out. 
     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention, and together with the description, serve to explain the principles thereof. Like items in the drawings are referred to using the same numerical reference. 
         FIG. 1  shows adjacent vertebrae with a bone graft; 
         FIG. 2  shows adjacent vertebrae with a bone graft and cervical plate; 
         FIG. 3  shows a perspective and cross-sectional view of a cervical plate with backout protection consistent with the present invention; 
         FIG. 4  shows a perspective view of a fixed angle bone screw; 
         FIG. 5  shows a perspective view of a variable angle bone screw; 
         FIG. 6  shows a perspective view of an embodiment of one possible bushing consistent with the present invention; 
         FIG. 7  shows a cross-sectional view of the bone screw and bushing as it would exist if the bone screw was threaded in the vertebral body; and 
         FIG. 8  shows a cross-section view similar to  FIG. 7  but prior to complete engagement. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described with reference to  FIGS. 3–8 . Referring first to  FIG. 3 , a perspective and cross-sectional view of a cervical plate  300  illustrative of an embodiment of the present invention is shown. Cervical plate  300  is shown with a construct that could span two intervertebral spaces (a.k.a. a two level cervical plate); however, cervical plate  300  could be constructed to span more or less intervertebral spaces. Because plate  300  spans two intervertebral spaces, cervical plate  300  is shown with two viewing windows  302 . More or less viewing windows  302  could be provided. For example, for a construct that spanned one intervertebral space, only one viewing window may be used. Moreover, for a construct that spanned three intervertebral spaces, three viewing windows may be provided. Furthermore, for cervical plate  300 , each viewing window  302  could be split into several smaller viewing windows as a matter of design choice. 
     Cervical plate  300  comprises a bone facing side  304 , a top side  306  opposite bone facing side  304 , and a plurality of through holes  308 . Through holes  308  generally have a diameter d1, at least at bone facing side  304  and top side  306 . Each of the plurality of through holes  308  has a channel  310  traversing a perimeter of through hole  308 . Channel  310  resides between bone facing surface  304  and top side  306 . Generally, channel  310  has a concave shape with a maximum diameter of diameter d2 greater than d1. While described separately, channel  310  may simply be a bowing or gradual increase in diameter along the sidewalls associated with through holes  308 . A bushing  312  resides in channel  310 , as will be explained further below. Bone screws  314  extend through through holes  308  such that bone screws  314  are threaded to vertebral bodies. A head  316  of bone screws  314  engage bushings  312 , as will be explained further below, inhibiting bone screws  314  from reverse threading or backing out. 
     Referring to  FIGS. 4 and 5 , fixed angle bone screw  400  and variable angle bone screw  500  are shown. Note, bone screws  314  could be either fixed angle bone screws  400  or variable angle bone screws  500 , and both are shown in  FIG. 3 . Fixed angle bone screws  400  and variable angle bone screws  500  are generally known in the art and will not be further explained herein. Head  316  will be explained further below with reference to  FIG. 7 . 
     Referring to  FIG. 6 , a perspective view of bushing  312  is shown in more detail. Bushing  312  comprises a top edge  602 , which would be located proximate top side  304 , and a bottom edge  604 , which would be located proximate bone facing surface  306 . Sidewall  606  extends between top edge  602  and bottom edge  604 . Bushing  312  is shown generally cylindrical in shape but generally would have a shape consistent with channel  310  to allow cervical plate  300  and bone screws  314  to align properly during surgery. In this case, channel  310  has a concave shape to cooperatively engage a convex shape of an outer surface  608  of sidewall  606 . Inner surface  610  has a shape consistent with heads  316 . Outer surface  608  has a diameter d2 at its maximum. Bushing  312  is compressible such that bushing  312  can be compressed to fit within diameter d1. To assist with compression, bushing  312  may have a gap  610 . Once the compressive force is removed, bushing  312  would expand such that outer surface  608  cooperatively engages channel  310 . 
     At least a bottom portion  612  of bushing  312  comprises a flexible material that can expand outward when impinged by head  316 , which will be explained further below. To facilitate the flexible movement, bottom portion  612  may comprises one or more slots  614 . When not impinged by head  316 , bottom edge  604  has a diameter d3. 
     Head  316  will be explained in more detail with reference to  FIGS. 7 and 8 . Head  316  has an internal matting surface  702 . Internal matting surface  702  is designed to allow a surgical tool to drive bone screw  314  into a vertebral body. Head  316  has an external surface  704  that cooperatively engages inner surface  610  of bushing  312 . In this case, inner surface  610  is concave and external surface  704  is convex. External surface  704  may extend over a portion or all of head  316 , but terminates at a transition edge  706  where a protrusion  708 , which may be a ledge or shoulder, extends. Transition edge  706  has a diameter d3 and protrusion  708  extends outward from transition edge  706  such that bottom edge  604  abuts protrusion  708 . Thus, bushing  312  inhibits reverse threading of bone screw  314  because bottom edge  604  abutting protrusion  708  inhibits upward movement of bone screw  314 . While bottom edge  604  could directly abut protrusion  708 , washers or other devices could be implanted as well. 
     While  FIG. 7  shows the implant after bone screw  314  has been threaded in the vertebral bodies,  FIG. 8  shows the implant prior to completion of the threading operation. As bone screw  314  is threaded in the vertebral bodies, head  316  advances through bushing  312 . A surface  802  on protrusion  708  tends to impinge on bottom portion  612  as head  316  advances. The impingement causes bottom portion  612  to flex. Once protrusion  708  advances past bottom edge  604 , bottom portion returns to it pre-flex position such that bottom edge  604  is aligned over protrusion  708 . 
     While the invention has been particularly shown and described with reference to an embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.