the present invention will be described with reference to fig3 - 8 . referring first to fig3 , 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 fig4 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 fig3 . 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 fig7 . referring to fig6 , 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 fig7 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 fig7 shows the implant after bone screw 314 has been threaded in the vertebral bodies , fig8 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 .
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 .