A bone plate assembly is provided having a fixation plate held in position in a bone by a plurality of fasteners. The fasteners extend through holes in the bone plate to engage the bone below. Further, the assembly includes a locking plate which is secured to the fixation plate by means of a lock screw extending through an opening in the locking plate. Further, the locking plate has a dovetailed connection to engage the flanges defining the exterior edges of a channel and the fixation plate. The locking plate, thus, has a sliding connection from a first position where it has at least partial openings corresponding to the openings of the fixation plate for the fasteners, and optionally also for a graft screw. In a second position, the edge of the plate surrounding the opening overlaps the fastener openings so as to lock these openings against the fastener backing out. The locking plate lock screw opening is counter sunk in the second position to enable the lock screws with the exterior surface of the locking plate.