A bone plate assembly utilizing at least one bone screw for fixation of adjacent bones of a spine including a base plate having at least one aperture extending therethrough and screw retaining mechanism mounted and movable on the plate between locked and unlocked positions relative to the aperture for preventing the bone screw from backing out from the base plate. Furthermore, a device for placement into an aperture of a base plate including a screw retaining mechanism mountable and movable on a plate between locked and unlocked positions relative to an aperture for preventing a bone screw from backing out from the base plate.