Abstract: A bone fixation plate assembly includes an elongated plate having two or more sets of two adjacent screw holes extending between its top and bottom surfaces. A landing extends through the top surface of between the adjacent screw holes of a set. An interior sidewall of each one of the screw holes is spherically shaped. Each one of the screw holes has a minimum diameter at its bottom edge that is less than a minimum diameter at its top edge. An anti-back-out device is fixedly attached to the elongated plate on each one of the landings. Opposing end portions of each anti-back-out device has one or more screw engaging structures integral therewith overhanging an adjacent one of the screw holes at the top edge thereof. The one or more screw engaging structures is deflectable with respect to a mounting portion of a respective one of the anti-back-out devices.