US Patent 9149311 Cervical plate fixation system

A bone fixation assembly comprises a plate having a top surface and a bottom surface and defining at least one bone screw bore, the bore including a circumferential rib situated between the top and bottom surfaces of the plate and at least one scallop interrupting the circumferential rib. A bone engaging fastener is provided for each bore, the fastener having a shank defining bone engaging threads and an enlarged head. The head defines threads for threaded engagement with the circumferential rib, with a lead thread configured to pass through a scallop to allow the threads to engage the circumferential rib.