A fastener, for example a bone screw, is sized to pass into an aperture of an implant, for example a bone plate, the fastener having an anchor portion engageable with body tissue, and a head portion formed by an assembly of an insert and an anchor extension. The implant has an aperture with a reduced entry dimension. The insert and anchor extension cooperate to form a reduced dimension passable into the aperture, and an expanded dimension not passable out of the aperture. The size of the assembly is configurable by bending resilient tabs of the anchor extension or the implant, using ramped surfaces, or an expanding diameter of a threadable insert.