A prosthesis includes an aperture with an arcuate wall incorporating a locking bone screw having an internally threaded annular collar with a circumferential cut-out allowing the collar to expand and contract. The collar may have an arcuate outer surface to engage and rotate in an arcuate wall of an aperture. The screw may include a head at one end of a screw shank, a first thread on the screw shank extending from the screw end remote from the head towards the head. A second thread on the shank may be adjacent the head. The screw may also include a threadless section of shank or lag between the first and second thread. The shank supporting the second thread may be tapered such that it expands the collar when screwed into the collar.