An anti-splay closure with a multi-surfaced aperture, such as a multi-lobular socket, includes a cylindrical body with an external, continuous, helically extending anti-splay guide and advancement flange and a breakaway installation head. The multi-lobular socket includes a plurality of circumferentially spaced lobes extending parallel to a closure axis of the plug and which circumferentially alternate with spline receiving grooves extending parallel to the closure axis. The closure is used with an open headed bone implant screw having arms that are provided with internal, helically extending mating guide and advancement structures complementary to the body flange to allow slidable mating with the body upon rotation thereof and radial interlocking between the arms and the body.
