An anti-splay closure with a left handed threaded removal bore, includes a cylindrical closure body with an external, continuous, helically extending anti-splay guide and advancement flange and a breakaway installation head. The removal bore is aligned with an axis of the closure body. The bore thread is sized and shaped to receive a removal tool having mating left handed thread. The closure is used with an open headed bone implant screw which is provided with internal, helically extending mating guide and advancement structure complementary to the flange of the closure body.