An anti-splay closure with a pair of spaced removal apertures, includes a cylindrical closure body with an external, continuous, helically extending anti-splay guide and advancement flange and a breakaway installation head. The removal apertures are spaced from and parallel to an axis of the closure body and are also spaced from a radially outer surface of the body. The apertures are sized and shaped to receive a removal tool having prongs with a complementary shape to the bores. 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.