A bone interface anchor is provided for use with a stabilizer rod for the internal fixation of a spine. The anchor has a seat which accommodates the stabilizer rod and which receives a bone screw for the fixation of the seat to the bone. A compression member cooperates with the seat external to the stabilizer rod and can be tightened to cause a compressive force on the stabilizer rod. The stabilizer rod bears on a rounded surface of the bone screw so as to cause a mating interface between the seat and the bone screw. Subsequently, the position of the seat relative to the bone screw can be locked.