A modular screw assembly for use in stabilizing a spinal column. The assembly has a fastener which couples to a screw head of a bone anchor, a screw head coupler, a retaining ring, and a friction ring. The fastener houses the screw head coupler, the retaining ring, and the friction ring. Translation of the fastener by the screw head results in the retaining ring elastically deforming and then snapping around the screw head.