The present invention involves a system and methods for assembling and implanting a polyaxial bone screw assembly. The polyaxial bone screw assembly includes, but is not necessarily limited to, a bone screw that can be initially secured to a bony structure and subsequently have a receiver assembly attached to the implanted bone screw. Additionally, a closure structure is a part of the polyaxial bone screw assembly and functions to secure an elongate member within the receiver assembly.