Abstract: A spinal fixation system for use in the mobilization of a sequence of spinal bones includes a unique pedicle screw assembly. The pedicle screw assembly includes a pedicle screw having a head portion pivotally secured within a base of a body and a threaded shaft portion extending therefrom for insertion into the spinal bone. The body includes a rod passageway adapted to receive a rod extending between two or more pedicle screw assemblies. The body includes oppositely threaded internal and external threads. A set screw having exterior threads engages the internal threads of the body so as to be selectively disposed therein and in contact with the rod. A nut having internal threads engages the external threads of the body. The set screw and nut are preferably simultaneously fastened by turning in opposite directions to counteract the forces applied to the assembly and spinal bones.