Bone screw assemblies include longitudinal connecting members that provide for dynamic stabilization, some including non-uniform portions that are configured to flex, contract or expand. Composite longitudinal connecting members include longitudinal segments made from different materials having different flexibilities. Polyaxial bone screw assemblies include change-out receivers for cooperating with replacement longitudinal connecting members having a different flexibility. Bone screw shanks for cooperating with one or more open receivers include treatment or coating to provide biologically active interface with bone.
