12. A pivotal bone anchor assembly for securing an elongate rod to a bone, the pivotal bone anchor assembly comprising: a receiver having a longitudinal axis, a lower body defining a cavity with a bottom opening and a pair of upwardly extending arms with opposed interior surfaces that define an open channel for receiving the elongate rod, the open channel communicating with the cavity to define a central bore, the open channel interior surfaces having a discontinuous helically wound guide and advancement structure formed thereon, the central bore including an interference structure formed therein that extends inwardly towards the longitudinal axis and is located below the guide and advancement structure; a shank having an upper end portion and an anchor portion extending distally from the upper end portion for attachment to the bone, the upper end portion being up-loadable into the receiver cavity through the bottom opening; a compression insert positionable within the central bore prior to the shank and having an upper surface configured to engage the elongate rod and a curved exterior surface configured to engage the receiver interference structure; and a retainer positionable within the receiver cavity prior to the shank and operable to capture the shank upper end portion, upon uploading through the bottom opening, in spaced relationship from an internal surface of the receiver cavity, the retainer having an upper portion that is at least partially receivable, in a side-to-side overlapping engageable relationship, within a recess formed into a lower portion of the compression insert, prior to the retainer capturing the shank upper end portion, wherein the retainer and compression insert are downwardly deployable within the receiver lower body after capture of the shank upper end portion until the exterior surface of the compression insert comes into a forced interference engagement with the receiver interference structure so as to prevent the compression insert from moving back up within the receiver central bore.
13. The pivotal bone anchor assembly of claim 12 , wherein the retainer gets wider to receive the upper end portion of the shank before the retainer is downwardly-deployed within the receiver central bore.