1. 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, an upper shank capture region, and a lower shank locking region, 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 closure-mating structure formed therein, the central bore including an interference wedging structure that extends inwardly toward the longitudinal axis below the closure-mating 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 retaining structure positionable within the receiver cavity upper shank capture region 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 retaining structure being held in the cavity upper shank capture region until the shank upper end portion is capture therein; and a compression insert having an upwardly facing surface sized and shaped for engaging the elongate rod, a laterally facing exterior surface for engaging the receiver interference wedging structure, and a recess formed into a lower portion of the insert for receiving and overlappingly engaging an upper portion of the retaining structure, wherein the compression insert is positionable within the receiver central bore prior to the shank and in a side-to-side overlapping relationship with the retaining structure, with the compression insert being prevented from moving downwardly within the central bore by at least partial engagement with the receiver interference wedging structure extending inwardly toward the longitudinal axis; and wherein after the shank upper end portion is captured within the retaining structure, the compression insert is downwardly deployable within the receiver by direct engagement with a tool until the exterior surface of the compression insert comes into a forced interference engagement with the receiver interference wedging structure so as to prevent the compression insert from moving back up within the receiver central bore.
1. 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, an upper shank capture region, and a lower shank locking region, 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 closure-mating structure formed therein, the central bore including an interference wedging structure that extends inwardly toward the longitudinal axis below the closure-mating 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 retaining structure positionable within the receiver cavity upper shank capture region 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 retaining structure being held in the cavity upper shank capture region until the shank upper end portion is capture therein; and a compression insert having an upwardly facing surface sized and shaped for engaging the elongate rod, a laterally facing exterior surface for engaging the receiver interference wedging structure, and a recess formed into a lower portion of the insert for receiving and overlappingly engaging an upper portion of the retaining structure, wherein the compression insert is positionable within the receiver central bore prior to the shank and in a side-to-side overlapping relationship with the retaining structure, with the compression insert being prevented from moving downwardly within the central bore by at least partial engagement with the receiver interference wedging structure extending inwardly toward the longitudinal axis; and wherein after the shank upper end portion is captured within the retaining structure, the compression insert is downwardly deployable within the receiver by direct engagement with a tool until the exterior surface of the compression insert comes into a forced interference engagement with the receiver interference wedging structure so as to prevent the compression insert from moving back up within the receiver central bore. 7. The pivotal bone anchor assembly of claim 1 , wherein the retaining structure frictionally engages the shank upper end portion upon capture within the receiver cavity to allow for an unlocked but non-floppy angular positioning of the shank with respect to the receiver prior to a final locking of the pivotal bone anchor assembly.