1. A pivotal bone anchor assembly comprising: a receiver having a lower body with a bottom opening and a pair of upwardly extending arms with opposed interior surfaces forming a receiver channel for receiving a longitudinal connecting member therebetween, the interior surfaces having a discontinuous helically wound guide and advancement structure thereon and at least one retaining recess formed into the interior surfaces below the helically wound guide and advancement structure, and the lower body having a cavity in communication with the receiver channel and partially defined by a partially spherical-shaped internal surface adjacent the bottom opening; a shank having a body with an upper end portion and threaded lower portion, the upper end portion having a capture structure with a partially spherical-shaped lower surface that extends radially outward and axially upward from the threaded lower portion; and an insert disposable within the receiver channel and having a connecting member- engaging upper surface, the insert being downwardly displaceable within the receiver channel and having at least one lateral projection configured to snap into the at least one retaining recess to prevent upward movement of the insert, wherein the partially spherical-shaped lower surface of the capture structure engages the partially spherical-shaped internal surface of the cavity to provide pivotal motion of the shank with respect to the receiver.
1. A pivotal bone anchor assembly comprising: a receiver having a lower body with a bottom opening and a pair of upwardly extending arms with opposed interior surfaces forming a receiver channel for receiving a longitudinal connecting member therebetween, the interior surfaces having a discontinuous helically wound guide and advancement structure thereon and at least one retaining recess formed into the interior surfaces below the helically wound guide and advancement structure, and the lower body having a cavity in communication with the receiver channel and partially defined by a partially spherical-shaped internal surface adjacent the bottom opening; a shank having a body with an upper end portion and threaded lower portion, the upper end portion having a capture structure with a partially spherical-shaped lower surface that extends radially outward and axially upward from the threaded lower portion; and an insert disposable within the receiver channel and having a connecting member- engaging upper surface, the insert being downwardly displaceable within the receiver channel and having at least one lateral projection configured to snap into the at least one retaining recess to prevent upward movement of the insert, wherein the partially spherical-shaped lower surface of the capture structure engages the partially spherical-shaped internal surface of the cavity to provide pivotal motion of the shank with respect to the receiver. 11. The pivotal bone anchor assembly of claim 1 , wherein a bottom surface of the insert is spaced from the upper end portion of the shank.