21. The polyaxial bone anchor assembly of claim 17 , further comprising a closure structure having a continuous helically wound external thread directly engaging the helically wound guide and advancement structure of the receiver.
17. A polyaxial bone anchor assembly comprising: a receiver having a base with a bottom opening and a pair of upwardly extending arms with opposed interior surfaces forming a U-shaped 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 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 lower portion; a retainer secured to the capture structure above the lower surface and having a partially spherical-shaped outer surface that conforms with and extends the partially spherical-shaped lower surface of the shank capture structure, with the retainer partially spherical-shaped outer surface and the shank partially spherical-shaped lower surface together having polyaxial engagement with the partially spherical-shaped internal surface of the cavity; and an insert disposable within the receiver and having a connecting member-engaging upper surface and a bottom surface spaced apart from the shank upper end portion, the insert being downwardly displaceable within the receiver channel and having at least one lateral projection configured to snap into the at least one recess to prevent upward movement of the insert.