1. A pivoting bone anchor assembly, comprising:a receiver defining a chamber communicating with a first channel, the first channel sized and shaped for receiving a portion of a longitudinal connecting member, the chamber communicating with a receiver bottom opening, the receiver chamber having a spherical seating surface adjacent the bottom opening and an upper expansion region, the first channel opening onto a top of the receiver;
a shank having a lower portion integral with an upper portion extending along a longitudinal axis, the upper portion including a circumferential capture recess with an external interface surface, the upper portion further including a first radiused surface above the recess and a second radiused surface below the recess;
an insert positioned within the receiver and configured to engage the shank upper portion; and
a resilient open retainer having an internal interface surface and a third radiused surface, the retainer being expandable over the first radiused surface and snapping into the shank capture recess within the upper expansion region of the receiver chamber, with the retainer internal interface surface frictionally engaging the shank external interface surface to maintain the retainer in an expanded state, when the shank upper portion is bottom loaded into the receiver chamber through the receiver bottom opening,
wherein the second radiused surface and the third radiused surface align to define a partially spherical bearing surface to pivotably engage the spherical seating surface when the retainer internal interface surface is frictionally engaged with the shank external interface surface, and
wherein both the insert and the retainer are top-loaded into the receiver before the shank upper portion is bottom-loaded into the receiver chamber through the bottom opening.