1. In a bone anchor, the improvement comprising: a) a shank having a body for fixation to a bone and an integral upper substantially spherical head; b) a receiver having a top portion and a base and an outer surface, the receiver top portion defining an open channel, the base defining a chamber and a lower opening, the chamber communicating with both the channel and the lower opening, the top portion having a pair of opposed through apertures formed therein; c) at least one insert disposed within the receiver, the insert having a curved surface sized and shaped for frictional mating cooperation with the shank head, the insert further having a pair of outwardly extending arm portions, each arm portion partially extending through one of the apertures of the receiver, the arm portions being engageable by manipulation tools located at the receiver outer surface for movement of the insert both downwardly into contact with the shank head and upwardly away from the shank head; and d) a resilient open retainer having a base, the retainer captured within the chamber and expandable about at least a portion of the shank head and wherein expansion-only locking engagement occurs between the shank upper portion and the retainer base and between the retainer base and the receiver.
2. The improvement of claim 1 wherein a pair of shallow grooves are formed in the outer surface of the receiver, each groove communicating with a respective aperture, each groove extending from the respective aperture to a top surface of the top portion of the receiver, each groove having a width for receiving a manipulation tool.