1. A polyaxial bone screw assembly for surgical implantation, the assembly comprising: a receiver having a base and opposed arms, the base having an inner cavity and the arms forming a longitudinal connecting member receiving channel, the channel communicating with the inner cavity; a shank having a threaded body and a head, the threaded body for implantation into bone and the head being captured within the inner cavity, the shank being pivotable with respect to the receiver; and a separate retaining structure located within the inner cavity that retains the shank head within the receiver, wherein: a) the inner cavity is partially defined by a lower seat and partially defined by a retaining structure expansion chamber located above the lower seat, the lower seat sized and shaped for operatively frictionally holding the retaining structure; b) the retaining structure being open and resilient, the shank head being bottom-loadable into the receiver, the retaining structure expandable about the head in the expansion chamber during loading of the head into the receiver; and c) the shank head having a first convex surface with a first radius that is sized and shaped for direct contact with a longitudinal connecting member to receive direct downward force from direct engagement with the longitudinal connecting member, the shank having a second lower convex surface having a second radius that is larger than the first radius, the second lower convex surface being operationally spaced from the receiver and in direct engagement with the retaining structure and fixed thereto when the longitudinal connecting member places direct downward force on the first convex surface, the second lower convex surface terminating at an upper perimetric edge and wherein one and up to a plurality of concave surface features are formed into the second lower convex surface at and near the upper perimetric edge, each concave surface feature sized and shaped for receiving a portion of the longitudinal connecting member to allow for an increased angle of pivot between the shank and the receiver, the upper perimetric edge being made discontinuous by the one and up to plurality of concave surface features.
1. A polyaxial bone screw assembly for surgical implantation, the assembly comprising: a receiver having a base and opposed arms, the base having an inner cavity and the arms forming a longitudinal connecting member receiving channel, the channel communicating with the inner cavity; a shank having a threaded body and a head, the threaded body for implantation into bone and the head being captured within the inner cavity, the shank being pivotable with respect to the receiver; and a separate retaining structure located within the inner cavity that retains the shank head within the receiver, wherein: a) the inner cavity is partially defined by a lower seat and partially defined by a retaining structure expansion chamber located above the lower seat, the lower seat sized and shaped for operatively frictionally holding the retaining structure; b) the retaining structure being open and resilient, the shank head being bottom-loadable into the receiver, the retaining structure expandable about the head in the expansion chamber during loading of the head into the receiver; and c) the shank head having a first convex surface with a first radius that is sized and shaped for direct contact with a longitudinal connecting member to receive direct downward force from direct engagement with the longitudinal connecting member, the shank having a second lower convex surface having a second radius that is larger than the first radius, the second lower convex surface being operationally spaced from the receiver and in direct engagement with the retaining structure and fixed thereto when the longitudinal connecting member places direct downward force on the first convex surface, the second lower convex surface terminating at an upper perimetric edge and wherein one and up to a plurality of concave surface features are formed into the second lower convex surface at and near the upper perimetric edge, each concave surface feature sized and shaped for receiving a portion of the longitudinal connecting member to allow for an increased angle of pivot between the shank and the receiver, the upper perimetric edge being made discontinuous by the one and up to plurality of concave surface features. 4. The polyaxial bone screw assembly of claim 1 wherein both the first convex surface and the second lower convex surface are substantially spherical.