US Patent 7476239 Polyaxial bone screw with compound articulation

A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a receiver and a retainer having spherical inner and outer surfaces. The retainer is compressible, having an opening with two ends that may be pressed toward one another. The retainer spherical surfaces slidingly mate with both the receiver and the capture structure, providing multiple or compound polyaxial articulation.