A polyaxial bone screw assembly includes a threaded shank body having an upper portion, a receiver, a compression insert and an articulation structure made from at least two discrete pieces. The shank upper portion cooperates with the compression insert to place the retainer in frictional engagement...http://www.google.com/patents/US7875065?utm_source=gb-gplus-sharePatent US7875065 - Polyaxial bone screw with multi-part shank retainer and pressure insert
