Abstract: The invention provides a bone screw for use in an implant assembly having a member including a hole through which the screw extends. The member preferably includes a socket in an anchor that receives a rounded bottom portion of the screw head in order to form a ball and socket connection. The screw has a necked area below the rounded bottom portion which projects through the hole and below that on the longitudinal axis of the screw. The screw includes a knurled collar that is sized so as to allow the anchor to be inserted onto the screw but which subsequently restrains the anchor from falling downward onto the threaded area of the screw.