A polyaxial bone anchor assembly, comprising: a threaded screw portion comprising a head end； a head body disposed concentrically about the head end of the threaded screw portion； and a saddle assembly disposed within the head body and selectively engaging the head end of the threaded screw portion； wherein, when the saddle assembly is translated within the head body into a first locked position, the head body is coupled to the threaded screw portion such that disengagement of the head body from the threaded screw portion is allowed； and wherein, when the saddle assembly is translated within the head body into a second locked position, the head body is coupled to the threaded screw portion such that disengagement of the head body from the threaded screw portion is prevented.
