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.