A polyaxial bone anchor assembly, comprising: a threaded screw portion comprising an externally threaded head end; a head body selectively disposed concentrically about the externally threaded head end of the threaded screw portion, wherein the head body defines an internally threaded hole in a bottom portion thereof; and an externally threaded compression saddle assembly selectively disposed within an interior portion of the head body and selectively engaging the externally threaded head end of the threaded screw portion; wherein the externally threaded compression saddle assembly and the externally threaded head end of the threaded screw portion are selectively disposed within the interior portion of the head body by engaging and passing through the internally threaded hole in the bottom portion of the head body.