A bone fixation apparatus adapted for use in spinal stabilization. The apparatus includes a threaded bone screw having a spherical head that is received in a receiver cup. The receiver cup also includes a slot or groove through which a support rod passes. A saddle is positioned in the receiver cup below the support rod and above the head of the threaded bone screw. Alignment flanges are positioned partially around an opening on the receiver cup opposite the bone screw. A cap having internal grooves corresponding to the alignment flanges mates with the receiver cup. The cap may be positioned onto the receiver cup in a first orientation and secured to the receiver cup by rotating approximately one quarter turn. A compression member is screwed into internal threads on an opening of the cup to provide pressure on the support rod, thereby fixing the rod to the receiver cup.
