17. A spine stabilization device comprising: a bone anchor; a deflectable mount; and a self-centering ball-joint connecting the bone anchor and the deflectable mount; wherein the self-centering ball-joint includes, a housing having a socket; a ball-rod received in the socket; and a centering rod having an inner core and an outer sheath, the centering rod received partially within the ball-rod and partially within the housing; whereby deflection of the ball-rod bends the centering rod and the centering rod exerts a restoring force on the ball-rod.
17. A spine stabilization device comprising: a bone anchor; a deflectable mount; and a self-centering ball-joint connecting the bone anchor and the deflectable mount; wherein the self-centering ball-joint includes, a housing having a socket; a ball-rod received in the socket; and a centering rod having an inner core and an outer sheath, the centering rod received partially within the ball-rod and partially within the housing; whereby deflection of the ball-rod bends the centering rod and the centering rod exerts a restoring force on the ball-rod. 18. The spine stabilization device of claim 17 , wherein said inner core of said centering rod is made of a super-elastic metal and said outer sheath is made of a biocompatible polymer.