22. A dynamic spine stabilization device comprising: a bone anchor having a housing and a longitudinal axis; a cavity in the housing coaxial with the longitudinal axis; a post received in the cavity; the post having a retainer at a distal end and a mount at a proximal end; the retainer being secured in a pocket of the cavity of the housing such that the post may pivot within the housing; a spring positioned in the cavity of the housing between the post and the housing such that deflection of the retainer in a direction perpendicular to the longitudinal axis causes compression of the spring in a direction parallel to the longitudinal axis; and wherein the spring applies a force upon the post pushing the post towards a position in which the post is coaxial with the longitudinal axis.
25. The device of claim 22 , wherein: said housing comprises a limit surface which contacts the post upon deflection of said post a first amount from the position in which the longitudinal axis of the post is coaxial with the bone anchor; and wherein further deflection of said post beyond said first amount requires a larger load per unit of deflection than deflection of said post up to said first amount.