6. The device of claim 1 , wherein: said housing has 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 at least double the load per unit of deflection than deflection of said post up to said first amount.
1. 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 be deflected within the housing; a spring positioned in the cavity of the housing between the post and the housing such that deflection of the post 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.