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.
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. 9. The device of claim 1 , wherein said spring has an anisotropic deflection profile.