Load-sharing bone anchor having a natural center of rotation and method for dynamic stabilization of the spine

A medical device comprising:

• a bone screw having a longitudinal axis, a proximal end and a distal end;
• the distal end of the bone screw adapted to engage a vertebra;
• a housing at the proximal end of the bone screw;
• a bore in the housing aligned with the longitudinal axis of the bone screw, the bore having an open end and the bore terminating in a spherical pocket;
• a post having a ball-shaped retainer at a first end, the post being positioned in the bore of the housing such that the ball-shaped retainer is trapped within the spherical pocket allowing the post to pivot about a pivot point at the center of the ball-shaped retainer, and a second end of the post extends through the open end of the bore;
• a compliant member positioned in the bore between the housing and post to flexibly align the post with the bore and the longitudinal axis of the bone screw;
• and
• wherein the bone screw and housing are configured so that, upon implantation of the bone screw in a bone, the pivot point is adapted to be positioned adjacent or below the surface of the bone, without obstruction of pivoting of the post.