Load-sharing bone anchor having a deflectable post and axial spring and method for dynamic stabilization of the spine
A dynamic spinal stabilization component which supports the spine while providing for the preservation of spinal motion. The component may be integrated in a bone anchor for implantation in a bone of the spine. The component and bone anchor provide load sharing while preserving range of motion and reducing stress exerted upon the bone anchors and spinal anatomy. The dynamic spinal stabilization component includes a deflectable post connected by a ball-joint to a threaded anchor. Deflection of the deflectable post is controlled by an axially compressible spring. The force/deflection properties of the dynamic bone anchor may be adapted to the anatomy and functional requirements of the patient. The dynamic spinal stabilization component may be used as a component of a dynamic stabilization system which supports the spine while providing for the preservation of spinal motion.
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.
2. The device of claim 1 , wherein said spring comprises a plurality of lever arms arranged in a circle.
3. The device of claim 1 , wherein said spring comprises a plurality of planar spring elements.
4. 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.
5. 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 a larger load per unit of deflection than deflection of said post up to said first amount.
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.
7. The device of claim 1 , wherein said post can be deflected about a point in said bone anchor that is adapted to be implanted adjacent a surface of a vertebra.
8. The device of claim 1 , wherein said spring has an isotropic deflection profile.
9. The device of claim 1 , wherein said spring has an anisotropic deflection profile.
10. The device of claim 1 wherein during deflection the post first is urged against said spring and then is subsequently urged against a limit surface of said housing.
11. The device of claim 1 , wherein the spring includes a plurality of arms that extend perpendicularly to the post.
12. A spine stabilization device comprising:
a bone anchor having a distal end adapted to engage a bone;
a housing at a proximal end of said bone anchor;
a longitudinal bore in said housing;
said bore being aligned with the bone anchor and having an open end and a closed end;
a deflectable post having a proximal end, an elongated body and a distal end;
the proximal end of said deflectable post extending from the open end of said longitudinal bore;
a curved retainer at the distal end of the deflectable post;
the curved retainer being received in the longitudinal bore;
a fastener which secures the curved retainer in the longitudinal bore but allows the deflectable post to pivot relative to the bone anchor; and
a spring positioned within the bore between the deflectable post and the housing such that the spring flexibly resists pivoting of the deflectable post towards the housing wherein pivoting of the deflectable post causes compression of the spring in a direction parallel to the axis of the longitudinal bore.
13. The spine stabilization device of claim 12 , further comprising:
a limit surface associated with the housing and positioned to contact the deflectable post after a first angle of pivoting of the deflectable post away from alignment with the bone anchor; and
wherein the limit surface resists further pivoting of said deflectable post beyond said first angle.
14. The spine stabilization device of claim 12 , further comprising:
a limit surface associated with the housing and positioned to contact the deflectable post after a first angle of pivoting of the deflectable post away from alignment with the bone anchor; and
wherein deflection of the proximal end of the deflectable post after contact between the deflectable post and the limit surface requires at least double the load per unit of deflection than deflection of said post prior to contact between the deflectable post and the limit surface.
15. The spine stabilization device of claim 12 , wherein said spring is made of PEEK.
16. The spine stabilization device of claim 12 , wherein the spring comprises a first portion in contact with the deflectable post, a second portion in contact with the housing and a flexible portion between the first portion and the second portion which is elastically deformed by pivoting of the deflectable post towards the housing.
17. The spine stabilization device of claim 12 , wherein the spring comprises a first portion in contact with the deflectable post, a second portion in contact with the housing and a flexible portion between the first portion and the second portion which is elastically deformed by pivoting of the deflectable post towards the housing and wherein the third portion comprises a plurality of lever arms.
18. The spine stabilization device of claim 12 , wherein the spring comprises a first portion in contact with the deflectable post, a second portion in contact with the housing and a flexible portion between the first portion and the second portion which is elastically deformed by pivoting of the deflectable post towards the housing and wherein the third portion comprises a coil.
19. The spine stabilization device of claim 12 , wherein the spring comprises one or more spring washers.
20. The spine stabilization device of claim 12 , wherein the spring is radially compressed by deflection of the deflectable post.
21. The device of claim 12 , wherein the spring includes a plurality of arms that extend perpendicularly to the post.
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.
23. The device of claim 22 , wherein said spring comprises a plurality of lever arms arranged in a circle.
24. The device of claim 22 , wherein said spring comprises a plurality of planar spring elements.
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.