Intervertebral spacer that dynamically promotes bone growth

1. A dynamic intervertebral spacer comprising:

a monolithic spacer having an anterior portion, a posterior portion, a superior surface, an inferior surface, and an open center portion, wherein said monolithic spacer is configured to be implanted in a space between adjacent vertebral bodies with the superior surface engaging a superior vertebral body and the inferior surface engaging an inferior vertebral body;

wherein the spacer is split to provide motion in the anterior/posterior direction between a bone attachment portion of the superior surface and a bone attachment surface of the inferior surface of between 1 and 5 degrees;

wherein a portion of the spacer is configured to act as a torsion spring to allow the anterior/posterior motion;

wherein the open center portion is configured to receive bone graft; and

wherein inferior and superior surfaces are angled with respect to one another in order to restore lordosis.

2. A spacer as in claim 1 , further comprising an attachment feature for attaching the bone attachment portion of at least one of the superior and inferior surfaces to the corresponding superior or inferior vertebral body.

3. A spacer as in claim 2 , wherein the attachment feature is a bone screw.

4. A spacer as in claim 2 , wherein the attachment feature is a surface feature or coating which promote bone ingrowth.

5. A spacer as in claim 2 , wherein first and second sides of the spacer each have at least one bone screw, wherein the bone screw(s) on one side are configured to attach to the superior vertebral body and the bone screw(s) on the another side are configured to attach to the inferior vertebral body.

6. A spacer as in claim 1 , wherein the monolithic spacer is in the shape of a split ring and the ring has first and second opposed faces at the split which move with respect to one another under load on the ring from the adjacent vertebral bodies.

7. A spacer as in claim 1 , wherein the spacer is configured to fit entirely within the space between the adjacent vertebral bodies.

8. A spacer as in claim 1 , wherein the spacer has a spacer body configured to fit within the space between the adjacent vertebral bodies and at least one plate configured to extend outside the space and providing an attachment feature for attaching the spacer to the adjacent vertebral bodies.

9. A spacer as in claim 1 , wherein the torsion spring is located at the posterior portion of the spacer.

10. A spacer as in claim 1 , wherein the monolithic spacer consists of a monolithic body configured to elastically resist flexion as a patient's spine goes through flexion and extension.

11. A spacer as in claim 10 , wherein the monolithic body consists of a polymer.

12. A spacer as in claim 11 , wherein the polymer is selected from the group consisting of polyether ether ketones (PEEK), polyaryl ether ketones (PAEK), and their composites, such as carbon fiber reinforced or with radiopaque compounds.

13. A spacer as in claim 10 , wherein the monolithic body consists of a metal.

14. A spacer as in claim 13 , wherein the metal is selected from the group consisting of titanium, and its alloys such as nitinol, cobalt chrome molybdenum and variants.

15. A spacer as in claim 1 , wherein the split is a vertical split which provides a vertical offset in the range from 0.05 mm to 3.0 mm.

16. A spacer as in claim 15 , wherein the vertical offset resists the compression with a spring force in the range from 20 N/mm to 40000 N/mm.

17. A spacer as in claim 1 , wherein the superior surface has a convex geometry.

18. A spacer as in claim 1 , wherein the spacer open center portion extends from the superior surface to the inferior surface to receive the bone graft material and allowing bone to grow through the spacer.

19. A system of dynamic intervertebral spacers comprising:

a first spacer comprising a monolithic ring having an anterior portion, a posterior portion, a first lateral portion, a second lateral portion opposite the first lateral portion, and an open center portion, wherein the ring is split in the anterior portion and surfaces adjoining the split are vertically offset from one another, wherein the posterior portion of the ring is configured to act as a torsion spring to allow the vertical offset to decrease under load on the ring, and wherein the first spacer has a first lordosis angle between superior and inferior surfaces of the spacer;

a second spacer comprising a monolithic ring having an anterior portion, a posterior portion, a first lateral portion, a second lateral portion opposite the first lateral portion, and an open center portion, wherein the ring is split in the anterior portion and surfaces adjoining the split are vertically offset from one another, wherein the posterior portion of the ring is configured to act as a torsion spring to allow the vertical offset to decrease under load on the ring, and wherein the second spacer has a second lordosis angle between superior and inferior surfaces of the spacer different from the first lordosis angle; and

a third spacer comprising a monolithic ring having an anterior portion, a posterior portion, a first lateral portion, a second lateral portion opposite the first lateral portion, and an open center portion, wherein the ring is split in the anterior portion and surfaces adjoining the split are vertically offset from one another, wherein the posterior portion of the ring is configured to act as a torsion spring to allow the vertical offset to decrease under load on the ring, and wherein the third spacer has attachment feature for attaching the at least one of the superior and inferior surfaces to the bone which is different from any attachment feature of the first and second spacer;

wherein each of said first, second and third spacers is configured to be implanted in a space between adjacent vertebral bodies to engage superior and inferior vertebral bodies.