Artificial spinal disc

1. A nucleus for an artificial disc for replacing the natural disc of a human spine having superior and inferior surfaces and anterior/posterior and medial/lateral sides and being adapted to fit between and move relative to upper and lower end plates that are attached to adjacent vertebrae, comprising:

a. a nucleus body having a superior side and an inferior side opposite the superior side, the superior side having a superior bearing surface for engaging a cooperating bearing surface formed on the upper end plate, and the inferior side having an inferior bearing surface for engaging a cooperating bearing surface formed on the lower end plate;

b. said superior bearing surface having an anterior section and a posterior section, and adapted to engage a cooperating bearing surface on said upper end plate and allow for anterior/posterior translation between the nucleus and said upper end plate;

c. said superior bearing surface having a straight section extending between the anterior section and the posterior section, the nucleus body at the anterior end of the straight section having a greater height measured between the superior side and the inferior side normal to the straight section than the height of the nucleus body at the posterior end of the straight section measured between the superior side and the inferior side normal to the straight section, to correct lordosis; and

d. an anterior nucleus portion projecting anteriorly from the anterior portion of the superior bearing surface, the projecting anterior nucleus portion having a planar superior side and a planar inferior side.

2. The nucleus of claim 1 , and further including upper and lower end plates for attachment to adjacent vertebrae.

3. The nucleus of claim 2 , wherein one of the endplates comprises a pair of parallel keels formed on an outer surface of the endplate.

4. The nucleus of claim 2 , wherein the lower end plate comprises an oversize shape relative to the nucleus, to allow limited anterior/posterior and medial/lateral translation.

5. The nucleus of claim 2 , wherein an interior surface of the upper end plate mates with the straight section, and sits at an angle that provides a deformity correction.

6. The nucleus of claim 5 , wherein there is surface contact between the upper end plate and the nucleus.

7. The nucleus of claim 5 , wherein during flexion/extension, there is line contact between the end plate and nucleus.

8. The nucleus of claim 5 , wherein the configuration of nucleus and endplate always has line or surface contact.

9. The nucleus of claim 1 , wherein the superior bearing surface is convexly curved to engage a cooperating concave bearing surface on said end plate.

10. The nucleus of claim 1 , wherein the nucleus has a second nucleus portion projecting from the superior bearing surface, wherein the second nucleus portion has a planar superior side.

11. The nucleus of claim 10 , wherein the second nucleus portion comprises a posterior nucleus portion projecting posteriorly from the superior bearing surface, the nucleus further comprising third and fourth lateral nucleus portions projecting from contralateral sides of superior bearing surface, the third and fourth lateral nucleus portions having planar superior sides.

12. A nucleus for an artificial disc for replacing the natural disc of a human spine having superior and inferior surfaces and anterior/posterior and medial/lateral sides and being adapted to fit between and move relative to upper and lower end plates that are attached to adjacent vertebrae, comprising:

a. a nucleus body having a first bearing surface for engaging a cooperating bearing surface formed on one of the end plates;

b. said first bearing surface having a curved posterior section and a curved anterior section, the curved sections each convexly curved anterior to posterior, and a straight section that extends between the curved posterior and anterior sections, the straight section oriented to provide a correction angle;

c. a posterior nucleus portion projecting outwardly from the posterior section of the first bearing surface for engaging a cooperating portion formed in the lower end plate to resist posterior migration of the nucleus from between the end plates, the posterior nucleus portion having a first side and a second side opposite the first side;

d. wherein the first side of the posterior nucleus portion is planar and the second side of the posterior nucleus portion is planar.

13. The nucleus of the claim 12 and further including said posterior nucleus portion comprising a tab extending outwardly from the inferior side of the nucleus.

14. The nucleus of claim 12 , wherein the first bearing surface is adapted to engage a cooperating bearing surface on said end plate.

15. The nucleus of claim 12 , and further including upper and lower end plates for attachment to adjacent vertebrae.

16. The nucleus of claim 15 , wherein an interior surface of the upper end plate mates with the straight section, and sits at an angle that provides a deformity correction.

17. The nucleus of claim 15 , wherein there is surface contact between the upper end plate and the nucleus.

18. The nucleus of claim 15 , wherein during flexion/extension, there is line contact between the end plate and nucleus.

19. The nucleus of claim 15 , wherein the configuration of nucleus and endplate always has line or surface contact.

20. The nucleus of claim 15 , wherein one of the endplates comprises a pair of parallel keels formed on an outer surface of the endplate.

21. The nucleus of claim 15 , wherein the lower end plate comprises an oversize shape relative to the nucleus, to allow limited anterior/posterior and medial/lateral translation.