Artificial spinal disc

1. An intervertebral disc prosthesis comprising:

a first end plate securable to a first vertebral body of a portion of a spine, the first end plate comprising a first articular surface;

a second end plate securable to a second vertebral body of the portion of the spine, the second end plate comprising a fourth articular surface; and

a nucleus positioned between the first and second end plates, the nucleus formed separately from the first and second end plates, the nucleus comprising a second articular surface that articulates with the first articular surface and a third articular surface that articulates with the fourth articular surface;

wherein the nucleus comprises an asymmetrical shape across a coronal plane of the first and second vertebral bodies, the asymmetrical shape tending to urge the first and second end plates to remain in a relative orientation that provides a preferred lordotic or kyphotic angle between the first and second vertebral bodies;

wherein line contact is provided between the first and second articular surfaces when the first articular surface articulates relative to the second articular surface in the anterior/posterior direction; and

wherein the third and fourth articular surfaces are substantially entirely planar to permit translational movement in at least one of the anterior-posterior and medial-lateral directions.

2. The intervertebral disc prosthesis of claim 1 , wherein the nucleus comprises a maximum vertical axis, wherein the maximum vertical axis is located anterior to the coronal plane of the first and second vertebral bodies to provide a lordotic correction to the relative orientation between the first and second end plates.

3. The intervertebral disc prosthesis of claim 1 , wherein the nucleus comprises a maximum vertical axis, wherein the maximum vertical axis is located posterior to the coronal plane of the first and second vertebral bodies to provide a kyphotic correction to the relative orientation between the first and second end plates.

4. The intervertebral disc prosthesis of claim 1 , wherein at least one of the first and second bone engagement surfaces comprises at least one bone engagement feature shaped to penetrate bone.

5. The intervertebral disc prosthesis of claim 4 , wherein the bone engagement features are distributed parasagittaly on the bone engagement surface.

6. The intervertebral disc prosthesis of claim 1 , wherein the second articular surface comprises a first straight section positioned between and contiguous with first and second curved sections of the second articular surface, the first straight section extending across the coronal plane of the first and second vertebral bodies.

7. The intervertebral disc prosthesis of claim 6 , wherein the first articular surface comprises a first straight portion in at least one cross section, wherein the first straight section rests against the first straight portion in a relative orientation between the first and second end plates that provides the preferred lordotic or kyphotic angle between the first and second vertebral bodies.

8. The intervertebral disc prosthesis of claim 6 , wherein the third articular surface rests against the fourth articular surface in a relative orientation between the first and second end plates that provides the preferred lordotic or kyphotic angle between the first and second vertebral bodies.

9. The intervertebral disc prosthesis of claim 6 , wherein the first straight section comprises a partial cylinder positioned between and contiguous with first and second curved sections of the second articular surface, wherein the first articular surface comprises a cylindrical surface shaped to mate with the partial cylinder.

10. The intervertebral disc prosthesis of claim 6 , wherein the line contact is between the first articular surface and at least one of the first and second curved sections of the second articular surface.

11. The intervertebral disc prosthesis of claim 6 , wherein the first straight section is oriented at an angle across the coronal plane of the first and second vertebral bodies.

12. The intervertebral disc prosthesis of claim 1 , wherein the nucleus and the second endplate are shaped to permit axial rotation between the third and fourth articular surfaces.

13. The intervertebral disc prosthesis of claim 12 , wherein the second endplate comprises a stop to limit the axial rotation between the third and fourth articular surfaces.