Cervical disc replacement

1 . An intervertebral implant comprising:

a first member having a first articulation surface that is defined by a plurality of concave arcs each having a respective radius of curvature about a first axis, and a plurality of convex arcs each having a respective radius of curvature about a second axis substantially perpendicular to the first axis; and

a second member having a second articulation surface that is defined by a plurality of convex arcs each having a radius of curvature about a third axis, and a plurality of concave arcs each having a radius of curvature about a fourth axis substantially perpendicular to the third axis,

each of the plurality of concave arcs of the first articulation surface opposing one of the plurality of convex arcs of the second articulation surface and each of the concave arcs of the second articulation surface opposing on of the convex arcs of the first articulation surface, the radius of curvature of each of the concave arcs of the first articulation surface being greater than the radius of curvature of the opposing convex arc of the second articulation surface,

wherein the first member has a first center of rotation that remains below the articulation surfaces during flexion/extension and a second center of rotation that remains above the articulation surfaces during lateral bending.

2 . The intervertebral implant of claim 1 , wherein the entirety of said first articulation surface is saddle-shaped, and the entirety of said second articulation surface is saddle-shaped.

3 . The intervertebral implant of claim 2 , wherein said first articulation surface includes a single concave surface and a single convex surface, and said second articulation surface includes a single concave surface and a single convex surface.

4 . The intervertebral implant of claim 3 , wherein said single concave surface of said first articulation surface engages said single convex surface of said second articulation surface, and said single convex surface of said first articulation surface engages said single concave surface of said second articulation surface.

5 . The intervertebral implant of claim 1 , wherein said first articulation surface includes continuous concave and convex surfaces extending between outer edges of said first articulation surface, and said second articulation surface includes continuous concave and convex surfaces extending between outer edges of said second articulation surface.

6 . The intervertebral implant of claim 1 , wherein the concave arcs of the second articulation surface are greater than the radius of curvature of the opposing convex arc of the first articulation surface

7 . The intervertebral implant of claim 1 , wherein said first and second members further include means for engaging a vertebral body.

8 . The intervertebral implant of claim 7 , wherein said means for engaging a vertebral body is selected from the group consisting of flanges, keels and spikes.

9 . The intervertebral implant of claim 1 , wherein rotation of said first member with respect to said second member occurs through at least a range of angles without said first and second members moving away from one another.

10 . The intervertebral implant of claim 9 , wherein said range of angles is approximately plus/minus three degrees from a resting position.

11 . An intervertebral implant comprising:

a first member having a first articulation surface; and

a second member having a second articulation surface,

wherein each of the first and second articulation surfaces has a toroidal saddle shaped surface, and the articulation surfaces are adapted to engage one another during at least one of flexion/extension and lateral bending, wherein the engaging articulation surfaces include opposing constant radius of curvature surfaces that are non-congruent.

12 . The intervertebral implant of claim 11 , wherein rotation of said first member with respect to said second member occurs through at least a range of angles without said first and second members moving away from one another.

13 . The intervertebral implant of claim 12 , wherein said range of angles is approximately plus/minus three degrees from a resting position.

14 . The intervertebral implant of claim 11 , wherein said first articulation surface includes continuous concave and convex surfaces extending between outer edges of said first articulation surface, and said second articulation surface includes continuous concave and convex surfaces extending between outer edges of said second articulation surface.

15 . The intervertebral implant of claim 11 , wherein said first articulation surface is defined by a concave arc having a substantially constant radius of curvature A about a first axis, and a convex arc having a substantially constant radius of curvature B about a second axis perpendicular to the first axis, and said second articulation surface is defined by a convex arc having a substantially constant radius of curvature C about a third axis, and a concave arc having a substantially constant radius of curvature D about a fourth axis perpendicular to the third axis.

16 . The intervertebral implant of claim 15 , wherein said radius of curvature A is greater than said radius of curvature C and said radius of curvature B is less than said radius of curvature D.

17 . The intervertebral implant of claim 15 , wherein at least one of: (i) said first and third axes are coaxial; and (ii) said second and fourth axes are coaxial.

18 . The intervertebral implant of claim 15 , wherein at least one of: (i) said first and third axes are coplanar; and (ii) said second and fourth axes are coplanar.

19 . The intervertebral implant of claim 11 , wherein the first implant portion has a center of rotation that remains below said articulation surfaces during flexion/extension and that remains above said articulation surfaces during lateral bending.

20 . The intervertebral implant of claim 11 , wherein said first and second member further include means for engaging a vertebral body.

21 . The intervertebral implant of claim 20 , wherein said means for engaging a vertebral body is selected from the group consisting of flanges, keels and spikes.

22 . An intervertebral implant comprising:

a first member having a first articulating surface, the entirety of the first articulating surface being a single saddle surface that is defined by a concave arc having a substantially constant radius of curvature A about a first axis perpendicular to leading and trailing ends of the first member and convex arc having a substantially constant radius of curvature B about a first axis perpendicular to lateral sides of the first member; and

a second member having a second articulating surface in contact with the first articulating surface, the entirety of the second articulating surface being a single saddle surface that is defined by a convex arc having a substantially constant radius of curvature C about a first axis perpendicular to leading and trailing ends of the second member and concave arc having a substantially constant radius of curvature D about a second axis perpendicular to lateral sides of the second member, wherein the constant radius of curvature A is non-congruent with the constant radius of curvature C and the constant radius of curvature B is non-congruent with the radius of curvature D;

the first and second members are operable to articulate relative to one another about at least one of: (i) a first center of rotation for at least one of flexion and extension that is located above the first and second articulating surfaces, and (ii) a second center of rotation for lateral bending that is located below the first and second articulating surfaces.

23 . The intervertebral implant of claim 22 , wherein rotation of said first member with respect to said second member occurs through at least a range of angles without said first and second members moving away from one another.

24 . The intervertebral implant of claim 23 , wherein said range of angles is approximately plus/minus three degrees from a resting position.

25 . An intervertebral implant comprising:

first and second members have opposing first and second articulating surfaces that are in contact with one another, the entirety of the first articulating surface being a single saddle surface that is defined by a concave arc having a substantially constant radius of curvature A about a first axis perpendicular to an axis passing through leading and trailing ends of the first member and a convex arc having a substantially constant radius of curvature B about a first axis perpendicular to an axis passing through lateral ends of the first member, the entirety of the second articulating surface being a single saddle surface that is defined by a convex arc having a substantially constant radius of curvature C about a first axis perpendicular to an axis passing through leading and trailing ends of the second member and a concave arc having a substantially constant radius of curvature D about a second axis perpendicular to an axis passing through lateral ends of the second member, the constant radius of curvature A being non-congruent with the constant radius of curvature C and the constant radius of curvature B being non-congruent with the constant radius of curvature D,

wherein the first and second members are adapted to articulate relative to one another about at least one of: (i) a first center of rotation for at least one of flexion and extension that is located above the first and second articulating surfaces, and (ii) a second center of rotation for lateral bending that is located below the first and second articulating surfaces.

26 . The intervertebral implant of claim 25 , wherein rotation of said first member with respect to said second member occurs through at least a range of angles without said first and second members moving away from one another.

27 . The intervertebral implant of claim 26 , wherein said range of angles is approximately plus/minus three degrees from a resting position.