SPINAL FACET IMPLANTS WITH MATING ARTICULATING BEARING SURFACE AND METHODS OF USE
Superior and/or inferior facets of one or more facet joints may be replaced by superior and/or inferior facet joint prostheses. In one embodiment, a kit of superior or inferior prostheses is provided, in which the prostheses have at least two dimensions that vary among members of the kit independently of each other. Each prosthesis may have a bone engaging surface having a surface that is polyaxially rotatable against a corresponding resection of a vertebra. Each prosthesis may also have an articulating surface shaped such that, after attachment to the spine, the replaced or partially replaced facet joints provide a larger medial-lateral range of motion when the spine is flexed than when the spine is extended. Crosslinks may be used to connect left and right prosthesis together in such a manner that they are stabilized in a position in which they are seated directly against the vertebra.
1 . A facet prosthesis system for replacing an inferior facet of a first vertebra of a spine, and a superior facet of a second vertebra of the spine, the system comprising:
an inferior prosthesis shaped to be coupled to the first vertebra, the inferior prosthesis having an inferior articulating surface; and
a superior prosthesis shaped to be coupled to the second vertebra, the superior prosthesis having a superior articulating surface;
wherein the articulating surfaces of the inferior and superior prostheses are shaped and relatively positioned to articulate against each other such that a medial-lateral range of relative motion between the first and second vertebrae decreases significantly with extension of the spine.
2 . The system of claim 1 , wherein each of the articulating surfaces has a curved shape, wherein one of the articulating surfaces has a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein the radius of curvature changes along the axis to provide greater clearance between the articulating surfaces when the spine is flexed.
3 . The system of claim 2 , wherein the articulating surface of the superior prosthesis has a concave shape having the cephalad end, caudal end, and radius of curvature, wherein the radius of curvature increases toward the cephalad end.
4 . The system of claim 1 , wherein each of the articulating surfaces has a curved shape, wherein one of the articulating surfaces has a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein the prosthesis is shaped such that, when the prosthesis is coupled to the vertebra, the axis is significantly anteriorly inclined to provide greater clearance between the articulating surfaces when the spine is flexed.
5 . The system of claim 4 , wherein the articulating surface of the superior prosthesis has a concave shape having the cephalad, caudal end, and radius of curvature, wherein the axis is anteriorly inclined toward the cephalad end.
6 . The system of claim 5 , wherein the axis is inclined at an angle of at least five degrees with respect to a longitudinal axis of the spine.
7 . The system of claim 1 , wherein each of the articulating surfaces has a curved shape, wherein one of the articulating surfaces has a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein the radius of curvature varies along a medial-lateral direction of the articulating surface.
8 . The system of claim 7 , wherein the articulating surface of the superior prosthesis has a concave shape having the cephalad end, caudal end, and radius of curvature, wherein the articulating surface of the superior prosthesis has a medial end, a lateral end, and a central portion between the medial and lateral ends, wherein the radius of curvature is larger toward the medial and lateral ends than at the central portion.
9 . The system of claim 8 , wherein the radius of curvature is substantially infinite toward the medial and lateral ends, such that the articulating surface of the superior prosthesis has a curved region proximate the central portion, a first tangent flat disposed medially of and tangent to the curved region, and a second tangent flat disposed laterally of and tangent to the curved region.
10 . The system of claim 7 , wherein the articulating surface of the superior prosthesis has a concave shape and the articulating surface of the inferior prosthesis has a convex shape.
11 . The system of claim 10 , wherein the articulating surface of the inferior prosthesis has a three-dimensionally curved elliptical shape.
12 . The system of claim 1 , wherein at least one of the articulating surfaces is constructed of a material selected from a group consisting of a polymeric material, a polymeric bearing material attached to a metal substrate, a ceramic bearing material, a metal bearing material, and combinations thereof.
13 . A method for replacing at least one facet of a first vertebra of a spine, the method comprising:
resecting the first vertebra to provide a resected portion of the first vertebra; and
attaching a coupling portion of a prosthesis to the first resected portion to position an articulating surface of the prosthesis to articulate against an articulating surface of a second vertebra;
wherein positioning the articulating surface of the prosthesis comprises providing a medial-lateral range of relative motion between the first and second vertebrae that decreases significantly with extension of the spine.
14 . The method of claim 13 , wherein the articulating surface of the prosthesis has a curved shape with a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein the radius of curvature changes along the axis, wherein providing the medial-lateral range of relative motion between the first and second vertebrae comprises providing greater clearance between the articulating surfaces when the spine is flexed.
15 . The method of claim 14 , wherein the prosthesis comprises a superior prosthesis, wherein the articulating surface of the prosthesis has a concave shape, wherein the radius of curvature increases toward the cephalad end, wherein resecting the first vertebra comprises removing at least a portion of a natural superior facet.
16 . The method of claim 13 , wherein the articulating surface of the prosthesis has a curved shape with a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein positioning the articulating surface of the prosthesis comprises significantly anteriorly inclining the axis to provide greater clearance between the articulating surfaces when the spine is flexed.
17 . The method of claim 16 , wherein the prosthesis comprises a superior prosthesis, wherein the articulating surface of the prosthesis has a concave shape, wherein resecting the first vertebra comprises removing at least a portion of a natural superior facet, wherein significantly anteriorly inclining the axis comprises inclining the axis toward the cephalad end.
18 . The method of claim 17 , wherein significantly anteriorly inclining the axis comprises inclining the axis at an angle of at least five degrees with respect to a longitudinal axis of the spine.
19 . The prosthesis of claim 13 , wherein the articulating surface of the prosthesis has a curved shape with a cephalad end and a caudal end, and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein the radius of curvature varies along a medial-lateral direction of the articulating surface, wherein providing the medial-lateral range of relative motion between the first and second vertebrae comprises providing greater clearance between the articulating surfaces when the spine is flexed.
20 . The prosthesis of claim 19 , wherein the prosthesis comprises a superior prosthesis, wherein the articulating surface of the prosthesis has a concave shape, wherein the articulating surface of the prosthesis has a medial end, a lateral end, and a central portion between the medial and lateral ends, wherein the radius of curvature is larger toward the medial and lateral ends than at the central portion, wherein resecting the first vertebra comprises removing at least a portion of a natural superior facet.
21 . The prosthesis of claim 20 , wherein the radius of curvature is substantially infinite toward the medial and lateral ends, such that the articulating surface of the prosthesis has a curved region proximate the central portion, a first tangent flat disposed medially of and tangent to the curved region, and a second tangent flat disposed laterally of and tangent to the curved region.
22 . A method for replacing at least one facet of a first vertebra of a spine, the method comprising:
resecting the first vertebra to provide a resected portion of the first vertebra; and
attaching a coupling portion of a prosthesis to the resected portion to position an articulating surface of the prosthesis to articulate against a second articulating surface of a second vertebra;
wherein the articulating surface of the vertebra comprises a cephalad end and a caudal end and a radius of curvature about an axis extending generally from the cephalad end to the caudal end, wherein positioning the articulating surface of the prosthesis comprises significantly inclining the axis anteriorly toward the cephalad end.
23 . The method of claim 22 , wherein the prosthesis comprises a superior prosthesis, wherein the articulating surface of the prosthesis has a concave shape, wherein resecting the first vertebra comprises removing at least a portion of a natural superior facet.
24 . The method of claim 23 , wherein significantly anteriorly inclining the axis comprises inclining the axis at an angle of at least five degrees with respect to a longitudinal axis of the spine.
25 . The method of claim 22 , wherein the articulating surface of the prosthesis has a medial end, a lateral end, and a central portion between the medial and lateral ends, wherein the radius of curvature is larger toward the medial and lateral ends than at the central portion.