Bone anchors having two or more portions exhibiting different performance characteristics and method of forming the same

1. A bone anchor, comprising:

a bone engaging element including external threads structured for engagement with bone and including:

a first portion formed of a first material and defining at least a portion of said external threads;

a second portion formed of a second material different from said first material and defining at least a portion of said external threads, said second portion extending at least partially along a length of said first portion and about a periphery of said first portion, said second portion of said bone engaging element positioned between said first portion and adjacent bone tissue when said bone engaging element is engaged with the bone to provide an interface with the adjacent bone tissue; and

wherein said second material comprises a moldable material that is molded about said periphery of said first portion of said bone engaging element to form a unitary, single-piece structure, said at least a portion of said external threads defined by said second portion comprising molded external threads formed of said moldable material that are molded into said second portion of said bone engaging element; and

wherein said first material exhibits a performance characteristic that differs from that of said second material; and

wherein said bone engaging element extends along an axis and includes at least three distinct axial regions, said second portion of said bone engaging element extending along at least one of said axial regions and providing a buffer between said first portion of said bone engaging element and the adjacent bone tissue, said first portion of said bone engaging element extending along at least one of said axial regions and interfacing directly with adjacent bone tissue, and wherein said axial regions defined by said second portion of said bone engaging element formed of said second material are positioned in an alternating manner relative to said axial regions defined by said first portion of said bone engaging element formed of said first material; and

wherein said at least a portion of said external threads defined by said first portion cooperates with said molded external threads defined by said second portion to form a substantially continuous threading extending along said at least three distinct axial regions of said bone engaging element; and

an implant engaging element structured for coupling with an implant to anchor the implant to the bone.

2. The bone anchor of claim 1 , wherein said performance characteristic comprises rigidity; and

wherein said second portion of said bone engaging element is less rigid than said first portion of said bone engaging element.

3. The bone anchor of claim 1 , wherein said performance characteristic comprises flexibility; and

wherein said second portion of said bone engaging element is more flexible than said first portion of said bone engaging element.

4. The bone anchor of claim 1 , wherein one of said first and second materials comprises a metallic material, and wherein the other of said first and second materials comprises a non-metallic material.

5. The bone anchor of claim 4 , wherein said first material comprises a metallic material; and

wherein said second material comprises a non-metallic material.

6. The bone anchor of claim 5 , wherein said second material comprises a polymeric material.

7. The bone anchor of claim 5 , wherein said second material comprises PEEK.

8. The bone anchor of claim 5 , wherein said second material comprises a shape-memory polymer material.

9. The bone anchor of claim 1 , wherein the bone anchor is configured to allow selective rotational or pivotal movement of said bone engaging element relative to said implant engaging element.

10. The bone anchor of claim 1 , wherein said bone engaging element includes a distal region, a proximal region, and an intermediate region positioned between said distal and proximal regions;

said second portion of said bone engaging element extending along said distal and proximal regions to provide said buffer between said first portion of said bone engaging element and the adjacent bone tissue; and

said first portion of said bone engaging element extending along said intermediate region and interfacing directly with the adjacent bone tissue.

11. The bone anchor of claim 1 , wherein said second material extends along an entire distal region of the bone engaging element including a distal end of said bone engaging element.

12. The bone anchor of claim 11 , wherein said second material surrounds a distal-most end of said bone engaging element.

13. The bone anchor of claim 11 , wherein said second material defines a tapered distal tip of said bone engaging element.

14. The bone anchor of claim 11 , wherein said second material defines a rounded distal tip of said bone engaging element.

15. The bone anchor of claim 1 , further comprising a head portion extending from said bone engaging element, said head portion including a pair of spaced apart arms defining a U-shaped passage having an open end.

16. The bone anchor of claim 15 , further comprising an elongate spinal rod positionable within said U-shaped passage of said head portion.

17. The bone anchor of claim 15 , wherein said pair of spaced apart arms each define internal threads configured for engagement with a set screw.

18. The bone anchor of claim 1 , wherein said substantially continuous threading extends along substantially an entire length of said bone engaging element.

19. A bone anchor, comprising:

a shank element having a length and defining external threads configured for engagement within an opening in bone, said shank element including:

a core portion formed of a first material; and

a bone interface portion formed of a second material less rigid than said first material, wherein said second material comprises a moldable material and wherein said bone interface portion is molded about said core portion to form a unitary, single-piece structure, said bone interface portion extending along a length of said core portion and about a periphery of said core portion, said bone interface portion defining at least a portion of said external threads of said shank element, wherein said at least a portion of said external threads comprise molded external threads that are molded into said bone interface portion;

wherein said core portion defines at least a portion of said external threads of said shank; and

wherein said at least a portion of said external threads defined by said core portion cooperates with said at least a portion of said external threads defined by said bone interface portion to form a substantially continuous threading extending along said shank element; and

an implant engaging element structured for coupling with an implant to anchor the implant to the bone.

20. The bone anchor of claim 19 , wherein said shank element extends along an axis and includes at least three distinct axial regions, said bone interface portion of said shank element extending along at least one of said axial regions and providing a buffer between said core portion of said shank element and adjacent bone tissue, said core portion of said shank element extending along at least one of said axial regions and interfacing directly with adjacent bone tissue, and wherein said axial regions defined by said bone interface portion of said shank element are positioned in an alternating manner relative to said axial regions defined by said core portion of said shank element.

21. The bone anchor of claim 19 , wherein said moldable material comprises a polymeric material.

22. The bone anchor of claim 19 , wherein said core portion is formed of a metallic material; and

wherein said bone interface portion is formed of a non-metallic material.

23. The bone anchor of claim 22 , wherein said non-metallic material comprises a polymeric material.

24. The bone anchor of claim 19 , wherein said substantially continuous threading extends along substantially an entire length of said shank element.

25. A method of forming a bone anchor, comprising:

providing a bone engaging element for anchoring to bone and an implant engaging element for coupling with an implant;

forming a first portion of the bone engaging element from a first material;

forming a second portion of the bone engaging element from a second material different from the first material, the forming of the second portion of the bone engaging element comprising molding the second material about a periphery of the first portion and along a length of the first portion to form a unitary, single-piece structure and to provide an interface between the first portion and adjacent bone when the bone engaging element is engaged with the bone; and

forming external threads along a length of the first portion of the bone engaging element by a machining process and forming external threads along a length of the second portion of the bone engaging element by a molding process, the external threads of the first portion cooperating with the external threads of the second portion to form a substantially continuous thread extending along the bone engaging element; and

wherein the forming of the external threads along the second portion of the bone engaging element is accomplished during the molding.

26. The method of claim 25 , wherein the forming of the second portion of the bone engaging element comprises molding the second portion entirely about the periphery of the first portion.

27. The method of claim 25 , wherein the second portion of the bone engaging element extends along substantially an entire length of the first portion of the bone engaging element.

28. The method of claim 25 , wherein the bone engaging element includes a distal region and a proximal region, the second portion of the bone engaging element extending along the proximal region to provide the interface between the first portion and the adjacent bone, the first portion of the bone engaging element interfacing directly with adjacent bone along the distal region of the bone engaging element.

29. The method of claim 28 , further comprising providing the first portion of the bone engaging element with cutting features along the distal region of the bone engaging element to provide the bone anchor with at least one of a self-cutting capability and a self-tapping capability.

30. The method of claim 25 , wherein the bone engaging element includes a distal region and a proximal region, the second portion of the bone engaging element extending along the distal region to provide the interface between the first portion and the adjacent bone, the first portion of the bone engaging element interfacing directly with adjacent bone along the proximal region of the bone engaging element.

31. The method of claim 25 , wherein the first portion of the bone engaging element exhibits a first level of rigidity, the second portion of the bone engaging element exhibiting a second level of rigidity less than the first level of rigidity.

32. The method of claim 25 , wherein the molding comprises injection molding.

33. The method of claim 32 , wherein the injection molding comprises plastic injection molding.

34. The method of claim 32 , wherein the injection molding comprises metal injection molding.

35. The method of claim 25 , wherein the molding comprises overmolding.

36. The method of claim 25 , wherein one of said first and second materials comprises a metallic material, and wherein the other of the first and second materials comprises a non-metallic material.

37. The method of claim 36 , wherein the first material comprises a metallic material, and wherein the second material comprises a polymeric material.

38. The method of claim 25 , wherein the substantially continuous thread extends along substantially an entire length of the bone engaging element.