Rotational offset oval vertebral rod
A flexible connection unit for use in a spinal fixation device is provided. The flexible connection unit includes an elongated member, such as a vertebral rod, having a first end portion and a second end portion opposite each other with a transition portion extending therebetween. The first end portion of the vertebral rod comprises an oval shape and the second end portion contains an oval or non-oval shape. The major diameter of the oval shaped first end is parallel to the sagittal plane of a patient's body and provides the higher stiffness for fusion to resist flexion in the sagittal plane. The second end portion of the vertebral rod is perpendicular to the sagittal end of a patient's body and provides the lower stiffness for fusion.
1. A flexible connection unit for use in a spinal fixation device comprising an elongated member having a first end portion, a second end portion opposite the first end portion and a transition portion extending therebetween, wherein the first end portion comprises an oval shape having a major diameter and the second end portion comprises an oval or non-oval shape, the major diameter of the oval shape of the first end portion configured to be positioned substantially parallel to a sagittal plane of a patient's body, wherein the major diameter of the first end portion is rotationally offset from a major diameter of the second end portion between 1 and 180 degrees when the elongated member is in an unstressed state.
2. A flexible connection unit of claim 1 , wherein the elongated member comprises a vertebral rod.
3. A flexible connection unit of claim 2 , wherein the second end portion comprises an oval shape.
4. A flexible connection unit of claim 2 , wherein (i) the first end portion is offset from the second end portion by about 90 degrees of rotation in an axial plane of the rod; (ii) the rod has a stiffness dependent upon a rotational offset between the first end portion and the second end portion; (iii) a relative stiffness difference of the rod between the first and second end portions can be increased by increasing a cross section of a stiffer section of the rod or decreasing the cross section of a less stiff section of the rod; or (iv) a relative stiffness difference of the rod between the first and second end portions can be increased by increasing a ratio of the major diameter to minor diameter of the first end portion and/or second end portion of the rod.
5. A flexible connection unit of claim 2 , wherein the major diameter of the oval shaped first end portion is rotationally offset from the major diameter of the second end portion about 90 degrees.
6. A flexible connection unit of claim 2 , wherein the rod comprises multiple offset angles from the first end portion to the second end portion.
7. A flexible connection unit of claim 2 , wherein the elongated member comprises a cross-section that is asymmetrically varied from the oval shaped first end portion to the oval or non-oval shaped second end portion.
8. A flexible connection unit of claim 2 , wherein the rod material comprises cobalt chromium alloys, titanium, titanium alloy, nitinol, stainless steel, PEEK, carbon reinforced PEEK or a mixture thereof.
9. A flexible connection unit of claim 1 , wherein the second end portion has a lower stiffness than the first end portion by at least 1% in the sagittal plane.
10. A flexible connection unit of claim 9 , wherein the second end portion is located caudally relative to the first end portion.
11. A flexible connection unit of claim 1 , wherein the second end portion has a lower stiffness than the first end portion by at least 5% in the sagittal plane.
12. A flexible connection unit of claim 1 , wherein the transition portion of the elongated member has a higher stiffness than the second end portion and a lower stiffness than the first end portion for spinal fusion.
13. A flexible connection unit of claim 1 , wherein the major diameter of the second end portion is less than the major diameter of the first end portion and a major diameter of the transition portion.
14. A flexible connection unit of claim 1 , wherein the elongated member comprises a straight rod.
15. A flexible connection unit of claim 1 , wherein the elongated member comprises a curved rod.
16. A flexible connection unit of claim 1 , wherein the elongated member has a flexibility of from about 0.1 N-m.sup.2 to about 75 N-m.sup.2 or from about 0.5 N-m.sup.2 to about 50 N-m.sup.2.
17. A flexible connection unit of claim 1 , wherein the elongated member comprises an outer surface treated by texturing, shot peening, burnishing or grit blasting.
18. A flexible connection unit of claim 17 , wherein the outer surface comprises an osteoconductive coating, an osteoinductive coating or a mixture thereof.
19. A method for treating a plurality of vertebrae regions in a patient in need of such treatment, the method comprising:
attaching to the vertebrae regions the flexible connection unit recited in claim 1 such that the major diameter of the first end portion is substantially parallel to the sagittal plane of the patient's body.
20. A method according to claim 19 , wherein the elongated member comprises a vertebral rod.