Spinal rods having different flexural rigidities about different axes and methods of use
A vertebral rod has an elongated body extending along a longitudinal axis. The rod also includes a cavity extending the length of the body. Either the body or the cavity may have an asymmetrical shape about a centroid in a plane perpendicular to the longitudinal axis. Alternatively, both may have the symmetrical shape about the centroid. The body of the rod may be bounded by an exterior surface and the cavity. The body has a first bending axis that is perpendicular to longitudinal axis. The body also has a second bending axis that is perpendicular to the longitudinal axis and to the first bending axis. The body of the rod may be distributed asymmetrically about the first and second bending axes. Also, the rod may have a different bending stiffness about the first and second bending axes.
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Spinal or vertebral rods are often used in the surgical treatment of spinal disorders such as degenerative disc disease, disc herniations, scoliosis or other curvature abnormalities, and fractures. Different types of surgical treatments are used. In some cases, spinal fusion is indicated to inhibit relative motion between vertebral bodies. In other cases, dynamic implants are used to preserve motion between vertebral bodies. For either type of surgical treatment, spinal rods may be attached to the exterior of two or more vertebrae, whether it is at a posterior, anterior, or lateral side of the vertebrae. In other embodiments, spinal rods are attached to the vertebrae without the use of dynamic implants or spinal fusion.
Spinal rods may provide a stable, rigid column that encourages bones to fuse after spinal-fusion surgery. Further, the rods may redirect stresses over a wider area away from a damaged or defective region. Also, a rigid rod may restore the spine to its proper alignment. In some cases, a flexible rod may be appropriate. Flexible rods may provide some advantages over rigid rods, such as increasing loading on interbody constructs, decreasing stress transfer to adjacent vertebral elements while bone-graft healing takes place, and generally balancing strength with flexibility.
Aside from each of these characteristic features, a surgeon may wish to control anatomic motion after surgery. That is, a surgeon may wish to inhibit or limit one type of spinal motion following surgery while allowing a lesser or greater degree of motion in a second direction. As an illustrative example, a surgeon may wish to inhibit or limit motion in the flexion and extension directions while allowing for a greater degree of lateral bending. However, conventional rods tend to be symmetric in nature and may not provide this degree of control.
SUMMARYIllustrative embodiments disclosed herein are directed to a vertebral rod having an elongated body extending along a longitudinal axis. The rod also includes a cavity extending the length of the body. Either the body or the cavity may have an asymmetrical shape about a centroid in a plane perpendicular to the longitudinal axis. Alternatively, both may have the symmetrical shape about the centroid. The body of the rod may be bounded by an exterior surface and the cavity. The body has a first bending axis that is perpendicular to longitudinal axis. The body also has a second bending axis that is perpendicular to the longitudinal axis and to the first bending axis. The body of the rod may be distributed asymmetrically about the first and second bending axes. Also, the rod may have a different bending stiffness about the first and second bending axes. The cavity may be contained within or intersect the exterior surface.
BRIEF DESCRIPTION OF THE DRAWINGS
The various embodiments disclosed herein are directed to spinal rods that are characterized by a cross section that provides different flexural rigidities in different directions. Various embodiments of a spinal rod may be implemented in a spinal rod assembly of the type indicated generally by the numeral 20 in
In one embodiment as illustrated in
The rod 10 may be constructed from a variety of surgical grade materials. These include metals such as stainless steels, cobalt-chrome, titanium, and shape memory alloys. Non-metallic rods, including polymer rods made from materials such as PEEK and UHMWPE, are also contemplated. Further, the rod 10 may be straight, curved, or comprise one or more curved portions along its length.
The structural characteristics of the rod 10 may be dependent upon several factors, including the material choice and the cross section shape of the rod 10. The flexural rigidity, which is a measure of bending stiffness, is given by the equation:
Flexural Rigidity=E×I (1)
where E is the modulus of elasticity or Young's Modulus for the rod material and I is the moment of inertia of a rod cross section about the bending axis. The modulus of elasticity varies by material and reflects the relationship between stress and strain for that material. As an illustrative example, titanium alloys generally possess a modulus of elasticity in the range between about 100-120 GPa. By way of comparison, implantable grade polyetheretherketone (PEEK) possesses a modulus of elasticity in the range between about 3-4 Gpa, which, incidentally, is close to that of cortical bone.
In general, an object's moment of inertia depends on its shape and the distribution of mass within that shape. The greater the concentration of material away from the object's centroid C, the larger the moment of inertia. In
Ix=∫y2dA (2)
Iy=∫x2dA (3)
where y is the distance between a given portion of the elliptical area and the x-axis and x is the distance between a given portion of the elliptical area and the y-axis. The intersection of the x-axis and y-axis is called the centroid C of rotation. The centroid C may be the center of mass for the shape assuming the material is uniform over the cross section. Since dimension h in
The actual bending stiffness of the rod 10a shown in
Ix=Ixo−Ixi (4)
Iy=Iyo−Iyi (5)
where Ixo and Ixi are the moments of inertia about the x-axis for the outer and inner areas, respectively. Similarly, Iyo and Iyi are the moments of inertia about the y-axis for the outer and inner areas, respectively.
In the present embodiment of the rod 10a shown in
It may be desirable to adjust the bending stiffness of the rod 10 by varying the size and shape of the inner aperture 30. For instance, a surgeon may elect to use the rods 10 disclosed herein with existing mounting hardware such as pedicle screws or hook saddles (not shown). Some exemplary rod sizes that are commercially available range between about 4-7 mm. Thus, the overall size of the rods 10 may be limited by this constraint.
The internal aperture 30 may be asymmetric as well. For example, the rod 10c shown in
The rods 10 may also have multiple inner apertures 30. For instance, the rod 10d shown in
The embodiments described above have all had a substantially similar, oval shaped outer surface 22. Certainly, other shapes are possible as illustrated by the embodiment of the rod 10e shown in
The rod 10f shown in
The rod 10 may also have substantially triangular outer surfaces 22 as evidenced by the embodiments 10h, 10i, and 10j. In
Other rods 10 may have polygonal shapes such as the embodiments illustrated in
The embodiments described thus far have included an aperture 30 that is substantially contained within the interior of the outer surface 22. In other embodiments, the aperture 30 may intersect with the outer surface 22. This can be seen in the exemplary embodiments shown in
The rods 10 may also have a substantially circular outer surface 22 similar to many conventional rods, thus accommodating existing rod securing hardware (not shown). This is illustrated by the exemplary rods 10q, 10r, and 10s shown in
In
Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description.
As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For example, embodiments described above have contemplated one or two inner apertures 30 to modify the moments of inertia about one axis relative to another. The rods 10 do not need to be limited to this number of apertures. The moment of inertia equations provided herein allow one to calculate moments of inertia for any number of apertures and flexural rigidity of the overall rod 10. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
Claims
1. A vertebral rod comprising:
- a body extending along a first axis and having a length along the first axis between a first end and a second end;
- a cavity extending the length of the body;
- at least one of the body and the cavity having an asymmetrical shape about a centroid in a plane perpendicular to the first axis.
2. The vertebral rod of claim 1 wherein the cavity is centered about the first axis.
3. The vertebral rod of claim 1 wherein the body is centered about the first axis.
4. The vertebral rod of claim 1 wherein the body and the cavity are centered about the first axis.
5. The vertebral rod of claim 1 wherein the cavity is defined by an inner surface having a first shape and the body is defined by an outer surface having a second shape, first and second shapes being different.
6. The vertebral rod of claim 1 wherein the cavity is defined by an inner surface having a first shape and the body is defined by an outer surface having a second shape, first and second shapes being the same.
7. The vertebral rod of claim 1 further comprising markings indicating an orientation of the asymmetrical shape.
8. A vertebral rod comprising:
- a body having a cavity, each extending along a first axis and each having a length along the first axis between a first end and a second end, the body bounded by an exterior surface and the cavity;
- the body having a first bending axis that is perpendicular to the first axis and a second bending axis that is perpendicular to the first axis and to the first bending axis, the body being distributed asymmetrically about the first and second bending axes.
9. The vertebral rod of claim 8 wherein the exterior surface is asymmetric about the first and second bending axes.
10. The vertebral rod of claim 8 wherein the cavity is asymmetric about the first and second bending axes.
11. The vertebral rod of claim 8 wherein the exterior surface and the cavity are each asymmetric about the first and second bending axes.
12. The vertebral rod of claim 8 wherein the cavity is interior to the exterior surface.
13. The vertebral rod of claim 8 wherein the cavity intersects with the exterior surface.
14. The vertebral rod of claim 8 further comprising markings indicating an orientation of the asymmetric distribution of the body.
15. A vertebral rod comprising:
- a body extending along a first axis and having a length along the first axis between a first end and a second end, the body having a first cross sectional shape substantially perpendicular to the first axis;
- a cavity extending the length of the body, the cavity having a second cross sectional shape substantially perpendicular to the first axis; and
- the first cross sectional shape and the second cross sectional shape being different.
16. The vertebral rod of claim 15 wherein the second cross sectional shape is interior to the first cross sectional shape.
17. The vertebral rod of claim 15 wherein the second cross sectional shape intersects the first cross sectional shape.
18. A vertebral rod comprising:
- a body having a cavity, each extending along a first axis and each having a length along the first axis between a first end and a second end, the body bounded by an exterior surface and the cavity;
- the body having a first bending axis that is perpendicular to the first axis and a second bending axis that is perpendicular to the first axis and to the first bending axis, the body having different area moments of inertia about the first and second bending axes.
19. The vertebral rod of claim 18 wherein the exterior surface defines an area having different area moments of inertia about the first and second bending axes.
20. The vertebral rod of claim 18 wherein the cavity has different area moments of inertia about the first and second bending axes.
21. The vertebral rod of claim 18 wherein the exterior surface and the cavity each have different area moments of inertia about the first and second bending axes.
22. The vertebral rod of claim 18 wherein the cavity is interior to the exterior surface.
23. The vertebral rod of claim 18 wherein the cavity intersects with the exterior surface.
24. The vertebral rod of claim 18 further comprising markings indicating an orientation of the asymmetric area moments of inertia.
Type: Application
Filed: Jan 27, 2006
Publication Date: Aug 16, 2007
Applicant:
Inventors: Jeff Justis (Germantown, TN), Fred Molz (Birmingham, AL), Michael Sherman (Memphis, TN)
Application Number: 11/342,195
International Classification: A61F 2/30 (20060101);