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  Spinal rods having different flexural rigidities about different axes and methods of useUSPTO Application #: 20070191841Title: Spinal rods having different flexural rigidities about different axes and methods of use Abstract: 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. (end of abstract) Agent: Coats & Bennett, PLLC - Cary, NC, US Inventors: Jeff R. Justis, Fred J. Molz, Michael C. Sherman USPTO Applicaton #: 20070191841 - Class: 606061000 (USPTO) Related Patent Categories: Surgery, Instruments, Orthopedic Instrumentation, Internal Fixation Means, Spinal Positioner Or Stabilizer The Patent Description & Claims data below is from USPTO Patent Application 20070191841. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] 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. [0002] 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. [0003] 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. SUMMARY [0004] Illustrative 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 [0005] FIG. 1 is a perspective view of first and second assemblies comprising spinal rods attached to vertebral members according to one or more embodiments; [0006] FIG. 2 is a lateral view of a spinal rod according to one or more embodiments; and [0007] FIGS. 3-20 are axial views of a spinal rod illustrating cross sections according to different embodiments. DETAILED DESCRIPTION [0008] 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 FIG. 1. FIG. 1 shows a perspective view of first and second spinal rod assemblies 20 in which spinal rods 10 are attached to vertebral members V1 and V2. In the example assembly 20 shown, the rods 10 are positioned at a posterior side of the spine, on opposite sides of the spinous processes S. Spinal rods 10 may be attached to a spine at other locations, including lateral and anterior locations. Spinal rods 10 may also be attached at various sections of the spine, including the base of the skull and to vertebrae in the cervical, thoracic, lumbar, and sacral regions. In one embodiment, a single rod 10 is attached to the spine. Thus, the illustration in FIG. 1 is provided merely as a representative example of one application of a spinal rod 10. [0009] In one embodiment as illustrated in FIG. 1, the spinal rods 10 are secured to vertebral members V1, V2 by pedicle assemblies 12 comprising a pedicle screw 14 and a retaining cap 16. The outer surface of spinal rod 10 is grasped, clamped, or otherwise secured between the pedicle screw 14 and retaining cap 16. Other mechanisms for securing spinal rods 10 to vertebral members V1, V2 include hooks, cables, and other such devices. Examples of other types of retaining hardware include threaded caps, screws, and pins. Spinal rods 10 are also attached to plates in other configurations. Thus, the exemplary assemblies 12 shown in FIG. 1 are merely representative of one type of attachment mechanism. [0010] 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. [0011] FIG. 2 shows a spinal rod 10 of the type used in the exemplary assembly 20 in FIG. 1. The rod 10 has a length between a first end 17 and a second end 18 extending along a longitudinal axis A. Other Figures described below show various embodiments of a spinal rod 10 characterized by different cross sections viewed according to the view lines illustrated in FIG. 2. For instance, FIG. 3 shows one example cross section of the spinal rod 10a. In this embodiment, the spinal rod 10a is comprised of an oval or elliptical outer surface 22a and an interior cavity or aperture 30a defined by an inner surface 32a. In one embodiment, the outer surface 22a and inner surface 32a are uniformly consistent along the entire length L of the rod 10a. That is, the cross section shown in FIG. 3 may be the same at all points along the length L of the rod 10a. The same may also be true of other cross sections described below. In one or more embodiments, the cross section of a rod 10 may vary along the length L of the rod 10. [0012] 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.times.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. [0013] 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 FIG. 3, the moments of inertia about the x-axis I.sub.x and the y-axis I.sub.y for the area inside the elliptical outer shape 22a (ignoring the inner aperture 30a for now) may be determined according to the following equations:I.sub.x=.intg.y.sup.2dA (2)I.sub.y=.intg.x.sup.2dA (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 FIG. 3 is larger than dimension b, it follows that the moment of inertia about the x-axis I.sub.x is larger than the moment of inertia about the y-axis I.sub.y. This means that the oval shape defined by the outer surface 22a has a greater resistance to bending about the x-axis as compared to the y-axis. [0014] The actual bending stiffness of the rod 10a shown in FIG. 3 may also depend upon the moment of inertia of the inner aperture 30a. Determining the overall flexural rigidity of the rod 10a requires an analysis of the composite shape of the rod 10a. Generally, the moment of inertia of a composite area with respect to a particular axis is the sum (or difference in the case of a void) of the moments of inertia of its parts with respect to that same axis. Thus, for the rod 10a shown in FIG. 3, the overall flexural rigidity is given by the following:I.sub.x=I.sub.xo-I.sub.xi (4)I.sub.y=I.sub.yo-I.sub.yi (5) where I.sub.xo and I.sub.xi are the moments of inertia about the x-axis for the outer and inner areas, respectively. Similarly, I.sub.yo and I.sub.yi are the moments of inertia about the y-axis for the outer and inner areas, respectively. [0015] In the present embodiment of the rod 10a shown in FIG. 3, the inner aperture 30a is symmetric about the centroid C. Consequently, the moments of inertia about the x and y axes for the area inside the outer surface 22a are reduced by the same amount according to equations (4) and (5). Still, the overall flexural rigidity of the rod 10a is greater about the x-axis as compared to the y-axis. Accordingly, a surgeon may elect to install the rod 10a in a patient to correspondingly control flexion, extension, or lateral bending. One may do so by orienting the rod 10a with the x-axis positioned perpendicular to the motion that is to be controlled. For example, a surgeon who elects to control flexion and extension may orient the rod 10a with the stiffer bending axis (x-axis in FIG. 3) approximately parallel to the coronal plane of the patient. Conversely, a surgeon who elects to control lateral bending may orient the rod 10a with the stiffer bending axis (x-axis in FIG. 3) approximately parallel to the sagittal plane of the patient. The surgeon may also elect to install the rod 10a with the x and y axes oriented at angles other than aligned with the sagittal and coronal planes of the patient. [0016] 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. [0017] FIG. 4 shows a rod 10b similar to rod 10a (i.e., outer surface 22b is substantially similar to surface 22a) with the exception that the inner aperture 30b defined by inner surface 32b is larger than the inner aperture 30a of rod 10a. Using the equations above, one is able to determine that the overall flexural rigidity about the x and y axes is greater for rod 10a as compared to rod 10b. Rods 10a and 10b may be available as a set with a common outer surface 22a, 22b. However, since the rods have a different internal aperture 30a, 30b configuration, a surgeon may select between the rods 10a, 10b to match a desired bending stiffness. [0018] The internal aperture 30 may be asymmetric as well. For example, the rod 10c shown in FIG. 5 includes an outer surface 22c that is substantially similar to the outer surface 22a of rod 10a. However, the inner aperture 30c defined by surface 32c is elliptical or oval shaped. The inner aperture 30c has a height h.sub.1 parallel to the x-axis that is less than the width b.sub.1 parallel to the y-axis. That is, the moment of inertia of the inner aperture 30c is greater about the y-axis than about the x-axis. This is in contrast to the outer surface 22c, which has a larger moment of inertia about the x-axis. [0019] The rods 10 may also have multiple inner apertures 30. For instance, the rod 10d shown in FIG. 6 comprises a plurality of apertures 30d, 130d defined by inner surfaces 32d, 132d. The outer surface 22d may be substantially similar to the outer surface 22a of rod 10a. Notably, the exemplary apertures 30d, 130d are disposed within the interior of the rod 10d. Further, the apertures 30d, 130d are offset from the centroid C. [0020] 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 FIG. 7. This particular rod 10e has a square outer surface 22e that is substantially symmetric relative to axes X and Y. However, the inner aperture 30e defined by inner surface 32e is asymmetric relative to these same X and Y axes. Inner surface 32e is substantially rectangular and defined by dimensions b and h. Specifically, dimension b (parallel to the Y-axis) is not equal to dimension h (parallel to the X-axis). In the embodiment shown, dimension b is larger than dimension h. Therefore, the aperture 30e has a larger moment of inertia relative to the Y-axis as compared to the X-axis. Consequently, according to equations (4) and (5), the rod 10e has a greater bending strength about the X-axis as compared to the Y-axis. Continue reading... Full patent description for Spinal rods having different flexural rigidities about different axes and methods of use Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Spinal rods having different flexural rigidities about different axes and methods of use patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. 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