Patent Publication Number: US-2016235446-A1

Title: Spinal Rods and Methods

Description:
CROSS-REFERENCE 
     This application is a claims the benefit of U.S. Provisional Application No. 62/116,482, filed on Feb. 15, 2015, which application is incorporated herein, in its entirety, by reference thereto. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of spinal surgery. More particularly, the present invention relates to spinal rods used in spinal surgery and methods of using the spinal rods. 
     BACKGROUND OF THE INVENTION 
     Spinal rods are commonly used in the surgical treatment of spinal disorders such as degenerative disc disease, scoliosis, fractures, other curvature abnormalities of the spine, other degenerative diseases of the spine, other diseases of the spine and other traumatic injuries to the spine. In performing fusion of two or more levels of the spine, and particularly in long fusion procedures where more than two levels are fused, it has been reported that accelerated degeneration of segments (levels) adjacent to those levels being fused has occurred, e.g., see Chou et al., “Adjacent segment degeneration after lumbar spinal posterolateral fusion with instrumentation in elderly patients”,  Arch Orthop Trauma Surg  (2002) 122:39-43; Hilibrand et al., “Adjacent segment degeneration and adjacent segment disease: the consequences of spinal fusion?”,  The Spine Journal , Vol. 4, Issue 6, Supplement, pp S190-S194, Nov-Dec 2004; and Kumar et al., “Long-term follow-up of functional outcomes and radiographic changes at adjacent levels following lumbar spine fusion for degenerative disc disease”,  Eur Spin J  (2001) 10:309-313, each of which is hereby incorporated herein, in its entirety, by reference thereto. 
     Accelerated segment degeneration is often noted particularly in the level or levels immediately above the fusion. For example, in a long fusion procedure fusing levels from the lower lumbar region to the upper thoracic region, the one or more levels immediately above the fusion (e.g., upper thoracic or cervical vertebra(ae)) have been found to rapidly degenerate, sometimes within a matter of weeks after the fusion procedure has been performed. 
     A major contributing factor to this accelerated degeneration is believed to be the abrupt transition from a very high stiffness provided at the top end of the spinal rod(s) implanted to rigidify the levels to be fused and the much lower stiffness of the level(s) adjacent the end of the rod(s) which are not fused. Attempts have been made at reducing the stress transition, each with its own drawbacks. 
     Simonson in U.S. Patent Application Publication No. 2012/0290013 discloses a tapered spinal rod that varies in cross-sectional diameter from 6.5 mm at the bottom to 3 mm at the top. A problem with this configuration is that, at almost every level of fusion, the screws used to fix the rods to the vertebrae do not set flat against the rods because of the taper of the external surfaces of the rods. This results in the unfortunate disadvantage that not every level of the fusion can be effectively locked to the rods. 
     Patterson et al. in U.S. Pat. No. 7,875,059 discloses spinal support members having various stiffnesses in which different portions of the support member are formed of different materials, with the different material having different stiffness characteristics. Problems with these designs include the greater cost of manufacture, compared to a support member made uniformly of one material, and, more importantly, and are more difficult to manufacture as the different components must be joined, by bonding or molding. More important, these designs run the risk of delamination after implantation in a patient, which can cause injury and/or require additional surgery for replacement or repair. 
     Other approaches have attempted to provide a dynamic fixation by one or more rods, at one or two levels above the fusion by providing a rod that bends or polymer rod (or rod extension, such as in Patterson) that can bend or flex. This requires the use of pedicle screws in the levels above the fusion that would otherwise not be required. As these levels above the fusion are smaller vertebrae, the patient can sometimes feel the pedicle screws protruding against the skin, causing discomfort. In some cases, the pedicle screws are even visible through the skin as they protrude into the layers of the skin. 
     There is a continuing need for improved products designed to reduce or eliminate the accelerated degeneration of segments adjacent to fused segments of a spine. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a spinal rod having an elongate body having first and second ends, a length and an outside diameter is provided. The outside diameter is substantially the same size over an entirety of the length of the elongate body. A cavity extends within the elongate body over a cavity length; wherein the cavity length is less than the length of the elongate body; and wherein a stiffness of the elongate body in portions containing the cavity is less than a stiffness of a portion of the elongate body not containing the cavity. 
     In at least one embodiment, the cavity length is in the range of 10% to 50% of the length of the elongate body. 
     In at least one embodiment, the cavity length is in the range about 15% to about 35% of the length of the elongate body. 
     In at least one embodiment, the cavity length is less than 50% of the length of the elongate body. 
     In at least one embodiment, the cavity extends from the first end to a location intermediate of the first and second ends. 
     In at least one embodiment, a diameter of the cavity varies along a length of the cavity. 
     In at least one embodiment, the cavity includes stepped portions, each stepped portion having a diameter that is substantially constant over a length of the respective step portion, and wherein the diameters of the stepped portions are each different from one another. 
     In at least one embodiment, the diameter of a first of the stepped portions is larger than the diameters of all other of the stepped portions and extends from the first end of the main body. 
     In at least one embodiment, the diameters of the stepped portions become decreasingly smaller in a direction from the first of the stepped portions toward the second end of the elongate body. 
     In at least one embodiment, the diameter of the cavity varies continuously over a length thereof. 
     In at least one embodiment, the diameter of the cavity decreases from a location nearest the first end of the elongate body to a location nearest the second end of the elongate body. 
     In at least one embodiment, the spinal rod is made entirely of a single material. 
     In at least one embodiment, the single material is metal. 
     In at least one embodiment, the metal comprises titanium. 
     In at least one embodiment, the metal comprises Cobalt Chromium. 
     In at least one embodiment, the single material comprises polymer. 
     In at least one embodiment, a first portion of the elongate body contains the cavity and a second portion of the elongate body is solid, and wherein the first portion has a stiffness in a range of from about 10 N/mm to about 250 N/mm and the second portion has a stiffness in a range of from about 40 N/mm to about 1000 N/mm. 
     In at least one embodiment, a first portion of the elongate body contains the cavity and a second portion of the elongate body is solid. The second portion has a stiffness in a range of from about 40 N/mm to about 1000 N/mm The first portion has at least first and second sub-portions, such that the first sub-portion portion has a stiffness in a range of from about 20 N/mm to about 500 N/mm and the second sub-portion has a stiffness in a range of from about 10 N/mm to about 250 N/mm, wherein the stiffness of the first sub-portion is greater than the stiffness of the second sub-portion. 
     In at least one embodiment, the first-sub portion is located intermediate of the second portion and the second sub-portion. 
     In at least one embodiment, a first portion of the elongate body contains the cavity and a second portion of the elongate body is solid, and wherein the second portion has a stiffness in a range of from about 14 GPa to about 250 GPa and wherein the first portion has at least first and second sub-portions, such that the first portion has a stiffness that varies from a range of from about 7 GPa to about 125 GPa in one of the at least first and second sub-portions to a range of from about 8.75 GPa to about 155 GPa in another of the at least first and second sub-portions. 
     In at least one embodiment, the spinal rod further includes a marker on an external surface of the elongate body, the marker identifying a location intermediate the first and second ends where the cavity ends. 
     In another aspect of the present invention, a method of performing spinal fusion is provided, including: selecting a spinal rod comprising an elongate body having first and second ends, a length and an outside diameter, the outside diameter being substantially the same size over an entirety of the length of the elongate body; and a cavity extending within the elongate body over a cavity length, wherein the cavity length is less than the length of the elongate body; and fixing the spinal rod to at least two adjacent spinal vertebrae. 
     In at least one embodiment, the fixing comprises fixing the spinal rod to three or more adjacent vertebrae. 
     In at least one embodiment, the fixing comprises orienting the spinal rod so that a portion of the elongate body containing the cavity is above a portion of the elongate body that does not contain the cavity. 
     In at least one embodiment, the fixing comprises fixing the spinal rod to each of the vertebrae spanned by the elongate body. 
     These and other features of the invention will become apparent to those persons skilled in the art upon reading the details of the embodiments as more fully described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a spinal rod according to an embodiment of the present invention. 
         FIG. 1B  is a view of  FIG. 1A  in which an upper portion of the elongate body is shown as a longitudinal sectional view. 
         FIG. 1C  is a cross-sectional view of  FIG. 1A  taken along line  1 C- 1 C. 
         FIG. 1D  is a cross-sectional view of  FIG. 1A  showing the solid lower portion of the elongate body. 
         FIG. 2  illustrates a spinal rod according to another embodiment of the present invention. 
         FIG. 3  illustrates a spinal rod according to another embodiment of the present invention. 
         FIGS. 4A and 4B  are partial views showing variants of the embodiment of  FIG. 3 . 
         FIG. 5  illustrates a pair of spinal rods having been implanted on a spine of a patient according to an embodiment of the present invention. 
         FIG. 6  illustrates a spinal rod that is curved along its length, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before the present devices and methods are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a vertebra” includes a plurality of such vertebrae and reference to “the rod” includes reference to one or more rods and equivalents thereof known to those skilled in the art, and so forth. 
     The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. The dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. 
     A “level” as used herein refers to fusion of two adjacent vertebrae to stop the motion at one segment, i.e. intervertebral disc; for example, an L4-L5 fusion is a one-level spinal fusion. 
       FIG. 1A  is a plan view of a spinal rod  10  according to an embodiment of the present invention. Spinal rod  10  comprises an elongate body  12  that is preferably formed as an integral unit so as to avoid risk of disintegration of parts during use of the rod  10  after implantation within a patient. Rod  10 , as well as all other embodiments described herein is preferably formed of metal, but could alternatively be formed of polymer or polymer composite. Preferably rod  10  and all other embodiments are formed of titanium or titanium alloy, but could alternatively be made from cobalt-chromium alloy, stainless steel, nickel-titanium, polymer such as polyetheretherketone (PEEK), carbon-reinforced PEEK or other metal, metal alloy, polymer or polymer alloy that is biocompatible and provides the required strength and other material properties required to function for this purpose. 
     Elongate body  12  is manufactured to have the same outside diameter  14  over the entire length thereof, except for the radiused ends  16 ,  18 . In one preferred embodiment, diameter  14  is about 6 mm. In another preferred embodiment, diameter  14  is about 5.5 mm. Diameter  14  is not limited to 5.5 mm or 6 mm but can be other values, depending upon the stiffness of the rod desired for a particular application, the hardware used for fixing the rod  10  to the spine, and the characteristics (e.g., size, condition, alignment/orientation, etc.) of the vertebrae to which the rod  10  is to be fixed. The length  20  of the rod  10 /elongate body  12  will vary, depending upon how many levels of vertebrae the rod  10  is intended to attach to including any number of levels within the range of one to fourteen. Typically, length  20  will be in a range of from about 6 cm to about 60 cm, more typically in a range of from about 30 cm to 45 cm. In one example, length  20  is about 30 cm for spanning the T3-L2 vertebrae. In another example, length  20  is about 45 cm for spanning T2 through the sacrum. In another example, length  20  is about 60 cm for spanning T2 to the ilium. 
       FIG. 1B  is a view of  FIG. 1A  in which an upper portion  22  of the elongate body  12  is shown as a longitudinal sectional view. This view illustrates the cavity or cannula  28  that is formed in the upper portion of the elongate body  12  to reduce the stiffness of portion  22 , relative to the stiffness of the lower portion  24 . The diameter  26  of the cavity  28  in the embodiment shown in  FIG. 1B  is about 3 mm, but can be in the range of about 1 mm to about 4 mm In the embodiment of  FIG. 1B , the diameter  26  of cavity  28  is substantially constant over the length thereof. The length  30  of the cavity may vary, depending upon the amount of stiffness reduction is desired over the upper portion of the rod  10  and how many vertebrae it is desired to attach a less stiff portion of the rod to. In the embodiment of  FIG. 1B , length  30  is about 75 mm, but may be in a range of from about 30 mm to about 100 mm Typically, one for levels (more typically, one to two levels) will be spanned by the less stiff portion of the rod at the top or at the bottom of a more stiff fusion construct. As shown, cavity  28  is concentric about the longitudinal axis  32  of rod  10  and rod  10  is also concentric about the longitudinal axis  32 .  FIG. 1C  is a cross-sectional view of  FIG. 1A  taken along line  1 C- 1 C and showing the cavity  28  surrounded by the elongate body  12 .  FIG. 1D  is a cross-sectional view of  FIG. 1A  showing the solid lower portion of the elongate body  12 . Alternatively, the rod  10  can be other than circular in cross-section, such as rectangular, oval, or other shape, or even a plate, but these conformations are less desirable as they would not be usable in deformity correction, and would be limited to a narrow range of spine fusion procedures. Further alternatively, rod  10  does not need to be concentric about the longitudinal axis, but could be offset. These alternatives are also less desirable for reasons provided above with regard to the shape of the cross-section. 
       FIG. 2  is a view of a spinal rod  40  according to another embodiment of the present invention. Like  FIG. 1B ,  FIG. 2  shows the upper portion of the elongate body  41  as a longitudinal sectional view. In this embodiment, the cavity  48  comprises stepped portions  48 A,  48 B,  48 C. Although the embodiment of  FIG. 2  shows three stepped portions, the cavity  48  could, alternatively have two stepped portions, or more than three stepped portions. Each stepped portion  48 A,  48 B,  48 C has a diameter ( 46 A,  46 B,  46 C, respectively) that is substantially constant over a length thereof. In the example shown in  FIG. 2 , diameter  48 A is about 3.5 mm, diameter  48 B is about 2.5 mm and diameter  48 C is about 1.5 mm, although each of these dimensions may vary and be larger or smaller. In a preferred embodiment, as shown in  FIG. 2 , diameter  46 A is larger than diameter  46 B and diameter  46 B is larger than diameter  46 C. In a preferred embodiment, the stepped portion having the largest diameter is nearest the upper end  36  of the rod  40  and the diameters of the stepped portions are successively smaller from the largest diameter portion in a direction toward the lower end  38  of the rod  40 . The length of each stepped portion can vary, and the percentage of the overall length of each portion, relative to the other portions can vary. In this way, the stiffness characteristics of portion  32  can be tailored as desired. The lengths  50 A,  50 B,  50 C of the stepped portions in  FIG. 2  are about 35 mm, 30 mm and 30 mm, respectively, although each may be varied to be longer or shorter. The length  32  of the portion of elongate body that contains the stepped portions  48 A,  48 B,  48 C is about 95 mm in the embodiment of  FIG. 2 , and the overall length of the elongate body  41  is about 450 mm The length  32  relative to the overall length  32  plus  34  is typically about 21%, but may be in a range of from about 10% to about 50% of the overall length of the elongate body  41 . A number of variant percentage lengths will be provided to accommodate different pathologies, bone quality, levels of fusion and patient heights. The lengths of portions  48 A,  48 B,  48 C may all be equal, but are typically unequal, as each portion  48 A,  48 B,  48 C, etc.(there may be more or less than three portions  48 A,  48 B,  48 C) is typically designed to span one level. 
       FIG. 3  is a view of a spinal rod  60  according to another embodiment of the present invention. Like  FIG. 1B ,  FIG. 3  shows the upper portion of the elongate body  61  as a longitudinal sectional view. In this embodiment, the cavity  68  comprises a continuously tapering diameter that varies from a largest diameter  68 A at the end of the cavity  68  nearest upper end  56  of the elongate body  61  to a smallest diameter  68 B at the end of the cavity  68  nearest the lower end  58  of the elongate body. In the embodiment shown, diameter  68 A is about 3.5 mm and diameter  68 B is about 1 mm, although each of these diameters may be larger or smaller. The continuous taper of the diameter of the cavity  68  in the embodiment of  FIG. 3  is linearly decreasing, as indicated by the straight lines of the cavity shown in the longitudinal sectional view of  FIG. 3 . Alternatively, the decreasing taper may be nonlinear, examples of which are illustrated in the partial, longitudinal, schematic views of  FIG. 4A and 4B , showing convex curvature of the walls  68 W of cavity  68  and concave curvature of walls  68 W of cavity  68 , respectively. Further alternatively, the walls  68 W of cavity  68  can be formed in any shape, to further tailor the stiffness characteristics of portion  52  of the rod  60 . There are an unlimited number of possibilities for design of the shapes and dimensions of the cavity walls  68 W, as will be apparent to those of ordinary skill in the art after reading the present disclosure. 
       FIG. 5  illustrates a pair of spinal rods  10  having been implanted on the spine  2  of a patient according to an embodiment of the present invention. It is noted that spinal rods  40  and  60 , and any embodiments of the present invention described herein, can be implanted in the same manner The spinal rods  10  are secured to the spine  2  by pedicle connectors  80  that include a pedicle screw  82  to secure the connector  80  to the pedicle  4  of a vertebra  6  and a retaining screw  84  that contacts the spinal rod  10  and compresses against it to lock it into position against the spine. Because the surface of the rod  10  is constant and does not taper in diameter, the retaining screw can establish full contact between its distal end surface and the surface of the rod  10  to establish a secure lock, along any location on the rod  10 . This is not possible with a tapering rod, as the full surface of the distal end of the retaining screw cannot establish contact with a tapered surface. In the embodiment shown, a five level fusion is being performed, as the rods  10  are secured to five adjacent vertebrae, as shown. It is noted that the present invention is not limited to five-level fusions, as more or less vertebrae may be secured in a fusion, e.g., two-level, three-level, four level, or more than five-level, by selecting rods  10  of the appropriate length and securing them in the same manner as shown in  FIG. 5 . As noted, the present invention allows locking of the spinal rod to every vertebrae in the span of vertebrae being fused, using standard hardware, without the need to change sizes of pedicle connectors. It is further noted that the present invention is not limited to implanting a pair of spinal rods on the spine as shown in  FIG. 5 . Alternatively, a single rod could be implanted on one side of the spine in the same manner as described with regard to  FIG. 5 , with no second rod being implanted. 
     The embodiments of the present invention described this far have all been shown as straight rods. Alternatively, rods having the characteristics described above can be formed a curved rods, if desired.  FIG. 6  illustrates a spinal rod that is curved along its length, according to an embodiment of the present invention. The lower portion  102  is curved in a first direction, and the upper portion is a curved in a second direction different from the first direction so that the rod  100  is configured to more closely conform to a curvature of the spine  2  that is desired to be achieved by the fusion. It is noted that the present invention is not limited to the curvatures shown I  FIG. 6 , as rod  100  could be made to have any curvature desired. Further alternatively, any of the spinal rods described herein can be made so that they can be bent by tools to deviate from a straight rod, so as to facilitate better alignment with a spinal curvature sought to be established by a fusion procedure. 
     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the invention as described herein.