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Publication numberUS20090194206 A1
Publication typeApplication
Application numberUS 12/322,521
Publication date6 Aug 2009
Filing date2 Feb 2009
Priority date31 Jan 2008
Publication number12322521, 322521, US 2009/0194206 A1, US 2009/194206 A1, US 20090194206 A1, US 20090194206A1, US 2009194206 A1, US 2009194206A1, US-A1-20090194206, US-A1-2009194206, US2009/0194206A1, US2009/194206A1, US20090194206 A1, US20090194206A1, US2009194206 A1, US2009194206A1
InventorsDong M. Jeon, Patrick D. Moore, Hee J. Yang, Sang K. Lee
Original AssigneeJeon Dong M, Moore Patrick D, Yang Hee J, Lee Sang K
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Systems and methods for wrought nickel/titanium alloy flexible spinal rods
US 20090194206 A1
Abstract
Dynamic, flexible wrought Nickel/Titanium alloy spinal rods for spinal fusion or dynamic stabilization vertebral implants and methods and processes related to their manufacture. The dynamic and flexibility properties of the wrought Nickel/Titanium alloy spinal rod may be varied by altering processing parameters during manufacture that develop the shape memory characteristics, mechanical properties, and product workability characteristics to achieve custom manufacture of spinal rods having desired flexion in desired lengths. Such a custom manufactured spinal rod may be affixed to an inferior vertebral body at a standard lamina or pedicle location and to one superior vertebral body at a standard lamina or pedicle location using pedicle screws, lamina hooks, or pedicle hooks to provide dynamic stabilization between superior and inferior vertebrae in connection with a spinal fusion procedure.
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Claims(19)
1. A process for manufacturing a dynamic and flexible spinal rod, the process comprising:
shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been hot worked from about 0% to about 20% and cold worked from about 0% to about 60%, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes; and
subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and in for a time of from about 1 minute to about 120 minutes.
2. The process of claim 1, wherein shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock comprises shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which contains less than about 54% Nickel.
3. The process of claim 1, wherein shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been hot worked from about 0% to about 20% and cold worked from about 0% to about 60%, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes comprises shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been about 10% hot worked and about 5% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
4. The process of claim 3, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 0.8462 Kg/mm2 to about 2.7501 Kg/mm2.
5. The process of claim 1, wherein shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been hot worked from about 0% to about 20% and cold worked from about 0% to about 60%, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes comprises shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been about 10% hot worked and about 20% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
6. The process of claim 5, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 1.8462 Kg/mm2 to about 5.7501 Kg/mm2.
7. The process of claim 1, wherein shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been hot worked from about 0% to about 20% and cold worked from about 0% to about 60%, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes comprises shaping a cylinder as a blank for a rod suitable for a spinal fusion procedure from a Nitinol alloy stock which has been about 10% hot worked and about 40% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
8. The process of claim 7, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 5.8462 Kg/mm2 to about 20.7501 Kg/mm2.
9. The process of claim 7, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment at a temperature of about 400 deg. C. for a time selected from the range of from about 1 minute to about 120 minutes to result in a spinal fusion rod which has a selected rigidity of from about 8.8078 Kg/mm2 to about 22.692 Kg/mm2.
10. A method of producing a dynamic flexible wrought Nickel/Titanium alloy rod for a spinal fusion procedure, the method comprising:
selecting a desired amount of flexibility required in the rod;
selecting a Nitinol alloy stock for forming the rod, the Nitinol alloy stock comprising a blank which has been hot worked from about 0% to about 20% and cold worked from about 0% to about 60%, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes;
shaping a cylinder as a blank for the rod from the selected Nitinol alloy stock; and
subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and in for a time of from about 1 minute to about 120 minutes.
11. The method of claim 10, wherein selecting a Nitinol alloy stock for forming the rod comprises selecting a Nitinol alloy stock which contains less than about 54% Nickel.
12. The method of claim 10, wherein selecting a desired amount of flexibility comprises selecting a flexibility for promoting formation of a spinal fusion mass based on the particular characteristics of a patient needing a spinal fusion procedure.
13. The method of claim 10, wherein selecting a Nitinol alloy stock for forming the rod comprises selecting a Nitinol alloy which has been about 10% hot worked and about 5% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
14. The method of claim 13, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 0.8462 Kg/mm2 to about 2.7501 Kg/mm2.
15. The method of claim 10, wherein selecting a Nitinol alloy stock for forming the rod comprises selecting a Nitinol alloy stock which has been about 10% hot worked and about 20% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
16. The method of claim 15, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 1.8462 Kg/mm2 to about 5.7501 Kg/mm2.
17. The method of claim 10, wherein selecting a Nitinol alloy stock for forming the rod comprises selecting a Nitinol alloy stock which has been about 10% hot worked and about 40% cold worked, and annealed at a temperature of about 800 deg. C. for a time of about 10 minutes.
18. The method of claim 17, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment for a time of about 10 minutes at a temperature selected from the range of from about 250 deg. C. to about 800 deg. C. to result in a spinal fusion rod which has a selected rigidity of from about 5.8462 Kg/mm2 to about 20.7501 Kg/mm2.
19. The method of claim 17, wherein subjecting the shaped blank to a shape setting heat treatment at a temperature of from about 250 deg. C. to about 800 deg. C. and for a time of from about 1 minute to about 120 minutes comprises subjecting the shaped blank to a shape setting heat treatment at a temperature of about 400 deg. C. for a time selected from the range of from about 1 minute to about 120 minutes to result in a spinal fusion rod which has a selected rigidity of from about 8.8078 Kg/mm2 to about 22.692 Kg/mm2.
Description
    CROSS-REFERENCE TO RELATED APPLICATION
  • [0001]
    This application claims the benefit of U.S. Provisional Application No. 61/025,204, filed Jan. 31, 2008, the disclosure of which is incorporated herein by reference in its entirety.
  • TECHNICAL FIELD
  • [0002]
    The present invention relates to devices and implants used in osteosynthesis and other orthopedic surgical procedures such as devices for use in spinal surgery, and, in particular, to orthopedic stabilization devices used to limit the relative motion of at least two vertebral bodies for the relief of pain which are manufactured from Nitinol, a dynamic/flexible wrought Nickel/Titanium alloy.
  • BACKGROUND
  • [0003]
    There have been many devices contrived to relieve pain associated with spinal injury or illness. Traditionally surgeons have fused the vertebral bodies with a pedicle screw and solid rod construct or a fusion cage. In attempting to fuse the spine using traditional methods, patients may experience a long and painful recovery process as well as the uncertainty of fusion mass formation. It is well known, where stress is allowed to transfer through the fusion site while the vertebral bodies are held in a limited range of motion, then fusion can occur much quicker aiding in patient recovery time. However, most rod and screw constructs and fusion cage constructs are very rigid, and do not allow transfer of stress into the fusion site that would aid in quicker recovery and the promotion of the boney fusion mass.
  • [0004]
    There are many devices that have been developed that attempt to allow relative motion, yet these devices have fallen short in preventing shear forces between the vertebral bodies from being stabilized. Another shortcoming is that such devices often forcibly channel relative motion through rather specific locations or hinge points in the mechanical construct. Some of these devices and their shortcomings are discussed in the following paragraphs.
  • [0005]
    U.S. Pat. No. 5,092,866, the disclosure of which is incorporated by reference herein in its entirety, discloses a pedicle screw system that is banded together with flexible ligaments. While these flexible ligaments allow for relative motion, they do not appear to resist compression or shear loads, as they appear to rely upon tension alone.
  • [0006]
    European Patent No. EP 06691091 A1/B1, the disclosure of which is incorporated by reference herein in its entirety, discloses a polycarbonate/urethane supporting element, compressed between two adjacent pedicle screws and passing over an elastic strap that acts as a flexible internal ligament. This flexible internal ligament is in the form of a nylon cord, which is pre-tensioned and fastened to the screw heads. While such a design provides flexural degrees of freedom and allows relative motion between the vertebral bodies, it does little to inhibit or prevent shearing between the vertebral bodies. Additionally, such a ligament appears to lack rigidity and relies on proper tensioning inter-operatively to gain its balance.
  • [0007]
    U.S. Pat. No. 6,267,764, the disclosure of which is incorporated herein by reference in its entirety, discloses a pedicle screw and rod system wherein the rod is flexible in translation. A dampening ball is not separate from the rods and has cutouts to allow bending, with no ligament passing through the centers of the rods. While flexibility in translation can be helpful, the spine loads in several planes at the same time and the translation spoken of in this patent would appear to inadequately redistribute stresses through the fusion site. As a result, motion is forcibly limited to one location, i.e., motion is constrained through a hinge point, which undesirably stresses the assembly construct.
  • [0008]
    As explained in S. M. Russell, Nitinol Melting and Fabrication, in Proceedings of the International Conference on Shape Memory and Superelastic Technologies, (International Organization on SMST-2001), the contents of which are incorporated by reference herein in their entirety, fabrication of Nitinol presents unique challenges because of the material's strong sensitivity to chemistry and processing.
  • [0009]
    Accordingly there exists a need for assemblies and devices that effectively resist torsion as well as shear forces while providing flexible stabilization. More specifically, it would be desirable to provide kits with such assemblies and devices, which work with existing pedicle screw arrangements if required. There is a further need to provide stabilization assemblies and devices manufactured from a shape memory material such as an alloy or other flexible polymer, which can withstand repeated loading of the spine without fatiguing, yet still maintain its flexibility.
  • SUMMARY
  • [0010]
    Dynamic, flexible wrought Nickel/Titanium alloy spinal rods for spinal fusion or dynamic stabilization vertebral implants and methods and processes related to their manufacture. The dynamic and flexibility properties of the wrought Nickel/Titanium alloy spinal rod may be varied by altering processing parameters during manufacture that develop the shape memory characteristics, mechanical properties, and product workability characteristics to achieve custom manufacture of spinal rods having desired flexion in desired lengths. For example, the amount of hot working of the alloy may be varied from about 0% to about 20%, the amount of cold working may be varied from about 0% to about 60%, and the final shape setting heat treatment of a shaped rod may be varied in temperature from about 250 deg. C. to about 800 deg. C. and in time from about 1 minute to about 120 minutes, to achieve desired characteristics.
  • [0011]
    Such a custom manufactured spinal rod may be affixed to an inferior vertebral body at a standard lamina or pedicle location and to one superior vertebral body at a standard lamina or pedicle location using pedicle screws, lamina hooks, or pedicle hooks to provide dynamic stabilization between superior and inferior vertebrae in connection with a spinal fusion procedure.
  • DESCRIPTION OF THE DRAWINGS
  • [0012]
    It will be appreciated by those of ordinary skill in the art that the elements depicted in the various drawings are not necessarily to scale, but are for illustrative purposes only. The nature of the present invention, as well as other embodiments of the present invention may be more clearly understood by reference to the following detailed description of the invention, to the appended claims, and to the several drawings attached hereto.
  • [0013]
    It will be appreciated by those of ordinary skill in the art that the elements depicted in the various drawings are not necessarily to scale, but are for illustrative purposes only. The nature of the present invention, as well as other embodiments of the present invention may be more clearly understood by reference to the following detailed description of the invention, to the appended claims, and to the several drawings attached hereto.
  • [0014]
    FIG. 1 is a diagram of the shape memory of Nitinol components.
  • [0015]
    FIG. 2 is a diagram of a loading/unloading curve for Nitinol.
  • [0016]
    FIG. 3 is a side view of an illustrative embodiment of a single-level spine rod including markings for identification and placement, which is manufactured in accordance with the principles of the present invention.
  • [0017]
    FIG. 4A is a side view of a second illustrative model of a single-level spine rod, manufactured in accordance with the principles of the present invention, in a relaxed or neutral state.
  • [0018]
    FIG. 4B is a side view of single-level spine rod of FIG. 4A in a flexed state.
  • [0019]
    FIG. 5 depicts a sectional side view of a single-level spine rod manufactured in accordance with the principles of the present invention retained in the connection channel of a poly-axial bone screw that may be used with embodiments in accordance with the present invention.
  • DETAILED DESCRIPTION
  • [0020]
    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
  • [0021]
    Dynamic stabilization of damaged or diseased spinal segments has long been desired. However, until recently, the technology has yet been underdeveloped. Numerous techniques and devices have been developed with varying degrees success. These dynamic stabilization applications include, but are not limited to, flexible rod systems, Interspinous Process Decompression devices and artificial disks. These different systems are successful in some aspects and failures in others as well as being indicated for a wide variety of uses; however, no device is all inclusive for all indications.
  • [0022]
    Some of the failures can be attributed to the devices' material of manufacture. By nature, dynamic stabilization requires movement in the device. These devices utilize relatively static materials for construction, therefore lacking inherent dynamic material qualities.
  • [0023]
    Apparatus systems in accordance with the present invention utilize rods constructed of Nitinol in conjunction with various pedicle screw, lamina hook, or pedicle hook based spinal fusion or dynamic stabilization vertebral implants.
  • [0024]
    Nitinol-based products have been on the market since the late 1960's. Nitinol possesses thermal shape memory behavior. Chilling a Nitinol component converts the Austenite structure of the Nitinol to a Martensite structure, becoming very malleable. Where the chilled component is then heated, the Martensite structure of the Nitinol returns to an Austenite structure and, thus, reverts the component to its original shape, as illustrated in the diagram shown in FIG. 1. Thus, in the medical device industry, Nitinol has been used for reusable medical instruments. Surgeons can shape an instrument on site to fit a patient's geometry, then after heat sterilization the device returns to its original shape for the next procedure.
  • [0025]
    In certain embodiments of the present invention, the unique thermal shape memory behavior of Nitinol may be utilized in the installation of the device. Where an embodiment in accordance with the present invention is used as a stand-alone device, that is to say, utilized without additional screw or hook attachment means, such a device may be chilled in saline, which converts the Austenite structure of the Nitinol to a Martensite structure, becoming very malleable. The surgeon then has the ability to deform the incorporated “hooks” of the device allowing easy installation at a lamina location or a pedicle location. Once installed, the surgeon may then flood the rod component with heated saline which converts the Martensite structure of the Nitinol to an Austenite structure and, thus, reverts the device to its original shape. This type of installation can be used where the embodiments in accordance with the present invention are formed of superelastic Nitinol. For such embodiments, chilling the device in a delivery system may keep the device in the soft martensite phase in a lower force state. After deployment, as the device warms to its new surroundings, it may recover its “programmed” shape and become superelastic.
  • [0026]
    Nitinol has an increased elasticity compared to stainless steel, allowing it to be bent more significantly than stainless steel without taking a set. Nitinol's elasticity or “springback” is some 10 times greater than stainless steel. Where embodiments in accordance with the present invention are formed of superelastic Nitinol, this unique property may be utilized to allow the embodiments, once installed, to be flexible without yielding under the stresses of the application. Superelastic Nitinol has an unloading curve that stays flat over large strains, thus, i.e. Nitinol devices can be designed that apply a constant stress over a wide range of shapes. FIG. 2 depicts a diagram of a loading/unloading curve for Nitinol.
  • [0027]
    Nitinol has been approved for many clinical applications including orthopedic bone anchors, vena cava filters, cardiovascular endoprostheses, and orthopedic archwires. Other Nitinol orthopedic applications include osteosynthesis staples and scoliosis correction rods. The biocompatibility of Nitinol results mainly from its tight intermetallic bounded structure, its chemically stable and homogeneous TiO2 surface layer, and its corrosion resistance, which is similar to other Titanium alloys.
  • [0028]
    The material specification for Nitinol conforms to ASTM standard ASTM F 2063-00, which is incorporated herein by reference in its entirety. While embodiments in accordance with the present invention may be made from Nitinol conforming to the ASTM standard, in other embodiments it may be desirable to alter the relative concentrations of copper and nickel in the alloy. For example, a typical Nitinol alloy contains at least 54% Nickel in order to ensure the desired ductile properties are present. By reducing the amount of Nickel from at least 54% to as low as about 51.0%, while increasing the amount of copper in the alloy, the applicants have been able to maintain the desired ductile properties, while reducing the potential for a nickel sensitivity reaction to occur after a device is implanted in a patient. The material specification for some acceptable Nitinol alloys is set forth in Table 1 below.
  • [0000]
    TABLE 1
    Material Specification for Nitinol
    Weight
    Element Percent
    Nickel 51~57
    Carbon, Max. 0.070
    Cobalt, Max. 0.050
    Copper, Max. 0.010~3.0 
    Chromium, Max. 0.010
    Hydrogen, Max. 0.005
    Iron, Max. 0.050
    Niobium, Max. 0.025
    Oxygen, Max. 0.050
    Titanium balance
  • [0029]
    The present invention relates to dynamic/flexible Wrought Nickel/Titanium Alloy (Nitinol) Spinal Rods for implantation within a patient for stabilization of the spine. Systems and apparatus in accordance with the present invention may provide posterior dynamic stabilization devices capable of achieving multiple angular axial orientations with respect to spinal bone tissue. Such systems and devices can be used to aid osteo-synthesis in combination with fusion devices, supplement other motion restoring devices such as disk implants or used solely to restrict the motion of vertebral bodies.
  • [0030]
    The fabrication and manufacturing process of a Nitinol component is generally composed of five manufacturing stages as follows. First, melting/alloying, second, hot working, third, cold working, fourth, machining (or forming), and fifth, shape setting heat treatment of the final product shape. The second, third and fifth processes are considered to be thermo-mechanical treatment of the product, which develop the specific shape memory characteristics, mechanical properties, and product workability characteristics of the final component). Quantitatively, the general condition ranges are as follows. For hot working the range is from about 0% to about 20%. For cold working the range is from about 0% to about 60%. For annealing time the range is from about 10 minutes to about 120 minutes. For annealing temperature the range is from about 100 deg. C. to about 850 deg. C. For shape setting heat treatment (or “final” heat treatment”) the time range is from about 1 minute to about 100 minutes and the temperature range is from about 250 deg. C. to about 800 deg. C.
  • [0031]
    It is noted that although the final properties of Nitinol strongly depend on the above conditions for fabrication, as well as varied chemical composition and working history, there also are optimum limitations relating to those conditions. The data summarized in Tables 2 through 5 below set forth specific conditions of thermo-mechanical treatment for production of a spinal rod formed from Nitinol in accordance with the principles of the present invention, and the resulting rigidity achieved in a spinal rod construct formed by such processing. It is noted that the percentages for hot and cold working used herein are taken to have the standard meaning in the art of referring to the percentage of processing. For example, the reduction of an alloy cylinder diameter by 10% resulting from drawing the cylinder at a temperature above the crystallization temperature of Nitinol would constitute 10% hot working.
  • [0032]
    As shown in Table 2, a spinal rod construct may be formed from Nitinol stock treated by about 10% hot working, and about 5% cold working, with an annealing time of 10 min. at an annealing temperature of 800 deg. C. The shaped rod is then subjected to shape setting treatment for a time of about 10 minutes. By selecting a temperature of from about 250 deg. C. to about 800 deg. C., the rigidity, reported as Stress/force related to Superelasticity/Ductility, of the final formed rod can be selected from about 0.8462 Kg/mm2 from a treatment at about 250 deg. C. to about 2.7501 Kg/mm2 from a treatment at about 450 deg. C.
  • [0000]
    TABLE 2
    Stress/force
    Annealing Shaping Heat related to
    Hot Cold Annealing Temperature Treatment Superelasticity/
    Working % Working % Time (min) (deg. C.) Time/Temp. Ductility (Kg/mm2)
    10% 5% 10 800 deg. C. 10;00 min./ 0.8462
    250 deg. C.
    10:00 min/ 1.2693
    350 deg. C.
    10:00 min/ 2.6924
    400 deg. C.
    10:00 min/ 2.7501
    450 deg. C.
    10:00 min/ 0.9078
    800 deg. C.
  • [0033]
    As shown in Table 3, a spinal rod construct may be formed from Nitinol stock treated by about 10% hot working, and about 20% cold working, with an annealing time of 10 min. at an annealing temperature of 800 deg. C. The shaped rod is then subjected to shape setting treatment for a time of about 10 minutes. By selecting a temperature of from about 250 deg. C. to about 800 deg. C., the rigidity of the final formed rod can be selected from about 1.8462 Kg/mm2 from a treatment at about 250 deg. C. to about 5.7501 Kg/mm2 from a treatment at about 450 deg. C.
  • [0000]
    TABLE 3
    Stress/force
    Annealing Shaping Heat related to
    Hot Cold Annealing Temperature Treatment Superelasticity/
    Working % Working % Time (min) (deg. C.) Time/Temp. Ductility (Kg/mm2)
    10% 20% 10 800 deg. C. 10;00 min./ 1.8462
    250 deg. C.
    10:00 min/ 2.2693
    350 deg. C.
    10:00 min/ 3.6924
    400 deg. C.
    10:00 min/ 5.7501
    450 deg. C.
    10:00 min/ 2.8078
    800 deg. C.
  • [0034]
    As shown in Table 4, a spinal rod construct may be formed from Nitinol stock treated by about 10% hot working, and about 40% cold working, with an annealing time of 10 min. at an annealing temperature of 800 deg. C. The shaped rod is then subjected to shape setting treatment for a time of about 10 minutes. By selecting a temperature of from about 250 deg. C. to about 800 deg. C., the rigidity of the final formed rod can be selected from about 5.8462 Kg/mm2 from a treatment at about 250 deg. C. to about 20.7501 Kg/mm2 from a treatment at about 450 deg. C.
  • [0000]
    TABLE 4
    Stress/force
    Annealing Shaping Heat related to
    Hot Cold Annealing Temperature Treatment Superelasticity/
    Working % Working % Time (min) (deg. C.) Time/Temp. Ductility (Kg/mm2)
    10% 40% 10 800 deg. C. 10;00 min./ 5.8462
    250 deg. C.
    10:00 min/ 6.2693
    350 deg. C.
    10:00 min/ 12.6924
    400 deg. C.
    10:00 min/ 20.7501
    450 deg. C.
    10:00 min/ 9.8078
    800 deg. C.
  • [0035]
    As shown in Table 5, a spinal rod construct may be formed from Nitinol stock treated by about 10% hot working, and about 40% cold working, with an annealing time of 10 min. at an annealing temperature of 800 deg. C. The rod is then subjected to shape setting treatment at a temperature of about 400 deg. C. By selecting a shape heating treatment time of from about one minute to about 120 minutes, the rigidity of the final formed rod can be selected from about 8.8078 Kg/mm2 from a treatment of about 120 minutes to about 22.692 Kg/mm2 from a treatment of about 30 minutes.
  • [0000]
    TABLE 5
    Stress/force
    Annealing Shaping Heat related to
    Hot Cold Annealing Temperature Treatment Superelasticity/
    Working % Working % Time (min) (deg. C.) Time/Temp. Ductility (Kg/mm2)
    10% 40% 10 800 deg. C.  1:00 min./ 10.8462
    400 deg. C.
    20:00 min/ 20.6924
    400 deg. C.
    30:00 min/ 22.6924
    400 deg. C.
    60:00 min/ 13.7501
    400 deg. C.
    120:00 min/ 8.8078
    400 deg. C.
  • [0036]
    Referring generally to FIG. 3, there is shown one illustrative embodiment of a wrought Nickel/Titanium alloy (Nitinol) flexible spinal rod 10 which is manufactured in accordance with the present invention. Rod 10 has a length L which may correspond to a number of spinal levels, such as one or two spinal levels, in order to allow the rod 10 to be attached to a bone anchor in the performance of a spinal fusion procedure. In the illustrated embodiment, rod 10 includes identification markings 102, and centerline marking 104 which aid in identification and placement during a surgical procedure.
  • [0037]
    While rods 10 may be manufactured in lengths of from about 40 mm to about 400 mm, a typical rod 10 will have a length of from about 40 mm to about 150 mm, which suffices for one to two spinal level constructs, based on specific patient anatomy. Longer rods up to about 400 mm may be offered for specialized uses. For example, such a long rod 10 could be used to create a long dynamic construct for treating certain scoliosis conditions. In typical applications, a rod 10 of from about 40.0 mm to about 70.0 mm may be used for a one level construct, a rod 10 of from about 70.0 mm to about 120.0 mm may be used for a two level construct, a rod 10 of from about 100.0 mm to about 200.0 mm may be used for a three level construct, and a rod 10 of from about 200.0 mm to about 400.0 mm may be used for a construct of four or more levels. Depending on a patient's anatomy, the length of rod compared to the number of spinal levels it is used for fusing can vary. Typical rod diameters may be in the range of from about 4.0 mm to about 6.0 mm. Where necessary, the rod 10 may be fitted into one or more sleeves for securing in a bone anchor.
  • [0038]
    FIGS. 4A and 4B depict another illustrative embodiment of a wrought Nickel/Titanium alloy flexible spinal rod 20 which is manufactured in accordance with the present invention. FIG. 4A depicts the rod 20 in a neutral relaxed state, and FIG. 4B depicts the rod in a fully-flexed state. By varying the parameters of the fabrication and manufacturing process of the rod 20 with respect to the thermo-mechanical treatment of the rod, (the hot working, cold working, and shape setting heat treatment of the final product shape), along the parameters set forth in Table 2, the characteristics of the rod 20 may be varied, including the rigidity, and superelasticity, to allow the fully-flexed state depicted in FIG. 4B, as well as the elastic modulus of the rod 20 to be varied as desired for the particular application for which the rod is used.
  • [0039]
    Turning to FIG. 5, there is shown one illustrative embodiment of an attachment means for a attaching a rod 10 or 20 in accordance with the present invention to a vertebral body in performing a spinal fusion. In the depicted embodiment a rod 10 is secured in the connection channel 400 of an appropriate bone anchor assembly. In the depicted embodiment, the attached bone screw assembly 40 is a poly-axial pedicle screw assembly, similar to those described in pending U.S. patent application Ser. No. 11/648,983 the disclosure of which is incorporated herein by reference in its entirety. It will be appreciated that other suitable bone anchor assemblies may be used, including poly-axial or mono-axial hooks, mono-axial or poly-axial pedicle screws, or other attachment means utilized in spinal surgery.
  • [0040]
    For use in a typical spinal fusion procedure, a practitioner will determine the proper size rod 10 for use. This will be based on the number of vertebral levels affected, the particular characteristic of particular patient's anatomy and physiology. The rod 10 selected having been manufactured in accordance with the present invention will have the specific desired flexibility characteristic appropriate for that patient. Additionally, by being flexible throughout the length of the rod 10, the creation of a hinge point is avoided.
  • [0041]
    For the purposes of clarity, this will be explained using a single level rod 10 and the installation of a single assembly including a rod 10 and two bone anchors 40. However, it will be appreciated that in a typical surgery, two rods 10 will be installed, one on either side of the spine, with a suitable number of bone anchors at the affected levels of the spine.
  • [0042]
    Where the rod is to be attached by a specific attachment means, the means is prepared, as by placement of pedicle screws 40 at the appropriate location, such as the standard pedicle location or lamina location for a spinal fusion procedure. The selected rod may then be attached to the pedicle screws 40 by securing the rod in the connection channels 400 of the rods.
  • [0043]
    In situations where the anatomy of the patient makes it desirable, the rod 10 may be chilled in saline, as by loading in saline of about 4 degrees C. for about 1 to 2 minutes, to convert the Austenite structure of the Nitinol to a Martensite structure. The now malleable rod 10 may then be bent to ease installation. The rod 10 may then be placed in the correct position, as by attachment to the attachment means, such as bone screws, and secured therein for installation. Once installed, the surgeon may then flood the rod 10 with heated saline, for example saline heated to from about 40 to about 45 degrees C., to convert the Martensite structure of the Nitinol to an Austenite structure and, thus, restoring the rod 10 to its original shape, becoming superelastic and exhibiting the desired flexibility.
  • [0044]
    It will be appreciated that other suitable the attachment means may include poly-axial, or mono-axial hooks, mono-axial pedicle screws, or any other attachment means utilized in spinal surgery.
  • [0045]
    While the present invention has been shown and described in terms of preferred embodiments thereof, it will be understood that this invention is not limited to any particular embodiment and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined and desired to be protected.
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Classifications
U.S. Classification148/563
International ClassificationC22F1/10
Cooperative ClassificationC22C19/03, C22F1/10
European ClassificationC22C19/03, C22F1/10