U.S. patent application number 12/322521 was filed with the patent office on 2009-08-06 for systems and methods for wrought nickel/titanium alloy flexible spinal rods.
Invention is credited to Dong M. Jeon, Sang K. Lee, Patrick D. Moore, Hee J. Yang.
Application Number | 20090194206 12/322521 |
Document ID | / |
Family ID | 40930496 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090194206 |
Kind Code |
A1 |
Jeon; Dong M. ; et
al. |
August 6, 2009 |
Systems and methods for wrought nickel/titanium alloy flexible
spinal rods
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.
Inventors: |
Jeon; Dong M.; (Draper,
UT) ; Moore; Patrick D.; (West Jordan, UT) ;
Yang; Hee J.; (Sungnam-si, KR) ; Lee; Sang K.;
(Seoul, KR) |
Correspondence
Address: |
MORRISS OBRYANT COMPAGNI, P.C.
734 EAST 200 SOUTH
SALT LAKE CITY
UT
84102
US
|
Family ID: |
40930496 |
Appl. No.: |
12/322521 |
Filed: |
February 2, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61025204 |
Jan 31, 2008 |
|
|
|
Current U.S.
Class: |
148/563 |
Current CPC
Class: |
C22F 1/10 20130101; C22C
19/03 20130101 |
Class at
Publication: |
148/563 |
International
Class: |
C22F 1/10 20060101
C22F001/10 |
Claims
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/mm.sup.2 to about 2.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 5.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 20.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 22.692
Kg/mm.sup.2.
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/mm.sup.2 to about 2.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 5.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 20.7501
Kg/mm.sup.2.
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/mm.sup.2 to about 22.692
Kg/mm.sup.2.
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 TiO.sub.2 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.
TABLE-US-00001 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/mm.sup.2 from a treatment at about 250 deg. C.
to about 2.7501 Kg/mm.sup.2 from a treatment at about 450 deg.
C.
TABLE-US-00002 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/mm.sup.2) 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/mm.sup.2 from a treatment at about 250 deg. C. to about
5.7501 Kg/mm.sup.2 from a treatment at about 450 deg. C.
TABLE-US-00003 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/mm.sup.2) 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/mm.sup.2 from a treatment at about 250 deg. C. to about
20.7501 Kg/mm.sup.2 from a treatment at about 450 deg. C.
TABLE-US-00004 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/mm.sup.2) 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/mm.sup.2 from a treatment of about
120 minutes to about 22.692 Kg/mm.sup.2 from a treatment of about
30 minutes.
TABLE-US-00005 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/mm.sup.2) 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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