U.S. patent application number 11/321337 was filed with the patent office on 2006-11-23 for mobile spine stabilization device.
Invention is credited to Patrick M. White.
Application Number | 20060264937 11/321337 |
Document ID | / |
Family ID | 37308321 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060264937 |
Kind Code |
A1 |
White; Patrick M. |
November 23, 2006 |
Mobile spine stabilization device
Abstract
An orthopedic device is described for stabilizing the spinal
column between first and second vertebral bodies. The device has
first and second screws adapted for fixation to the first and
second vertebral bodies, respectively. The device further includes
an elongated ligament with a first end connected to the first screw
and the second end operatively connected with the second screw. The
ligament is made preferably of a nickel titanium alloy selected to
have ductile inelastic properties at body temperature and is
capable of continuous plastic deformation to allow relative
constrained motion between the vertebral bodies. In a preferred
embodiment the second pedicle screw includes a bearing for
receiving the ligament in a slideably engageable relationship. The
device further includes optional first and second dampening members
surrounding the ligament for restraining the spinal column during
flexion and extension. Other preferred devices and kits containing
such devices are also described.
Inventors: |
White; Patrick M.; (West
Chester, PA) |
Correspondence
Address: |
Patrick M. White
511 Price Street
West Chester
PA
19382
US
|
Family ID: |
37308321 |
Appl. No.: |
11/321337 |
Filed: |
December 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11244184 |
Oct 5, 2005 |
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11321337 |
Dec 29, 2005 |
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60677699 |
May 4, 2005 |
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Current U.S.
Class: |
606/257 ;
606/259; 606/263; 606/280; 606/281; 606/907; 606/910 |
Current CPC
Class: |
A61B 17/7031 20130101;
A61B 2017/00867 20130101; A61B 17/702 20130101; A61B 17/7029
20130101; A61B 17/7011 20130101; A61B 17/7032 20130101; A61B
17/7004 20130101; A61B 17/7046 20130101 |
Class at
Publication: |
606/061 |
International
Class: |
A61F 2/30 20060101
A61F002/30 |
Claims
1. An orthopedic device for stabilizing a first and second bone of
the spinal column, the device comprising: an elongated ligament
having first and second ends, the ligament selected to exhibit
inelastic characteristics at body temperature and further capable
of continuous plastic deformation; a first screw adapted to
securely fasten the first end of the ligament to the first bone; a
second screw presenting a bearing for receiving the second end of
the ligament and securing it in a mobily constrained fashion to the
second bone, wherein plastic deformation in the ligament allows
relative constrained motion between the bones.
2. The orthopedic device of claim 1 wherein the implantable
ligament is in the form of at least one wire, rod, tube, cable,
band or plate.
3. The orthopedic device of claim 1 further comprising a dampening
member surrounding the ligament and sandwiched between the first
and second screw.
4. The orthopedic device of claim 1 wherein the second end of the
ligament has an abutment.
5. The orthopedic device of claim 4 further comprising a dampening
member oriented around the ligament and sandwiched between the
second screw and the abutment.
6. The orthopedic device of claim 1 wherein the bearing further
comprises a plastic material selected from polyethylene or
polyetheretherketone.
7. The orthopedic device of claim of claim 1 wherein the ligament
further comprises a nickel titanium alloy.
8. The orthopedic device of claim 7 wherein the ligament exhibits
shape memory characteristics with a transition temperature above
body temperature.
9. An orthopedic device for stabilizing first and second vertebral
bodies of the spinal column, the device comprising: an elongated
shape memory nickel titanium ligament having a transformation
temperature above body temperature and exhibiting ductile
characteristics during use, the ligament formed in the shape of a
rod with first and second ends; a first screw adapted to securely
fasten the first end of the rod to the first vertebral body; a
second screw presenting a plastic linear bearing for receiving the
second end of the rod and securing it in a slideably constrained
fashion to the second vertebral body, wherein ductile deformation
in the ligament allows slideably constrained motion between the
vertebral bodies.
10. The orthopedic device of claim 9 further comprising a dampening
member surrounding the rod and sandwiched between the first and
second screw.
11. The orthopedic device of claim 9 wherein the second end of the
rod has an abutment.
12. The orthopedic device of claim 11 further comprising a
dampening member oriented around the ligament and sandwiched
between the second screw and the abutment.
13. The orthopedic device of claim 9 wherein the bearing further
comprises a plastic material selected from polyethylene or
polyetheretherketone.
14. An orthopedic device for stabilizing first and second vertebral
bodies of the spinal column, the device comprising: an elongated
shape memory nickel titanium ligament having a transformation
temperature above body temperature and exhibiting ductile
characteristics during use, the ligament formed in the shape of a
rod with first and second end, the second end including an
abutment; a first screw adapted to securely fasten the first end of
the rod to the first vertebral body; a second screw presenting a
plastic linear bearing for receiving the second end of the rod and
securing it in a slideably constrained fashion to the second
vertebral body, a first dampening member surrounding the rod and
sandwiched between the first screw and the second screw, a second
dampening member surrounding the rod and sandwiched between the
second screw and the abutment, wherein ductile deformation in the
ligament allows slideably constrained motion between the vertebral
bodies.
Description
RELATED APPLICATION
[0001] This application is a continuation in part of U.S. Ser. No.
11/244,184 filed Oct. 5, 2005.
[0002] The present inventor has previously filed U.S. application
Ser. No. 11/244,184 entitled "Orthopedic Stabilization Device" on
Oct. 5, 2005 and provisional application 60/677,699 entitled
"Dynamic spine stabilization device" on May 4, 2005, the entire
disclosures of which are expressly incorporated by reference herein
and relied upon.
BACKGROUND OF THE INVENTION
[0003] 1. Technical Field of the Invention
[0004] The present invention relates to orthopedic stabilization
devices used to limit the relative motion of at least two vertebral
bodies for the relief of pain. These 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 without other
devices.
[0005] 2. Description of the Related Art
[0006] In the field of spine surgery there have been many attempts
to relieve pain associated with spinal injury or illness.
Traditionally surgeons have fused the vertebral bodies with a
pedicle screw and rod construct or a fusion cage. In attempting to
fuse the patient there is a long and painful recovery process. Most
rod and screw constructs and fusion cage constructs are very rigid,
not allowing transfer of stress into the fusion site that would
otherwise aid in a quicker recovery. These approaches defy Wolfe's
law stating: bone that is not stressed will degrade. As a
corollary, 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.
[0007] Many are working to develop devices that allow relative
motion, yet these have fallen short in preventing shear forces
between the vertebral bodies being stabilized. Another shortcoming
is that relative motion has been forcibly channeled through a
rather specific location or hinge point in the mechanical
construct. The following discussion more particularly summarizes
these efforts.
[0008] U.S. Pat. No. 5,092,866 (U.S. Re. 36,221) discloses a
pedicle screw system that is banded together with flexible
ligaments. While the ligaments allow for relative motion, they do
not appear to resist compression or shear loads, instead relying
upon tension alone.
[0009] European Patent No. EP 06691091 A1/B1 and the "DYNESYS"
product brochure disclose a polycarbonate/urethane supporting
element, compressed between two adjacent pedicle screws and passing
over an elastic strap that acts as a flexible internal ligament.
The flexible internal ligament is in the form of a nylon cord,
which is pre-tensioned and fastened to the screw heads. This design
provides flexural degrees of freedom, allows relative motion
between the vertebral bodies but does little to inhibit or prevent
shearing between the vertebral bodies. While flexibility is
desirable, the "DYNASES" ligament would appear to lack rigidity and
rely on proper tensioning inter-operatively to gain its
balance.
[0010] U.S. Pat. No. 6,267,764 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.
[0011] U.S. Pat. No. 6,241,730 discloses a construction that lacks
a ligament element, particularly a ligament extending through the
center of rod members. There is a compressible dampening element.
The '730 design attempts to accomplish a multidirectional
redistribution of force for aiding in quicker fusion rates, however
its constructs were not designed for use in conjunction with a disk
implant. The '730 approach overly limits motion of the vertebral
bodies to one location, i.e., forces motion unnaturally through a
hinge point.
[0012] U.S. Pat. Nos. 6,293,949 and 6,761,719 disclose embodiments
seeking to elastically constrain range of motion using a continuous
super-elastic nitinol rod and pedicle screw system. Due to the
super-elastic state of the rod, motion is always pushed-back to a
neutral, pre-set position. This constrains force through the rod in
a manner causing early fatigue failure. In order to provide the
correct elasticity of the rod, its diameter must be so small that
it cannot withstand the continuous loads. Further, the rod cannot
be bent at the time of surgery to a preformed shape holding the
vertebral bodies in a desired relative position while also limiting
their relative motion.
[0013] Accordingly there exists a need for assemblies and devices
that effectively resist torsion as well as shear forces while
providing flexible spine stabilization. More specifically, it would
be desirable to provide kits with such assemblies and devices,
which work with existing pedicle screw arrangements.
[0014] There is another need for flexible assemblies and devices
having rigid portions deformable to fit a patient's anatomical
contours while maintaining flexibility of the orthopedic
construct.
[0015] There is yet another need for assemblies and devices to
stabilize vertebrae while providing multi-directional flexibility,
without imparting elastic stresses to the bone.
[0016] There is a further need yet to provide a spine stabilization
device that can allow natural flexion and extension motion while
effectively restraining torsional and shear forces.
[0017] There is a further need to provide spine 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 OF THE INVENTION AND ADVANTAGES
[0018] According to one embodiment of the present invention, there
is provided an orthopedic device for stabilizing the spinal column
between anchorage locations on respective first and second
vertebral bodies. The device includes an elongated bridge having
first and second ends operatively connected at the respective
anchorage locations. The bridge contains an implantable ligament
selected to be inelastic at body temperature. The ligament is
further capable of continuous plastic deformation to allow relative
constrained motion while resisting forces exerted upon the
vertebral bodies. In a preferred embodiment, the bridge contains an
implantable nickel titanium alloy. In another preferred embodiment
the device further includes a dampening member surrounding at least
a portion of the ligament. In yet another preferred embodiment, the
ligament is in the form of a wire, tube, or band. In still another
preferred embodiment the device includes rigid rod members each
correspondingly retained with either end of the ligament, and
independently attached to the vertebral bodies with anchors. The
rigid rod members are correspondingly connected to either end of
the ligament. In still yet another preferred embodiment, the device
includes a plate segment retained with an end of the ligament and
independently attached to a vertebral body with the plurality of
anchors; more preferably, a plurality of plate segments are
correspondingly connected to either end of the ligament.
[0019] In another embodiment of the present invention, an
orthopedic device for stabilizing the spinal column includes an
elongated implantable ligament with two ends, the ligament
partially formed of an implantable nickel titanium alloy capable of
continuous plastic in-elastic deformation at body temperature.
Either end of the ligament is attached to a vertebral body with a
screw at an anchor location. A compression-dampening member
surrounds the ligament and is sandwiched between the screws.
Plastic deformation in the ligament allows relative constrained
motion between the vertebral bodies.
[0020] In yet another embodiment of the present invention, an
orthopedic device for stabilizing the spinal column is disclosed.
The device includes an implantable elongated ligament with two
enlarged end portions. The ligament is partially formed of a nickel
titanium alloy capable of continuous plastic in-elastic deformation
at body temperature. Two rigid rod members each contain a bore
sized for the ligament, the rigid rod members being retained with
either end of the ligament and engageable with two vertebral bodies
by a plurality of anchors. A compression-dampening member surrounds
the ligament and is sandwiched between the rods. Two
tension-dampening members are captured within the rigid rod bores,
surround the ligament and abut the enlarged end portions
respectively. Plastic deformation in the ligament allows relative
constrained motion between the vertebral bodies.
[0021] In still another embodiment of the present invention, a
surgical kit is disclosed. The kit includes at least one bone
anchor and a flexible spine stabilization device. The device
includes a ligament partially formed of an implantable nickel
titanium alloy capable of continuous plastic in-elastic deformation
at body temperature. In a preferred embodiment, the surgical kit
includes at least one rigid fusion rod. The anchor, ligament and
rigid fusion rod mentioned above are provided in various sizes to
accommodate a given patient's anatomy.
[0022] An orthopedic device for stabilizing first and second
vertebral bodies of the spinal column, the device comprising:
[0023] In a further embodiment an orthopedic spine stabilization
device is disclosed having an elongated ligament with two ends. The
ligament is manufactured to exhibit inelastic characteristics at
body temperature while further being capable of continuous plastic
deformation and can be in the form of a wire, rod, tube, cable,
band or plate. The device includes a first screw adapted to
securely fasten one end of the ligament to a vertebral body and a
second screw with a bearing for receiving the opposite end of the
ligament securing it in a mobile fashion to another vertebral body.
Plastic deformation in the ligament allows relative constrained
motion between the vertebral bodies.
[0024] In still yet a further embodiment of a spine stabilization
device is disclosed with an elongated shape memory nickel titanium
ligament having a transformation temperature above body
temperature. The nickel titanium ligament in the form of a rod
exhibits a ductile characteristic during use allowing motion. One
end of the rod is fixed to one vertebral body with a first screw.
The other end of the rod is secured to a second vertebral body with
a second screw containing a plastic linear bearing. As the body
moves the ductile nature of the ligament resists bending and shear
motions in the vertebral column while at the same time the rod
slides in a translational relationship to the second screw further
allowing flexion and extension motions.
[0025] In another preferred embodiment of the present invention an
orthopedic device for stabilizing the spinal column is shown. The
device includes an elongated shape memory nickel titanium ligament
having a transformation temperature above body temperature and
exhibiting ductile characteristics during use. The ligament is
formed in the shape of a rod with first and second ends and the
second end includes an abutment. The device also includes a first
screw adapted to securely fasten the first end of the rod to a
vertebral body and a second screw presenting a plastic linear
bearing for receiving the second end of the rod and securing it in
a slideably constrained fashion to the other vertebral body.
Surrounding the rod and sandwiched between the first and second
screw is one dampening member and a second dampening member is
found surrounding the rod and sandwiched between the second screw
and the abutment. As the body moves the ductile nature of the
ligament resists bending and shear motion in the vertebral column
while at the same time the rod can slideably translate in
relationship to the second screw allowing flexion and extension
motion. The dampening members act as cushions for flexion and
extension motions and controllably resist the sliding motion
between the ligament and the bearing.
[0026] An advantage of the present invention is a device that
limits the range of relative motion between two vertebral bodies
and works with existing pedicle screw assemblies.
[0027] Another advantage of the invention is to constrain the
motion between vertebral bodies in a ductile manor.
[0028] In still another advantage is to allow controlled flexion
and extension motions of the spine while constraining bending and
shear forces.
[0029] Another advantage of the invention is to provide a kit to
the surgeon that has a variety of pedicle screws, rigid fusion rods
and elongated implantable ductile ligaments. Further it is
desirable that the ligaments provide a variety of stiffness and
flexibility options so the surgeon can select the appropriate
stiffness and range of motion to achieve the desired surgical
result whether it is for aiding fusion or restoring normal range of
motion to a patient.
[0030] Other objects and advantages will become apparent to a
reader skilled in the art, with reference to the following Figures
and accompanying Detailed Description wherein textual reference
characters correspond to those denoted on the Drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] In the accompanying drawings:
[0032] FIG. 1 is an exploded perspective view of a flexible
inelastic spine stabilization device, according to the present
invention;
[0033] FIG. 1A is a perspective view of a representative plate
segment for securing the device of FIG. 1;
[0034] FIG. 2 is a perspective view of the device of FIG. 1, shown
in its assembled state;
[0035] FIG. 3 is an elevational view of the device of FIGS. 1 and
2, further including pedicle screws for attaching the device to
adjacent vertebral bodies as schematically shown;
[0036] FIG. 4 is an elevational view of the device of FIG. 3, upon
application of load;
[0037] FIG. 5 is a kit of spinal implant components including a
pedicle screw, a rigid fusion rod, and a ligament of the present
invention, selected from among various ranges of flexibility;
[0038] FIG. 6 is a top view of a device employing an elongated
implantable ligament attached to vertebral bodies (schematically
shown) with pedicle screws that directly secure the ligament
between the screws;
[0039] FIG. 7 is an elevational view of the device of FIG. 6;
[0040] FIG. 8 is an exploded perspective view of another device of
the present invention, employing a ligament surrounded by
compression and tension-damping members;
[0041] FIG. 9 is an elevational view of the assembled structures
shown in FIG. 8 prior to application of a load; and
[0042] FIG. 10 is a sectional view taken longitudinally along Lines
10-10 of FIG. 9.
[0043] FIG. 11 is an elevation view of the present invention
employing a ligament which is slideably constrained using a pedicle
screw and bearing.
[0044] FIG. 12 is a sectional view taken longitudinally along Lines
12-12 of FIG. 11.
[0045] FIG. 13 is a sectional view taken longitudinally along Lines
13-13 of FIG. 11 showing a pedicle screw with a bearing sleeve.
DETAILED DESCRIPTION OF THE INVENTION
[0046] With reference generally to FIGS. 1-13, the Applicant's
invention provides flexible spinal stabilization allowing
controlled relative vertebral motion for the relief of pain, while
preventing intervertebral shear forces. Moreover, the invention
evenly distributes mechanical stresses throughout its structure
rather than constraining motion within a limited portion of its
structure, by virtue of its distinctive design.
[0047] Referring to FIGS. 1 and 2-4, an elongated bridge member is
generally shown as an assembly at 10. Assembly 10 includes a
ligament 28 shown in the form of a wire. It will be understood that
the ligament 28 may also take the form of a tube, solid rod or a
band, having different cross sectional shapes and sizes. The
ligament 28 is made of an implantable material selected to be
inelastic at body temperature and allows relative constrained
motion while resisting bodily shear forces. The ligament 28 has
opposed first 30 and second 32 ends received within washer type
connectors 34, 36 that engage counter-bores 38, 40 formed within
rigid rod members 42, 44, respectively. Those in the art will
appreciate that the rigid rod members 42, 44 could have differing
sizes and/or lengths. Washer-shaped connectors 34, 36 are
preferably made of a shape-memory alloy in its super-elastic state
at body temperature. Alternatively other means for attaching the
in-elastic ligament 28 to rigid rod potions 42, 44 may include
welding, threading, gluing or crimping instead of using connectors
34, 36. Thus, assembly 10 operatively functions as a bridge between
a first anchor 20 and a second anchor 22, respectively. Ligament 28
is preferably made of an implantable shape memory alloy, more
preferably a nickel titanium alloy, which is selected to be
inelastic at body temperature. That is, ligament 28 is not in a
super-elastic state. Preferably, assembly 10 may include a
dampening member 46 that has an inner diameter 48 surrounding
ligament 28. Referring to FIGS. 3-4, first and second screw anchors
20, 22 are adapted for respectively affixing assembly 10 to first
and second vertebral bodies 24, 26.
[0048] Referring to FIG. 1A, a representative plate segment 12 has
openings 14 that receive anchoring screws for attachment to a
vertebral body not shown. Each plate segment 12 has a passageway 16
configured to receive one end of a ligament not shown. It will be
understood that the ligament used in conjunction with the plate 12
could have a variety of forms as elucidated in the above discussion
of FIGS. 1 and 2-4. Passageway 16 could have a rectangular cross
section as shown, or could have a variety of forms for receiving
the ligament. Preferably a plurality of plates 12 can be employed
with the ligament, across a corresponding plurality of vertebral
bodies to form a bridge similar to assembly 10 in FIGS. 1 and
24.
[0049] Referring to FIGS. 3-4, plastic deformation in ligament 28
is in response to external stimulus indicated by arrows 54, 54,
which for the sake of illustration is shown as direct uniform axial
compression. However, as will be appreciated, the external stimulus
often consists of combined bending and twisting motions of a
patient's body.
[0050] With continuing reference to FIGS. 1-4, the present assembly
10 resists shear forces exerted between vertebral bodies 24, 26
during the bending and twisting motions of a patient without
creating elastic forces that otherwise would exert unnatural
stresses forcing the vertebral bodies back into some prior
position. The present invention instead allows the body's own
motion to return it to the natural position without undue elastic
impetus. This natural return to body position is therefore distinct
from prior approaches that rely upon super-elastic members such as
those discussed above; moreover, the present invention is distinct
from prior approaches that do not resist both shear and direct
torsional movements while yet bending themselves. The present
assembly 10 does not exert resultant forces that are opposite to
the motion input 54, 54 and yet the assembly is repetitively
plastically deformable due to the material and design employed
herein.
[0051] FIG. 5 depicts still another embodiment of the present
invention, that is, a surgical kit generally shown at 58. Kit 58
includes an array of bone anchors 60 and an elongated bridge
assembly 10 preferably of the type shown in FIGS. 1-4 although it
will be appreciated that an array of assemblies having various
sizes and stiffness can be provided. The assembly 10 is capable of
continuous plastic in-elastic deformation at body temperature. In a
preferred embodiment, the surgical kit 58 includes an array of
semi-rigid fusion rods such as the representative rod shown at 62.
In another preferred embodiment, an alternative surgical kit not
shown may include an array of plates similar to those described in
conjunction with FIG. 1A. The arrays mentioned above are provided
in various sizes to accommodate a given patient's anatomy.
[0052] Referring to FIGS. 6-7, an orthopedic device 110 for
stabilizing the spinal column includes an elongated bridge member
that takes the form of ligament 128 instead of the assembly 10 as
previously discussed in conjunction with FIGS. 1 and 2-4. It will
be understood that the ligament 128 may also take the form of a
tube, solid rod or a band, having different cross sectional shapes
and sizes. The ligament 128 is at least partially made of an
implantable material that is preferably a nickel titanium alloy
capable of continuous plastic in-elastic deformation at body
temperature in similar fashion as ligament 28 (FIG. 1). Device 110
has no distinct rigid rod members as depicted, for example, at 42
and 44 in FIGS. 1 and 2-4. Nor are there any plate segments as at
12 in FIG. 1A to operatively anchor ligament 28. Instead, ligament
128 extends between and directly interconnects screws 120, 122,
which affix it to vertebral bodies 124 and 126. An optional
compression-dampening member 146 is shown surrounding ligament 126
and is sandwiched between the screw heads 150, 152 in FIGS. 6-7.
Plastic deformation in the ligament 128 allows relative motion
while preventing shear stresses between vertebral bodies 124, 126.
FIGS. 6-7 show a tubular dampening member 146 preferably made of an
in implantable elastomer such as silicone or polycarbonate
urethane, through which elongated ligament 128 passes.
[0053] Referring to FIGS. 8-10 there is yet another embodiment of
the present invention. An elongated bridge member is shown in the
form of an assembly 210, for stabilizing the spinal column.
Assembly 210 includes an elongated ligament 228 with an enlarged
fixed end portion 229 and a free end 231. Ligament 228 is at least
partially formed of an implantable inelastic material, preferably
nickel titanium alloy capable of continuous plastic in-elastic
deformation at body temperature, i.e., not in a super-elastic
state. Bore 240 of rigid rod member 244 is sized for passage of
ligament 228, the rod members 242, 244 are retained with ends 229,
231 of the ligament for attachment at respective anchorage
locations to vertebral bodies (not shown). Those in the art will
appreciate that the rigid rod members 242, 244 could have differing
sizes and/or lengths. A compression-dampening member 246 has a bore
247 that surrounds ligament 228 and is sandwiched between
proximally chamfered rigid rod members 242, 244. Tension-dampening
members 248, 250 have respective bores 252, 254 sized to allow
passage of ligament 228 housed within the bores of the rigid rods
242, 244. Tension-dampening members 248, 250 surround ligament 228
and are respectively captured by rigid rods 242, 244 along with
enlarged ends 229, 256 as shown in FIG. 10. Plastic deformation in
ligament 228 allows relative constrained motion between, while
resisting shear forces exerted upon the vertebral bodies. Motion is
transmitted along the entire length of ligament 228 from its
enlarged fixed ends 229, 256. Dampening members 246, 248, 250 are
preferably made of an implantable elastomer such as silicone or
polycarbonate urethane.
[0054] Inelastic ligament 28, 128, 228 is preferably manufactured
from a nickel titanium alloy preferably having a diameter in the
range of 3-6 mm. Other cross sectional shapes and sizes of ligament
28, 128, 228 may be made available for different surgical
applications. Nickel Titanium can be alloyed to have varying
properties, some alloys exhibiting super-elastic behavior at body
temperature while other alloys are continuously in-elastic at body
temperature. These inelastic alloys are commonly referred to as
shape memory alloys by those skilled in the art. Shape memory
alloys may further have transition temperatures either above or
below body temperature; however, the applicable transition
temperature for the present invention is selected to be higher than
body temperature. The present inventor has determined that within
the operative size range, in-elastic ligaments 28, 128, 228 made
from shape memory alloys having a transition temperature above body
temperature, exhibit acceptable fatigue resistance. This is because
there are no elastic forces exerted by the ligament 28, 128, 228 of
the present invention, against the body. It is intended that a
surgeon determine how much in-elastic resistance is necessary for
each individual patient's needs and then pre-selects an assembly or
device 10, 110, 210 at the time of surgery to ensure the best
resistance. Preferably the ligament 28, 128, 228 is non-braided and
is formed as a unitary contiguous member enabling the ligament to
resist shear forces. In the instance where a surgeon may be
supplementing fixation of two vertebral bodies 24, 26 and 124, 126
with a fusion cage (not shown), a flexible inelastic assembly or
device 10, 110, 210 with pedicle screws 20, 22 and 120, 122 is
preferable to limit motion and allow stress transfer through the
fusion site in accordance to Wolfe's law. In this instance the
surgeon would select a less flexible assembly or device 10, 110,
210 with a larger inelastic ligament 28, 128, 228 such as 5-6 mm in
diameter. The diameter and length of the inelastic ligament 28,
128, 228 determine the flexibility of the surgical construct. In
another instance a surgeon may selectively remove the facia from
two adjacent vertebral bodies 24, 26 and 124, 126 to eliminate
arthritis caused by bony contact at the facia. To replace the
support for the vertebral bodies after the faciaectomy the surgeon
would use a flexible inelastic assembly or device 10, 110, 210 with
pedicle screws 20, 22 and 120, 122 to ensure that axial spacing
between posterior segments (not shown) of vertebral bodies 24, 26
and 124, 126 is maintained. In this instance it would be preferable
for the surgeon to select a more flexible assembly or device 10,
110, 210 that has an inelastic ligament 28, 128, 228 with a
diameter closer to 3-4 mm. This surgical construct would allow a
patient to have constrained motion but would limit contact between
the facia of the two vertebral bodies 24, 26 and 124, 126. The
rigid rod portions 42, 44 and 242, 244 are typically manufactured
from stainless steel or titanium and are preferably in the diameter
range of 4-7 mm. This size range is typical of other commercially
available spinal implant hardware so that flexible inelastic
assembly or device 10, 110, 210 of the present type is universally
received by existing pedicle screws 20, 22 and 120, 122.
[0055] FIGS. 3-7 illustrate use of pedicle screws 20, 22 and 120,
122 to fasten the flexible assembly 10 or device 110 to vertebral
bodies 24, 26 or 124, 126. However, other attachment means are
possible as well as a variety of alternate locations for mounting.
In a traditional spinal stabilization system the rods are placed
posterior to and on either side of the spinous process. Depending
on the pathology observed, a surgeon might select a unitary
flexible ligament 128 or assembly 10 to mount posterior to the
theoretical centerline of a patient between two spinous processes.
Or alternatively a flexible inelastic ligament 128 or assembly 10
may be placed on the anterior side of the vertebral bodies 24, 26;
124, 126.
[0056] Referring to FIGS. 11-13 an embodiment is shown with a
device 310 allowing the spine to bend under dynamic constraint. As
seen in FIGS. 11-12 a first pedicle screw 320 is designed to be
mounted in a first vertebral body (not shown). The pedicle screw
320 has a head portion 350 which is designed to receive the
ligament 328 securely with a set screw 355 or any other locking
mechanisms that lock the ligament 328 to the screw 320. The
ligament 328 is being shown in the form of a rod, however it is
important to realize that the form of the ligament is not as
important as its' mechanical characteristics and could be made in
the shape of wire, tube, cable, band or plate. Preferably the
ligament 328 should be made from a material that can withstand
repeated cyclical loading without failing and should have a ductile
nature while at body temperature. Nickel and Titanium or Nickel
Titanium as it is referred to can be alloyed to result in a
material property that has this ductility which can also be
classified as having an in-elastic behavior with continuous plastic
deformation. Nickel Titanium is known to be manufactured in two
general categories. The first is super-elastic; these alloys have
an elastic behavior at body temperature but for this application
reapply unwanted stresses into the vertebral column during motion
and are undesirable. The additional stresses also lead to lower
fatigue resistance during use. The second category of Nickel
Titanium is classified as having a shape memory characteristic. The
temperature at which the material will exhibit the memory
characteristics is set during the manufacturing process and this
temperature is often referred to as the transition temperature at
which a phase transformation between martensite and austenite
occur. For this application it is desirable to set the transition
temperature above body temperature. It is known that the higher the
transition temperature of the material the higher the fatigue
resistance. So, below the transition temperature the ligament 328
can be bent with restraint and takes on a ductile nature allowing
it to be reshaped on a continuous basis without fatiguing allowing
it to support the mobile spinal column. In FIGS. 11 and 13 a second
pedicle screw 322 is shown for adaptation to another vertebral body
(not shown), however both pedicle screws 320, 322 could also be
used to treat a fracture within one vertebral body when the bone is
fractured or cut into two or more fragments. The second pedicle
screw 322 is adapted with a bearing 360 which can be manufactured
from any known implantable bearing material such as plastic, metal
or ceramic. If a plastic were selected polyethylene or
polyetheretherketone materials have shown good characteristics as a
bearing material in orthopedic devices. The bearing 360 can be
manufactured as an integral part of the pedicle screw 322 for
instance as a simple hole (not shown) drilled through the head 352,
or the bearing can be mounted with a set screw 357 as shown. The
bearing 360 can be in the form of a ring, washer, ball or any other
bearing that will allow the ligament 328 to be received and allow
relative movement between the ligament and the pedicle screw 322
during use. As can be appreciated the bearing 360 could be fully
closed or split to accommodate the relative motion and could be
used to receive other rods known in the art. For instance titanium
alloy rods 62 shown in the kit 58 in FIG. 5 used for fusion could
be received within the bearing 360 to allow slight relative
movement between the pedicle screws 320, 322. It is contemplated
that other bridge members such as plastic rods currently under
development could also be used in conjunction with the bearings 360
and this description should not be limiting in nature. The ligament
328 can be manufactured to have an abutment 329 and the ligament
can receive optional dampening members 346, 347. While the bearing
360 allows relative movement between the pedicle screws 320, 322 in
flexion and extension of the spinal column the optional dampening
members 346, 347 are useful for additional constraint. The first
dampening member 346 can be sandwiched between the head 350 of the
first pedicle screw 320 and the second head 352 of pedicle screw
322. This first dampening member 346 is used to constrain motion
while the spinal column is in extension. The second dampening
member 347 surrounds the ligament 328 and is sandwiched between the
second head 352 of the pedicle screw 322 and the abutment 329. The
second dampening member 347 can be used to constrain motion in
flexion.
[0057] The present invention is by no means restricted to the above
described preferred embodiments, but covers all variations that
might be implemented by using equivalent functional elements or
devices that would be apparent to a person skilled in the art, or
modifications that fall within the spirit and scope of the appended
claims.
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