Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEMS AND METHODS FOR SPINE STABILIZATION
BACKGROUND OF THE INVENTION
1. Technical Field
The present disclosure relates to advantageous methods and apparatus for
spinal
stabilization. More particularly, the present disclosure relates to methods
and apparatus for
providing dynamic stabilization to the spine so as to provide clinically
efficacious results.
2. Background Art
Low back pain is one of the most expensive diseases afflicting industrialized
societies. With the exception of the common cold, it accounts for more doctor
visits than any
other ailment. The spectrum of low back pain is wide, ranging from periods of
intense
disabling pain which resolve to varying degrees of chronic pain. The
conservative treatments
available for lower back pain include: cold packs, physical therapy,
narcotics, steroids and
chiropractic maneuvers. Once a patient has exhausted all conservative therapy,
the surgical
options generally range from micro discectomy, a relatively minor procedure to
relieve
pressure on the nerve root and spinal cord, to fusion, which takes away spinal
motion at the
level of pain.
Each year, over 200,000 patients undergo lumbar fusion surgery in the United
States.
While fusion is effective about seventy percent of the time, there are
consequences even to
these successful procedures, including a reduced range of motion and an
increased load
transfer to adjacent levels of the spine, which may accelerate degeneration at
those levels.
Further, a significant number of back-pain patients, estimated to exceed seven
million in the
U.S., simply endure chronic low-back pain, rather than risk procedures that
may not be
appropriate or effective in alleviating their symptoms.
New treatment modalities, collectively called motion preservation devices, are
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New treatment modalities, collectively called motion preservation devices, are
currently being developed to address these limitations. Some promising
therapies are in the
form of nucleus, disc or facet replacements. Other motion preservation devices
provide
dynamic internal stabilization of the injured and/or degenerated spine, e.g.,
the Dynesys
stabilization system (Zimmer, Inc.; Warsaw, IN) and the Graf Ligament. A major
goal of this
concept is the stabilization of the spine to prevent pain while preserving
near normal spinal
function. The primary difference in the two types of motion preservation
devices is that
replacement devices are utilized with the goal of replacing degenerated
anatomical structures
which facilitate motion while dynamic internal stabilization devices are
utilized with the goal
of stabilizing and controlling abnormal spinal motion.
Over ten years ago a hypothesis of low back pain was presented in which the
spinal
system was conceptualized as consisting of the spinal column (vertebrae, discs
and
ligaments), the muscles surrounding the spinal column, and a neuromuscular
control unit
which helps stabilize the spine during various activities of daily living.
Panjabi MM. "The
stabilizing system of the spine. Part I. Function, dysfunction, adaptation,
and enhancement."
.T Spiiaal Disord 5 (4): 383-389, 1992a. A corollary of this hypothesis was
that strong spinal
muscles are needed when a spine is injured or degenerated. This was especially
true while
standing in neutral posture. Panjabi MM. "The stabilizing system of the spine.
Part II.
Neutral zone and instability hypothesis." .I Spiraal Disord 5 (4): 390-397,
1992b. In other
words, a low-back patient needs to have sufficient well-coordinated muscle
forces,
strengthening and training the muscles where necessary, so they provide
maximum protection
while standing in neutral posture.
Dynamic stabilization (non-fusion) devices need certain functionality in order
to
assist the compromised (injured or degenerated with diminished mechanical
integrity) spine
of a back patient. Specifically, the devices must provide mechanical
assistance to the
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compromised spine, especially in the neutral zone where it is needed most. The
"neutral
zone" refers to a region of low spinal stiffness or the toe-region of the
Moment-Rotation
curve of the spinal segment (see Figure 1). Panjabi MM, Goel VK, Takata K.
1981 Volvo
Award in Biomechanics. "Physiological Strains in Lumbar Spinal Ligaments, an
in vitro
Biomechanical Study." Spirae 7 (3): 192-203, 1982. The neutral zone is
commonly defined
as the central part of the range of motion around the neutral posture where
the soft tissues of
the spine and the facet joints provide least resistance to spinal motion.
This concept may be visualized with reference to load-displacement or moment-
rotation curves for an intact spine and an injured spine, as shown in Figure
1. The curves are
non-linear; that is, the spine mechanical properties change with the amount of
angulations
and/or rotation. If the curves on the positive and negative sides are
understood to represent
spinal behavior in flexion and extension, respectively, then the slope of the
curve at each
point represents spinal stiffness. As seen in Figure 1, the neutral zone is
the low stiffness
region of the range of motion.
Experiments have shown that after an injury to the spinal column and/or
degeneration
of the spine, neutral zones, as well as ranges of motion, increase (see Figure
1). However,
the neutral zone increases to a greater extent than does the range of motion,
when described
as a percentage of the corresponding intact values. This implies that the
neutral zone may be
a better measure of spinal injury and instability than the range of motion.
Clinical studies
have also found that range of motion does not correlate well with low back
pain. Therefore,
an unstable spine needs to be stabilized, especially in the neutral zone.
With the foregoing in mind, those skilled in the art will understand that a
need exists
for spinal stabilization devices, systems and/or methods that overcome the
shortcomings of
prior art devices, systems and methods. The present invention provides
devices, systems and
methods for enhanced and efficacious spinal stabilization. More particularly,
the present
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disclosure provides advantageous dynamic internal stabilization devices,
systems and
methods that are flexible so as to move with the spine, thus allowing the
disc, the facet joints,
and the ligaments normal (or improved) physiological motion and loads
necessary for
maintaining their nutritional well-being. The devices, systems and methods of
the present
disclosure also advantageously accommodate different physical characteristics
of individual
patients and anatomies to achieve a desired and/or improved posture for each
individual
patient.
SUMMARY OF THE PRESENT DISCLOSURE
According to the present disclosure, advantageous devices, systems and methods
for
spinal stabilization are provided. According to preferred embodiments of the
present
disclosure, the disclosed devices, systems and methods provide dynamic
stabilization to the
spine so as to provide clinically efficacious results. In addition, the
disclosed devices,
systems and methods offer clinical advantages, including ease of installation,
versatility/flexibility in application, and superior clinical results for
individuals encountering
lower back pain and other spine-related difficulties.
According to exemplary implementations of the present disclosure, devices,
systems
and methods are provided that encompass one or more pedicle screws for
attachment to
spinal structures. The pedicle screw(s) of the present disclosure are
typically employed as
part of a spine stabilization system that includes one or more of the
following advantageous
structural and/or functional attributes:
= A dynamic junction between at least one pedicle screw and at least one
elongated
member (or multiple elongated members), e.g., rod(s), that engage and/or
otherwise cooperate with the pedicle screw;
= Advantageous assembly mechanisms that facilitate assembly/installation of a
ball/sphere or other accessory component relative to the pedicle screw and
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provide advantageous functional attributes as part of a spinal stabilization
system.
Exemplary mechanisms include advantageous collet-based mechanisms,
cooperatively threaded mechanisms, mechanisms that apply bearing forces
against
a ball/sphere or other accessory component, and/or mechanisms that include a
snap ring or analogous structure;
= Advantageous multi-level dynamic spine stabilization
systems/implementations,
including multi-level systems that permit one or more adjustments to be made
(e.g., in situ and/or prior to clinical installation), e.g., adjustments as to
the
magnitude and/or displacement-response characteristics of the forces applied
by
the stabilization system; according to exemplary multi-level implementations
of
the present disclosure, different stabilization modalities may be employed at
individual stabilization levels, e.g., by mixing of dynamic and non-dynamic
stabilizing structures between adjacent pedicle screws at different
stabilization
levels;
= Advantageous installation accessories (e.g., cone structures) for
facilitating
placement/installation of spine stabilization system components, such
accessories
being particularly adapted for use with conventional guidewire(s) to
facilitate
alignment/positioning of system components relative to the pedicle screw;
= Dynamic stabilization systems and/or other surgical implants that include a
cover
and/or sheath structure that provides advantageous protection to inner force-
imparting component(s), e.g., one or more springs, while exhibiting clinically
acceptable interaction with surrounding anatomical fluids and/or structures,
e.g., a
cover and/or sheath structure that is fabricated (in whole or in part) from
ePTFE,
UHMWPE and/or alternative polymeric materials such as polycarbonate-
polyurethane copolymers and/or blends;
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= Advantageous dynamic spine stabilization connection systems that facilitate
substantially rigid attachment of an elongated member (e.g., a rod) relative
to the
pedicle screw while simultaneously facilitating movement relative to adjacent
structures (e.g., an adjacent pedicle screw) to permit easy and efficacious
intra-
operative system placement;
= An advantageous "pre-load" arrangement for a securing structure (e.g., a set
screw) that may be used in situ to mount a ball joint or other accessory
component
relative to the pedicle screw, thereby minimizing the potential for clinical
difficulties associated with location and/or alignment of such securing
structure(s).
As noted above, advantageous spine stabilization devices, systems and methods
may
incorporate one or more of the foregoing structural and/or functional
attributes. Thus, it is
contemplated that a system, device and/or method may utilize only one of the
advantageous
structures/functions set forth above, a plurality of the advantageous
structures/functions
described herein, or all of the foregoing structures/functions, without
departing from the spirit
or scope of the present disclosure. Stated differently, each of the structures
and functions
described herein is believed to offer benefits, e.g., clinical advantages to
clinicians and/or
patients, whether used alone or in combination with others of the disclosed
structures/functions.
Generally, the structures/funetions of the threaded shaft portions of the
pedicle screws
disclosed herein are of conventional design. Thus, installation of the pedicle
screws is
generally undertaken by a clinician in a conventional manner. Selection and
placement of the
pedicle screws is generally based on conventional criteria, as are known to
persons skilled in
the art. However, unlike conventional pedicle-screw based systems, the
devices, systems,
and methods of the present disclosure offer advantageous clinical results,
e.g., based on ease
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and flexibility of rod/elongated member placement, dynamic attributes of the
rod/elongated
member in situ relative to the pedicle screws, and/or dynamic force delivery
in response to
spinal displacement stimulus.
Additional advantageous features and functions associated with the devices,
systems
and methods of the present disclosure will be apparent to persons skilled in
the art from the
detailed description which follows, particularly when read in conjunction with
the figures
appended hereto. Such additional features and functions, including the
structural and
mechanistic characteristics associated therewith, are expressly encompassed
within the scope
of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
To assist those of ordinary skill in the art in making and using the disclosed
devices,
systems and methods for spinal stabilization and other applications, reference
is made to the
appended figures wherein:
Figure 1 is a Moment-Rotation curve for a spinal segment (intact and injured),
showing relatively low spinal stiffness within the neutral zone.
Figure 2 is a schematic representation of a spinal segment in conjunction with
a
Moment-Rotation curve for a spinal segment, showing relatively low spinal
stiffness within
the neutral zone. ,
Figure 3a is a schematic representation of an exemplary device/system
according to
the present disclosure in conjunction with a Force-Displacement curve,
demonstrating
increased resistance provided within the central zone of a dynamic spine
stabilizer according
to the present disclosure.
Figure 3b is a Force-Displacement curve demonstrating a change in profile
achieved
througli replacement of springs according to an exemplary embodiment of the
present
disclosure.
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Figure 3c is a posterior or dorsal view of the spine with a pair of exemplary
stabilizers
secured thereto.
Figure 3d is a lateral or side view showing an exemplary stabilizer according
to the
present disclosure in tension.
Figure 3e is a lateral or side view showing an exemplary stabilizer according
to the
present disclosure in compression.
Figure 4 is a schematic representation of an exemplary dynamic spine
stabilizer
according to the present disclosure.
Figure 5 is a schematic representation of an alternate exemplary embodiment of
a
qynamic spine stabilizer in accordance with one aspect of the present
disclosure.
Figure 6 is a Moment-Rotation curve demonstrating the manner in which an
exemplary dynamic spine stabilizer according to the present disclosure assists
spinal
stabilization.
Figures 7a and 7b are, respectively, a free body diagram of an exemplary
dynamic
stabilizer according to the present disclosure and a diagram representing the
central zone of
such exemplary stabilizer.
Figure 8 is an exploded view of an exemplary dynamic spine stabilization
system in
accordance with an embodiment of the present disclosure.
Figure 9 is a perspective view of the exemplary dynamic spine stabilization
system
shown in Figure 8.
Figures 10 and 11 are perspective views showing exemplary attachment members
for
use with dynamic spine stabilizations of the present disclosure.
Figure 12 is a schematic representation showing a guidewire assembly technique
in
accordance with an exemplary implementation of the spine stabilization
techniques of the
present disclosure.
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Figures 13 is a schematic side view of a pair of pedicle screws according to
an
exemplary embodiment of the present disclosure.
Figure 14 is a side view of a pair of pedicle screws in combination with
guidewire
assemblies according to an exemplary embodiment of the present disclosure.
Figures 15a is a perspective view of an attachment member that is adapted to
facilitate
alignment with elongated member(s), e.g., rod(s), according to exemplary
embodiments of
the present disclosure.
Figure 15b is a side view of a spherical element for use in an attachment
member
according to an exemplary embodiment of the present disclosure.
Figure 16 is a top view of a pair of single level spinal stabilization systems
according
to an exemplary embodiment of the present disclosure.
Figure 17 is an illustrative Force-Displacement curve for an exemplary dynamic
spine
stabilization system according to the present disclosure.
Figure 18 is a schematic top view of an exemplary multiple level, dynamic
spine
stabilization system in accordance with an implementation of the present
disclosure.
Figure 19 is a schematic, exploded side view of a portion of the exemplary
dynamic
spine stabilization system of Figure 18.
Figure 20 is a schematic side view of an aspect of the exemplary dynamic spine
stabilization system of Figure 18.
Figure 21 is a perspective view of the exeniplary multiple level, dynamic
spine
stabilization system of Figures 18 to 20.
Figure 22 is a further perspective view of the exemplary multiple level,
dynamic spine
stabilization system of Figure 19.
Figure 23 is a side view of exemplary portions of a pedicle screw/ball joint
subassembly (partially exploded) according to the present disclosure.
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Figures 24a, 24b and 24c are views of an alternative collet-based mechanism
according to the present disclosure;
Figures 25a, 25b and 25c are views of a non-spreading collet-based mechanism
according to the present disclosure;
Figures 26a, 26b and 26c are views of a further alternative mechanism for
mounting a
ball/sphere relative to a pedicle screw according to the present disclosure;
Figure 27 is a cross-sectional side view an additional alternative mechanism
for
mounting a ball/sphere relative to a pedicle screw according to the present
disclosure.
Figure 28 is a perspective view of an exemplary socket member and spring cap
according to an exemplary embodiment of the present disclosure.
Figure 29 is an exploded view of an alternative dynamic junction between a
pedicle
screw and accessory component(s) according to the present disclosure.
Figure 30 is a perspective view of a spring cap rod according to an exemplary
embodiment of the present disclosure.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
The present disclosure provides advantageous devices, systems and methods for
spinal
stabilization and/or alternative surgical implant applications. More
particularly, the present
disclosure provides devices, systems and methods that deliver dynamic
stabilization to the
spine so as to provide clinically efficacious results. The exemplary
embodiments disclosed
lierein are illustrative of the advantageous spine stabilization systems and
surgical implants of
the present disclosure, and methods/techniques for implementation thereof. It
should be
understood, however, that the disclosed embodiments are merely exemplary of
the present
invention, which may be embodied in various forms. Therefore, the details
disclosed herein
with reference to exemplary dynamic spinal stabilization systems and
associated
methods/techniques are not to be interpreted as limiting, but merely as the
basis for teaching
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one skilled in the art how to make and/or use the advantageous dynamic spinal
stabilization
systems and alternative surgical implants of the present disclosure.
With reference to Figures 2, 3a-e and 4, an exemplary method and apparatus for
spinal stabilization are disclosed. Although the description which follows is
primarily
directed to spinal stabilization, it is expressly contemplated that the
disclosed methods and
apparatus may be advantageously employed in alternative surgical applications,
e.g., any
long bone application. Thus, throughout the detailed disclosure which follows,
it is to be
understood that references and teachings with respect to spinal stabilization
are merely
illustrative and that the disclosed systems, devices and methods find
application in a
multitude of a surgical/anatomical settings, including specifically long bone
applications
involving the femur, tibia, fibula, ulna, and/or humerus.
In accordance with an exemplary embodiment of the present disclosure, the
spinal
stabilization method is achieved by securing an internal dynamic spine
stabilizing member 10
between adjacent vertebrae 12, 14, thereby providing mechanical assistance in
the form of
elastic resistance to the region of the spine to which the dynamic spine
stabilizing member 10
is attached. The elastic resistance is applied as a function of displacement
such that greater
stiffness, i.e., greater incremental resistance, is provided while the spine
is in its neutral zone
and lesser mechanical stiffness, i.e., lesser incremental resistance, is
provided while the spine
bends beyond its neutral zone. Although the term elastic resistance is
generally used
throughout the body of the present specification, other forms of resistance
may be employed
without departing from the spirit of the present invention.
As those skilled in the art will certainly appreciate, and as mentioned above,
the
"neutral zone" is understood to refer to a region of low spinal stiffness or
the toe-region of
the Moment-Rotation curve of the spinal segment (see Figure 2). That is, the
neutral zone
may be considered to refer to a region of laxity around the neutral resting
position of a spinal
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segment where there is minimal resistance to inter-vertebral motion. The range
of the neutral
zone is considered to be of major significance in determining spinal
stability. Panjabi, MM.
"The stabilizing system of the spine. Part II. Neutral zone and instability
hypothesis." J
Spinal Disorders 1992; 5(4): 390-397.
In fact, Dr. Panjabi (a presently named inventor) has previously described the
load
displacement curve associated with spinal stability through the use of a "ball
in a bowl"
analogy. According to this analogy, the shape of the bowl indicates spinal
stability. A deeper
bowl represents a more stable spine, while a more shallow bowl represents a
less stable spine.
Dr. Panjabi previously hypothesized that for someone without spinal injury
there is a normal
neutral zone (that part of the range of motion where there is minimal
resistance to inter-
vertebral motion) with a normal range of motion and, in turn, no spinal pain.
In this instance,
the bowl is not too deep nor too shallow. However, when an injury occurs to an
anatomical
structure associated with the spine, the neutral zone of the spinal column
increases and "the
ball" moves freely over a larger distance. By the noted analogy, the bowl
would be shallower
and the ball less stable; consequently, pain would result from the enlarged
neutral zone.
In general, pedicle screws 16, 18 are used to attach the dynamic spine
stabilizing
member 10 to the vertebrae 12, 14 of the spine using well-tolerated and
familiar surgical
procedures known to those skilled in the art. The pedicle screws 16, 18 in
combination with
a dynamic spine stabilizing member 10 comprise a stabilizing system 11. In
accordance with
an exemplary embodiment, and as those skilled in the art will certainly
appreciate, paired
stabilizing systems 11 are commonly used to balance the loads applied to the
spine (see
Figure 3c). The dynamic spine stabilizing members 10 assist the compromised
(injured
and/or degenerated) spine of a back-pain patient, and help her/him perform
daily activities.
The dynamic spine stabilizing member 10 does so as part of stabilizing system
11 by
providing controlled resistance to spinal motion, particularly around neutral
posture in the
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region of neutral zone. As the spine bends forward (flexion) the stabilizing
member 10 is
tensioned (see Figure 3d) and when the spine bends backward (extension) the
stabilizing
member 10 is compressed (see Figure 3e).
The resistance to displacement provided by the dynamic spine stabilizing
member 10
is non-linear, being greatest in its central zone so as to correspond to the
individual's neutral
zone; that is, the central zone of the stabilizing member 10 provides a high
level of
mechanical assistance in supporting the spine. As the individual moves beyond
the neutral
zone, the increase in resistance decreases to a more moderate level. As a
result, the
individual encounters greater resistance to movement (or greater incremental
resistance)
while moving within the neutral zone.
The central zone of the dynamic spine stabilization system 11, that is, the
range of
motion in which the spine stabilization system 11 provides the greatest
incremental resistance
to movement, may be adjustable at the time of surgery according to exemplary
embodiments
of the present disclosure to suit the neutral zone of each individual patient.
Thus, according
to exemplary embodiments of the present disclosure, the resistance to movement
provided by
the dynamic spine stabilizing member 10 is adjustable pre-operatively and/or
intra-
operatively. This adjustability helps to tailor the mechanical properties of
the dynamic spine
stabilizing system 11 to suit the compromised spine of the individual patient.
In addition,
according to exemplary embodiments of the present disclosure, the length of
the dynamic
spine stabilizer 10 may also (or alternatively) be adjustable intra-
operatively to suit
individual patient anatomy and to achieve desired spinal posture. In such
exemplary
embodiments, the dynamic spine stabilizing element 10 can be re-adjusted post-
operatively
with a surgical procedure to adjust its central zone, e.g., to accommodate a
patient's altered
needs.
With reference to Figure 4, ball joints 36, 38 may be employed according to
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exemplary embodiments of the present disclosure to link or otherwise join the
dynamic spine
stabilizing member 10 with pedicle screws 16, 18. The junction of the dynamic
spine
stabilizing member 10 and pedicle screws 16, 18 is free and rotationally
unconstrained.
Thus, three rotational degrees of freedom are provided by advantageous dynamic
junctions
according to the present disclosure. Alternative structural arrangements are
contemplated to
provide the desired rotational degrees of freedom of the disclosed dynamic
joints, e.g.,
universal joint structures of the type disclosed in Figure 29 and discussed
herein below. The
structures mounted with respect to the pedicle screw that support or
accommodate motion
relative to the pedicle screw, e.g., the disclosed spherical elements and
universal joint
mechanisms, are exemplary motion interface elements according to the present
disclosure.
Therefore, first of all, by providing the dynamic junctions of the present
disclosure, the spine
is allowed all physiological motions of bending and twisting and, second, the
dynamic spine
stabilizing member 10 and pedicle screws 16, 18 are protected from potentially
harmful
bending and/or torsional forces, or moments. As previously stated, while ball
joints are
disclosed in accordance with an exemplary embodiment of the present
disclosure, the present
disclosure is not limited to use of one or more ball joints, and other linking
structures/mechanisms may be utilized without departing from the spirit or
scope of the
present disclosure.
As there are ball joints 36, 38 mechanically cooperating with each end of the
stabilizing member 10 according to the exemplary embodiment of Figure 4,
bending
moments are generally not transferred from the spine to the stabilizing member
10 within
stabilizing system 11. Further, it is important to recognize that the only
forces associated
with operation of stabilizing member 10 are the forces due to the forces of
springs 30, 32 that
form part of stabilizing member 10. These forces are solely dependent upon the
tension
and/or compression of the stabilizing member 10 as determined by spinal
motion. In
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summary, the forces associated with operation of stabilizing member 10 are
limited to the
spring forces. Irrespective of the large loads on the spine, such as when a
person carries or
lifts a heavy load, the loads experienced by stabilizing member 10 are only
associated with
the spring forces developed within stabilizing member 10, which are the result
of spinal
motion and not the result of the spinal load. The stabilizing member 10 is,
therefore,
uniquely able to assist the spine without enduring the high loads of the
spine, allowing a wide
range of design options.
The loading of the pedicle screws 16, 18 in the presently disclosed
stabilizing system
11 is also quite different from that in prior art pedicle screw fixation
devices. The only load
experienced by the pedicle screws 16, 18 of stabilizing system 11 is the force
delivered by
the stabilizing member 10 which translates into pure axial force at the ball
joint-screw
interface. The design and operation of the disclosed stabilizing system 11
thus greatly
reduces the bending moments placed onto pedicle screws 16, 18, as compared to
prior art
pedicle screw fusion systems. Due to the free motion associated with ball
joints 36, 38, the
bending moment within each pedicle screw 16, 18 is theoretically zero at ball
joints 36, 38,
respectively, and the potential for failure is therefore advantageously
reduced. In sum, the
pedicle screws 16, 18, when used as part of the exemplary dynaniic spine
stabilization
systems of the present disclosure, carry significantly less load and are
placed under
significantly less stress than typical pedicle screws.
In Figure 2, the Moment-Rotation curve for a healthy spine is shown in
configurations
with an exemplary stabilizing member 10 as part of a dynamic spine stabilizing
system. This
curve shows the low resistance to movement encountered in the neutral zone of
a healthy
spine. However, when the spine is injured, this curve changes and the spine
becomes
unstable, as evidenced by the expansion of the neutral zone (see Figure 1).
In accordance with exemplary embodiments of the present disclosure, people
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suffering from spinal injuries are best treated through devices, systems and
methods that
provide increased mechanical assistance in the neutral zone. As the spine
moves beyond the
neutral zone, the necessary mechanical assistance decreases and becomes more
moderate. In
particular, and with reference to Figure 3a, an exemplary support profile
contemplated
through implementation of advantageously disclosed devices, systems and
methods is
depicted.
Three different profiles are shown in Figure 3a. The disclosed profiles are
merely
exemplary and demonstrate the possible support requirements within the neutral
zone.
Profile 1 is exemplary of an individual requiring great assistance in the
neutral zone and the
central zone of the stabilizing system of the present disclosure is therefore
increased,
providing a high level of resistance over a great displacement; Profile 2 is
exemplary of an
individual where less assistance is required in the neutral zone and the
central zone of the
stabilizing system of the present disclosure is therefore more moderate,
providing increased
resistance over a more limited range of displacement; and Profile 3 is
exemplary of situations
where only slightly greater assistance is required in the neutral zone and the
central zone of
the stabilizing system of the present disclosure may therefore be decreased to
provide
increased resistance over even a smaller range of displacement.
As those skilled in the art will certainly appreciate, the mechanical
assistance required
and the range of the neutral zone will vary from individual to individual.
However, the basic
tenet of the present invention remains; that is, greater mechanical assistance
for those
individuals suffering from spinal instability is required within the
individual's neutral zone.
This assistance is provided in the form of greater resistance to movement
provided within the
neutral zone of the individual and the central zone of the dynamic spine
stabilizing member
10 which advantageously forms part of a dynamic spine stabilizing system.
Exemplary dynamic spine stabilizing member 10 of the present disclosure
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advantageously provides mechanical assistance in accordance with the desired
support
profile. Further, exemplary embodiments of dynamic spine stabilizing member 10
provide
for adjustability, e.g., via a concentric spring design. More specifically and
with reference to
exemplary embodiments of the present disclosure, spine stabilizing system 10
provides
assistance to the compromised spine in the form of increased stiffness, i.e.,
greater
incremental resistance to movement (provided by springs in accordance with a
preferred
embodiment) as the spine moves from the neutral posture, in any physiological
direction. As
mentioned above, the Force-Displacement relationship provided by exemplary
stabilizing
system 10 and dynamic spine stabilizing member 10 are non-linear, with greater
incremental
resistance around the neutral zone of the spine and central zone of the
stabilizing system 11,
and decreasing incremental resistance beyond the central zone of the dynamic
spine
stabilizing system 11 as the individual moves beyond the neutral zone (see
Figure 3a).
The relationship of the present stabilizing system 11 to forces applied during
tension
and compression is further shown with reference to Figure 3a. As discussed
above, the
behavior of the present stabilizing system 11 is non-linear. The Load-
Displacement curve
has three zones: tension, central and compression. If Kl and K2 define the
stiffness values in
the tension and compression zones, respectively, the advantageous stabilizing
systems
according to the present disclosure are designed such that high stiffiness is
delivered in the
central zone, i.e., "Kl + K2". Depending upon the "preload" of stabilizing
member 10, as
discussed below in greater detail, the width of the central zone and,
therefore, the region of
high stiffness, can be adjusted.
With reference to Figure 4, an exemplary dynamic spine stabilizing system 11
that
includes a dynamic spine stabilizing member 10 in accordance with the present
disclosure is
schematically depicted. Dynamic spine stabilizing system 11 includes a support
assembly
associated with spine stabilizing member 10 in the form of a housing 20
composed of a first
17
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housing member 22 and a second housing member 24. The first housing member 22
and the
second housing member 24 are telescopically connected via external threads
formed upon the
open end 26 of the first housing member 22 and internal threads formed upon
the open end
28 of the second housing member 24. In this way, the housing 20 is completed
by screwing
the first housing member 22 into the second housing member 24. As such, and as
will be
discussed below in greater detail, the relative distance between the first
housing member 22
and the second housing member 24 can be readily adjusted for the purpose of
adjusting the
compression of first spring 30 and second spring,32 contained within the
housing 20.
Although springs are employed in accordance with a preferred embodiment of the
present
invention, other elastic members may be employed without departing from the
spirit or scope
of the present invention. A piston assembly 341inks the first spring 30 and
the second spring
32 relative to first and second ball joints 36, 38. The first and second ball
joints 36, 38 are in
turn shaped and designed for selective attachment to pedicle screws 16, 18,
which may
extend from the respective vertebrae 12, 14 (as shown, e.g., in Figure 2).
The first ball joint 36 is secured relative to the closed end 39 of the first
housing
member 22 via a threaded engagement member 40 that is shaped and dimensioned
for
coupling with first housing member 22. According to an exemplary embodiment of
the
present disclosure, an aperture 42 is formed in the closed end 39 of the first
housing member
22 and is provided with threads for engaging the threaded portion of
engagement member 40.
In this way, the first ball joint 36 substantially closes off the closed end
39 of the first
housing member 22. The length of dynamic spine stabilizing system 11 may be
readily
adjusted by rotating the first ball joint 36 relative to first housing member
22 to adjust the
extent of overlap between the first housing member 22 and the engagement
member 40 of the
first ball joint 36, i.e., the degree to which engagement member 40 is nested
within first
housing member 22. As those skilled in the art will certainly appreciate, a
threaded
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engagement between the first housing member 22 and the engagement member 40 of
the first
ball joint 36 is disclosed in accordance with an exemplary embodiment of the
present
disclosure, although other coupling structures (e.g., welding attachment, a
bayonet lock or the
like) may be employed without departing from the spirit or scope of the
present invention.
In an exemplary embodiment of the present disclosure, the closed end 44 of the
second housing member 24 is provided with a cap 46 having an aperture 48
formed therein.
As will be discussed below in greater detail, the aperture 48 is shaped and
dimensioned to
accommodate passage of a piston rod 50 associated with piston assembly 34
therethrough.
Exemplary piston assembly 34 includes a piston rod 50; first and second
springs 30, 32; and
retaining rods 52. The piston rod 50 includes a stop nut 54 and an enlarged
head 56 at its
first end 58. The enlarged head 56 is rigidly connected to the piston rod 50
and includes
guide holes 60 through which the retaining rods 52 extend during operation of
the present
dynamic spine stabilizing member 10. As such, the enlarged head 56 is guided
along the
retaining rods 52 while the second ball joint 38 moves toward and away from
the first ball
joint 36, i.e., in connection with relative motion between first and second
ball joints 36, 38.
As will be discussed below in greater detail, the enlarged head 56 interacts
with the first
spring 30 to create resistance as the dynamic spine stabilizing member 10 is
extended and the
spine is moved in flexion.
A stop nut 54 is fit over the piston rod 50 for free movement relative
thereto.
However, movement of the stop nut 54 toward the first ball joint 36 is
prevented by the
retaining rods 52 that support the stop nut 54 and prevent the stop nut 54
from moving toward
the first ball joint 36. As will be discussed below in greater detail, the
stop nut 54 interacts
with the second spring 32 to create resistance as the dynamic spine
stabilizing member 10 is
compressed and the spine is moved in extension.
The second end 62 of the piston rod 50 extends from the aperture 48 at the
closed end
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44 of the second housing member 24, and is attached to an engagement member 64
associated with the second ball joint 38. In an exemplary embodiment of the
present
disclosure, the second end 62 of the piston rod 50 is coupled to the
engagement member 64
of the second ball joint 38 via a threaded engagement. As those skilled in the
art will
certainly appreciate, a threaded engagement between the second end 62 of the
piston rod 50
and the engagement member 64 of the second ball joint 38 is disclosed in
accordance with an
exemplary embodiment, although other coupling structures may be employed
without
departing from the spirit or scope of the present invention.
As briefly mentioned above, first and second springs 30, 32 are held or
captured
within housing 20. In particular, the first spring 30 extends between the
enlarged head 56 of
the piston rod 50 and the cap 46 of the second housing member 24. The second
spring 32
extends between the distal end of the engagement member 64 of the second ball
joint 38 and
the stop nut 54 of the piston rod 50. A preloaded force applied by the first
and second
springs 30, 32 generally holds the piston rod in a static position within the
housing 20, and
the piston rod 50 is able to move relative to housing 20 during either
extension or flexion of
the spine.
In use, when the vertebrae 12, 14 are moved in flexion and the first ball
joint 36 is
drawn away from the second ball joint 38, i.e., there is relative motion
between first and
second ball joints 36, 38 such that they are moving away from each other, the
piston rod 50 is
pulled within the housing 24 against the force being applied by the first
spring 30. In
particular, the enlarged head 56 of the piston rod 50 is moved toward the
closed end 44 of the
second housing member 24. This movement causes compression of the first spring
30,
creating resistance to the movement of the spine. With regard to the second
spring 32, the
second spring 32 moves with the piston rod 50 away from second ball joint 38.
As the
vertebrae move in flexion within the neutral zone, the height of the second
spring 32 is
CA 02571573 2006-12-18
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increased, reducing the distractive force, and in effect increasing the
resistance of the device
to movement. Through this mechanism, as the spine moves in flexion from the
initial
position both spring 30 and spring 32 resist the distraction of the device
directly, either by
increasing the load within the spring (i.e. first spring 30) or by decreasing
the load assisting
the motion (i.e. second spring 32).
However, when the spine is in extension, and the second ball joint 38 is moved
toward the first ball joint 36, the engagement member 64 of the second ball
joint 38 moves
toward the stop nut 54, which is held in place by the retaining rods 52 as the
piston rod 50
moves toward the first ball joint 36. This movement causes compression of the
second spring
32 held between the engagement member 64 of the second ball joint 38 and the
stop nut 54,
to create resistance to the movement within the dynamic spine stabilizing
member 10. With
regard to the first spring 30, the first spring 30 is supported between the
cap 46 and the
enlarged head 56, and as the vertebrae move in extension within the neutral
zone, the height
of the second spring 30 is increased, reducing the compressive force, and in
effect increasing
the resistance of the device to movement. Through this mechanism, as the spine
moves in
extension from the initial position both spring 32 and spring 30 resist the
compression of the
device directly, either by increasing the load within the spring (i.e. second
spring 32) or by
decreasing the load assisting the motion (i.e. first spring 30).
Based upon the use of two concentrically positioned elastic springs 30, 32 as
disclosed in accordance with an exemplary embodiment of the present invention,
an
assistance (force) profile as shown in Figure 2 is provided by the present
dynamic spine
stabilizing member 10. That is, the first and second springs 30, 32 work in
conjunction to
provide a large elastic force when the dynamic spine stabilizing member 10 is
displaced
within the central zone of the stabilizing system 11. However, once
displacement between
the first ball joint 36 and the second ball joint 38 extends beyond the
central zone of the
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stabilizing system 11 and the neutral zone of the individual's spinal
movement, the
incremental resistance to motion is substantially reduced as the individual no
longer requires
the substantial assistance needed within the neutral zone. This is
accomplished by setting the
central zone of the device disclosed herein. The central zone of the force
displacement curve
is the area of the curve, which represents when both springs are acting in the
device as
described above. When the motion of the spine is outside the neutral zone and
the correlating
device elongation or compression is outside the set central zone, the spring,
which is
elongating, reaches its free length. Free length, as anybody skilled in the
art will appreciate,
is the length of a spring when no force is applied. In the advantageous,
exemplary
mechanism of the present disclosure, the resistance to movement of the device
outside the
central zone (where both springs are acting to resist motion) is only reliant
on the resistance
of one spring: either spring 30 in flexion or spring 32 in extension.
As briefly discussed above, exemplary dynamic spine stabilizing member 10 may
be
adjusted by rotation of the first housing member 22 relative to the second
housing member
24. This movement changes the distance between the first housing member 22 and
the
second housing member 24 in a manner which ultimately changes the preload
placed across
the first and second springs 30, 32. This change in preload alters the
resistance profile of the
present dynamic spine stabilizing member 10 from that shown in Profile 2 of
Figure 3a to an
increase in preload (see Profile 1 of Figure 3a), which enlarges the effective
range in which
the first and second springs 30, 32 act in unison. This increased width of the
central zone of
the stabilizing member 10 correlates to higher stiffness over a larger range
of motion of the
spine. This effect can be reversed, as is evident in Profile 3 of Figure 3a.
The present dynamic spine stabilizing member 10 is attached to pedicle screws
16, 18
extending from the vertebral section requiring support. During surgical
attachment of the
dynamic spine stabilizing member 10, the magnitude of the stabilizer's central
zone can be
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adjusted for each individual patient according to exemplary embodiments of the
present
disclosure, as judged by the surgeon and/or quantified by an instability
measurement device.
This adjustable feature of the dynamic spine stabilizing member 10 is
exemplified in the
three explanatory profiles that have been generated in accordance with an
exemplary
embodiment of the present invention (see Figured 3a and 3b; note the width of
the device
central zones).
Pre-operatively, the first and second elastic springs 30, 32 of the dynamic
spine
stabilizing member 10 can be replaced by a different set of springs (in whole
or in part) to
accommodate a wider range of spinal instabilities. As expressed in Figure 3b,
Profile 2b
demonstrates the force displacement curve generated with a stiffer set of
springs when
compared with the curve shown in Profile 2a of Figure 3b.
Intra-operatively, the length of exemplary dynamic spine stabilizing member 10
may
be adjustable, e.g., by turning engagement member 40 of the first ball joint
36 to lengthen the
stabilizing member 10 in order to accommodate different patient anatomies and
desired
spinal posture. Pre-operatively, the piston rod 50 may be replaced with piston
rods of
differing lengths/geometries to accommodate an even wider range of anatomic
variation.
The exemplary dynainic spine stabilizing member 10 disclosed herein has been
tested
alone for its load-displacement relationship. When applying tension, the
dynamic spine
stabilizing member 10 demonstrated increasing resistance up to a pre-defined
displacement,
followed by a reduced rate of increasing resistance until the device reached
its fully elongated
position. When subjected to compression, the dynamic spine stabilizing member
10
demonstrated increasing resistance up to a pre-defined displacement, followed
by a reduced
rate of increasing resistance until the device reached its fully compressed
position.
Therefore, the dynamic spine stabilizing member 10 exhibits a load-
displacement curve that
is non-linear with the greatest resistance to displacement offered around the
neutral posture.
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This advantageous behavior helps to normalize the load-displacement curve of a
compromised spine.
In another exemplary embodiment of the present disclosure, with reference to
Figure
5, the stabilizing member 110 may be constructed with an in-line spring
arrangement. In
accordance with this embodiment, the housing 120 is composed of first and
second housing
members 122, 124 which are coupled with threads allowing for adjustability. A
first ball
joint 136 extends from or relative to the first housing member 122. The second
housing
member 124 is provided with an aperture 148 through which the second end 162
of piston
rod 150 extends. The second end 162 of the piston rod 150 is attached relative
to the second
ball joint 138. For example, the second ball joint 138 may be screwed onto the
piston rod
150.
The piston rod 150 includes an enlarged head 156 at its first end 158. The
first and
second springs 130, 132 are respectively secured between the enlarged head 156
and the
closed ends 139, 144 of the first and second housing members 122, 124. In this
way, the
stabilizing member 110 provides resistance to both expansion and compression
using the
same mechanical principles described for the previous embodiment, i.e.,
stabilizing membe'r
10.
Adjustment of the resistance profile in accordance with this alternate
embodiment
may be achieved by rotating the first housing member 122 relative to the
second housing
member 124. Rotation in this way alters the central zone of high resistance
provided by
stabilizing member 110. As previously described, one or both springs may also
be
exchanged to change the slope of the force-displacement curve in two or three
zones,
respectively.
To explain how the exemplary stabilizing members 10, 110 assist a compromised
spine (increased support in the neutral zone), reference is made to the moment-
rotation
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curves (Figure 6). Four curves are shown: 1. Intact, 2. Injured, 3. Stabilizer
("DSS") and, 4.
Injured + Stabilizer ("DSS"). These are, respectively, the Moment-Rotation
curves of the
intact spine, injured spine, stabilizer alone, and stabilizer plus injured
spine. Of note, the
latter curve (i.e., injured spine plus stabilizing system of the present
disclosure) is close to the
intact curve. Thus, the stabilizer/stabilizing system of the present
disclosure, which provides
greater resistance to movement around the neutral posture, is well suited to
compensate for
the instability of the spine.
With reference to Figures 8 to 17, further embodiments of the advantageous
stabilizing system 211 of the present disclosure (and associated force profile
characteristics)
are schematically depicted and described herein. This exemplary stabilizing
system 211
includes first and second concentric springs 212, 214 as part of stabilizing
member 210 that is
positioned between first and second pedicle screws 216, 218, as generally
shown in the
exploded view of Figure 8. As those skilled in the art will appreciate, the
springs that are
incorporated in stabilizing member 210 may take a variety of forms known to
those skilled in
the art, for example, machine springs, wire coil springs, wave springs, and
the like, without
departing from the spirit or scope of present the invention. In addition, it
is contemplated that
other resistance devices may be incorporated in stabilizing member 210, for
example,
elastomeric materials and/or elastomeric structures, Belleville washers, and
the like (such
alternative resistance devices being used alone or in combination with the
foregoing springs),
without departing from the spirit or scope of the present invention.
Stabilizing system 211 generally defines a first end 220 and a second end 222.
The
schematic depiction of Fig. 8 includes a pair of pedicle screws (216, 218),
but it is to be
understood that the "first end" and/or the "second end" may form intermediate
locations, with
additional pedicle screw and/or stabilizing members positioned therebeyond.
Toward the
first end 220, a first attachment member 224 is provided that is configured
and dimensioned
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to receive a first ball (or spherical elenient) 262a to define a first ball
joint 226 that
accommodates relative movement between the first attachment member 224 and
pedicle
screw 216. Indeed, the dynamic junction formed at ball joint 226
advantageously provides
three rotational degrees of freedom. Toward the second end 222 of stabilizing
system 211, a
second attachment member 228 is provided that is configured and dimensioned to
receive a
second ball (or spherical element) 262b to define a second ball joint 230. The
second ball
joint advantageously accommodates relative movement between the second
attachment
member 228 and pedicle screw 218, i.e., defines a dynamic junction that
provides three
rotational degrees of freedom.
In the exemplary embodiment of Fig. 8, ball joints 226, 230 include a socket
232, 234
formed integrally with the respective first and second attachment members 224,
228 and a
ball or sphere 236, 238 positioned therein. Of course, sockets 232, 234 maybe
fabricated as
separate components from first and second attachment members 224, 228 without
departing
from the spirit or scope of the present disclosure. In implementations wherein
the sockets are
fabricated separately from the attachment members, appropriate mechanisms for
joining/connection such sub-assemblies may be employed, e.g., welded
connections, threaded
engagements, bayonet locking mechanisms or the like.
According to the exemplary embodiment of Figs. 8-17, the first attachment
member
224 is structured for supporting the inner first spring 212 for operation in
accordance with the
present stabilizing system 211. As best seen in Figs. 16 and 28, the first
attachment member
224 includes a body member 240 having an aperture 242 extending therethrough.
The inner
surface of aperture 242 defines socket 232 and is shaped and dimensioned for
receipt of ball
(or spherical element) 236. The assembly of the ball/spherical element is
achieved by
rotating the ball 90 degrees off of the normal position of the ball relative
to socket 232. At
this position the ball/spherical element can slide through two opposed slots
232a cut in the
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internal spherical race of the socket. In exemplary embodiments of the present
disclosure,
the opposed slots are substantially arcuate and extend for a distance that
accommodates the
height of the spherical element. Once positioned within the socket, the
ball/spherical element
is generally rotated relative to the socket to prevent disengagement
therefrom. Indeed, once
assembled onto the pedicle screw, there is no possibility of the balUspherical
element coming
disassembled from the internal spherical race formed in the socket member. In
exemplary
embodiments of the present disclosure, aperture 242 is sized such that
ball/spherical element
236 engages socket 232 at or near a plane that defines the diameter of
ball/spherical element
236. In this way, ball/spherical element 236 is centrally positioned relative
to socket 232 and
is not permitted to pass through socket 232. The inner first spring 212
extends from, and in
an exemplary embodiment is integrally formed with, the body member 240 of the
first
attachment member 224.
The second attachment member 228 similarly includes a body member 244 having
an
aperture 246 extending therethrough. The inner surface of the aperture 246
defines a socket
234 that is shaped and dimensioned for receipt of the bal1238. Thus, in
exemplary
embodiments, socket 234 includes opposed slots to accommodate introduction of
a
ball/spherical element, as described above with reference to socket 232. As
with the
dimensional relationship between bal1236 and socket 232, aperture 246 is
advantageously
dimensioned such that ba11238 is engaged by socket 232 at or near a plane that
defines the
diameter of ba11238 (and ball 238 is not permitted to pass through socket
232). The second
attachment member 228 further includes a rod connector 248 with a transverse
aperture or
channe1250 extending therethrough. The transverse aperture or channe1250 is
shaped and
dimensioned for passage of spring cap rod 252 therethrough. The spring cap rod
252 is
secured within the transverse aperture 250, e.g., via a set screw 254
extending through a
threaded aperture that provides a channel from the external surface of the rod
connector 248
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and the transverse aperture/channe1250 within which is positioned spring cap
rod 252.
In accordance with an alternate embodiment, and with reference to Figure 10,
set
screw 254' interacts with a wedge member 249'. The wedge member 249' is seated
within
transverse aperture/channe1250' and is shaped and dimensioned for engaging the
spring cap
rod 252 as it passes through the transverse aperture/channe1250'. More
particularly, the
wedge member 249' includes an exposed arcuate surface that is shaped and
dimensioned to
interact with spring cap rod 252' to substantially prevent movement of the
spring cap rod
relative to the second attachment member 228' when set screw 254' is tightened
against
wedge member 249'.
With reference to Figures 11, 15a and 15b, a further alternative structural
arrangement
for securing a spring cap rod relative to an attachment member according to
the present
disclosure is schematically depicted. The structural arrangement of Figs. 11,
15a and 15b
may be particularly advantageous when it is desirable to provide flexible
loading of the
spring cap rod within the attachment member. The alternate embodiment of Figs.
11, 15a and
15b employs a selectively rotatable ba11249" within transverse
aperture/channe1250"
defined in attachment member 228". The bal1249" includes a transverse
compression slot
251" extending therethrough. A plurality of internal grooves 253 opening into
opening 255
are also formed in bal1249" to further facilitate gripping of a spring cap rod
252" positioned
therewithin, as described in greater detail below. Of note, opening 255 formed
in bal1249"
and shown in Fig. 15b is advantageously elliptical in geometry, with a minor
axis "Y" and a
major axis "Z". Compression slot 251" is substantially aligned with the minor
axis "Y" and
grooves 253 are deployed in an arcuate manner in facing relation to
compression slot 251",
i.e., on the opposite side of opening 255.
In use, after an element is positioned within opening 255, e.g., an elongated
member
such as a rod, a mechanism (e.g., set screw 254") is used to apply a force to
the exterior of
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ball 249". The force is advantageously applied to ball 249" in substantial
alignment with the
major axis "Z" of elliptical opening 255. As force is applied to the outer
surface of ball 249",
the elliptical opening 255 is deformed and assumes a circular (or
substantially circular)
geometry. Deformation into a circular geometry is facilitated by the
positioning of
compression slot 251" and grooves 253 relative to opening 255. Indeed, the
positioning of
compression slot 251" and grooves 253 acconunodates preferential deformation
of ball 249"
to a desired circular (or substantially circular) opening 255. By assuming a
circular/substantially circular geometry, the inner wall of ball 249" around
opening 255
engages an elongated member/rod of circular cross section around substantially
the entire
circumference of the elongated member/rod., By engaging the elongated
member/rod around
substantially the entire circumference thereof, greater security is imparted
between the ball
and the elongated member/rod.
Thus, the slot 251" and grooves 253 allow the ball 249" to be compressed and
deformed to a limited degree by force imparted by the set screw 254", thereby
locking the
ball 249" and spring cap rod 252" in position within the transverse
aperture/channel 250".
The ball 249" allows the spring cap rod 252" to extend therethrough while the
orientation of
the ball 249" and spring cap rod 252" relative to the second attachment member
228" is
adjusted to a desired orientation. Stated differently, ball 249" has three
degrees of rotational
freedom within aperture/channel 250" such that the ball 249" can be oriented
at essentially
any angle to acconunodate alignment with spring cap rod 252" (or another
elongated
member/rod), thereby greatly enhancing the ease and flexibility of assembly
associated with a
spinal stabilization system. Indeed, a rod positioned within bal1249" is
generally trimmed-
to-length by a clinician/surgeon once assembled with an attachment member; if
trimmed very
close to the exiting edge of ball 249", the ball/rod combination will exhibit
essentially 180 of
rotational freedom relative to attachment member 228". High degrees/levels of
angulation,
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as are accommodated by the exemplary embodiments disclosed herein, are
generally
advantageous in clinical applications. The combination of ba11249" with
aperture/channel
250" of attachment member 228" may be termed a "ball-in-a-box." Once the
desired
orientation is achieved for the rod relative to other components of a spinal
stabilization
system, the set screw 254" may be tightened and the assembly is thereby locked
in position.
With further reference to Figure 8, the first and second attachnient members
224, 228
are adapted to be mounted upon pedicle screws 216, 218. Each of the pedicle
screws 216,
218 includes a proximal end 256 and a distal end 258 (inasmuch as the first
and second
pedicle screws 216, 218 in the exemplary embodiment depicted herein are
identical, the same
numeric designations will be used in describing both pedicle screws; however,
it is
contemplated that pedicle screws having differing structural and/or functional
features may
be incorporated into stabilizing system implementations according to the
present disclosure
without departing from the spirit or scope hereof). The distal end 258
includes traditional
threading adapted for secure attachment along the spinal column of an
individual. According
to exemplary embodiments of the present disclosure and with further reference
to Figure 23,
the proximal end 256 of pedicle screw 216 is provided with a collet 260 that
is sized for
receipt in a substantially cylindrical receiving aperture/channe1262a formed
within
ball/spherical element 236.
Collet 260 is fabricated and/or formed with an ability to expand and contract,
e.g.,
under the control of medical practitioner(s) involved in using stabilizing
system 211.
Exemplary collet 260 includes a plurality of upstanding segments 264 that are
arranged in a
substantially arcuate manner around a central cavity 266, i.e., around the
periphery of central
cavity 266. Adjacent upstanding segments 264 are separated by a slot or
channel 265. As
shown in Figure 23, slot 265 may define an enlarged, substantially circular
region 265a at a
base thereof. In exemplary embodiments of the present disclosure, circular
region 265a
CA 02571573 2006-12-18
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further facilitates relative movement of adjacent upstanding segments 264.
With further reference to Figures 8 and 23, exemplary collet 260 defines three
(3)
upstanding segments 264 that are substantially identical in
geometry/dimension, although
alternative numbers, spacings and/or arrangements of upstanding segments 264
may be
utilized and/or employed without departing from the spirit or scope of the
present disclosure.
As will be explained below in greater detail, the upstanding segments 264 are
adapted for
movement between: (i) an expanded (or outwardly deflected) state for locking
collet 260
within a receiving channel 262a, 262b of a ball/spherical element 236, 238 and
(ii) an
unexpanded (or rest) state wherein the collet 260 may be selectively inserted
or removed
from a receiving channe1262a, 262b of a ball/spherical element 236, 238. Of
note, the
"expanded state" is generally not associated with a fixed or predetermined
degree of
expansion, but rather is generally defined by the level of expansion (i.e.,
outward deflection)
required to achieve a desired frictional engagement between collet 260 and
ball/spherical
element 236, 238.
According to exemplary embodiments of the present disclosure, each of the
receiving
channels 262a, 262b of the respective balls/spherical elements 236, 238 is
configured and
dimensioned for receiving a collet 260 associated with a pedicle screw 216,
218 while in its
unexpanded (or substantially unexpanded) state. Retention of the collet 260
may be further
enhanced by the provision of a lip 268 at (or adjacent) the distal or upper
end of upstanding
segments 264 of collet 260. A lip 268 is generally formed on each upstanding
segment 264,
e.g., during the molding or machining of collet 260, and generally extends
around the
available perimeter of collet 260. Each of the receiving channels 262a, 262b
generally
includes first and second chamfered regions at opposite ends thereof. The
chamfered regions
facilitate alignment and connection of components of the disclosed stabilizing
system, e.g.,
interaction between pedicle screws 216, 218 and balls/spherical elements 236,
238. To
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facilitate flexibility in use of the disclosed stabilizing system,
balls/spherical elements 236,
238 are generally symmetric around or relative to a mid-plane (designated by
phantom line
"MP" in Figure 23). Accordingly, the chamfered regions at either end of
receiving channels
262a, 262b are substantially identical in geometry and dimension.
As noted above, lips 268 are formed on the outer walls of upstanding segments
264
and are advantageously configured and dimensioned to cooperate with the
chamfered regions
of receiving channels 262a, 262b. Thus, once collet 260 is extended through a
receiving
channel 262a, 262b, the lips 268 associated with upstanding segments 264 are
generally
positioned in a chamfered region associated with the receiving channe1262a,
262b.
Frictional interaction between the lips 268 and the chamfered face of the
receiving channel
262a, 262b generally helps to maintain relative positioning of the collet 260
and the receiving
channel 262a, 262b, e.g., both before and after expansion of the collet 260 as
described
herein.
According to exemplary embodiments of the present disclosure, structural
features
and/or elements are provided on ball/spherical element 236, 238 and/or collet
260 to facilitate
interaction with one or more tools, e.g., tools for securing a ball/spherical
element 236, 238
relative to a pedicle screw 216, 218 and/or other components associated with
stabilizing
system 211. With reference to the exemplary system of Figures 8 and 23,
alignment tabs or
cut-outs 270, 272 are formed in upstanding segments 264 for tool interaction.
The alignment
tabs/cut-outs 270, 272 shown in Figure 23 have a substantially L-shaped
geometry, although
alternative geometries may be employed to accommodate specific tool designs
and/or tool
interactions. In the exemplary embodiment of Figures 8 and 23, a tool (not
pictured) may
advantageously interact with adjacent alignment tabs/cut-outs 270, 272, e.g.,
through
arcuately arranged gripping extensions that are spaced, configured and
dimensioned to
engage/cooperate with adjacent alignment tabs/cut-outs. As noted above,
balls/spherical
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elements 236, 238 are generally symmetric relative to a mid-plane ("MP") and
the disclosed
alignment tabs/cut-outs 270, 272 are typically formed at both ends of
balls/spherical elements
236, 238. Indeed, the provision of alignment tabs/cut-outs 270, 272 on both
ends of
balls/spherical elements 236, 238 advantageously facilitates the mounting of a
ba11236, 238
in either orientation without sacrificing functionality/interactivity, e.g.,
interaction with an
ancillary tool or the like. According to exemplary embodiments of the present
disclosure,
complementary notches 271 may be formed in balls 236, 238 to facilitate tool
interaction.
Notches 271 are generally spaced around the periphery of ba11262a, 262b, and
may be
brought into alignment with cut-outs 270, 272, e.g., by rotational
reorientation of ba11262a,
262b relative to collet 260, by a tool (not shown) in connection with tool-
related
manipulation thereof. Also, there can be geometry and/or structure on the
pedicle screw
which is configured to interact with the cut-outs on the ball/spherical
element to
automatically orient and provide rotational stability to allow for counter
torque, e.g., when
fixing the ball/spherical element relative to the pedicle screw.
Expansion of the exemplary collet 260 associated with pedicle screw 216, 218
may be
achieved by the insertion of a set screw 274 within the central aperture 266
defined within
upstanding segments 264 of collet 260. In accordance with an exemplary
embodiment, set
screw 274 is secured within the central aperture 266 via mating threads formed
along the
inner surface of the central aperture 266 and the outer surface of the set
screw 274. Set screw
274 generally includes an outwardly tapered portion 274a, e.g., at or adjacent
the non-
threaded end thereof, which is configured and dimensioned to engage upstanding
segments
264 of collet 260 as screw 274 is threaded relative to pedicle screw 216, 218.
Thus, as set
screw 274 moves downwardly within the central aperture 266, the upstanding
segments 264
are contacted by the outwardly tapered portion 274a of screw 274 and are
forced/deflected
outwardly. Outward deflection of upstanding segments 264 increases the
effective diameter
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of the collet 260, increasing (or establishing) interference contact between
the outer surface
of collet 260 and the inner wall of receiving channel 262a, 262b. By further
insertion of set
screw 274, collet 260 may be brought into locking engagement with the
receiving channel
262a, 262b of ball/spherical element 236, 238. As noted previously, lips 268
may be
provided on the outer surface of upstanding segments 264 to, ifater alia,
enhance the
"locking" forces imparted by collet 260.
With reference to Figures 24a, 24b and 24c, an alternative collet-based system
for
securing or mounting a ball/spherical element relative to a pedicle screw
according to the
present disclosure is depicted. The collet-based system of Figs. 24a-24c is
similar to the
system depicted in Figure 23. However, in the system of Figs. 24a-24c, an
internal snap ring
273 is provided that is configured to cooperate with an external ring groove
277 formed in
the outer wall of upstanding segments 264 and an internal ring groove 279
formed in
ball/sphere 236. Snap ring 273 defines a partial circle, with opening 273a
facilitating
expansion of the diameter of snap ring 273. Typically, snap ring 273 is
fabricated from an
appropriate metallic material, e.g., titanium or stainless steel, that
provides a desired degree
of elasticity. The depths of external and internal ring grooves 277, 279,
respectively, are
generally selected to ensure seating of snap ring 273.
In use, snap ring 273 is typically positioned in the internal groove formed in
the
ball/spherical element and essentially "snaps" into place with the outer
groove formed in the
collet, i.e., when the components reach the desired alignment. This "snap"
connection
between the ball/spherical element and the collet/pedicle screw allows the
clinician to take
appropriate steps to more permanently secure the components relative to each
other (e.g.,
locate and position appropriate tools) without risk that the components will
become
misaligned. Thus, the snap ring advantageously aligns with and partially nests
within both
ring grooves 277, 279, thereby providing a further engagement between
ball/sphere 236. As
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set screw 274 is screwed into place, the upstanding segments 264 deflect
outward, thereby
providing a greater engagement between ball/sphere 236 and pedicle screw 216.
In
alternative embodiment hereof, the snap ring may be initially positioned on
the outer surface
of the collet (i.e., in the outer groove), in which case the snap ring "snaps"
into the inner
groove of the ball/spherical alignment when the desired alignment is achieved.
Of note, with a snap ring included in the disclosed assembly, the collet is no
longer
required to deform both inwardly and outwardly. The function of the lip on the
collet may be
replaced by the snap ring which separates the function of the temporary snap
fit and final
securement. Due to this separation of mechanical function imparted by snap
ring 273, the
depth of slots/channels 265 may be reduced in the exemplary embodiment of
Figs. 24a-24c
relative to the embodiment of Fig. 23, without diminishing the effectiveness
of secure
interaction between the ball/spherical element and the collet. The potential
for reducing the
depth of slots/channels 265 arises because the slots/channels no longer need
to allow
deformation inward. Since only outward deflection of upstanding segments 264
is required
to achieve the requisite securing force, the slot/channel depth may be
reduced, thereby
stiffening and strengthening the collet. The selection of an appropriate depth
for
slots/channels 265 is well within the skill of persons skilled in the art
based on the present
disclosure. By reducing the depth of slots/channels 265, greater strength may
be imparted to
collet 260.
With reference to Figures 25a-25c, a further alternative mechanism is depicted
wherein the collet is non-deflecting, i.e., the slots/channels from the
preceding embodiments
are eliminated. Thus, collet 260' defines a substantially cylindrical
structure, rather than a
plurality of upstanding, deflectable segments that are separated by
slots/channels 265, as
described with reference to the preceding embodiments. The cylindrical
structure imparts
additional strength to collet 260', relative to the previously described
slotted embodiments.
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As with the embodiment of Figs. 24a-24c, an internal snap ring 273 is provided
and is
adapted to nest within internal and external ring grooves 277, 279 in the
manner described
above. Interaction between snap ring 273 and ring grooves 277, 279 provides a
securing
force between collet 260' and ball/sphere 236.
With particular reference to the exploded view of Fig. 25b and the cross-
sectional
view of Fig. 25c, set screw 274' defines an enlarged head 274a that is
dimensioned to
cooperate with the chamfered opening to ball/sphere 236. A tapered,
circumferential bearing
surface 274b is defined on the lower portion of head 274a, which is adapted to
engage
ball/sphere 236 as set screw 274' is screwed into collet 260'. Cooperating
screw threads are
generally defined on the exterior of the downwardly extending portion of set
screw 274' (e.g.,
6-32 thread) and on the inner surface of collet 260'. Thus, as set screw 274'
is advanced into
collet 260', bearing surface 274b engages a cooperating chamfered surface on
balUsphere
236. At the same time, an angled, circumferential bearing surface 261 that is
defined by (or
associated with) pedicle screw 216 is brought into engagement with the
symmetrically
defined, chamfered surface at the opposite end of ball/sphere 236. Thus, the
ball/sphere 236
is effectively captured between the enlarged head of set screw 274' and
bearing surface 261
is positioned adjacent the base of collet 260'.
According to the alternative embodiment of Figs. 25a-25c, the strength of the
collet is
increased through elimination of the slots/channels. In addition, the greater
size of the
enlarged head of set screw 274' permits a larger hexagonal (or other
geometrically shaped)
tool engagement feature relative to the previously described embodiments.
Moreover, a
"tissue-friendly" surface feature 274c may be defined on the upper surface of
the enlarged
head to shield tissue from the space within ball/spherical element 236.
However, according
to the embodiment of Figs. 25a-25c, it is not possible to "preload" set screw
274' within the
central aperture formed within pedicle screw (as described in greater detail
below) because it
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is not possible to pass the ball/spherical element thereover.
With reference to Figures 26a-26c, a further exemplary mechanism for securing
or
mounting a ball/sphere relative to a pedicle screw is depicted according to
the present
disclosure. As with the embodiment of Figs. 25a-25c, a non-slotted collet is
provided in
association with pedicle screw. Also, as with the preceding embodiment, an
angled,
circumferential bearing surface 261 is positioned adjacent the base of the
collet and is
configured and dimensioned to engage an inner surface defined by the
ball/sphere. Bearing
surface 261 is defined by (or associated with) pedicle screw 216 and is
positioned below the
screw threads discussed below.
With particular reference to Figs 26b and 26c, ball/spherical element 236'
defines a
threaded inner surface 236a that is adapted to cooperate with an outwardly
threaded surface
260a formed on collet 260". The cooperating threads obviate the need for, and
utility of, the
snap rings discussed with reference to prior embodiments. Of note, one or more
features are
generally formed at the openings of ball/sphere 236' to facilitate interaction
with a tool (not
pictured) for imparting rotational motion of ball/sphere 236' relative to
pedicle screw 216. In
like measure, one or more features are generally formed at (or near) the top
of collet 260" to
facilitate interaction with a counter-torque tool (not pictured) to ensure
that rotation of
ball/sphere 236 results in the desired tightening of ball/sphere 236' relative
to collet 260". As
ball/sphere 236' is tightened relative to collet 260", the bottom portion of
the ball/sphere
engages bearing surface 261, thereby providing further frictional engagement
therebetween.
In use, the mounting mechanism of Figs. 26a-26c obviates the need for a set
screw (as
described in previous embodiments) and utilizes a non-slotted collet, thereby
imparting
additional strength to the collet structure relative to previously disclosed
slotted collets.
Assembly of the ball/sphere and the pedicle screw requires thread alignment
and appropriate
tool interaction to effect the desired rotation of the ball/sphere relative to
the collet/pedicle
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screw.
With reference to Figure 27, a further alternative mounting mechanism is
depicted
wherein entry threads 236b on the ball/sphere 236"are configured to interact
with cooperative
threads 260x at (or near) the base of slotted collet 260k. A snap ring 273 is
provided to
supply further mounting security as the upstanding segments of the slotted
collet 260k are
deflected outward, i.e., when set screw 274 is advanced downward relative to
pedicle screw
216. According to exemplary embodiments of the disclosed mechanism, the entry
threads are
"left-handed" threads, thereby minimizing the potential for disengagement
thereof as set
screw 274 is introduced. Indeed, as the set screw is advanced, the ball/sphere
is urged into a
locked position due to the oppositely oriented threading thereof.
Alternatively, the set screw
could be provided with left-handed threads, and the entry threads could be
right-handed to
achieve the same result. In use, the mounting mechanism of Fig. 27 provides
enhanced
mounting security between the ball/sphere and the collet/pedicle screw through
the combined
contributions of the deflectable upstanding segments of the collet (in
response to set screw
introduction), the inclusion of the snap ring, and the inclusion of entry
threads on the
ball/sphere.
According to exemplary embodiments of the present disclosure, set screw 274 is
advantageously "preloaded" within central aperture 266, i.e., set screw 274 is
partially
threaded into central aperture 266 prior to commencing the clinical procedure.
For purposes
of the mounting mechanisms described above, only the design of Figs. 25a-25c
is not
susceptible to a "preloaded" set screw (because of the enlarged head on set
screw 274'). An
interference may be provided on the surface of set screw 274 to maintain the
set screw 274 in
an initial "preloaded" position, e.g., during shipment and initial clinical
positioning/introduction of the pedicle screw relative to a patient. An
exemplary interference
according to the present disclosure involves a deformation in the helical
thread, e.g., at or
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near a distal end thereof. The deformation may be effected by striking the
formed thread in
one or more locations (e.g., two opposed locations) with a rigid surface. In
an exemplary
embodiment, a pair of deformations or "pings" are formed in the screw thread
at or near the
distal end of the set screw. It is further contemplated that a desired
interference may be
achieved by providing a limited region of "off-pitch" threading along the
length of the screw
thread. Alternative structures and/or mechanisms may be employed to achieve
the desired
interference (which is easily overcome by the clinician when he/she advances
the set screw
relative to the pedicle screw), as will be readily apparent to persons skilled
in the art from the
present disclosure.
By-"preloading" the set screw as described herein, clinical use of the
disclosed system
is facilitated, e.g., potential difficulties associated with aligning set
screw 274 with central
aperture 266 during a clinical procedure and/or the potential for
misplacing/dropping and/or
cross-threading the set screw in connection with clinical activities are
substantially
eliminated. Of note, the length of set screw 274 and/or the relative
dimensions and/or
positioning of the outwardly tapered region of set screw 274 may be
advantageously selected
so as prevent or limit outward deflection of upstanding segments 264 in the
"preloaded"
configuration of set screw 274.
In general, tightening and/or locking of a ball/spherical element relative to
a pedicle
screw is thus undertaken according to exemplary embodiments of the present
disclosure by
threading a set screw into a central aperture positioned at or near the head
of the pedicle
screw. The set screw may be advantageously pre-loaded into the central
aperture to facilitate
clinical use thereof. Threading of the set screw into the central aperture
causes an outward
deflection of a series of upstanding segments associated with a collet
mechanism associated
with the pedicle screw. To facilitate movement of the set screw relative to
the pedicle screw,
it is generally desirable to impart a "counter-torque" force to the pedicle
screw so as to
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prevent/limit rotational motion of the pedicle screw as the set screw is
inserted or withdrawn
relative to the central aperture. Tools for providing a desired counter-torque
(and for
inserting/withdrawing a set screw) are known. According to exemplary
embodiments of the
present disclosure, cut-outs/alignment tabs may be formed or associated with
the collet and
cooperative notches may be formed or associated with the ball/spherical
element to facilitate
interaction with such tools, e.g., a tool for imparting a desired counter-
torque force to the
pedicle screw during set screw insertion/withdrawal.
Although the present disclosure has described a series of exemplary
embodiments
wherein a ball/spherical element is mounted with respect to a pedicle screw
and cooperates
with a socket member to support motion relative to the pedicle screw (i.e.,
act as a motion
interface element) and provide an advantageous dynamic junction, it is to be
understood that
the present disclosure is not limited to dynamic junctions formed through
interaction between
a ball/spherical element and a socket member. For example, as shown in Fig.
29, a pedicle
screw 216 having an outwardly threaded collet 260a may engage an inwardly
threaded cavity
23 6a that is mounted or jointed to a first universal joint mechanism 241
which functions as a
motion interface element. A rod 252 cooperates with first universal joint
mechanism 241 at a
first end thereof and a second universal joint mechanism 243 at an opposite
end thereof. The
design and operatiori of universal joint mechanisms are well known to persons
skilled in the
art and implementation thereof in connection with pedicle screw mounting
structures of the
type disclosed herein provide advantageous alternative dynamic junctions for
use in
stabilization systems/applications. Alternative dynamic junction assemblies
may also be
employed without departing from the spirit or scope of the present disclosure,
as will be
readily apparent to persons skilled in the art from the detailed description
provided herein.
As those skilled in the art will certainly appreciate, efficient and reliable
alignment of
ball/spherical element 236, 238 relative to collet 260 and within socket 232,
234 is desirable.
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In accordance with exemplary embodiments of the present disclosure and with
reference to
Figures 12 and 14, alignment activities are facilitated by providing
clinicians with an
advantageous guidewire system 275. Exemplary guidewire system 275 includes a
guidewire
276 and a tapered guide member 278 that defines an outwardly tapered guiding
surface (e.g.,
a conical surface) that is shaped and dimensioned to facilitate positioning of
a ball relative to
a pedicle screw and/or socket systems, as described herein. Guidewire 276
generally defines
a proximal end 280 and a distal end 282 with a central portion 284
therebetween. In
exemplary embodiments of the present disclosure, the proximal and distal ends
280, 282 of
guidewire 276 are substantially similar to conventional guidewires that are
used in
conventional pedicle screw installations. However, the central section 284 is
provided with
an advantageous tapered guide member 278, as described herein.
Tapered guide 278 generally defines a sloped outer surface and a base 279 that
is
substantially planar. Base 279 is generally dimensioned to have a maximum
diameter that is
slightly smaller than that of the diameter of receiving channel 262a, 262b (as
measured in the
non-chamfered regions). Typically, the difference in diameter between base 279
of tapered
guide 278 and the central channel of receiving channe1262a, 262b is about
.001" to about
.020", thereby facilitating alignment of a ball relative to a pedicle screw
while simultaneously
ensuring non-obstructed passage of the ball relative to the base of the
tapered guide. In
exemplary embodiments of the present disclosure, the distal end 282 of
guidewire 276
extends within the pedicle screw 216, 218, e.g., to a position short of the
distal end 258 of the
pedicle screw 216, 218. The tapered guide member 278 is then advantageously
positioned on
guidewire 276 such that base 279 is adjacent the proximal end 256 of the
pedicle screw, e.g.,
adjacent or in contact with collet 260.
In use, a pedicle screw may be introduced into a desired anatomical location.
The
disclosed guidewire system may then be advantageously employed to facilitate
efficient and
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reliable positioning of a ball/sphere relative to the pedicle screw. The
guidewire is generally
fed into the pedicle screw such that the base of the disclosed tapered guide
member is
brought into close proximity and/or contact with the proximal end of the
pedicle screw, e.g.,
the collet positioned at or near the head thereof. In percutaneous
applications, however, the
guidewire is generally positioned first, with the pedicle screw introduced to
a desired
anatomical location over the guidewire. A ball/spherical element (or
alternative accessory
structure) is then fed along the guidewire, i.e., the guidewire passes through
the receiving
channel of a ball/spherical element. The tapered guide member advantageously
guides the
ball into alignment with the proximal end of the pedicle screw, e.g., into
alignment with a
collet positioned at the head of the pedicle screw. The ball/sphere then
passes over the base
of the tapered guide member into position at the head of the pedicle screw,
e.g., with an
advantageous collet of the present disclosure positioned within the receiving
channel of the
ball.
It is contemplated that the tapered guide member of the present disclosure may
be
formed with various shapes designed to suit specific needs and/or
applications. For example,
the tapered guide member may be spirally shaped and provided with additional
guides for
ensuring that a ball has a proper orientation/registration when seated upon
the collet. Such an
embodiment might be used in minimally invasive procedures, e.g., to facilitate
proper
alignment with a set screw of an attachment member. In addition, the tapered
guide member
may advantageously include structures and/or features to facilitate rotational
alignment or
registration of a component, e.g., a component having at least one
asyminetrical
characteristic, relative to a pedicle screw. Thus, for example, a spiral may
be provided on the
tapered guide member that ensures proper alignment/registration with
feature(s) on the
pedicle screw.
In addition, a guiding cone or tapered guide member may be used according to
the
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present disclosure to guide a screwdriver and/or a counter-torque device down
the guidewire,
e.g., to facilitate accessing of the set screw with limited or non-existent
visualization. In an
additional advantageous embodiment of the present disclosure, the guidewire
system may
facilitate tool alignment/guidance to an off-axis location, e.g., a laterally
spaced attachment
member and/or rod connector, based on a known lateral/off-axis direction and
distance
relative to the pedicle screw in which the guidewire is positioned. Thus, a
guide member
may be slid along the guidewire that effects a predetermined and advantageous
off-axis
positioning of, for example, a tool (e.g., a screw driver) relative to the
guidewire.
Further, a tapered guide member according to the present disclosure may have a
star-
shaped or triangular profile. In addition, the tapered guide member may be
provided as a
separate component, i.e., for assembly with the guidewire at a desired point
in time, e.g.,
during installation of a stabilization system according to the present
disclosure. In
implementations where the tapered guide member is provided as a distinct
component
relative to the guidewire (as opposed to a pre-assembled guidewire system),
the tapered guide
member is advantageously passed over the guidewire and positioned at a desired
axial
position during the stabilization system installation process. Indeed, it is
further
contemplated that the tapered guide member may be formed and used separately
from a
guidewire, e.g., by placing the tapered guide member in juxtaposition with the
proximal end
of a pedicle screw, e.g., by mounting a tapered guide member relative to a
collet that is
associated with a pedicle screw.
With further reference to the biasing structures of exemplary stabilizing
member 210,
a piston assembly 286 is provided that includes concentric springs 212, 214.
The concentric
springs take the form of an inner first spring 212 and an outer second spring
214. As will be
described below in greater detail, the piston assembly 286 further includes a
spring cap 288
and a spring cap rod 252 which translate and/or transmit forces between piston
assembly 286
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and pedicle screws 216, 218. Inasmuch as pedicle screws 216, 218 are
substantially integral
with spinal structures of a patient, the structural arrangement described
herein effectively
translates and/or transmits forces to and from a patient's spine.
The inner first spring 212 generally defines a first end 290 and a second end
292. As
mentioned above, in exemplary embodiments of the present disclosure, first
spring 212 is
captured with respect to first attachment member 224. The second end 292 of
the inner first
spring 212 is captured with respect to abutment surface 294 of spring cap rod
252. The outer
second spring 214 also defines a first end 296 and a second end 298. In
exemplary
embodiments of the present disclosure, the first end 296 of the outer second
spring 214 is
rigidly secured to spring cap 288 and the second end 298 of outer second
spring 214 is rigidly
secured to abutment surface 294 of spring cap rod 252.
As discussed above, the respective first and second springs 212, 214 are
coupled to
one or more structures associated with the exemplary stabilizing member 210.
According to
exemplary embodiments hereof, one or both springs 212, 214 may be rigidly
(i.e., fixedly)
coupled with respect to one or more component(s) associated with stabilizing
member 210.
In accordance with a preferred embodiment of the present disclosure, the
springs are welded
to structures at one or both ends thereof, although those skilled in the art
will appreciate that
other coupling techniques (e.g., nesting and/or capturing techniques) may be
used without
departing from the spirit or scope of the present invention.
The springs 212, 214 are generally positioned within a sheath 300, e.g., a
substantially
cylindrical member, to prevent undesirable interaction or interference between
the springs
and anatomical structures in situ. Thus, sheath member 300 is advantageously
substantially
inert with respect to surrounding anatomical structures and fluids. In
accordance with
exemplary embodiments of the present disclosure, sheath 300 is fabricated (at
least in part) of
ePTFE (expanded polytetrafluoroethylene), UHMVWPE (Ultra-High Molecular Weight
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Polyethylene), polycarbonate-urethane composite materials (e.g., copolymers
and/or blends
thereof), or combinations thereof, although those skilled in the art will
appreciate that other
materials may be used without departing from the spirit or scope of the
present invention.
Sheath 300 is generally fabricated from a material with sufficient elasticity
to accommodate
axial elongation/contraction of stabilizing member 110, although structural
arrangements to
accommodate such axial motion, e.g., a bellows-like structure, may also be
employed. It is
contemplated that sheath 300 may include a surface treatment, e.g., a drug
and/or medicinal
agent, to facilitate or promote desired clinical results.
Abutment surface 294 of spring cap rod 252 is generally secured with respect
to
sheath 300 at a first end thereof, and spring cap 288 is generally secured
with respect to
sheath 300 at an opposite end thereof. Washers or C-clamps 302 are generally
positioned at
the junction between sheath 300 and the end member (i.e., spring cap 288 and
abutment
surface 294) to facilitate interaction therebetween. In an exemplary
embodiment of the
present disclosure, spring cap 288 is further rigidly secured with respect to
body member 240
of first attachment member 224.
As shown in Figures 8 and 9, first and second springs 212, 214, spring cap 288
and
spring cap rod 252 generally couple piston assembly 286 to pedicle screws 216,
218 in a
manner providing a desirable and advantageous force profile, despite the
limited anatomical
space available in spine applications. For example, when the spine moves in
extension,
pedicle screws 216, 218 encounter forces that bias the pedicle screws toward
each other. The
forces experienced by pedicle screws 216, 218 are translated to forces on
first and second
attachment members 224, 228, which similarly are biased to move toward each
other. The
foregoing forces (that originate from spinal activity) generate a compressive
force on
stabilizing member 210. In response to the compressive force experienced by
stabilizing
member 210, a counterforce is generated within stabilizing member 210 through
the spring
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force generated as spring cap rod 252 pushes and compresses outer second
spring 214
between spring cap 288 and abutment surface 294 of spring cap rod 252. An
additional
counterforce is generated by stabilizing member 210 as spring cap rod 252
pushes and
compresses the inner first spring 212 between the body 240 of the first
attachment member
224 and the abutment surface 294 of the spring cap rod 252. As shown in Figure
17, the
combined spring forces of first spring 212 and second spring 214 creates a
substantially
uniform force profile in response to spine movement in tension, while
extension generates
compression across the spring member(s).
When the spine moves in flexion, pedicle screws 216, 218 are subject to forces
that
bias the pedicle screws away from each other. The forces experienced by
pedicle screws 216,
218 as the spine moves in flexion are translated to first and second
attachment members 224,
228, which similarly experience a force that biases such components of
stabilizing system
211 away from each other. A counterforce is generated by stabilizing member
210 in
response to flexion motion of the spine. The counterforce is generated in part
as a result of
the spring force generated when the spring cap rod 252 pulls upon and extends
outer second
spring 214 between the spring cap 288 and abutment surface 294 of spring cap
rod 252. An
additional counterforce is generated in response to flexion movement of the
spine as spring
cap rod 252 allows extension of the inner first spring 212 between the body
240 of first
attachment member 224 and abutment surface 294 of spring cap rod 252. As the
force profile
of Figure 17 shows, the operation of springs 212, 214 within stabilizing
member 210 creates
a force profile that advantageously decreases in intensity as overall spinal
displacement
increases/continues. At a certain point the inner spring reaches its free
length and the
resistance to motion is only in response to the increased elongation of the
outer spring.
Referring to Figures 8 and 13-16, and in accordance with an exemplary
embodiment
of the present disclosure, stabilizer system 211 is generally installed in the
following manner.
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Pedicle screws 216, 218 are positioned'within the vertebrae using traditional
techniques.
The use of fluoroscopy for guidance of the pedicle screws is generally
employed and strongly
recommended. The pedicle screws 216, 218 are typically placed lateral to the
facets in order
to ensure that there is no interference between a facet and the implanted
system. The pedicle
is first opened with a high-speed burr or an awl. Thereafter, a stabilizer
pedicle probe may be
used to create a channel for pedicle screws 216, 218. The pedicles screws 216,
218 are
generally self-tapping and therefore tapping of the pedicle screw channel
typically is not
required. The integrity of the pedicle channel wall is then typically checked
and an
appropriately sized pedicle screw 216, 218 is installed by attaching the screw
to a screw
driver and introducing the screw lateral to the facets. The pedicle screw 216,
218 is generally
advanced until the head of the screw is in contact with the pedicle.
Typically, placement of
the pedicle screw 216, 218 as low as possible,is very important, especially in
the L5 and S1
pedicles. The placement of the pedicle screws 216, 218 is then generally
checked with
fluoroscopy, X-ray and/or other surgical navigation/viewing technique.
Once the pedicle screws 216, 218 are properly installed, the distance between
the
pedicle screws 216, 218 is generally measured and rod 252 of stabilizing
member 210 may be
cut to proper dimension, as appropriate. Alternatively, rods 252 of varying
length may be
provided to permit a clinician to select a rod of desired length. Still
further, means for
adjusting the length of a rod 252 may be employed, e.g., a telescoping rod
with mechanism(s)
for securing the rod at one or more desired lengths (e.g., detent mechanisms
at fixed intervals,
set screw systems for fixing the telescoping rod members relative to each
other, or the like).
In installation procedures that employ a guidewire system to guide alignment
and/or
installation of system components, guidewire(s) 276 are positioned within one
or both of the
pedicle screws 216, 218. According to exemplary embodiments of the present
disclosure, a
tapered guide member 278 is advantageously positioned adjacent the top of
collet 260.
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However, as noted previously, a tapered guide member may be directly
associated with the
pedicle screw and/or collet to facilitate alignment and/or installation of
system components
(e.g., in implementations that do not employ a guidewire).
An attachment member 224, 228 (which encompasses a ball/sphere 236) may be
slid
down along a guidewire 276 until a tapered guide 278 is reached. Once the
attachment
member 224, 228 reaches the tapered guide 278, a more exact guiding function
is imparted to
the attachment member. Indeed, tapered guide 278 advantageously functions to
guide the
ball/sphere 236 associated with attachment member 224, 228 into alignment with
collet 260
such that it is positioned/aligned for efficient sliding passage thereover.
Thus, tapered guide
278 brings the center line of the channel formed in ball/sphere 236 into
substantial alignment
with the center line of collet 260 so that collet 260 can readily slide
through the ball/sphere
236. Depending on the mounting mechanism associated with interaction between
the collet
and the ball/sphere (see Figs. 23-27), the aligned components are then mounted
with respect
to each other.
Thus, in the exemplary embodiment of Figs. 8 and 15-16, set screw 274 is
advantageously tightened within collet 260 to effect outward deflection of the
upstanding
segments, thereby locking/securing the ball 236, 238 in position relative to
the collet/pedicle
screw. Of note, in the exemplary embodiment of Figs. 8 and 15-16, set screw
274 may be
advantageously preloaded relative to collet 260, thereby facilitating the
mounting process as
described previously. For alternative mounting mechanisms described herein,
appropriate
steps may be undertaken to secure the ball/sphere relative to the collet,
e.g., rotational motion
of ball 236, 238 relative to the collet. Of note, ball 236, 238 is adapted for
freely rotational
motion relative to attachment member 224, 228, thereby facilitating rotational
mounting of
the ball, if desired.
At this stage of assembly/installation, a first ball is secured relative to a
first
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collet/pedicle screw. However, according to the present disclosure, a dynamic
junction is
nonetheless established because the attachment member is free to move, e.g.,
rotate, relative
to the ball. Indeed, a "race" is generally defined therebetween to facilitate
relative movement
between the ball and attachment member. As such, realignment and/or
reorientation of the
attachment member is possible so as to facilitate alignment with an adjacent
pedicle screw,
i.e., for assembly of a dynamic stabilization level. Of particular note, even
after mounting of
an attachment member relative to an adjacent pedicle screw, the dynamic
junction remains
operative at the initial pedicle screw described herein, thereby accommodating
anatomical
shifts that may arise after installation of the disclosed dynamic
stabilization system.
With further reference to Figs. 15-16, rod 252 is aligned with a receiving
portion of
rod connector 248 that is associated with second attachment member 228. As
with the first
attachment member discussed above, a dynamic junction is advantageously
defined between
socket 232 and ball/sphere 238 such that alignment between rod connector 248
and rod 252 is
facilitated. Moreover, the functionality of the dynamic junction is unaffected
by mounting of
rod 252 relative to rod connector 248, i.e., rotational motion therebetween is
not affected
when a rod is secured/assembled according to the disclosed dynamic
stabilization system.
When rod 252 is properly aligned within rod connector 248, set screw 254 is
tightened within
transverse aperture 250 to lock rod 252 in position. The installation
procedure is generally
repeated on the opposite side of the vertebrae to complete a single level
dynamic
stabilization. Thus, at this stage in the assembly process, a dynamic
stabilization is
established for a single level, i.e., the level defined by the location of
pedicle screws 216, 218
(and the associated counterparts on the opposite side of the vertebrae).
With reference to Figs. 28 and 30 (and corresponding structures in Figs. 8 and
19),
additional structural and assembly details associated with an exemplary
embodiment of the
disclosed dynamic stabilizing member are now provided. As noted above, first
attachment
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member 224 includes spring cap 228. As shown in Fig. 28, spring cap 228
includes a helical
groove 229 on the outer periphery of the flange-like structure of spring cap
228. The width
and depth of groove 229 are generally sized so as to accommodate the wire gage
of a helical
outer spring (e.g., second spring 214 of Fig. 8 or second spring 456 of Fig.
19). In addition, a
post 231 extends from the flange-like structure of spring cap 228. Post 231 is
generally
centrally located on the flange-like structure and extends away from socket
232. An annular
cavity 233 may be formed around post 231: According to exemplary embodiments
of the
present disclosure and with reference to Fig. 30, abutment surface 294 of
spring cap rod 252
includes a helical groove 295 (akin to helical groove 229), post 297 (akin to
post 231) and
annular cavity 299 (233). An elongated member (rod) 301 extends from abutment
surface
294 in a direction opposite to post 297. The foregoing structures and features
facilitate
assembly and operation of exemplary dynamic stabilizing members according to
the present
disclosure.
More particularly, according to exemplary embodiments of the present
disclosure,
inner first spring 212 is initially positioned within second (outer) spring
214, and is then
positioned around or on post 231 and the opposed post 297 that extends from
abutment
surface 294. According to exemplary assemblies of the present disclosure,
inner first spring
212 advantageously extends into annular cavity 233 and the opposed cavity 299
formed in
abutment surface 294. In this way, inner first spring 212 is effectively
captured between
spring cap 288 and spring cap rod 252, and essentially floats relative to the
opposing posts
231, 297. Thereafter, second spring 214 is threaded into groove 229 formed in
spring cap
288 (or the opposed groove 295 formed in abutment surface 294). Ultimately,
second spring
214 is typically fixed with respect thereto, e.g., by welding, and may be
trimmed so as to be
flush relative to an outer edge of the flange-like structure to which it is
mounted. The outer
second spring 214 is then extended so as to be threaded onto the opposing
groove, i.e., the
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groove associated with abutment surface 294 or spring cap 288, e.g., by
rotating abutment
surface 294 or spring cap 288 relative to second spring 214, as the case may
be. Once
threaded into the opposing groove, the second spring 214 is typically fixed
with respect
thereto, e.g., by welding, and may be trimmed to establish a flush edge.
Of note, outer second spring 214 is typically shorter than inner first spring
212. Thus,
as abutment surface 294 and spring cap 288 are brought toward each other (to
permit second
spring 214 to be mounted on both), inner first spring 212 is placed in
compression. The
degree to which first spring 212 is compressed is generally dependent on the
difference in
length as between springs 212, 214. Thus, the preload compression of first
spring 212 may
be controlled and/or adjusted in part through selection of the relative
lengths of springs 212,
214. In addition to the preload compression of inner spring 212, the mounting
of outer spring
214 with respect to both spring cap 288 and abutment surface 294 places outer
spring 214 in
tension. The overall preload of a dynamic stabilizing member according to this
exemplary
embodiment corresponds to the equal and opposite forces experienced by springs
212, 214,
i.e., the initial tension of outer spring 214 and the initial compression of
inner spring 212.
According to exemplary embodiments of the present disclosure, inner spring 212
reaches its free length (i.e., non-compressed state) at or about the point at
which a patient's
movement exceeds the neutral zone. Beyond this point, inner spring 212 is free
floating (on
the opposed posts) and contributes no resistance to spinal movement. As
described
previously, the advantageous force profile supplied by the dynamic
stabilization system of
the present disclosure is achieved through utilization of inner and outer
springs working
synergistically. In particular, the force profiles for the springs are chosen
to produce a
reduction in the increase of mechanical resistance as the displacement moves
beyond the
neutral zone.
As briefly mentioned above, an axial spring configuration may be employed
which
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generates the Force-Displacement curves shown with reference to Figure 17,
while allowing
for a shorter distance between the first and second attachment members. As
noted above, the
Force-displacement curve is not exactly the same as that disclosed with
reference to the
embodiment of Figures 1 to 7. That is, the curve is substantially uniform
during extension of
the back and compression of the stabilizer, but the curve is substantially
similar to that -
described with reference to Figure 3a and 3b when the back is in flexion and
the stabilizer is
elongated. The exemplary concentric spring design of the present disclosure
allows a shorter
distance between the first and second attachment members, eliminates the
overhang on some
previous embodiments, but this concentric spring orientation dictates that the
extension curve
be uniform or straight (i.e., no elbow). This profile characteristic results
from the fact that
both springs are loaded in extension, thus creating the exact same curve when
both springs
are loaded in the neutral zone, as compared to a situation wherein only one
spring is loaded in
flexion, i.e., while being elongated once outside the central zone of the
device.
The advantageous dynamic stabilization systems disclosed herein may also be
used in
the stabilization of multiple level systems. Multiple level stabilization may
be achieved
through installation of a plurality stabilizing members coupled through a
plurality of
elongated members (e.g., rods) and a plurality of pedicle screws. For example
and with
reference to Figures 18 to 22, a multiple level, dynamic stabilization system
410 is
schematically depicted. Multi-level stabilization system 410 may employ a
variety of
different attachment members 412, 414, 416. The different attachment member
designs may
be selected based on anatomical considerations, e.g., the spinal location for
installation,
and/or the position within the multi-level system. In other words, certain
attachment member
designs are better utilized at a first end or a second end, whereas other
attachment member
designs are suited for intermediate locations. While a specific combination of
elements
and/or components are disclosed in accordance with the exemplary multi-level
stabilization
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system of Figs. 18-22, those skilled in the art will readily understand from
the present
disclosure how the various attachment members and related
structures/components may be
employed to achieve dynamic stabilization at various spinal locations and/or
in alternative
deployment schemes.
Exemplary multi-level dynamic stabilization system 410 employs three distinct
attachment members 412, 414, 416 dynamically linked by piston assemblies 418,
420 in the
creation of a two level system. Of course, additional levels may be stabilized
by extending
the assembly with additional pedicle screws, collet/ball mounting mechanisms,
dynamic
stabilizing members, and elongated members/rods. The various attachment
members are
secured to the vertebrae through interaction with pedicle screws (not shown),
as described
above. Typically a dynamic junction is advantageously established between each
pedicle
screw (through cooperation with a ball/collet mechanism) and the attachment
member
mounted with respect thereto. The dynamic junction facilitates alignment with
adjacent
pedicle screw/attachment member subassemblies during installation/assembly of
the multi-
level dynamic stabilization system, and accommodates limited anatomical
shifts/realignments
post-installation.
With regard to dynamic stabilization between the first attachment member 412
and
the second attachment member 414, the first attachment member 412 is
structured for
supporting inner first spring 428 and includes a body member 430 having an
aperture 432
that extends therethrough. Body member 430 defines a socket 434 which is
configured and
dimensioned for receipt of ball 436, thereby establishing a first dynamic
junction. According
to the exemplary embodiment depicted herein, the inner first spring 428
extends from, and
may be integrally formed with (or otherwise positioned with respect to), body
member 430 of
the first attachment member 412.
The second attachment member 414 similarly includes a body member 438 having
an
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aperture 440 that extends therethrough. Body member 438 defines socket 442
which is
configured and dimensioned for receipt of ball 444, thereby establishing a
second dynamic
junction. Second attachment member 414 further includes or defines a rod
connector 446
with a transverse slot or channel 448 that extends therethrough. Transverse
slot/channel 448
is configured and dimensioned to accommodate positioning and/or passage of
stabilizer
spring cap rod 450 therewithin. Spring cap rod 450 is generally secured within
the transverse
slot/channel 448 via a set screw 452 that extends between the external surface
of rod
connector 446 and the transverse slot/channel 448 formed by rod connector 446.
As those
skilled in the art will certainly appreciate, the transverse channel/slot may
be structured in a
variety of ways (e.g., as discussed above with reference to Figures 8-11).
Second attachment
member 414 is further associated with an inner first spring 454 that extends
therefrom for
interaction with third attachment member 416 (discussed below).
Piston assembly 418, which is positioned between first and second attachment
members 412, 414, generally includes a pair of concentric springs. An inner
first spring 428
and an outer second spring 456 are typically provided. As with the embodiment
described
above, inner first spring 428 and outer second spring 456 are secured with
respect to an
abutment surface 458 of spring cap rod 450 and body member 430 of first
attachment
member 412. Thus, first and second springs 428, 456 supply forces that act on
(or with
respect to) first and second attachment members 412, 414 during spinal
movement, e.g.,
during extension and flexion of the spine. As is readily apparent from the
discussion herein,
the forces exerted on first and second attachment members 412, 414 are
translated to forces
on the associated pedicle screws, thereby stabilizing the vertebrae to which
the pedicle
screws are mounted.
Referring now to the relationship between second attachment member 414 and
third
attachment member 416, it is noted that the structural features of third
attachment member
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416 are substantially similar to those of second attachment member 414.
However, in
exemplary two-level stabilization systems disclosed herein, third attachment
member 416
does not have an inner and outer springs extending therefrom. In such
embodiments, the
"second" level is not subject to dynamic stabilization. Piston assembly 420
positioned
between second and third attachment members 414, 416 is similar to the
previously described
piston assemblies. Generally, piston assembly 420 includes an inner first
spring 454 that
extends from second attachment member 414 and spring cap rod 464 extends from
third
attachment member 416.
As mentioned above, first, second and third attachment members 412, 414, 416
may
have particular utility at particular anatomical locations. For example, it is
contemplated that
first attachment member 412 could be most useful at position S 1 and below
position L5,
whereas second and third attachment members 414, 416 may be advantageously
employed at
L5 and above. Alternative implementations of the foregoing attachment members
may be
undertaken based on particular clinical needs and/or judgments.
Of note, single or multi-level dynamic spine stabilization
systems/implementations
according to the present disclosure permit one or more adjustments to be made
(e.g., in situ
and/or prior to clinical installation). For example, adjustments as to the
magnitude and/or
displacement-response characteristics of the forces applied by the
stabilization system may
be implemented, e.g., by substituting springs within one or more of the
stabilizing members
and/or adjusting the first/second housings, as described with reference to
Fig. 8. The
adjustments may be made prior to initiating a clinical procedure, e.g., based
on an evaluation
of a particular patient, or after a clinical procedure, e.g., based on post-
surgical experiences
of a patient.
According to further exemplary embodiments of the present disclosure, multi-
level
spinal stabilizations may be undertaken wherein the same or differing
stabilization modalities
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may be employed at each of the individual levels. Thus, for example, a dynamic
stabilizing
member according to the present disclosure may be employed at a first
stabilization level, a
non-dynamic stabilizing member (e.g., a rigid structure/assembly such as a
rigid rod or plate
connection) at a second stabilization level, and a dynamic or non-dynamic
stabilizing element
at a third stabilization level. The advantageous flexibility and versatility
of the disclosed
systems/designs for mounting relative to a pedicle screw enhance the ability
to vary the
stabilization modalities from level-to-level according to the present
disclosure. For example,
upwardly extending collets disclosed herein readily accommodate cooperative
mounting with
respect to both dynamic and non-dynamic stabilizing members/elements. Indeed,
it is
contemplated according to the present disclosure that decisions as to
stabilization modalities
may be made at the time of surgery, e.g., based on clinical observations
and/or limitations.
Moreover, it is contemplated that dynamic and non-dynamic modalities may be
interchanged
at a point in time post-surgery. In such applications, a first stabilizing
member (whether
dynamic or non-dynamic) may be disengaged from a clinically installed
stabilization system,
and a second stabilizing member that offers a different modality may be
installed in its place.
Thus, systems according to the present disclosure encompass multi-level
stabilizations that
include at least one level that includes a dynamic stabilizing member and at
least one level
that includes a non-dynamic stabilizing element.
A kit may be advantageously provided that contains the components that may be
necessary to perform clinical procedures according to the present disclosure,
i.e., spine
stabilization procedures. The kit contents are typically sterilized, as is
known in the art, and
may include appropriate labeling/indicia to facilitate use thereof. Typical
kit contents
include: (i) two or more attachment members (wherein one of the attachment
members may
include an extension member that incorporates a stabilizing member), (ii) two
or more
balls/spheres, and (iii) two or more pedicle screws. Alternative kits
according to the present
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disclosure may include one or more of the following additional items: (iv) a
variety or
assorlrnent of replacement springs for potential use in the dynamic
stabilizing members of the
present disclosure, (v) one or more tools for use in the dynamic stabilization
procedures of
the present disclosure (e.g., a screw driver, counter-torque device,
measurement tools, tools
for placement of the pedicle screws, etc.), (vi) one or more guidewires, (vii)
one or more
tapered guides or cones, and/or (viii)one or more set screws. The enclosures
for the
foregoing kits are typically configured and dimensioned to accommodate the
foregoing
components, and are fabricated from materials that accommodate sterilization,
as are known
in the art. A single kit may be broken into multiple enclosures, without
departing from the
spirit or scope of the present disclosure.
For exemplary embodiments of the present disclosure wherein springs are
utilized in
fabricating the disclosed dynamic stabilizing members, spring selection is
generally guided
by the need or desire to deliver a particular force profile or force profile
curve, as described
above. Generally, spring selection is governed by basic physical laws that
predict the force
produced by a particular spring design/material. However, the particularly
advantageous
dynamic spinal stabilization achieved according to the present disclosure (as
described above
and schematically depicted in Figures 3a, 3b and 17) require a recognition of
the conditions
and stimuli to be encountered in a spinal environment.
A first design criterion is the fact that the dynamic stabilizing member must
function
both in compression and tension. Second, the higher stiffness (Kl + K2)
provided by a
disclosed dynamic stabilizing member in the central zone is generally achieved
through the
presence of a spring preload. Both springs'are made to work together when the
preload is
present. As the dynamic stabilizing member is either tensioned or compressed,
the
responsive force increases in one spring and decreases in the other. When the
decreasing
force reaches a zero value, the spring corresponding to this force no longer
contributes to the
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stabilizing functionality. An engineering analysis, including the diagrams
shown in Figures
7a and 7b, is presented below. This analysis specifically relates to the
exemplary
embodiment disclosed in Figure 5, although those skilled in the art will
appreciate the way in
which the analysis applies with equal force to all embodiments disclosed
herein.
Fo is the preload within the dynamic stabilizing member, introduced by
shortening
the body length of the housing as discussed above.
Kl and K2 are stiffness coefficients of the compression springs, active during
tensioning and compression of the dynamic stabilizing member, respectively.
F and D are respectively the force and displacement of the disc of the dynamic
stabilizing member with respect to the body of the dynamic stabilizing
member.
The suni of forces must equal zero. Therefore,
F+(Fo -D x K2) -(Fo + D x Kl) =0, and
F=Dx(K1+K2).
With regard to the central zone (CZ) width (see Figure 3a):
On Tension side CZT is:
CZT= Fo/K2,
On Compression side CZc is:
CZc= Fo/Ki.
While the foregoing analysis is useful in understanding the physical
properties and
forces associated with operation of the disclosed dynamic stabilizing member,
the present
disclosure is not limited to any theoretical or quantitative characterization
of spring design or
function. Rather, desired force profiles/force profile curves may be achieved
through
quantitative analysis, empirical study, or combinations thereof. In addition,
as those skilled
in the art will certainly appreciate, the concepts underlying the dynamic
stabilization systems
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and associated components/assemblies may be applied to other clinical needs
and/or
medical/surgical procedures. As such, the disclosed devices, systems and
methods may be
utilized beyond spinal treatments without departing from the spirit or scope
of the present
invention.
Having described exemplary embodiments of the present disclosure, it is
specifically
noted that the present invention embodies a series of advantageous features
and functions
having particular utility in spinal stabilization devices/systems and
associated methods,
including the following: I
= Devices, systems and methods that provide a dynamic junction between at
least one
pedicle screw and at least one elongated member (or multiple elongated
members),
e.g., rod(s), that engage and/or otherwise cooperate with the pedicle screw.
In
exemplary embodiments of the present disclosure, the dynamic junction is
provided
through interaction between a collet/ball mechanism and a socket that is
associated
with an attachment member. The dynamic junction facilitates assembly of a
spinal
stabilization system and permits the pedicle screw/elongated member to
accommodate
limited degrees of anatomical realignment/reorientation post-installation.
= Devices, systems and methods that provide or incorporate ball assembly
mechanisms
that facilitate assembly/installation of a ball/sphere relative to a pedicle
screw and
provide advantageous functional attributes as part of a spinal stabilization
system.
Exemplary mechanisms include advantageous collet-based mechanisms (e.g.,
slotted
and non-slotted collets), cooperatively threaded mechanisms (e.g., an
externally
threaded collet cooperating with an internally threaded ball/sphere),
mechanisms that
apply bearing forces against the ball/sphere (e.g., a circumferential bearing
surface
formed on a set screw having an enlarged head), and/or mechanisms that include
a
snap ring or analogous structure. The disclosed mechanisms permit reliable
mounting
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of a ball/sphere relative to a pedicle screw.
= Devices, systems and methods that provide dynamic spine stabilization
systems/implementations over a single level and/or multiple levels, including
single
and multi-level systems that permit one or more adjustments to be made (e.g.,
in situ
and/or prior to clinical installation), e.g., adjustments as to the magnitude
and/or
displacement-response characteristics of the forces applied by the
stabilization
system.
= Devices, systems and methods that provide multi-level dynamic stabilization
systems
that include different stabilization modalities at different levels, e.g., at
least one level
including a dynamic stabilizing member and at least one level including a non-
dynamic stabilizing member. According to exemplary embodiments of mixed multi-
level stabilization systems, the dynamic and non-dynamic stabilizing elements
are
mounted with respect to common, i.e., identical, pedicle screws as disclosed
herein.
= Devices, systems and methods that provide or utilize advantageous
installation
accessories (e.g., cone structures) for facilitating placement and/or
installation of
spine stabilization system components, such accessories being particularly
adapted for
use with a conventional guidewires to facilitate alignment/positioning of
system
components relative to the pedicle screw.
= Devices, systems and methods that provide or utilize dynamic spring
stabilization
components that include a cover and/or sheath structure that provides
advantageous
protection to inner force-imparting component(s) while exhibiting clinically
acceptable interaction with surrounding anatomical fluids and/or structures,
e.g., a
cover and/or sheath structure that is fabricated (in whole or in part) from
ePTFE,
UHMWPE and/or alternative polymeric materials such as polycarbonate-
polyurethane
copolymers and/or blends.
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= Devices, systems and methods that provide advantageous dynamic spine
stabilization
connection systems that facilitate substantially rigid attachment of an
elongated
member (e.g., a rod) relative to the pedicle screw while simultaneously
facilitating
movement relative to adjacent structures (e.g., an adjacent pedicle screw) to
permit
easy and efficacious intra-operative system placement;
= Devices, systems and methods that provide an advantageous "pre-load"
arrangement
for a securing structure (e.g., a set screw) that may be used in. situ to
mount a ball joint
relative to a pedicle screw, thereby minimizing the potential for clinical
difficulties
associated with location and/or alignment of such securing structure(s).
= Devices, systems and methods that embody or utilize advantageous kits that
include
an enclosure and necessary components for implementing dynamic spine
stabilization
in the manner described herein, such enclosure/components being supplied in a
clinically acceptable form (e.g., sterilized for clinical use).
Although the present disclosure has been disclosed with reference to exemplary
embodiments and implementations thereof, those skilled iri the art will
appreciate that the
present disclosure is susceptible to various modifications, refinements and/or
implementations without departing from the spirit or scope of the present
invention. In fact,
it is contemplated the disclosed connection structure may be employed in a
variety of
environments and clinical settings without departing from the spirit or scope
of the present
invention. Accordingly, while exemplary embodiments of the present disclosure
have been
shown and described, it will be understood that there is no intent to limit
the invention by
such disclosure, but rather, the present invention is intended to cover and
encompass all
modifications and alternate constructions falling within the spirit and scope
hereof.
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