U.S. patent application number 12/535560 was filed with the patent office on 2010-02-11 for methods and systems for increasing the bending stiffness and constraining the spreading of a spinal segment.
This patent application is currently assigned to Simpirica Spine, Inc.. Invention is credited to Todd Alamin, Ian Bennett, Colin Cahill, Louis Fielding, Manish Kothari, Craig Litherland, Hugues Malandain.
Application Number | 20100036424 12/535560 |
Document ID | / |
Family ID | 43544633 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100036424 |
Kind Code |
A1 |
Fielding; Louis ; et
al. |
February 11, 2010 |
METHODS AND SYSTEMS FOR INCREASING THE BENDING STIFFNESS AND
CONSTRAINING THE SPREADING OF A SPINAL SEGMENT
Abstract
A system for restricting spinal flexion includes superior and
inferior tether structures joined by a pair of compliance members.
Compliance members comprise tension members which apply a
relatively low elastic tension on the tether structures. By placing
the tether structures on or over adjacent spinous processes,
flexion of a spinal segment can be controlled in order to reduce
pain.
Inventors: |
Fielding; Louis; (San
Carlos, CA) ; Bennett; Ian; (San Francisco, CA)
; Kothari; Manish; (San Rafael, CA) ; Alamin;
Todd; (Woodside, CA) ; Malandain; Hugues;
(Mountain View, CA) ; Litherland; Craig; (Palo
Alto, CA) ; Cahill; Colin; (Portola Valley,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
Simpirica Spine, Inc.
San Carlos
CA
|
Family ID: |
43544633 |
Appl. No.: |
12/535560 |
Filed: |
August 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12106103 |
Apr 18, 2008 |
|
|
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12535560 |
|
|
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|
60936897 |
Jun 22, 2007 |
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Current U.S.
Class: |
606/263 ;
606/279; 606/86R |
Current CPC
Class: |
A61B 17/7067 20130101;
A61B 17/842 20130101; A61B 17/7062 20130101; A61B 2090/064
20160201; A61B 17/7068 20130101; A61B 17/7053 20130101 |
Class at
Publication: |
606/263 ;
606/86.R; 606/279 |
International
Class: |
A61B 17/70 20060101
A61B017/70; A61B 17/58 20060101 A61B017/58; A61B 17/88 20060101
A61B017/88 |
Claims
1. A method for relieving symptoms of lumbar pain associated with
flexion of a spinal segment of a patient, said method comprising:
increasing the bending stiffness of the spinal segment by an amount
in the range from 0.1 Nm/deg to 2 Nm/deg.
2. A method as in claim 1, wherein the bending stiffness is
increased by coupling an elastic constraint between a superior
spinous process and an inferior spinous process or between an L5
spinous process and a sacrum.
3. A method as in claim 2, wherein the elastic constraint has an
elastic stiffness in the range from 7.5 N/mm to 40 N/mm and the
constraint is positioned at a lateral distance in the range from 25
mm to 75 mm in a posterior direction from a center of rotation of
the spinal segment.
4. A method as in claim 2, further comprising adjusting the elastic
constraint so that it is taut but not stretched over the spinous
processes or L5 spinous process and sacrum when the spinal segment
is in its neutral position.
5. A method as in claim 4, wherein adjusting comprises changing the
length of the elastic constraint after it has been coupled to the
spinous processes or L5 spinous process and sacrum.
6. A method as in claim 1, wherein the bending stiffness is
increased over at least a portion of the full flexion range of
motion of the spinal segment.
7. A method as in claim 7, wherein the bending stiffness is
increased over the entire full flexion range of motion of the
spinal segment.
8. A method for relieving symptoms of lumbar pain associated with
flexion of spinal segment, said method comprising: constraining
spreading of the spinous processes of the spinal segment to a
maximum distance in the range from 1 mm to 10 mm from a neutral
position of the segment.
9. A method as in claim 8, wherein spreading is constrained to a
maximum distance in the range from 2 mm to 8 mm.
10. A method as in claim 8, wherein constraining comprises coupling
an elastic constraint between a superior spinous process and an
inferior spinous process or between a spinous process and a sacrum
of the spinal segment.
11. A compliance member for attaching inelastic tethers
circumscribing spinal processes, said compliance member comprising:
a body having a first tether attachment element and a second tether
attachment element, said body defining an axial tension spring
between said attachments; wherein the body has a maximum axial
length of 38 mm, a maximum depth in an anterior-posterior direction
of 18 mm, and a maximum width in a direction normal to the depth of
15 mm.
12. A compliance member as in claim 11, wherein at least the first
tether attachment element releasably secures the tether.
13. A compliance member as in claim 11, wherein at least the first
tether attachment element allows bidirectional axial displacement
of the tether relative to the body.
14. A compliance member as in claim 13, wherein the at least first
tether attachment comprises a mechanism selected from the group
consisting of rollers and ratchets.
15. A system for elastically constraining a spinal segment of a
patient, said system comprising: first and second compliance
members as in claim 9; a first inelastic tether adapted to attach
to the first tether attachment element of the first compliance
member and to the second tether attachment element of the second
compliance member; and a second inelastic tether adapted to attach
to the first tether attachment element of the second compliance
member and to the second tether attachment element of the first
compliance member; wherein the inelastic tethers have a central
region adapted to be received over the spinous processes, said
central region having a thickness no greater than 2 mm and a width
in the range from 3 mm to 10 mm.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 12/106,103 (Attorney Docket No. 026398-000410US), filed on
Apr. 18, 2008, which claims the benefit of provisional application
60/936,897, (Attorney Docket No. 026398-000400US), filed on Jun.
22, 2007, the full disclosures of which are incorporated herein by
reference.
[0002] The present invention is related to but does not claim
priority from application Ser. No. 11/076,469, filed on Mar. 9,
2005, now U.S. Pat. No. 7,458,981, which claimed the benefit of
prior provisional application 60/551,235, filed on Mar. 9, 2004;
application Ser. No. 11/777,366 (Attorney Docket No.
026398-000110US); filed on Jul. 13, 2007; application Ser. No.
11/827,980 (Attorney Docket No. 026398-000120US); filed on Jul. 13,
2007; PCT application no. US 2007/081815 (Attorney Docket No.
026398-000130PC); filed on Oct. 18, 2007; PCT application no. US
2007/081822 (Attorney Docket No. 026398-000140PC); filed on Oct.
18, 2007; and application Ser. No. 11/975,674 (Attorney Docket No.
026398-000150US); filed on Oct. 19, 2007, the full disclosures of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to medical methods
and apparatus. More particularly, the present invention relates to
methods and devices for restricting spinal flexion in patients
having back pain or other spinal conditions.
[0005] A major source of chronic low back pain is discogenic pain,
also known as internal disc disruption. Patients suffering from
discogenic pain tend to be young, otherwise healthy individuals who
present with pain localized to the back. Discogenic pain usually
occurs at the lower lumbar discs of the spine (FIG. 1). Pain is
typically exacerbated when patients put their lumbar spines into
flexion (i.e. by sitting or bending forward) and relieved when they
put their lumbar spines into extension (i.e. the standing position,
or arching backwards). Discogenic pain can be quite disabling, and
for some patients, can dramatically affect their ability to work
and otherwise enjoy their lives.
[0006] Such discogenic low back pain can be thought of as flexion
instability and is related to flexion instability that is
manifested in other conditions. The most prevalent of these is
spondylolisthesis, a spinal condition in which abnormal segmental
translation is exacerbated by segmental flexion.
[0007] Current treatment alternatives for patients diagnosed with
chronic discogenic pain are quite limited. Many patients follow a
conservative treatment path, such as physical therapy, massage,
anti-inflammatory and analgesic medications, muscle relaxants, and
epidural steroid injections, but typically continue to suffer with
a significant degree of pain. Other patients elect to undergo
spinal fusion surgery, which commonly requires discectomy (removal
of the disk) together with fusion of adjacent vertebrae. Fusion is
not usually recommended for discogenic pain because it is
irreversible, costly, associated with high morbidity, and of
questionable effectiveness. Despite its drawbacks, however, spinal
fusion for discogenic pain remains common due to the lack of viable
alternatives.
[0008] An alternative method, that is not commonly used in
practice, but has been approved for use by the FDA, is the
application of bone cerclage devices that can encircle the spinous
processes or other vertebral elements and thereby create a
restraint to motion. Physicians typically apply a tension or
elongation to the devices that applies a constant and high force on
the anatomy, thereby fixing the segment in one position and
allowing effectively no motion. The lack of motion allowed after
the application of such a device is thought useful to improve the
likelihood of fusion performed concomitantly; if the fusion does
not take, these devices will fail through breakage of the device or
of the spinous process to which the device is attached. These
devices are designed for static applications and are not designed
to allow for a dynamic elastic resistance to flexion across a range
of motion. The purpose of bone cerclage devices and the other
techniques described above is to almost completely restrict
measurable motion of the vertebral segment of interest. This loss
of motion at a given segment gives rise to abnormal loading and
motion at adjacent segments, leading eventually to adjacent segment
morbidity.
[0009] Recently, a less invasive and potentially more effective
treatment for discogenic pain has been proposed. A spinal implant
has been designed which inhibits spinal flexion while allowing
substantially unrestricted spinal extension. The implant is placed
over one or more adjacent pairs of spinous processes and provides
an elastic restraint to the spreading apart of the spinous
processes which occurs during flexion. Such devices and methods for
their use are described in U.S. Patent Publication No.
2005/02161017A1, published on Sep. 29, 2005, and having common
inventors with the present application.
[0010] As illustrated in FIG. 2, an implant 10 as described in the
'017 application typically comprises an upper strap component 12
and a lower strap component 14 joined by a pair of compliance
members 16. The upper strap 12 is shown disposed over the top of
the spinous process SP4 of L4 while the lower strap 14 is shown
extending across the bottom of the spinous process SP5 of L5. The
compliance member 16 will typically include an internal element,
such as elastomeric members 72a and 72b (FIG. 7 of the '017
application) which are attached to inelastic cables 76a and 76b in
such a way that the cables may be "elastically" or "compliantly"
pulled apart as the spinous processes SP4 and SP5 move apart during
flexion. In particular, the compliance or elasticity is provided by
the cables compressing the elastomeric members 72a and 72b between
stoppers elements 78a, 78b, 80a, and 80b at their respective ends.
In this way, the implant provides an elastic tension on the spinous
processes which provides a force that resists flexion. The force
increases as the processes move further apart and the rubber or
elastomeric blocks become more compressed. Usually, the straps or
cables themselves will be essentially non-compliant so that the
degree of elasticity or compliance may be controlled and provided
solely by the nature of the elastomeric members in compliance
members 16.
[0011] While potentially robust over millions of cycles of use, the
"compressive" compliance members of the '017 application can have
difficulty in providing controlled elastic tension within the
relatively low 25 N/mm to 75 N/mm range set forth in the
application. The use of compressive rubber or elastomeric blocks in
the compliance members also limits the length of device elongation
which can be achieved. Even if the initial compression provided by
the block is within the target elastic resistance range, the
stiffness of the compressive block would be expected to rise
quickly and potentially fall outside of the target range as the
block is further compressed by pulling of the spinous processes on
the upper and lower straps. Moreover, even such relatively "low"
stiffnesses above 25N/mm can present some risk of damage or trauma
to the spinous processes and other parts of the vertebrae and
spine. In order to reduce the compressive force and increase the
compressive length, the size of the compressive block may be
increased. Increasing the size of the compressive block, however,
increases the overall size of the device and is undesirable. The
need to have the straps or cables traverse the entire length of the
compressive block also increases the size and complexity of the
implant structure. Increasing the size of the device is undesirable
for many reasons, including making implantation more difficult,
while increasing the complexity of the device is undesirable as it
increases the risk of failure.
[0012] For these reasons, it would be desirable to provide improved
spinal implants and methods for their use in inhibiting flexion in
patients suffering from discogenic pain. It would be particularly
desirable if the improved devices could reliably and repeatedly
provide relatively low initial tension on the spinous processes and
a relatively low elastic resistance to flexion, even over
relatively long lengths of travel. Moreover, any risk of damage to
the vertebrae of spine should be minimized. In addition, the
devices should have a relatively small size with a decreased
complexity in order to facilitate implantation and reduce the risk
of failure. Furthermore, the devices should be designed to continue
to function even after being cycled for long periods of time (e.g.
up to multiple years of implantation) through high numbers of
cycles (e.g. up to millions of cycles) and as such should exhibit
primarily elastic behavior with minimal plasticity, i.e., low
creep. At least some of these objectives will be met by the
inventions described hereinbelow.
[0013] 2. Description of the Background Art
[0014] US Patent Publication No. 2005/0216017A1 has been described
above. US 2005/0192581 describes an orthopedic tether which can
have a stiffness from at least 1 N/mm to at least 200N/mm and which
can be used for many purposes, including wrapping spinous
processes. Other patents and published applications of interest
include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600;
5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374;
5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318;
5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582;
5,902,305; Re. 36.221; 5,928,232; 5,935,133; 5,964,769; 5,989,256;
6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030;
6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944;
6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533;
6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720;
6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application
Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US
2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US
2004/0243239; US 2005/0033435; US 2005/0049708; US 2006/0069447; US
2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096;
Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO
02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO 2004/052246 A1;
WO 2004/073532 A1; and Published Foreign Application Nos. EP
0322334 A1; and FR 2 681 525 A1. The mechanical properties of
flexible constraints applied to spinal segments are described in
Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine
22:596-604; and Garner et al. (2002) Eur. Spine J. S186-S191; A1
Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997)Arch.
Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al.
(2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5:
489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka
et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000)
Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23,
No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol
2:259-264.
SUMMARY OF THE INVENTION
[0015] The present invention provides methods and apparatus for
relieving symptoms of lumbar pain associated with flexion of a
spinal segment of a patient. The lumbar pain may arise from a
variety of particular conditions such as those described previously
herein. The devices and methods will dynamically limit flexion of
at least one spine segment by increasing the bending stiffness of
the spinal segment by a preselected amount, typically in the range
from 0.1 Nm/deg to 2 Nm/deg, preferably from 0.4 Nm/deg to 1 Nm/deg
Usually, the bending stiffness is increased by coupling an elastic
constraint between a superior spinous process and an inferior
spinous process or between an L5 spinous process and a sacrum of
the patient. The elastic constraint may have an elastic tensile
stiffness in the range from 7.5 N/mm to 40 N/mm, where the
constraint may be positioned at a lateral distance in the range
from 25 mm to 75 mm in a posterior direction from a center of
rotation of the spinal segment. The bending stiffness will be
increased during flexion (but not extension) of the spinal segment,
usually being increased over the full range of flexion. The full
flexion-extension range of motion of the spinal segment will
typically be from 3 to 20 degrees, usually from 5 to 15 degrees.
The flexion portion of the total range of motion of the spinal
segment is expressed as an angle measured relative to the neutral
position (defined below) and will typically be from 2 degrees to 15
degrees, usually from 4 degrees to 10 degrees. The bending
stiffness will be increased over at least 75% of the full range
flexion, usually over the full range of flexion as well as 25% of
the extension range of motion.
[0016] In another aspect of the present invention, the symptoms of
lumbar pain associated with flexion may be relieved by constraining
flexion of a spinal segment by limiting spreading of the spinous
processes of a spinal segment to a maximum distance in the range
from 1 mm to 10 mm, preferably from 2 mm to 8 mm. Optionally, the
bending stiffness will be increased over the constrained range of
flexion which is allowed. For example, the range of movement may be
limited and the bending stiffness increased using a device having
an elastic component together with stops or other mechanical
constraints which provide a "hard stop" to prevent extension of the
device beyond the allowed limited spreading distance of the spinous
processes.
[0017] The present invention still further provides a compliance
member for attaching tethers, typically being substantially
inelastic, which circumscribe spinal processes for use in the
methods of the present invention. The compliance member will
comprise a body having a first tether attachment element and a
second tether attachment element, where the body defines an axial
tension spring between said attachments. The compliance members
will typically be used in pairs, and systems according to the
present invention will include first and second compliance members
together with first and second tethers, typically inelastic tethers
adapted to attach between the attachment elements on the compliance
members so that the tethers may be placed over a superior spinous
process and beneath an inferior spinous process in order to provide
the elastic constraint and/or bending stiffness required by the
methods herein. With such systems, compliance members will
typically be located laterally adjacent to and vertically spanning
the spinous processes of the spinal segment being treated. It has
been found that compliance members having a maximum axial length of
34 mm (typically being in the range from 15 mm to 30 mm), a maximum
depth in an anterior-posterior direction of 18 mm (typically being
in the range from 8 mm to 15 mm), and a maximum width in the
direction normal to the depth of 15 mm (typically being in the
range from 7 mm to 10 mm), have been found to be particularly
useful in conforming to the anatomy of most patients. Systems
comprising of a pair of compliance members in combination with
first and second inelastic tethers are also provided. The inelastic
tethers usually have central regions adapted to be received over
the spinous processes, with a thickness no greater than 2 mm and a
width typically in the range from 3 mm to 10 mm, preferably from 5
mm to 8 mm.
[0018] The preferred methods and systems of the present invention
will provide a minimum and preferably no elastic resistance to
extension of the spinal segments. The preferred methods and systems
of the present invention will usually be coupled to the spinous
processes via flexible straps which, by virtue of their placement
around the spinous processes and their flexible nature, make it
very difficult for the preferred methods and systems of the present
invention to provide any resistance to extension. Furthermore, the
implants of the present invention will usually be free from
structure located between adjacent spinous processes, although in
some cases structure may be provided where the structure does not
substantially interfere with or impede the convergence of the
spinous processes as the spine undergoes extension. While some
small amount of elastic resistance to extension might be found, it
will preferably be below 3 N/mm, more preferably below 1 N/mm, and
usually below 0.5 N/mm.
[0019] Similarly, the preferred methods and systems of the present
invention will provide a minimum and preferably no elastic
resistance to lateral bending or rotation of the spinal segments.
The preferred methods and systems of the present invention will
usually be coupled to the spinous processes via flexible straps
which, by virtue of their placement around the spinous processes
and their flexible nature, make it very difficult for the preferred
methods and systems of the present invention to provide any
resistance to lateral bending or rotation. This is particularly
true in the lumbar spine where the range of motion in rotation is
usually limited to .+-.3.degree.. While some small amount of
elastic resistance to lateral bending or rotation might be found,
it will preferably be small.
[0020] As used herein, the phrase "spinal segment" refers to the
smallest physiological motion unit of the spine which exhibits
mechanical characteristics similar to those of the entire spine.
The spinal segment, also referred to as a "functional spinal unit"
(FSU), consists of two adjacent vertebrae, the intervertebral disk,
and all adjoining ligaments and tissues between them. For a more
complete description of the spinal segment or FSU, see White and
Panjabi, Clinical Biomechanics of the Spine, J. B. Lippincott,
Philadelphia, 1990.
[0021] As used herein, "neutral position" refers to the position in
which the patient's spine rests in a relaxed standing position. The
"neutral position" will vary from patient to patient. Usually, such
a neutral position will be characterized by a slight curvature or
lordosis of the spine where the spine has a slight anterior
convexity and slight posterior concavity. In some cases, the
presence of the constraint of the present invention may modify the
neutral position, e.g. the device may apply an initial force which
defines a new neutral position having some small extension of the
untreated spine. As such, the use of the term "neutral position" is
to be taken in context of the presence or absence of the device. As
used herein, "neutral position of the spinal segment" refers to the
position of a spinal segment when the spine is in the neutral
position.
[0022] As used herein, "segmental flexion" refers to the motion
between adjacent vertebrae in a spinal segment as the patient bends
forward. Referring to FIG. 1A, as a patient bends forward from the
neutral position of the spine, i.e. to the right relative to a
curved axis A, the distance between individual vertebrae L on the
anterior side decreases so that the anterior portion of the
intervertebral disks D are compressed. In contrast, the individual
spinous processes SP on the posterior side move apart in the
direction indicated by arrow B. Segmental flexion thus refers to
the relative movement between adjacent vertebrae as the patient
bends forward from the neutral position illustrated in FIG. 1A.
[0023] As used herein, "segmental extension" refers to the motion
of the individual vertebrae L as the patient bends backward and the
spine extends from the neutral position illustrated in FIG. 1A. As
the patient bends backward, the anterior ends of the individual
vertebrae will move apart. The individual spinous processes SP on
adjacent vertebrae will move closer together in a direction
opposite to that indicated by arrow B.
[0024] As used herein, the phrases "elastic resistance" and
"elastic stiffness" refer to an application of constraining force
to resist motion between successive, usually adjacent, spinous
processes such that increased motion of the spinous processes
results in a greater constraining force. The elastic resistance or
stiffness will, in the inventions described herein, inhibit motion
of individual spinal segments by, upon deformation, generating a
constraining force transmitted directly to the spinous processes or
to one or more spinous process and the sacrum. The elastic
resistance or stiffness can be described in units of stiffness,
usually in units of force per deflection such as Newtons per
millimeter (N/mm). In some cases, the elastic resistance will
generally be constant (within .+-.5%) over the expected range of
motion of the spinous processes or spinous process and sacrum. In
other cases, typically with elastomeric components as discussed
below, the elastic resistance may be non-linear, potentially
varying from 33% to 100% of the initial resistance over the
physiologic range of motion. Usually, in the inventions described
herein, the pre-operative range of motion of the spinous process
spreading from the neutral or upright position to a maximum
flexion-bending position will be in the range from 2 mm to 20 mm,
typically from 4 mm to 12 mm. With the device implanted, the
post-operative range of motion of the spinous process spreading
from the neutral or upright position to a maximum flexion-bending
position will be reduced and will usually be in the range from 1 mm
to 10 mm, typically from 2 mm to 5 mm. Such spinous process
spreading causes the device to undergo deformations of similar
magnitude.
[0025] As used herein, the phrase "bending stiffness" is defined as
the resistance of the spinal segment to bending. The incremental
bending stiffness which is provided by the constraints of the
present invention may be calculated based on the elastic tensile
stiffness (or elastic resistance) of the constraint circumscribing
the spinous processes (or coupling the L5 spinous process to
sacrum) and the distance or moment arm between a center of rotation
(COR) of the spinal segment and the location at which the elastic
constraint is located on the spinous processes. As used herein, the
moment arm distance D will be expressed in meters (m) and the
elastic stiffness ES will be expressed in Newtons per millimeter
(N/mm). The units of bending stiffness, as used herein, will be
Newton-meters per degree (Nm/deg.). The increase in bending
stiffness IBS provided by the constraint of the present invention
can be calculated by the formula:
IBS=1000ESD.sup.2 (.pi./180.degree.)
where the elastic stiffness ES of the device can be measured by
testing the device on an Instron.RTM. or other tensile strength
tester, and the moment arm length D can be measured from
radiographs.
[0026] Alternatively, the increase in bending stiffness of a device
could be measured directly by placement on a cadaveric spine
segment or a suitable vertebral model. The bending stiffness of the
spine segment could be measured with and without the elastic
constraint and the increase in bending stiffness provided by the
constraint would be the difference between the two values. It would
also be possible to calculate the increase in bending stiffness by
finite element analysis.
[0027] The bending stiffness increase can thus be adjusted by
changing the tensile stiffness of the elastic constraint and/or the
distance of the moment arm. For example, once the treating
physician determines the location of the elastic constraint and the
distance between that location and the center of rotation (COR),
the physician can then choose an elastic constraint having an
appropriate elastic tensile stiffness in order to achieve a target
therapeutic increase in the bending stiffness. The location of the
center of rotation and the distance of the moment arm can be
determined from radiographic images of the target spinal segment,
typically taken in at least two positions or postures, such as in
flexion and in extension. Typically, the center of rotation will be
an average or calculated value determined by measuring
translational vectors between the two radiographic positions for
two points on a vertebra. Such techniques are described in detail,
for example, in Musculoskeletal Biomechanics. Paul Brinckmann,
Wolfgang Frobin, Gunnar Leivseth (Eds.), Georg Thieme Verlag,
Stuttgart, 2002; p. 105. It would also be possible to employ the
instantaneous axis of rotation (IAR), which location varies
depending on the degree of spinal flexion or extension. Generally,
however, using the COR is preferred since it is a fixed and readily
determined value, although the device may affect the location of
the COR in some cases.
[0028] Thus, the bending stiffness applied by a constraining
structure according to the present invention is increased when the
spinal segment moves beyond the neutral position and will depend on
several factors including the elastic characteristics of the
constraining structure, the position of the constraining structure
on the spinous processes, the dimensions of the constraining
structure, and the patient's anatomy and movement. The constraining
structure will usually be positioned so that the upper and lower
tethers engage the middle anterior region of the spinous process
(25 mm to 75 mm posterior of the COR), and the dimensions of the
constraining structure will usually be adjusted so that the tethers
are taut, i.e. free from slack, but essentially free from tension
(axial load) when the spinal segment is in its neutral position,
i.e., free from flexion and extension. As the segment flexes beyond
the neutral position, the constraining structure will immediately
provide an elastic resistance in the ranges set forth above.
[0029] In some cases, the dimensions and assembly of the construct
will be selected so that the tethers and compliance members are
slightly pre-tensioned even before the compliance members are under
load. Thus, the constraining structure may apply a predetermined
resistive force, typically in the range from 7.5N to 40N, as soon
as the spinal segment flexes from the neutral position. In the
absence of such pre-tensioning, the compliance members would apply
a zero resistive force at the instant they are placed under load.
In all cases, as the segment flexes beyond the treated neutral
position, the constraining structure will provide increasing
bending stiffness in the ranges set forth above.
[0030] Usually, the constraining structures will apply minimal or
no bending stiffness when the spinal segment is in the neutral
position. In some instances, however, it may be desirable to
tighten the constraining structure over the spinous processes so
that a relatively low finite bending stiffness force (typically in
the range from 0.1 Nm/deg to 2 Nm/deg, usually from 0.4 Nm/deg to 1
Nm/deg) is applied even before flexion while the spinal segment
remains at a neutral position. In this case, the additional
stiffness afforded by the constraining structure will affect the
entire flexion range of motion; as well as a portion of the
untreated extension range of motion of the spinal segment.
[0031] The relative increase in bending stiffness afforded by the
constraining structures of the present invention is advantageous
because it allows the constraining structure to cause the treated
segment to resist flexion sufficiently to relieve the underlying
pain or instability with a reduced risk of injury from excessive
force. In particular, the preferred bending stiffness ranges set
forth above provide sufficient constraint to effect a significant
change in flexion in the typical patient while allowing a
significant safety margin to avoid the risk of injury. The bending
stiffness provided by the constraints of the present invention will
limit the separation of the spinous processes on the treated spinal
segment which is desirable both to reduce flexion-related pain and
spinal instability.
[0032] The resistance to flexion provided by the elastic
constraints of the present invention may reduce the angular
range-of-motion (ROM) relative to the angular ROM in the absence of
constraint. Angular ROM is the change in angle between the inferior
end plate of the superior vertebral body of the treated segment and
the superior endplate of the inferior vertebral body of the treated
segment when the segment undergoes flexion. Thus, the treatments
afforded by the elastic constraints of the present invention will
provide a relatively low angular ROM for the treated segment, but
typically a ROM higher than that of a fused segment.
[0033] While the constraint structures of the present invention
will limit flexion, it is equally important to note that in
contrast to spinal fusion and immobilizing spinal spacers, the
methods and devices of the present invention will allow a
controlled degree of flexion to take place. Typically, the methods
and devices of the present invention will allow a degree of flexion
which is equal to at least about 20% of the flexion that would be
observed in the absence of constraint, more typically being at
least about 33%. By reducing but not eliminating flexion, problems
associated with fusion, such as increased pain, vertebral
degeneration, instability at adjacent segments, and the like, may
be overcome.
[0034] The constraint structures of the present invention will act
to restore the stiffness of a spinal segment which is "lax"
relative to adjacent segments. Often a patient with flexion-related
pain or instability suffers from a particular looseness or laxity
at the painful segment. When the patient bends forward or sits
down, the painful, lax segment will preferentially flex relative to
the stiffer adjacent segments. By adjusting the length, position,
or other feature of the devices of the present invention so that
constraint structure is taut over the spinous processes when the
spinal segment is in its neutral position, the stiffness of the
treated segment can be "normalized" immediately as the patient
begins to impart flexion to the spine. Thus, premature and/or
excessive flexion of the target spinal segment can be inhibited or
eliminated.
[0035] The protocols and apparatus of the present invention allow
for individualization of treatment. Compliance members with
different stiffnesses, elongations (lengths of travel), placement
location in the anterior posterior direction along the spinous
processes and other characteristics can be selected for particular
patients based on their condition. For example, patients suffering
from a severe loss of stiffness in the target spinal segment(s) may
be treated with devices that provide more elastic resistance.
Conversely, patients with only a minimal loss of natural segmental
stiffness can be treated with devices that provide less elastic
resistance. Similarly, bigger patients may benefit from compliance
members having a greater maximum elongation, while smaller patients
may benefit from compliance members having a shorter maximum
elongation.
[0036] For some patients, particularly those having spinal segments
which are very lax, having lost most or all of their natural
segmental stiffness, the present invention can provide for
"pre-tensioning" of the constraining structure. As described above,
one way to accomplish this is by shortening the constraining
structure such that a small amount of tension is held by the
constraining structure when the spine is in the neutral or slightly
extended initial position. Alternatively, pre-tensioned compliance
elements can be provided to pre-tension the constraining structure
without changing its length. The tension or compression elements
utilized in the compliance members of the present invention, such
as coil springs, elastomeric bodies, and the like, will typically
present little or no elastic resistance when they are first
deformed. Thus, there will be some degree of elongation of the
compliance members prior to the spinal segment receiving a
therapeutic resistance. To provide a more immediate relief, the
tension or compression members may be pre-tensioned to have an
initial static resistive force which must be overcome to initiate
deformation. In this way, a constrained spinal segment will not
begin to flex at the instant the patient begins to flex her or his
spine which is an advantage when treating lax spinal segments.
Certain specific embodiments for achieving such pre-tensioning are
described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a schematic diagram illustrating the lumbar region
of the spine including the spinous processes (SP), facet joints
(FJ), lamina (L), transverse processes (TP), and sacrum (S).
[0038] FIG. 1A is a schematic illustration illustrating a portion
of the lumbar region of the spine taken along a saggital plane.
[0039] FIGS. 1B and 1C illustrate a spinal segment having a center
of rotation (COR) both in a neutral position (FIG. 1B) and in a
fully flexed position (FIG. 1C).
[0040] FIG. 2 illustrates a spinal implant of the type described in
US 2005/0216017A1.
[0041] FIG. 3 is a schematic illustration of the systems of the
present invention comprising superior and inferior tether
structures and right and left compliance members.
[0042] FIG. 4 illustrates an exemplary coil spring tension
member.
[0043] FIG. 4A illustrates the coil spring tension member of FIG. 4
illustrating the preferred dimensions.
[0044] FIGS. 5A and 5B illustrate a sheath and placement of the
sheath over the coil spring tension member of FIG. 4.
[0045] FIGS. 6A-6C illustrate the use of a locking mechanism
incorporated in the tension member of FIG. 4 for removably securing
a band member of a tether structure.
[0046] FIGS. 7A-7C illustrate a second exemplary tension element
suitable for incorporation in a compliance member in accordance
with the principles of the present invention. In this embodiment,
the tension element comprises an elastomeric body having superior
and inferior passages which define tether connectors.
[0047] FIGS. 8A-8C illustrate a third exemplary tension element
suitable for incorporation in a compliance member in accordance
with the principles of the present invention. In this embodiment,
the tension member comprises a ring having a single central opening
which defines superior and inferior tether connectors.
[0048] FIGS. 8D-8G illustrate a fourth exemplary tension element
suitable for incorporation in a compliance member in accordance
with the principles of the present invention. In this embodiment,
the tension member comprises an elastomeric body having upper and
lower cap members for attachment to superior and inferior tether
connectors.
[0049] FIGS. 8H and 8I illustrate alternative embodiments for
elastomeric tension elements suitable for incorporation into both
tension and compression compliance members.
[0050] FIGS. 9A-9B illustrate a fifth exemplary tension element
suitable for use as a compliance member in accordance with the
principles of the present invention. In this embodiment, the
tension element comprises an S-shaped spring having integral
superior and inferior tether structure connectors.
[0051] FIGS. 10A and 10B illustrate a sixth exemplary tension
element which is used as part of a compliance member in accordance
with the principles of the present invention. In this embodiment,
the tension element comprises a helical spring having a lever arm
or cam-locking tether connector and including a braided sheath for
protecting the spring.
[0052] FIGS. 10C and 10D illustrate an alternative method for
joining a coil spring tension member to a connector.
[0053] FIG. 11 illustrates a particular technique for connecting a
coil spring tension member to upper and lower connector
members.
[0054] FIGS. 12A and 12B illustrate a constraint assembly similar
to that shown in FIGS. 10A and 10B where the sheath contains
elements which minimize sheath interaction with the tension element
and/or limit the maximum elongation of the assembly under
tension.
[0055] FIGS. 13A and 13B illustrate an accordion-type sheath which
could potentially also limit maximum elongation.
[0056] FIGS. 14A and 14B illustrate tension and compression members
having pre-tensioned tension elements.
[0057] FIGS. 15A and 15B are force-displacement graphs which
illustrate the difference between the pre-tensioned and
non-pre-tensioned tension and compression members.
[0058] FIG. 16 illustrates a spinous process constraint structure
incorporating a rigid frame work for coupling compliance members to
adjacent spinous processes.
[0059] FIG. 17 illustrates a spinous process constraint structure
having the superior and inferior tether structures each of which
comprise a plurality of individual coupling elements.
[0060] FIGS. 18 and 19 illustrate the use of indicators which
provide readings of displacement and/or force between the
compliance member and the associated tether, where the indicated
information is useful in initial positioning and/or subsequent
monitoring of the performance of the spinous process constraint
system.
DETAILED DESCRIPTION OF THE INVENTION
[0061] Exemplary spinous process constraints according to the
present invention are illustrated schematically in FIG. 3. The
systems 10 comprise a superior tether structure 12, and inferior
tether structure 14, and right compliance member 16 and a left
compliance member 18. The superior tether structure 12 will
typically be a continuous band, cable, strap, cord, or other
structure which extends between the two compliance members and
provides a saddle region 20 which is adapted to lie over and
conform to a superior surface of a superior spinous process SSP as
described in more detail in the related prior applications which
have been incorporated herein by reference. The inferior tether
structure 14 will typically comprise a band, cable, or the like
which is constructed similarly if not identically to the superior
tether structure 12 and has a saddle region 22 adapted to lie over
and conform to an inferior surface of an inferior spinous process
22. In certain instances, however, the inferior tether structure 14
may comprise separate bands, cables, straps, cords, or the like,
14a and 14b, shown in broken line, which have anchors 15a and 15b
at their lower ends and are adapted to be separately attached to an
inferior vertebrae or more commonly to a sacrum. The use of such
separate tether structures for inferior attachment are described in
more detail in co-pending application Ser. No. 11/827,980 (Attorney
Docket No. 026398-000120US), the full disclosure of which has been
previously incorporated herein by reference. The tether structures
will usually be flexible but effectively non-compliant so that they
allow minimum elongation under tensile load.
[0062] The right and left compliance members 16 and 18 will usually
have similar or identical constructions and include an adjustable
attachment component 32 and a fixed attachment component 34 for
securing connecting segments of the superior and inferior tether
structures 12 and 14. Usually, each compliance member 16 and 18
will have one of the tether structures 12 and 14 pre-attached to
the fixed attachment component 34. The two subassemblies can then
be introduced onto opposite sides of the spinous processes, and the
tether structures placed over the spinous processes or otherwise
attached to the vertebral bodies, as generally described in
co-pending application Ser. No. 11/875,674 (Attorney Docket No.
026398-000150US), the full disclosure of which is incorporated
herein by reference.
[0063] The present invention is particularly concerned with the
nature of the tension elements 30, and a number of specific
embodiments will be described hereinbelow. In general, the tension
elements 30 will elastically elongate as tension is applied by the
superior and inferior tether structures 12 and 14 through the
attachments 32 and 34, in the direction shown by arrow 36. As the
spinous processes or spinous process and sacrum move apart during
flexion of the constrained spinal segment, the superior and
inferior tether structures 12 and 14 will also move apart, as shown
generally in broken line in FIG. 3. A tension element 30 will
elastically resist the spreading with a force determined by the
mechanical properties of the tension member. In particular, the
tension members will be selected to have a tensile or elastic
stiffness, also known as a spring constant, in the relatively low
ranges set forth above. Such low elastic constricting forces
provide a number of advantages when compared to complete
restriction or constriction with a high elastic force as described
above.
[0064] The tension elements of the present invention will be
positioned over adjacent spinous processes, or over the L5 spinous
process and adjacent sacrum, in order to increase the bending
stiffness of the spinal segment. Referring to FIGS. 1B and 1C, the
bending resistance is the resistance to bending of the spinal
segment about a center of rotation (COR) positioned generally
within or adjacent to the disk between adjacent vertebral bodies.
The center of rotation can be determined from radiographic images,
generally as described above, and it can be seen that a point PS on
the superior spinous process SPS and a similar point PI on the
inferior spinous process SPI will move generally along a curved
line or arc A as shown in FIG. 1C. While the center of rotation COR
is not fixed during flexion or extension of the spinal segment, and
the points will not travel on a true arc, the motion of the spinous
processes is nonetheless arcuate in nature as illustrated.
[0065] Thus, the positioning of any of the elastic constraints as
described herein at a position on the spinous processes SPS and SPI
generally indicated by line L will define a moment arm distance dm,
as illustrated in FIG. 1B. The position L will generally be
selected so that the moment arm length dm will be in the range from
25 mm to 75 mm, preferably from 40 mm to 60 mm. By thus selecting
an elastic constraint having a stiffness in the range from 7.5 N/mm
to 40 N/mm, the desired bending stiffness of the spinal segment can
be increased by an amount in the range from 0.1 Nm/deg to 2 Nm/deg,
preferably from 0.4 Nm/deg to 1 Nm/deg.
[0066] As also shown on FIG. 1C, the spinous processes SPS and SPI
will spread to a maximum distance d.sub.s upon full flexion of the
spinal segment. In accordance with other aspects of the present
invention, it may be desirable to constrain the spreading of the
spinous processes to a maximum distance above the distance in the
neutral position (as shown in FIG. 1B) in the range from 1 mm to 10
mm, preferably from 2 mm to 8 mm. Certain of the elastic
constraints in the present invention can provide for both increased
bending stiffness and for a complete stop of flexion. See, for
example, the device described in FIGS. 12A and 12B hereinafter.
[0067] A first exemplary tension element 40 constructed in
accordance with the principles of the present invention is
illustrated in FIGS. 4, 5A and 5B. The tension element 40 comprises
a helical spring structure 41 formed from a single piece of
material. The tension member 40 includes an adjustable tether
connector 42 and a fixed tether connector 44, both of which are
preferably formed integrally or monolithically with the helical
spring structure 41. Typically, the helical spring structure 41 and
both tether connectors 42 and 44 will be formed from one piece of
material, usually being a metal such as titanium, but optionally
being a polymer, ceramic, reinforced glass or other composite, or
other material having desired elastic and mechanical properties and
capable of being formed into the desired geometry. In a preferred
embodiment, the tension member 40 is machined or laser cut from a
titanium rod. Alternatively, a suitable polymeric material will be
polyethylene ether ketone (PEEK). Other features may be built into
the tension member 40, such as a stress relief hole 46. Components
that mate with the adjustable tether connector may potentially
include a roller and a lock-nut; such components could be made from
the same material as the tension element and adjustable tether
connector (e.g. titanium components if the tension member is
titanium), or they could be made from a different material (e.g.
injection molded PEEK).
[0068] The exterior of the tension member 40 may be covered with a
protective cover, such as the elastomeric sheath 50 illustrated in
FIG. 5A. The sheath 50 may be placed over the body of the tension
member 40, as illustrated in FIG. 5B, in order to prevent the
intrusion of tissue and body materials into the spaces between the
turns of the coil and interior of the element.
[0069] Referring now to FIG. 4A, preferred dimensions for the
tension member 40 are illustrated. In order to accommodate the
patient anatomy when the tension members are arranged laterally of
and vertically between adjacent spinous processes, as generally
shown in FIG. 3, the compliance member will have a length l of 38
mm or less, preferably in the range from 20 mm to 30 mm, a depth d
in the anterior-posterior direction no greater than 18 mm,
preferably in the range from 8 mm to 15 mm, and a width in the
direction normal to depth no greater than 15 mm, preferably in the
range from 7 mm to 10 mm.
[0070] A free end 53 of the tether structure 52 may be attached to
the adjustable tether connector 42, as illustrated in FIG. 6A
through 6C. Initially, a barrel locking mechanism 54 is
rotationally aligned such that a slot 56 is aligned with an inlet
opening 58 on the top of the connector 42 and an outlet opening 60
on the side of the connector. The inlet opening 58 is located
centrally and providing a primarily axial load on the compliance
member, thereby evenly loading the compliance member and having the
advantages described above. The free end 55 of tether 52 is then
advanced through the inlet opening 58, slot 56, and outlet opening
60, as illustrated in FIG. 6C. By then rotating the barrel lock 54
90.degree. to 180.degree., the tether 52 will be locked in place in
the connector 42, as shown in FIG. 6A. It will be appreciated that
this simple locking mechanism allows tether 52 to be appropriately
tensioned for the individual patient before locking the tether in
place. A locking feature, e.g. set screw, nut, or pin (not shown)
would then be used to lock the tether and roller in place,
providing additional resistance to unfurling and opening. The
tensioning could be performed separately and/or simultaneously
during implantation of the constraint assembly. Additional features
of the mechanism such as pins, shoulders, or other features which
control the travel of the roller or lock-nut may aid in the
alignment and operation of the mechanism.
[0071] Another tether structure (not illustrated) will be attached
to the fixed connector 44 at the other end of the tension element
40, typically using a pin (not illustrated). The pin may be
anchored in a pair of receiving holes 62, and a free end of the
tether wrapped over the pin and firmly attached. Usually, the fixed
tether structure will be pre-attached at the time of manufacture so
that the treating physician can implant each of the pair of tension
members, with one tether structure attached to the fixed tether
connector. The remaining free ends of each tether structure 52 may
then be deployed around the spinous processes (or attached to a
sacrum) in a pattern generally as shown in FIG. 3.
[0072] An alternative tension element 66 comprising an elastomeric
body 68 is illustrated in FIGS. 7A-7C. The elastomeric body 68
comprises a central tensile segment 70 joined by a pair of ring
connectors 72 and 74. The entire structure will be molded or cast
from an elastomeric material having mechanical properties that
provide the desired elastic stiffness or spring force, as set forth
above. A particularly suitable elastomer is silicone rubber, but
other thermoplastics and thermosetting elastomers could also be
used.
[0073] The tension elements 66 may be joined to tether structures
76 and 78, as shown in FIG. 7B. As with the prior embodiments, each
tether structure 76 and 78 is fixably attached to one tension
element 66 at one end and adjustably attached to the other tension
element at the other end. In particular, one end of the superior
tether structure 76 is fixedly attached to the upper end of the
left tension member 66 by wrapping around a shackle 80 which is
attached to the ring connector 72 with a pin or bolt 82. Similarly,
one end of the inferior tether structure 78 is fixedly attached to
ring connector 72 on the lower end of the right tension element 66
using a shackle 82.
[0074] In contrast, adjustable attachment of the tether structures
76 and 78 is provided by a cord 84 which may be loosened or
tightened in a locking structure comprising mating surfaces on a
nut 86 and pin 88 assembly, as shown in FIG. 7C. The pin 88 is
received in the ring connector 74 and holds a threaded cup 90 in
place. The nut 86 is threadably received in the cup 90 and can be
axially translated relative to the mating surface of pin 88. Thus,
the cord 84 may be passed freely through the assembly when the nut
86 is loosened. Once the desired tension is placed on the tether
structure 76 or 78, the nut 86 can be tightened to hold the cord 84
in place.
[0075] A further alternative embodiment of a tension element 100 is
illustrated in FIGS. 8A-8C. The tension element 100 comprises a
single elastomeric ring structure 102 having a large central
opening 104. The elastomeric ring can be formed from any of the
elastomers listed above for element 66. A pair of the tension
elements 100 may be held in place by tether structures 106, as
illustrated in FIG. 8B. Ends of the tether structures 106 may be
looped through the central opening 104 to provide a continuous
circumferential structure. As illustrated in FIG. 8B, there is no
adjustability of the circumferential length of the structure. It
will be appreciated, however, that at least one of the tether ends
may be left free so that the loop may be tightened and then held in
place, for example using crimping structure 110, as illustrated in
FIG. 8C. Alternatively, four tether structures could be used, each
pre-attached in the form of a permanent, closed loop around each
end of each ring. Two inferior structures could then be attached
(e.g. by crimping) to each other, and two superior structures could
similarly be attached to each other.
[0076] Referring now to FIGS. 8D-8G, an alternative elastomeric
tension element 200 comprises an elongate elastomeric body 202
having an upper cap member 204 and a lower cap member 206. The body
202 is formed from any of the elastomeric materials listed above
and will provide an elastic resistance to elongation when opposite
tensile forces are placed on the caps 204 and 206.
[0077] The elastomeric tension elements 200 may be incorporated
into a superior tether structure 210 and an inferior tether
structure 212, as seen in FIGS. 8E and 8F. Each of the tether
structures 210 and 212 comprises a sheath 214 which is formed from
a braided polymer or other substantially non-distensible fabric,
textile or other material, typically being formed from polyester or
polyethylene. The sheath has a generally tubular structure, and the
braided or other fabric structure allows it to be radially expanded
to accommodate an elastic tension element 200 at one end. As best
seen in FIG. 8G, the elastic tension element 200 is placed in an
end of the sheath 214 and secured by rings or bands 220 which are
placed over the exterior of the sheath and which are held in place
by collars 222. The collars 222 are typically formed from
biologically inert polymer or metal, such as PEEK or titanium, and
serve to transfer load from the sheath 214 to the cap members 204
and 206 and thus the elastomeric body 202 as a tensile load is
placed on the sheath 214.
[0078] While the superior and inferior tether structures 210 and
212 could be joined in a variety of ways, a particularly convenient
approach is to form a connecting loop 230 at the end of the sheath
214 which holds the elastic tensioning element 200. The loop 230
may be formed simply by stretching and folding the end of the
sheath and attaching the end to the body of the sheath, by heat
sealing, adhesives, crimps, or the like. After the loop is formed,
the two tether structures 210 and 212 may be joined into a
continuous loop for placement over the spinous processes by drawing
distal ends 232 of each sheath 214 through the loop 230 of the
opposite tether structure, as best seen in FIG. 8F. Once the proper
tension is applied to the tether structures 210 and 212 by pulling
on the distal ends, the distal end may be fixed in place, typically
using an anchor 234 which can be crimped in place. Preferably but
not necessarily, a slack region 240 will be provided in the sheath
214 between the retention rings 220 to allow desired elongation of
the elastomeric body 202.
[0079] FIGS. 8H and 8I illustrate particular embodiments for
elastomeric tension and compression elements suitable for use in
the spinous process constraint systems of the present invention. In
FIG. 8H the elastomeric tension element 300 is formed similarly to
tension element 200 and includes an elastomeric body 302 having an
upper cap member 304 and a lower cap member 306. The tension member
300 will typically also include collars 308 which are used to
couple the tension member to tether structures. In order to enhance
and control the elasticity or spring constant of the elastomeric
body 302, the body is formed with a corrugated or "accordion"
profile. The accordion profile allows the degree of elasticity to
be increased relative to a similar sized elastomeric body having a
cylindrical profile.
[0080] An elastomeric compression member 320 is illustrated in FIG.
8I. The elastomeric compression member 320 may be formed from any
of the elastomeric materials described above, but will be formed to
have a number of holes or voids 322. The formation of elastomers
having such holes or voids may be accomplished by molding or
extruding the elastomer with materials that are later removed to
leave the voids in place. The presence of such voids in the
elastomeric body 320 serves to enhance or help control the
compressive elasticity of the member. Typically, the superior and
inferior tether members 330 and 332 will pass through the body and
be anchored on opposite ends to end caps 334 and 336, respectively,
so that axial tension on the tether structures, as indicated by the
outward arrows, will compress the elastomeric body 320, as
indicated by the inwardly facing arrows.
[0081] Another flexion restriction system 120, as illustrated in
FIGS. 9A-9B, comprises a pair of leaf spring structures 122, each
of which includes an S-shaped center portion 122 and two tether
connectors 126. Superior and inferior tether structures 130 and 132
respectively, each have two free ends which are adjustably received
in the tether connectors 126. Each of the tether connectors 126
includes a screw with clamping surfaces 132 which may be loosened
or tightened in order to permit adjustment of the tension on the
tether structure as desired, as shown in FIG. 9B. The S-shaped
center portion 124 of the leaf spring structure 122 may be formed
from a metal, polymer, reinforced composite, or any other material
which can be fabricated to provide an elastic stiffness or spring
constant within the ranges described above. The tether connectors
126 may be formed integrally or monolithically with the center
portions 124, or alternatively may be formed separately and adhered
using adhesives, fasteners, or the like.
[0082] Referring now to FIGS. 10A and 10B, a further flexion
restriction system 140 in accordance with the principles of the
present invention will be described. As with all previously
described systems, the system 140 comprises a pair of compliance
members 142 attached to superior and inferior tether structures 144
and 146, respectively. The compliance members 142 each comprise a
fixed tether connector 148 and an adjustable tether connector 150.
The tether connectors 148 and 150 are joined by a coil spring 152
(best seen in FIG. 10B) which is enclosed within a textile sheath
154. The superior tether structure 144 is fixably connected to the
fixed tether connector 148 on the left hand side compliance member
142, while the inferior tether structure 146 is fixedly connected
to the fixed connector 148 on the right compliance member 142. Each
of the adjustable tether connectors 150 includes a latch arm cam
lock 156 which may be lifted or opened, as shown in FIG. 10A, to
allow a free end of the tether structure 144 or 146 to be advanced
therebeneath so that the tether can be tightened or cinched over an
adjacent spinous process. Once the tether structure 144 or 146 has
been sufficiently tightened, the latch arm 156 may be closed, as
shown in FIG. 10B, to hold the tether structure 144 or 146 firmly
in place. To prevent loosening, each latch arm cam lock 146 may be
provided with surface textures or other gripping features such as
spikes or chevrons 158.
[0083] In the embodiment of FIGS. 10A and 10B, the coil spring 152
may be secured to the fixed and adjustable tether connectors 148
and 150 by any conventional technique. In certain cases, however,
it may be desirable to provide a pivotable or adjustable
connection, as shown in FIG. 11. There, ball joints 160 may be
formed on superior and inferior connectors 162 and 164,
respectively. A coil spring 166 may have converging ends 168 which
can be secured over the ball joints to provide a universal joint
therebetween.
[0084] Coil spring tension members may be secured to both fixed and
adjustable tether connectors, such as connectors 148 and 150 in
FIGS. 10A and 10B, in a variety of ways. As shown in FIGS. 10C and
10D, a tether connector 250 may be attached to an end of a coil
spring 252 using a threaded receiving component 254 which can be
screwed in to the coil spring, as illustrated in FIG. 10D. The
threaded receiving component 254 mates with the coil spring,
typically by threadably engaging the internal grooves on the coil
spring, thus evenly spreading the tension across the end of the
spring. Optionally, or alternatively, the receiving component 254
can be welded in place, held in place by a suitable adhesive, or be
held in place by various secondary fasteners, such as screws,
rivets, or the like.
[0085] Referring now to FIGS. 12A and 12B, yet another alternative
construction of a flexible restraint system 170 will be described.
The flexible restraint system 170 may be identical in all respects
to the flexible restraint system 140 as described previously.
Instead of a mesh sheath, however, the flexible restraint system
170 includes a sheath having a plurality of battens or wires 172
which reduce interactions between the sheath and restraint system,
as well as provide an axial constraint to limit the maximum axial
separation of the fixed and adjustable tether connectors 174 and
176, respectively. As shown in FIG. 12A, the battens 172 are
axially compressed so that they bow outwardly, distancing the
sheath from the tensile member. In FIG. 12B, the fixed and
adjustable tether connectors 174 and 176 have moved to their
maximum axial separation, straightening the battens 172.
[0086] Referring now to FIGS. 13A and 13B, yet another flexible
restraint system 180 constructed in accordance with the principles
of the present invention will be described. The flexible restraint
system 180 is similar to those of both systems 170 and 140, except
that the sheath structure has an accordion fold to provide for
lengthening and shortening together with the movement of fixed and
adjustable tether connectors 182 and 184, respectively. The
accordion folds both permit greater gross elongation of the sheath
with lower material strains than in a purely cylindrical sheath and
potentially reduce interaction between the sheath and tensile
member. The sheath with the accordion fold may or may not act as a
constraint on maximum elongation of the compliance members. The
sheath could also be used with separate tension members for
providing the maximum elongation limit.
[0087] Pre-tensioning or pre-loading of compliance members is
illustrated in FIGS. 14A and 14B. In FIG. 14A, a compliance member
260 includes a spring compression member 262 placed in a housing
264 and a superior tether structure 266 which is secured to a
piston 268 which is free to slide within an interior chamber 270 of
the housing 264. The coil spring 262 is disposed between an upper
surface of the piston 268 and the lower surface of the top end of
the housing 260. If the coil spring 262 is sized so that it
occupies the space between the piston and the top end of the
housing without any compression, then the compliance member 260
will have no pre-tensioning or pre-loading. If, however,
pre-tensioning is desired, the spring 262 will be chosen to be
slightly longer than the distance between the piston and the top
end of the housing so that the spring 262 is under compression even
when there is no tension being placed on the superior tether 266 or
the inferior tether 267. Note that the degree of pre-tensioning can
be controlled by selecting the position of retaining shoulders 270
formed on the interior surface of the housing 264. The compliance
member 260 will apply elastic resistance to spreading of tension
members 266 and 267 as the spring compresses in the direction of
the arrows.
[0088] An alternative compliance member 280 is illustrated in FIG.
14B. Compliance member 280 includes a tension spring member 282
received in the interior 284 of housing 286. A superior tether
structure 288 is attached to an upper end of the housing 286 and an
inferior tether structure 290 is attached to a piston 292 slidably
received in the interior 284 of the housing. When tension is
applied to the superior and inferior tether structures 288 and 290,
tension will be transferred to the spring in the direction of the
arrows which will elastically resist spreading apart of the tether
structures.
[0089] Movement of the piston 292 is constrained by a shoulder 294
formed about the circumference of the interior 284. If the spring
282 is selected so that its length is equal to the length between
the piston (when engaged against the shoulder 294) and the upper
end of the interior 284, then there will be no pre-tensioning of
the spring. If, however, the spring is selected so that it is
shorter than the distance between the piston 292 and the upper end
of the chamber 284, then the spring will be in tension at all
times, even prior to elongation by placing a force between the
tether structures. In this way, the initial displacement of the
tether structures relative to each other will act to overcome the
pre-tensioning force of the spring.
[0090] The effect of pre-tensioning on the kinematics of the
compliance members is best understood with reference to FIGS. 15A
and 15B. In FIG. 15A, the relationship between force F and
displacement D for a compliance member without pre-tensioning is
illustrated. Prior to displacement, when the displacement is zero,
the spring force will be essentially zero. The spring force will
increase linearly from zero depending on the spring constant k as
illustrated. When the tension member 262 or 284 is pre-tensioned,
however, the initial force imparted by the compliance member will
be F.sub.0 (greater than zero), as shown in FIG. 15B. The magnitude
of F.sub.0 is determined by the degree of pre-tensioning, typically
being in the range from 0N to 50N, usually from 5N to 25N, for the
compliance members herein. Once displacement begins, however, the
increase in force (F-F.sub.0) will be linear and again determined
by the spring constant k.
[0091] As illustrated thus far, spinous constraint structures of
the present invention have generally included flexible, typically
non-distensible, tethers or bands adjoining the superior and
inferior ends of the compliance members. Instead of employing such
flexible tether structures, the compliance members could be joined
by a rigid frame structure 340, as illustrated in FIG. 16. For
example, compliance members 342 could be joined to superior and
inferior yokes 344 and 346, each of which include a central
engagement member 348 for placement over the superior and inferior
spinous processes. Optionally, the engagement members 348 could be
pivotally attached between a pair of adjacent wing members 350. The
wings members 350, in turn, could be coupled to the compliance
members 342 using rods or posts 352, where the rods or posts 352
are optionally threaded to allow adjustment and tightening of the
yokes 344 and 346 over the spinous processes. The compliance
members 342 could have any of the tension members and coupling
structures described previously in order to connect to the posts
352.
[0092] Referring now to FIG. 17, as a further alternative to the
tether structures which have been previously described, compliance
members 360 could be joined by superior and inferior tether
structures 362 and 364 each of which comprises a plurality of
individual coupling elements 366. The individual coupling elements
366 could include filaments, strands, fibers, wires, small diameter
cables, and the like, composed of polymers, metals, metal-polymer
composites, and the like. Coupling elements could be simple
constant diameter elongate elements, but could alternatively
comprise regions of different characteristics, including elastic
regions, spring-like regions, rigid regions, or the like. The
individual fibers will typically be free to move relative to each
other so that they independently function to couple the compliance
members 360 together. In that way, should any of the coupling
elements 366 fail, the remaining coupling elements would not be
compromised. Alternatively, the individual coupling elements 366
could be woven or braided together along a portion of or their
entire lengths. An advantage of the use of individual coupling
elements is that the elements may spread and conform to the
particular geometry of the spinous process providing a more stable
connection. Certain embodiments, in the coupling elements could be
composed entirely or in part of a material that promotes tissue in
growth, such as tantalum.
[0093] In some instances, it may be desirable to incorporate the
ability to monitor displacement and/or tension force between the
tether structures and the compliance members. As illustrated in
FIG. 18, an upper tether member 380 can be provided with a scale or
other indicia 382 that indicates the displacement or band length.
Alternatively, the scale or indicia 382 could be calibrated to show
displacement force.
[0094] The displacement or force measurement could also be provided
in an indicator window 390 in a compliance member 392, as shown in
FIG. 19. Often, the indicia will be visible by the physician during
the implantation procedure. Alternatively, the indicia could be
transmitted for reading during implantation procedure and
optionally after implantation. The indicia could be configured so
that it is visible on imaging procedures, such as x-rays, MRI'S and
the like.
[0095] While the above is a complete description of the preferred
embodiments of the invention, various alternatives, modifications,
and equivalents may be used. Therefore, the above description
should not be taken as limiting the scope of the invention which is
defined by the appended claims.
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