U.S. patent application number 11/342195 was filed with the patent office on 2007-08-16 for spinal rods having different flexural rigidities about different axes and methods of use.
This patent application is currently assigned to SDGI Holdings, Inc.. Invention is credited to Jeff R. Justis, Fred J. IV Molz, Michael C. Sherman.
Application Number | 20070191841 11/342195 |
Document ID | / |
Family ID | 37983604 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070191841 |
Kind Code |
A1 |
Justis; Jeff R. ; et
al. |
August 16, 2007 |
Spinal rods having different flexural rigidities about different
axes and methods of use
Abstract
A vertebral rod has an elongated body extending along a
longitudinal axis. The rod also includes a cavity extending the
length of the body. Either the body or the cavity may have an
asymmetrical shape about a centroid in a plane perpendicular to the
longitudinal axis. Alternatively, both may have the symmetrical
shape about the centroid. The body of the rod may be bounded by an
exterior surface and the cavity. The body has a first bending axis
that is perpendicular to longitudinal axis. The body also has a
second bending axis that is perpendicular to the longitudinal axis
and to the first bending axis. The body of the rod may be
distributed asymmetrically about the first and second bending axes.
Also, the rod may have a different bending stiffness about the
first and second bending axes.
Inventors: |
Justis; Jeff R.;
(Germantown, TN) ; Molz; Fred J. IV; (Birmingham,
AL) ; Sherman; Michael C.; (Memphis, TN) |
Correspondence
Address: |
COATS & BENNETT, PLLC
1400 Crescent Green, Suite 300
Cary
NC
27518
US
|
Assignee: |
SDGI Holdings, Inc.
|
Family ID: |
37983604 |
Appl. No.: |
11/342195 |
Filed: |
January 27, 2006 |
Current U.S.
Class: |
606/250 ;
606/254 |
Current CPC
Class: |
A61B 17/7002 20130101;
A61B 17/701 20130101; A61B 17/7029 20130101 |
Class at
Publication: |
606/061 |
International
Class: |
A61F 2/30 20060101
A61F002/30 |
Claims
1. A vertebral rod comprising: a body extending along a first axis
and having a length along the first axis between a first end and a
second end; a cavity extending the length of the body; at least one
of the body and the cavity having an asymmetrical shape about a
centroid in a plane perpendicular to the first axis.
2. The vertebral rod of claim 1 wherein the cavity is centered
about the first axis.
3. The vertebral rod of claim 1 wherein the body is centered about
the first axis.
4. The vertebral rod of claim 1 wherein the body and the cavity are
centered about the first axis.
5. The vertebral rod of claim 1 wherein the cavity is defined by an
inner surface having a first shape and the body is defined by an
outer surface having a second shape, first and second shapes being
different.
6. The vertebral rod of claim 1 wherein the cavity is defined by an
inner surface having a first shape and the body is defined by an
outer surface having a second shape, first and second shapes being
the same.
7. The vertebral rod of claim 1 further comprising markings
indicating an orientation of the asymmetrical shape.
8. A vertebral rod comprising: a body having a cavity, each
extending along a first axis and each having a length along the
first axis between a first end and a second end, the body bounded
by an exterior surface and the cavity; the body having a first
bending axis that is perpendicular to the first axis and a second
bending axis that is perpendicular to the first axis and to the
first bending axis, the body being distributed asymmetrically about
the first and second bending axes.
9. The vertebral rod of claim 8 wherein the exterior surface is
asymmetric about the first and second bending axes.
10. The vertebral rod of claim 8 wherein the cavity is asymmetric
about the first and second bending axes.
11. The vertebral rod of claim 8 wherein the exterior surface and
the cavity are each asymmetric about the first and second bending
axes.
12. The vertebral rod of claim 8 wherein the cavity is interior to
the exterior surface.
13. The vertebral rod of claim 8 wherein the cavity intersects with
the exterior surface.
14. The vertebral rod of claim 8 further comprising markings
indicating an orientation of the asymmetric distribution of the
body.
15. A vertebral rod comprising: a body extending along a first axis
and having a length along the first axis between a first end and a
second end, the body having a first cross sectional shape
substantially perpendicular to the first axis; a cavity extending
the length of the body, the cavity having a second cross sectional
shape substantially perpendicular to the first axis; and the first
cross sectional shape and the second cross sectional shape being
different.
16. The vertebral rod of claim 15 wherein the second cross
sectional shape is interior to the first cross sectional shape.
17. The vertebral rod of claim 15 wherein the second cross
sectional shape intersects the first cross sectional shape.
18. A vertebral rod comprising: a body having a cavity, each
extending along a first axis and each having a length along the
first axis between a first end and a second end, the body bounded
by an exterior surface and the cavity; the body having a first
bending axis that is perpendicular to the first axis and a second
bending axis that is perpendicular to the first axis and to the
first bending axis, the body having different area moments of
inertia about the first and second bending axes.
19. The vertebral rod of claim 18 wherein the exterior surface
defines an area having different area moments of inertia about the
first and second bending axes.
20. The vertebral rod of claim 18 wherein the cavity has different
area moments of inertia about the first and second bending
axes.
21. The vertebral rod of claim 18 wherein the exterior surface and
the cavity each have different area moments of inertia about the
first and second bending axes.
22. The vertebral rod of claim 18 wherein the cavity is interior to
the exterior surface.
23. The vertebral rod of claim 18 wherein the cavity intersects
with the exterior surface.
24. The vertebral rod of claim 18 further comprising markings
indicating an orientation of the asymmetric area moments of
inertia.
Description
BACKGROUND
[0001] Spinal or vertebral rods are often used in the surgical
treatment of spinal disorders such as degenerative disc disease,
disc herniations, scoliosis or other curvature abnormalities, and
fractures. Different types of surgical treatments are used. In some
cases, spinal fusion is indicated to inhibit relative motion
between vertebral bodies. In other cases, dynamic implants are used
to preserve motion between vertebral bodies. For either type of
surgical treatment, spinal rods may be attached to the exterior of
two or more vertebrae, whether it is at a posterior, anterior, or
lateral side of the vertebrae. In other embodiments, spinal rods
are attached to the vertebrae without the use of dynamic implants
or spinal fusion.
[0002] Spinal rods may provide a stable, rigid column that
encourages bones to fuse after spinal-fusion surgery. Further, the
rods may redirect stresses over a wider area away from a damaged or
defective region. Also, a rigid rod may restore the spine to its
proper alignment. In some cases, a flexible rod may be appropriate.
Flexible rods may provide some advantages over rigid rods, such as
increasing loading on interbody constructs, decreasing stress
transfer to adjacent vertebral elements while bone-graft healing
takes place, and generally balancing strength with flexibility.
[0003] Aside from each of these characteristic features, a surgeon
may wish to control anatomic motion after surgery. That is, a
surgeon may wish to inhibit or limit one type of spinal motion
following surgery while allowing a lesser or greater degree of
motion in a second direction. As an illustrative example, a surgeon
may wish to inhibit or limit motion in the flexion and extension
directions while allowing for a greater degree of lateral bending.
However, conventional rods tend to be symmetric in nature and may
not provide this degree of control.
SUMMARY
[0004] Illustrative embodiments disclosed herein are directed to a
vertebral rod having an elongated body extending along a
longitudinal axis. The rod also includes a cavity extending the
length of the body. Either the body or the cavity may have an
asymmetrical shape about a centroid in a plane perpendicular to the
longitudinal axis. Alternatively, both may have the symmetrical
shape about the centroid. The body of the rod may be bounded by an
exterior surface and the cavity. The body has a first bending axis
that is perpendicular to longitudinal axis. The body also has a
second bending axis that is perpendicular to the longitudinal axis
and to the first bending axis. The body of the rod may be
distributed asymmetrically about the first and second bending axes.
Also, the rod may have a different bending stiffness about the
first and second bending axes. The cavity may be contained within
or intersect the exterior surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a perspective view of first and second assemblies
comprising spinal rods attached to vertebral members according to
one or more embodiments;
[0006] FIG. 2 is a lateral view of a spinal rod according to one or
more embodiments; and
[0007] FIGS. 3-20 are axial views of a spinal rod illustrating
cross sections according to different embodiments.
DETAILED DESCRIPTION
[0008] The various embodiments disclosed herein are directed to
spinal rods that are characterized by a cross section that provides
different flexural rigidities in different directions. Various
embodiments of a spinal rod may be implemented in a spinal rod
assembly of the type indicated generally by the numeral 20 in FIG.
1. FIG. 1 shows a perspective view of first and second spinal rod
assemblies 20 in which spinal rods 10 are attached to vertebral
members V1 and V2. In the example assembly 20 shown, the rods 10
are positioned at a posterior side of the spine, on opposite sides
of the spinous processes S. Spinal rods 10 may be attached to a
spine at other locations, including lateral and anterior locations.
Spinal rods 10 may also be attached at various sections of the
spine, including the base of the skull and to vertebrae in the
cervical, thoracic, lumbar, and sacral regions. In one embodiment,
a single rod 10 is attached to the spine. Thus, the illustration in
FIG. 1 is provided merely as a representative example of one
application of a spinal rod 10.
[0009] In one embodiment as illustrated in FIG. 1, the spinal rods
10 are secured to vertebral members V1, V2 by pedicle assemblies 12
comprising a pedicle screw 14 and a retaining cap 16. The outer
surface of spinal rod 10 is grasped, clamped, or otherwise secured
between the pedicle screw 14 and retaining cap 16. Other mechanisms
for securing spinal rods 10 to vertebral members V1, V2 include
hooks, cables, and other such devices. Examples of other types of
retaining hardware include threaded caps, screws, and pins. Spinal
rods 10 are also attached to plates in other configurations. Thus,
the exemplary assemblies 12 shown in FIG. 1 are merely
representative of one type of attachment mechanism.
[0010] The rod 10 may be constructed from a variety of surgical
grade materials. These include metals such as stainless steels,
cobalt-chrome, titanium, and shape memory alloys. Non-metallic
rods, including polymer rods made from materials such as PEEK and
UHMWPE, are also contemplated. Further, the rod 10 may be straight,
curved, or comprise one or more curved portions along its
length.
[0011] FIG. 2 shows a spinal rod 10 of the type used in the
exemplary assembly 20 in FIG. 1. The rod 10 has a length between a
first end 17 and a second end 18 extending along a longitudinal
axis A. Other Figures described below show various embodiments of a
spinal rod 10 characterized by different cross sections viewed
according to the view lines illustrated in FIG. 2. For instance,
FIG. 3 shows one example cross section of the spinal rod 10a. In
this embodiment, the spinal rod 10a is comprised of an oval or
elliptical outer surface 22a and an interior cavity or aperture 30a
defined by an inner surface 32a. In one embodiment, the outer
surface 22a and inner surface 32a are uniformly consistent along
the entire length L of the rod 10a. That is, the cross section
shown in FIG. 3 may be the same at all points along the length L of
the rod 10a. The same may also be true of other cross sections
described below. In one or more embodiments, the cross section of a
rod 10 may vary along the length L of the rod 10.
[0012] The structural characteristics of the rod 10 may be
dependent upon several factors, including the material choice and
the cross section shape of the rod 10. The flexural rigidity, which
is a measure of bending stiffness, is given by the equation:
Flexural Rigidity=E.times.I (1) where E is the modulus of
elasticity or Young's Modulus for the rod material and I is the
moment of inertia of a rod cross section about the bending axis.
The modulus of elasticity varies by material and reflects the
relationship between stress and strain for that material. As an
illustrative example, titanium alloys generally possess a modulus
of elasticity in the range between about 100-120 GPa. By way of
comparison, implantable grade polyetheretherketone (PEEK) possesses
a modulus of elasticity in the range between about 3-4 Gpa, which,
incidentally, is close to that of cortical bone.
[0013] In general, an object's moment of inertia depends on its
shape and the distribution of mass within that shape. The greater
the concentration of material away from the object's centroid C,
the larger the moment of inertia. In FIG. 3, the moments of inertia
about the x-axis I.sub.x and the y-axis I.sub.y for the area inside
the elliptical outer shape 22a (ignoring the inner aperture 30a for
now) may be determined according to the following equations:
I.sub.x=.intg.y.sup.2dA (2) I.sub.y=.intg.x.sup.2dA (3) where y is
the distance between a given portion of the elliptical area and the
x-axis and x is the distance between a given portion of the
elliptical area and the y-axis. The intersection of the x-axis and
y-axis is called the centroid C of rotation. The centroid C may be
the center of mass for the shape assuming the material is uniform
over the cross section. Since dimension h in FIG. 3 is larger than
dimension b, it follows that the moment of inertia about the x-axis
I.sub.x is larger than the moment of inertia about the y-axis
I.sub.y. This means that the oval shape defined by the outer
surface 22a has a greater resistance to bending about the x-axis as
compared to the y-axis.
[0014] The actual bending stiffness of the rod 10a shown in FIG. 3
may also depend upon the moment of inertia of the inner aperture
30a. Determining the overall flexural rigidity of the rod 10a
requires an analysis of the composite shape of the rod 10a.
Generally, the moment of inertia of a composite area with respect
to a particular axis is the sum (or difference in the case of a
void) of the moments of inertia of its parts with respect to that
same axis. Thus, for the rod 10a shown in FIG. 3, the overall
flexural rigidity is given by the following:
I.sub.x=I.sub.xo-I.sub.xi (4) I.sub.y=I.sub.yo-I.sub.yi (5) where
I.sub.xo and I.sub.xi are the moments of inertia about the x-axis
for the outer and inner areas, respectively. Similarly, I.sub.yo
and I.sub.yi are the moments of inertia about the y-axis for the
outer and inner areas, respectively.
[0015] In the present embodiment of the rod 10a shown in FIG. 3,
the inner aperture 30a is symmetric about the centroid C.
Consequently, the moments of inertia about the x and y axes for the
area inside the outer surface 22a are reduced by the same amount
according to equations (4) and (5). Still, the overall flexural
rigidity of the rod 10a is greater about the x-axis as compared to
the y-axis. Accordingly, a surgeon may elect to install the rod 10a
in a patient to correspondingly control flexion, extension, or
lateral bending. One may do so by orienting the rod 10a with the
x-axis positioned perpendicular to the motion that is to be
controlled. For example, a surgeon who elects to control flexion
and extension may orient the rod 10a with the stiffer bending axis
(x-axis in FIG. 3) approximately parallel to the coronal plane of
the patient. Conversely, a surgeon who elects to control lateral
bending may orient the rod 10a with the stiffer bending axis
(x-axis in FIG. 3) approximately parallel to the sagittal plane of
the patient. The surgeon may also elect to install the rod 10a with
the x and y axes oriented at angles other than aligned with the
sagittal and coronal planes of the patient.
[0016] It may be desirable to adjust the bending stiffness of the
rod 10 by varying the size and shape of the inner aperture 30. For
instance, a surgeon may elect to use the rods 10 disclosed herein
with existing mounting hardware such as pedicle screws or hook
saddles (not shown). Some exemplary rod sizes that are commercially
available range between about 4-7 mm. Thus, the overall size of the
rods 10 may be limited by this constraint.
[0017] FIG. 4 shows a rod 10b similar to rod 10a (i.e., outer
surface 22b is substantially similar to surface 22a) with the
exception that the inner aperture 30b defined by inner surface 32b
is larger than the inner aperture 30a of rod 10a. Using the
equations above, one is able to determine that the overall flexural
rigidity about the x and y axes is greater for rod 10a as compared
to rod 10b. Rods 10a and 10b may be available as a set with a
common outer surface 22a, 22b. However, since the rods have a
different internal aperture 30a, 30b configuration, a surgeon may
select between the rods 10a, 10b to match a desired bending
stiffness.
[0018] The internal aperture 30 may be asymmetric as well. For
example, the rod 10c shown in FIG. 5 includes an outer surface 22c
that is substantially similar to the outer surface 22a of rod 10a.
However, the inner aperture 30c defined by surface 32c is
elliptical or oval shaped. The inner aperture 30c has a height
h.sub.1 parallel to the x-axis that is less than the width b.sub.1
parallel to the y-axis. That is, the moment of inertia of the inner
aperture 30c is greater about the y-axis than about the x-axis.
This is in contrast to the outer surface 22c, which has a larger
moment of inertia about the x-axis.
[0019] The rods 10 may also have multiple inner apertures 30. For
instance, the rod 10d shown in FIG. 6 comprises a plurality of
apertures 30d, 130d defined by inner surfaces 32d, 132d. The outer
surface 22d may be substantially similar to the outer surface 22a
of rod 10a. Notably, the exemplary apertures 30d, 130d are disposed
within the interior of the rod 10d. Further, the apertures 30d,
130d are offset from the centroid C.
[0020] The embodiments described above have all had a substantially
similar, oval shaped outer surface 22. Certainly, other shapes are
possible as illustrated by the embodiment of the rod 10e shown in
FIG. 7. This particular rod 10e has a square outer surface 22e that
is substantially symmetric relative to axes X and Y. However, the
inner aperture 30e defined by inner surface 32e is asymmetric
relative to these same X and Y axes. Inner surface 32e is
substantially rectangular and defined by dimensions b and h.
Specifically, dimension b (parallel to the Y-axis) is not equal to
dimension h (parallel to the X-axis). In the embodiment shown,
dimension b is larger than dimension h. Therefore, the aperture 30e
has a larger moment of inertia relative to the Y-axis as compared
to the X-axis. Consequently, according to equations (4) and (5),
the rod 10e has a greater bending strength about the X-axis as
compared to the Y-axis.
[0021] The rod 10f shown in FIG. 8 has rectilinear inner 32f and
outer 22f surfaces. However, in contrast to rod 10e, the inner
surface 32f is substantially square and outer surface 22f is
substantially rectangular. This configuration is analogous to rod
10a shown in FIG. 3 in that the inner aperture 30f is symmetric
about the X and Y axes while the outer surface 22f is asymmetric
about the X and Y axes. The rod 10g shown in FIG. 9 has both an
inner aperture 30g and an outer surface 22g that are asymmetric
about the X and Y axes. The same is true of the rod 10c shown in
FIG. 5. However, rod 10g has an inner aperture 30g and an area
inside the outer surface 22g that have larger moments of inertia
about the same X-axis. This is due, in part, to the fact that the
rectangular inner aperture 30g and outer surface 22g are
substantially aligned.
[0022] The rod 10 may also have substantially triangular outer
surfaces 22 as evidenced by the embodiments 10h, 10i, and 10j. In
FIG. 10, the outer surface 22h is shown as an isosceles triangle
that has a larger height h (parallel to the X-axis) than base b
(parallel to the Y-axis). This may tend to yield a rod 10h having a
greater moment of inertia about the X-axis. By comparison, the rod
10i shown in FIG. 11 comprises a triangular outer surface 22i that
is substantially equilateral. The rod 10j shown in FIG. 12
comprises a substantially triangular outer surface 22j that is
substantially equilateral, albeit with non-linear sides. The inner
apertures 30h, 30i, 30j may be shaped as shown in FIGS. 10-12 or as
desired in accordance with the discussion provided above.
[0023] Other rods 10 may have polygonal shapes such as the
embodiments illustrated in FIGS. 13 and 14. The rod 10k shown in
FIG. 13 comprises a hexagonal outer surface 22k while rod 10m in
FIG. 14 comprises a pentagonal outer surface 22m. The rods 10 may
have more sides if desired.
[0024] The embodiments described thus far have included an aperture
30 that is substantially contained within the interior of the outer
surface 22. In other embodiments, the aperture 30 may intersect
with the outer surface 22. This can be seen in the exemplary
embodiments shown in FIGS. 15 and 16. In FIG. 15, the rod 10n
comprises two apertures 30n, 130n that are defined by inner
surfaces 32n, 132n. As indicated, the inner surfaces 32n, 132n
intersect the outer surface 22n resulting in open apertures 30n,
130n. The rod 10n is shaped similar to an I-beam that has a greater
moment of inertia and bending stiffness about the X-axis. By way of
comparison, the rod 10p shown in FIG. 16 also has a single open
aperture 30p defined by an inner surface 32p that intersects with
the outer surface 22p.
[0025] The rods 10 may also have a substantially circular outer
surface 22 similar to many conventional rods, thus accommodating
existing rod securing hardware (not shown). This is illustrated by
the exemplary rods 10q, 10r, and 10s shown in FIGS. 17, 18, and 19.
In each case, the outer surface 22q-s of the rod 10q-s is
substantially circular and/or characterized by a substantially
constant radius. As such, the moment of inertia about axes X and Y
is substantially the same for the areas within the outer surface
22q-s. However, the moment of inertia about the X and Y axes for
the rod 10q-s may be altered by including an asymmetric inner
aperture 30q-s.
[0026] In FIG. 17, the inner aperture 30q defined by inner surface
32q has a larger moment of inertia about the X-axis. Thus, the rod
10q has a larger moment of inertia about the Y-axis (pursuant to
equations (4) and (5)). In FIG. 18, the inner aperture 30r defined
by inner surface 32r is also substantially circular. However, the
inner aperture 30r is offset from centroid C. Further, the inner
surface 32r is tangent to the Y-axis, but spaced away from the
X-axis. Thus, the moment of inertia of the inner aperture 30r is
larger with respect to the X-axis as compared to the Y-axis.
Consequently, the moment of inertia and bending stiffness of the
overall rod 10r is larger about the Y-axis.
[0027] FIG. 19 shows another embodiment of a rod 10s having an open
inner aperture 30s. In this embodiment, the inner surface 32s has a
substantially constant radius and intersects the substantially
circular outer surface 22s. The inner aperture 30s is offset from
the centroid C, but aligned with the Y-axis in the orientation
shown. Therefore, the inner aperture 30s has a larger moment of
inertia about the X-axis. The bending stiffness of the overall rod
10s is therefore greater about the Y-axis.
[0028] FIG. 20 shows the same rod 10q as illustrated in FIG. 17. In
this particular view, the rod 10q comprises a first set of markings
34 (the - sign in the embodiment shown) and a second set of
markings 36 (the + sign in the embodiment shown). The markings 34,
36 may be stamped, engraved, or otherwise included on the rod as an
indication of the bending stiffness in the direction of the
marking. The markings 34, 36 may be included on an end 17, 18 of
the rod 10q as shown or on the outer surface 22q.
[0029] Spatially relative terms such as "under", "below", "lower",
"over", "upper", and the like, are used for ease of description to
explain the positioning of one element relative to a second
element. These terms are intended to encompass different
orientations of the device in addition to different orientations
than those depicted in the figures. Further, terms such as "first",
"second", and the like, are also used to describe various elements,
regions, sections, etc and are also not intended to be limiting.
Like terms refer to like elements throughout the description.
[0030] As used herein, the terms "having", "containing",
"including", "comprising" and the like are open ended terms that
indicate the presence of stated elements or features, but do not
preclude additional elements or features. The articles "a", "an"
and "the" are intended to include the plural as well as the
singular, unless the context clearly indicates otherwise.
[0031] The present invention may be carried out in other specific
ways than those herein set forth without departing from the scope
and essential characteristics of the invention. For example,
embodiments described above have contemplated one or two inner
apertures 30 to modify the moments of inertia about one axis
relative to another. The rods 10 do not need to be limited to this
number of apertures. The moment of inertia equations provided
herein allow one to calculate moments of inertia for any number of
apertures and flexural rigidity of the overall rod 10. The present
embodiments are, therefore, to be considered in all respects as
illustrative and not restrictive, and all changes coming within the
meaning and equivalency range of the appended claims are intended
to be embraced therein.
* * * * *