U.S. patent application number 10/333284 was filed with the patent office on 2003-09-11 for semirigid linking piece for stabilizing the spine.
Invention is credited to Le Couedic, Regis, Pasquet, Denis.
Application Number | 20030171749 10/333284 |
Document ID | / |
Family ID | 8852865 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030171749 |
Kind Code |
A1 |
Le Couedic, Regis ; et
al. |
September 11, 2003 |
Semirigid linking piece for stabilizing the spine
Abstract
The invention concerns a linking piece designed to maintain a
spacing between at least two anchoring elements screwed into
vertebrae. The piece comprises at least: a flexible part (10)
divided into two branches (12, 14), the ends of said branches being
interconnected in pairs and defining a first neutral plane (Pm),
and, two rigid parts (20, 22) forming rods, having a first fixing C
portion (20', 22') and a second fixing portion (20", 22"), each
said second portion (20", 22") of said rigid parts (20, 22)
extending respectively in two opposite directions said ends of said
branches interconnected in pairs, such that said linking piece,
whereof the fixing portions (20', 22') are respectively fixed on
each of the anchoring elements is designed to bend elastically
perpendicularly to said neutral plane (Pm).
Inventors: |
Le Couedic, Regis;
(Bordeaux, FR) ; Pasquet, Denis; (Pessac,
FR) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
8852865 |
Appl. No.: |
10/333284 |
Filed: |
January 16, 2003 |
PCT Filed: |
July 25, 2001 |
PCT NO: |
PCT/FR01/02425 |
Current U.S.
Class: |
606/254 ;
606/257; 606/261; 606/272; 606/907 |
Current CPC
Class: |
A61B 17/7026 20130101;
A61B 17/7032 20130101; A61B 17/7011 20130101 |
Class at
Publication: |
606/61 |
International
Class: |
A61B 017/70 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2000 |
FR |
00/09705 |
Claims
1. A connecting member for maintaining the spacing between at least
two anchor members screwed into vertebrae, the connecting member
being characterized in that it comprises: a flexible part divided
into two spaced continuous branches that are substantially
symmetrical about a longitudinal axis of said member, the ends of
said branches being interconnected in pairs and defining a first
median plane, and two rigid rod-forming parts each having a fixing
first portion and a second portion, each of said second portions of
said two rigid parts respectively extending said ends of said
branches interconnected in pairs in opposite directions, the
cross-section of each of said branches being less than the
cross-section of said rigid parts so that said connecting member,
said fixing portions of which are fixed to respective anchor
members, is able to bend elastically about an axis that is
perpendicular to the longitudinal axis, said perpendicular axis
being contained in said median plane of the connecting member on
relative movement of the vertebrae, whereby the vertebrae, which
are held spaced relative to each other, are movable relative to
each other.
2. A connecting member according to claim 1, adapted to
interconnect n anchor members, the connecting member being
characterized in that it comprises n rigid parts between which
there are disposed n-1 flexible parts along the longitudinal axis
of said member, each rigid part situated between two flexible parts
having a first or fixing portion and two second portions, each of
said second portions being situated at a respective end of said
first or fixing portion, said second portions extending respective
ends of the branches of said two flexible parts so that the median
planes of all the flexible parts are substantially the same, and in
that the rigid parts situated at the ends of said member have
respective single second portions extending the ends of the
branches of the flexible parts.
3. A connecting member according to claim 1 or claim 2,
characterized in that the sum of the surface areas of the sections
of said two branches is less than the surface area of the section
of said rigid rod-forming parts.
4. A connecting member according to any one of claims 1 to 3,
characterized in that said branches have portions parallel to each
other and to the longitudinal axis of said member and in that the
distance between said branches is at least equal to the length of
said portions.
5. A connecting member according to claim 4, characterized in that
the distance between the ends of said branches connected in pairs
lies in the range 1.5 times to 2.5 times the distance between said
branches.
6. A connecting member according to any one of claims 1 to 5,
characterized in that the section of said rigid rod-forming parts
is circular.
7. A connecting member according to any one of claims 1 to 6,
characterized in that it is made of titanium alloy.
8. A vertebral stabilization system for fastening together at least
two vertebrae each having a median plane substantially
perpendicular to the axis of the spine of which they are part and a
posterior wall defining a posterior median plane of said spine,
said system comprising at least two anchor members each adapted to
be fixed into the posterior wall of a vertebra so that a line which
intersects said two anchor members is substantially parallel to
said axis of the spine, the system being characterized in that it
further comprises a connecting member according to any one of
claims 1 to 7 whose two rigid parts are adapted to interconnect
said two anchor members so that said median plane defined by said
two branches is substantially parallel to said posterior median
plane of said spine, whereby said vertebrae, which are
interconnected in their posterior portions, are relatively movable
along said axis of said spine.
Description
[0001] The present invention relates to a connecting member for
maintaining the spacing between at least two anchor members which
are interconnected by said connecting member.
[0002] Fields of application of the invention include stabilization
and arthrodesis of segments of the vertebral column in degenerative
pathologies of the spine.
[0003] Systems for stabilizing the vertebral column which brace at
least two consecutive vertebrae by means of anchor members fixed
into said vertebrae and connected by rigid connecting rods are well
known in the art. Systems of this kind are generally coupled
systems such that two consecutive vertebrae are interconnected by
two substantially parallel rods fixed one on each side of the
spinous processes. The anchor members are screwed into the
posterior portion of the vertebrae and pass through the pedicles
and a substantial portion of the vertebral bodies and therefore
provide a fixed and durable connection.
[0004] The above stabilizing systems are routinely used to
consolidate several consecutive vertebrae. Thus the vertebrae are
interconnected by rigid rods over a substantial length of the
vertebral column. Such assemblies hold the vertebrae correctly
relative to each other; however, they considerably stiffen the
spine in terms of bending. It has been shown that a more flexible
stabilizing system, which confers greater relative mobility on the
vertebrae, is beneficial in some pathologies.
[0005] Naturally enough, to increase the amplitude in bending of
the stabilizing system, it has been proposed to reduce the section
of the connecting rods between the anchor members to increase the
amount by which said connecting rods deform for the same stress.
This increases the mobility of the vertebrae relative to each other
and also increases the amplitude of forward/rearward bending of the
vertebral column for the same force. However, the vertebral column
is not so well stabilized, in particular with regard to lateral
stabilization of the vertebrae relative to each other. Also,
stresses are higher in connecting rods of smaller section and the
connecting rods may deteriorate prematurely.
[0006] A first object of the present invention is to provide a
connecting member for maintaining the spacing between existing
anchor members and which can bend more than the rods used at
present without increasing the internal stresses in said connecting
rods.
[0007] To achieve the above object, a connecting member in
accordance with the invention for maintaining the spacing between
at least two anchor members screwed into vertebrae comprises: a
flexible part divided into two spaced continuous branches that are
substantially symmetrical about a longitudinal axis of said member,
the ends of said branches being interconnected in pairs and
defining a first median plane, and two rigid rod-forming parts each
having a fixing first portion and a second portion, each of said
second portions of said two rigid parts respectively extending said
ends of said branches interconnected in pairs in opposite
directions, the cross-section of each of said branches being less
than the cross-section of said rigid parts so that said connecting
member, said fixing portions of which are fixed to respective
anchor members, is able to bend elastically about an axis that is
perpendicular to the longitudinal axis, said perpendicular axis
being contained in said median plane of the connecting member on
relative movement of the vertebrae, whereby the vertebrae, which
are held spaced relative to each other, are movable relative to
each other.
[0008] Thus one characteristic feature of the connecting member is
its shape, which is such that the stresses exerted on its rigid
parts by bending of the spine cause the two branches to bend in a
direction perpendicular to the median plane that they define. This
is because the connecting member can bend only in a direction
perpendicular to the median plane that the two branches define
because the two branches are joined at each end by the rigid parts
and bending about another axis would longitudinally elongate one of
the branches and longitudinally compress the other one; given the
stresses, this would cause little deformation. In this way, the
connecting member has a specific bending direction perpendicular to
its main axis and is disposed so that the median plane defined by
the two branches is substantially perpendicular to the plane in
which the spine is able to bend. As explained in more detail below,
the section of each of the continuous branches, which is
substantially constant, is smaller than the section of the rigid
parts, which increases the amplitude by which the member bends for
the same force. Also, the bending stresses, representative of the
internal forces in the two branches, are lower than the stresses to
which a single branch would be subjected when bent by the same
amount by the same force. This reduces fatigue of the connecting
member of the invention.
[0009] The connecting member is advantageously adapted to
interconnect n anchor members and comprises n rigid parts between
which there are disposed n-1 flexible parts along the longitudinal
axis of said member, each rigid part situated between two flexible
parts having a first or fixing portion and two second portions,
each of said second portions being situated at a respective end of
said first or fixing portion, said second portions extending
respective ends of the branches of said two flexible parts so that
the median planes of all the flexible parts are substantially the
same, and the rigid parts situated at the ends of said member
advantageously have respective single second portions extending the
ends of the branches of the flexible parts.
[0010] Thus, by virtue of this feature, the connecting member
maintains the spacing between all the anchor members that it
interconnects, each of which is fixed to a vertebra, to align them.
Each rigid part is fixed to an anchor member and between the anchor
members of each pair there is a flexible part extended by said
rigid parts. In this way, a single connecting member stabilizes a
plurality of vertebrae, which reduces the time needed to assemble
the stabilizing system as a whole, and consequently the operating
time. Also, this feature of the connecting member stabilizes a
plurality of consecutive vertebrae by connecting them together,
while at the same time allowing them great relative
flexibility.
[0011] In a particular embodiment of said connecting member, the
sum of the surface areas of the sections of said two branches is
less than the surface area of the section of said rigid rod-forming
parts. Thus the rigid rod-forming parts are more rigid than the two
branches which bend more readily, said rigid parts being securely
connected to said anchor members.
[0012] In another particular embodiment of the invention said
branches have portions parallel to each other and to the
longitudinal axis of said member and the distance between said
branches is at least equal to the length of said portions. This
configuration provides a connecting member which can bend much more
in a direction perpendicular to the median plane defined by the
branches than in any other direction; thus, allowing for the forces
exerted on it, the connecting member has a single bending
direction.
[0013] In a further particular embodiment of the invention, the
distance between the ends of said branches connected in pairs lies
in the range 1.5 times to 2.5 times the distance between said
branches. This allows increased bending compared to the connecting
rods routinely used, while at the same time obtaining a single
bending direction perpendicular to the median plane defined by the
two branches.
[0014] Also, the section of said rigid rod-forming parts is
advantageously circular, which facilitates manufacture of the
member, which is preferably made of titanium alloy. Also, if prior
art circular section connecting rods are to be replaced by
connecting members conforming to the invention without making it
necessary to replace the anchor members, it is necessary for said
rigid parts to have sections identical to the sections of prior art
connecting rods.
[0015] Titanium alloys have mechanical and corrosion resistant
properties compatible with the technical specifications required of
the connecting member.
[0016] The present invention also provides a vertebral
stabilization system for fastening together at least two vertebrae
each having a median plane substantially perpendicular to the axis
of the spine of which they are part and a posterior wall defining a
posterior median plane of said spine, said system comprising at
least two anchor members each adapted to be fixed into the
posterior wall of a vertebra so that a line which intersects said
two anchor members is substantially parallel to said axis of the
spine, which system further comprises a connecting member of the
invention whose two rigid parts are adapted to interconnect said
two anchor members so that said median plane defined by said two
branches is substantially parallel to said posterior median plane
of said spine, whereby said vertebrae, which are interconnected in
their posterior portions, are relatively movable along said axis of
said spine.
[0017] Other features and advantages of the invention will emerge
on reading the following description of particular embodiments of
the invention, which is given by way of non-limiting example and
with reference to the accompanying drawings, in which:
[0018] FIG. 1 is a diagrammatic perspective view of a connecting
member of the invention,
[0019] FIG. 2 is a diagrammatic perspective view showing anchor
members connected by the connecting member, and
[0020] FIG. 3 is a diagrammatic side elevation view of a vertebral
column showing two consecutive vertebrae into which there are
screwed anchor members interconnected by a connecting member of the
invention.
[0021] The various portions of a connecting member of the invention
are described initially with reference to FIG. 1.
[0022] The connecting member 8 comprises a flexible part 10 having
two parallel branches 12 and 14 interconnected at their ends at two
points 16 and 18. Two rigid rod-forming parts 20 and 22 have
respective fixing first portions 20', 22' and second portions 20",
22" extending the joined together ends of the two branches 12 and
14 from the two points 16 and 18 in opposite directions along a
longitudinal axis A. The two parallel branches 12 and 14 joined at
their ends therefore surround a void 24. They are also parallel to
the axis A and symmetrical about that axis. The sum of the sections
of the two branches 12 and 14 is preferably less than the section
of the rigid parts which have substantially the same circular
section, with the result that the diameter of the cross-section of
a branch is less than 70% of the diameter of the rigid parts. For
example, the diameter of the cross-section of a branch lies in the
range 50% to 70% of the diameter of the rigid parts.
[0023] Also, the two branches 12 and 14 define a median plane Pm
intersecting the two rigid parts 20 and 22 axially.
[0024] Thus the connecting member having the first fixing portions
20' and 22' of its rigid parts 20 and 22 in fixed bearing
engagement, is itself able to bend elastically in a direction Dp
perpendicular to the median plane Pm when a force F is exerted on
each of the branches 12 and 14. This is because, given the section
of the branches 12 and 14 relative to the section of the rigid
parts 20 and 22, the radius of curvature of the connecting member
when it bends is at a minimum at the center of the flexible part
10.
[0025] The sum of the forces F exerted on the branches in a
direction perpendicular to the median plane Pm causing the
connecting member to bend without breaking would not cause the
connecting member to bend in a direction perpendicular to the axis
A of the member and contained in the median plane Pm because, in
this case, one of the branches would be in tension and the other in
compression, which would prevent bending. A preferential bending
direction is therefore obtained, which is advantageous in the
context of the intended application, as explained in more detail
below.
[0026] To obtain a preferential bending direction and an optimum
bending amplitude, given the dimensions of the connecting portion,
the two branches 12 and 14 are separated by a distance that is not
less than the length of their parallel portions. Obviously, the
closer together the branches 12,14, the greater the potential for
bending about an axis not strictly perpendicular to the median
plane Pm, and the force necessary to obtain said bending increases
as the total length of the branches 12 and 14 decreases. However,
the spacing between the two branches 12 and 14 is limited by the
space available for inserting said member, both transversely and
along the longitudinal axis, and consequently the distance between
the ends of said branches connected in pairs is in the range 1.5
times to 2.5 times the distance between said branches.
[0027] To compare the stress to which a connecting member of the
invention is subjected with the stress to which a simple prior art
connecting rod is subjected, a rod of diameter 1 was made whose
bending amplitude was to be increased by 50%. To achieve this, the
diameter of the simple connecting rod had to be reduced by 10%, the
consequence of which was a 35% increase in the stress in it. In
contrast, to obtain the same amplitude of bending, the diameter of
the branches of the connecting member of the invention had to
represent 75% of the original diameter of the rod of diameter 1,
although the stress in the branches was increased by only 13%.
[0028] The above measurements showed that, for the same bending,
the connecting member of the invention was subjected to lower
stresses than a conventional connecting rod, thereby reducing metal
fatigue, so that its service life should be longer than prior art
rods. Also, the lower the stresses in the connecting member, the
better it retains its elastic properties.
[0029] Deformation of the connecting member on relative movement of
the anchor members 26 and 28 is described below with reference to
FIG. 2.
[0030] FIG. 2 shows the two rigid parts 20 and 22 of the connecting
member interconnecting two anchor members 26 and 28. The two anchor
members 26 and 28 are parallel to each other in a common axial
plane Pa. The connecting member is fixed to the anchor members 26
and 28 so that the median plane Pm defined by the branches 12 and
14 is substantially perpendicular to the common axial plane Pa.
[0031] Each anchor member 26, 28 has a threaded shank 30 with a
U-shaped head 32 at the top whose inside wall is threaded so that a
screw-forming member 34 can be screwed into it. Thus the first or
fixing portions 20' and 22' of the rigid parts 20 and 22 are
accommodated in the heads 32 of the respective anchor members 26
and 28 and are locked to them by tightening the screw-forming
members 34.
[0032] In this way, when the threaded shanks 30 of the anchor
members tend to move towards each other due to the effect of
opposite forces T and -T in the plane Pa and substantially parallel
to the axis A, the anchor members 26 and 28 deform the connecting
member to an arcuate shape in the plane Pa. The stress induces
bending of the branches 12 and 14 of the connecting member in a
direction perpendicular to the median plane Pm. When the stress is
removed, the connecting member reverts to its original rectilinear
shape and the threaded shanks of the anchor members 30 return to
their former relative position.
[0033] The mechanism of elastic bending of the connecting member
and the anchor members described above is the same if the threaded
shanks 30 of the anchor members 26 and 28 move away from each
other, the connecting member forming a reversed arcuate shape.
[0034] The use of the connection member 8 in a vertebral
stabilizing system for fastening together at least two vertebrae V1
and V2 is described below with reference to FIG. 3.
[0035] Each vertebra V1, V2 has a respective median plane PV1, PV2
substantially perpendicular to the axis Ar of the spine of which it
is a part and a respective posterior wall PPV1, PPV2 defining a
posterior median plane PPr of said spine.
[0036] The stabilizing system includes at least two anchor members
26 and 28 respectively screwed into the posterior walls PPV1 and
PPV2 of the vertebrae V1 and V2 so that a line L that intersects
the two anchor members 26 and 28 is substantially parallel to said
axis Ar of the spine. The two first or fixing portions 20' and 22'
of a connecting member 8 interconnect the two anchor members 26 and
28 so that said median plane Pm defined by said branches 12 and 14
is substantially parallel to said posterior median plane PPr of
said spine. In this way, the vertebrae V1 and V2, which are
interconnected in their posterior portions, are relatively movable
along the axis Ar of the spine.
[0037] Thus when the spine is stretched, the vertebrae V1 and V2
move away from each other in respective directions E and -E, which
causes the threaded shanks 30 to move away from each other,
deforming the connecting member 8, and in particular its flexible
part 10. The deformed connecting member has its concave side facing
away from the spine.
[0038] When the spine is bent, the inverse effect occurs and the
vertebrae V1 and V2 move towards each other, which induces
deformation of the connecting member with its concave side facing
toward the spine.
[0039] Obviously, because it can bend more than prior art
connecting rods, the connecting member 8 of the invention increases
the mobility of the vertebrae relative to each other. Also, because
of its construction, with two parallel branches 12, 14 connected
together at their ends and extended by the rigid parts 20, 22, the
connecting member 8 allows the spine to bend and extend in the
plane Pa while at the same time limiting bending in the plane Pm
perpendicular to the plane Pa. Thus the spine is stabilized
laterally and relative movement of the vertebrae is limited.
[0040] In a particular embodiment, not shown, the connecting member
has three rigid rod-forming parts interconnected by flexible parts.
To this end, the central rigid part has two second portions
extending respective ends of said fixing portion, said second
portions respectively extending the ends of the two interconnected
branches of the two flexible parts. The rigid parts extend the ends
of the interconnected branches so that the median planes of both
flexible parts are substantially the same. A longitudinal member is
thus obtained having two rigid first portions, one at each end, and
a central rigid part between the two rigid first portions, the
rigid parts being interconnected in pairs by flexible parts.
[0041] The connecting member therefore maintains the spacing
between the three anchor members that it interconnects, which
members are fixed to three substantially equidistant vertebrae, to
align them. Each rigid part of the connecting member is fixed to an
anchor member so that there is a flexible part between pairs of
vertebrae. In this way a single connecting member stabilizes three
vertebrae, which reduces the time needed to assemble the
stabilizing system as a whole and consequently the operating time.
Also, because the three vertebrae are interconnected by a single
connecting member, their mobility relative to one another is better
controlled.
[0042] It goes without saying that providing connecting members
having more than three rigid parts separated by flexible parts
would not depart from the scope of the invention.
* * * * *