U.S. patent application number 10/356711 was filed with the patent office on 2004-05-27 for intervertebral prosthesis especially for a neck vertebral support.
This patent application is currently assigned to Waldemar Link GmbH & Co.. Invention is credited to Keller, Arnold, McAfee, Paul C..
Application Number | 20040102846 10/356711 |
Document ID | / |
Family ID | 27763370 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040102846 |
Kind Code |
A1 |
Keller, Arnold ; et
al. |
May 27, 2004 |
Intervertebral prosthesis especially for a neck vertebral
support
Abstract
Intervertebral prosthesis, in particular for the cervical spine,
consisting basically of a first cover plate (1) to be connected to
a first vertebral body, a second cover plate (2) to be connected to
the second vertebral body, and a prosthesis core (10) which is held
by a seat (5, 6, 7) of the first cover plate (1) and forms an
articulation (20, 21) with the second cover plate (2). The core
(10) is movable in the AP direction relative to the first cover
plate (1). Mobility can also be provided in the lateral direction
and rotational direction.
Inventors: |
Keller, Arnold; (Kayhude,
DE) ; McAfee, Paul C.; (Baltimore, MD) |
Correspondence
Address: |
Barry E. Bretschneider
Morrison & Foerster LLP
Suite 300
1650 Tysons Boulevard
McLean
VA
22102
US
|
Assignee: |
Waldemar Link GmbH &
Co.
Barkhausenweg 10
Hamburg
DE
D-22339
|
Family ID: |
27763370 |
Appl. No.: |
10/356711 |
Filed: |
February 3, 2003 |
Current U.S.
Class: |
623/17.11 |
Current CPC
Class: |
A61F 2220/0025 20130101;
A61F 2002/304 20130101; A61F 2002/30392 20130101; A61F 2002/443
20130101; A61F 2002/30649 20130101; A61F 2310/00011 20130101; A61F
2002/30616 20130101; A61F 2002/30397 20130101; A61F 2/4425
20130101; A61F 2002/30517 20130101; A61F 2002/30387 20130101; A61F
2002/30578 20130101 |
Class at
Publication: |
623/017.11 |
International
Class: |
A61F 002/44 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2002 |
EP |
02 005 632.1 |
Claims
1. Intervertebral prosthesis, in particular for the cervical spine,
consisting basically of a first cover plate (1, 31) to be connected
to a first vertebral body, of a second cover plate (2, 32) to be
connected to the second vertebral body, and of a prosthesis core
(10, 34) which is held by a seat of the first cover plate (1, 31)
and forms an articulation with the second cover plate (2, 32),
characterized in that the core (10, 34) is movable relative to the
first cover plate (1, 31) at least in the AP direction.
2. Prosthesis according to claim 1, characterized in that the seat
has a movement-limiting device cooperating with an edge (11, 36) of
the core (10, 34).
3. Prosthesis according to claim 2, characterized in that the
movement-limiting device is designed as a guide device in the AP
direction.
4. Prosthesis according to one of claims 1 to 3, characterized in
that the guide device is formed by two opposite, parallel lateral
guide rails (7) for guiding in the AP direction.
5. Prosthesis according to claim 4, characterized in that the
lateral guide rails (7) are undercut, and the core (10) has a ridge
(11) engaging in the undercut.
6. Prosthesis according to claim 4 or 5, characterized in that the
first cover plate (1) has a dorsal limit stop (21) limiting the
movement of the core (10).
7. Prosthesis according to one of claims 4 to 6, characterized in
that the first cover plate (1) has a ventral limit stop (14)
limiting the movement of the core (10) and removable from its
limit-stop position.
8. Prosthesis according to claim 2 or 3, characterized in that the
core (34) has a recess (35) which cooperates with a projection (37,
37a) of the first cover plate (31).
9. Prosthesis according to claim 8, characterized in that the
recess (35) has an elongate shape extending in the AP
direction.
10. Prosthesis according to claim 8 or 9, characterized in that the
projection (37, 37a) and the recess (35) have interacting undercuts
(36, 38).
11. Prosthesis according to one of claims 1 to 10, characterized in
that the articulation impedes or inhibits the rotation movement
between the upper cover plate (2, 32) and the prosthesis core (10,
34), and the core (10, 34) can pivot about the vertical axis
relative to the first cover plate (1).
12. System of intervertebral prostheses, in particular for the
spinal column, consisting basically of a first cover plate to be
connected to a first vertebral body, of a second cover plate to be
connected to the second vertebral body, and of a prosthesis core
which is held by a seat of the first cover plate and forms an
articulation with the second cover plate, characterized in that, in
addition to the intervertebral prosthesis types according to one of
claims 1 to 11, the system includes types of corresponding external
configuration which do not have AP mobility between the prosthesis
core and the first cover plate.
13. System according to claim 12, characterized in that the cover
plates of the corresponding types with/without AP mobility are
identical and the core is different.
14. System according to claim 12, characterized in that the second
cover plate and the prosthesis core of the corresponding types
with/without AP mobility are identical and the first cover plate is
different.
15. System according to claim 12, characterized in that all three
components are identical and a limit stop, which limits the
mobility of the prosthesis core in the ventral direction, is
different.
Description
[0001] Intervertebral prostheses are used for replacing the
intervertebral disk. They comprise two cover plates, whose outer
surfaces are designed for connection to adjacent vertebral bodies,
and an articulation device enclosed by the cover plates. In a known
prosthesis (EP-B 471 821), the upper cover plate forms a concavely
spherical articulation surface on its inner side, which cooperates
with the convexly spherical top surface of a prosthesis core of
polyethylene in order to form an articulation. The core has a flat
underside and a cylindrical edge which are received with matching
fit in a seat which is formed on the inside by the lower cover
plate.
[0002] In order to permit easy flexion movement, it would be useful
to choose a small radius of curvature of the articulation surfaces.
However, this would lead to a small corresponding surface area of
the articulation surfaces and would lessen the capacity for load
transmission. In said known prosthesis, the radius of curvature of
the articulation surface is chosen approximately equal to half the
prosthesis diameter. This has the disadvantage that the prosthesis
has a considerable structural height and in many cases cannot be
accommodated in the restricted intervertebral space. If, in said
type of prosthesis, one were to choose a still greater articulation
radius, so that the core assumed the shape of a flat sphere
section, it would be found that the articulation properties of the
prosthesis leave something to be desired.
[0003] The object of the invention is to make available a
prosthesis which has a small structural height and yet has good
articulation properties, in particular for use in the cervical
spine.
[0004] The solution according to the invention lies in the features
of claim 1 and preferably those of the dependent claims.
Accordingly, an intervertebral prosthesis of the type mentioned in
the introduction is characterized in that the core is movable at
least in the AP direction relative to the cover plate forming the
seat. Mobility can also be provided in the lateral direction and in
rotation. The invention is based on the knowledge that, with a
comparatively large radius of the articulation surfaces, the
articulation movement is associated with a translation movement of
the cover plates in relation to one another, the extent of this
movement increasing the greater the articulation radius, and that
this translation movement is countered by the resistance of the
ligament apparatus and the facet articulations acting between the
vertebrae.
[0005] The invention has recognized that this problem can be
overcome by allowing the core to execute a displacement, which
compensates for the translation offset of the cover plates relative
to one another, in relation to the cover plate holding it. For
example, upon a flexion movement relative to the core, the upper
cover plate not only pivots about a transverse axis, but also
executes a displacement in the ventral direction. This displacement
can be compensated by a corresponding dorsal displacement of the
core (together with the upper cover plate) relative to the lower
cover plate.
[0006] The invention also has the advantage that the particular
relative position of the upper and lower cover plates can be
adapted to the particular anatomical conditions. This applies in
particular to those cases where the ventral end faces of the
adjacent vertebral bodies determine or influence the position of
the cover plates assigned to them and protrude to different extents
in the ventral direction.
[0007] The core must be held securely in the prosthesis, so that it
cannot protrude into the spinal canal for example. For this
purpose, movement-limiting devices, which restrict the extent of
the movement which the core is allowed, can be provided on one or
both cover plates. These movement-limiting devices can interact for
example with the outer edge of the core. For example, the lower
cover plate can have a raised collar which extends completely or
partially around it and which interacts with the outer edge of the
core and is so high that the core, even upon a certain expansion of
the intervertebral space, cannot slide over it. As is known per se
(DE-C 30 23 353), this edge can also be so high in places that it
forms projections which engage in corresponding recesses of the
opposite cover plate in order to form a cage for retention of the
core.
[0008] The mobility of the core relative to the cover plate holding
it is particularly important in the AP direction because the
greatest relative movements (flexion and extension) take place in
the sagittal plane, whereas the lateral bending movements are
comparatively slight. In an advantageous embodiment of the
invention, provision is therefore made for the movement-limiting
device to be designed as a guide device in the AP direction. In
particular, it can be formed by opposite, parallel lateral guide
rails between which the core is held in such a way that it can move
only in the AP direction. The guide rails are in this case
expediently undercut in order to interact with a ridge of the core
which engages in the undercut. In this way, it is ensured that the
core does not lift from the cover plate holding it. This has the
advantage that the devices provided for restricting the movement of
the core do not have to be very high and, for this reason, there is
also no risk that they could impede the relative movement of the
cover plates with respect to one another. To ensure that the core
does not slide out from the rails in the dorsal or ventral
direction, suitable limit stops can be provided. The dorsal limit
stop is expediently connected rigidly to the cover plate forming
the seat (i.e. the guide rails). At the ventral end, a limit stop
should be provided which can be removed from its limit-stop
position so that the core can be more easily inserted after
implantation of the cover plate. The limit stop is then fixed in
the position in which it prevents the escape of the core.
[0009] Instead of a movement-limiting device which interacts with
the outer edge of the core, it is also possible to provide one
which interacts with an inner edge of a recess of the core. For
example, on the face directed toward the cover plate holding it,
the core can have a recess which interacts with a projection on
this cover plate. The recess can have an elongate shape extending
in the AP direction. In this case, it is expedient to design the
articulation to be rotatable about the vertical axis. If the
projection is made short in the AP direction or is limited in a
circular way in cross section, so that it can pivot in relation to
the recess, the core is able to pivot, relative to the cover plate
holding it, with respect to the vertical axis, so that a
possibility of rotation of the prosthesis articulation about this
axis can be dispensed with. This allows for greater freedom in the
design of the articulation. If it connects the core pivotably to
the upper cover plate, the AP direction of the core is then
determined by the AP direction of the upper cover plate.
[0010] A particular aspect of the inventive concept resides in the
fact that a system of intervertebral prostheses includes, in
addition to those which have the described AP mobility, also other
types, preferably of corresponding external configuration, which do
not have AP mobility between the prosthesis core and the cover
plate holding it. This allows the physician to decide, during the
operation, whether or not he wishes to provide AP mobility. The
cover plates of the prostheses movable or immovable in the AP
direction are expediently of uniform configuration, and only the
core is different. However, provision can also be made for the
prosthesis core and the cover plate forming the articulation with
it to be uniform in all types, while the AP mobility is afforded by
differences in the cover plate holding the prosthesis core.
Finally, there is also the possibility that all three components
are uniform, and that only the limit stop limiting the ventral
movement of the prosthesis core in the AP direction is differently
located.
[0011] Where the terms lower and upper cover plate are used here,
this is not intended to imply that the cover plate forming the seat
for the core would always have to be arranged at the bottom.
Rather, the arrangement can also be chosen the other way round. The
claims therefore talk more generally of a first cover plate and a
second cover plate.
[0012] In order to prevent lifting of the core from the cover plate
holding it, provision can be for the projection and the recess to
be designed with interacting undercuts.
[0013] Preferred embodiments are explained below with reference to
the drawings, in which:
[0014] FIG. 1 shows a frontal cross section,
[0015] FIG. 2 shows a sagittal cross section,
[0016] FIG. 3 shows an exploded view of a first embodiment,
[0017] FIG. 4 the exploded view of a second embodiment,
[0018] FIG. 5 shows a variant of the lower cover plate belonging to
the embodiment according to FIG. 4;
[0019] FIG. 6 shows two cover plates of a further embodiment
without core.
[0020] The lower cover plate 1 and the upper cover plate 2 of the
first embodiment have outer surfaces 3 and 4, respectively, which
are intended for anchoring to the associated vertebral body. They
are preferably plane. However, other substantially flat
configurations including suitable surface structures for better
anchoring to the bone are also conceivable. The cover plates are
preferably made of metal.
[0021] The lower cover plate 1 has a plane upper surface 5 facing
toward the upper cover plate 2 and enclosed on three sides by a
collar 6 which, above an inner undercut, forms an inwardly
projecting ridge 7. The lower cover plate 1 is of approximately
rectangular shape in plan view. In the area of its sides 8, 9, the
branches of the collar 6 located there extend parallel to one
another and rectilinearly.
[0022] The upper surface 5 and the collar 6 of the lower cover
plate form a seat for the prosthesis core 10, which is made of a
material with good sliding properties, for example polyethylene. It
has a plane lower surface which matches the surface 5 and which is
delimited by an edge ridge 11 above which a groove 12 is situated.
The ridge 11 engages in the undercut of the collar 6 below the
ridge 7. The ridge 7 engages in the groove 12. Sliding play is
provided between the collar 6 of the lower cover plate 1 and the
edge of the core 10.
[0023] The core 10 has the same contour shape as the lower cover
plate 1 on both sides (right and left in FIG. 1) and dorsally (left
in FIG. 2). The shape of its edge ridge 11 and of its groove 12
exactly follows the shape of the collar 6. Ventrally (right in FIG.
2), the core is slightly shorter than the lower cover plate, so
that play remains between its ventral end face 13 and the limit
stop 14.
[0024] On their ventral edge, the cover plates 1, 2 each have a
flange 15, 16 which issues from them approximately at right angles
and which has screw holes 17 for fastening to the vertebral body.
Located in the flange 15 of the lower cover plate 1 there is a slot
18 in which a limit-stop plate 14 is held displaceably. It can
assume the locking position shown in FIG. 2 in which it forms a
limit stop for the forwardly directed movement of the core 10. It
can also be pushed so far down into the slot 18, or completely
removed from the latter, as to allow the prosthesis core to be
introduced easily between the cover plates from the ventral
direction. It has two bores 19 which, in the locking position of
the plate 14, are flush with the screw holes 17. When the lower
cover plate 1 is secured on the vertebral body via the screw holes
17, the fastening screws also pass through the holes 19 and thus
secure the plate 14 in its locking position.
[0025] The lateral branches of the collar 6 form a guide for the
prosthesis core, in which guide said prosthesis core can move a
certain distance in the AP direction indicated in FIG. 3, namely by
the width of the free space between the ventral limit-stop surface
13 of the prosthesis core and the limit-stop plate 14. The ventral
part 21 of the collar 6 acts as a securing limit stop which
prevents the core from escaping in the dorsal direction from the
space between the cover plates 1 and 2. The presence of the
undercut on the collar 6 and on the edge of the core 10 is only of
importance in the lateral areas 8 and 9 of the lower cover plate 1
and of the core, but not in the dorsal extent 21 of the collar
6.
[0026] At its top, the core 10 has a preferably convexly spherical
articulation surface 22 which, in order to form an articulation,
interacts with the concavely spherical slide surface 23 on the
underside of the upper cover plate 2.
[0027] Upon flexion movement, the upper cover plate 2 pivots
slightly clockwise in relation to the lower cover plate 1 in the
view according to FIG. 2, and, upon extension movement, it moves in
the opposite direction. If the upper cover plate 2 exactly follows
the direction predetermined by the slide surfaces 22, 23, this
pivot movement is associated with a translation movement which is
directed forwardly upon flexion (toward the right in FIG. 2) and
directed rearwardly upon extension (toward the left in FIG. 2).
Part of this translation movement may be inconsistent with the
physiological situation and may lead to undesired stresses. These
stresses cause restoring forces which, in the prosthesis design
according to the invention, result in the upper cover plate moving
in the opposite direction relative to the lower cover plate and
thereby compensating for the undesired component of movement.
[0028] Between the interacting guide devices of the core and of the
lower cover plate, so much clearance can be left in the lateral
direction that a certain relative movement is possible also in this
direction.
[0029] The extent of the movement clearance in the AP direction is
preferably between one and four, more preferably of the order of
two to three millimeters. If a relative mobility in the lateral
direction is provided, the extent of this should not be more than
two millimeters.
[0030] In the second embodiment according to FIG. 4, the prosthesis
consists of a lower cover plate 31 and of an upper cover plate 32.
The lower cover plate has an upper, plane surface 33 on which the
prosthesis core 34 lies. Whereas this core in the first embodiment
is guided at its outersides, in the second embodiment it has a
recess 35 with undercut side edges 36 which interact with an
elongate projection 37 of the lower cover plate with
correspondingly undercut edges 38. The core 34 is thus movable in
the AP direction relative to the lower cover plate 31, in the same
way as was explained with reference to the first illustrative
embodiment. In addition, the interaction of the undercuts protects
it against lifting from the lower cover plate. Suitable limit stops
(not shown) can be provided which prevent the prosthesis core from
escaping from the space between the plates.
[0031] The lower cover plate 31 can be replaced by the lower cover
plate 31a which is shown in FIG. 5 and which differs from the lower
cover plate 31 in that its projection 37a is not elongate, but
limited in a circular way in plan view. This means that the
prosthesis core 34, which is assumed to be connected in terms of
rotation to the upper cover plate 32 with respect to a vertical
axis, can rotate about the projection 37a without impeding the
desired AP movement. This may be desirable in the case of an
aspherical configuration of the slide surfaces between core and
upper cover plate.
[0032] This ability of the core to rotate relative to the lower
cover plate can also be provided in the embodiment according to
FIGS. 1 through 3, by means of the edge 11, 12 of the core 10 being
made circular. It can then not only move in the AP direction
between the parallel, lateral branches 8, 9 of the collar 6, but
can also pivot. Instead of this, it is also possible for both the
edge of the prosthesis core and also the collar of the lower cover
plate to be made circular. The core is then able to rotate relative
to the cover plate, without having mobility in the AP
direction.
[0033] FIG. 6 shows only the cover plates of a further illustrative
embodiment, without the prosthesis core and without the devices
which allow the latter to move in the AP direction relative to the
lower cover plate. This figure serves merely to demonstrate an
embodiment of the means which ensure that a prosthesis core held
between the cover plates cannot escape in the dorsal direction.
These consist of one or more tongues 40, 41 projecting upward from
the lower cover plate, and of one or more tongues 42 projecting
downward from the upper cover plate, which tongues are offset
relative to one another so that they each engage in the space
between or alongside the opposite tongues. The tongues are of such
a length in the vertical direction that, even with the greatest
possible pivoting of the cover plates, they do not move away from
each other to such an extent that the prosthesis core could escape
from between them. Corresponding devices can also be provided at
the sides and at the ventral end.
* * * * *