U.S. patent application number 13/327589 was filed with the patent office on 2012-04-05 for cervical intervertebral prosthesis.
This patent application is currently assigned to CERVITECH, INC.. Invention is credited to Arnold Keller.
Application Number | 20120083891 13/327589 |
Document ID | / |
Family ID | 34933451 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120083891 |
Kind Code |
A1 |
Keller; Arnold |
April 5, 2012 |
CERVICAL INTERVERTEBRAL PROSTHESIS
Abstract
A spinal implant includes two connection plates dimensioned for
connection to adjacent vertebral bodies and a hinge core. The hinge
core together with one connection plate forms a hinge between a
generally spherical hinge extending from the one connection plate
into an opening of a generally spherical depression in the hinge
core. The opening is dimensioned to be elastically deformed to
allow passage of the generally spherical hinge into the generally
spherical depression. The hinge core bears on a slide surface which
extends substantially parallel to the direction of the other
connection plate and permits relative translatory movement of the
hinge core. The walled receiving area limits this relative
translatory movement of the hinge core. The stability of the spinal
implant with respect to laterally incident forces is increased.
Inventors: |
Keller; Arnold; (Kayhude,
DE) |
Assignee: |
CERVITECH, INC.
San Diego
CA
|
Family ID: |
34933451 |
Appl. No.: |
13/327589 |
Filed: |
December 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11125312 |
May 10, 2005 |
8088164 |
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13327589 |
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Current U.S.
Class: |
623/17.16 |
Current CPC
Class: |
A61F 2/4425 20130101;
A61F 2310/00179 20130101; A61F 2002/30578 20130101; A61F 2002/30904
20130101; A61F 2220/0025 20130101; A61F 2002/443 20130101; A61F
2002/30649 20130101; A61F 2310/00011 20130101; A61F 2002/30604
20130101; A61F 2002/30662 20130101; A61F 2220/0033 20130101; A61F
2002/30369 20130101; A61F 2002/305 20130101; A61F 2002/30574
20130101 |
Class at
Publication: |
623/17.16 |
International
Class: |
A61F 2/44 20060101
A61F002/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2005 |
EP |
05001557.7 |
Claims
1. A spinal implant, comprising: a first connection plate having a
first surface dimensioned to contact a first vertebral body, a
second surface opposite the first surface, and a generally
spherical hinge extending from the second surface; a second
connection plate having a first surface dimensioned to contact a
second vertebral body, a second surface opposite the first surface,
and a walled section extending from the second surface to define a
receiving area; and a hinge core dimensioned to be received at
least partially within the receiving area of the second connection
plate, the hinge core including an opening and a generally
spherical depression extending from the opening, wherein the
opening is dimensioned to be elastically deformed to allow the
generally spherical hinge of the first connection plate to pass
through the opening and into the generally spherical
depression.
2. The spinal implant of claim 1, wherein at least one of first and
second connection plate includes a flange disposed generally
perpendicularly to the connection plate with at least one aperture
to receive a fixation element to aid in affixing the connection
plate to the respective vertebral body.
3. The spinal implant of claim 1, wherein the hinge core includes a
base portion and a collar, the base portion positioned within the
receiving area of the second connection plate and the collar
extending radially outward from the base portion.
4. The spinal implant of claim 1, wherein the hinge core is made of
a non-metal material.
5. The spinal implant of claim 1, wherein the walled section of the
receiving area is rectangular.
6. The spinal implant of claim 1, wherein the walled section of the
receiving area is square.
7. The spinal implant of claim 1, wherein the walled section of the
receiving area is square, and the hinge core is dimensioned with
one pair of sides which has a side length substantially
corresponding to a side length of the receiving area, and one pair
of sides which has a shorter side length than the side length of
the receiving area.
8. The spinal implant of claim 1, wherein the walled section of the
receiving area is square, and the hinge core is dimensioned with
sides that have a shorter side length than the receiving area.
9. The spinal implant of claim 1, wherein the walled section of the
receiving area and the hinge core have substantially the same
dimensions.
10. The spinal implant of claim 1, wherein the hinge core lies
substantially completely inside the receiving area.
11. The spinal implant of claim 1, wherein the generally spherical
depression of the hinge core encloses the generally spherical hinge
of the first connection plate within an enclosure angle of more
than 180.degree..
Description
FIELD AND BACKGROUND OF THE INVENTION
[0001] A cervical intervertebral prosthesis is known (U.S. Pat. No.
6,368,350) which is composed of an upper connection plate and a
lower connection plate for connection to the adjacent vertebral
bodies, and of a hinge core which, with one connection plate, forms
a hinge with a convex hinge surface on the underside of the upper
connection plate and with a concave hinge surface on the top side
of the hinge core. The hinge core bears on the other connection
plate via a slide surface which extends parallel to the direction
of extension of the connection plate and which permits a relative
translatory movement of the hinge core in this direction with
respect to the lower connection plate. This relative movement is
limited by means of the slide surface being restricted by a
vertical edge. In the known prosthesis, the hinge surface of the
hinge core has a very shallow dish-like configuration. This is
customary, because it is possible in this way to achieve a low
specific surface load. However, this entails the risk that, if the
ligaments connecting the relevant vertebral bodies to one another
are too weak or stretched, the prosthesis parts may come loose from
one another. Although is known (U.S. Pat. No. 5,895,428) to connect
the prosthesis parts to one another by means of a screw passing
centrally through the hinge surfaces, this cannot be done in
conjunction with the abovementioned translatory freedom of movement
of the hinge core parallel to the extension of the connection
plates, and, in any case, this is impossible in a cervical
prosthesis, because of the confined space.
SUMMARY OF THE INVENTION
[0002] It is therefore an object of the invention to make available
a prosthesis, of the type mentioned above, which promises
sufficient stability even in the case of weak ligaments. The
solution lies in the features of the invention, which broadly
includes a cervical intervertebral prosthesis including first and
second connection plates configured for connection to adjacent
vertebral bodies and hinge core which forms together with the first
connection plate a hinge with a concave hinge surface on the hinge
core and with a complementary convex hinge surface on the first
connection plate. The convex hinge surface bears on the second
connection plate via a slide surface which extends substantially
parallel to the direction of the second connection plate and which
permits relative movement in the direction of the second connection
plate. This slide surface is surrounded by an edge limiting the
relative movement. The concave hinge surface encloses the convex
hinge surface within a solid angle of at least 90.degree., and the
part of the convex hinge surface lying within this solid angle lies
inside a recess surrounded by the edge. Other features of the
invention are disclosed below.
[0003] Accordingly, it is provided that the concave hinge surface
encloses the convex hinge surface within a solid angle of at least
90.degree.. This ensures that forces acting obliquely from the side
cannot push the convex hinge surface out of the concave hinge
surface. Since the stated angle indicates the area within which the
concave hinge surface encloses the convex hinge surface, it is
referred to below as the enclosure angle. It is expediently at
least 110.degree., more preferably 135.degree.. If, according to a
further feature of the invention, it is over 180.degree., the
convex hinge surface is secured completely within the concave hinge
surface. The enclosure angle or solid angle is measured as the
central angle of a circular cone between radii issuing from
opposite points of the edge of the concave hinge surface.
[0004] As the stabilization of the prosthesis in the area of the
hinge surfaces would be futile if the attachment of the hinge core
to the other connection plate was unstable, the invention further
provides that the part of the concave hinge surface located within
the solid angle of at least 90.degree. lies inside the recess
enclosed by the edge. That is to say, this part of the hinge
surface lies below the surface spanned by the top face of the edge.
The effect is that it is highly unlikely that the hinge core will
be able to be pushed, by laterally incident forces, out of the
recess of the connection plate in question. If, as has been
described above as being advantageous, an enclosure angle of over
90.degree. is chosen, that part of the convex hinge surface
extending beyond 90.degree. does not then have to lie in the
recess. However, it is advantageous if it does so.
[0005] The positioning of the convex hinge surface inside the
recess serves not only to secure the hinge core on the connection
plate holding it, but also to reduce the overall height of the
prosthesis. The radius of the convex hinge surface is so small in
relation to the cross section of the hinge core that it is for the
most part inside that part of the hinge core surrounded by the
recess of the lower connection plate. The hinge core can be
configured such that it lies almost exclusively inside the recess.
However, it can be provided with a collar which rests on the top
face of the edge of the recess in order to rule out direct contact
between the metal parts of the upper and lower connection plates
upon flexion of the joint.
[0006] The recess should accommodate the hinge core at least to one
third, preferably to about half, of its hinge surface radius. A
configuration is especially preferred in which the recess
accommodates the hinge core substantially to the height of the
center point of its hinge surface. The minimum height of the edge
surrounding the recess, above the slide surface, should be least
one quarter, preferably at least one third, of the antero-posterior
(hereinafter "AP") extent of the recess.
[0007] Through the interaction of the hinge core with the edge of
the recess, the freedom of movement of the hinge core can be
predetermined in a desired manner. In an advantageous embodiment,
recess is rectangular, and preferably square, and its boundaries
run parallel to the AP direction and transversely thereto. This has
the advantage that a recess of the greatest possible dimension can
be accommodated in the lower connection plate. In a first variant
of this embodiment, a rectangularly delimited hinge core is
available which in one direction has a shorter side length than the
recess, while its other site length substantially corresponds to
the side length of the recess. In this way, depending on the or of
the hinge core in the recess, the freedom of movement of the hinge
core can be restricted to the AP direction (this is the most common
scenario) or to the direction perpendicular to the AP direction.
If, in a further variant, the translatory movement of the hinge
core is to be ruled out entirely, a hinge core is used which is
square and has the same dimensions as the recess. If, by contrast,
freedom of movement is desired in all directions, the dimensions of
the hinge core in all directions are chosen smaller than those of
the recess.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The invention is explained in greater detail below with
reference to the drawing, in which:
[0009] FIG. 1 shows a sagittal section through a first
embodiment,
[0010] FIG. 2 shows a sagittal section through a second embodiment,
and
[0011] FIG. 3 shows an exploded perspective view of two connection
dates and, shown between them, different versions of hinge
cores.
DETAILED DESCRIPTION OF THE INVENTION
[0012] A first connection plate 1 has a surface 2 intended for
connection to the cover plate of a first vertebral body. It can be
equipped with teeth or other means permitting intimate connection
with the bone tissue. A ventral flange includes a screw bore 4 for
a bone screw. It will be appreciated that these structural details
of the prosthesis will be able to be configured differently. A
second cover plate 5 has a surface 6 for connection to the second
vertebral body, and a ventral flange 7 with screw bore 8. The first
connection plate 1 is depicted at the top, and it is also in most
cases used at the top. In the following, therefore, it is
designated as the upper connection plate, and the other one is
designated as the lower connection plate. The arrangement, however,
can also be the other way round.
[0013] The underside of the upper connection plate carries a hinge
head 10 which forms a convex hinge surface 11 with center point 12.
The lower connection plate carries a peripheral edge 15 which
surrounds a square recess 16 above a plane slide surface 17. The
inner surfaces of the edge 15 extend perpendicularly with respect
to slide surface 17 forming the bottom of the recess 16, and
parallel to the AP direction or perpendicularly thereto. The
corners 18 are rounded. The dimensions 26, 27 in the AP direction
and transversely thereto, respectively, are identical. The
connection plates 1, 5 are expediently made of a durable material,
such, as metal.
[0014] The hinge core 20, expediently made of polyethylene, ceramic
or another material that favors sliding, is located inside the
recess 16. It has a plane bottom surface which bears on the slide
surface 17 across the greatest possible area and, together with
this, forms a sliding bearing for translatory movement. Its side
surface 21 extending in the AP direction, and the side surface 22
extending transversely thereto, form a rectangular boundary similar
and complementary to the demarcation of the recess 16 by the inner
surfaces of the edge 15.
[0015] At the upper edge, the hinge core 20 carries a collar 25
which rests on the top face of the edge 15. In the event of a
flexion movement, it protects the underside of the upper connection
plate 1 from direct contact with parts of the lower connection
plate 5. In the lateral area 24 between connection plates 1, 5,
sufficient space remains for a relative flexion movement.
[0016] The hinge core 20 includes a spherical depression which is
complementary to the convex hinge surface 11 and forms the
associated concave hinge surface. Its center point 12 coincides
with that of the convex hinge surface 11. It encloses the convex
hinge surface 11 within an enclosure angle 28 of more than 180
degrees. Above the center point 12, it narrows slightly so that the
hinge head 10 can escape from it only under elastic deformation and
is therefore held securely in it under normal force conditions.
[0017] By virtue of the fact that the hinge head 10 is
substantially enclosed by the concave hinge surface 11 of the hinge
core 20, the flexion joint has a high degree of stability even
under the effect of forces acting from the side. Likewise, the
hinge core 20 is held with a high degree of stability inside the
recess 16 by the edge 15. This is because the hinge core 20 is
almost completely surrounded, at any rate to a considerable height,
by the edge 15. Its top edge and the surface 23 spanned by the
latter are higher than the center point 12 of the concave hinge
surface 11. The height 29 of the edge 15 above the slide surface 17
is slightly (greater than two fifths of the AP dimension 26 of the
recess 16. Even if the hinge core were to be lifted slightly by
unanticipated forces, the edge 15 always returns it to the position
shown.
[0018] The connection plates 1, 5 can be combined with hinge cores
of different side lengths. This is illustrated in FIG. 3. The
dimensions of the side surfaces of the hinge core "a" are equal
(with reasonable play) to the inner surfaces of the edge 15. When
it is fitted into the recess 16 of the lower connection plate, it
has no translatory mobility relative to the lower connection plate
5. It is therefore used in the case of greatly weakened ligaments,
if the prosthesis has to ensure the stabilizing of the
intervertebral joint.
[0019] The hinge core "c" is smaller than the recess 16 both in the
AP direction and transversely thereto. Is a result, there is a
possibility of limited movement in all directions. This hinge core
is used in those cases where the joint can be allowed a translators
movement by a certain distance, but where stabilizing is desired
after this distance.
[0020] The hinge core "b" represents a cross between the
aforementioned possibilities, since the side length of its site
surfaces 21 is equal to the side lengths 26, 27 of the recess 16,
whereas the pair of side surfaces 22 transverse thereto is shorter.
This means that the hinge core "b" is not movable relative to the
associated lower connection plate 5 in the direction of the side
surfaces 21, but is movable to a limited extent in the direction of
the site surfaces 22. This hinge core is used when the need for
stabilization differs in different directions. The mobility is in
most cases provided in the AP direction, while the connection in
the transverse direction is fixed. The reverse option, however, can
also be achieved.
[0021] FIG. 2 shows an illustrative embodiment which differs from
that in FIG. 1 in terms of a more shallow design of the hinge
surfaces 11' and a lower hinge core 20'. The enclosure angle 28' is
only about 110.degree.. The area of the hinge surface 11' defined
by it lies entirely inside the recess 16', i.e. not higher than the
upper boundary surface 231. With its portion 31, the radius 30 of
the hinge surface lies more than one third below the boundary
surface 23' inside the recess 16'. The height 29' of the recess 16'
is equal to about one third. of the AP dimension of the recess. The
total height of the prosthesis is slightly less than that of the
first illustrative embodiment. Nonetheless, it affords a high level
of lateral stability.
* * * * *