Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Dr. Hans Jorg Meisel
M60157CA
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Parts assembly and part for a prosthesis
The invention relates to a parts assembly and to a part for a pros-
thesis, particularly a cervical spine intervertebral disc prosthe-
sis.
Background of the invention
Prostheses based on one or more parts are used to support the func
tionality of, or even replace, parts of the bone skeleton, for exam
ple the spine or a joint.
Degenerative damage to the cervical spine which is associated with a
herniated disc or compresses the spinal cord as a result of bone
constriction is usually operated on ventrally when an operation is
required. When operating in this way, the cervical disc usually has
to be completely removed in order to relieve the pressure on the
spinal cord and the nerve root. To date, this operative procedure
has of course been associated with a loss of function of the af-
fected vertebral motor segment. In order to prevent an additional
loss of height of the disc, which can lead to an increase in degen-
erative and neurological changes, the ventral fusion operation has
until now been the method of choice.
By using bone cement, iliac crest bone or cage-shaped spacers (so-
called cages) adapted to the height of the disc, at least the height
of the disc is reconstructed in this case, wherein a further aim of
this procedure is to achieve bone fusion of the vertebral bodies of
the affected vertebral motor segment. This has the disadvantage that
the connecting vertebral motor segments are worse affected than be-
fore by the lifting effect of the fused vertebrae obtained after fu-
sion, which promotes increasing degeneration in these connecting
segments. When suitably indicated, a future aim of this treatment
method is to retain the function of the segment by providing a full
disc prosthesis.
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Developments proposed to date for cervical spine intervertebral disc
prostheses, which have led to market-ready products, are not very
widely used in terms of clinical use since complicated operational
procedures are required to implant them and these are associated
with irreversible changes to the lower end plate and upper end plate
of the affected vertebral bodies. For example, in order to use the
prosthesis according to Bryan et al., parts of the adjoining verte-
bral bodies have to be removed in order on the one hand to fix the
prosthesis in place but also to accommodate the relatively large
height of the implant. The reason for the high size of this implant
is the very complex structure, which performs a shock-absorbing
function, and also the fact that the structure is composed of a
large number of individual parts, which are made of different mate-
rials. Implantation takes a great deal of time and requires the use
of more than 30 implantation instruments. The spontaneous fusion
rate after implantation can be reduced by the post-operative admini-
stration of cortisone.
Summarv of the invention
It an object of the invention to provide an improved parts assembly
and an improved part for a prosthesis, which are cost-effective and
can be manufactured with as little complexity as possible.
According to the invention, this task is solved by a parts assembly
according to independent claim 1 and by a part according to inde-
pendent claim 11. Advantageous embodiments of the invention can be
found in the dependent claims.
According to the invention, there is provided a parts assembly for a
prosthesis, particularly a cervical spine intervertebral disc pros-
thesis, comprising two base parts, which are coupled to one another
in an articulated manner by means of coupling parts formed on the
base parts, wherein the base parts are in each case formed in one
piece with an associated coupling part. By means of the design of
the parts assembly for the prosthesis with two base parts, which are
coupled to one another in an articulated manner, a mechanically sim-
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ple structure is selected. Furthermore, it is provided that the base
parts and the coupling parts are made of a material selected from
the following group of materials: polyetherketone (PEK), polyethere-
therketone (PEEK), polyacryletherketone (PAEK), polyetherketoneke-
tone (PEKK), polyetherketoneetherketoneketone (PEKEKK) and poly-
etherketoneetherketone (PEKEK).
According to another aspect of the invention, there is provided a
part for a prosthesis parts assembly, particularly a cervical spine
intervertebral disc prosthesis part, comprising a base part and a
coupling part formed on the base part for articulated coupling to
another base part, wherein the base part and the coupling part are
formed in one piece. Furthermore, it is provided that the base part
and the coupling part are made of a material selected from the fol-
lowing group of materials: polyetherketone (PEK), polyetherether-
ketone (PEEK), polyacryletherketone (PAEK), polyetherketoneketone
(PEKK), polyetherketoneetherketoneketone (PEKEKK) and poly-
etherketoneetherketone (PEKEK).
Compared to known parts for cervical spine intervertebral disc pros-
theses in which the coupling part is mounted on the base part, the
one-piece design of the base part together with the coupling part
has the advantage that the production method is simplified since op-
erating steps for the separate manufacture of base part and coupling
part and the subsequent connection of the two parts can be omitted.
The part for the prosthesis can be manufactured as a whole part in
one production process. This can be carried out using one tool.
Moreover, problems concerning the stable and permanent support of
the coupling part on the base part, as may arise if the two parts
are manufactured separately and then assembled, are avoided as a re-
sult of the one-piece design.
As a result of making the base part (s) and the coupling part (s) of
one material, it is possible during manufacture to make use only of
tools, which can be used to process the material employed. There is
no need to use different tools for different materials, and this
leads to a cost saving.
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One advantage lies in the material properties, that is to say the
similar modulus of elasticity to that of cortical bone. In order to
further improve the tribological and mechanical properties, it may
be provided to use PAEK with a filler material, for example carbon
or glass fibres, and/or to modify the polymer matrix, for example by
means of crosslinking or iron implantation.
One advantageous embodiment of the invention provides that an ana-
tomically adapted contact surface is formed on a respective outer
side of the two base parts. By means of the anatomically adapted
contact surface, implantation of the parts assembly as a prosthesis
in the skeleton is made possible in such a way that the prosthesis
is integrated in the skeleton in as natural a manner as possible.
The anatomically adapted contact surface helps the prosthesis to in-
tegrate into the skeleton with a precise fit. Slipping of the pros-
thesis is prevented by means of the anatomically adapted contact
surface. Moreover, the arrangement of the bone on the anatomically
adapted contact surface when the prosthesis is implanted counteracts
any undesirable rotation of the prosthesis relative to the bones,
which are adjacent to the prosthesis, so that stable support of the
prosthesis is promoted.
It may advantageously be provided that an anti-rotation means is
formed on each of the two base parts in order to prevent any rota-
tion of the base parts relative to the bone parts arranged adjacent
to the base parts when the prosthesis is implanted. The anti
rotation means preferably comprises a web arranged on the respective
outer side of the base parts, wherein openings may be provided in
the web. The bone can grow into the openings.
In order to make it possible for the two base parts to move relative
to one another, which then makes it possible for the bone parts to
move relative to one another when the parts assembly is used in an
implanted prosthesis, it may be provided that the two base parts are
coupled to one another by means of a sliding connection. The sliding
connection is preferably embodied by means of sliding surfaces on
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the coupling parts. One preferred embodiment of the invention pro-
vides that one of the sliding surfaces is formed on a hemispherical
protrusion on one of the coupling parts. Once the parts assembly has
been assembled, the rounded sliding surface is supported on a coun-
tersliding surface on another coupling part, the shape of said coun-
tersliding surface being adapted to that of the first sliding sur-
face .
In order to achieve the highest possible abrasion resistance of
sliding surface and countersliding surface, these are advantageously
coated with a material based on a chromium-nickel alloy.
In one embodiment of the invention, the material used for producing
the base parts is preferably a polyetheretherketone. This material
has the advantage that a modulus of elasticity is thus provided
which is similar to that of cortical bone.
Description of preferred embodiments of the invention
The inventionwill be explained in more detail below on the basis
of
preferred emb odiments and with reference to a drawing, in which:
Fig. 1 shows a perspective view of a base part for a parts
assembly for use as a prosthesis;
Fig. 2 shows a perspective view of a further base part for
a parts assembly, together with the base part of
Fig. 1, for use as a prosthesis;
Fig. 3 shows a perspective view of a parts assembly com-
prising the base part of Fig. 1 and the further
base part of Fig. 2 in the coupled state;
Figs. 4A and 4B show a view of two bone parts, which are joined
by
means of a parts assembly according to Fig. 3, in
a
coupled and decoupled state; and
Figs. 5A and 5B show a view of two further bone parts, which are
joined by means of a parts assembly according to
Fig. 3, in a coupled and decoupled state.
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Figs. 1 and 2 show a perspective view of a base part 1 and of a fur-
ther base part 2 for a parts assembly for use as a prosthesis, par-
ticularly a cervical spine intervertebral disc prosthesis. Fig. 3
shows a perspective view of a parts assembly 30 in which the base
part 1 and the further base part 2 are connected to one another in
an articulated manner.
As shown in Fig. 1, the base part 1 has a dorsal section la and a
ventral section 1b. An anatomically adapted contact surface 5 is
formed on an outer side 3. During production of the base part 1, the
anatomically adapted contact surface 5 is adapted to the surface
contour of the bone in which the prosthesis is to be implanted. When
used in connection with an intervertebral disc, the geometry of the
contact surface 5 permits as congruent an adaptation as possible to
the upper end plates that have been carefully freed from the in-
tervertebral disc cartilage during the operation (intervertebral
disc removal).
As shown in Fig. 2, the further base part 2 has a dorsal section 2a
and a ventral section 2b. An anatomically adapted contact surface 6
is formed on an outer side 4 of the base part 2. During production
of the base part 2, the anatomically adapted contact surface 6 is
adapted to the surface contour of the bone in which the prosthesis
is to be implanted. When used in connection with an intervertebral
disc, the geometry of the contact surface 6 permits as congruent an
adaptation as possible to the upper end plates that have been care-
fully freed from the intervertebral disc cartilage during the opera-
tion (intervertebral disc removal).
In order to prevent dislocation of the base parts 1, 2 relative to
the bone in an improved manner, a material coating may be provided
on the anatomically adapted contact surfaces 5, 6, for example using
hydroxylapatite.
As shown in Figs. 1 and 2, a web 7, 8 is in each case arranged on
the outer side 3, 4 of the base parts 1, 2, said web being designed
as an anti-rotation means. When the parts assembly is implanted, the
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respective web 7, 8 engages in a bone depression, so that rotation
of the base parts 1, 2 relative to the respectively adjacent bone is
not possible. The application of a coating to the webs 7, 8 may be
provided in order to prevent dislocation of the prosthesis in situ
in an improved manner. The webs 7, 8 have respective openings 7a, 7b
and 8a, 8b, through which the bone can grow.
As shown in Figs. 1 and 2, the base parts 1, 2 have a respective
coupling part 11, 12, which is formed in one piece with the associ-
ated base part 1, 2. By means of the coupling parts 11, 12, a con-
nection between the two base parts 1, 2 is produced in such a way
that the two base parts 1, 2 are connected to one another in an ar-
ticulated manner. For this purpose, the coupling part 11 shown in
Fig. 1 has a protrusion 13, which is shaped in a hemispherical man-
ner. A sliding surface 14 is formed on the protrusion 13, which
sliding surface lies on a countersliding surface 15 on the other
coupling part 12 once the parts assembly has been assembled (cf.
Fig. 3), so that an articulated sliding connection is created be-
tween the two base parts 1, 2. By means of this connection, when
used as a prosthesis, the parts assembly 30 ensures the mobility and
articulation of the skeleton section in which the parts assembly 30
is implanted as a prosthesis.
The base parts 1, 2 and the coupling parts 11, 12 may be varied in
terms of their specific design, for example their size, in order to
provide different implant sizes and angles, which permit the best
possible approximation to the individual anatomy.
The sliding surfaces 14, 15 which provide the articulation are pref-
erably coated with a Co-Cr alloy, which ensures low abrasion under
the loads occurring in the cervical spine and thus ensures long-term
movement ability.
The two base parts 1, 2, including the coupling parts 11, 12 formed
in one piece therewith, are made of polyetherketone (PEK), polyacry-
letherketone (PAEK), polyetheretherketone (PEEK), polyetherketoneke-
tone (PEKK), polyetherketoneetherketoneketone (PEKEKK) or poly-
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etherketoneetherketone (PEKEK). One advantage of lies in the mate-
rial properties, that is to say the similar modulus of elasticity to
that of cortical bone.
In order to further improve the tribological and mechanical proper-
ties, it may be provided to use PAEK with a filler material, for ex-
ample carbon or glass fibres, and/or to modify the polymer matrix,
for example by means of crosslinking or iron implantation.
Figs. 4A, 4B and 5A, 5B show perspective views of bone parts, which
are joined to one another in an articulated manner via a parts as-
sembly 30 as shown in Fig. 3, in a coupled state and a decoupled
state, wherein the coupling parts are shown detached from the base
parts in Fig. 4A.
The described base parts, in connection with a cervical spine in-
tervertebral disc prosthesis based thereon, lead to the following
advantages compared to known prostheses: reconstruction of the seg-
ment mobility; reconstruction of the individual cervical spine lor-
dosis by means of possible different angles of the implants; recon-
struction of the individual intervertebral disc height by means of
possible different implant heights (modular system technology); sim-
plification of the operative procedure for installing the implant;
reduction in implant costs due to the use of two materials and modu-
lar technology; ability to adapt prosthesis shape to the existing
anatomical conditions; no early spontaneous fusion; and implant less
susceptible to dislocation.
The instrumentation required for implantation when using a prosthe-
sis based on the described parts can be kept simple and requires for
example, in addition to adaptation distractors and an implant
holder, only an upper end plate curette and special cervical spine
punches, which are able to perform bone decompression on the dorsal
side of the vertebral canal even with little distraction of the ver-
tebral motor segment.
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The features of the invention, which are disclosed in the above de-
scription, the claims and the drawing may be important both indi-
vidually and in any combination for implementing the invention in
its various embodiments.
