Note: Descriptions are shown in the official language in which they were submitted.
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ZM0169 MODULAR IMPLANT AND METHOD OF ASSEMBLY
BACKGROUND OF THE INVENTION
The present invention relates to the attachment of modular
orthopaedic implant components to one another and, more
specifically, to the use of plastic materials to isolate and adhere
these components.
In applying orthopaedic implants to the human body, especially
hip and knee implants, a surgeon often encounters a less than
optimal fit of the implant to a bone. To accommodate the
variations in patients and surgical situations that lead to poor
fit, many designers have developed modular implant systems.
Modular implant systems allow opt:ional, pre-operative or intra-
operative assembly of components by a surgeon as needed to fit a
specific patient.
Previous modular implants have employed a variety of
mechanical fixation means as illustrated in U.S. 4,482,606 and U.S.
4,995,883 directed to hip implants and U.S. 4,~50,298 and U.S.
4,936,847 directed to knee implants. These mechanical connecting
means can be problematic in that they allow small relative motion
between components and between the connecting members. This
motion in turn generates wear debris, compromises the implant
fixation to the bone, and concentrates stress on the components and
connecting members. All of these results of component relative
motion have been implicated in failure of implant procedures.
It is also known in the field of modular joint implants to use
polymethylmethacrylate "bone" cement to attach components. The
cement is applied in liquid form, the components assembled, and the
cement allowed to cure. The cement, in this case, simply fills the
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space between the components and forms a mechanical interlock so
far as interlocking surfaces are provided. There is substantially
no adhesion of the cement to the implant surface. This form of
connection is characterized by low overall fixation strength as
well as potential separation between the cement and component
resulting in relative motion and debris. In addition, liquid
cement is messy and awkward to work with for this purpose in that
it must be mixed and then used within a specified working time, it
tends to run onto articular and bone interface surfaces from which
it must be removed and it gives off noxious fumes.
SUMMARY OF THE INVENTION
The invention of this disclosure addresses the deficiencies of
the above-mentioned attachment means by providing a solid, heat
activated, adhesive to join modular implant components. The
present invention isolates components to prevent debris generation
between components and adheres tightly to the components surfaces
to prevent motion and debris generation between the components and
the adhesive. In addition the secure bond distributes stress
evenly between the components. It does this while being clean and
convenient to use both pre-operatively and intra-operatively.
BRIEF DESCRIPTION OF THE DRAWINGS
The before mentioned features and advantages of the present
invention are apparent from the following detailed description and
the drawings wherein:
FIG. 1 is an exploded perspective view of a tibial knee
embodiment of the invention.
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FIG. 2 is a perspec-tive view of a tibial augmentation
component.
FIG. 3 is an exploded side view of a ~emoral knee embodiment
of the invention.
FIG. 4 is an exploded perspective view of a femoral hip
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention as shown in FIG. 1
comprises a modular orthopaedic implant with a tibial base
component 1, a tibial augment component 3, ancl a thin plastic wafer
or film 2 for adhesively joining and preventing relative motion
between components l and 3. The film 2 is a thermoplastic with a
lower melting point than the components 1 and 3. It also exhibits
good adhesive bonding characteristics with the components l and 3.
Experiments have shown members of the polyaryletherketone (PAEK)
family of thermoplastic polymers to develop strong adhesive bonds
with commonly used implant metals such as cobalt-chromium-
molybdenum alloy (Co~Cr-Mo~ and titanium-vanadium-aluminum alloy
(Ti-6Al-4V). Speci~ically, a PAEK sold by BASF under the tradename
Ultrapek~ KR4177 was tested to determine its bonding strength with
Co Cr-Mo and Ti-6Al-4V. Pairs of cylindrical rod samples with .G
square inch faces were adhered together with Ultrapek KR~177 fully
engaging the faces. The polymer was placed between the faces and
the assembly heated in a furnace at 770 F for 45 minutes. The
2~ samples were then cooled to room temperature and subjected to
tensile testing to determine the stress required to separate the
sample pairs. The testing showed that for Co-Cr-Mo samples that
were cleaned, glass bead blasted and passivated prior to bonding;
the separation stress was equal to the ultimate tensile strength of
the PA~X of 17 KSI. For similarly treated Ti-6Al-4V samples the
separation stress was 7 KSI. The lower strength bond was
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determined to be due to the build-up of an oxide layer on the
samples during heating in the furnace. When the experiment was
repeated with Ti-6Al 4V samples that were titanium nitride coated,
the stress increased to lO KSI. In addition, when the samples were
heated by induction for 4 minutes rather than in the furnace, the
stress increased to 13 XSI.
The preferred embodiment of the present invention comprises
the tibial base component 1 and a tibial augment component 5
(FIG. 2) with a thin layer of PAEK 4 pre-bonded to it. After a
surgeon has selected the proper components for use with a specific
patient, he assembles the chosen components and positions them with
a bonding fixture such as a clamp as is well Xnown in the art. He
then applies heat to the assembly such as in a furnace or by
induction heating. The assembly is allowed to cool and the custom
assembled implant is implanted.
While the foregoing has described a pre~erred embodiment
of the present invention, variations in design, materials and
method are possible to attach any modular implant component. For
example, FIG. 3 shows a ~emoral knee component 6 with a variety of
augmentation components. An anterior augmentation component 7, a
distal augmentation component 8, and a stem extension component 9
can all be attached to the ~emoral component 6 via bonding with a
thermoplastic polymer as described above. FIG. 4 shows a femoral
hip stem component 10 with a lateral augmentation component ll, an
anterior/posterior augmentation 12, a distal stem augmentation 13,
and a femoral head 14 all of which can be assembled according to
the present invention in order to securely at~tach them and prevent
relative motion and debris generation between components.
As noted previo~lsly the thermoplastic layer may be supplied
pre-bonded to one of the components of a mating pair or the layer
may be supplied as a separate wafer shaped to conform to the
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corresponding components. The present invention is also useful in
combination with a primary mechanical fixation means, such as
screws, dovetail interlocks, snap fits, etc. For example, in FIG.
1, the thermoplastic adhesive layer 18 prevents micromotion and
debris generation between system components such as ~he tibial base
plate 1 r an augmentation block 16 and an attachment screw 17. In
addition, materials other than metals may be joined in the
disclosed manner so long as the thermoplastic layer has a lower
melting point than the components to be joined and exhibits good
adhesive bonding characteristics with the components. The tibial
base component 1 can be joined to a polymer tibial articular
surface 20 with a thermoplastic bonding layer 21. It is also
within the scope of this invention to use an operating room
autoclave to heat the components provided that the thermoplastic
has a sufficiently low melting point to be within the range of
autoclave temperatures. Finally, various surface modifications
such as grooving, roughening or porous coating may be employed to
compliment the adhesion of the thermoplastic to the component.
It will be understood by those s~illed in the art that the
aforementioned emkodiments and modifications and numerous others
may be made without departing from the spirit and scope of the
invention defined by the appended claims.
