ORTHOPAEDIC IMPLANT

IMPLANT ORTHOPEDIQUE

English Abstract

A plurality of polymer layers are stacked together and
subjected to a consolidation operation to generate an
orthopaedic bearing component. Each layer is formed by
longitudinally extending polymer fibers that are placed or woven
together to define a predetermined orientation of the fibers.
The consolidation operation fuses the layers together while at
the same time retaining the predetermined orientation of the
fibers.

Claims

CLAIMS:
1. An orthopaedic bearing component uniformly composed of a single
polymer and defining an articular bearing surface wherein the polymer
comprises consolidated fibres, the molecules of which are oriented in the
lengthwise direction of said fibres, the majority of fibres at said bearing
surface
of the bearing component being oriented substantially parallel to one another
so
that intersecting fibres are substantially eliminated at said bearing surface,
said
majority of the fibres also being orientated substantially parallel to the
intended
direction of articulation.
2. The component of claim 1 in which the fibres include a plurality of
layers stacked on top of each other to form the bearing surface.
3. The component of claim 1 or 2 in which the polymer comprises
polyethylene fibres.
4. The component of any one of claims 1-3 in which individual discrete
fibres are arranged together to form a tibial component for knee arthroplasty.
5. The component of claim 1 in which the fibre arrangement is a weave
pattern.
6. A method for constructing a tibial component defining an articular
bearing surface comprising the steps of arranging a plurality of fibres
composed
of a single polymer to form a multiplicity of fibre layers, the molecules of
said
fibres being oriented in the lengthwise direction thereof; and consolidating
the
multiplicity of fibre layers together sufficiently to fuse the fibres together
while
retaining the molecular orientation thereof, the fibres further being arranged
such that the fibres at said bearing surface are oriented substantially
parallel to


-2-
one another and to the intended direction of articulation and so that
intersecting
fibres are substantially eliminated at the bearing surface.
7. The method of claim 6 in which the multiplicity of fibre layers is at least
equal to twenty or more layers before said consolidating step.
8. A polymer tibia bearing plate having an articular bearing surface
adapted for sliding engagement with a femoral component, the polymer tibia
bearing plate comprising a laminated structure with a multiplicity of
single-polymer fibre layers joined together wherein the polymer comprises
consolidated fibres, the molecules of which are linearly oriented, the
majority
of fibres at said bearing surface being oriented substantially parallel to one
another and to the intended direction of articulation so that intersecting
fibres
are substantially eliminated at said bearing surface.
9. The polymer tibia bearing plate of claim 8 in which the majority of the
fibres at said bearing surface are substantially parallel to the bearing
surface.
10. The polymer tibia bearing plate of claim 9 in which the polymer
comprises polyethylene fibres substantially parallel to the bearing surface.
11. An orthopaedic implant component defining a rigid bearing surface
comprising a plurality of longitudinally extending single-polymer fibres, the
molecules of which are linearly oriented, said fibres also being oriented
substantially parallel to one another so that intersecting fibres are
substantially
absent from the rigid bearing surface and so that a majaority of the fibres at
the
bearing surface extend parallel to the intended direction of articulation with
other forms of the polymer being remote from the rigid bearing surface, and
the


-3-
other forms of the polymer being fused together with the longitudinally
extending fibres to form a rigid structure for the component.
12. The orthopaedic implant of claim 11 in which the plurality of
longitudinally extending fibres occupy less volume of the component than the
other forms.
13. The orthopaedic implant of claim 11 in which the plurality of
longitudinally extending fibres are arranged together to form individual
layers
of longitudinally extending fibres and the layers are fused together.
14. The orthopaedic implant of claim 11 in which one of the other forms of
the polymer is polymer flake.

Description

CA 02011724 1999-OS-12
ZM0133
ORTHOPAEDIC IMPLANT
The present invention relates to an orthopaedic implant to
be used in surgical repair and/or reconstruction of human
joints. In particular the invention is concerned with a unique
construction for a tibia bearing plate for use in total knee
arthroplasty, and, in the alternative, other bearing components
in orthopaedic implants.
It has been the practice for many years in knee arthroplasty
to reconstruct knee joints by attaching a femoral component to
the distal end of a femur and attaching a tibial component to
the proximal end of a tibia. The tibia component retains a
bearing plate with an articulating surface forming a sliding
engagement with the femoral component. Heretofore, the bearing
plate was constructed from polymer flakes or polymer powder that
were compression molded to the desired geometry of the bearing
plate. In addition, it was possible to machine a block of
molded or extruded polymer to the desired geometry. With
polymer flakes, the polymer molecules are orientated in a random
fashion such that some molecules extend in the direction of
sliding motion between the femoral component and the bearing
plate while other molecules intersect the articulating surface
in a direction normal to the sliding direction. It is believed
that the orientation of the molecules in a direction normal to
the sliding direction creates resistance to sliding movement and
such resistance results in wear debris in response to sliding
movement of the femoral component relative to the bearing plate.
-1 -


CA 02011724 1999-08-25
The invention in one aspect includes an orthopaedic bearing component
uniformly composed of a single polymer and defining an articular bearing
surface wherein the polymer comprises consolidated fibres, the molecules of
which are oriented in the lengthwise direction of said fibres, the majority of
S fibres at said bearing surface of the bearing component being oriented
substantially parallel to one another so that intersecting fibres are
substantially
eliminated at said bearing surface, said majority of the fibres also being
orientated substantially parallel to the intended direction of articulation.
An embodiment of the present invention provides for a predetermined
orientation of polymer fibers in a tibial bearing plate such that the
predetermined orientation substantially parallels sliding movement direction
in
a knee joint. The predetermined
20
- la-

~t~~tl~~,
orientation is controlled by using longitudinally extending
polymer fibers that are woven or arranged into layers and each
fiber is formed with molecules extending in the longitudinal
direction. In a preferred embodiment, a multiplicity of the
layers are stacked on top of each other in a laminated manner
with the fibers of each woven layer orientated in a
predetermined direction. Thereafter, consolidation tightly
binds the layers together and a die utilized during or after
consolidation imparts a contoured outer surface to the final
structure. In particular a tibial bearing plate is formed by
stacking together a plurality of layered ultra high molecular
weight polyethylene (UHMWPE) ~ fibers. In the preferred
embodiment, each layer is formed with a fine weave and the
UHMWPE is preferably Spectra 900 as marketed by the Allied
Signal Corporation. The consolidation process preferably
includes compression molding but it is also possible to utilize
pultrusion, isostatic compression or extrusion to consolidate
the fibers.
In an alternative embodiment of the present invention the
tibial bearing plate is formed from a combination of polymer
flake, and/or chopped polymer fibers with layers of woven
longitudinally extending fibers. The layers are compression
molded to the chopped fibers and/or polymer flake such that the
articulating surface of the tibial bearing plate is formed by
the layers of longitudinally extending fibers and the chopped
fibers and/or polymer flake are provided to "back-up'~ the
layered longitudinally extending fibers and remain spaced from
the articulating surface.
It is an advantage of the present invention that the tibial
bearing plate is formed from longitudinal fibers that retain
molecular orientation in the longitudinal direction following

20~~. r ~;<~
compression molding. Therefore, the fiber molecules are
orientated in a plane substantially parallel to the interface
between the femoral component and the bearing plate and
resistance to movement at the interface is reduced. With less
resistance it is believed that wear debris will also be reduced
to minimize the release of debris into the tissue surrounding
the reconstructed knee joint.
In the drawings Fig. 1 shows a frontal cross sectional view
of a knee joint schematically illustrating the tibial bearing
plate of the present invention. Fig. 2 is a top view of the
tibial bearing plate and tibial tray shown in Fig. 1. Fig. 3 is
a cross sectional view taken along line 3-3 of Fig. 1. Fig. 4
is a schematic illustration of the process utilized to construct
the tibial bearing plate of the present invention. Fig. 5 is a
view- similar to Fig. 3 showing an alternative embodiment of the
present invention.
A knee joint prosthesis 10 in Fig. 1 includes a femoral
component 12 made from metallic material and a tibial component
14 comprising a metal tray 16 and a tibial bearing plate 18.
The femoral component 12 is secured to the distal femur of a
patient receiving the knee joint prosthesis by any suitable
means such as bone cement or bone ingrowth via a porous surface
on the side of the component in intimate contact with the bone.
In a similar fashion the tibial component 14 is secured to the
proximal tibia by any suitable means so that the tibial tray 16
is secured to the tibia. Optional pegs 20 can be provided to
enhance fixation to the tibia.
The tibial tray 16 includes a circumferential rim 22
cooperating with a bottom surface 24 to form a recess 26. The
tibial bearing plate 18 fits within the recess 26 and is secured

to the tibia tray via suitable means to prevent separation
therebetween. If a modular tibial component is desired it is
also possible to modify the tibial tray 16 so that different
sizes of tibial bearing plates may be secured to the tibial tray
in response to different sizes of femoral components 12. The
superior contour of the tibial bearing plate includes arcuate
depressions 28 and 30 which slidingly engage the arcuate
condylar portions 32 and 34, respectively, of the femoral
component 12.
In order to form the tibial bearing plate 18, a plurality of
longitudinally extending fibers are woven into a cloth layer 40
(Fig. 4) such that the fiber orientation is maintained by the
weave pattern and each individual fiber 36 extends across the
width or length of the layer 40. As shown in Fig. 2, the fibers
36 extend in a lateral=medial direction and in an
anterior-posterior direction. The length and width dimension
for each layer is approximately equal to the maximum length and
width dimension for the tibial bearing plate 18. A plurality of
layers 40 are stacked on top of each other as shown in Fig. 4
between a die 42 and a jig 44. The die 42 includes an outer
surface 46 identical in contour to the contour of the arcuate
depressions 28 and 30 so that when the die 42 is forced against
the plurality of layers 40 the arcuate depressions 28 and 30
will be imparted to, the piece removed from the jig. When the
die 42 is impacted against the layers 40 in a compression
molding operation, the jig 44 and die 42 are suitably equipped
with heating elements to heat the fibers to a temperature that
permits fusion and consolidation of all the fibers without a
loss of identity for most of the discrete fibers. After the die
imparts a contour to the plurality of heated layers 40, the
tibial plate 18 is formed as a rigid structure from the
plurality of layers 40. The tibial bearing plate 18 is then

~~U~.17~L:~
removed from the jig 44 for final cutting and machining to
remove burrs and/or impart the final geometry to the tibial
bearing plate 18 before utilization with the tibial tray 14.
A tibial bearing plate 18 constructed in accordance with the
aforegoing procedure retains evidence of a substantial number of
individual fibers within the rigid structure so that it is
possible to identify a weave or orientation pattern in the
tibial bearing plate 18. It is contemplated that such
identification may include visual examination, light scattering
techniques, x-ray diffraction or polarized light microscopy. A
suitable fiber for constructing the tibial bearing plate 18 is
Spectra 900 polyethylene (Allied Signal Corporation) with a
density of .97 grams per cubic centimeter and a fiber diameter
equal to 38 microns. As an alternative, it is possible to
construct the tibial bearing plate 18 from Spectra 1000
polyethylene (Allied Signal Corporation) with a density of .97
grams per cubic centimeter and a fiber diameter of 20-25
microns. Each layer of fiber cloth 40 is also available from
Allied Signal Corporation in cloth form as Spectra 900 Plain
Weave S902 Scoured cloth.
In the alternative embodiment of Fig. 5, the tibial bearing
plate 118 includes a plurality of woven layers 140 of
longitudinally extending fibers at the articulating surface
128. A first region 102 is made from the longitudinally
extending fibers and a second region 104 is made from chopped
polymer fibers and/or polymer flake 106. The depth of the first
region 102 is less than the depth of the second region so that
only the region of the tibial bearing plate immediately adjacent
the articulating surface 128 is provided with the woven layers
of longitudinally extending fibers.



I~ ~ .~ A'~~~~.r S
Although the aforegoing description proceeds with reference
to a tibial bearing plate, it is within the scope of the
appended claims to construct other polymer bearing components in
accordance with the present invention. For example, an
acetabular cup prosthesis includes a metal shell for attachment
to an acetabulum and a polymer bearing retained within the metal
shell for articulating engagement with the head of a hip
prosthesis. This polymer bearing can be formed from a plurality
of polymer longitudinally extending fibers in cloth layer form
with a corresponding jig and die to generate the appropriate
contour for cooperation with the metal shell and hip prosthesis.
In addition, other polymers may be used in place of polyethylene
so long as the polymer material is available in fiber form.

Representative Drawing