Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02437282 2003-07-29
COMPOSITE PROSTHESIS
Related Applications
This is a divisional application of CA 2,095,762 filed
May 7, 1993.
Field of the Invention
The present invention relates to a composite structure
that is particularly suitable for use as a component of an
artificial joint prosthesis and more particularly to a
component having a shaft or stem that is to be inserted
into a cavity in a bone such as the femoral component of
a hip prosthesis, the tibial component of a knee
prosthesis, the glenoid component of a shoulder prosthesis
and an elbow prosthesis.
Background of the Invention
The role of the hip joint in the skeletal system is to
transmit loads between the pelvis and the femur and allow
for motion between the two such that .normal activities
such as walking and stair climbing can occur. The loads_
are transmitted through a joint similar to a ball and
socket joint. The head of the femur provides the ball
portion of this joint. Its role is to provide a mode of
transmitting the axial, torsional and bending forces that
are developed during normal human activities: The
structure of a normal femur is a tubular construction in
which the. wall (or cortical shell of the femur) carries
most of the' load and the load is transmitted from the
femoral head along this wall or cortical shell.
The occurrence of arthritis in the hip joint or the
fracture of the femoral neck often times requires that a
prosthesis be implanted in the femur to perform this
structural role. Current prosthetic designs require that
the femoral head and neck be removed and the cancellous
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bone and marrow that make up the core of the femur are
also removed, leaving only a shell made up primarily of
cortical bone. A metallic prosthesis is inserted into the
cavity thus formed. It is secured in place by being press
fit, cemented in place; or held in place by tissue
ingrowth into a porous coating on the surface of the
prosthesis.
The presence of a metallic prosthesis in the femur changes
the mode in which the loads are carried by the bone. This
will result in the loads being carried initially by the
implant and subsequently transmitted to the wall of the
bone from the inside out, as opposed to always through the
cortical wall as occurs with the normal femur.
The metal implants have modulus values that range from 100
to 200 GPa. In contrast, cortical bone has a modulus of
approximately 20 GPa: Due to this modulus mismatch the
stem of the implant carries loads that in a normal femur
the cortical shell would carry. A consequence of this
modified load transfer is that the cortical bone does not
experience the proper mechanical environment that it
requires to maintain its normal structure. over a long
enough period of time in this condition the bone will
slowly resorb.
A complication that arises from bone resorption is
cortical wall thinning (i.e., an increase in the size of
the medullary canal of the femur). The consequences of
this is that the prosthesis loses some of its support, and
becomes loose and can be painful to the patient. This
loss of support also can be a contributing factor to the
fatigue failure of the devices.
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It has been proposed that the modulus mismatch between the
stem of a prosthesis and the cortical shell of the bone
could be overcome to some degree by the use of a fiber
reinforced composite structure whose modulus more closely
conformed to that of bone. Although many such composite
structures have been proposed, no composite structure has
been widely used in implantable joint prosthesis.
U.S. Patent No. 4,750,905, "Beam Construction and Method"
to discloses a composite femoral component for a hip joint
which is constructed of a composite of carbon fiber and
polysulfone. The construction includes a core formed of
continuous filament fibers oriented substantially along
the length of the core and embedded in a polymer matrix.
The core is encased with a sheath of braided or woven
filaments. The sheathed core is embedded in a polymer
which fills the space of a bone cavity. The proximal end
of the core is the neck of the device. The neck includes
a tapered thimble which can carry the ball-like hip joint
head of the prosthesis. The device disclosed in this
patent does not provide for the smooth transition of
forces through the device.
EPO No. 277,727 discloses a composite orthopaedic implant
constructed with a polymer matrix and continuous filament
carbon fiber reinforcement. A number of different
polymers are disclosed including polysulfones and
polyaryletherketones. The device is constructed from
uniplanar sheets or prepregs which are stacked and molded
to form blocks from which the devices are machined. The
composite may include a sheath of continuous fiber to
inhibit delamination: The sheath may be a woven or
braided mantle or sock fitted over the molded prepreg.
Constructions of this type have a problem in the disparity
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in the flexural trength and stiffness in and out of the
plane of the sheets from which the product is constructed.
U.S. Patent 4,892,552 and the corresponding WO No.
85/04323 also disclose a composite prosthesis made from
various biocompatible polymers and reinforced with carbon
fibers. The prosthesis is made of uniplanar stacked
sheets molded and shaped and is similar to the prosthesis
disclosed in EPO No. 0277,727.
U.S. Patent No: 4,662,887 discloses a high modulus
implantable prosthesis made from fiber reinforced
polyaryletherketones.
U.S. Patents 4,902,297, and 4,978,360 and GB2216425,
disclose a composite prosthesis with a unidirectional
carbon fiber core, an inner casing of braided carbon
fibers, and an outer casing of an injected molded polymer.
U.S. Patent 4,978,358 discloses a composite prosthesis
comprising an outer component made of metal and an inner
component made of a carbon fiber reinforced composite.
The inner component. may be made of a material of lower
bending stiffness and/or higher strength than material of
the outer component.
Mathys Jr., et al., Current Interdisciplinary Res.,
(Perren M., et al. eds.) Martinus Nyhoff, Boston (1985)
pp. 371-376 ~and.Morscher et al.,'"Clinical Orthopaedics
and Related Research, Number 176, June 1983, pp. 77-87
disclose a hip prosthesis made from a plastic material,
polyacetal, reinforced with a metallic core.
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Summary of the Invention
The present invention relates to a composite orthopaedic
prosthesis, particularly a femoral component of a
replacement hip prosthesis which avoids the problems
associated with previously known composite hip prosthesis.
The prosthesis of the present invention provides uniform
strength and stiffness ire all planes of the prosthesis.
In addition, the prosthesis is better able to transmit
force or load developed during normal activities between
the pelvis and the cortical bona of the femur.
The composite prosthesis of the present invention
minimizes the modulus mismatch of the stem portion of the
component with the cortical shell of the femur. In use,
the stiffness of the inner member (prosthesis] of the
femur-prosthesis combination is minimized which forces the
outer member (cortical shell] to carry a greater portion
of the load and react closer to the normal physiological
manner thereby reducing the problem of bone resorption.
The prosthesis of the present invention includes a metal
core which extends from the distal end of the stem of the
prosthesis to the proximal end of the stem and to the neck
of the prosthesis. The proximal end of the metal core has
the standard neck geometry, including a trunnion, used on
standard metal hip prosthesis to allow the ball component
of the prosthesis to be attached to the stem. The metal
core may also include a cap which overlies and covers the
proximal end of the composite shell. The cross sectional
area of the metal core can be varied over the length of
the stem to control the flexural properties of the stem.
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The metal core is covered or encased by a composite shell.
The composite shell is composed of high tensile strength
filaments in a matrix of a biocompatible thermoplastic
resin. The thickness of the shell may be varied from the
proximal end to the distal end of stem.
The composite shell is fabricated so that the shell has
substantially equal strength in all directions i.e.
isotropic. This can be accomplished by arranging the high
strength filaments so there are filaments substantially
longitudinally oriented along the length of the stem and
transverse to the length of the stem. Such fiber
orientation is shown in U:S. Patent 4,892,552 and EP
277,727 cited above. The preferred method of arranging
the filaments is by the filament braiding techniques
described in more detail below.
At the proximal end of the stem, adjacent the neck, there
is an insert between the braided shell and the metal core.
The insert is preferably made from a composite material,
but can be made of any biocompatible material, i. e. , metal
or polymer. The stiffness of the insert may be selected
to increase or decrease the stiffness of the proximal
region of the stem as may be required for various
modalities. The use of an insert also provides a method
to define or vary the proximal geometry of the stem. In
some configurations, the insert can be formed as an
integral part of the core.
30. The three components of the device are assembled by
bonding all three components together by heat and pressure
or by adhesives or by a combination of heat and pressure
and adhesives.
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The present invention provides a process of making a stem for
a prosthesis having a metal core and composite fiber shell
surrounding said core, the process comprising providing a
tapered metal core and a composite shell, applying an
adhesive to the surface of said tapered metal core, placing
the tapered metal core into a cavity in the composite shell,
said cavity having the same configuration as said tapered
metal core, said core having a length greater than the length
of the cavity in said composite shell so that a portion of
the core extends beyond each of the ends of the composite
shell, placing the composite shell in an insulated fixture
having a length less than the length of said core so that the
metal core extends beyond the ends of the fixture, heating
the metal core and the interior of the shell, applying
pressure along the length of the core to force the core
against the adhesive within the composite shell thereby
bonding the core to said shell.
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The three components of the device contribute attributes
or benefits to the complete device which are not found in
prior art hip stems, either metallic or composite.
The first benefit is the increase in the strain of the
proximal cortical wall of the femur. The decreased
moduius of the present stem will cause the strains to be
closer to normal physiological strains. The ramifications
of this is that bone resorption will be reduced and the
device will be provided with a more consistent means of
support. This should result in a less painful device
since a gap will not develop between the device and the
cortical shell of the bone. Hence, there will be no
movement between the device and the bone which would be a
possible source of pain to the patient.
The composite shell offers several options for device
construction that do not exist in other designs. The
stiffness of the device over its length can be varied in
order to respond to various modalities. A less stiff
proximal region in the normal stem will allow the proximal
cortical shell o carry more of the load. This will
result in much higher strains in the proximal bone and
should eliminate proximal cortical bone resorption. A
braided composite revision stem could have a stiffer
proximal section than the normal stem in order to make up
for the lack of cortical bone support that can occur in
revision cases.
The use of a metallic neck in a composite stem results in
a higher strength neck, for a fixed volume, than is
possible with a fiber-resin composite. In the unsupported
region of the hip joint the neck of the device is
subjected to a high stress region as a result of the
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bending, compressive, and torsional stresses caused by
normal daily activities such as rising from a chair and
stair climbing. This also allows for a minimal Morse
taper type femoral head to be used without any concern
about the long-term behavior of the composite. A metal
cap can be affixed.to the core and neck which provides a
solid surface to apply force to seat the stem in the
medullary canal. The cap may also be an integral portion
of the core.
Brief Description of the Drawings
Fig. 1 is an isometric view of the prosthetic device of
the present invention.
Fig. 2 is a cross sectional: view of the prosthetic device
of the present invention taken along lines 2-2 of Fig. 1.
Fig. 3 is a cross sectional view of the core of the
prosthetic device taken along lines 3-3 of Fig. 2.
Fig. 4 is a side view of a mandrel used to,make the
prosthesis of the present invention showing an insert on
the lateral side of the prosthesis:
Fig. 4a-4d are cross sectional views of the Metal core of
Fig. 4 taken along lines 4A-4A, 4B-4B, 4C-4C and 4D-4D
respectively in Fig. 4.
Fig. 5 is a fragmentary view of the winding of the braided
sheath on the stem of the present invention.
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Fig. 6 is across sectional view of another embodiment of
the present invention with an insert on the medial side of
the prosthesis.
Fig. 7 is a full cross sectional view of embodiment of the
invention shown in Fig. 6, taken at a position along the
stem of the prosthesis shown as line 7-7 of Fig. 6.
Fig. 8 is a side view of an embodiment of a mandrel used
to make the prosthesis in the present invention with an
insert on the medial side.
Fig. 8A - 8D are cross sectional views of the mandrel of
Fig: 8 taken along lines 8A-8A, 8B-8B, 8C-8C and 8D-8D
respectively in Fig. 8.
Fig. 9 is a side view, partially in section, of the
apparatus used to assemble the prosthesis of the present
invention.
Fig. 10 is a fragmentary detail view of the apparatus of
Fig. 9.
Detailed Description of tl~e invention
As previously indicated, the prosthesis of the present
invention is particularly directed to the femoral
component of an artificial hip prosthesis. It should be
understood that the invention could also be used in the
manufacture of other implantable prostheses, specifically
those which have a stem which is implanted into a bone
cavity. Tmplantable knee, elbow and shoulder prostheses
may also have such stems. The present invention finds
particular advantage for use in an implantable hip
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prosthesis for the reasons previously explained and will
be described in. reference to such hip prosthesis.
Fig. 1 shows an isometric view of the prosthesis 10 of the
present invention. Tt should be understood that the ball
11 on neck 16 shown in Fig. 1 is a standard ball which is
sized to fit into the acetabular cup component of a total
hip prosthesis. Metal or ceramic balls of the types
commonly used in implantable hip prostheses can be readily
used with the hip stem of the present invention. Also,
the acetabular cup component of a total hip prosthesis of
the types commonly used with other femoral components can
be used with the hip stem component of the present
invention.
As shown in Fig. 2 the stem 12 of the prosthesis has an
inner core 13, a composite outer shell 14 and an insert 15
adjacent the proximal end of the stem between the core 13
and the composite shell 14.
In describing the prosthesis and the method of making the.
prosthesis, the term "insert" describes a component of the
finished prosthesis and the term "spacer" describes a
fixture that is used as part of the mandrel during the
manufacture of the composite shell but that is not
necessarily a part ~of the finished prosthesis.
The core 13 may be constructed of metal or a combination
of metal and a polymer or a composite of polymer and
reinforcing fiber. The preferred material of construction
of the core is metal of the type previously used in joint
prosthesis such as a cobalt-chrome alloy or a titanium
alloy. The titanium alloy Ti-6A1-4v is preferred. The
use of a metal core also facilitates the construction of
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the neck portion 16 and cap portion 19 of the stem as the
metal can be machined to provide a Morse-type taper to
allow the ready attachment of the ball 11 to the neck 16.
The cross section of the core can be tapered or otherwise
varied along the length of the stem to control the
flexural properties of the stem. The metal core can be
made by casting or forging depending on the material
selected. It may be surface finished to the desired
dimensions. The -stiffness or flexural rigidity of the
stem is provided by the core, the insert and the composite
shell. In order to minimize the mismatch between the
stiffness of the stem and the bone into which the stem is
implanted, the core should provide only about 5 to 25%,
preferably about 10% of the stiffness of the entire stem.
The particular shape or cross sectional configuration of
the core can be varied depending on the particular
configuration of the finished prosthesis. Tapered rods
with a circular or ovoid cross section are suitable. A
core with a circular cross section shown in Fig. 7 is
preferred.
The composite shell allows for the 'selection of the
mechanical properties that are desired for a particular
prosthesis, that is, the stiffness of the composite shell,
(compared to the stiffness of the core) can be chosen such
that the strains transmitted to the cortical shell of the
femur are as close as possible to the normal physiological
strains. The composite shell is a composite of a high
strength fiber and a biocompatible semi-crystalline
thermoplastic polymer. The high strength fiber such as
carbon fiber has a strength of at least 3600 MPa and a
modulus of 220-240 GPa. The carbon fibers have a diameter
of less than 10 microns, usually in the range of 6 to 9
microns. The polymer may be any of the thermoplastic
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polymers previously proposed for use in composite
orthopaedic implants. However, the use of
polyaryletherketone, polyetherketone, polyetherketone-
ketone or polyetheretherketone offer significant
advantages in environmental stability over other polymers.
The composite shell is preferably abraided structure made
from a commercially available composite material of carbon
fiber in a polyetheretherketone thermoplastic polymer
20 matrix. The material is available as APC-2 from Imperial
Chemical Industries PLC in a variety of different forms,
i.e. unidirectional and quasi isotropic sheets, tapes and
tows. The polymer and fiber reinforced devices formed
from the polymer are described in U.S. Patents 4,662,887
and 4,721,945. All forms mentioned above are suitable
for use in the present invention. The tow is preferred.
The tow and sheets are available in various widths e.g.
2mm to 30Omm. The tape or sheet is slit to a narrow
width and wound onto bobbins of the type used in a
braiding machine. A width of about 2mm is a suitable
width. The particular width of tape employed is a
function of the number of carriers on the braider and the
desired diameter of the prosthesis and the desired fiber
angle. The tape is then braided onto a mandrel which has
the geometry of the desired prosthesis. The braid is
applied to the mandrel one ply at a time until the
desired diameter is obtained.
A carbon fiber tow impregnated with the polymer is
available with variations in resin content of from 30 to
60% by volume, 24 to 52% by weight. The tow is preferred
to other forms of impregnated fiber because it is more
consistent in thickness and width. The fibers in the tow
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are also continuous and there are no ends that can split
off during the braiding operation.
The plys of the impregnated carbon (fiber can be laid on
the mandrel at various angles to the longitudinal axis of
the mandrel. A typical construction would include laying
the fibers (the tape: or tow) in a triaxial construction
with fibers in the longitudinal direction, i . e. , o ° , and
other fibers in two directions symmetrical about the 0°
fibers i.e. 45° in either direction. It should be
understood that the angle of the fibers will change over .
the length of the stem as the diameter of the stem varies
from the distal end to' the proximal end and because of the
limitations of the braiding process. The advantage of a
braided shell compared to a laminated structure of the
type used in previous composites such as disclosed in EPO
0277,727 is that in the laminated structure all of the
fibers are parallel to each other in any given ply, while
in a braided structure the fibers in any given ply are at
various angles within a given ply and are interlocked and
transversely isotropic. This results in a device that
will flex uniformly in the medial/lateral and the
anterior/posterior planes as compared to a laminated
structure which would not be isotropic in both planes. It
should also be understood that the thickness of the
braided shell will vary with the cross section of the
mandrel. That is, as the cross section and circumference
of the mandrel increases, the thickness of the braided
shell will decrease. This is because the same volume of
fiber as tape or tow, is applied at any point along the
length of the mandrel and if the circumference of the
mandrel increases, the tape thickness will decrease. The
cross sectional area of the braided shell is relatively
constant throughout the length of the stem.
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The braided structure also provides mare resistance to
delamination because each ply has fewer fiber ends. The
stiffness of the braided shell is determined by the number
of plys of the fibers. in the braid and the ratio of fiber
to resin in the tow or tape from which the shell is made.
The stiffness in any particular direction will depend on
the fiber content in the particular direction. Generally,
the number of plys will vary from approximately 2 plys to
16 plys. The fiber content will be between about 40% and
70% by volume of the, composite shell. The fiber
orientation in the bias direction can be varied over wide
angles but will be between about 30° and 75° to the
longitudinal axis of the stem. The fiber orientation in
the axial direction can be varied by about 10° from the
longitudinal axis of the stem.
The spacer 15 acts as an addition to the mandrel to
determine the outer geometry of the proximal portion of
the stem. The design of the device is such that there is
a significant flair in the proximal--medial contour of the
stem. This geometry change can not be accomplished by
using only a braided structure. The major limitation to
the braiding process is that it can only apply material in
a symmetric manner about the part. Therefore, creating an
asymmetric or variable cross section structure with a
braid alone is not possible and this change in volume of
the stem has to be created fn some manner other than with
the braid, This volumetric change is best accomplished
with the use of a spacer in the braiding process. In some
constructions, the spacer can be an integral part of the
core when construction allows the core to be used as the
mandrel in the braiding process. In other constructions,
the spacer is used only during the braiding process and
removed before the core is joined to the composite shell.
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In such constructions, an insert is placed between the
core and the shell in the finished prosthesis as will be
later described.
The insert can be constructed from any one of a number of
materials depending on the desired proximal stiffness. A
prosthesis with a flexible proximal geometry could have a
pure resin insert. A revision prosthesis with a stiffer
stem in the proximal area would have a metal insert such
as Cobalt-Chromium. For this type of prosthesis, the
braid could be applied directly to a metal core, and the
insert would be integral with the core. A third and the
preferred option is to have a fiber reinforced insert that
has a stiffness similar to that of the braid. However the
stiffness of the insert may also be varied by changing the
fabrication technique and the fiber reinforcement to
correspond to a particular design.
Other alternative constructions for the insert would
include making the insert out of two or more materials .
The proximal portions could be made out of Titanium or
cobalt chromium and the distal portion could be a
composite or resin. This would result in a shifting of
the load transferred to the stem to a slightly more distal
location. In particular, such construction could be used
for a custom device where most of the proximal calcar of
the patient has resorbed and the surgeon would not desire
the bone to carry much of the load.
The metal components of the device consist of a tapered
Chrome-Cobalt or Titanium core and a Chrome-Cobalt or
Titanium cap neck and trunnion component. These parts may
be fabricated as a single piece or may be separate pieces
that are welded together.
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Fig. 2 shows a metal core 13 with an extension 17 machined
in the proximal end of the core. The extension 17 is
fitted into an opening 18 in the cap 19. The parts are
then welded into a single component using electron beam or
other welding techniques such as arc, laser, resistance
and friction welding: Brazing and diffusion bonding may
also be used to join the metal components. The cap 19
would subsequently be polished so the extension 17 would
not be visible. The extension 17 is shown with a shoulder
in Fig. 2 and without a shoulder in Fig. 6. The use of a
shoulder on the extension 17 is optional.
The composite shell may be attached or applied directly to
the metal core or formed on a mandrel. The choice of
construction depends on whether the insert is a different
material than the core and the configuration of the
finished prosthesis. If the core is curved, e.g: when the
insert is placed on the lateral side of the prosthesis as
shown in Fig. 2, a core with an integral insert could not
be inserted into the composite shell. In this situation
the prosthesis could be constructed with a separate insert
or by applying the fibers directly onto the core to form
the prosthesis. The braided shell would be removed from
the mandrel and a core component would be fitted into the
shell in the manner later described. Fig. 4 shows a
mandrel 20 with an spacer 21 in a position on the lateral
side of the mandrel. The braid is applied from the
smaller end of the mandrel to the larger end which is from
the distal end to the proximal end of the prosthesis. A
spacer 21, which is the size of the insert used in the
final prosthesis, may be used during the braiding process.
As indicated above, the braid varies in thickness with the
diameter or circumference of the mandrel. As shown in Fig
4A-4D, the braid thickness is at a maximum at the smaller
CA 02437282 2003-07-29
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end of the mandrel, Fig. 4A, and decreases in thickness,
Fig.'s 4B and 4C, to the larger end of the mandrel Fig.
4D. The distal end portion of the mandrel is longer than
the desired length of the stem for ease in processing. If
the braid is applied directly to a core excess metal
extending beyond the braider shell will be removed to form
the desired final. configuration of the prosthesis. Fig.
5 illustrates the braid positioned on the core.
Fig. 6 shows a hip prosthesis with the insert 15 on the
medial side of the prosthesis.
The cross sectional area of the metal core 13 at a
position immediately adjacent the cap of the prosthesis of
Figs. 6 and 7 is circular. The cross sectional area of
the core at the same position in the design shown in Figs.
2 and 3 is ovoid or egg shaped.
Fig. 8 and Fig.'s 8A-8.D show a mandrel that could be used
to make the prosthesis of Fig. 6 and cross sectional areas
of the mandrel at different points along its length. The
cross sectional area of the core in this design is always
circular.
The total stem is fabricated by the following procedure:
The braided shell is made by braiding over a mandrel as
many plys as necessary to create the distal outer
diameter. The braid may be adjusted with regard to the
width of the tow being braided and the number of carriers
being used to control the thickness of the individual ply,
the fiber orientation within the ply and the ratio of
fiber to resin in the tow in order to control the
stiffness of the device. The insert may be an integral
part of the mandrel or may be a separate piece affixed to
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the mandrel prior to braiding. The core, insert and
braided shell are then assembled with a layer of
thermoplastic resin applied to any metal components. The
resin may be applied in the form of a piece of film, a
plasma sprayed coating, or a molded coating on the.metal
core, or a combination of the three. The resin may either
be the same as the matrix materials or miscible in the
matrix material. The source of the resin may not need to
be separately applied to the components but may be applied
to the composite components during the composite
fabrication process rather than during the joining
process.
In the assembly of the prosthesis, the braided shell is
removed from the braiding mandrel and machined to
dimensionally match the length of the containment mold 31
in Fig. 9, which is about the length of the finished stem.
The inside taper of the composite is lightly sanded to
remove any contaminant from the surface and to provide a
roughened surface for the adhesive to which the adhesive
will readily bond, The surface is then washed with water
rinsed with a solvent such as acetone or isopropyl alcohol
and air dried or oven dried to remove the residual
solvents. The metal core that will be used is then sand
blasted to provide a roughened surface for adhesive
bonding. The core is then cleaned in an ultrasonic
cleaner with a similar type of organic solvent that is
used to clean the composite shell and then vapor
degreased. The adhesive is-then applied to either the
metal core or to the inside of the composite shell or to
both. The adhesive may be in the form of a thin
polyetheretherketone resin film cut into a conical pattern
to fit the interior of the shell. The adhesive may also
be sprayed on to the core or otherwise applied to the core
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or the shell. If the adhesive is the form of a film, the
film is also cleaned with a solvent and oven dried to
remove any absorbed moisture.
The procedure to produce the composite is described in
more detail below. .
The apparatus to bond the core, insert and shell is
depicted in Fig. 9 and 10. A bonding fixture_30 contains
a containment mold 31 which has a cavity 32 which
corresponds in shape to the outer surface of the
prosthesis. The mold 31 is held in the fixture 30 in a
block 34 of insulation material. The plates 35 and 36 are
joined by bolts 37 which contain the block 34. There are
ceramic insulators 38 in each plate with an opening of
sufficient size to allow the ends of the metal core to
pass through the opening when the core-shell assembly is
placed in the fixture, and when the core moves during the
bonding operation.
The plates 35 and 36 are secured to a base 39. Also
attached to the base 39 is a pneumatic or hydraulic
cylinder with a piston 40 which can apply force to the
core. An electric motor with a ball-screw actuator may be
used in place of a pneumatic or hydraulic cylinder.
An electrical power supply provides current to the core
which provides the necessary heat to bond the assembly.
The shell, insert and core are then assembled and placed
in the end station of the bonding fixture. At the distal
end of the core there is placed a seal 50, best shown in
Fig. 10, to prevent the flow of the adhesive out of the
interface between the core and the braided shell upon the
CA 02437282 2003-07-29
20 -
application of heat and pressure. This seal can be made
from a thin piece of ductile metal such as titanium or
brass approximately .005-.007 inches thick which has an
opening slightly smaller than the diameter of the core at
the point where the core contacts the seal. The core in
contact with the seal prevents.the flow of adhesive out of
the mold when pressure is applied. When the components
are flitted into the bonding ffixture, the wires 41 from the
power supply 42 are attached to both ends of the core.
Electricity is then allowed to flow through the core and
at the same time force is applied through the piston 40 to
push the core in the direction of diminishing taper
against the braided shell. The flow of electric current
through the core heats the core. This bonds the shell to
the core. When the bonding procedure is completed the
piece is allowed to cool and the excess portions of the
core are removed by cutting and the finished stem is
machined to its proper shape.
Example 1
An example of the above procedure is as follows:
A Ti-6A1-4V core and a polyetheretherketone/Carbon
fiber braided shell were fitted together with a piece of
polyetheretherketone film between the core and the shell.
The composite shell had a hole drilled through the wall
and a thermocouple was inserted until it was in contact
with the polyetheretherketone film. The electrical
contacts were connected to the core and a current of 75
amps was applied to the core which resulted in a measured
initial heatup rate of 40°C per minute. Concurrently a
force of 2, 800 Newtons was applied to the assembly. After
13 minutes the measured bond line temperature had reached
290°C. The load was gradually increased over the next 6
minutes until a load of 6,000 Newtons was obtained and the
measured bond line temperature was 300°C. This condition
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was then held for 1 minute at which point the power was
turned off and the sample was allowed to cool to room
temperature under load. The bond strength of this sample
was 28.1 MPa.
An example of a modified process is the following
example.
Example 2
A Ti-6A1-4V core and a polyetheretheketone-Carbon
fiber braided shell were fitted together with a film of
polyetheretherketone between the core and the shell. The
composite shell had a hole drilled through the shell and
a thermocouple was inserted until it was in contact with
the polyetheretherketone film. The electrical contacts
were connected to the core and a current of 45-48 amps was
applied to the core which resulted in a measured initial
heatup rate of 10°C per. minute. Concurrently a force of
1800 Newtons was applied to the assembly. The temperature
at the bondline reached the Tg of polyetheretheketone at
9 minutes at which time a minor deflection of the core was
noted. At this point, the load was increased to 12,400
Newtons and the resulting deflection was noted. Current
was increased to 75 amps and a load was adjusted during
the subsequent heatup to maintain a constant deflection.
When the measured bondline temperature reached 255°C, the
load was then slowly increased to 6800 Newtons over a five
minute period. At this point the measured bond line
temperature had reached a maximum of 290°C. The current
was then shut off and the part cooled to room temperature
under Load over a 40 minute time period. The bond
strength of this sample was 25.5 MPa.
