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BACKGROUND OF THE INVENTION
The present invention relates generally to biomedical implant devices, and
more particularly to a method for forming biocompatible components.
A natural joint in the human body such as a knee joint may undergo
degenerative changes due to a variety of etiologies. When these
degenerative changes become advanced and are irreversible, it may
ultimately become necessary to replace the natural joint with a prosthetic
joint. Such a prosthetic joint often includes several biocompatible
components which are formed from high strength synthetic materials. These
materials are not only able to accommodate the various loading conditions
that the prosthetic joint may encounter, but are also biocompatible with
the human body. An example of such high strength synthetic materials is
ultra-high molecular weight polyethylene which is often used when there is
relative movement between the adjacent metallic surface of a prosthetic
joint.
Biocompatible components which are made from ultra-high molecular weight
polyethylene are often formed using one of two different techniques. In
one technique, a relatively precise amount of polyethylene powder is
placed between two halves of a die which are then simultaneously
compressed and heated. After the powder is densified using standard
sintering techniques, the die is allowed to cool. The biocompatible
component is then removed from the die and is sterilized in a manner
well-known to those skilled in the art.
In the second technique, a substantially completely consolidated
polyethylene stock is first formed and then the biocompatible component is
machined from the substantially completely consolidated stock. Several
methods exist which may be used to form the substantially completely
consolidated stock. In one method, the substantially completely
consolidated stock is extruded by placing polyethylene powder in a
cylindrical chamber having an opening of a particular shape at one end of
the chamber. A hydraulically operated piston located at the other end of
the cylinder is then used to compress the polyethylene powder. The force
exerted by the piston on the polyethylene powder causes the powder to
compact. Heat is also applied to solidify the powder as it moves through
the cylinder. In another method for forming a substantially completely
consolidated stock, polyethylene powder is placed between two flat plates
which are compressed while heat is applied. As this occurs, the
polyethylene powder is densified so as to form the substantially
completely consolidated stock.
While these two techniques for forming biocompatible components are
effective, they nevertheless have certain disadvantages. With respect to
the first technique described above, it will be appreciated that only one
biocompatible component can be made at one time. Accordingly, this
technique is relatively inefficient in terms of the amount of time
required to make the biocompatible component. With respect to the second
technique in which the biocompatible component is formed from a
substantially completely consolidated stock, the resulting consolidated
stock may often require a stress relief operation or an annealing
operation prior to machining. In addition, when polyethylene stock is
formed by heating polyethylene powder between two plates acting under
pressure, the resulting may have density gradients or voids due to the
relatively nonuniform pressure applied to the powder across the plates.
In addition, methods are also known for treating ultrahigh molecular weight
polyethylene prior to being machined into a biocompatible component. One
such method is disclosed in U.S. Pat. No. 5,037,928. However, during the
procedure described in this reference, the polyethylene stock is placed
under a sufficient pressure so as to induce pressure crystallization of
the stock. This pressure crystallization tends to cause increased
susceptibility to wear. In addition, the use of this relatively high
pressure required that relatively expensive pressure containment vessels
be used. Furthermore, this method describes processing preformed
polyethylene stock which often has unwanted density gradients or voids as
described above.
SUMMARY OF THE INVENTION
An advantage of the present invention is to provide a method for forming
biocompatible components using a multiple-step technique which can produce
biocompatible components relatively quickly at a reduced cost.
A further advantage of the present invention is to provide a method for
forming biocompatible components which produces a stock of consolidated
ultra-high molecular weight polyethylene which can be machined without
being subjected to a stress relief or annealing operation.
Another advantage of the present invention is to provide a method for
forming biocompatible components which does not substantially increase the
crystallization of the stock used to form the biocompatible component.
A further advantage of the present invention is to provide a method for
forming biocompatible components which uses both a cold isostatic pressure
treatment as well as a hot isostatic pressure treatment.
A further advantage of the present invention is to provide a method for
forming biocompatible components which enhances the bonding between the
composite materials from which the biocompatible component is made.
A further advantage of the present invention is to provide a method for
forming biocompatible components which facilitates the adhesion of a
porous metal coating.
In one form thereof, the present invention provides a method for forming
biocompatible components from a powder such as ultra-high molecular weight
polyethylene. The method includes enclosing the powder in a first
container and subjecting the first container to a cold isostatic pressure
treatment which forms an incompletely consolidated stock from the powder.
The incompletely consolidated stock is removed from the first container
and is placed in a second container which is then located within a hot
isostatic press and is subjected to a hot isostatic pressure treatment.
The hot isostatic press treatment forms the relatively completely
consolidated stock from the incompletely consolidated stock. The
relatively completely consolidated stock is then machined into a
biocompatible component.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sagittal elevational view of a knee joint prosthesis including
a biocompatible component in the form of a tibial bearing formed from
ultra-high molecular weight polyethylene by the preferred embodiment of
the present invention;
FIG. 2 is a cross-sectional view of a cold isostatic press of the type used
in accordance with the teachings of the preferred embodiment of the
present invention;
FIG. 3 is a perspective view of the first container used with the cold
isostatic press shown in FIG. 2 according to the preferred embodiment of
the present invention;
FIG. 4 is a cross-sectional view of a hot isostatic press of the type used
in accordance with the teachings of the preferred embodiment of the
present invention;
FIG. 5 of the second container used in conjunction with the hot isostatic
press shown in FIG. 4 according to the preferred embodiment of the present
invention;
FIG. 6 is a flow diagram illustrating the steps for forming a biocompatible
component according to the preferred embodiment of the present invention;
FIG. 7 is a perspective view of the container which is used in accordance
with the preferred embodiment of the present invention to enhance binding
of the layers of composite material of a biocompatible component;
FIG. 8 is a perspective view of the container used in accordance with the
preferred embodiment of the present invention to reduce voids in a stock
of biocompatible composite material;
FIG. 9 is a perspective view of the container used in accordance with the
preferred embodiment of the present invention to enhance adhesion of a
porous coating to a biocompatible component; and
FIG. 10 is a perspective view of the container used in accordance with the
preferred embodiment of the present invention to facilitate adhesion of
porous coated pads on a biocompatible component.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
It should be understood that while this invention is described in
connection with a particular example thereof, the scope of the invention
need not be so limited. Rather, those skilled in the art will appreciate
that the following teachings can be used in a much wider variety of
applications than the examples specifically mentioned herein.
Referring now to FIG. 1, a knee joint prosthesis is shown which is
generally designated by the numeral 10. The knee joint prosthesis 10 is
functionally depicted as being secured to a tibia 12 and a femur 14 of a
surgically resected knee joint, with the tibia 12 and femur 14 being shown
in phantom. The knee joint prosthesis 10 is shown to include a femoral
component 16 having a bearing surface 18. The femoral component 16 is
secured to the femur 14 by means of an inferiorly extending femoral stem
20 inserted into a matching bore created within the femur 14 in a manner
well-known to those skilled in the art.
The knee joint prosthesis 10 is further shown to include a tibial component
22 that is secured to the tibia 12 by means of an interiorly extending
tibial stem 24 inserted into a matching bore created within the tibia 12
in a similar manner as that described above. The tibial component 22
includes a platform-like tibial tray 26 which is used to support a tibial
bearing 28 constructed by the method of the present invention. The tibial
bearing 28 is formed to be symmetrically oriented about the sagittal
plane. In operation, the tibial bearing 28 provides a bearing surface 30
that is operable to accept a rotatable, low friction contact relationship
with the bearing surface 18 of the femoral component 16.
The tibial bearing 28 is formed of a low friction material having enhanced
wear resistance properties. In a preferred embodiment, the tibial bearing
28 is machined from a substantially completely consolidated stock that is
molded from an ultra-high molecular weight polyethylene powder having a
molecular weight of from about 3 million to about 6 million. The
ultra-high molecular weight polyethylene powder may be any powder
conforming to ASTM F-648, though preferable powders include Hifax 1900
resin available from Himont and GUR 405 or 415 resin available from
Hoechst Celanese. It will be understood, however, that other suitable
materials may be used to form a stock from which a tibial bearing 28 may
be machined. For example, the formation of articles from this method can
be accomplished using other polymer materials in powder form, preferably
having a molecular weight of from about 3 million to about 6 million. The
specific method used to form the tibial bearing 28 includes several steps
which are more fully described below. However, several of these steps
involve the use of either a cold isostatic press or a hot isostatic press.
Accordingly, the structure and operation of the cold isostatic press and
the hot isostatic press will now be described.
Referring to FIG. 2, a cold isostatic press 32 according to the preferred
embodiment of the present invention is shown which includes a pressure
chamber 34 that has an upper cover 36. The upper cover 36 includes a
threaded closure 38 that enhances a sealed condition within the pressure
chamber 34 when the pressure chamber 34 is pressurized. When the pressure
chamber 34 is sealed in this manner, the length of the pressure chamber 34
is approximately 24-30 inches and the diameter of the pressure chamber 34
is approximately 12 inches. The pressure chamber 34 is substantially
surrounded by an annular wall 40 which is operable to define the pressure
chamber 34, and has a thickness which is sufficient to contain the
pressure within the pressure chamber 34.
The cold isostatic press 32 further includes a pressure inlet line 42 and a
pressure relief line 44. The pressure inlet line 42 and pressure relief
line 44 are preferably tubular passageways each regulated by a pressure
control mechanism (not shown) that are operable to accommodate a
pressurized transfer of a gas or liquid fluid from an external source (not
shown) into and out of the pressure chamber 34.
The cold isostatic press 32 is preferably designed to operate at pressures
capable of compacting the powder to about 60-80% of its desired final
density, with the preferable range being between 65-75%. The cold
isostatic press 32 may be that which is available from National Forge,
Andover, Mass. or Models IP6-24-60 and IP8-36-60 which are available from
ABB Autoclave Systems, Inc. of Columbus, Ohio. However, other suitable
cold isostatic presses may be used.
The hot isostatic press 46 will now be described with reference to FIG. 3.
The hot isostatic press 46 is shown to include a pressure chamber 48 which
is defined in part by an annular wall 50, the thickness of which is
between about 6 inches and about 3 inches. In addition, the pressure
chamber 48 is about 18 inches in diameter and is about 53 inches in
length. The hot isostatic press 46 further includes a lower closure 52 and
upper closure 54 which are threadedly attached to the annular wall 50 by
matching buttress threads 56 and 58. It is to be understood, however, that
a pin locking mechanism may also be employed for securing the lower
closure 52 and upper closure 54 to the annular wall 50. The lower closure
52 and upper closure 54 are operable to maintain a heated and pressurized
condition within the pressure chamber 48 during the hot isostatic pressure
treatment described below.
The hot isostatic press 46 further includes a plurality of heating elements
60 that are operable to generate thermal energy within the pressure
chamber 48. Alternatively, the hot isostatic press 46 may include another
heating means, such as a solution jacket adjacent to the pressure chamber
48, that is operable to contain a hot fluid for providing thermal energy
to the pressure chamber 48. The hot isostatic press 46 is also shown to
include a cooling jacket 62 which comprises a plurality of coils
encircling the annular wall 50. The cooling jacket 62 is operable to
contain a suitable heat transfer fluid for removing thermal energy from
the hot isostatic press 46 by a transfer of thermal energy into the
cooling fluid. It will be understood, however, that the cooling function
accomplished by the cooling jacket 62 can be performed by another cooling
means disposed at a different location within the hot isostatic press 46,
such as within the pressure chamber 48 or between the pressure chamber 48
and the annular wall 50.
The hot isostatic press 46 further includes a heat shield 64 which is
located between the annular wall 50 and the heating elements 60. The heat
shield 64 is operable to limit heat losses from within the pressure
chamber 48 and to assist in controlling the temperature within the
pressure chamber 48. The hot isostatic press 46 also includes a pressure
system (not shown) of a type well-known to those skilled in the art that
is operable to pressurize the pressure chamber 48. The pressure system
also communicates with the pressure chamber 48 by means of a pressure
input/output line 68 that is connected to an inert gas source and
compressor of a type well-known to those skilled in the art. In a
preferred embodiment, the inert gas is argon, though nitrogen, helium and
neon gases may also be used.
The hot isostatic press 46 further includes a power distribution system
(not shown) of a type well-known to those skilled in the art. The power
distribution system is used for controlling the heat and pressure within
the pressure chamber 48. In addition, the electrical energy required by
the heating elements 60 is provided in this embodiment by an electrical
power line 72 that is connected to an electrical power source (not shown).
The hot isostatic press 46 is operable to change the temperature within the
pressure chamber 48 from an initial room temperature of from about
60.degree. F. to about 70.degree. F. to an operating temperature of from
about 365.degree. F. to about 420.degree. F. In addition, the hot
isostatic press 46 is also operable to change the pressure within the
pressure chamber 48 from approximately atmospheric pressure to an
operating pressure of preferably from about 7,500 pounds per square inch
to about 10,000 pounds per square inch. The hot isostatic press 46 may be
one of several well-known to those skilled in the art, such as Model
HP6-30, available from Iso-Spectrum, Inc. of Columbus, Ohio. Other
suitable hot isostatic presses are available from National Forge of
Andover, Mass. However, other suitable hot isostatic presses may be used.
The method of the preferred embodiment of the present invention will now be
described with reference to FIG. 4 which comprises the steps 80 through
96. At step 80, an ultra-high molecular weight polyethylene powder is
introduced into a first container 98 (see FIG. 3) so as to substantially
fill the first container 98. The first container 98 is preferably both
flexible and collapsible, and is made from a material that has sufficient
strength to contain the powder over the operating pressure ranges during
the cold isostatic pressure treatment without exhibiting any physical
deterioration, chemical degradation or chemical interaction with the
powder disposed therein. It is also preferred that the first container 98
be made of a material that will not adhere to the powder at any time
during the cold isostatic pressure treatment.
In a preferred embodiment, the first container 98 is a cylindrical
polyurethane container of dimensions approximately inches in diameter, 18
inches in length, and has a wall thickness of approximately one-half inch
to three-fourths inch. The first container 98 is sealed by means of a plug
100 that is inserted into a matching port 102 at one end of the first
container 98. The plug 100 is secured to the first container 98 by means
of an adhesive, such as a hot melt glue, located on the top of the
interface of the plug 100 and the matching port 102. Because it is
desirable that the first container 98 be substantially evacuated prior to
the cold isostatic pressure treatment, the plug 100 of the first container
98 preferably includes an evacuation/de-airing tube 104. The
evacuation/deairing tube 104 is operable to be connected to an evacuation
pump (not shown) and subsequently sealed by any suitable means prior to
the cold isostatic pressure treatment.
It will be noted that the size and shape of the first container 98 will
vary depending upon the desired size and shape of the consolidated stock
being formed. It will also be noted that other suitable materials may be
used to form the first container 98 and that other suitable means may be
used for substantially sealing and evacuating the first container 98. For
example, a flexible and collapsible rubber material may be employed for
constructing the first container 98. When constructed of polyurethane, the
first container 98 may be reused provided it is not subjected to extended
periods of high temperature.
Once the first container 98 has been filled with powder, the first
container 98 is sealed in the manner described above. The first container
98 is then substantially evacuated and then the evacuation/de-airing tube
104 is sealed by any suitable means such as by a clamp 106. As is
illustrated by the step 82, the first container 98 is then located within
the pressure chamber 34 of the cold isostatic press 32. The pressure
chamber 34 is substantially sealed at step 84 to enclose the first
container 98 by threading the upper cover 36 onto the matching threads 38
disposed upon the annular wall 40.
The first container 98 is then subjected to a cold isostatic pressure
treatment as indicated by the step 86 during which a uniform pressure is
applied to the first container 98. In this regard, the pressure applied to
the first container 98 is developed by introducing a pressurized fluid
into the pressure chamber 34. This pressurized fluid may be water, mineral
oil or other oils having similar compressive properties, as well as inert
gases such as argon, nitrogen, helium and neon. In addition, the pressure
chamber 34 may be partially filled with water while the pressurized gas
may be used to fill the remainder of the pressure chamber 34. The pressure
within the pressure chamber 34 is preferably increased as quickly as
possible from approximately atmospheric pressure to a pressure sufficient
to form the powder into an incompletely consolidated stock that can be
manipulable for further processing without substantial degradation.
Suitable maximum pressures range from 1100 psi to 10,000 psi which are
generally sufficient to compact the powder to 60-80% of its final density.
Below this range the incompletely consolidated stock is structurally
unstable and above this range gases may become trapped within the
incompletely consolidated stock during evacuation of the first container
98. In a preferred embodiment, the maximum pressure applied to the first
container 98 is approximately 1500 psi, and the typical length of time for
increasing the pressure to this level may be approximately 2 to 5 minutes.
However, maximum pressure applied to the first container 98 is dependent
upon several factors including the size of the first container 98, the
amount of powder within the first container 98, the size of the resulting
stock needed to manufacture the tibial bearing 28 and the size of the
pressure chamber 34. The pressure is preferably held at the maximum
pressure for approximately one minute, though longer times can be used.
After the maximum pressure within the cold isostatic press 32 is maintained
for approximately one minute, the pressure is slowly reduced so as to
allow the resulting incompletely consolidated stock to relax within the
first container 98 without yielding to outward internal pressure which can
cause the incompletely consolidated stock to lose integrity. The pressure
is preferably released over a period of from approximately 10 to
approximately 30 minutes, although longer times can be used.
The cold isostatic press treatment enhances a uniform density within the
incompletely consolidated stock and reduces internal stresses from
appearing within the material being formed during the subsequent hot
isostatic pressure treatment. In addition, the shape of the incompletely
consolidated stock is in large part dependent upon the shape of the first
container 98. The incompletely consolidated stock resulting from the cold
isostatic press treatment is typically compacted to a preferred density of
about 70% of its desired final density following the hot isostatic
pressure treatment.
After the incompletely consolidated stock has been removed from the first
container 98, the incompletely consolidated stock is placed in a second
container 108 (see FIG. 5) as indicated by the step 88. The second
container 108 is preferably a collapsible container made from a material
that has sufficient strength to contain the incompletely consolidated
stock over the temperature and pressure ranges encountered in the hot
isostatic press treatment without exhibiting any physical deterioration,
chemical degradation or chemical interaction with the incompletely
consolidated stock. It is also preferred that the second container 108 be
made of a material that will not adhere to the incompletely consolidated
stock at any time during the hot isostatic pressure treatment. In a
preferred embodiment, the second container 108 is a foilized heat sealable
bag that has an external surface formed from a layer of an aluminum foil
with a polyester vapor barrier, and has an internal surface formed from a
heat-sealable, low density polyethylene layer on its internal surface. The
second container 108 may typically be approximately 18 inches in length,
approximately 12 inches in width and have a wall thickness of between
approximately 2-3 mils. As will be appreciated by those skilled in the
art, the second container 108 may be made from other suitable materials as
well.
Because it is desirable to have the second container 108 be substantially
evacuated prior to the hot isostatic pressure treatment, the second
container 108 preferably includes an evacuation tube 110 that is operable
to be connected to a vacuum pump (not shown). In this regard, the
evacuation tube 110 is placed in the second container 108 and then a heat
sealer is used to seal that region of the second container 108 which is
not immediately adjacent to the evacuation tube 110. Hot melt glue is then
placed around the region of the second container 108 which is adjacent to
the evacuation tube 110.
It will be noted that the size and shape of the second container 108 will
vary depending upon the desired size and shape of the consolidated stock
being formed. It will also be noted that other suitable materials may be
used for the second container 108 and that other suitable means may be
used for sealing and evacuating the second container 108.
After the incompletely consolidated stock is placed in the second container
108, the second container 108 is evacuated in similar fashion to the
evacuation of the first container 98 as indicated by the step 90. A heat
sealer is then used to substantially enclose the second container 108 at a
region below the evacuation tube 110. The evacuation tube 110 may then be
removed from the second container 108.
Once the incompletely consolidated stock is placed within the second
container 108 and the second container 108 is sealed and evacuated. The
second container 108 is then placed into the pressure chamber 48 of the
hot isostatic press 46 as indicated by the step 92 of the present
invention. The lower closure 52 and the upper closure 54 of the hot
isostatic press 46 are then closed to substantially enclose the second
container 108 within the hot isostatic press 46.
At step 94, the incompletely consolidated stock undergoes the hot isostatic
pressure treatment. In this regard, the pressure within the pressure
chamber 48 is initially raised to approximately 24 psi while the
temperature of the hot isostatic press 46 is raised between 365.degree. F.
and 420.degree. F. Below this range the incompletely consolidate stock
does not melt and above this range the polyethylene may degrade.
Preferably, the temperature of the hot isostatic press is raised to
between 365.degree.-385.degree. F. to minimize the possibility that
degradation will occur. Most preferably, the temperature is raised to
365.degree. F. Once 365.degree. C. is reached, the temperature of the hot
isostatic press 46 is raised as quickly as possible and may typically heat
between one to three hours.
When the temperature of the hot isostatic press 46 reaches approximately
365.degree. C. the pressure within the pressure vessel 46 is also
increased over a 1-2 hours period to a pressure preferably between about
7,500 to about 10,000 psi. It will be appreciated that the maximum
pressure may range from about 3,000 psi to about 40,000 psi. However,
pressures below 3,000 psi or above 40,000 psi tend to cause consolidation
errors to occur or may cause the resulting completely consolidated stock
to have an undesirable crystalline structure. The preferred maximum
pressure between 7,500 psi and 10,000 psi is dependent upon several
factors including the size, shape and construction of the second
container, the dimensions of the pressure chamber 48 and the desired final
diameter of the resulting completely consolidated stock. In addition, the
duration of the hot isostatic pressure treatment may also depend on the
size of the resulting completely consolidated stock. For example, smaller
diameters of the completely consolidated stock (e.g., 11/2 inches)
typically require less time to become fully compacted, while larger
diameters of completely consolidated stock, such as 4 inches, typically
require more time to become fully cured. In addition, the use of the
lowest satisfactory pressure is desirable as it would tend to prolong
equipment life. An inert gas such as argon is preferably used in the hot
isostatic press 46 as the pressure medium. Alternative selections for the
pressure medium include nitrogen, helium and neon gases, although these
gases can be chemically reactive under certain conditions.
Once the temperature and pressure have reached the desired levels, the
temperature and pressure of the pressure chamber 48 remains relatively
constant for a given dwell time. During this dwell time, the powder is
further compressed so as to minimize any compression release that may
occur following termination of the application of heat and pressure.
Preferred dwell times are dependent upon the desired final diameter of the
consolidated stock being produced, and range from approximately 45 minutes
to several hours or more. For example, typical desired dwell times may be
approximately 45 minutes to approximately 1 hour for a 1 inch diameter
consolidated stock, approximately 2 hours for a 21/2 inch diameter
consolidated stock, and approximately 5 hours for a 4 inch diameter
consolidated stock.
After the second container 108 has been subjected to the desired
temperature and pressure for the given dwell time, the hot isostatic press
46 is allowed to cool to room temperature. After the temperature of the
hot isostatic press 46 cools to approximately 100.degree. F., the pressure
within the pressure chamber 48 is gradually decreased to approximately
atmospheric pressure over a period of time that is dependent upon the
desired final diameter of the consolidated stock being produced. In this
regard, the pressure for larger diameters of consolidated stock may be
reduced more slowly because they may typically have a larger internal
compression and larger potential energy that are more likely to release
upon removal of pressure. For example, a 1 hour pressure release time is
preferred for a 4 inch diameter consolidated stock, while a 20 minute
pressure release time may be sufficient for a 11/2 inch diameter
consolidated stock. After the release time has elapsed, the pressure
chamber 48 is then opened and the second container 108 is removed from the
pressure chamber 48.
The consolidated stock is removed from the second container 108 and is
machined at step 96 under methods well-known to those skilled in the art
to produce the desired product, such as the tibial bearing 28, acetabular
cup replacement or other biocompatible component. After machining the
consolidated stock at step 92 to form the tibial bearing 28, the tibial
bearing 28 is then sterilized in a manner well-known to those skilled in
the art.
In addition to using of the hot isostatic press 46 in forming a completely
consolidated stock, the hot isostatic press 46 may be also used to enhance
the adhesion between materials which form a composite biocompatible
component. For example, as shown in FIG. 7, a biocompatible component 112
representing a femoral hip stem of a hip joint prosthesis is shown. The
biocompatible component 112 is preferably formed of a biocompatible
thermoplastic having a biocompatible fibrous material disposed therein.
The biocompatible thermoplastic may be polysulfone, poly ether ether
ketone (PEEK), or poly aryl ether ketone (PAEK), though other suitable
materials may be used. The amount and orientation of the biocompatible
fibrous material within the biocompatible component 112 is selected to
achieve the desired structural modulus for the biocompatible component.
The biocompatible fibrous material may be either continuous or chopped
fibers, though other suitable materials may be used.
When used in this manner, a sheet of the biocompatible thermoplastic such
as polysulfone is first formed into two portions, each portion having a
shape generally corresponding to one-half of the biocompatible component
112. Each portion of the biocompatible thermoplastic is then placed within
the container 114 with the biocompatible fibrous material disposed between
the portions. The container 114 is preferably made from copper or
stainless steel. However, other suitable materials such as high
temperature silicon, which does adhere to the polysulfone, may also be
used. The container 114 is then filled with zirconium oxide beads (i.e.,
Zr.sub.2 O.sub.3) and is then evacuated using the evacuation tube 115
which is then sealed. It will be appreciated that zirconium oxide beads do
not have to be used when the container 114 is made from a very pliable
material such as high temperature silicon. The container 114 is then
placed in the hot isostatic press 46 and is subjected to the hot isostatic
pressure treatment in a manner similar to that described above. In this
regard, the maximum temperature of the isostatic pressure treatment is
preferably slightly above the melting temperature of the biocompatible
thermoplastic. In addition, the pressure applied and the duration of the
hot isostatic pressure treatment should be sufficient to cause the
biocompatible thermoplastic to encapsulate the biocompatible fibrous
material. Preferably, the temperature will fall within the range of
400.degree.-440.degree. F. while the pressure will be greater than between
5000 psi and 7500 psi, and most preferably greater than 7500 psi. It will
be understood by those skilled in the art, however, that the temperature,
pressure and duration of the hot isostatic pressure treatment will depend
upon the specific materials being used.
The isostatic press 46 may also be used to reduce the voids in a stock of
biocompatible composite material prior to being machined into a
biocompatible component. For example, as shown in FIG. 8, a biocompatible
material stock 116 is shown as being disposed within a container 118. The
biocompatible material stock 116 may be made from polysulfone, poly ether
ether ketone (PEEK), or poly arly ether ketone (PAEK), though other
suitable materials may be used. The container 118 may be a stainless steel
or a copper container. However, the container may also be a stainless
steel heat treat bag of the type which is available from Sentry Company,
Foxboro, Mass., other suitable containers may be used.
When used in this manner, the biocompatible material stock 116 is first
placed within the container 118 and then the container 118 is filled with
zirconium oxide beads. It will be appreciated, however, that zirconium
oxide beads do not have to be used if the container 118 is made from a
pliable material such as a heat treat bag. The container 118 is then
sealed. A vacuum is then drawn on the container 118 through the evacuation
tube 119 and then the evacuation tube 119 is then sealed by closing a
valve connected to the evacuation tube 119. The container 118, with the
biocompatible stock material 116 located therein, is placed in the
isostatic press 46 and the temperature and pressure of the isostatic press
46 are raised to such an extent that the voids formed within the
biocompatible stock material 116 are reduced. This reduction in voids
occurs because of the external pressure applied to the exterior of the
container 118. By using the isostatic press 46 in this manner, the
resulting biocompatible material has improved consolidation. The
temperature to which the hot isostatic press 46 is raised will be
substantially that of the glass transition temperature of the resin of the
biocompatible stock material 116, while the pressure applied by the hot
isostatic press 46 is as high as reasonably possible. Preferably, the
temperature will fall within the range of 400.degree.-440.degree. F. while
the pressure will be greater than between 5000 psi and 7500 psi, and most
preferably greater than 7500 psi. However, other suitable temperatures and
pressures may be used.
The isostatic press 46 may also be used to enhance the adhesion of a porous
coating on a biocompatible component formed from a composite material. As
shown in FIG. 9, the biocompatible component 120 includes a porous coated
surface 122 which is used to facilitate adhesion of the biocompatible
component immediately after surgery. The porous coated surface 122 may be
applied by a plasma spray operation and may comprise an alloy of
Ti-6Al-4V, commercially pure titanium, a cobalt chrome alloy or other
biocompatible materials.
When the isostatic press 46 is used to enhance adhesion of a porous coating
122 onto the biocompatible component 120, the porous coated surface 122 is
first applied to the biocompatible component 120 by a plasma spray
operation. The biocompatible component 120 is then placed in a container
124 and then the container 124 is filled with zirconium oxide beads. The
container 124 is then sealed and then is evacuated through the evacuation
tube 125. The container 124 is preferably made of stainless steel or
copper. However, other suitable materials such as high temperature
silicon, which does not adhere to the component 120 may also be used. It
will be appreciated, however, that zirconium oxide beads do not have to be
used if the container 124 is made from a pliable material such as high
temperature silicon. The container 124, with the biocompatible component
120 inside, is then placed in a hot isostatic press 46 which is then
operated in a manner similar to that described above. Preferably, the
temperature will fall within the range of 400.degree.-440.degree. F. while
the pressure will be greater than between 5000 psi and 7500 psi, and most
preferably greater than 7500 psi. As a result, the adhesion of the porous
coated surface 122 to the biocompatible component 120 is improved.
The hot isostatic press 46 may also be used to enhance adhesion of porous
coated pads on a biocompatible component. As shown in FIG. 10, the
biocompatible component 126 includes a plurality of porous coated pads 128
which are used to facilitate fixation of the biocompatible component 126
immediately after surgery. The porous coated pads 128 may be made from a
titanium alloy such as Ti-6A1-4V, commercially pure titanium, a cobalt
chrome alloy or other biocompatible metal alloys. While the porous coating
on the porous coated pads 128 may be applied by a flame spray, plasma
spray or sputtering techniques, it will be appreciated that other suitable
methods may be used.
When the hot isostatic press 46 are used to enhance adhesion of the porous
coated pads 128 onto the biocompatible component 126, the regions on the
biocompatible component 126 where the porous coated pads 128 are to be
placed are first coated with methylene chloride to partially dissolve
those regions at the biocompatible component 126. The porous coated pads
128 are then applied to the biocompatible component 126 and are
temporarily secured thereto. The biocompatible component 126, together
with the porous coated pads 128, are then placed in the container 130 and
then the container 130 is filled with zirconium oxide beads. The container
may be made from stainless steel or copper, though other suitable
materials may be used. In this regard, pliable materials such as high
temperature silicon which is able to withstand the operating temperatures
and pressures may be used which do not necessarily require the use of
zirconium oxide beads. After the container 130 is evacuated through the
evacuation tube 132 and the evacuation tube 132 is sealed, the container
130 is placed in the hot isostatic press 46 which is then operated in a
manner similar to that described above. Preferably, the temperature will
fall within the range of 400.degree.-440.degree. F. while the pressure
will be greater than between 5000 psi and 7500 psi, and most preferably
greater than 7500 psi. As a result, the porous coated pads 128 are
relatively securely attached to the biocompatible component 126.
The principles of the present invention described broadly above will now be
described with reference to the following specific example, without
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