U.S. patent application number 10/935353 was filed with the patent office on 2005-03-24 for medical implant or medical implant part comprising porous uhmwpe and process for producing the same.
This patent application is currently assigned to DePuy Products, Inc.. Invention is credited to Hanes, Mark D., King, Richard S..
Application Number | 20050065307 10/935353 |
Document ID | / |
Family ID | 37776541 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050065307 |
Kind Code |
A1 |
King, Richard S. ; et
al. |
March 24, 2005 |
Medical implant or medical implant part comprising porous UHMWPE
and process for producing the same
Abstract
The invention provides a medical implant or medical implant part
comprising porous ultrahigh molecular weight polyethylene having a
weight average molecular weight of about 400,000 atomic mass units
or more and a porosity of about 15% to about 65%. The invention
further provides a process for producing a medical implant or
medical implant part.
Inventors: |
King, Richard S.; (Warsaw,
IN) ; Hanes, Mark D.; (Winona Lake, IN) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
TWO PRUDENTIAL PLAZA, SUITE 4900
180 NORTH STETSON AVENUE
CHICAGO
IL
60601-6780
US
|
Assignee: |
DePuy Products, Inc.
Warsaw
IN
|
Family ID: |
37776541 |
Appl. No.: |
10/935353 |
Filed: |
September 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60504355 |
Sep 19, 2003 |
|
|
|
Current U.S.
Class: |
526/352 ; 264/49;
623/23.5; 623/23.58 |
Current CPC
Class: |
A61L 27/56 20130101;
C08L 23/06 20130101; A61L 27/16 20130101; A61L 27/16 20130101; C08L
23/06 20130101 |
Class at
Publication: |
526/352 ;
264/049; 623/023.58; 623/023.5 |
International
Class: |
C08F 110/02; A61F
002/28 |
Claims
What is claimed is:
1. A medical implant or medical implant part comprising porous
ultrahigh molecular weight polyethylene, wherein (i) the ultrahigh
molecular weight polyethylene has a weight average molecular weight
of about 400,000 atomic mass units or more, (ii) the ultrahigh
molecular weight polyethylene has a porosity of about 20% or more,
(iii) the average diameter of the pores in the ultrahigh molecular
weight polyethylene is about 400 .mu.m or less, and (iv) at least
some of the ultrahigh molecular weight polyethylene comprises
hydrophilic functional groups attached thereto.
2. The medical implant or medical implant part of claim 1, wherein
the ultrahigh molecular weight polyethylene has a weight average
molecular weight of about 1,000,000 atomic mass units or more.
3. The medical implant or medical implant part of claim 1, wherein
the hydrophilic functional groups comprise one or more functional
groups selected from the group consisting of amino functional
groups, carboxylic acid functional groups, hydroxyl functional
groups, hydroxysulfuryl functional groups, and combinations
thereof.
4. The medical implant or medical implant part of claim 3, wherein
the hydrophilic functional groups comprise one or more carboxylic
acid functional groups.
5. The medical implant or medical implant part of claim 1, wherein
the hydrophilic functional groups are attached to the surface of
the ultrahigh molecular weight polyethylene through hydrophilic
monomers.
6. A medical implant or medical implant part comprising porous
ultrahigh molecular weight polyethylene, wherein (i) the ultrahigh
molecular weight polyethylene has a weight average molecular weight
of about 400,000 atomic mass units or more, (ii) the ultrahigh
molecular weight polyethylene has a porosity of about 15% to about
65%, and (iii) at least about 5% (by volume) of the pores in the
ultrahigh molecular weight polyethylene have a diameter of about
200 .mu.m or more.
7. The medical implant or medical implant part of claim 6, wherein
the ultrahigh molecular weight polyethylene has a weight average
molecular weight of about 1,000,000 atomic mass units or more.
8. The medical implant or medical implant part of claim 6, wherein
the ultrahigh molecular weight polyethylene has a porosity of about
20% to about 60%.
9. The medical implant or medical implant part of claim 6, wherein
about 5% or more (by number) of the pores in the ultrahigh
molecular weight polyethylene have a diameter of about 200 .mu.m or
more.
10. The medical implant or medical implant part of claim 6, wherein
the average diameter of the pores in the ultrahigh molecular weight
polyethylene is about 400 .mu.m or less.
11. The medical implant or medical implant part of claim 6, wherein
at least some of the ultrahigh molecular weight polyethylene
comprises hydrophilic functional groups attached thereto.
12. The medical implant or medical implant part of claim 11,
wherein the hydrophilic functional groups comprise one or more
functional groups selected from the group consisting of amino
functional groups, carboxylic acid functional groups, hydroxyl
functional groups, hydroxysulfuryl functional groups, and
combinations thereof.
13. The medical implant or medical implant part of claim 12,
wherein the hydrophilic functional groups comprise one or more
carboxylic acid functional groups.
14. The medical implant or medical implant part of claim 11,
wherein the hydrophilic functional groups are attached to the
ultrahigh molecular weight polyethylene through hydrophilic
monomers.
15. The medical implant or medical implant part of claim 14,
wherein the hydrophilic monomer is selected from the group
consisting of acrylic acid, poly(ethylene glycol), 2-hydroxyethyl
methacrylate, and combinations thereof.
16. A process for producing a medical implant or medical implant
part comprising porous ultrahigh molecular weight polyethylene, the
process comprising: (a) providing a compression mold for the
medical implant or medical implant part having an internal volume,
(b) providing a matrix comprising ultrahigh molecular weight
polyethylene, wherein the ultrahigh molecular weight polyethylene
has a weight average molecular weight of about 400,000 atomic mass
units or more, (c) dispersing a porogen comprising a
melt-processable polymer in the matrix to produce a mixture
comprising at least one porogen and ultrahigh molecular weight
polyethylene, (d) filling at least a portion of the internal volume
of the compression mold with the mixture obtained in step (c), (e)
compressing the mixture contained within the compression mold for a
time and under conditions sufficient to form a medical implant or
medical implant part therefrom, (f) removing the medical implant or
medical implant part from the compression mold, and (g) immersing
the medical implant or medical implant part obtained in step (f) in
a solvent for a time and under conditions sufficient to extract at
least a portion of the porogen from the medical implant or medical
implant part.
17. The process of claim 16, wherein the ultrahigh molecular weight
polyethylene has a weight average molecular weight of about
1,000,000 atomic mass units or more.
18. The process of claim 16, wherein the melt-processable polymer
has a melt index of about 5 g/10 min or less.
19. The process of claim 16, wherein the melt-processable polymer
is selected from the group consisting of polyethylene glycol,
poly(ethylene oxide), polyvinylpyrrolidone, poly(vinyl alcohol),
and mixtures thereof.
20. The process of claim 16, wherein the porogen further comprises
a water-soluble salt.
21. The process of claim 16, wherein the process further comprises:
(h) modifying at least a portion of the ultrahigh molecular weight
polyethylene contained within the medical implant or medical
implant part to make the ultrahigh molecular weight polyethylene
hydrophilic after the completion of step (g).
22. The process of claim 21, wherein the ultrahigh molecular weight
polyethylene is modified by introducing hydrophilic functional
groups onto at least a portion of the surface of the ultrahigh
molecular weight polyethylene.
23. The process of claim 22, wherein the hydrophilic functional
groups comprise one or more functional groups selected from the
group consisting of amino functional groups, carboxylic acid
functional groups, hydroxyl functional groups, hydroxysulfuryl
functional groups, and combinations thereof.
24. The process of claim 23, wherein the hydrophilic functional
groups comprise one or more carboxylic acid functional groups.
25. The process of claim 22, wherein the hydrophilic functional
groups are introduced onto the surface of the ultrahigh molecular
weight polyethylene by grafting hydrophilic monomers onto at least
a portion of the surface of the ultrahigh molecular weight
polyethylene.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This patent application claims the benefit of U.S.
Provisional Patent Application No. 60/504,355, filed Sep. 19,
2003.
FIELD OF THE INVENTION
[0002] This invention pertains to medical implants and medical
implant parts comprising porous ultrahigh molecular weight
polyethylene and processes for producing the same.
BACKGROUND OF THE INVENTION
[0003] The success of orthopaedic implants surgically implanted in
living bone substantially depends on achieving and maintaining an
enduring bond between the confronting surfaces of the implant and
the host bone. Surgical procedures for preparing living bone to
receive a surgically implanted orthopaedic device have been known
for twenty years or more, but the ideal properties of the surface
of the orthopaedic implant which confronts the host bone and
processes of preparing the implant surface are the subjects of
considerable disagreement.
[0004] Ultrahigh molecular weight polyethylene is widely used in
the orthopaedics industry for the production of orthopaedic
implants due to its relatively high wear-resistance and
biocompatibility. For example, ultrahigh molecular weight
polyethylene frequently is used to produce the acetabular cup of
artificial hip joints. However, virgin ultrahigh molecular weight
polyethylene is bio-inert, and living cells (e.g., bone cells,
osteoblast-like cells, or soft tissue cells) show relatively
little, if any, affinity towards such virgin ultrahigh molecular
weight polyethylene. Accordingly, orthopaedic implants comprising
ultrahigh molecular weight polyethylene components that must be
anchored to the host bone are at risk for potential failure of the
orthopaedic implant unless additional means are undertaken to
ensure the establishment and maintenance of a bond between the
ultrahigh molecular weight polyethylene component and the host
bone.
[0005] Many techniques have been or currently are used to establish
such a bond between the ultrahigh molecular weight polyethylene
component and the host bone. For instance, early ultrahigh
molecular weight polyethylene components, such as the acetabular
cup of an artificial hip, were bonded to the host bone (i.e., the
acetabulum of the pelvis) using a bone cement. Currently, some
commercially available ultrahigh molecular weight polyethylene
components have been provided with complex surface geometries,
which surface geometries comprise ridges and/or projections,
intended to provide sites for the anchoring of the ultrahigh
molecular weight polyethylene component to the host bone.
Alternatively, ultrahigh molecular weight polyethylene components
can be anchored to the host bone using a mechanical fastener or
attached to a metallic "shell" or "tray," which shell or tray
usually comprises a roughened and/or porous coating that confronts
the host bone. While each of these means for effectively "bonding"
the ultrahigh molecular weight polyethylene component to the host
bone have enjoyed varying degrees of success, orthopaedic implants
relying on such means are at risk for failure if the particular
means (e.g., the ridge, protrusion, or mechanical fastener)
fails.
[0006] A need therefore exists for a medical implant or medical
implant part comprising ultrahigh molecular weight polyethylene
that provides a substrate suitable for bone in-growth and/or soft
tissue in-growth. A need also exists for a medical implant or
medical implant part comprising ultrahigh molecular weight
polyethylene having a structure that permits bone cement to
penetrate into the medical implant or medical implant part, which
can increase the strength of the bond between the medical implant
or medical implant part and the host bone. The invention provides
such an implant and a process for making the same. These and other
advantages of the invention, as well as additional inventive
features, will be apparent from the description of the invention
provided herein.
BRIEF SUMMARY OF THE INVENTION
[0007] The invention provides a medical implant or medical implant
part comprising porous ultrahigh molecular weight polyethylene,
wherein (i) the ultrahigh molecular weight polyethylene has a
weight average molecular weight of about 400,000 atomic mass units
or more, (ii) the ultrahigh molecular weight polyethylene has a
porosity of about 20% or more, (iii) the average diameter of the
pores in the ultrahigh molecular weight polyethylene is about 400
.mu.m or less, and (iv) at least some of the ultrahigh molecular
weight polyethylene comprises hydrophilic functional groups
attached thereto.
[0008] The invention also provides a medical implant or medical
implant part comprising porous ultrahigh molecular weight
polyethylene, wherein (i) the ultrahigh molecular weight
polyethylene has a weight average molecular weight of about 400,000
atomic mass units or more, (ii) the ultrahigh molecular weight
polyethylene has a porosity of about 15% to about 65%, and (iii) at
least about 5% (by volume) of the pores in the ultrahigh molecular
weight polyethylene have a diameter of about 200 .mu.m or more.
[0009] The invention further provides a process for producing a
medical implant or medical implant part comprising porous ultrahigh
molecular weight polyethylene, the process comprising: (a)
providing a compression mold for the medical implant or medical
implant part having an internal volume, (b) providing a matrix
comprising ultrahigh molecular weight polyethylene, wherein the
ultrahigh molecular weight polyethylene has a weight average
molecular weight of about 400,000 atomic mass units or more, (c)
dispersing a porogen comprising a melt-processable polymer in the
matrix to produce a mixture comprising at least one porogen and
ultrahigh molecular weight polyethylene, (d) filling at least a
portion of the internal volume of the compression mold with the
mixture obtained in step (c), (e) compressing the mixture contained
within the compression mold for a time and under conditions
sufficient to form a medical implant or medical implant part
therefrom, (f) removing the medical implant or medical implant part
from the compression mold, and (g) immersing the medical implant or
medical implant part obtained in step (f) in a solvent for a time
and under conditions sufficient to extract at least a portion of
the porogen from the medical implant or medical implant part.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a Scanning Electron Microscopy (SEM) micrograph
(35 times magnification) of the surface of porous ultrahigh
molecular weight polyethylene (porosity of about 23%) made by a
conventional sintering process.
[0011] FIG. 2 is an SEM micrograph (300 times magnification) of the
surface of porous ultrahigh molecular weight polyethylene (porosity
of about 33%) made by a conventional sintering process. The
micrograph also includes measurements of some of the pores present
in the porous ultrahigh molecular weight polyethylene.
[0012] FIG. 3 is an SEM micrograph (35 times magnification) of the
surface of porous ultrahigh molecular weight polyethylene (porosity
of about 30%) made using the process of the invention. The
micrograph also includes measurements of some of the pores present
in the porous ultrahigh molecular weight polyethylene.
[0013] FIG. 4 is an SEM micrograph (180 times magnification) of the
surface of porous ultrahigh molecular weight polyethylene (porosity
of about 30%) made using the process of the invention. The
micrograph also includes measurements of some of the pores present
in the porous ultrahigh molecular weight polyethylene.
[0014] FIG. 5 is an SEM micrograph (300 times magnification) of the
surface of porous ultrahigh molecular weight polyethylene (porosity
of about 30%) made using the process of the invention. The
micrograph also includes measurements of some of the pores present
in the porous ultrahigh molecular weight polyethylene.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The invention provides a medical implant or medical implant
part comprising porous ultrahigh molecular weight polyethylene. The
medical implant or medical implant part can be any suitable medical
implant or medical implant part. Suitable medical implants or
medical implant parts include, but are not limited to, the
acetabular cup, the insert or liner of the acetabular cup, or
trunnion bearings of artificial hip joints, the tibial plateau,
patellar button (patello-femoral articulation), and trunnion or
other bearing components of artificial knee joints, the talar
surface (tibiotalar articulation) and other bearing components of
artificial ankle joints, the radio-numeral joint, ulno-humeral
joint, and other bearing components of artificial elbow joints, the
glenoro-humeral articulation and other bearing components of
artificial shoulder joints, intervertebral disk replacements and
facet joint replacements for the spine, temporo-mandibular joints
(jaw), and finger joints. Alternatively, the medical implant or
medical implant part can be a drug delivery device that is adapted
to be implanted within a host. In such an embodiment, the medical
implant or medical implant part can comprise one or more active
ingredients, for example, contained in the pores of the ultrahigh
molecular weight polyethylene.
[0016] As noted above, the medical implant or medical implant part
comprises ultrahigh molecular weight polyethylene. Preferably, the
ultrahigh molecular weight polyethylene has a weight average
molecular weight of about 400,000 atomic mass units or more, more
preferably about 1,000,000 (e.g., about 2,000,000 or about
3,000,000) atomic mass units or more. Typically, the weight average
molecular weight of the ultrahigh molecular weight polyethylene is
about 10,000,000 atomic mass units or less, more preferably about
6,000,000 atomic mass units or less. Ultrahigh molecular weight
polyethylene suitable for use in the invention includes, but is not
limited to, commercially available ultrahigh molecular weight
polyethylene, such as GUR 1050 powdered ultrahigh molecular weight
polyethylene (weight average molecular weight of about 4,000,000 to
about 6,000,000 atomic mass units) and GUR 1020 powdered ultrahigh
molecular weight polyethylene (weight average molecular weight of
about 2,000,000 to about 4,000,000 atomic mass units) from Ticona
(Summit, N.J.). Preferably, the ultrahigh molecular
weight-polyethylene does not contain stabilizers, antioxidants, or
other chemical additives which may have potential adverse effects
in medical applications.
[0017] The medical implant or medical implant part comprises
ultrahigh molecular weight polyethylene that is porous. As utilized
herein, the term "porous" refers to a mass of ultrahigh molecular
weight polyethylene comprising open pores on at least a portion of
its exterior surface. The ultrahigh molecular weight polyethylene
preferably comprises pores distributed throughout the mass. The
pores can be distributed throughout the ultrahigh molecular weight
polyethylene in any suitable manner. In certain embodiments, the
pores are concentrated at or near the surface of the ultrahigh
molecular weight polyethylene. Preferably, the pores are
substantially uniformly distributed throughout the ultrahigh
molecular weight polyethylene comprising the medical implant or
medical implant part of the invention. Preferably, at least a
portion of the pores present in the ultrahigh molecular weight
polyethylene are interconnected (i.e., the pores are connected to
one or more of the adjacent pores).
[0018] The ultrahigh molecular weight polyethylene can have any
suitable porosity. As utilized herein, the term "porosity" refers
to the ratio of the total volume of the pores (e.g., void volume)
in the ultrahigh molecular weight polyethylene to the overall
volume of the ultrahigh molecular weight polyethylene. Preferably,
the ultrahigh molecular weight polyethylene has a porosity of about
15% or more, more preferably about 20% or more, even more
preferably about 25% or more (e.g., about 30% or more or about 35%
or more). It will be understood that, as the porosity of the
ultrahigh molecular weight polyethylene increases, the mechanical
strength of the medical implant or medical implant part can
decrease. Accordingly, the porosity of the ultrahigh molecular
weight polyethylene preferably is not so high as to substantially
compromise the mechanical strength of the medical implant or
medical implant part. To that end, the ultrahigh molecular weight
polyethylene typically has a porosity of about 65% or less (e.g.,
about 60% or less), preferably about 55% or less, more preferably
about 50% or less (e.g., about 45% or less or about 40% or less).
In a preferred embodiment, the ultrahigh molecular weight
polyethylene has a porosity of about 20% to about 50%.
[0019] The pores present in the ultrahigh molecular weight
polyethylene can have any suitable diameter. Preferably, at least a
portion of the pores in the ultrahigh molecular weight polyethylene
have a diameter of about 200 .mu.m or more (e.g., about 250 .mu.m
or more, about 300 .mu.m or more, or about 350 .mu.m or more). Any
suitable percentage of the pores can have a diameter falling within
one of the above-recited ranges (e.g., about 200 .mu.m or more).
Indeed, it will be understood that the preferred percentage of
pores in the ultrahigh molecular weight polyethylene having a
diameter falling within one of the above-recited ranges will
depend, at least in part, on the intended use of the medical
implant or medical implant part. Accordingly, the preferred
percentage for a medical implant or medical implant part that will
be fixated using a bone cement may be different from the preferred
percentage for a medical implant or medical implant part that will
be fixated using a press-fit technique, in which the medical
implant or medical implant part is anchored to the host bone by
osseointegration and/or soft tissue attachment. Preferably, the
percentage (by volume) of pores having a diameter falling within
one of the above-recited ranges (e.g., having a diameter of about
200 .mu.m or more) is about 5% or more (e.g., about 10% or more, or
about 15% or more), more preferably about 25% or more (e.g., about
35% or more, about 40% or more, or about 50% or more). Typically,
the percentage (by volume) of pores having a diameter falling
within one of the above-recited ranges (e.g., having a diameter of
about 200 .mu.m or more or about 200 .mu.m to about 800 .mu.m) will
not exceed about 85% (e.g., will not exceed about 80%). The
percentage (by volume) of pores having a particular diameter can be
determined using any suitable technique. Preferably, the percentage
(by volume) of pores falling within any of the ranges recited
herein is determined using mercury intrusion porosimetry, for
example, using a PoreMaster.RTM. 33 automatic pore size analyzer
from Quantachrome Instruments (Boynton Beach, Fla.).
[0020] Generally, the average diameter of the pores in the
ultrahigh molecular weight polyethylene is about 400 .mu.m or less.
It will be understood that the preferred diameter of the pores in
the ultrahigh molecular weight polyethylene will depend, at least
in part, on the intended use of the medical implant or medical
implant part. Accordingly, the preferred average pore diameter for
a medical implant or medical implant part that will be fixated
using a bone cement may be different from the preferred average
pore diameter for a medical implant or medical implant part that
will be fixated using a press-fit technique, in which the medical
implant or medical implant part is anchored to the host bone by
osseointegration and/or soft tissue attachment. Preferably, the
average diameter of the pores in the ultrahigh molecular weight
polyethylene is about 350 .mu.m or less, more preferably about 300
.mu.m or less (e.g., about 250 .mu.m or less). The average diameter
of the pores in the ultrahigh molecular weight polyethylene
typically is about 1 .mu.g/m or more, preferably about 10 .mu.m or
more, more preferably about 25 .mu.m or more, and most preferably
about 50 .mu.m or more (e.g., about 75 .mu.m or more, or about 100
.mu.m or more). The average diameter of the pores can be determined
using any suitable technique. Preferably, the average diameter of
the pores is determined using mercury intrusion porosimetry, for
example, using a PoreMaster.RTM. 33 automatic pore size analyzer
from Quantachrome Instruments (Boynton Beach, Fla.).
[0021] Preferably, at least a portion (e.g., at least a portion of
the surface) of the porous ultrahigh molecular weight polyethylene
is hydrophilic. It will be understood that, when referring to the
ultrahigh molecular weight polyethylene, the term "hydrophilic" is
used to refer to ultrahigh molecular weight polyethylene that is
more hydrophilic than similar, "untreated" ultrahigh molecular
weight polyethylene. The ultrahigh molecular weight polyethylene
can be made hydrophilic by any suitable means. In certain
embodiments, the ultrahigh molecular weight polyethylene comprises
hydrophilic functional groups attached to at least a portion
thereof. The hydrophilic functional groups can be directly attached
to at least a portion of the ultrahigh molecular weight
polyethylene, or the hydrophilic functional groups can be part of
hydrophilic monomers attached to at least a portion of the
ultrahigh molecular weight polyethylene. Preferably, the
hydrophilic functional groups comprise one or more functional
groups selected from the group consisting of amino functional
groups, carboxylic acid functional groups, hydroxyl functional
groups, hydroxysulfuryl functional groups, and combinations
thereof. More preferably, the hydrophilic functional groups
comprise one or more carboxylic acid functional groups. Suitable
hydrophilic monomers that can be used to modify the surface of the
ultrahigh molecular weight polyethylene include, but are not
limited to, acrylic acid, poly(ethylene glycol), 2-hydroxyethyl
methacrylate, and combinations thereof.
[0022] While not wishing to be bound to any particular theory, it
is believed that the porous and/or hydrophilic nature of the
ultrahigh molecular weight polyethylene of the medical implant or
medical implant part provides an ideal substrate for bone
in-growth, soft tissue in-growth, and/or penetration of bone
cement. More specifically, it is believed that the porous nature of
the ultrahigh molecular weight polyethylene provides a featured
surface to which cells (e.g., bone cells, osteoblast-like cells, or
soft tissue cells) can attach themselves. Furthermore, when the
ultrahigh molecular weight polyethylene of the medical implant or
medical implant part comprises hydrophilic functional groups, it is
believed that the hydrophilicity of the ultrahigh molecular weight
polyethylene attracts and further promotes the attachment of cells
as compared to untreated ultrahigh molecular weight polyethylene.
It is also believed that the porous nature of the ultrahigh
molecular weight polyethylene allows bone cement to penetrate, at
least partially, into the interior portions of the medical implant
or medical implant part, thereby improving the ability of the bone
cement to anchor the medical implant or medical implant part to the
host bone. For example, it is believed that the porous nature of
the ultrahigh molecular weight polyethylene can allow the
monomer(s) of the bone cement to diffuse into the interior portions
of the medical implant or medical implant part, where they can be
polymerized to form a polymer network distributed throughout the
porous structure of the ultrahigh molecular weight polyethylene.
Such a network of the bone cement can provide a link or connection
between the interior portions of the medical implant or medical
implant part and the bone cement on the exterior portions of the
medical implant or medical implant part (e.g., the bone cement
confronting the host bone).
[0023] The invention further provides a process for producing a
medical implant or medical implant part comprising porous ultrahigh
molecular weight polyethylene, the process comprising: (a)
providing a compression mold for the medical implant or medical
implant part having an internal volume, (b) providing a matrix
comprising ultrahigh molecular weight polyethylene, wherein the
ultrahigh molecular weight polyethylene has a weight average
molecular weight of about 400,000 atomic mass units or more, (c)
dispersing a porogen comprising a melt-processable polymer in the
matrix to produce a mixture comprising at least one porogen and
ultrahigh molecular weight polyethylene, (d) filling at least a
portion of the internal volume of the compression mold with the
mixture obtained in step (c), (e) compressing the mixture contained
within the compression mold for a time and under conditions
sufficient to form a medical implant or medical implant part
therefrom, (f) removing the medical implant or medical implant part
from the compression mold, and (g) immersing the medical implant or
medical implant part obtained in step (f) in a solvent for a time
and under conditions sufficient to extract at least a portion of
the porogen from the medical implant or medical implant part.
[0024] The characteristics of the medical implant or medical
implant part produced by the method of the invention (e.g., the
molecular weight of the ultrahigh molecular weight polyethylene,
the porosity of the ultrahigh molecular weight polyethylene, the
size and/or shape of the pores, etc.) can be the same as those set
forth above for the medical implant or medical implant part of the
invention.
[0025] As noted above, the method of the invention comprises
providing a compression mold for the medical implant or medical
implant part having an internal volume. The term "compression mold"
is utilized herein to refer to a mold typically having two halves
which, when joined together, define an internal volume (i.e., mold
cavity). The compression mold can be provided in any suitable
configuration. Generally, the compression mold is configured such
that the internal volume of the compression mold (i.e., the mold
cavity) defines the medical implant or medical implant part in a
substantially complete form (i.e., in substantially the same form
as will be used for implantation in the host). However, it will be
understood that the medical implant or medical implant part
produced by the method of the invention also can be subjected to
further processing (e.g., machining) to provide the medical implant
or medical implant part in the final form used for implantation in
the host.
[0026] The matrix of ultrahigh molecular weight polyethylene can be
provided in any suitable form. Preferably, the matrix of ultrahigh
molecular weight polyethylene comprises, consists essentially of,
or consists of ultrahigh molecular weight polyethylene in a
powdered or pelletized form.
[0027] As noted above, the method of the invention utilizes a
porogen. As utilized herein, the term "porogen" refers to a labile,
pore-generating material (i.e., a material capable of readily
undergoing, under the appropriate conditions, chemical and/or
physical changes to form pores in the ultrahigh molecular weight
polyethylene). Preferably, the porogen comprises a melt-processable
polymer. As utilized herein to refer to the melt-processable
polymer present in the porogen, the term "melt-processable" refers
to a polymer that can be processed in its molten state using
processes such as injection molding, extrusion molding, blow
molding, and/or compression molding. Preferably, a melt processable
polymer does not exhibit significant oxidative degradation,
decomposition, or pyrolysis at the processing temperatures
typically used in such molding processes. The porogen can comprise
any suitable melt-processable polymer. Suitable melt-processable
polymers include, but are not limited to, poly(ethylene oxide),
polyethylene glycol, polyethylene glycol copolymers (e.g.,
poly(ethylene glycol)-poly(propylene glycol) copolymers,
poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol)
block copolymers, or poly(propylene glycol)-poly(ethylene
glycol)-poly(propylene glycol) block copolymers), poly(propylene
glycol), poly(2-hydroxyethyl methacrylate), poly(vinyl alcohol),
poly(acrylic acid), poly(methacrylic acid), polyvinylpyrrolidone,
cellulose ether, alginate, chitosan, hyaluronate, collagen, and
mixtures or combinations thereof. Preferably, the melt-processable
polymer is selected from the group consisting of polyethylene
glycol, poly(ethylene oxide), polyvinylpyrrolidone, poly(vinyl
alcohol), and mixtures thereof. In certain embodiments, the
melt-processable polymer preferably is water-soluble. As utilized
herein, the term "water-soluble" refers to a polymer having a
solubility in water (at 25.degree. C.) of about 1 mg/L or more,
preferably about 10 mg/L or more, more preferably about 30 mg/L or
more, even more preferably about 100 mg/L or more, and most
preferably about 1,000 mg/L or more.
[0028] In order to ensure that the melt-processable polymer remains
distributed throughout the ultrahigh molecular weight polyethylene
during the compression molding step of the inventive process, the
melt-processable polymer preferably has a low melt index.
Typically, the melt-processable polymer has a melt index of about 5
g/10 min or less (i.e., equal to or less than about 5 g/10 min),
such as about 4 g/10 min or less, about 3 g/10 min or less, about 2
g/10 min or less, or about 1 g/10 min or less (e.g., about 0.1 g/10
min to about 1 g/10 min). Preferably, the melt-processable polymer
has a melt index of about 0.5 g/10 min or less, more preferably
about 0.45 g/10 min or less, even more preferably about 0.425 g/10
min or less, and most preferably about 0.4 g/10 min or less.
[0029] The melt index of the melt-processable polymer can be
determined using any suitable method. Preferably, the melt index of
the melt-processable polymer is determined in accordance with ASTM
Standard D1238-88 (entitled, "Flow Rates of Thermoplastics by
Extrusion Plastometer") using the following conditions: (i)
190.degree. C., (ii), 21.6 kg weight, (iii) 20 cm.sup.3 sample, and
(iv) 5.5 minute preheat time. More specifically, the equipment used
to determine the melt index of the melt-processable polymer (i.e.,
the plastometer, the cylinder, the die, the piston, the heater, the
thermometer, etc.) is the same as that defined in ASTM Standard
D1238-88; however, the conditions under which the melt index is
measured differ from those specified in the aforementioned
standard. In particular, a 20 cm.sup.3 sample of the
melt-processable polymer, which sample is provided as a homogeneous
powder or pellets, is used to determine the melt index of the
melt-processable polymer. The 20 cm.sup.3 sample is placed in the
barrel of the plastometer while tamping the material to ensure that
the entire sample is placed in the barrel and at least 90% of the
volume of the barrel is filled with the sample. After the sample is
loaded into the barrel of the plastometer, the piston is inserted
into the barrel, and 10.8 kg of weight is placed on the piston. The
timing of the 5.5 minute preheating cycle is then begun. During the
preheat cycle, the weight on the piston is adjusted so that the
sample extrudes from the barrel at a rate such that the lower two
scribed marker lines on the piston reach the top of the barrel by
the end of the 5.5 minute preheat cycle (+/-15 seconds).
Immediately prior to the completion of the preheat cycle, the
weight on the piston is increased to 21.6 kg. Once the lower of the
two scribed marker lines on the piston reaches the top of the
barrel, the extrudate is cut from the barrel, and a timed sampling
cycle is begun. Typically, the sampling cycle is about 1 minute;
however, the length of the sampling cycle can be shorter (e.g., 30
seconds) or longer (e.g., 2 minutes) depending on the rate at which
the sample extrudes from the barrel of the plastometer. At the end
of the sampling cycle, the extrudate is cut from the barrel and
weighed. The melt index (in g/10 min) is then calculated using the
weight of the extrudate and the duration of the sampling cycle.
[0030] In addition to the melt-processable polymer, the porogen can
also comprise other suitable labile, pore-generating materials.
Such materials include, but are not limited to, hydrocarbons (e.g.,
aliphatic, cyclic, and aromatic hydrocarbons), waxes (e.g.,
polyethylene waxes, paraffin waxes, and synthetic waxes), and
soluble salts (e.g., sodium chloride). Typically, in order to
lessen the risk of adverse reactions following implantation of the
medical implant or medical implant part, it is desirable to avoid
the use of organic solvents in preparing the medical implant or
medical implant part of the invention. Accordingly, any additional
materials included in the porogen preferably are water-soluble.
Suitable water-soluble porogens include, but are not limited to,
water-soluble salts, such as sodium chloride or potassium
chloride.
[0031] When the porogen comprises a mixture of a melt-processable
polymer and another labile, pore-generating material, the
melt-processable polymer can comprise any suitable portion of the
total volume of the porogen. The melt-processable polymer typically
comprises about 5 vol. % or more, preferably about 10 vol. % or
more, more preferably about 15 vol. % or more (e.g., about 20 vol.
% or more) of the total volume of the porogen. Typically, the
melt-processable polymer comprises about 50 vol. % or less,
preferably about 45 vol. % or less, more preferably about 40 vol. %
or less (e.g., about 35 vol. % or less, or about 33 vol. % or less)
of the total volume of the porogen.
[0032] The porogen is dispersed in the matrix of ultrahigh
molecular weight polyethylene to produce a mixture comprising at
least one porogen and ultrahigh molecular weight polyethylene. In a
preferred embodiment, the mixture consists essentially of, or
consists of, the porogen and ultrahigh molecular weight
polyethylene. The porogen can be dispersed in the ultrahigh
molecular weight polyethylene using any suitable means. Typically,
the porogen and the ultrahigh molecular weight polyethylene are
provided in a powdered or pelletized form, and the porogen is
dispersed in the ultrahigh molecular weight polyethylene by dry
blending the two components to form a mixture comprising the
porogen and ultrahigh molecular weight polyethylene. The porogen
and ultrahigh molecular weight polyethylene can be dry blended
using any suitable apparatus, such as the Turbula.RTM.
shaker-mixers manufactured by Glen Mills Inc. (Clifton, N.J.).
[0033] As noted above, at least a portion of the internal volume of
the compression mold (i.e., mold cavity) is filled with the mixture
comprising, consisting essentially of, or consisting of the porogen
and ultrahigh molecular weight polyethylene. Preferably, the
portion of the internal volume of the compression mold filled with
the mixture comprises a portion of a surface layer of the medical
implant or medical implant part. In such an embodiment, the surface
layer preferably corresponds to a surface of the medical implant or
medical implant part that, after implantation into the host, is
adjacent to (e.g., abuts) bone and/or soft tissue. In another
embodiment, substantially all of the internal volume of the
compression mold preferably is filled with the mixture comprising
the porogen and ultrahigh molecular weight polyethylene. When only
a portion of the internal volume of the compression mold is filled
with the mixture comprising the porogen and ultrahigh molecular
weight polyethylene, the remaining portion of the internal volume
of the compression mold preferably is filled with ultrahigh
molecular weight polyethylene.
[0034] After at least a portion of the internal volume of the
compression mold is filled, the mixture comprising the porogen and
ultrahigh molecular weight polyethylene contained within the
compression mold is compressed for a time and under conditions
sufficient to form a medical implant or medical implant part
therefrom. It will be understood that the mixture comprising the
porogen and ultrahigh molecular weight polyethylene is compressed
by any suitable means, such as by mating the two halves of a
two-part compression mold and applying an external force in a
direction such that any substance contained within the mold (e.g.,
mixture comprising the porogen and ultrahigh molecular weight
polyethylene) is subjected to a compressive force. It will be
further understood that the particular time and conditions (e.g.,
force applied to the compression mold) necessary to form a medical
implant or medical implant part will depend upon several factors,
such as the composition of the mixture (i.e., the type and/or
amount of porogen(s) added to the mixture), and the size (e.g.,
thickness) of the desired medical implant or medical implant part,
as well as other factors. Typically, the mixture is subjected to a
pressure of about 3,400 kPa to about 28,000 kPa during the
compression molding step. Preferably, the mixture is subjected to a
pressure of about 3,400 kPa to about 14,000 kPa (e.g., about 3450
kPa to about 6,900 kPa) or about 3,800 kPa to about 14,000 kPa.
During the compression molding step, the mixture typically is
subjected to a temperature of about 140.degree. C. to about
220.degree. C. at a heating rate of about 5.degree. C./min to about
12.degree. C./min. Preferably, the mixture is subjected to a
temperature of about 160.degree. C. to about 210.degree. C. (e.g.,
about 160.degree. C. to about 200.degree. C.), more preferably a
temperature of about 170.degree. C. to about 190.degree. C., during
the compression molding step. The mixture can be compressed in the
compression mold for any amount of time sufficient to form a
medical implant or medical implant part therefrom. Typically, the
mixture is compressed for about 5 to about 30 minutes, more
preferably about 5 to about 20 minutes (e.g., about 15 minutes),
during the compression molding step. Once the mixture has been
compressed for the desired amount of time, the resulting medical
implant or medical implant part typically is cooled (e.g., to a
temperature of about 38.degree. C. (100.degree. F.)) at a rate of
about 2.degree. C./min to about 6.degree. C./min.
[0035] The invention further comprises the step of (g) immersing
the medical implant or medical implant part obtained in step (f) in
a solvent for a time and under conditions sufficient to extract at
least a portion of the porogen from the medical implant or medical
implant part. Preferably, the medical implant or medical implant
part is immersed in the solvent for a time and under conditions to
remove substantially all of the porogen (e.g., about 90% or more,
about 95% or more, or about 98% or more). It will be understood
that the particular solvent used to remove (e.g., extract) the
porogen from the medical implant or medical implant part will
depend, at least in part, on the particular porogen used.
Furthermore, it will be understood that the particular solvent used
to remove (e.g., extract) the porogen from the medical implant or
medical implant part should not damage (e.g., significantly swell
and/or dissolve) the ultrahigh molecular weight polyethylene of the
medical implant or medical implant part. In certain embodiments,
such as when the porogen is water-soluble (e.g., the porogen
comprises a water-soluble, melt-processable polymer), the solvent
preferably comprises or is water. In other embodiments, the solvent
can comprise a mixture of two or more different solvents. For
example, the solvent can comprise a mixture of water and a polar,
bio-inert, organic solvent. Suitable polar, bio-inert, organic
solvents include, but are not limited to, ethyl acetate, ethanol,
acetone, methyl ethyl ketone, N-methyl-2-pyrrolidone.
Alternatively, the medical implant or medical implant part can be
sequentially immersed in two or more different solvents (e.g., a
first solvent comprising, consisting essentially of, or consisting
of water and a second solvent comprising, consisting essentially
of, or consisting of ethyl acetate).
[0036] The medical implant or medical implant part can be immersed
in the solvent for any suitable amount of time and under any
conditions suitable for the extraction of at least a portion of the
porogen from the medical implant or medical implant part. For
example, the medical implant or medical implant part typically is
immersed in the solvent for up to about 170 hours, preferably for
about 48 hours to about 170 hours. The solvent can be maintained at
any suitable temperature during the extraction step. It will be
understood that the particular temperature at which the solvent is
maintained during the extraction step will depend, at least in
part, on the particular solvent(s) used, as well as the composition
of the porogen. In certain embodiments, such as when the solvent
comprises water, the solvent typically is maintained at a
temperature of about 20.degree. C. to about 100.degree. C.,
preferably about 20.degree. C. to about 60.degree. C. (e.g., about
20.degree. C. to about 30.degree. C.). During the extraction step,
one or more nozzles can be used to generate jet(s) of the solvent
that are sprayed onto the surface of the medical implant or medical
implant part under pressure (e.g., at a pressure of about 350 kPa
or less). The extraction process can additionally or alternatively
comprise the step of passing ultrasonic waves through solvent
(e.g., by immersing the medical implant or medical implant part in
a solvent contained in a suitable ultrasonic bath).
[0037] The process of the invention optionally, but preferably,
further comprises the step of (h) modifying at least a portion of
the ultrahigh molecular weight polyethylene to make the ultrahigh
molecular weight polyethylene hydrophilic. The ultrahigh molecular
weight polyethylene can be modified at any suitable point in time.
Preferably, the ultrahigh molecular weight polyethylene contained
within the medical implant or medical implant part is modified
after the desired amount of the porogen has been extracted
therefrom. The ultrahigh molecular weight polyethylene can be
modified using any suitable method. Preferably, the ultrahigh
molecular weight polyethylene is modified by introducing
hydrophilic functional groups onto at least the ultrahigh molecular
weight polyethylene exposed on the surface of the medical implant
or medical implant part. The hydrophilic functional groups can be
introduced onto the ultrahigh molecular weight polyethylene using
any suitable method. For example, the hydrophilic functional groups
can be directly introduced onto the ultrahigh molecular weight
polyethylene, or the hydrophilic functional groups can be
introduced onto the ultrahigh molecular weight polyethylene by
grafting hydrophilic monomers (e.g., monomers containing one or
more hydrophilic functional groups) onto the ultrahigh molecular
weight polyethylene. The hydrophilic functional groups preferably
comprise one or more functional groups selected from the group
consisting of amino functional groups, carboxylic acid functional
groups, hydroxyl functional groups, hydroxysulfuryl functional
groups, and combinations thereof. More preferably, the hydrophilic
functional groups comprise one or more carboxylic acid functional
groups. Preferably, the ultrahigh molecular weight polyethylene is
modified using a gas plasma method, such as the methods set forth
in U.S. Pat. Nos. 4,656,083, 4,919,659, 5,080,924, and
6,379,741.
[0038] In certain embodiments, the process of the invention further
comprises the step of irradiating the medical implant or medical
implant part for a time and under conditions sufficient to
cross-link at least a portion of the ultrahigh molecular weight
polyethylene contained in the medical implant or medical implant
part. The medical implant or medical implant part can be irradiated
at any suitable point in time. Preferably, the medical implant or
medical implant part is irradiated after the porogen has been
removed from the medical implant or medical implant part. While not
wishing to be bound to any particular theory, it is believed that
removing the porogen from the medical implant or medical implant
part before irradiating the ultrahigh molecular weight polyethylene
increases the amount of porogen that can be removed from the
medical implant or medical implant part. The medical implant or
medical implant part also is preferably irradiated before the
ultrahigh molecular weight polyethylene contained within the
medical implant or medical implant part is modified to make the
ultrahigh molecular weight polyethylene hydrophilic. While not
wishing to be bound to any particular theory, it is believed that
irradiating the medical implant or medical implant part before
modification of the ultrahigh molecular weight polyethylene reduces
the potential negative effects of chain-scission and/or free
radical generation in the ultrahigh molecular weight polyethylene
caused by the irradiation process.
[0039] The medical implant or medical implant part can be
irradiated by any suitable means, such as by exposing the medical
implant or medical implant part to a suitable amount of gamma,
x-ray, or electron beam radiation. Preferably, medical implant or
medical implant part is irradiated by exposure to about 0.5 to
about 10 Mrad (e.g., about 1.5 to about 6 Mrad) of gamma radiation
using methods known in the art. While the medical implant or
medical implant part can be exposed to amounts of radiation falling
outside of the aforementioned range, such amounts of radiation tend
to produce a medical implant or medical implant part with
unsatisfactory properties. In particular, radiation doses of less
than about 0.5 Mrad generally provide insufficient cross-linking of
the ultrahigh molecular weight polyethylene. Furthermore, while
doses of greater than 10 Mrad may be used, the additional
cross-linking that is achieved generally is offset by the increased
brittleness of portions of the medical implant or medical implant
part (e.g., the surface layer of the medical implant or medical
implant part).
[0040] When irradiated, the medical implant or medical implant part
preferably is irradiated in an inert or reduced-pressure
atmosphere. Irradiating the medical implant or medical implant part
in an inert (i.e., non-oxidizing) or reduced-pressure atmosphere
reduces the effects of oxidation and chain scission reactions which
can occur during irradiation in an oxidative atmosphere. Typically,
the medical implant or medical implant part is placed in an
oxygen-impermeable package during the irradiation step. Suitable
oxygen-impermeable packaging materials include, but are not limited
to, aluminum, polyester-coated metal foil (e.g., the Mylar.RTM.
product available from DuPont Teijin Films), polyethylene
terephthalate, and poly(ethylene vinyl alcohol). In order to
further reduce the amount of oxidation which occurs during the
irradiation of the medical implant or medical implant part, the
oxygen-impermeable packaging may be evacuated (e.g., the pressure
within the packaging may be reduced below the ambient atmospheric
pressure) and/or flushed with an inert gas (e.g., nitrogen, argon,
helium, or mixtures thereof) after the medical implant or medical
implant part has been placed therein.
[0041] When the medical implant or medical implant part is
irradiated, the free radicals generated in the ultrahigh molecular
weight polyethylene preferably are quenched following the
irradiation of the medical implant or medical implant part using
methods known in the art. For example, the free radicals contained
within the irradiated portion of the medical implant or medical
implant part can be quenched by heating the irradiated medical
implant or medical implant part to a temperature between room
temperature and the melting point of ultrahigh molecular weight
polyethylene in an oxygen-reduced, non-reactive atmosphere for a
length of time sufficient to reduce the number of free radicals
present in the medical implant or medical implant part (see, e.g.,
U.S. Pat. Nos. 5,414,049, 6,174,934, and 6,228,900). Alternatively,
the free radicals contained within the irradiated portion of the
medical implant or medical implant part can be quenched by heating
the irradiated medical implant or medical implant part to a
temperature at or above the melting point of ultrahigh molecular
weight polyethylene in an oxygen-reduced, non-reactive atmosphere
for a length of time sufficient to reduce the number of free
radicals present in the medical implant or medical implant part
(see, e.g., U.S. Pat. Nos. 6,017,975, 6,228,900, 6,242,507, and
6,316,158). Lastly, the free radicals contained within the
irradiated portion of the medical implant or medical implant part
can be quenched by immersing the irradiated portion of the medical
implant or medical implant part in a non-polar solvent for a time
and under conditions sufficient to quench a substantial portion of
the free radicals contained therein. The aforementioned process is
explained more fully in copending U.S. patent application Ser. Nos.
10/609,749 and 10/795,755.
[0042] The method of the invention can further comprise the steps
of sterilizing the medical implant or medical implant part by any
suitable means, preferably using a non-irradiative process. The
medical implant or medical implant part can be sterilized at any
suitable point in time, but preferably is sterilized after the
completion of step (g) or step (h). Sterilizing the medical implant
or medical implant part using a non-irradiative method avoids the
formation of additional free radicals in the ultrahigh molecular
weight polyethylene, which free radicals could undergo oxidative
reactions resulting in the chain scission of the ultrahigh
molecular weight polyethylene. Suitable non-irradiative
sterilization techniques include, but are not limited to, gas
plasma or ethylene oxide methods known in the art. For example, the
packaged medical implant or packaged medical implant part can be
sterilized using a PlazLyte.RTM. Sterilization System (Abtox, Inc.,
Mundelein, Ill.) or in accordance with the gas plasma sterilization
processes described in U.S. Pat. Nos. 5,413,760 and 5,603,895.
[0043] The medical implant or medical implant part can be packaged
in any suitable packaging material. Desirably, the packaging
material maintains the sterility of the medical implant or medical
implant part until the packaging material is breached. If the
medical implant or medical implant part has not been irradiated or
if the medical implant or medical implant part has been irradiated
and a substantial portion of the free radicals contained within the
medical implant or medical implant part have been quenched, the
medical implant or medical implant part will be relatively stable
to atmospheric oxidation. Under such circumstances, it would not be
necessary to package the medical implant or medical implant part in
an inert atmosphere and, therefore, the medical implant or medical
implant part could be packaged in an air-impermeable or
air-permeable packaging material.
EXAMPLE
[0044] This example further illustrates the invention but, of
course, should not be construed as in any way limiting its scope.
This example demonstrates a process for producing a medical implant
or medical implant part according to the invention, as well as the
properties of the medical implant or medical implant part of the
invention. Three samples of porous ultrahigh molecular weight
polyethylene (Samples 1-3) were prepared from GUR 1020 powdered
ultrahigh molecular weight polyethylene using three different
processes.
[0045] Sample 1 (comparative) was prepared by a sintering process
in which 14 grams of GUR 1020 powdered ultrahigh molecular weight
polyethylene was placed into a cylindrical mold (approximately 10
cm in diameter) and heated in a high-vacuum oven to a temperature
of approximately 200.degree. C. under a load of approximately 5.6
kg for 180 minutes. The resulting sample was measured and weighed
to determine its porosity, which was determined to be approximately
23%. The surface of the sample was then analyzed using a Hitachi
S3500 N variable pressure scanning electron microscope and Quartz
PCI image acquisition software (Version 4.1). An SEM micrograph
(35.times. magnification) of the surface of Sample 1 is provided in
FIG. 1.
[0046] Sample 2 (comparative) was prepared by a sintering process
in which 14 grams of GUR 1020 powdered ultrahigh molecular weight
polyethylene was placed into a cylindrical mold (approximately 10
cm in diameter) and heated in a high-vacuum oven to a temperature
of approximately 150.degree. C. under a load of approximately 2.4
kg for 180 minutes. The resulting sample was measured and weighed
to determine its porosity, which was determined to be approximately
33%. The surface of the sample was then analyzed using SEM. An SEM
micrograph (300.times. magnification) of the surface of Sample 2 is
provided in FIG. 2. The SEM micrograph was further analyzed using
the Quartz PCI image acquisition software to determine the diameter
of the pores present on the surface of the sample. The pore
diameter measurements for selected pores are provided in FIG.
2.
[0047] Sample 3 (invention) was prepared by dry blending GUR 1020
powdered ultrahigh molecular weight polyethylene and a porogen
comprising POLYOX.TM. WSR-303 poly(ethylene oxide) (available from
The Dow Chemical Company, Midland, Mich.) and sodium chloride. The
polyethylene, poly(ethylene oxide), and sodium chloride were mixed
in a volume ratio of approximately 70:5:25, respectively. The
resulting mixture was placed in a cylindrical compression mold
(approximately 10 cm in diameter) having an internal volume. The
mixture was then packed into the mold by subjecting the compression
mold to a pressure of approximately 340-690 kPa (50-100 psi) for
approximately 1-2 minutes. Following the packing step, the
temperature within the compression mold was increased from room
temperature to approximately 165.degree. C. (330.degree. F.) at a
rate of approximately 6.degree. C./min (11.degree. F./min) while
the pressure on the compression mold was increased from 0 kPa to
about 3450 kPa (500 psi). Once the temperature within the mold
reached 165.degree. C. (330.degree. F.), the temperature was
increased from 165.degree. C. (330.degree. F.) to approximately
205.degree. C. (400.degree. F.) at a rate of approximately
2.8.degree. C./min (5.degree. F./min), and the pressure was
increased from 3450 kPa (500 psi) to approximately 6900 kPa (1000
psi). The temperature and pressure within the compression mold were
then maintained at approximately 205.degree. C. (400.degree. F.)
and 6900 kPa (1000 psi) for approximately 15 minutes. Following the
compression step, the resulting molded disk was cooled from
205.degree. C. (400.degree. F.) to approximately 38.degree. C.
(100.degree. F.) at rate of approximately 5.5.degree. C./min
(10.degree. F./min) while the pressure was reduced from 6900 kPa
(1000 psi) to approximately 2280 kPa (330 psi). The resulting
molded disk was then removed from the compression mold and immersed
in water for approximately 168 hours to extract the porogen (i.e.,
poly(ethylene oxide) and sodium chloride) therefrom. After the
extraction step, the resulting sample was dried under vacuum at a
temperature of approximately 60.degree. C. for approximately 240
minutes. The resulting sample was measured and weighed to determine
its porosity, which was determined to be approximately 30%. The
surface of the sample was then analyzed using SEM. SEM micrographs
(35.times., 180.times., and 300.times. magnification) of the
surface of Sample 3 are provided in FIGS. 3-5. The SEM micrographs
were further analyzed using the Quartz PCI image acquisition
software to determine the diameter of the pores present on the
surface of the sample. The pore diameter measurements for selected
pores are provided in FIGS. 3-5.
[0048] A comparison of the SEM micrographs and pore size
measurements for each of the samples reveals that the porous
ultrahigh molecular weight polyethylene produced by the process of
the invention exhibits a surface morphology and pore size
characteristics that are different from porous ultrahigh molecular
weight polyethylene having a similar porosity that was produced by
a sintering process. In particular, the surfaces of Samples 1 and 2
(comparative) are comprised of a network of roughly round
polyethylene particles fused about their outer surfaces to several
adjacent polyethylene particles. The interstitial spaces in the
network of particles form the pores present in the samples. The
pores at the surface of Sample 2 (33% porosity) measured up to
approximately 100 .mu.m in diameter. By way of contrast, Sample 3
(invention), which had approximately the same porosity as Sample 2
(comparison), comprised a substantially continuous expanse of
ultrahigh molecular weight polyethylene having pores distributed
across its surface. The pores in the surface of Sample 3 measured
from approximately 25 .mu.m up to approximately 425 .mu.m in
diameter. The foregoing results demonstrate that, unlike porous
ultrahigh molecular weight polyethylene made by a sintering
process, the porous ultrahigh molecular weight polyethylene of the
invention comprises a combination of pores having relatively small
diameters (e.g., about 100 .mu.m or less) and pores having
relatively large diameters (e.g., about 200 to about 400
.mu.m).
[0049] All references, including publications, patent applications,
and patents, cited herein are hereby incorporated by reference to
the same extent as if each reference were individually and
specifically indicated to be incorporated by reference and were set
forth in its entirety herein.
[0050] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the invention (especially in
the context of the following claims) are to be construed to cover
both the singular and the plural, unless otherwise indicated herein
or clearly contradicted by context. The terms "comprising,"
"having," "including," and "containing" are to be construed as
open-ended terms (i.e., meaning "including, but not limited to,")
unless otherwise noted. Recitation of ranges of values herein are
merely intended to serve as a shorthand method of referring
individually to each separate value falling within the range,
unless otherwise indicated herein, and each separate value is
incorporated into the specification as if it were individually
recited herein. All methods described herein can be performed in
any suitable order unless otherwise indicated herein or otherwise
clearly contradicted by context. The use of any and all examples,
or exemplary language (e.g., "such as") provided herein, is
intended merely to better illuminate the invention and does not
pose a limitation on the scope of the invention unless otherwise
claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of
the invention.
[0051] Preferred embodiments of this invention are described
herein, including the best mode known to the inventors for carrying
out the invention. Variations of those preferred embodiments may
become apparent to those of ordinary skill in the art upon reading
the foregoing description. The inventors expect skilled artisans to
employ such variations as appropriate, and the inventors intend for
the invention to be practiced otherwise than as specifically
described herein. Accordingly, this invention includes all
modifications and equivalents of the subject matter recited in the
claims appended hereto as permitted by applicable law. Moreover,
any combination of the above-described elements in all possible
variations thereof is encompassed by the invention unless otherwise
indicated herein or otherwise clearly contradicted by context.
* * * * *