U.S. patent application number 11/183456 was filed with the patent office on 2006-01-19 for pulsed current sintering for surfaces of medical implants.
This patent application is currently assigned to Smith & Nephew Inc.. Invention is credited to Michael B. Cooper, Daniel A. Heuer, Gordon Hunter, Vivek Pawar, Abraham Salehi.
Application Number | 20060015187 11/183456 |
Document ID | / |
Family ID | 35789150 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060015187 |
Kind Code |
A1 |
Hunter; Gordon ; et
al. |
January 19, 2006 |
Pulsed current sintering for surfaces of medical implants
Abstract
A porous medical implant and a method of making same is
described. The medical implant comprises a porous surface formed by
application of pulsed electrical energy ins such a way as to cause
a localized heating in the surface of the material comprising
portions of the implant. The method comprises a pulsed current
sintering technique.
Inventors: |
Hunter; Gordon; (Memphis,
TN) ; Pawar; Vivek; (Memphis, TN) ; Heuer;
Daniel A.; (Memphis, TN) ; Salehi; Abraham;
(Bartlett, TN) ; Cooper; Michael B.; (Nesbit,
MS) |
Correspondence
Address: |
SMITH & NEPHEW, INC.
1450 E. BROOKS ROAD
MEMPHIS
TN
38116
US
|
Assignee: |
Smith & Nephew Inc.
Memphis
TN
|
Family ID: |
35789150 |
Appl. No.: |
11/183456 |
Filed: |
July 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60589143 |
Jul 19, 2004 |
|
|
|
Current U.S.
Class: |
623/23.5 ;
264/112; 264/113; 264/460; 623/23.51; 623/23.73 |
Current CPC
Class: |
B22F 3/105 20130101;
A61F 2002/30677 20130101; B22F 7/08 20130101; A61F 2002/30968
20130101; B22F 3/26 20130101; A61F 2002/2817 20130101; B22F 2998/00
20130101; B22F 2998/00 20130101; A61L 27/56 20130101; A61F
2002/30957 20130101; A61L 27/30 20130101; A61F 2310/00293
20130101 |
Class at
Publication: |
623/023.5 ;
623/023.73; 623/023.51; 264/460; 264/112; 264/113 |
International
Class: |
A61F 2/28 20060101
A61F002/28 |
Claims
1. A method of making a medical implant having a porous surface and
a solid substrate, comprising the steps of: placing a finite number
of individual bodies in continuous contact with one another, said
finite number of individual bodies comprising a first material;
sintering said first material by applying pulsed electrical energy
across at least a portion of the aggregate mass of said individual
bodies, thereby creating a cohesive porous structure; and,
attaching said first material to a second material, said second
material comprising said solid substrate.
2. The method of claim 1, wherein said step of attaching said first
material to a second material comprises sintering said first
material to said second material by applying pulsed electrical
energy across at least a portion of the aggregate mass of the first
material and the second material while the first material and the
second material are in physical contact with one another.
3. The method of claim 1, wherein said steps of sintering and
attaching are performed simultaneously by applying pulsed
electrical energy across at least a portion of the aggregate mass
of the first material and the second material while the first
material and the second material are in physical contact with one
another.
4. The method of claim 1, wherein said steps of sintering and
attaching are performed sequentially by first applying pulsed
electrical energy across at least a portion of the aggregate mass
of the first material and thereafter applying pulsed electrical
energy across at least a portion of the aggregate mass of the first
material and the second material while the first material and the
second material are in physical contact with one another.
5. The method of claim 1, wherein said step of attaching said first
material to a second material comprises a step selected from the
group consisting of welding, soldering, diffusion bonding, brazing,
adhering using an adhesive or grouting material or both, and any
combination thereof.
6. The method of claim 1, wherein said step of placing a finite
number of individual bodies in continuous contact with one another
comprises placing a finite number of individual bodies of at least
two materials in continuous contact with one another.
7. The method of claim 6, further comprising the step of removing
at least a portion of at least one of said at least two materials
either during or after said step of sintering, thereby creating a
cohesive porous structure where said material was removed.
8. The method of claim 1, further comprising the step of applying a
mechanical load to at least a portion of said first material or to
at least a portion of said second material or to at least a portion
of both said first material and said second material.
9. The method of claim 8, wherein said step of applying a
mechanical load is performed during said step of sintering.
10. The method of claim 1, wherein said step of sintering is
performed at an elevated temperature.
11. The method of claim 1, wherein said step of sintering comprises
applying pulsed electrical energy at high frequencies.
12. The method of claim 1, wherein said first material and said
second material are selected from the group consisting of metal,
ceramic, polymer, composite materials, and any combination
thereof.
13. The method of claim 1, wherein the composition of said first
material and said second material are different.
14. The method of claim 1, wherein the first material and the
second material are refractory materials.
15. The method of claim 1, wherein one or both of the first
material and the second material are non-refractory materials.
16. The method of claim 1, wherein a portion of said individual
bodies of said first material are of different composition from
another portion of said individual bodies of said first
material.
17. The method of claim 1, wherein a portion of said individual
bodies of said first material comprises a refractory material and
another portion of said individual bodies of said first material
comprises a non-refractory material.
18. The method of claim 1, wherein one of said first material and
said second material is refractory and the other is
non-refractory.
19. The method of claim 1, wherein said first material has a form
selected from the group consisting of symmetric particles,
asymmetric particles, single fibers, multiple fibers, flat porous
sheets, deformed porous sheets, reticulated open-celled structures,
and any combination thereof.
20. The method of claim 19, wherein said first material has a
symmetric particle form and is a spherical particle.
21. The method of claim 1, wherein said step of sintering is
performed in a controlled environment.
22. The method of claim 21, wherein said controlled environment is
a pressure less than atmospheric pressure.
23. The method of claim 21, wherein said controlled environment
comprises an atmosphere of an inert gas.
24. The method of claim 21, wherein said controlled environment
comprises an atmosphere of a reactive gas.
25. The method of claim 21, wherein said controlled environment is
varied during said step of sintering.
26. The method of claim 1, wherein said step of placing comprises
using a binder.
27. The method of claim 1, further comprising the step of infusing
at least a portion of the porous region with a material.
28. The method of claim 27, wherein said step of infusing comprises
infusing with a method selected from the group consisting of direct
compression molding, injection, solution deposition, vapor
deposition, and any combination thereof.
29. The method of claim 27, wherein said material to be infused is
a polymer.
30. The method of claim 27, wherein said material to be infused
comprises a growth factor or antibiotic.
31. The method of claim 27, wherein said material to be infused is
selected from the group consisting of hydroxyapatite,
fluoroapatite, chloroapatite, bromoapatite, iodoapatite, calcium
sulfate, calcium phosphate, calcium carbonate, calcium tartarate,
bioactive glass, and any combination thereof.
32. A method of making a medical implant having a porous surface
comprising the steps of: placing a finite number of non-spherical
individual bodies in continuous contact with one another; and,
sintering said individual bodies by applying pulsed electrical
energy across at least a portion of the aggregate mass of said
individual bodies, thereby creating a cohesive porous
structure.
33. The method of claim 32, wherein said step of placing a finite
number of non-spherical individual bodies in continuous contact
with one another further comprises placing said individual bodies
in contact with at least one other material.
34. The method of claim 33, further comprising the step of removing
at least a portion of said at least one other material either
during or after said step of sintering, thereby creating a cohesive
porous structure where said material was removed.
35. The method of claim 32, further comprising the step of applying
a mechanical load to at least a portion of said individual
bodies.
36. The method of claim 35, wherein said step of applying a
mechanical load is performed during said step of sintering.
37. The method of claim 32, wherein said step of sintering is
performed at an elevated temperature.
38. The method of claim 32, wherein said step of sintering
comprises applying pulsed electrical energy at high
frequencies.
39. The method of claim 32, wherein said individual bodies are
selected from the group consisting of metal, ceramic, polymer,
composite materials, and any combination thereof.
40. The method of claim 32, wherein the composition of a portion of
said individual bodies is different from the composition of another
portion of said individual bodies.
41. The method of claim 32, wherein at least a portion of said
individual bodies comprise a refractory material.
42. The method of claim 32, wherein said individual bodies have a
form selected from the group consisting of symmetric particles,
asymmetric particles, single fibers, multiple fibers, flat porous
sheets, deformed porous sheets, reticulated open-celled structures,
and any combination thereof.
43. The method of claim 32, wherein said step of sintering is
performed in a controlled environment.
44. The method of claim 43, wherein said controlled environment is
a pressure less than atmospheric pressure.
45. The method of claim 43, wherein said controlled environment
comprises an atmosphere of an inert gas.
46. The method of claim 43, wherein said controlled environment
comprises an atmosphere of a reactive gas.
47. The method of claim 43, wherein said controlled environment is
varied during said step of sintering.
48. The method of claim 32, wherein said step of placing comprises
using a binder.
49. The method of claim 32, further comprising the step of infusing
at least a portion of the porous structure with a material.
50. The method of claim 49, wherein said step of infusing comprises
infusing with a method selected from the group consisting of direct
compression molding, injection, solution deposition, vapor
deposition, and any combination thereof.
51. The method of claim 49, wherein said material to be infused is
a polymer.
52. The method of claim 49, wherein said material to be infused
comprises a growth factor or antibiotic.
53. The method of claim 49, wherein said material to be infused is
selected from the group consisting of hydroxyapatite,
fluoroapatite, chloroapatite, bromoapatite, iodoapatite, calcium
sulfate, calcium phosphate, calcium carbonate, calcium tartarate,
bioactive glass, and any combination thereof.
54. A medical implant comprising a solid substrate and a porous
sintered surface, wherein said solid substrate possesses
substantially the same bulk mechanical and tribological properties
after sintering which existed prior to sintering.
55. The medical implant of claim 54, wherein said material
possesses substantially the same microstructure after sintering
which existed prior to sintering.
56. A medical implant having a porous surface produced by the
process comprising the steps of: placing a finite number of
non-spherical individual bodies in continuous contact with one
another; and, sintering said individual bodies by applying pulsed
electrical energy across at least a portion of the aggregate mass
of said individual bodies, thereby creating a cohesive porous
structure.
57. A medical implant having a porous surface produced by the
process comprising the steps of: placing a finite number of
individual bodies in continuous contact with one another, said
finite number of individual bodies comprising a first material;
sintering said first material by applying pulsed electrical energy
across at least a portion of the aggregate mass of said individual
bodies, thereby creating a cohesive porous structure; and,
attaching said first material to a second material, said second
material comprising said solid substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional
application Ser. No. 60/589,143, filed on Jul. 19, 2004.
TECHNICAL FIELD
[0002] The present invention is directed toward the fabrication of
a porous sintered surface for medical implants.
BACKGROUND OF THE INVENTION
[0003] For a variety of reasons, it is sometimes necessary to
surgically correct an earlier implanted medical implant (most
commonly a prosthetic joint) or replace it with an entirely new
medical implant. Typically, this results from either a loosening of
the implant in the implant site, or the deterioration of the
implant due to forces such as abrasion. Ideally, an medical implant
is often formed from a high-strength material which is not only
able to accommodate the various loading conditions that it may
encounter, but is also non-toxic to, and otherwise biocompatible
with, the human body. It is also preferable to implant the device
in such a way as to enhance fixation over the long term.
[0004] A number of advances have been made to increase service life
of medical implants by increasing their resistance to forces such
as abrasion. The advent of oxidized zirconium, first described by
Davidson in U.S. Pat. No. 5,037,438 has provided a surface with
superior hardness which is also resistance to brittle fracture,
galling, fretting and attack by bodily fluids. A similar advance in
the area of fixation stability will address the other major source
of implant failure and would represent a significant advance in
implant service life.
[0005] In cases of extreme loading conditions as is often the case
for artificial hips, prosthetic joints may be made from metal
alloys such as titanium, zirconium, or cobalt chrome alloys. Not
only are these metal alloys of sufficient strength to withstand
relatively extreme loading conditions, but due to their metallic
nature, a metallic porous coating typically of titanium or cobalt
chrome may be secured to the metal alloy by a metallic bond. Such
metallic porous coatings are useful for providing initial fixation
of the implant immediately after surgery, but also serve to
facilitate long-term stability by enhancing bone ingrowth and
ongrowth.
[0006] While medical implant devices made from biocompatible metal
alloys are effective, they may lack certain desirable
characteristics. For example, metal alloys have poor flexibility
and therefore do not tend to distribute load as evenly as would be
desired. Uneven loads tend to result in a gradual loosening of the
implant. As such loosening becomes more severe, revision or
replacement becomes necessary. For this reason, it is desirable to
design medical implants generally and prosthetic joints
specifically in such a way as to improve their in vivo fixation
stability.
[0007] One way this problem has historically been addressed in the
past is through the use of modified surfaces for medical implants
which increase surface contact area and promote bone ingrowth and
ongrowth. Another more recent technique involves the use of
depositing material onto the surface of an implant, the material
being the emission of a plasma spray source. This is discussed in
U.S. Pat. Nos. 5,807,407, 6,087,553, and 6,582,470, among others,
which are incorporated by reference as though fully disclosed
herein.
[0008] A promising way to form porous products involves fusing
materials in such as way as to effect a porous finished material.
Such approaches have been the subject of past work. Electrical
discharge is one mechanism by which this has been performed, as in
U.S. Pat. Nos. 5,294,769, 5,352,385, and 5,421,943. Sintered
materials have also been the subject of investigation as a
potential solution to the issue of fixation stability improvement
through the use of porous materials which allow for tissue ingrowth
and ongrowth. For example, Chowdhary in U.S. Pat. No. 5,104,410,
describes a prosthesis having a metallic substrate and multiple
sintered layers. The sintered layers were formed by conventional
methods of sintering, using temperatures of 1100.degree. C. for one
hour at 10.sup.-5-10.sup.-6 torr. While such sintered surface
imparts desirable porosity, sintering at such extreme conditions of
temperature and time fundamentally alter the nature of the
substrate in undesirable ways.
BRIEF SUMMARY OF THE INVENTION
[0009] A porous medical implant and a method of making same is
described. The medical implant comprises a porous surface formed by
application of pulsed electrical energy in such a way as to cause a
localized heating in the surface of the material comprising
portions of the implant.
[0010] In one aspect of the present invention, there is a method of
making a medical implant having a porous surface and a solid
substrate, comprising the steps of placing a finite number of
individual bodies in continuous contact with one another, the
finite number of individual bodies comprising a first material;
sintering the first material by applying pulsed electrical energy
across at least a portion of the aggregate mass of the individual
bodies, thereby creating a cohesive porous structure and, attaching
the first material to a second material, the second material
comprising the solid substrate. In some embodiments, the step of
attaching said first material to a second material comprises
sintering said first material to said second material by applying
pulsed electrical energy across at least a portion of the aggregate
mass of the first material and the second material while the first
material and the second material are in physical contact with one
another. In some embodiments, the steps of sintering and attaching
are performed simultaneously by applying pulsed electrical energy
across at least a portion of the aggregate mass of the first
material and the second material while the first material and the
second material are in physical contact with one another. In some
embodiments, the steps of sintering and attaching are performed
sequentially by first applying pulsed electrical energy across at
least a portion of the aggregate mass of the first material and
thereafter applying pulsed electrical energy across at least a
portion of the aggregate mass of the first material and the second
material while the first material and the second material are in
physical contact with one another. In some embodiments, the step of
attaching said first material to a second material comprises a step
selected from the group consisting of welding, soldering, diffusion
bonding, brazing, adhering using an adhesive or grouting material
or both, and any combination thereof. In some embodiments, the step
of placing a finite number of individual bodies in continuous
contact with one another comprises placing a finite number of
individual bodies of at least two materials in continuous contact
with one another. The method may further comprise the step of
removing at least a portion of at least one of said at least two
materials either during or after said step of sintering, thereby
creating a cohesive porous structure where said material was
removed. Preferably, the method further comprises the step of
applying a mechanical load to at least a portion of said first
material or to at least a portion of said second material or to at
least a portion of both said first material and said second
material. In cases where a mechanical load is applied, it is
preferably applied during said step of sintering. In some
embodiments, the step of sintering is performed at an elevated
temperature. In some embodiments, the step of sintering comprises
applying pulsed electrical energy at high frequencies. In some
embodiments, the first material and said second material are
selected from the group consisting of metal, ceramic, polymer,
composite materials, and any combination thereof. The first
material and second material may or may not be different.
Preferably the first material and the second material are
refractory materials. Alternatively, one or both of the first
material and the second material may be non-refractory materials.
In some embodiments, a portion of the individual bodies of the
first material are of different composition from another portion of
the individual bodies of the first material. Accordingly in some
embodiments, a portion of the individual bodies of the first
material comprises a refractory material and another portion of the
individual bodies of the first material comprises a non-refractory
material. In some embodiments, one of the first material and the
second material is refractory and the other is non-refractory. In
some embodiments, the first material has a form selected from the
group consisting of symmetric particles, asymmetric particles,
single fibers, multiple fibers, flat porous sheets, deformed porous
sheets, reticulated open-celled structures, and any combination
thereof. In some embodiments, the first material has a symmetric
particle form and is a spherical particle. In some embodiments, the
sintering step is performed in a controlled environment. The
controlled environment may be one having a pressure less than
atmospheric pressure. The controlled environment may be one
comprising an atmosphere of an inert gas. The controlled
environment may be one comprising an atmosphere of a reactive gas.
In some embodiments of the method, the controlled environment is
varied during the step of sintering. In some embodiments of the
method, the step of placing comprises using a binder. In some
embodiments, the method further comprises the step of infusing at
least a portion of the porous region with a material. In some
embodiments where an infusing step is used, the step of infusing
comprises infusing with a method selected from the group consisting
of direct compression molding, injection, solution deposition,
vapor deposition, and any combination thereof. In some embodiments
where an infusing step is used, the material to be infused is a
polymer. In some embodiments where an infusing step is used, the
material to be infused comprises a growth factor or antibiotic. In
some embodiments where an infusing step is used, the material to be
infused is selected from the group consisting of hydroxyapatite,
fluoroapatite, chloroapatite, bromoapatite, iodoapatite, calcium
sulfate, calcium phosphate, calcium carbonate, calcium tartarate,
bioactive glass, and any combination thereof.
[0011] In another aspect of the present invention, there is a
method of making a medical implant having a porous surface
comprising the steps of placing a finite number of non-spherical
individual bodies in continuous contact with one another; and,
sintering said individual bodies by applying pulsed electrical
energy across at least a portion of the aggregate mass of said
individual bodies, thereby creating a cohesive porous structure. In
some embodiments, the step of placing a finite number of
non-spherical individual bodies in continuous contact with one
another further comprises placing said individual bodies in contact
with at least one other material. In some embodiments, the method
further comprises the step of removing at least a portion of said
at least one other material either during or after said step of
sintering, thereby creating a cohesive porous structure where said
material was removed. The method may further comprise the step of
applying a mechanical load to at least a portion of said individual
bodies. In some embodiments, the step of applying a mechanical load
is performed during said step of sintering. In some embodiments,
the step of sintering is performed at an elevated temperature. In
some embodiments, the step of sintering comprises applying pulsed
electrical energy at high frequencies. In some embodiments, the
individual bodies are selected from the group consisting of metal,
ceramic, polymer, composite materials, and any combination thereof.
In some embodiments, the composition of a portion of the individual
bodies is different from the composition of another portion of the
individual bodies. In some embodiments, at least a portion of said
individual bodies comprise a refractory material. In some
embodiments, the individual bodies have a form selected from the
group consisting of symmetric particles, asymmetric particles,
single fibers, multiple fibers, flat porous sheets, deformed porous
sheets, reticulated open-celled structures, and any combination
thereof. In some embodiments, the sintering step is performed in a
controlled environment. The controlled environment may be one
having a pressure less than atmospheric pressure. The controlled
environment may be one comprising an atmosphere of an inert gas.
The controlled environment may be one comprising an atmosphere of a
reactive gas. In some embodiments of the method, the controlled
environment is varied during the step of sintering. In some
embodiments, the step of placing comprises using a binder. In some
embodiments, the method further comprises the step of infusing at
least a portion of the porous structure with a material. In some
embodiments, the step of infusing comprises infusing with a method
selected from the group consisting of direct compression molding,
injection, solution deposition, vapor deposition, and any
combination thereof. In some embodiments where an infusing step is
used, the material to be infused is a polymer. In some embodiments
where an infusing step is used, the material to be infused
comprises a growth factor or antibiotic. In some embodiments where
an infusing step is used, the material to be infused is selected
from the group consisting of hydroxyapatite, fluoroapatite,
chloroapatite, bromoapatite, iodoapatite, calcium sulfate, calcium
phosphate, calcium carbonate, calcium tartarate, bioactive glass,
and any combination thereof.
[0012] The present invention also includes a medical implant
comprising a solid substrate and a porous sintered surface, wherein
the solid substrate possesses substantially the same bulk
mechanical and tribological properties after sintering which
existed prior to sintering. Preferably, the material possesses
substantially the same microstructure after sintering which existed
prior to sintering.
[0013] There is also a medical implant having a porous surface
produced by the process comprising the steps of placing a finite
number of non-spherical individual bodies in continuous contact
with one another; and, sintering the individual bodies by applying
pulsed electrical energy across at least a portion of the aggregate
mass of the individual bodies, thereby creating a cohesive porous
structure.
[0014] There is also a medical implant having a porous surface
produced by the process comprising the steps of placing a finite
number of individual bodies in continuous contact with one another,
said finite number of individual bodies comprising a first
material; sintering the first material by applying pulsed
electrical energy across at least a portion of the aggregate mass
of the individual bodies, thereby creating a cohesive porous
structure; and, attaching the first material to a second material,
the second material comprising said solid substrate.
[0015] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It should be appreciated that the conception and
specific embodiment disclosed may be readily utilized as a basis
for modifying or designing other structures for carrying out the
same purposes of the present invention. It should also be realized
that such equivalent constructions do not depart from the invention
as set forth in the appended claims. The novel features which are
believed to be characteristic of the invention, both as to its
organization and method of operation, together with further objects
and advantages will be better understood from the following
description when considered in connection with the accompanying
figures. It is to be expressly understood, however, that each of
the figures is provided for the purpose of illustration and
description only and is not intended as a definition of the limits
of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings.
[0017] FIG. 1 is a demonstrating the result of the use of
conventional sintering on an medical implant.
[0018] FIG. 2 is a schematic illustration demonstrating the result
of the use of pulsed current sintering on an medical implant.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention describes a medical implant and a
method of making a medical implant comprising a porous surface for
tissue ingrowth and ongrowth. Specifically, sintered medical
implant product is described. The sintered product avoids the
changes in bulk microstructure and the corresponding changes in the
mechanical and tribological properties of a solid substrate which
occurs when high temperature sintering is required to create and
bond a porous tissue ingrowth and ongrowth surface to an
implantable medical device.
[0020] As used herein, "a" or "an" is defined herein as one or
more. Unless otherwise indicated or apparent by the context, the
singular includes the plural and the plural includes the singular
herein.
[0021] As used herein, "metal" means any material comprising a
metal and includes, but is not limited to, metals and metal
alloys.
[0022] As used herein, "non-refractory" means a material that melts
at a relatively low temperature, typically, a temperature lower
than that defined by the melting points of iron, cobalt and
nickel.
[0023] As used herein, "refractory" means a material that melts at
a high temperature, typically, a temperature higher than that
defined by the melting points of iron, cobalt and nickel.
[0024] As used herein, "porous" means a material, or a portion
thereof, having at least 20% surface-connected porosity with an
average pore size ranging from about 10 microns to about 1000
microns. The term "porous" may connote regions within a material,
i.e., a material may have regions of porosity while having other
regions which are non-porous.
[0025] As used herein, "solid" means a material have less than 5%
porosity.
[0026] As used herein, "tissue" means any and all bodily tissue,
including bone and soft tissue.
[0027] Sintering is a simple process whereby particular material in
powder form is heated to a high temperature less than the melting
point, whereby the particles bond to each other, producing a porous
(on a microscopic scale) material. These materials include but are
not limited to, metals, metal alloys, and ceramics. Sintering is a
method for making strong ceramic objects from ceramic powder. The
process typically includes mixing water, binder, antiflocculant,
and ceramic powder to form a slurry. The slurry is spray dried and
put into a mold and pressing it to form a green body (an unsintered
ceramic material) The green body is heated at low temperature to
burn the binder off. The material is heated at high temperature
(but lower than its melting point) to fuse the ceramic particles
together. A similar process of sintering is sometimes used to form
metallic objects. Sintered bronze in particular is frequently used
as a material for bearings since it is porous and thus allows
lubricants to flow through. The result is a fairly low density
material which can be cut and shaped fairly easily, can hold small
loads in compression, and provides good thermal insulation, but
cannot take much stress in tension and is brittle. Sintering allows
production of parts without melting and liquid casting processes,
i.e., dealing with only powder or fine sand. Sintering is the most
common technique for consolidating powders.
[0028] Sintering techniques have been used to produce porous
surfaces for medical implants. The porous surfaces of such implants
exhibit excellent tissue ingrowth and ongrowth properties. However,
conventional sintering methods result in degradation in mechanical
and tribological properties. Attempts to address this problem in
the prior art have focused on the use of alternative, less
desirable porous surfaces, and the use of sintering aides to
attempt to decrease the sintering temperatures and lessen the
changes to the microstructure of the bulk of the material. In the
context used herein, the surface of the implant materials extends
to a first approximation to about one micron. The bulk exists at
deeper levels.
[0029] There are many methods for sintering a component. The most
important are: vapor-phase sintering; solid-state sintering;
liquid-phase sintering; reactive liquid sintering. Overpressure
sintering uses pressure to accelerate densification. The biggest
problem of this technique is shrinkage which causes cracking and
distortion. Importantly, where sintering is used to create a porous
surface on a substrate, the somewhat harsh conditions necessary for
sintering result in unwanted changes in the substrate material for
given applications.
[0030] The inventors have found that sintering methods which
utilize pulsed electrical current to effect a substantially
localized heating of the interfaces between portions of material to
be sintered result in a superior sintered device for medical
implant applications. While heating may not be completely limited
to these interfaces, it is at least kept to a minimum in other
regions. Implants produced using pulsed current techniques can
produce strongly bonded porous surfaces while maintaining or only
minimally changing the microstructure of other material regions of
the implant. Sintering techniques which utilize the application of
pulsed electrical energy are known by a variety of names, including
spark plasma sintering (SPS), pulsed electric current sintering
(PECS), and field activated sintering technique (FAST). In this
general technique, it is possible to produce high quality sintered
materials in short periods by charging the intervals between powder
particles (or other material forms) with electrical energy and, in
some cases, a high mechanical load between the materials to be
sintered.
[0031] In pulsed current sintering, sufficient current is supplied
such that electrical arcing occurs across interfaces, especially
the spaces between portions of the material(s) to be sintered. The
interfacial resistivity causes a localized heating to occur. Such
heating is localized to the spaces between portions and the
surfaces of the material portion. It is possible to use this
technique and minimize the more general resistive heating (Joule
heating) that occurs in the bulk of the material. It is this latter
form of heating which modifies the bulk of the material in unwanted
ways, including, but not limited to, grain growth in the bulk of
the material. In this technique, small particles or beads are
preferred. A finite number of such bodies are placed in continuous
contact with one another and a pulse of electrical energy is
applied across at least a portion of the aggregate mass of the
bodies. A localized heating occurs at the contact areas between the
bodies, resulting in their union at the contact points. The
resulting structure is porous. The bodies can be sintered to one
another and/or to a solid substrate material.
[0032] In the present invention, the thermal energy so transferred
to the material is ideally just enough to cause bonding of the
material. Any excess energy should be minimized, as such energy
will contribute to further heating of the bulk and potentially
affect the bulk microstructure. Sintering with pulsed electrical
energy allows one to achieve or approximate this condition of
energy transfer if sintering is performed under appropriate
conditions. The frequency of the electrical pulse is one parameter
which may be manipulated to achieve this result. By increasing the
frequency of the pulse, the result will be to drive the current to
the surface. The current under these conditions skims the surface
and will effect the desired bonding to form a porous surface.
Accordingly, high frequencies are preferred. A pulse rate of at
least 1 pulse per second is preferred, although lower frequencies
may be acceptable for particular applications. More preferably,
much higher pulse rates are desired, on the order of 10
pulses/second (10 Hz) to 1 pulse/microsecond (1 MHz) and higher.
Frequencies of 10 pulses/second and above are considered high
frequencies herein. The time between pulses may, but need not, be
equal to the time duration of an individual pulse. The asymmetry
may favor either the "on" time or the "off" time. These parameters,
like all others in pulsed current sintering of medical implants,
may be varied to best suit the materials being used to fabricate
the implant.
[0033] Another parameter which can be controlled to effect a more
localized heating of the surface as opposed to a general heating of
the surface and bulk, is to accelerate the bonding process. The
shorter the duration of the application of thermal energy to the
implant, the less will be the change in the microstructure of the
bulk of the material. This may be accomplished, for example, by
applying and/or increasing the mechanical load on the implant. This
forces the particles to be bound more closely together. This
hastens the bonding and permits the process to be completed with
the addition of a minimum of electrical (and therefore, thermal)
energy.
[0034] In one embodiment of the present invention, the material is
placed in an graphite tube housing ("outer die") with two graphite
plugs on either end of the tube. The outer die, or "tube housing"
as stated here, could also be made from other materials. For
example it could be made of a non-conductive material, such as a
ceramic. Having a graphite die allows some of the current to be
used to heat the die (through resistive or Joule heating). With a
non-conductive die, more of the current would go through the sample
itself. However, graphite also has a higher thermal conductivity,
and can remove heat from the sample more quickly than a ceramic.
Electrodes which are used to apply the pulsed current must always
be conductive. In the case of a sintered surface on a solid
substrate, the surface material is placed in contact with the
substrate, either with or without a binder material. It is
important that the graphite plugs, or other conductive material,
contact the material(s) to be sintered. Electrodes contact the
material such that a pulsed current may be applied. The current
pulses travel through the material and arc across gaps in the
material. These gaps are most often the spaces between the material
to be sintered. The current encounters resistance at these
interfaces. This interfacial resistance causes a localized heating
where it occurs.
[0035] In the basic method, a first material is sintered by
electrically charging it with pulsed electrical energy. The
electrical energy is pulsed at high frequencies, preferably greater
than 1 pulse per second, and preferable, the material is under a
mechanical load. Preferably, when a mechanical load is used, it is
a compressive load of at least 1 N. The magnitude of the pulse can
be varied as well to optimize results. Conditions are optimized
when sintering occurs and the microstructure of the subsurface bulk
of the material is not changed or is not substantially changed from
that which existed prior to the application of the pulsed
electrical energy. Most preferably, the absence of a change in
microstructure is evidenced by no grain growth or by substantially
no grain growth or when there is no change or substantially no
change in the distribution of any of the component phases in the
subsurface bulk of the material after application of the pulsed
electrical energy when compared to that which existed before
application of the pulsed electrical energy. When so pulsing with
electrical energy, the spaces between portions of the material are
heated, and the heating being substantially localized to said
spaces and to the surface of said portions.
[0036] In the case where a medical implant having a solid substrate
and a porous surface is desired, a second material is used. The
second material may be the same or different from the first
material. In some cases, the second material may be electrically
charged with pulsed electrical energy.
[0037] It may be preferable to perform the process at elevated
temperatures. In this way, the material to be sintered required
less electrical energy to reach the bonding energy. When elevated
temperatures are used, they should be below those temperature which
cause substantial change in the substrate microstructure (or a
change in the microstructure of the subsurface bulk in the case of
a free-standing porous implant).
[0038] As discussed above, efforts to improve medical implants by
application of porous surfaces has found limited success but
improvements are needed. Conventional sintering techniques have
been used, but the conditions necessary for conventional sintering
techniques have unwanted effects on substrate materials that form
the bulk of the implant. By using electrical sintering and
maintaining the proper conditions, these unwanted effects may be
reduced or eliminated, resulting in a superior porous medical
implant.
[0039] The inventors have applied this technology to the
fabrication of medical implants with porous surfaces and have found
that superior medical implants can be so made. By avoiding the high
temperature sintering cycle required to bond most porous tissue
ingrowth coatings, bonding can be achieved while maintaining a
refined substrate microstructure, better preserving the original
mechanical, tribological, and oxidation properties of the
substrate. Microstructure and grain size in the substrate are
unchanged or are substantially unchanged. This is schematically
illustrated in FIGS. 1 and 2. FIG. 1 demonstrates the results of
conventional sintering, while FIG. 2 demonstrates the results of
pulsed current sintering. In some cases, bonding is achieved while
maintaining an average final substrate grain size of less than 1
mm. Also, since this process is less dependent on differences in
melting points, it is possible to join refractory and
non-refractory materials without the use of an intermediate layer
to enhance or enable bonding. Although FIGS. 1 and 2 demonstrate
the preservation of the mechanical and tribological properties of
the substrate before and after pulse electrical sintering, the same
can also be said for the bulk (i.e., non-surface region) of the
sintered bodies (depicted as the spheres in FIGS. 1 and 2). It is
possible to preserve the mechanical and tribological properties of
the bulk region of the sintered bodies also. Depending upon the
conditions used, the surface areas of the sintered bodies
(particularly those areas in physical contact with other surfaces)
may experience a significant change in mechanical and tribological
properties. Changes in mechanical and tribological properties are
typically manifest by a change in microstructure such as a change
in grain size or an altered distribution of crystal phases, and/or
other properties. Such changes are often deleterious to the
performance of medical implants. Thus, avoiding these changes will
lead to improved implants.
[0040] Another advantage of the pulsed electrical sintering method
is that non-spherical particles can be used to produce an equally
strong, but more porous structure. Packed spherical particles of
uniform size typically produce a porosity of only 25-35%. The
packing density of uniformly-sized non-spherical particles can
produce much greater porosity, which is desirable for stand-alone
porous implants. However, in conventional sintering, irregular
particles, for example, typically have fewer and smaller necking
regions, or regions where the particles sinter together, giving
irregular particle porous coatings lower attachment strength than
spherical particle porous coatings. Increasing the sintering
temperature or applying pressure during sintering could increase
bonding strength of irregular powder porous coatings, however both
methods are likely to have detrimental consequences. For example,
either method increases the likelihood of collapse of the porous
structure being sought. Furthermore, if being bonded to a solid
substrate, increasing the sintering temperature increases the
likelihood that the substrate microstructure will be detrimentally
affected. Spherical bodies would normally be preferred because they
are inherently self-supporting, reducing the likelihood of collapse
of the porous structure, and pack more uniformly and repeatably
than non-spherical bodies, particularly under the mechanical loads
preferred by this method, resulting in greater porosity and a more
regular and uniform distribution of porosity of the resulting
sintered product. The inventors have found that the sintering
performance of non-spherical particles is much improved in pulsed
electrical sintering in comparison to conventional sintering. As a
result, medical implants can more readily be made non-spherical
bodies using pulsed electrical sintering.
[0041] The medical device is made by bonding or simultaneously
creating and bonding a metallic, ceramic, polymer, or composite
porous structure to a solid metallic, ceramic, polymer, or
composite substrate using pulsed electrical sintering. The bonding
in the device is achieved using pulsed electrical sintering in a
vacuum or inert gas environment to prevent material/environment
reactivity or to modify heat flow behavior. The bonding may be
achieved using pulsed electrical sintering in combination with
pressure and/or additional heat. In the fabrication of the device,
a porous surface can be created with lower temperatures and/or
pressures than traditional sintering or diffusion bonding methods
used for medical implants.
[0042] In a preferred embodiment, the medical implant comprises a
solid substrate and a porous sintered surface. The solid substrate
and porous surface may be composed of substantially the same
material(s). For example, titanium metal or titanium metal alloy
may be sintered onto a titanium or titanium alloy surface.
Alternatively, they may be composed of substantially different
materials. For example, titanium may be sintered onto a
cobalt-chromium surface, or a metal or metal alloy may be sintered
onto a ceramic surface. Alternatively, the medical implant may
comprise a purely porous component. In either case, the medical
implant may comprise a variety of materials, including but not
limited to, metal, ceramic, polymer, composite materials, and any
combination thereof. The present invention is applicable to all
conventional implant materials. The material and their precursors
may have a variety of forms, including but not limited to,
particles, fibers, flat porous sheets, deformed porous sheets,
reticulated open-celled structures, and any combination thereof.
Where the medical implant comprises more than one material, the
materials may be the same or different (for example, both may be
titanium or a titanium alloy). Additionally, where the medical
implant comprises more than one material, the materials may have
the same or different forms (e.g., particles, fibers, etc.). In
some applications, the final medical implant may comprise bioactive
ceramic materials such as hydroxyapatite, fluoroapatite,
chloroapatite, bromoapatite, iodoapatite, calcium sulfate, calcium
phosphate, calcium carbonate, calcium tartarate, bioactive glass,
and combinations thereof.
[0043] To fabricate the implant, a first material is sintered by
electrically charging it with pulsed electrical energy under
conditions in which spaces between portions of it are heated. As a
result, the heating is substantially localized to said spaces and
to the surface of said portions. The first material is then
attached to a second material. The second material is preferably a
solid substrate. It should be noted that the attachment may occur
as a consequence of the sintering step in some cases. In such
cases, the sintering of the first and second materials may occur
simultaneously to the sintering of the bodies of the first material
(for example, wherein the pulsed electrical energy sinters the
bodies of the first material to each other and to the second
material. Alternatively, sequential sintering steps may be used. In
other cases, the sintered material may be attached to the substrate
by some other means. This may be accomplished by any of the various
ways known to those of skill in the art, including but not limited
to, diffusion boding, welding, soldering, brazing, attaching with
adhesive or grouting material, or any combination thereof, etc. The
first and second materials may be the same or different, both in
terms of composition and properties. Each material, whether it be
the first material, the second material, or any other material(s),
may be a pure material, or it may comprise a mixture. In other
words, each material, as that term is used herein, may comprise one
or more than one material. The term "material" includes both the
singular and the plural. This is true for all embodiments of the
present invention.
[0044] The implant may have a porous structure made from a
refractory material and a substrate made from a non-refractory
material. The implant may have a porous structure made from a
non-refractory material and a substrate made from a refractory
material. In the case where the medical implant comprises a purely
porous component (e.g., a stand alone porous structure without a
solid substrate), it may be made of a refractory material, a
non-refractory material, or both.
[0045] Creating the porous surface is accomplished by sintering a
precursor material. The form of the porous structure precursor may
vary in the present invention. The porous structure precursor may
be any of a number of different forms. These include, but are not
limited to, beads, particles, single or multiple fibers, flat or
deformed porous sheets, reticulated open-celled structures, and
others. The beads and particles may be of any shape and form, such
as spherical or non-spherical, symmetric or asymmetric.
Combinations of any of these forms are also possible. The porous
structure precursor may be comprised of a variety of materials,
including, but not limited to, metals, ceramics, polymers, and
composite materials. Combinations of any of these materials are
also possible.
[0046] In some embodiments, the porous structure precursor is
temporarily attached or positioned to the substrate using a binder.
The binder could be cellulose or other commonly used binders in the
sintering field. Wax may be used in some cases. In some cases, the
porous structure is created by the removal of an interconnected
pore-creating secondary material during or after the bonding
process.
[0047] Although use of the present invention allows for the
production of a substrate material directly bonded to a porous
surface, it is also within the scope of the invention to produce a
stand-alone porous structure with the pulsed current techniques
herein described and bond that structure to a substrate using an
intermediate bonding layer. Bonding in the present invention may be
achieved without the use of an intermediate layer whose main
purpose is to enhance or enable bonding. Such structures and
methods of making them are known in the art, e.g., see U.S.
Published Application No. 2003/0232124 to Medlin et al.; U.S. Pat.
No. 6,063,442 to Cohen et al. Boron-containing compounds may also
be an intermediate layers with nickel-based metals. Stand-alone
porous structures without a bound substrate may also be produced.
These are useful in particular applications as medical
implants.
[0048] The basic method may also be modified to include the use of
a pore-creating material. The pore-creating material may be mixed
or otherwise combined with the implant materials. The pore-creating
material can be any volatile, dissolvable, and/or decomposable
material. The pore-creating material forms a matrix with the
implant material and is subsequently removed by decomposition,
volatilization, dissolution, any combination thereof, etc. Examples
included titanium hydride, which decomposes through loss of the
hydride hydrogen; naphthalene, which is removed through
sublimation; and various salts, which may be washed out of the
matrix.
[0049] Where desirable, the methodology can be used to produce an
medical device in which the same or different morphologies are
bonded to different regions of the device. The use of different
surface morphologies enables optimization of the surface for
interaction with certain types of tissue. The medical device may be
manufactured such that different portions of it are optimized for
specific ingrowth results. For example, the device can be
fabricated such that at least one region is intended for
soft/fibrous tissue ingrowth, while at least one region is intended
for bone tissue ingrowth. In another embodiment, the medical
implant is infused with ultra-high molecular weight polyethylene or
other load-bearing implantable polymers. Typically, this is done
through a direct compression molding process and at least one
region is intended for bone or soft/fibrous tissue ingrowth. Other
possible methods to infuse, include, but are not limited to,
solution deposition, vapor deposition, or various injection
techniques such as injection molding or injection of a curable
polymer. The medical implant may also be infused with other active
biomolecules such as growth factors or drugs such as antibiotics.
These materials may be infused into the medical implant in a
polymer matrix which may be infused as discussed above or by other
means.
[0050] The process for creating the porous surface is pulsed
electrical sintering. As discussed above, the technique uses a
pulsed frequency current to create a localized heating that results
in sintering without significant perturbation of the substrate
phase. The process may comprise the bonding of multiple substrate
surfaces simultaneously. The process may comprise bonding surfaces
that are non-planar, such as an acetabular shell or a hip stem. The
process may comprise two or more non-coplanar substrate surfaces
simultaneously, for example, two or more of the fixation surfaces
of a knee femoral component.
[0051] The invention also includes a medical implant made by
bonding together metallic, ceramic, polymer, or composite
non-spherical particles, fibers, or flat or deformed porous sheets
using pulsed current sintering. In some cases, the particles,
fibers, or sheets of the device are comprised of substantially the
same materials. Alternatively, the particles, fibers, or sheets of
the device are comprised of two or more substantially different
materials. In the case of different materials, the combination of
materials may be chosen such that at least one material provides
enhanced mechanical properties and at least one material provides
enhanced tissue ingrowth properties. The medical implants of the
present invention include those that incorporate only particles,
fibers, or sheets, or any combination of particles, fibers, or
sheets. Alternatively, the medical device incorporates spherical
beads with any combination of particles, fibers, and sheets.
[0052] Implants which have regions of porosity and non-porous
regions are also possible. The different regions may have different
characteristics also. For example, the medical implant may have
comprise a titanium alloy substrate with a porous region having
sintered titanium and another porous regions with sintered
zirconium. Medical implants having other final constructions are
possible. For example, the present invention includes an implant
having porous regions sandwiched between solid substrates and vice
versa.
[0053] Additionally, implants which possess porosity everywhere,
instead of those having a porous surface on a solid substrate are
within the scope of the present invention. Purely porous implants
formed by pulsed current sintering have the advantage of a more
refined subsurface bulk.
[0054] The present invention is applicable to all medical implants.
However, its most important application is expected to be in the
area of joint prostheses and other orthopaedic implants. For
example, fixation stability is a common problem for hip and knee
prostheses. Other applications include but are not limited to
shoulder, elbow, ankle, finger, wrist, and toe prostheses. The
ability to produce a stable, porous surface for tissue ingrowth and
ongrowth, while preserving the integrity of the bulk will lead to a
superior prosthesis. The invention is applicable to other joint
prostheses as well, including, but not limited to, shoulder and
elbow prostheses. Other medical implants that can be improved
through the use of the invention include vertebral implants and
dental implants. Also, the present invention can be applied to
maxillofacial and tempromandibular implants. It can also be applied
to bone implant hardware, including, but not limited to, nails,
screws, rods, pins, plates, spacers, wedges, void fillers, and any
combination thereof.
[0055] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the invention as defined by the appended claims. Moreover, the
scope of the present application is not intended to be limited to
the particular embodiments of the process, machine, manufacture,
composition of matter, means, methods and steps described in the
specification. As one will readily appreciate from the disclosure,
processes, machines, manufacture, compositions of matter, means,
methods, or steps, presently existing or later to be developed that
perform substantially the same function or achieve substantially
the same result as the corresponding embodiments described herein
may be utilized. Accordingly, the appended claims are intended to
include within their scope such processes, machines, manufacture,
compositions of matter, means, methods, or steps. All patents and
patent applications cited herein are incorporated by reference as
though fully set out herein.
* * * * *