U.S. patent application number 13/126737 was filed with the patent office on 2011-10-06 for porous surface layers with increased surface roughness and implants incorporating the same.
This patent application is currently assigned to Smith & Nephew, Inc.. Invention is credited to Carie Fincher Alley, Laura J. Gilmour.
Application Number | 20110245930 13/126737 |
Document ID | / |
Family ID | 42153481 |
Filed Date | 2011-10-06 |
United States Patent
Application |
20110245930 |
Kind Code |
A1 |
Alley; Carie Fincher ; et
al. |
October 6, 2011 |
POROUS SURFACE LAYERS WITH INCREASED SURFACE ROUGHNESS AND IMPLANTS
INCORPORATING THE SAME
Abstract
Systems and methods for providing tissue-interfacing surface
layers with increased roughness can be attained by providing a
metallic powder to a machined or previously machined
tissue-interfacing surface of a porous foam structure. The metallic
powder can have sizes and characteristics such that the porous
structure can have an increased roughness at the tissue-interfacing
machined surface while inhibiting the occlusion of the open pores
in the porous metallic foam structure.
Inventors: |
Alley; Carie Fincher;
(Memphis, TN) ; Gilmour; Laura J.; (Memphis,
TN) |
Assignee: |
Smith & Nephew, Inc.
Memphis
TN
|
Family ID: |
42153481 |
Appl. No.: |
13/126737 |
Filed: |
October 23, 2009 |
PCT Filed: |
October 23, 2009 |
PCT NO: |
PCT/US2009/061881 |
371 Date: |
June 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61109395 |
Oct 29, 2008 |
|
|
|
Current U.S.
Class: |
623/23.74 ;
427/2.24 |
Current CPC
Class: |
A61L 27/306 20130101;
A61L 27/56 20130101; A61F 2/3859 20130101; A61F 2310/00089
20130101; A61F 2002/3092 20130101; A61F 2310/00029 20130101; A61F
2002/30968 20130101; A61F 2/3094 20130101; A61L 2400/18 20130101;
A61F 2/44 20130101; Y10T 156/10 20150115; A61F 2310/00131 20130101;
A61F 2/389 20130101 |
Class at
Publication: |
623/23.74 ;
427/2.24 |
International
Class: |
A61F 2/02 20060101
A61F002/02; B05D 3/02 20060101 B05D003/02 |
Claims
1.-9. (canceled)
10. A method for increasing the surface roughness of a porous
structure, comprising: bonding a plurality of powder particles to
at least a portion of a machined tissue-interfacing outer surface
of a machined porous structure having a desired shape, wherein at
least a portion of the plurality of powder particles has sufficient
dimensions to increase the roughness of the machined
tissue-interfacing outer surface of the machined porous structure
without occluding a plurality of pores of the porous structure.
11. The method of claim 10 wherein at least a portion of the
plurality of powder particles comprises asymmetric powder
particles.
12. The method of claim 10 wherein the size of at least a portion
of the plurality of powder particles is from about 75 micrometers
to about 106 micrometers.
13. The method of claim 10 wherein said bonding step comprises
applying a binder to the machined tissue-facing outer surface of
the machined porous structure; subsequently applying said plurality
of powder particles by a means selected from the group consisting
of dipping, spraying, sprinkling, and any combination thereof; and
sintering said porous structure to bond a portion of said plurality
of powder to at least a portion of the machined tissue-facing outer
surface of the porous structure.
14. The method of claim 10 wherein the porous structure comprises a
metal foam structure.
15. The method of claim 10 further comprising the step of attaching
the porous structure to a substrate.
16. The method of claim 15, wherein the substrate comprises a
metallic foam-coated implant selected from the group consisting of
a knee implant, hip implant, a shoulder implant, a spinal implant,
a tibial tray, an acetabular shell, a femoral stem, and a stem
collar.
17. The method of claim 10 further comprising the step of applying
one or more additional layers of said plurality of powder particles
to one or more non-tissue-interfacing surfaces of the porous
structure.
18. The method of claim 17, wherein the step of applying one or
more additional layers occurs after the step of bonding a powder to
a machined tissue-interfacing outer surface of the machined porous
structure.
19. The method of claim 17, wherein at least one layer of the one
or more additional layers comprises a plurality of fine spherical
particles, wherein at least a portion of said particles has a size
of less than about 45 micrometers.
21. A porous structure with increased surface roughness comprising:
a porous structure having a machined tissue-interfacing outer
surface; a plurality of powder particles bonded to at least a
portion of said machined tissue-interfacing outer surface of the
porous structure, wherein at least a portion of the plurality of
powder particles has sufficient dimensions to increase the
roughness of the machined tissue-interfacing outer surface of the
machined porous structure without occluding a plurality of pores of
the porous structure.
22. The structure of claim 21 wherein at least a portion of the
plurality of powder particles comprises asymmetric powder
particles.
23. The structure of claim 21 wherein the size of at least a
portion of the plurality of powder particles is from about 75
micrometers to about 106 micrometers.
24. The structure of claim 21 wherein the porous structure
comprises a metal foam structure.
25. The structure of claim 21 further comprising a substrate
attached a portion of said porous structure.
26. The structure of claim 25, wherein the substrate is a metallic
foam-coated implant selected from the group consisting of a knee
implant, hip implant, a shoulder implant, a spinal implant, a
tibial tray, an acetabular shell, a femoral stem, and a stem
collar.
27. The structure of claim 21 further comprising one or more
additional layers of said plurality of powder particles bonded to
one or more non-tissue-interfacing surfaces of the porous
structure.
28. The structure of claim 21, wherein at least one layer of the
one or more additional layers comprises a plurality of fine
spherical particles, wherein at least a portion of said particles
has a size of less than about 45 micrometers.
29. The structure of claim 21, wherein the size of at least a
portion of the plurality of powder particles is between about 10%
and 30% of the pore size of the porous structure.
30. The structure of claim 21, wherein the size of at least a
portion of the plurality of powder particles is between about 30%
and 70% of the pore size of the porous structure.
31. The structure of claim 21, wherein the size of at least a
portion of the plurality of powder particles is between about 40%
and 60% of the pore size of the porous structure.
32. The structure of claim 21, wherein the plurality of powder
particles comprises material selected from a group consisting of
titanium, titanium hydride, titanium dehydride, titanium alloy,
cobalt-chrome alloy, tantalum, zirconium, zirconium alloy, and any
combination thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/109,395, filed 29 Oct. 2008. The disclosure of
this prior application is incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to surface layers with
increased roughness, and more particularly to a method for
increasing the roughness of a tissue-engaging outer surface of a
porous structure without altering the pore size and porosity of the
structure, and to medical implants incorporating said porous
structure with increased surface roughness.
[0004] 2. Description of the Related Art
[0005] Especially in the medical fields, the surface of an implant,
device, or other implement can significantly affect function. For
example, attempts have been made to improve bone implant stability
by increasing the roughness of the implant. Other attempts have
been made to improve bone implant stability by providing pores in
the implant for bone ingrowth.
[0006] One method of achieving bone ingrowth in implants that
contact bone (e.g., orthopedic implants) includes sintering
metallic bead surfaces onto a substrate. Other methods of achieving
bone ingrowth in implants includes using a reticulated foam porous
coating fabricated from titanium that incorporates an electrical
discharge machined (EDM) surface treatment, an EDM surface with
axial grooves, an EDM surface with cross-hatching, or a
photo-etched surface. Foam metal implants have been shown to
achieve greater bone ingrowth than sintered bead implants. See,
Urban, Robert M. et al., "Biomechanical and Histological Response
to a Novel Foam Metal Porous Coating with Comparison of Two Methods
for Measuring Bone Ingrowth," Transactions of the 54th Annual
Meeting of the Orthopaedic Research Society, p. 1854, Mar. 2-5,
2008.
[0007] However, production of a porous metallic foam ingrowth
structure (e.g., one created by applying fine metal powder
particles to all surfaces of a porous structure) can require a
secondary machining step to obtain the desired shape and dimensions
(e.g., tolerances) of the machined metal foam structure. Such
machining can cause a loss of roughness on the machined surfaces
(e.g., tissue-engaging outer surfaces). The roughness can be
maintained or recovered using textured molds during sintering to
pressure-sinter particles to a substrate without sacrificing
texture for porous bead-coated implants. Alternatively, the
roughness for a metallic foam can be maintained or recovered using
electrical discharge machining ("EDM"), creating a cross-hatch
pattern and, upon implantation, gaps between the grooves in the
coating and bone. These mechanisms have thus far proved
unsatisfactory in increasing the roughness of machined
tissue-engaging outer surfaces of a porous metallic foam ingrowth
structure while maintaining the pore size and porosity of the
structure.
[0008] Therefore, there is a need for an improved method for
providing a porous metallic foam structure with improved bone
ingrowth characteristics that avoids the drawbacks discussed
above.
SUMMARY OF THE INVENTION
[0009] Embodiments of the invention are directed to increasing the
surface roughness of a machined tissue-interfacing outer surface of
a porous structure without altering the pore size or porosity of
the porous structure.
[0010] In one embodiment a prosthetic implant comprises a machined
reticulated porous structure. A powder comprising asymmetric
particles can be disposed on a machined tissue-interfacing outer
surface of the porous structure. The asymmetric particles can have
a size of between about 30% and about 70% of the pore size in the
porous structure so as to increase the surface roughness of the
machined tissue-interfacing outer surface of the implant while
substantially inhibiting the occlusion of the open pores of the
porous structure and/or without substantially modifying the
porosity of the porous structure. In one embodiment, the porous
structure can be a porous metal body. Similarly, the powder can in
one embodiment be a metallic powder. In other embodiments, the
porous structure and powder can be of non-metallic materials.
[0011] In another embodiment a prosthetic implant comprises a
previously machined reticulated porous structure to which one or
more additional layers of powder have been applied to all surfaces
of the previously machined reticulated porous structure. A powder
comprising asymmetric particles can be disposed on a previously
machined tissue-interfacing outer surface of the porous structure.
The asymmetric particles can have a size of between about 30% and
about 70% of the pore size in the porous structure so as to
increase the surface roughness of the previously machined
tissue-interfacing outer surface of the implant while substantially
inhibiting the occlusion of the open pores of the porous structure
and/or without substantially modifying the porosity of the porous
structure
[0012] In accordance with another embodiment, a prosthetic implant
is provided comprising a machined reticulated porous construct
applied to a solid surface. A powder comprising asymmetric powder
particles can be adhered to a machined tissue-interfacing outer
surface of the porous construct. The powder comprises a particle
size configured to increase the surface roughness of the machined
tissue-interfacing outer surface of the porous construct while
substantially maintaining the open pores of the porous
construct.
[0013] In accordance with still another embodiment, a prosthetic
implant is provided comprising a previously machined reticulated
porous construct to which one or more additional layers of powder
have been applied to all surfaces and the construct applied to a
solid surface. A powder comprising asymmetric powder particles can
be adhered to a previously machined tissue-interfacing outer
surface of the porous construct. The powder of asymmetric particles
comprises a particle size configured to increase the surface
roughness of the previously machined tissue-interfacing outer
surface of the porous construct while substantially maintaining the
open pores of the porous construct.
[0014] In accordance with yet another embodiment, a surface layer
is provided comprising a machined reticulated structure and a
powder bonded to a machined tissue-interfacing outer surface of the
reticulated structure. The powder comprises asymmetric titanium
particles with a size of between about 75 microns and about 106
microns.
[0015] In accordance with another embodiment, a surface layer is
provided comprising a previously machined reticulated structure to
which one or more additional layers of powder have been applied to
all surfaces of the previously machined reticulated structure. A
powder comprising asymmetric titanium particles with a particle
size of between about 75 microns and about 106 microns can be
bonded to a machined tissue-interfacing outer surface of the
reticulated structure.
[0016] In accordance with still another embodiment, a method for
increasing the surface roughness of a porous structure is provided.
The method comprises machining a porous structure to a desired
shape and bonding a powder, comprising asymmetric powder particles,
to a machined tissue-interfacing outer surface of the machined
porous structure. The powder particles are sized to increase the
roughness of the machined tissue-interfacing outer surface of the
machined porous structure, while preventing the occlusion of the
pores of the porous structure and/or maintaining the porosity of
the porous structure. In one embodiment, the porous structure is a
porous metal foam and the powder comprises a metallic powder. In
another embodiment the porous structure and powder are of a
non-metallic material.
[0017] In accordance with yet another embodiment, a method for
increasing the surface roughness of a porous structure is provided.
The method comprises machining a porous structure to a desired
shape and applying one or more additional layers of powder to all
surfaces of the porous structure. The method also comprises bonding
a powder, comprising asymmetric powder particles, to a previously
machined tissue-interfacing outer surface of the machined porous
structure, said powder particles being sized to increase the
roughness of the previously machined tissue-interfacing outer
surface of the machined porous structure, while preventing the
occlusion of the pores of the porous structure and/or maintaining
the porosity of the porous structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention will be described herein below by means of
example embodiments which are explained in detail with reference to
the drawings, in which:
[0019] FIG. 1 depicts an enlarged image of a sintered metal foam
pre-form of the prior art. The sintered metal foam pre-form shown
in FIG. 1 is formed using the steps of: 1) providing a 60 ppi
polyurethane (PU) foam skeleton, 2) using a binder, coating said 60
ppi polyurethane (PU) foam skeleton on all of its surfaces with
three layers of fine spherical metallic powder (e.g., spherical
titanium powder) to create a "Pre-form A", 3) subsequently burning
out the PU skeleton from "Pre-form A" as described in reference to
Table 1 at 50.times. magnification to form a green metal foam, 4)
subsequently machining said green metal foam to a desired shape
using a wire electrical discharge machining (WEDM) process, and
then 5) subsequently sintering the machined green metal foam to
form said prior art sintered metal foam pre-form;
[0020] FIG. 2 depicts an enlarged image of an improved sintered
metal foam pre-form according to one embodiment of the present
invention. The improved sintered metal foam pre-form shown in FIG.
2 may be formed using the steps of: 1) providing a 60 ppi
polyurethane (PU) foam skeleton, 2) using a binder, coating said 60
ppi PU foam skeleton on all of its surfaces with two layers of fine
spherical metallic powder (e.g., spherical Ti powder), 3)
subsequently burning out the PU skeleton from the resulting
construct to form a green metal foam, 4) subsequently machining
said green metal foam to a desired shape using a wire electrical
discharge (WEDM) process or the like to form a machined green metal
foam, 5) subsequently applying an additional layer of fine
spherical metallic powder (e.g., spherical Ti powder) to all
surfaces of said machined green metal foam to form a "Pre-form B"
as described in reference to Table 1 at 50.times. magnification,
and then 6) subsequently sintering the Pre-form B to form said
improved sintered metal foam;
[0021] FIG. 3 depicts an enlarged image of a "roughened metal foam"
according to another embodiment of the present invention. The
"roughened metal foam" may be formed using the steps of: 1)
providing "Pre-form A" as discussed above, 2) machining "Pre-form
A" to a desired shape using a wire electrical discharge machining
(WEDM) process or the like, wherein the step of machining forms at
least one machined tissue-interfacing outer surface, 3) applying at
least one layer of asymmetric metallic powder particles (e.g.,
titanium or Ti dehydride particles) to said at least one machined
tissue-interfacing outer surface as described in reference to Table
1 at 50.times. magnification, and 4) sintering the resulting
construct to form said "roughened metal foam";
[0022] FIG. 4 is an enlarged image of a cross-section of the
Roughened Metal Foam of FIG. 3 showing a roughened porous metal
foam structure with a roughened tissue-interfacing outer surface at
50.times. magnification (large image) and 85.times. magnification
(inset image);
[0023] FIG. 5 depicts topographical relief maps of the outer
tissue-interfacing surfaces of "Pre-form A", "Pre-form B, and
"Roughened Metal Foam", respectively, as described in reference to
Table 2;
[0024] FIG. 6 shows SEM images (25.times.) of machined and sintered
metal foam produced using (A) 60 ppi starting polyurethane foam and
(B) 45 ppi starting polyurethane foam, with reference to Table
4.
[0025] FIG. 7 depicts one embodiment of a method for preparing a
porous foam structure with a tissue-engaging outer surface having
increased roughness.
[0026] FIG. 8 depicts another embodiment of a method for preparing
a porous foam structure with a tissue-engaging outer surface having
increased roughness without affecting the porosity and pore size of
the porous structure.
[0027] FIG. 9 depicts an embodiment of a femoral stem of a hip
joint prosthesis with a roughened tissue-interfacing outer
surface;
[0028] FIG. 10 depicts an embodiment of an acetabular shell of a
hip joint prosthesis with a roughened tissue-interfacing outer
surface;
[0029] FIG. 11 depicts an embodiment of a shoulder prosthesis with
a roughened tissue-interfacing outer surface; and
[0030] FIG. 12 depicts one embodiment of a knee joint prosthesis
with a tissue-interfacing outer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] The embodiments disclosed herein provide a porous structure
with increased surface roughness on a machined tissue-interfacing
outer surface of the structure and methods of fabricating the same.
The machined tissue-interfacing outer surface generally benefits
from an increased roughness created by the application of a powder
to a porous structure (e.g., porous metal body, porous foam
material).
[0032] Generally, a tissue-interfacing outer surface with increased
roughness can be applied to a porous metallic structure, a formed
structure, the surface of a pre-formed structure, or some other
object. In the case of medical articles, a bioinert material such
as titanium, titanium alloys, tantalum, tantalum alloys,
cobalt-chromium alloys, zirconium, zirconium alloys, and the like
can be used for the porous structure. However, other suitable
metallic and non-metallic materials can be used. Such non-metallic
materials can include osteoconductive ceramics such as, for
example, calcium phosphates (e.g., alpha and beta tricalcium
phosphates, hydroxyapatite, etc). The material may be molded,
machined, or processed in any known manner to a desired shape.
Further, the material may be solid, in foam form (such as, for
example, polyurethane foam), or a foam previously applied to a
solid metal substrate composed e.g. of titanium, titanium alloys,
tantalum, tantalum alloys, cobalt-chromium alloys, zirconium,
zirconium alloys, or other suitable metallic and non-metallic
materials.
[0033] Notably, as discussed above, machining (e.g. wire electrical
discharge machining ("WEDM")) can reduce the surface roughness
initially provided to a structure. When the structure is for
example a medical article to be implanted in bone, the reduced
roughness can decrease any scratch-fit against the bone surface and
reduce implant stability. As discussed above, roughness can be
recovered using textured molds or using WEDM to cut grooves into
the structure. Additionally, as known in the art, surface roughness
can be recovered following machining with fine powder (e.g.,
particle size <45 .mu.m) layer(s) that can be applied to all
surfaces of a pre-form foam structure. However, this process does
not achieve the desired level of surface roughness in the machined
tissue-interfacing outer surface of the pre-form foam structure
(see Table 1, below) to increase the scratch-fit of the pre-form
structure against an interfacing surface (e.g., bone). Moreover,
such a process disadvantageously reduces the porosity of the
pre-form structure, which may result in the clogging or occlusion
of the pores in the pre-form structure, thereby reducing the
ability of bone to intergrow within the porous structure.
[0034] In some embodiments the powder can be chosen to optimally
increase roughness while maintaining pores open to the surface. In
a preferred embodiment a coarse powder having a particle size of
between about 75 and 106 .mu.m can be applied to a machined
tissue-interfacing outer surface of the pre-form metal foam
structure, as further described below, to increase the roughness of
said tissue-interfacing outer surface without altering the porosity
and pore size of the porous structure. However, said coarse
particles can have other suitable sizes. In one embodiment, the
porous structure can have a porosity of between about 40% and about
85%. In another embodiment, the porous structure can have a
porosity of between about 60% and about 80%.
[0035] In one embodiment, the porous structure can have an average
pore size of between about 50 .mu.m and about 1000 .mu.m measured
using a scanning electron microscope (SEM) or 2D metallographic
techniques. In another embodiment, the porous structure can have an
average pore size of between about 100 .mu.m and about 500 .mu.m.
In still another embodiment, the porous structure can have an
average pore size of about 200 .mu.m. However, the porous structure
can have other pore sizes. Additionally, the pore size of the
porous structure (e.g., polyurethane foam) used to create the
pre-form metal foam can be varied to affect the end pore size.
[0036] In a preferred embodiment, the size of the coarse powder
particles can be between about 10% and 30% of the pore size of the
porous structure. In another embodiment, the size of the coarse
powder particles can be between about 30% and 70% of the pore size
of the porous structure. In still another embodiment, the size of
the coarse powder particles can be between about 40% and about 60%
of the pore size of the porous structure. However, the coarse
powder particles can have other suitable sizes relative to the pore
size of the porous structure so as to allow particles that are not
bound to the machined tissue-interfacing outer surface of the
porous structure to easily pass through the pores of the porous
structure to inhibit (e.g., prevent) the clogging or occlusion of
the pores in the porous structure.
[0037] The powder particles can be applied by dipping, spraying,
sprinkling, electrostatic methods, or any other appropriate
methods. In one embodiment, a binder can be applied to the machined
tissue-interfacing outer surface of the machined metal foam
structure. The porous structure can then be dipped into a layer of
coarse powder particles to coat the machined tissue-interfacing
outer surface with said coarse powder particles. In another
embodiment, the coarse powder particles can be sprinkled onto the
machined tissue-interfacing outer surface of the porous structure
after the binder has been applied to said surface. As discussed
above, the coarse powder particles are preferably sized to allow
particles that do not adhere to the tissue-interfacing outer
surface to easily pass through the porous structure so as to
inhibit (e.g., prevent) the clogging or occlusion of the pores in
the porous structure. In still another embodiment, the coarse
powder particles can be sprayed onto the machined
tissue-interfacing outer surface of the porous structure after the
binder has been applied to said surface.
[0038] Further, the powder can have other properties. In one
embodiment the coarse powder particles can be generally asymmetric,
which can provide additional roughness for a given particle size.
The fine and coarse powders can be of a variety of materials, such
as titanium powder, commercially pure titanium powder ("cpTi"),
titanium hydride, and titanium dehydride. However the powder can
include other suitable metallic materials, such as titanium alloy,
cobalt-chrome alloy, tantalum, zirconium, and zirconium alloy, and
suitable non-metallic materials, such as calcium phosphates,
hydroxyapatite, etc.
[0039] The fine and coarse powder can be applied by a variety of
methods. For example, a binder can first be applied to the porous
structure, such as a polyurethane foam. Then, a layer of powder can
be applied to the porous structure. The porous structure can then
be sintered such that the powder bonds to the structure. In other
embodiments, the metal foam structure to which the fine and coarse
powder particles have been applied can be attached to some other
structure (e.g., implant substrate), if desired.
[0040] More specifically, in one embodiment a polyurethane foam can
be provided, which can be cut to a desired size. The cut
polyurethane foam can then be impregnated with a binder. A fine
powder, such as cpTi, can then be applied to all surfaces of the
polyurethane foam to form a starting metal foam structure. In one
embodiment, the fine powder can be applied in one or two layers, or
more if desired, with binder applied to the porous structure before
application of each layer of powder. In another embodiment, the
fine powder can be applied in one to four layers, or more if
desired. Preferably, the fine powder is applied in sufficient
layers to the polyurethane foam to form a porous structure having
the desired characteristics (e.g., cell size, interconnecting pore
size, average pore diameter, porosity, strength) for a particular
application (e.g., medical applications where the structure
provides for bone ingrowth) after the final sintering step. As used
herein, a pore can be an interstitial pore in the exterior or
interior of the foam or porous structure, struts can be the
structural elements that define the pores, and the cell can be the
volume defined by struts with the pores defined on an outer
circumference of the cell. The starting metal foam is then heated
at a temperature substantially above the decomposition temperature
of polyurethane to burn out the polyurethane and form a green metal
foam structure. The green metal foam structure can then be machined
(e.g. WEDM) to desired shape to form a pre-form metal foam
structure, which as described above, can result in a reduction of
the roughness of the machined tissue-interfacing outer surface of
the pre-form metal foam structure. In one embodiment, the number of
layers of powder applied to the polyurethane foam prior to
machining or wire EDM is just enough to increase foam strength to
allow for machining of the green metal foam structure while
inhibiting damage to the foam structure.
[0041] Following the machining of the green metal foam structure,
additional layers of fine powder can in one embodiment be applied
to all surfaces of the pre-form metal foam structure to further
strengthen and roughen the porous structure in order to achieve a
desired structure strength and pore size (e.g., for a particular
application) upon final sintering. Again, the powder here can be
applied in one or more layers, as desired.
[0042] Once the machined pre-form metal foam structure has the
desired strength and pore size (e.g., via the application of powder
layers, as discussed above), a binder can be applied to a machined
tissue-interfacing outer surface of the porous structure. In a
preferred embodiment, one or more layers of coarse powder particles
(e.g., asymmetric particles) can be applied to the binder-coated
machined tissue-interfacing outer surface of the pre-form metal
foam structure, as described above, to form a roughened pre-form.
The coarse powder particles can be applied to the binder-coated
machined tissue-interfacing outer surface by spraying, brushing, or
sprinkling the coarse powder onto the binder-coated outer surface,
or by dipping the binder-coated outer surface into a layer of
coarse powder. The coarse powder can then be sintered onto the
binder-coated outer surface to form a roughened metal foam. In
another embodiment, the metal powder particles can be coated with
binder and applied to the machined tissue-interfacing outer surface
of the pre-form metal foam structure. In one embodiment, the
roughened pre-form structure can be attached to a substrate before
the coarse powder is sintered onto the binder-coated outer surface
of the roughened pre-form structure.
[0043] In another embodiment, a porous titanium foam pre-form that
has been machined to size can be provided. A layer of binder can be
applied to the machined tissue-interfacing outer surface of the
pre-form structure, followed by a coarse metal powder (such as, for
example, cpTi or titanium hydride), to form a roughened pre-form
structure. The roughened pre-form structure can then be put through
a final sintering, bonding the coarse powder to the pre-form to
produce a roughened metal foam structure.
[0044] Samples of machined and sintered titanium foam pieces with
and without added powder layers have been tested. The texture of
the machined tissue-interfacing outer surface of the samples was
determined by measuring the coefficient of linear friction of said
surface. The linear friction was measured against rigid
polyurethane foam (used to simulate cancellous bone) using an
orthopedic friction and wear testing machine (OrthoPod), where a
normal load of approximately 44 N was applied to the sample part
against the polyurethane foam and the foam rotated in an arc shaped
motion at a displacement rate of about 3.8 mm/sec. Further details
of the linear friction test methodology used can be found in
"Friction Evaluation of Orthopedic Implant Surfaces Using a
Commercially Available Testing Machine," Gilmour et al., abstract
#464 World Biomaterials Congress 2008, the contents of which are
incorporated herein by reference in their entirety and should be
considered a part of this specification, and which is attached as
Appendix A.
[0045] Table 1 shows the friction results for three types of
sintered Ti foam surfaces: (1) a pre-form machined by WEDM from a
green metal foam formed by coating a 60 ppi PU foam on all its
surfaces with three layers of fine (<45 .mu.m) spherical Ti
powder, in which all three layers were applied before machining
("Pre-form A"), illustrated in FIG. 1; (2) a pre-form machined by
WEDM from a green metal foam formed by coating a 60 ppi PU foam on
all its surfaces with three layers of fine (<45 .mu.m) spherical
Ti powder, in which two powder layers were applied before machining
and one was applied after machining ("Pre-form B"), illustrated in
FIG. 2; and (3) Pre-form A with one layer of coarse (75-106 .mu.m)
asymmetric Ti (Ti dehydride) powder applied after machining to the
outer tissue-interfacing surfaces ("Roughened Metal Foam"),
illustrated in FIGS. 3-4. As shown, the surface with a large
asymmetric powder applied after machining had the highest
coefficient of linear friction as compared to the other
surfaces.
TABLE-US-00001 TABLE 1 Linear Friction Testing Coefficient of Test
Sample Linear Friction Pre-form A 0.90 .+-. 0.09 Pre-form B 0.98
.+-. 0.02 Roughened Metal Foam 1.09 .+-. 0.10 (n = 3 per group)
[0046] FIG. 1 shows sintered metal foam "Pre-form A" where the
machined tissue-interfacing outer surface of the porous metal foam
structure has not been roughened, as discussed in embodiments
herein. The pre-form metal foam structure has a cell size diameter
of approximately 600 .mu.m with interconnecting pores of
approximately 200 .mu.m in diameter.
[0047] The overall average pore diameter (mean void intercept
length (MVIL)) is approximately 464.4.+-.95.4 .mu.m. The average
thickness of a strut (e.g., the support element that defines the
cell) of the non-roughened metal foam is approximately 150 .mu.m.
The average gravimetric porosity of the metal foam was
75.2.+-.2.7%. Linear friction tests of the machined
tissue-interfacing outer surface of "Pre-form A" resulted in a
maximum linear friction coefficient of 0.90.+-.0.09.
[0048] FIG. 2 shows sintered metal foam "Pre-form B" with a fine
metal powder applied to all surfaces of the machined porous metal
foam structure (i.e., the pre-form metal foam structure). Pre-form
B in FIG. 2 includes one layer of fine (<45 .mu.m) spherical
cpTi powder applied to the all surfaces of the pre-form structure
after machining of the green metal foam structure. Linear friction
tests of the machined tissue-interfacing outer surface of "Pre-form
B" with the layer of fine spherical Ti powder applied after
machining resulted in a maximum linear friction coefficient of
0.98.+-.0.02.
[0049] FIGS. 3 and 4 illustrate a sintered "Roughened Metal Foam"
structure with a roughened machined tissue-interfacing outer
surface achieved according to a preferred embodiment of the
invention. As shown in FIGS. 3-4, a layer of metal powder was
applied to the machined tissue-interfacing outer surface of a
pre-form metal foam structure such that the overall pore size and
porosity of the porous metal foam are not substantially altered.
The metal powder applied to the pre-form metal foam illustrated in
FIGS. 3 and 4 for increasing the roughness of the machined
tissue-interfacing outer surface of the pre-form metal foam was
asymmetric titanium powder with particles approximately 75-106
.mu.m in size. Because the powder was applied only to the machined
tissue-interfacing outer surface, the average cell size diameter
and interconnecting pore size was not substantially different from
the pre-form metal foam structure following application of the
powder (e.g., MVIL of Roughened Metal Foam is approximately
448.9.+-.34.5). Furthermore, the average gravimetric porosity of
the roughened pre-form metal foam structure was substantially
unchanged from that of the pre-form metal foam structure and is
approximately 75.3.+-.2.2%. Linear friction tests of the machined
tissue-interfacing outer surface of the "Roughened Metal Foam" with
the layer of coarse asymmetric Ti powder applied after machining
resulted in a maximum linear friction coefficient of
1.09.+-.0.10.
[0050] As depicted in FIG. 5, white light interferometry was used
to determine the difference in surface roughness of the metal foam
struts on the machined tissue-interfacing outer surface of the
sintered Ti Foam structures under the following conditions:
"Pre-form A" (Wire EDM Surface) shown in FIG. 1; "Pre-form B" (Wire
EDM surface plus one layer of fine spherical Ti powder on all
surfaces after machining of the green state metal foam structure),
as shown in FIG. 2; and "Roughened Metal Foam" (Pre-form A plus one
layer of coarse (75-106 .mu.m) asymmetric Ti (Ti dehydride) powder
applied to the outer tissue-interfacing surfaces after machining of
the green state metal foam structure), as shown in FIGS. 3-4. The
results are given in Table 2, with "Ra" representing the average
roughness of all points from a plane fit to the test part surface,
and "SRz" representing the average of the largest half of the
radial peak-to-valley areal roughness results. The Roughened Metal
Foam Ti Foam surface had the largest roughness values, followed by
the Ti Foam "Pre-form B" with the fine spherical powder applied to
all surfaces after machining of the green state metal foam
structure and the machined "Pre-form A" Ti Foam. These results are
reflective of the tactile feel of the surfaces, with the large
asymmetric powder coated Ti Foam sample having the roughest
feel.
TABLE-US-00002 TABLE 2 White Light Interferometry Results Test
Sample Ra(.mu.m) SRz(.mu.m) Pre-form A 2.3 .+-. 0.5 19.6 Pre-form B
6.2 .+-. 0.7 40.6 Roughened Metal Foam 9.9 .+-. 2.1 57.7
[0051] With reference to FIG. 1, white light interferometry
roughness measurements of the machined tissue-interfacing outer
surfaces of the "Pre-form A" metal foam structure resulted in an
average roughness (Ra) of 2.3.+-.0.50 .mu.m.
[0052] With reference to FIG. 2, white light interferometry
roughness measurements of the machined tissue-interfacing outer
surface of the "Pre-form B" metal foam structure with said
additional layer of fine spherically-shaped metal particles applied
to all surfaces of the pre-form structure resulted in an average
roughness (Ra) of about 6.2 .mu.m.
[0053] With reference to FIGS. 3-4, white light interferometry
roughness measurements of the roughened metal foam structure
resulted in an increase in average roughness (Ra) of 9.9.+-.2.1
significantly greater than the roughness of either non-roughened
metal foam (Pre-form A or Pre-form B).
[0054] A summary of the properties describing the pre-form metal
foam structure and roughened metal foam structure as shown in FIGS.
1 and 3-4, respectively, is given in Table 3.
TABLE-US-00003 TABLE 3 Properties of Sintered Pre-form Metal Foam
and Roughened Metal Foam Pre-form A Roughened Metal Foam Metal Foam
Cell Size Diameter (microns) ~600 ~600 Interconnecting Pore Size
(microns) ~200 ~200 Average Pore Diameter (MVIL) (microns) 464.4
.+-. 95.4 448.9 .+-. 34.5 Gravimetric Porosity (%) 75.2 .+-. 2.7
75.3 .+-. 2.2 Strut Roughness (Ra) (microns) 2.3 .+-. 0.50 9.9 .+-.
2.1 Maximum Coefficient of Friction 0.90 .+-. 0.09 1.09 .+-.
0.10
[0055] Of the powders used to roughen the Ti Foam surface, the
Titanium Dehydride Powder -140 +200 Mesh (75-106 .mu.m), resulted
in the bone interface surface with the highest friction, largest
roughness value, and roughest texture as assessed by tactile
feel.
[0056] In other embodiments, the pre-form metal foam structure can
have variations in pore size and strut thickness. Additionally, the
powder applied to the machined tissue-interfacing outer surface to
increase its roughness can, in other embodiments, have a particle
size greater than 106 .mu.m or smaller than 75 .mu.m. In another
embodiment, the shape of the metal powder particles deposited on
the machined tissue-interfacing outer surface of the pre-form metal
foam structure can be shapes other than asymmetric. Additionally,
the metal powder particles need not have a uniform shape.
[0057] Additional variations can involve the types of powder used
and steps taken after the application of the powder. For example,
different types and sizes of powder can be applied to different
portions of an implant, for example where different portions of the
implant will interface with different types of tissue. Further,
different types and sizes of powder can be layered, so as to
produce, for example, a fractal-like effect of roughness at varying
sizes overlaid on one-another. Varying roughness sizes can allow
different mechanisms of attachment with surrounding body tissue,
such as simultaneously allowing tissue ingrowth at a macroscopic
scale, while also allowing cellular adhesion to an implant surface
at a smaller scale. To accomplish such varying roughness sizes, the
different powders can be applied sequentially, creating for example
a size gradient with a top surface of small-scale roughness and
larger roughness directly beneath. Alternatively, in one embodiment
the different powders can be applied simultaneously, creating a
heterogeneous mix of roughness sizes.
[0058] In some embodiments, as the pore size increases, the strut
thickness can also increase (see Table 4 and FIG. 6). Both
properties dictate the size range of powder that can be used to
roughen the machined tissue-interfacing outer surface of the
pre-form metal foam structure while maintaining an open surface
porosity. The powder applied to the tissue-interfacing outer
surface of the pre-form metal foam structure is preferably sized to
inhibit (e.g., prevent) surface pore occlusion. In a preferred
embodiment, powder applied to the tissue-interfacing outer surface
of the machined foam metal structure has a size of approximately
<100% of the strut thickness and about .ltoreq.50% of the pore
size, so as to advantageously inhibit pore occlusion.
TABLE-US-00004 TABLE 4 Pore Size and Strut Thickness for Two Metal
Foams of Different Pore Densities. Starting Polyurethane Pore Size
Strut Thickness Foam Density (MVIL) (microns) (microns) 60 ppi
464.4 .+-. 95.4 146 .+-. 26 45 ppi 618.4 .+-. 57.9 365 .+-. 73
(Note: Starting Polyurethane Foam was coated with the same number
of metal powder layers to produce the 60 pores per inch (ppi) and
45 ppi Pre-form Metallic Foams.)
[0059] The shape and size of the surface roughening powder affects
the roughness and frictional values of the roughened metal foam.
Roughness and friction properties of a sintered Pre-form A metal
foam structure (a WEDM surface) and a sintered Pre-form B metal
foam structure (a WEDM surface with a layer of fine (<45)
spherical powder applied after machining to all surfaces) are
compared to Roughened Metal Foam with either fine asymmetric powder
(<45 .mu.m) or coarse asymmetric powder (75-106 .mu.m), as shown
in Table 5.
TABLE-US-00005 TABLE 5 Properties of Sintered Pre-form Metal Foams
A and B (Not Roughened), Fine Asymmetric Powder Roughened Metal
Foam, and Coarse Asymmetric Roughened Metal Foam Fine Coarse
Asymmetric Asymmetric Pre-form A Pre-form B Roughened Roughened
Metal Foam Metal Foam Metal Foam Metal Foam Strut 2.3 .+-. 0.50 6.2
.+-. 0.70 6.4 .+-. 0.98 9.9 .+-. 2.1 Roughness (Ra) (microns)
Maximum 0.90 .+-. 0.09 0.98 .+-. 0.02 0.97 .+-. 0.01 1.09 .+-. 0.10
Coefficient of Friction
[0060] Use of powders also provides advantages over other methods.
For example, the application of such powders can be simpler,
easier, and cost effective and does not introduce grooves that
would result in gaps between the bone and ingrowth structure upon
implantation. Unlike overlying grids, the powder can be easily
applied to almost any arbitrary geometry. Further, the powders can
allow increases of roughness with relative precision (e.g., close
tolerances) in regard to the end roughness of the piece, as well as
the final geometry of the piece.
[0061] The layers described herein can be used with a number of
medical articles. For example, the layer can be applied to a bulk
metal foam augment to fill a bone void, a metallic foam-coated
implant for a knee implant, hip implant, shoulder or spinal
application, a tibial tray, acetabular shell, femoral stem, stem
collar, other knee femoral components, or other medical implants or
articles.
[0062] FIG. 7 illustrates one embodiment of a method 100 for
preparing a roughened metal foam structure with a tissue-engaging
machined outer surface having increased roughness without affecting
the porosity and pore size of the porous structure. The method 100
includes cutting 110 a polyurethane foam having a desired pore size
to a desired size and impregnating 120a the foam with a binder
(e.g., a thermally decomposing binder), after which a first layer
of fine powder (e.g., a bioinert metallic powder such as titanium,
titanium alloy, tantalum, tantalum alloy, cobalt-chromium alloys,
zirconium, zirconium alloys, etc.) is applied to the foam to form a
starting metal foam. In the illustrated embodiment, the fine powder
having a particle size of less than 45 .mu.m is applied 130a to all
surfaces of the porous polyurethane foam. The method 100 further
includes impregnating 120b the starting metal foam with binder and
applying 130b a second layer of fine powder, after which the
starting metal foam is further impregnated 120c with binder and a
third layer of fine powder is applied 130c. However, more or fewer
than three layers of fine powder can be applied so as to achieve
the desired characteristics (e.g., pore size and strength
requirements) of the starting metal foam, as discussed above. The
method 100 additionally includes burning out 140 the polyurethane
to provide a green metal foam structure. The green metal foam
structure can then be machined 150 to provide a pre-form metal foam
structure. The steps 110-150 above for providing a pre-form metal
foam structure are known in the art.
[0063] Advantageously, in the embodiments of the invention
disclosed herein, the method 100 further includes applying 180 a
binder to bone-interfacing machined outer surface of the pre-form
metal foam structure and applying 190 a layer of coarse asymmetric
powder with a particle size of between about 75 .mu.m and 106 .mu.m
thereonto to form a roughened pre-form structure. Preferably, the
layer of coarse asymmetric powder is deposited only on the
bone-interfacing machined outer surface (e.g., the coarse particles
are sized relative to the pores so that particles that are not
deposited on the bone-interfacing machined outer surface pass
through the pores of the metallic foam structure without clogging
or occluding the pores of the structure). Though the method 100
discloses applying one layer of coarse powder particles, one of
ordinary skill in the art will recognize that any suitable number
of layers of coarse metal powder particles can be applied. The
method 100 optionally includes attaching 195 the roughened pre-form
structure to a substrate. The layer of coarse powder is then
sintered 200 on the bone-interfacing outer surfaces of the
roughened pre-form structure to form the roughened metal foam.
[0064] FIG. 8 illustrates another embodiment of a method 100' for
preparing a porous foam structure with a tissue-engaging outer
surface having increased roughness without affecting the porosity
and pore size of the porous structure. The method 100' is similar
to the method 100 illustrated in FIG. 7 so that similar steps are
identified with identical numerical identifiers. The method 100'
differs from the method 100 in that the starting metal foam is
twice impregnated 120a, 120b with a binder, and only two layers of
fine powder are applied 130a, 130b to all surfaces of the starting
metal before machining of the green state metal foam to provide a
pre-form metal foam structure. As discussed above, the process of
forming the pre-form metal foam structure is known in the art.
[0065] Advantageously, the method 100' includes impregnating 160
the pre-form metal foam structure with binder and applying 170 a
third layer of fine powder to all surfaces of the pre-form metal
foam structure. However, one of ordinary skill in the art will
recognize that any suitable number of layers of metal powder can be
applied before and/or after the machining of the green state metal
foam structure to achieve the desired characteristics of the metal
foam structure, as discussed above. A layer of binder 180 and
asymmetric powder 190 is similarly applied and sintered 200 to the
machined tissue-interfacing outer surface to increase the roughness
of the pre-form metal foam so as to provide a roughened metal foam
without altering the overall pore size and porosity of the
structure so as to inhibit (e.g., prevent) clogging of the pores in
the roughened metal foam structure.
[0066] Embodiments of medical implants that can incorporate the
roughened tissue-interfacing outer surface on a porous structure,
as described in the embodiments above, are depicted in FIGS.
9-12.
[0067] FIG. 9 depicts an embodiment of a femoral stem 310 of a hip
joint prosthesis with a roughened tissue-interfacing porous outer
surface, as further described in U.S. Pat. No. 6,540,788, the
contents of which are hereby incorporated by reference and should
be considered a part of this specification. For example, the outer
surface of one or more of the anterior/posterior sides 312, lateral
side 314 and medial side 316 of the femoral stem 310 can include a
roughened porous structure having a roughened tissue-interfacing
outer surface, as described above, to improve its fixation in a
femoral cavity. In one embodiment, the substrate material of the
femoral stem 310 can undergo a surface treatment (e.g., grit
blasting), after which the roughened porous structure (e.g.,
roughened metal foam, as described above) can be applied to the
substrate.
[0068] Similarly, FIG. 10 depicts an embodiment of an acetabular
shell 320 for a hip joint prosthesis, as further described in U.S.
Pat. No. 6,537,321, the contents of which are hereby incorporated
by reference and should be considered a part of this specification.
The outer surface 322 of the acetabular shell 320 can include a
roughened porous structure with a roughened tissue-interfacing
outer surface, as discussed above, to advantageously increase the
scratch fit of the acetabular shell 320 against the bone (e.g., the
acetabulum) into which its implanted, as well as allow for bone
ingrowth into the porous structure to provide for greater stability
of the implanted acetabular shell 320.
[0069] FIG. 11 depicts an embodiment of a shoulder prosthesis
including a glenoid prosthesis 330, as further described in U.S.
Publication No. 2006-0111787, the contents of which are hereby
incorporated by reference and should be considered a part of this
specification. The anchoring surfaces 332, 334 of the glenoid
prosthesis 330 can include a roughened porous structure with a
roughened tissue-interfacing outer surface, to facilitate anchoring
of the glenoid prosthesis in the scapula of a shoulder blade.
Similarly, bone engaging surfaces 342, 344 of the humerus stem 340
of the shoulder prosthesis can have a roughened porous structure
with a roughened tissue-interfacing outer surface, as described in
the embodiments above, which can advantageously improve the
scratch-fit of the stem in bone, as well as allow bone ingrowth
into the porous structure to provide improve stability of the stem
following implantation.
[0070] FIG. 12 depicts an embodiment of a knee joint prosthesis 350
including a femoral component 352 and a tibial component 360, as
further described in U.S. Pat. No. 5,954,770, the contents of which
are hereby incorporated by reference and should be considered a
part of this specification. The bone engaging surfaces of the
femoral component prosthesis 352, including the internal anterior
354 and posterior 356 condyle surfaces, the interior surface of the
patellar shield 358, and the femoral anchoring stem 359 can include
a roughened porous structure with a roughened bone-interfacing
outer surface that can be formed as disclosed in embodiments
herein. Similarly, bone engaging surfaces of the tibial stem
prosthesis 360, including exterior surfaces of the tibia plateau
362, 364 and tibia shaft 366 can include a roughened porous
structure with a bone-interfacing outer surface formed as described
in the embodiments above, to provide an increase scratch fit of the
tibial stem prosthesis 360 in bone, as well as to allow for bone
ingrowth into the porous structure, thereby providing improved
stability of the tibial stem prosthesis 360 following
implantation.
[0071] The embodiments of the invention described herein can also
be incorporated into a porous augment that can be implanted into a
void in bone or can be used to fill a void, crack, cavity or other
opening in bone, whether naturally occurring or surgically
created.
[0072] Although the foregoing systems and methods have been
described in terms of certain preferred embodiments, other
embodiments will be apparent to those of ordinary skill in the art
from the disclosure herein. Additionally, other combinations,
omissions, substitutions and modifications will be apparent to the
skilled artisan in view of the disclosure herein. While certain
embodiments of the inventions have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
methods and systems described herein may be embodied in a variety
of other forms without departing from the spirit thereof.
Accordingly, other combinations, omissions, substitutions and
modifications will be apparent to the skilled artisan in view of
the disclosure herein.
* * * * *