U.S. patent application number 14/078614 was filed with the patent office on 2021-01-21 for porous composite biomaterials and related methods.
The applicant listed for this patent is Gabriel L. Converse, Ryan K. Roeder, Stephen M. Smith. Invention is credited to Gabriel L. Converse, Ryan K. Roeder, Stephen M. Smith.
Application Number | 20210015977 14/078614 |
Document ID | / |
Family ID | 1000005313071 |
Filed Date | 2021-01-21 |
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United States Patent
Application |
20210015977 |
Kind Code |
A9 |
Roeder; Ryan K. ; et
al. |
January 21, 2021 |
POROUS COMPOSITE BIOMATERIALS AND RELATED METHODS
Abstract
Synthetic composite materials for use, for example, as
orthopedic implants are described herein. In one example, a
composite material for use as a scaffold includes a thermoplastic
polymer forming a porous matrix that has continuous porosity and a
plurality of pores. The porosity and the size of the pores are
selectively formed during synthesis of the composite material. The
example composite material also includes a plurality of a
anisometric calcium phosphate particles integrally formed, embedded
in, or exposed on a surface of the porous matrix. The calcium
phosphate particles provide one or more of reinforcement,
bioactivity, or bioresorption.
Inventors: |
Roeder; Ryan K.; (Granger,
IN) ; Converse; Gabriel L.; (Basehor, KS) ;
Smith; Stephen M.; (South Bend, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Roeder; Ryan K.
Converse; Gabriel L.
Smith; Stephen M. |
Granger
Basehor
South Bend |
IN
KS
IN |
US
US
US |
|
|
Prior
Publication: |
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Document Identifier |
Publication Date |
|
US 20140236299 A1 |
August 21, 2014 |
|
|
Family ID: |
1000005313071 |
Appl. No.: |
14/078614 |
Filed: |
November 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12039666 |
Feb 28, 2008 |
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14078614 |
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60904098 |
Feb 28, 2007 |
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60939256 |
May 21, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29C 67/202 20130101;
A61L 27/56 20130101; A61L 27/26 20130101; A61L 27/54 20130101; A61L
27/18 20130101; A61L 27/12 20130101; A61F 2/4455 20130101 |
International
Class: |
A61L 27/56 20060101
A61L027/56; A61F 2/44 20060101 A61F002/44; A61L 27/18 20060101
A61L027/18; A61L 27/12 20060101 A61L027/12; A61L 27/26 20060101
A61L027/26; A61L 27/54 20060101 A61L027/54; B29C 67/20 20060101
B29C067/20 |
Claims
1. A porous reinforced composite scaffold material, comprising: a
thermoplastic polymer material having essentially uniform porosity
and comprising a plurality of anisometric calcium phosphate
particles distributed essentially uniformly throughout, wherein the
composite comprises a plurality pores that are defined by voids
interconnected by struts, the size of the pores being either
homogenous or heterogeneous and within the range from about 10 to
500 .mu.m, and wherein the calcium phosphate particles comprise
single crystals, dense polycrystals or combinations thereof, and
wherein the calcium phosphate particles are embedded within the
thermoplastic polymer material and exposed on the struts within the
pore voids.
2. A porous reinforced composite scaffold material as described in
claim 1, wherein the porosity ranges between 1 to 95 percent by
volume.
3. A porous reinforced composite scaffold material as described in
claim 1, wherein the anisometric calcium phosphate comprises one or
more of hydroxyapatite, calcium-deficient hydroxyapatite,
carbonated calcium hydroxyapatite, beta-tricalcium phosphate
(beta-TCP), alpha-tricalcium phosphate (alpha-TCP), amorphous
calcium phosphate (ACP), octacalcium phosphate (OCP), tetracalcium
phosphate, biphasic calcium phosphate (BCP), anhydrous dicalcium
phosphate (DCPA), dicalcium phosphate dihydrate (DCPD), anhydrous
monocalcium phosphate (MCPA), monocalcium phosphate monohydrate
(MCPM), and combinations thereof, and, wherein the polymer
comprises one or more of polyaryletherketone, polyetheretherketone
(PEEK), polyetherketonekteone (PEKK), polyetherketone (PEK),
polyethylene, high density polyethylene (HDPE), ultra high
molecular weight polyethylene (UHMWPE), low density polyethylene
(LDPE), polyethylene oxide (PEO), polyurethane, polypropylene,
polypropylene oxide (PPO), polysulfone, polymethylmethacrylate
(PMMA), and other polyacrylics from monomers such as bisphenol a
hydroxypropylmethacrylate (bis-GMA) or tri(ethylene glycol)
dimethacrylate (TEG-DMA), polypropylene, poly(e-caprolactone)
(PCL), poly(dioxanone) (PDO), poly(glyconate),
poly(hydroxybutyrate) (PHB), poly(hydroxyvalerate (PHV),
poly(orthoesters), poly(carboxylates), poly(propylene fumarate),
poly(phosphates), poly(carbonates), poly(anhydrides),
poly(iminocarbonates), poly(phosphazenes), poly(DL-lactide) (PDLA),
poly(L-lactide) (PLLA), poly(glycolide) (PGA), copolymers thereof,
and blends thereof, and wherein the porogen is selected from NaCl
particles, wax, polysaccharides, and cellulose.
4. A porous reinforced composite scaffold material as described in
claim 3, wherein the anisometric calcium phosphate particles
comprise hydroxyapatite whiskers, and wherein the scaffold
comprises between 1 to 50 percent by volume of hydroxyapatite
whiskers.
5. A porous reinforced composite scaffold material as described in
claim 1, wherein the reinforcement particles have a mean aspect
ratio (length along c-axis/length along a-axis) of greater than 1
and less than 100.
6. A porous reinforced composite scaffold material as described in
claim 1, wherein the size of the reinforcement particles ranges
between 20 nm and 2 mm.
7. A porous reinforced composite scaffold material as described in
claim 1, comprising one or more additives selected from growth
factors, transcription factors, matrix metalloproteinases,
peptides, proteins, bone cells, progenitor cells, blood plasma,
bone marrow aspirate, and combinations thereof, and wherein the one
or more additives coat the scaffold material, or are contained
within some or all of the pore void space, or combinations of
these.
8. A porous reinforced composite scaffold material as described in
claim 7, comprising a Bone Morphogenetic Protein (BMP).
9. A porous reinforced composite scaffold material as described in
claim 1, wherein the scaffold material is part of a synthetic bone
graft, a bone ingrowth surface applied to an existing implant, or
tissue scaffold.
10. A porous reinforced composite scaffold material as described in
claim 1, wherein the porosity is either variable or uniform from a
center of the material to an exterior surface.
11. A porous reinforced composite scaffold material as described in
claim 10, wherein the porosity is either functionally graded
radially from a highly porous center to a relatively dense exterior
surface or functionally graded radially from a relatively dense
center to a highly porous exterior surface.
12. A porous reinforced composite scaffold material as described in
claim 1, further comprising one or a plurality of any one or
combination of features selected from holes, notches, pins,
radiographic markers, gripping features for positioning of the
implantable device by surgical tools.
13. A porous reinforced composite scaffold material as described in
claim 1, wherein the material is formed according to the method set
forth in claim 14.
14. A method of forming a reinforced composite scaffold material,
the method comprising the steps of: providing a thermoplastic
polymer and anisometric calcium phosphate particles; uniformly
mixing the polymer and the calcium phosphate particles; providing a
selection of porogen materials having homogenous or heterogenous
particle size within the range from about 10 to 500 .mu.m selecting
a porogen material to achieve desired scaffold porosity, and
uniformly mixing the selected porogen with the polymer and calcium
phosphate mixture; processing the mixture to form a solid composite
material; leaching the porogen material from the solid composite
form; whereby, a composite material of essentially uniform
continuous porosity is formed, the composite material comprising
pores sized from about 10 to 500 .mu.m.
15. A method as described in claim 14, wherein the anisometric
calcium phosphate comprises one or more of hydroxyapatite,
calcium-deficient hydroxyapatite, carbonated calcium
hydroxyapatite, beta-tricalcium phosphate (beta-TCP),
alpha-tricalcium phosphate (alpha-TCP), amorphous calcium phosphate
(ACP), octacalcium phosphate (OCP), tetracalcium phosphate,
biphasic calcium phosphate (BCP), anhydrous dicalcium phosphate
(DCPA), dicalcium phosphate dihydrate (DCPD), anhydrous monocalcium
phosphate (MCPA), monocalcium phosphate monohydrate (MCPM), and
combinations thereof, and, wherein the polymer comprises one or
more of polyaryletherketone, polyetheretherketone (PEEK),
polyetherketonekteone (PEKK), polyetherketone (PEK), polyethylene,
high density polyethylene (HDPE), ultra high molecular weight
polyethylene (UHMWPE), low density polyethylene (LDPE),
polyethylene oxide (PEO), polyurethane, polypropylene,
polypropylene oxide (PPO), polysulfone, polymethylmethacrylate
(PMMA), and other polyacrylics from monomers such as bisphenol a
hydroxypropylmethacrylate (bis-GMA) or tri(ethylene glycol)
dimethacrylate (TEG-DMA), polypropylene, poly(e-caprolactone)
(PCL), poly(dioxanone) (PDO), poly(glyconate),
poly(hydroxybutyrate) (PHB), poly(hydroxyvalerate (PHV),
poly(orthoesters), poly(carboxylates), polypropylene fumarate),
poly(phosphates), poly(carbonates), poly(anhydrides),
poly(iminocarbonates), poly(phosphazenes), poly(DL-lactide) (PDLA),
poly(L-lactide) (PLLA), poly(glycolide) (PGA), copolymers thereof,
and blends thereof, and wherein the porogen is selected from NaCl
particles, wax, polysaccharides, and cellulose.
16. A method as described in claim 14, wherein processing the
mixture comprises one or more steps of vacuum filtering, drying and
densifying under uniaxial compression.
17. A method as described in claim 14, wherein processing includes
one or a combination of injection molding, reaction injection
molding, compression molding, transfer molding, extrusion, blow
molding, pultrusion, casting/potting, solvent casting, sintering,
and rapid prototyping processes
18. A method as described in claim 14, wherein mixing the polymer
and the calcium phosphate particles comprises using ultrasonication
under constant stirring, and the method further comprising the step
of sintering the composite material at elevated temperatures while
the composite material is under uniaxial compression, wherein
sintering is at a temperature range between approximately
25.degree. C. to approximately 1000.degree. C.
19. A method as described in claim 14, wherein leaching of the
porogen is achieved by a method selected from dissolution in a
solvent and heating.
20. An implantable orthopedic device, comprising: an interbody
spinal fusion cage, bone ingrowth scaffolding for implant fixation,
fracture fixation, synthetic bone graft substitute, or a tissue
engineering scaffold, that is completely or at least partially
formed with a porous reinforced composite scaffold that comprises a
thermoplastic polymer material having essentially uniform porosity
and comprising a plurality of reinforcing anisometric calcium
phosphate particles distributed essentially uniformly throughout,
wherein the composite comprises a plurality pores that are defined
by voids interconnected by struts, the size of the pores being
either homogenous or heterogeneous and within the range from about
10 to 500 .mu.m, and wherein the calcium phosphate particles
comprise single crystals, dense polycrystals or combinations
thereof, and wherein the calcium phosphate particles are embedded
within the thermoplastic polymer material and exposed on the struts
within the pore voids.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/904,098, filed on Feb. 28, 2007,
entitled "Reinforced Porous Polymer Scaffolds," U.S. Provisional
Patent Application Ser. No. 60/939,256, filed on May 21, 2007,
entitled "Interbody Spinal Fusion Cages Containing Anisometric
Calcium Phosphate Reinforcements and Related Methods," and U.S.
patent application Ser. No. 12/039,666, filed on Feb. 28, 2008,
entitled "Porous Composite Biomaterials and Related Methods," each
of which is incorporated herein by reference in its entirety.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates generally to composite
biomaterials and more particularly to porous composite biomaterials
and related methods.
BACKGROUND
[0003] Natural bone grafts such as autogenous bone grafts
(autografts) are commonly used in procedures for repairing or
replacing bone defects because they provide good structural support
and osteoinductivity. Natural bone grafts involve removing or
harvesting tissue from another part of a host's body (e.g.,
typically from the iliac crest, hip, ribs, etc.) and implanting the
harvested tissue in the defect site. Not only do these grafts
require an added surgical procedure needed to harvest the bone
tissue, these grafts have limitations, including for example,
transplant site morbidity. One alternative to autografts is
allografts, which involve removing and transplanting tissue from
another human (e.g., from bone banks that harvest bone tissue from
cadavers) to the defect site. However, allografts are known to
induce infection and immunotoxicity, suffer from limited supply and
variability, and have a lessened effectiveness because the cells
and proteins that promote bone growth are lost during the
harvesting process (e.g., during cleansing and disinfecting
process). Demineralized bone matrix (DBM) is typically used to
induce bone growth at defect sites, but DBM lacks the mechanical
properties (e.g., stiffness, strength, toughness, etc.) necessary
to be considered a viable option for load-bearing applications.
[0004] Synthetic bone substitute materials have been researched in
the treatment of diseased bone (e.g., osteoporosis), injured bone
(e.g., fractures), or other bone defects in lieu of natural bone
grafts. Synthetic bone substitutes are viable alternatives to the
more traditional methods described above. However, synthetic
substitute materials used to repair diseased bones and joints
should function or perform biologically and mechanically (i.e., as
the structural support role of the bone itself) by, for example,
mimicking the density and overall physical structure of natural
bone to provide a framework for ingrowth of new tissue. One type or
application of a synthetic bone substitute is a scaffold, which
provides support for bone-producing cells. Scaffolds may be
biodegradable, which degrade in vivo, or they may be
non-biodegradable to provide permanent implant fixation (e.g.,
spinal fusion cages). In addition, scaffolds are typically
biocompatible, and some may be bioactive, bioresorbable,
osteoconductive, and/or osteoinductive. The shapability,
deliverability, cost, and ability to match the mechanical
properties of the surrounding host tissue are other factors that
vary among different types of scaffolds and other bone
substitutes.
[0005] Problems may arise when there is a mechanical mismatch
between the bone substitute and the surrounding tissue. For
example, metallic implants and dense ceramics have mechanical
properties that are typically an order of magnitude greater than
the bone tissue. As a result, a stiff metal bone substitute implant
acts to "shield" the adjacent bone tissue from mechanical stresses,
resulting in a weakened bone at the bone-implant interface.
Furthermore, efforts to utilize porous ceramics or polymer bone
cement in place of stiffer materials have been limited. For
example, ceramics possess low fracture toughness, thereby making
the orthopedic implant brittle (i.e., susceptible to fracture).
Polymers are limited by higher compliance and lower strength,
thereby limiting their ability to support physiological load
levels. Additionally, conventional orthopedic implant biomaterials
are not osteoconductive and bioactive, resulting in a lack of
bonding between the implant and the peri-implant tissue.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1A is a perspective view of an example porous composite
material described herein.
[0007] FIG. 1B is a cross-sectional view of a portion of the
example porous composite material of FIG. 1A.
[0008] FIG. 2A-2C are scanning electron micrographs of a portion of
the example porous composite material shown in increasing
magnification.
[0009] FIG. 3 is graphical representation of the elastic modulus of
example apatite reinforced polymer composites versus the
reinforcement volume fraction.
[0010] FIG. 4 shows scanning electron micrograph of a portion of
the surface of an example composite material reinforced with
calcium phosphate whiskers embedded within and exposed on the
surface, and schematically showing the orientation of the whiskers
relative to the loading direction of the material or scaffold
strut.
[0011] FIG. 5A is a schematic illustration of a known spinal fusion
cage inserted between spinal vertebrae.
[0012] FIG. 5B illustrates the example known spinal fusion cage of
FIG. 5A.
[0013] FIG. 6 illustrates another example scaffold described
herein.
[0014] FIG. 7 illustrates yet another example scaffold described
herein.
[0015] FIG. 8 is a flow diagram illustrating an example process of
creating an example composite material apparatus described
herein.
DETAILED DESCRIPTION
[0016] In general, the example methods, apparatus, and materials
described herein provide a biocompatible, bioactive synthetic
porous composite for use as synthetic bone substitute materials.
The synthetic composite may provide a synthetic porous scaffold for
use an orthopedic implant and/or be injectable via percutaneous or
surgical injection to cure in vivo. Because the example composite
material is used to form a scaffold or matrix that is used in an
implantable device, the descriptions of one or more of these
structures may also describe one or more of the other structures.
The synthetic porous composites are tailored to mimic biological
and mechanical properties of bone tissue for implant fixation,
synthetic bone graft substitutes, tissue engineering scaffolds,
interbody spinal fusion, or other orthopedic applications. An
example porous composite material described herein reduces
subsidence and/or bone resorption resulting from mechanical
mismatch problems between a synthetic scaffold of an implant device
and the peri-implant tissue. Additionally, porosity and/or the pore
sizes of the example synthetic composite are tailorable to specific
applications to effectively promote the vascularization and growth
of bone in the pores and/or void spaces of the example scaffolds,
thereby improving bonding between the scaffolds and peri-implant
tissue.
[0017] The example composite material or scaffolds are synthesized
or made through a process that enables reinforcement particles to
be integrally formed with or embedded within polymer matrices. In
this manner, the polymer matrices embedded with the reinforcement
material provide improved material properties (e.g., stiffness,
fatigue strength, and toughness). The reinforcement particles are
also exposed on a surface of the matrices, which promotes
bioactivity and/or bioresorption. Additionally, the process
provides flexibility to tailor the level of reinforcement particles
and porosity for a desired application. For example, a porogen
material may be used to vary the porosity, while the pore size is
tailored by, for example, sieving the porogen to a desired
size.
[0018] By varying the volume of the reinforcement particles and the
porosity of the example scaffold, the mechanical properties (e.g.,
stiffness, strength, toughness, etc.) of the example scaffold of
the implant device may be tailored to match those of the adjacent
peri-implant bone tissue to reduce mechanical mismatch problems.
Reducing mechanical mismatch provides a decreased risk of
subsidence, stress shielding, bone resorption, and/or subsequent
failure of adjacent peri-implant bone tissue. Additionally, the
example scaffold of the implant device may include a significantly
high porosity to promote bone ingrowth, while exhibiting
significantly higher effective mechanical properties such as, for
example, the mechanical properties of trabecular bone.
[0019] In particular, the example composite material includes a
continuous porous biocompatible matrix having a thermoplastic
polymer matrix reinforced with anisometric calcium phosphate
particles. More specifically, in one example, a composite material
includes a polyetheretherketone (PEEK) or a polyetherketoneketone
(PEKK) matrix reinforced with various volume fractions of
hydroxyapatite (HA) whiskers (e.g., 20 or 40 volume percent),
wherein the matrix is approximately between 70% and 90% porous. In
another example, the porous matrix includes a biocompatible,
microporous polymer cage reinforced with anisometric calcium
phosphate particles and bone morphogenic protein (BMP) such as, for
example, rhBMP-2, which can be dispersed or accommodated by the
void spaces and/or pores of the example porous scaffold and/or
exposed on the surface of the example porous scaffold.
Additionally, the BMP binds to the calcium phosphate further
localizing the BMP to the surface of the scaffold or matrix.
[0020] The example composite materials described herein may be used
for applications such as, for example, synthetic bone graft
substitutes, bone ingrowth surfaces applied to existing implants,
tissue engineering scaffolds, interbody spinal fusion cages, etc.
In each of the applications, carrier materials (e.g., collagen,
hydrogels, etc.) containing growth factors, such as BMP, may be
incorporated into the pore space of the scaffold of the implant
device to further enhance osteoinduction and/or osteoconduction to
promote osteointegration.
[0021] Human bone tissues exhibit substantial variation in
mechanical properties depending on the tissue density and
microstructure. The properties are highly dependent on anatomic
location and apparent density of the measured specimen. For
example, a femur includes a cortical bone that has a relative
porosity on the order of about 5-15%, and a trabecular bone that
has a porosity on the order of about 75-95%. Due to the highly
significant porosity differences, the trabecular bone exhibits
significantly lower effective mechanical properties compared to the
cortical bone. Therefore, depending on the application, synthetic
composite materials for use as scaffolds and/or spinal fusion cages
or other implant devices should possess the mechanical properties
exhibited by the cortical bone or the trabecular bone, but must
also have effective porosity to promote bone growth.
[0022] To avoid the mechanical mismatch problems, such as stress
shielding, the example scaffold of the implant device described
herein may be tailored to substantially match or mimic the
mechanical properties (e.g., stiffness, strength, toughness, etc.)
of the adjacent and/or substituted bone tissue. Several factors may
be varied during the synthesis of the composite material and
scaffold of the implant device to tailor the mechanical properties
including the calcium phosphate reinforcement volume fraction,
aspect ratio, size and orientation; the polymer; and the size,
volume fraction, shape and directionality of the void space and/or
porosity. Tailoring the mechanical properties of the scaffold
reduces the likelihood of mechanical mismatch leading to a
decreased risk of subsidence, stress shielding, bone resorption
and/or subsequent failure of adjacent vertebrae.
[0023] FIG. 1A illustrates the example synthetic porous composite
material 100 described herein. FIG. 1B is a cross-sectional view of
a portion of the example porous composite material 100 of FIG. 1A.
The example synthetic composite material 100 provides a synthetic
porous scaffold 101 for use or in as an orthopedic implant.
[0024] The example synthetic porous composite material 100 includes
a porous thermoplastic polymer (e.g., a PEEK polymer) matrix 102
having anisometric calcium phosphate reinforcement particles 104
integrally formed or embedded with the matrix 102 and/or exposed on
a surface of the matrix. In this manner, the polymer matrix 102
embedded with the reinforcement particles 104 provides high
material strength, and the reinforcement particles 104 exposed on
the surface of the matrix 102 promote bioactivity and/or
bioresorption. The porous polymer matrix 102 includes a
substantially continuous porosity and a plurality of pores 106 to
enable bone ingrowth into the porous matrix 102. In addition, the
matrix 102 is substantially continuously interconnected via a
plurality of struts 108. Furthermore, at least one of the plurality
of struts 108 may be a load-bearing strut.
[0025] FIGS. 2A-2C are scanning electron micrographs showing
increasing magnification of a portion of an example scaffold 200
with struts 202. The example scaffold 200 is a PEEK scaffold
reinforced with 40% by volume HA whiskers 204. FIG. 2A illustrates
the architecture or matrix 206 of the scaffold 200 and FIG. 2B
illustrates an enlarged portion of the struts 202. In the
illustrated example, the HA whiskers 204 are integrally formed
and/or embedded within the matrix 206 of the scaffold 200 for
reinforcement. The HA whiskers 204 are also exposed on a surface
208 of the matrix of the scaffold 200 for bioactivity and/or
bioresorption, as noted above. As shown in FIG. 2C, the HA whiskers
204 are aligned in a sheet texture and are exposed on the surface
208 of the struts 202.
[0026] The thermoplastic polymer of the example scaffolds described
herein may be a biodegradable polymer for synthetic bone graft
substitute applications, or nonbiodegradable for implant fixation
applications. The thermoplastic polymer includes a continuous
matrix of a composite material and is biocompatible and/or
bioresorbable as described above. Additionally or alternatively,
the polymer may be a radiolucent polymer, bioresorbable (i.e., a
material capable of being resorbed by a patient under normal
physiological conditions) and/or non-bioresorbable, as desired.
Further, the thermoplastic polymer matrix may include a polymer
suitable for injection via percutaneous or surgical injection so
that the composite material 100 cures in vivo.
[0027] Suitable non-resorbable polymers include, without
limitation, polyaryletherketone (PAEK), polyetheretherketone
(PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK),
polyethylene, high density polyethylene (HDPE), ultra high
molecular weight polyethylene (UHMWPE), low density polyethylene
(LDPE), polyethylene oxide (PEO), polyurethane, polypropylene,
polypropylene oxide (PPO), polysulfone, polypropylene, copolymers
thereof, and blends thereof. Suitable bioresorbable polymers
include, without limitation, poly(DL-lactide) (PDLA),
poly(L-lactide) (PLLA), poly(glycolide) (PGA), poly( -caprolactone)
(PCL), poly(dioxanone) (PDO), poly(glyconate),
poly(hydroxybutyrate) (PHB), poly(hydroxyvalerate (PHV),
poly(orthoesters), poly(carboxylates), polypropylene fumarate),
poly(phosphates), poly(carbonates), poly(anhydrides),
poly(iminocarbonates), poly(phosphazenes), copolymers thereof, and
blends thereof. Suitable polymers that are injectable via
percutaneous or surgical injection that cure in vivo include,
without limitation, polymethylmethacrylate (PMMA), and other
polyacrylics from monomers such as bisphenol a
hydroxypropylmethacrylate (bis-GMA) and/or tri(ethylene glycol)
dimethacrylate (TEG-DMA).
[0028] Although synthetic substitute composite materials made of
polymers satisfy the functional criteria of an implantable device
because they are, for example, formable and inexpensive, polymers
alone lack biological efficacy to promote bone growth and/or may
lack requisite mechanical properties to support load levels. To
increase the mechanical properties of the polymers, the polymers
are reinforced with calcium phosphates. The aspect ratio, size,
volume fraction and degree of preferred orientation of the calcium
phosphate particles 104 (e.g., HA whisker particles) may be
tailored for the desired material properties and implant
performance. For example, consider the information presented in
FIG. 3, which is a graphical illustration of the elastic modulus of
HA whisker and powder reinforced polymer composite materials versus
the volume fraction percentage of the apatite calcium phosphates
that is mixed with the polymer matrix. The shaded areas of FIG. 3
show approximate regions for the given mechanical property of the
human cortical bone tissue. As depicted in the graph, the elastic
modulus of the composite materials increases with increasing HA
content. In addition, increasing the level of HA reinforcement in
polymer composites increases cellular activity during
osteointegration.
[0029] The calcium phosphate reinforcement particles 104 may be in
the form of single crystals or dense polycrystals, but are at least
in some portion anisometric. "Anisometric" refers to any particle
morphology (shape) that is not equiaxed (e.g., spherical), such as
whiskers, plates, fibers, etc. Anisometric particles are usually
characterized by an aspect ratio. For example, HA single crystals
are characterized by the ratio of dimensions in the c- and a-axes
of the hexagonal crystal structure. Thus, the anisometric particles
in the present disclosure have an aspect ratio greater than 1. In
one example, the mean aspect ratio of the reinforcement particles
is from about 1 to about 100. By further example, the reinforcement
particles can be provided in an amount of from about 1% by volume
of the composite biomaterial to about 60% by volume, and for
example, from about 20% by volume of the composite to about 50% by
volume.
[0030] Due to their morphology, the calcium phosphate
reinforcements particles 104 may be oriented in bulk or near the
surface of the polymer matrix 102 to provide directional
properties, if desired. For example, if the reinforcement particles
104 are predominately aligned within the matrix 102 the
morphological alignment of the particles 104 provides anisotropy
for the overall composite 100, which can be tailored to be similar
to the anisotropic mechanical properties of bone tissues. For
example, FIG. 4 shows a micrograph of anisometric calcium phosphate
reinforcement 400 on the surface of a dense composite polymer
matrix 402. Also shown in FIG. 4 is a schematic illustration of a
portion of matrix 402 and illustratively showing the orientation of
the reinforcement particles 400 relative to the loading direction
of the material and/or a scaffold strut, which is, for example, at
an angle .theta..
[0031] Furthermore, there are no limits on the size or amount of
the calcium phosphate particles 104 in matrix 102, provided that
the calcium phosphate particles 104 are dispersed within and/or
exposed at the surface of the polymer matrix 102. For example, the
reinforcement particles 104 may have a maximum dimension from about
20 nm to about 2 mm, and for example, between 20 nm to about 100
.mu.m. While both nano- and micro-scale calcium phosphate particles
improve the mechanical properties of the example synthetic
composite material 100 described herein, nano-scale calcium
phosphate particles are particularly effective for enhancing
bioresorbability and cell attachment, and micro-scale particles are
particularly effective for obtaining a uniform dispersion within
the matrix 102.
[0032] Suitable calcium phosphates may include, without limitation,
calcium HA, HA whiskers, HA, carbonated calcium HA, beta-tricalcium
phosphate (beta-TCP), alpha-tricalcium phosphate (alpha-TCP),
amorphous calcium phosphate (ACP), octacalcium phosphate (OCP),
tetracalcium phosphate, biphasic calcium phosphate (BCP), anhydrous
dicalcium phosphate (DCPA), dicalcium phosphate dihydrate (DCPD),
anhydrous monocalcium phosphate (MCPA), monocalcium phosphate
monohydrate (MCPM), and combinations thereof.
[0033] As described above, a synthetic composite material 100 not
only bears physiological levels of load, but also promotes
oseteointegration--the direct structural and functional connection
between the living bone and the surface of the load-bearing
implant. The bioactive calcium phosphate particles 104 (e.g., HA
whiskers) exposed on the surface of the example porous matrix 102
promote a stable bone-implant interface. Osteointegration also
requires the vascularization and growth of bone into an implant via
interconnected and/or continuous porosity.
[0034] Thus, the size, volume fraction, shape, and directionality
of the void spaces and/or pores 106 may be tailored to optimize
osteoconduction and implant mechanical properties. The pores 106
may be any size or shape, while maintaining a continuous network to
promote a fusion through the formation of new bone tissue in the
void spaces and/or pores 106. For example, the pores 106 may be
present throughout the matrix 102 as illustrated in FIG. 1A. Also,
the pores 106 may be functionally graded in any material or implant
direction, for example, radially, as shown in FIGS. 6 and 7, from a
highly porous region to a relatively dense region, or may include a
void space. The change in porosity from one region to another may
be very distinct, for example as shown in FIGS. 6 and 7, or
gradual. Furthermore, the graded change may be uniform or variant.
In addition, instead of a graded change there may be a combination
of materials having two or more densities of pores. The central
void may be any shape or size, and may receive (e.g., be filled) a
material, a structure, or the composite material 100 (i.e., the
composite material graded from the porous outer surface to a dense
center), thereby forming a porous outer perimeter and a dense
central region. Examples are further described in connection with
FIGS. 6 and 7 below.
[0035] As discussed in greater detail below, the porosity and/or
pore sizes 106 may be selectively formed by the inclusion of, for
example, a porogen material during synthesis of the composite
material 100. Pores spaces may range from about 100 .mu.m to about
500 .mu.m, and, for example, from about 250 .mu.m to about 500
.mu.m. The example composite biomaterial 100 may additionally
contain some fraction of microporosity within scaffold struts that
is less than about 10 .mu.m in size. The total amount of porosity
within porous regions may range from about 1% to about 90% by
volume, and, for example, between about 70% and 90% by volume.
However, the porosity may also be tailored via other processes such
as, for example, microsphere sintering, fiber weaving, solvent
casting, electrospinning, freeze drying (lyophilization), thermally
induced phase separation, gas foaming, and rapid prototyping
processes such as, for example, solid freeform fabrication, robotic
deposition (aka, robocasting), selective laser sintering, fused
deposition modeling, three-dimensional printing, laminated object
manufacturing, stereolithography, etc., or any other suitable
process(es) or combination(s) thereof.
[0036] Additionally, the example composite material 100 may
optionally include additives, if desired. For example, the
composite material 100 may include one or more surface-active
agents to enhance interfacial bonding between the reinforcement
particles 104 and the polymer matrix 102. The void spaces and/or
pores 106 may accommodate and deliver one or more growth factors
such as, for example, BMP, to enhance osteoinductivity and/or bone
regeneration. Furthermore, the void spaces and/or pores 106 may
also accommodate and deliver one or more transcription factors,
matrix metalloproteinases, peptides, proteins, bone cells,
progenitor cells, blood plasma, bone marrow aspirate, or
combinations thereof, to improve or speed bone regeneration, or
resorption and replacement of the biomaterial. In some examples,
the void spaces and/or pores 106 may further accommodate a carrier
material that may be incorporated into the void spaces and/or pores
106. The carrier material may include, for example, a collagen
sponge, membrane, or a hydrogel material to deliver the growth
factor material such as, for example, the BMP. The calcium
phosphate reinforcements 104 exposed on the surface of the porous
matrix 102, along with the porosity, improves the retention of the
BMP within the matrix 102 and at the peri-implant interface.
[0037] FIG. 5A is an illustration showing a known interbody spinal
fusion cage 500. The example spinal fusion cage 500 is implanted in
the inter-vertebral space 502 between two adjacent vertebrae 506
and 508. A disc 510, due to degeneration, herniation, etc., is
typically removed and replaced by the spinal fusion cage 500. The
spinal fusion cage 500 is used to support or restore vertebral
height, and, thus, stabilize or retain adjacent vertebrae 506 and
508 in a desired position. Additionally, the spinal fusion cage 500
is to promote fusion between the vertebrae 506 and 508.
[0038] FIG. 5B is an enlarged illustration of the known spinal
fusion cage 500 of FIG. 5A. A typical spinal fusion cage 500
includes a body 510 having a dense outer region 512 and a void 514
at its center. The dense outer surface 512 may be made of PEEK,
titanium, or other material that can be used to support the
vertebrae 506 and 508. However, the spinal fusion cage 500 made of
a PEEK, titanium, etc., cannot attach to the bone. Thus, to promote
bone ingrowth, the center void 514 is typically provided with a
packing material (not shown) such as, a natural bone graft, a
collagen sponge that retains bone growth factors, or the spinal
fusion cage 500 is coated with the bone growth factors or other
agents that promote osteoinduction.
[0039] The example porous scaffold 101 having the composite
material 100 described herein, and with respect to FIG. 1A, can be
implemented with the spinal fusion cage 500 of FIGS. 5A and 5B to
replace the packing materials, such as natural bone graft. As noted
above, the porous scaffold 101 promotes bone ingrowth and the pores
106 may accommodate or deliver for example, a BMP, to further
improve rate of growth (fusion rate). Furthermore, the calcium
phosphate (e.g., HA whisker) binds to the body 510 and localizes
the BMP within the central void 514, which further promotes
osteointegration.
[0040] FIG. 6 illustrates another example scaffold or matrix 600
implemented with the example composite material 100 described
herein. The example scaffold 600 may be implemented as an interbody
spinal fusion cage. The scaffold 600 includes a body 602 having a
porous polymer matrix 604 integrally formed or embedded with
anisometric calcium phosphate particles 606. Furthermore, the
matrix 604 of the example scaffold 600 includes a radiolucent
polymer (e.g., PEEK) integrally formed or embedded with anisometric
calcium phosphate reinforcements 606 such as, for example, HA
whiskers. The radiolucent polymer provides improved radiographic
analysis of fusion following implantation. The example scaffold 600
may also include a BMP such as, for example, rhBMP-2. In another
example, the example scaffold 600 is a biocompatible, microporous
polymer scaffold or matrix supplemented with anisometric calcium
phosphate reinforcements and BMP.
[0041] Additionally, the example scaffold 600 may formed so that
the pores are functionally graded in any material or implant
direction such as, for example, radially, as shown in FIG. 6, from
a highly porous center or central region 608 to relatively dense
outer region or surface 610. The change in porosity from one region
to another may be distinct or gradual from the central region 608
to the outer region 610. Further, the graded change may be uniform
or variant. The dense outer region 610 provides structural
integrity along with the advantages of the composite material 100
described herein. In the illustrated example, the porous structure
604 has pore sizes that range between about 100 .mu.m and about 500
.mu.m, and preferably, between about 250 .mu.m and about 500 .mu.m,
and a porosity that ranges between about 1% to about 90%.
Furthermore, the spinal fusion cage material 600 may include
microporosity having pore sizes less than about 10 .mu.m.
[0042] FIG. 7 illustrates another example scaffold 700. The porous
scaffold 700 includes a porous matrix 702 having the composite
material 100 described herein. The porous scaffold 700 includes a
center or central void 704 that may be any shape or size.
Additionally or alternatively, the central void 704 may receive a
material 706, a stem, or any other substance or structure,
illustratively depicted by dashed lines. For example, the central
void 704 may receive a stem 706 (e.g., an implant) such as, for
example, a titanium stem, a dense composite stem (e.g, a PEEK
composite stem), or any other suitable material or structure.
[0043] Additionally, in other examples, the scaffold 700 is formed
so that the pores are functionally graded in any material or
implant direction, for example, radially as shown in FIG. 7, from a
the high porous outer region or surface 702 to a relatively dense
center or central region 708. The change in porosity from one
region to another may be distinct or gradual from the central
region 708 to the highly porous outer region 702. Further, the
graded change may be uniform or variant. In this manner, the
example scaffold 700 forms a porous perimeter having a dense core,
where the material is continuous from the porous perimeter to the
dense core. The dense central region 708 provides structural
integrity along with the advantages of the composite material 100
described herein. In the illustrated example, the porous matrix 702
and the dense central region 708 may have pore sizes that range
between about 100 .mu.m and about 500 .mu.m, and preferably,
between about 250 .mu.m and about 500 .mu.m, and a porosity that
ranges between about 1% to about 90%. Furthermore, the spinal
fusion cage 700 may include a microporosity having pore sizes less
than about 10 .mu.m.
[0044] In other examples, the composite material 100 and/or the
scaffolds 101, 600, 700 may also include a roughened surface such
as, for example, serrated teeth, that come into direct contact with
the adjacent peri-implant tissue to prevent movement relative to
the peri-implant tissue after implantation. Additionally or
alternatively, although not shown, the scaffolds 101, 600, 700 may
include holes, notches, pins, radiographic markers, or other
features that may be gripped or otherwise used for positioning of
the scaffolds 101, 600, 700 by minimally invasive surgical tools
and procedures.
[0045] The example composite material 100 and/or the scaffolds 101,
600, 700 may be manufactured by methods common to reinforced
thermoplastic and thermosetting polymers, including but not limited
to injection molding, reaction injection molding, compression
molding, transfer molding, extrusion, blow molding, pultrusion,
casting/potting, solvent casting, microsphere sintering, fiber
weaving, solvent casting, electrospinning, freeze drying
(lyophilization), thermally induced phase separation, gas foaming,
and rapid prototyping processes such as solid freeform fabrication,
robotic deposition (aka, robocasting), selective laser sintering,
fused deposition modeling, three-dimensional printing, laminated
object manufacturing, stereolithography, etc., or any other
suitable process(es) or combination(s) thereof.
[0046] FIG. 8 is a flowchart of an example method 800 that may be
used to synthesize the example composite material 100 and/or
scaffolds 101, 600, 700 described herein. While an example manner
of synthesizing the example composite material 100 and/or scaffolds
101, 600, 700 has been illustrated in FIG. 8, one or more of the
steps and/or processes illustrated in FIG. 8 may be combined,
divided, re-arranged, omitted, eliminated and/or implemented in any
other way. Further still, the example method of FIG. 8 may include
one or more processes and/or steps in addition to, or instead of,
those illustrated in FIG. 8, and/or may include more than one of
any or all of the illustrated processes and/or steps. Further,
although the example method is described with reference to the flow
chart illustrated in FIG. 8, persons of ordinary skill in the art
will readily appreciate that many other methods of synthesizing the
example composite material 100 and/or scaffolds 101, 600, 700 may
alternatively be used.
[0047] The composite material 100 and/or the scaffolds 101, 600,
700 are processed using a powder processing approach in conjunction
with compression molding and particle leaching techniques and is
particularly suited for achieving a high concentration (e.g.,
>40 vol %) of well-dispersed (and aligned, if desired)
anisometric calcium phosphate reinforcements (e.g., HA whiskers) in
a thermoplastic matrix (e.g., PEEK) with minimal degradation of the
calcium phosphate size/shape during processing. In this manner, the
calcium phosphate reinforcement volume fraction, aspect ratio, size
and orientation; the polymer; and the size, volume fraction, shape
and directionality of the void space and/or porosity may be
tailored to vary the mechanical properties of the composite
material 100 and/or scaffolds 101, 600, 700.
[0048] A polymer such as, for example, PEEK, and anisometric
calcium phosphate particles, such as HA whiskers, are provide in
powder form (block 802). The PEEK polymer powder may have, for
example, a mean particle size of about 26 .mu.m. The HA whiskers
may be synthesized (block 801) using, for example, the chelate
decomposition method.
[0049] The PEEK powder and the synthesized HA whiskers are
co-dispersed in a fluid (block 804) such as, for example ethanol,
and mixed (block 804) using, for example, ultrasonication under
constant stirring--forming a viscous suspension.
[0050] After the polymer powder and the HA whiskers are mixed, the
porosity of the mixture is selectively varied and/or tailored
(block 806). In one example, the porosity may be formed and
tailored by the addition of a suitable porogen material such as,
for example, NaCl, wax, polysaccharides (sugars), cellulose, etc.
The extent of the porosity can be controlled by varying the amount
of porogen used (block 805), while the pore size could be tailored
by sieving the porogen (block 807) to a desired size prior to
mixing the porogen with the polymer mixture. In another examples,
the porosity and/or the pore size of the polymer matrix may be
selectively varied using any other suitable methods and/or
process(es) such as, for example, microsphere sintering, fiber
weaving, solvent casting, electrospinning, freeze drying
(lyophilization), thermally induced phase separation, gas foaming,
or rapid prototyping processes such as, for example, solid freeform
fabrication, robotic deposition (aka, robocasting), selective laser
sintering, fused deposition modeling, three-dimensional printing,
laminated object manufacturing, stereolithography, etc., or any
other suitable process(es) or combination(s) thereof.
[0051] The viscous suspension is wet-consolidated (block 808) by,
for example, vacuum filtration and drying to remove any residual
fluid (i.e., ethanol). The composite mixture is densified (block
810) by, for example, uniaxial compression, to form a composite
preform.
[0052] Following the initial densification, the preform is
compression molded (block 812) and/or sintered at elevated
temperatures (e.g., approximately 20.degree. C. to 400.degree. C.)
sufficient to fuse the polymer particles with minimal damage to the
calcium phosphate reinforcements. The process or composite material
may be heated to a desired processing temperature and the implant
may be shaped or formed (block 814). Densifying and molding the
composite material includes aligning the calcium phosphate
reinforcement particles (e.g., HA whiskers) morphologically and/or
crystallographically within the scaffold struts.
[0053] The scaffold may have any shape and/or size (e.g., any
polygonal shape) and can be formed by methods common to reinforced
thermoplastic and thermosetting polymers, including but not limited
to injection molding, reaction injection molding, compression
molding, transfer molding, extrusion, blow molding, pultrusion,
casting/potting, solvent casting, and rapid prototyping processes
such as, for example, solid freeform fabrication, robotic
deposition (aka, robocasting), selective laser sintering, fused
deposition modeling, three-dimensional printing, laminated object
manufacturing, stereolithography, etc., or any other suitable
process(es). The composite material 100 and/or the scaffolds 101,
600, 700, are formed by the mold walls and/or machining after
molding.
[0054] The composite material undergoes a leaching process (block
816) to remove, for example, the porogen used during synthesis of
the composite material. The leaching may occur, for example, via a
dissolution method, heating method, and/or any other suitable
methods and/or process(es). More specifically, dissolution may
include immersing the scaffold in a fluid, such as, for example,
deionized water.
[0055] Furthermore, viscous flow of the polymer/reinforcement
mixture during molding can be designed to tailor the preferred
orientation of the anisometric reinforcements in the implant.
Additionally surface-active agents may be added during the mixing
process and/or to the surface of the composite material to enhance
interfacial bonding between reinforcement particles and the
matrix.
[0056] The following example is provided to further illustrate the
example apparatus and methods described herein and, of course,
should not be construed as in any way limiting in scope. It is to
be understood by one of ordinary skill in the art that the
following examples are neither comprehensive nor exhaustive of the
many types of methods and apparatus which may be prepared in
accordance with the present disclosure.
[0057] In the example, commercially available PEKK and sodium
chloride (NaCl) powders with mean particle sizes of 70 and 250
.mu.m, respectively, were used as-received. HA whiskers were
synthesized using the chelate decomposition method. The
as-synthesized HA whiskers were measured by optical microscopy to
have a mean length of 21.6 .mu.m, width of 2.8 .mu.m and aspect
ratio of 7.6.
[0058] In the example, composite scaffolds with 75, 82.5 and 90%
porosity were processed with 0-40 vol % HA whisker reinforcement.
Appropriate amounts of polymer powder and HA whiskers were
co-dispersed in ethanol via a sonic dismembrator and mechanical
stirring at 1200 rpm. Following dispersion, the appropriate amount
of the NaCl (i.e, porogen) was added to the suspension and mixed by
hand using a Teflon coated spatula. The total scaffold volume
consisted of the material volume plus the pore volume. Thus, the
reinforcement level was calculated based the desired material
volume, while the porosity level was calculated based on the total
scaffold volume. After mixing, the viscous suspension was
wet-consolidated using vacuum filtration. The powder mixture was
dried overnight in a forced convection oven at 90.degree. C. and
densified at 125 MPa in a cylindrical pellet die using a hydraulic
platen press. The die and densified powder mixture was heated in a
vacuum oven to the desired processing temperature and transferred
to a hydraulic platen press for compression molding. Scaffolds with
82.5 and 90% porosity were molded at 350.degree. C., while
scaffolds with 75% porosity were molded at 350, 365 and 375.degree.
C. A pressure of 250 MPa was applied to the die as the polymer
solidified. After cooling to room temperature, the sintered
composite pellet was ejected from the die and placed approximately
300 mL deionized water for at least 72 h to dissolve the NaCl
crystals. The deionized water was changed daily. The as-molded
composite scaffolds had a diameter of 1 cm and were machined to a
height of 1 cm.
[0059] In the example, un-confined compression tests were performed
to investigate the mechanical properties of the composite
scaffolds. Specimens were tested on an electromagnetic test
instrument in phosphate buffered saline (PBS) at 37.degree. C.
using a crosshead speed of 1 mm/min. Force-displacement data was
used to calculate the elastic modulus, compressive yield stress
(CYS), and failure strain of the composite scaffolds. One-way
analysis of variance (ANOVA) was used to compare mechanical
properties between experimental groups. The compressive properties
of HA whisker reinforced PEKK scaffolds were tabulated in table
1.
[0060] The table below provides mechanical properties of the
example PEKK scaffold reinforced with HA whiskers that was
processed using a compression molding/particle leaching method such
as, for example, the method 800 of FIG. 8 as implemented in the
description above. The mechanical properties of HA whisker
reinforced PEKK were evaluated in uniaxial compression. Tensile
properties of the HA whisker reinforced PEKK scaffolds were
evaluated prior to scaffold fabrication.
TABLE-US-00001 HA Molding Elastic Yield Porosity Content
Temperature Modulus Strain (%) (Vol %) (.degree. C.) (MPa) CYS
(MPa) (%) 75 0 350 69.5 (12.2) 1.25 (0.16) 5.7 (0.4) 75 0 365 100.7
(10.7) 2.04 (0.26) 4.4 (1.2) 75 0 375 95.6 (10.5) 2.55 (0.34) 4.4
(1.1) 75 20 350 106.3 (15.0) 1.28 (0.13) 3.7 (1.1) 75 20 365 115.4
(13.4) 1.77 (0.30) 2.9 (0.6) 75 20 375 141.1 (39.2) 2.28 (0.35) 2.9
(0.7) 75 40 350 54.0 (18.3) 0.52 (0.17) 2.2 (0.3) 75 40 365 84.3
(41.5) 1.15 (0.38) 2.7 (0.5) 75 40 375 120.2 (29.8) 1.65 (0.34) 2.3
(1.9) 82 40 350 15.7 (6.7) 0.13 (0.03) 2.6 (1.9) 90 0 350 0.75
(0.18) 0.15 (0.04) 30 (0.0) 90 40 350 0.23 (0.13) 0.04 (0.01) 30
(0.0)
[0061] As shown in the table, for a given reinforcement level, the
compressive modulus decreased with increased porosity, and the
yield strength decreased with increased porosity. Scaffolds with 0%
vol % HA whisker reinforcement and 75% and 90% porosity exhibited
moduli of 69.5 and 0.75 MPa, while scaffolds with 40 vol % HA
whisker reinforcement and 75%, 82% and 90% porosity exhibited
moduli of 54.0, 15.7 and 0.23 MPa, respectively. Scaffolds with 0
vol % HA whisker reinforcement and 75% and 90% porosity exhibited
yield strengths of 1.25 MPa and 0.15 MPa, respectively. Scaffolds
with 40 vol % HA whisker reinforcement and 75%, 82% and 90%
porosity exhibited yield strengths of 0.52 MPa, 0.13 MPa and 0.04
MPa, respectively. The HA content also affected the modulus and
failure strain of the scaffolds. A scaffold having 75% porosity and
20 vol % reinforcement HA whisker exhibited modulus of 106.3 MPa,
compared to a modulus of 69.5 MPa for scaffolds with 0 vol % HA
whisker reinforcement.
[0062] The example methods and apparatus described herein offer
synthetic porous composite material that may be used for synthetic
bone substitutes for impant fixation, fraction fixation, synthetic
bone graft substitutes, interbody spinal fusion, tissue engineering
scaffolds, or other applications. Many aspects of the of the porous
composite material may be tailored to provide specific mechanical,
biological, and surgical functions, such as, the polymer
composition and molecular orientation, porosity and pore size of
the porous matrix, or the HA reinforcement content, morphology,
preferred orientation, and size.
[0063] Although the teachings of the present disclosure have been
illustrated in connection with certain examples, there is no intent
to limit the present disclosure to such examples. On the contrary,
the intention of this application is to cover all modifications and
examples fairly falling within the scope of the appended claims
either literally or under the doctrine of equivalents.
* * * * *