U.S. patent application number 10/116961 was filed with the patent office on 2002-10-24 for self-setting calcium phosphate pastes and related products.
This patent application is currently assigned to Etex Corporation. Invention is credited to Aiolova, Maria, Lee, Dosuk D., Rey, Christian, Tofighi, Aliassghar.
Application Number | 20020155167 10/116961 |
Document ID | / |
Family ID | 46254384 |
Filed Date | 2002-10-24 |
United States Patent
Application |
20020155167 |
Kind Code |
A1 |
Lee, Dosuk D. ; et
al. |
October 24, 2002 |
Self-setting calcium phosphate pastes and related products
Abstract
The present invention provides a novel process for producing a
calcium phosphate cement or filler which hardens in a temperature
dependent fashion in association with an endothermic reaction. In
the reaction a limited amount of water is mixed with dry calcium
phosphate precursors to produce a hydrated precursor paste.
Hardening of the paste occurs rapidly at body temperature an is
accompanied by the conversion of one or more of the reactants to
poorly crystalline apatitic calcium phosphate. The hardened
cements, fillers, growth matrices, orthopedic and delivery devices
of the invention are rapidly resorbable and stimulate hard tissue
growth and healing.
Inventors: |
Lee, Dosuk D.; (Brookline,
MA) ; Rey, Christian; (Castanet, FR) ;
Aiolova, Maria; (Brookline, MA) ; Tofighi,
Aliassghar; (Belmont, MA) |
Correspondence
Address: |
HALE AND DORR, LLP
60 STATE STREET
BOSTON
MA
02109
|
Assignee: |
Etex Corporation
Cambridge
MA
02139
|
Family ID: |
46254384 |
Appl. No.: |
10/116961 |
Filed: |
April 5, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10116961 |
Apr 5, 2002 |
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09568760 |
May 11, 2000 |
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10116961 |
Apr 5, 2002 |
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08729344 |
Oct 16, 1996 |
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6117456 |
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10116961 |
Apr 5, 2002 |
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08650764 |
May 20, 1996 |
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6214368 |
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10116961 |
Apr 5, 2002 |
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08446182 |
May 19, 1995 |
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5676976 |
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Current U.S.
Class: |
424/602 ;
424/423 |
Current CPC
Class: |
A61F 2/28 20130101; A61L
27/38 20130101; A61L 2300/22 20130101; A61F 2002/30062 20130101;
A61L 27/12 20130101; C01B 25/325 20130101; A61F 2310/00293
20130101; C04B 28/344 20130101; A61F 2002/30762 20130101; C01B
25/32 20130101; Y10S 977/91 20130101; A61F 2002/4648 20130101; A61L
2430/02 20130101; A61L 24/02 20130101; Y10S 977/926 20130101; A61L
2300/414 20130101; A61L 2300/406 20130101; A61F 2210/0004 20130101;
Y10S 977/775 20130101; A61L 2300/452 20130101; A61F 2/30756
20130101; C01B 25/321 20130101; A61L 2400/06 20130101; A61L 27/425
20130101; C04B 2111/00836 20130101; A61L 2300/252 20130101; A61K
9/2009 20130101; A61L 27/58 20130101; Y10S 977/915 20130101; A61L
27/54 20130101; C01B 25/16 20130101; Y10S 977/70 20130101 |
Class at
Publication: |
424/602 ;
424/423 |
International
Class: |
A61K 033/42; A61F
002/00 |
Claims
What is claimed is:
1. A formable paste, suitable for use as a bone substitution
material, comprising: a reactive amorphous calcium phosphate
material having at least 90% amorphous character and characterized
in that, when prepared 1:1 as a mixture with dicalcium phosphate
dihydrate (DCPD) in water, wherein at least 20% by weight of said
DCPD is retained by a 106.mu. sieve, the mixture remains injectable
and formable for a time greater than about 60 minutes at about
18-19.degree. C. and hardens at about 37.degree. C. within about 10
to 60 minutes; a second calcium phosphate; and an aqueous-based
fluid in an amount to provide a paste having a formable or
injectable consistency at a temperature of about 18-19.degree.
C.
2. The paste of claim 1, wherein said amorphous calcium phosphate
has a Ca/P molar ratio of about 1.1 to 1.65.
3. The paste of claim 1, wherein said second calcium phosphate has
a Ca/P molar ration of less than or equal to 1.67.
4. The paste of claim 1, 2, or 3, further comprising a
supplementary material selected to change a physical parameter of
the paste or the hardened product, said parameter selected from the
group consisting of strength, resorption time, adherence,
injectability, frictional characteristics, release kinetics,
tensile strength, hardness, fracture toughness, elasticity, imaging
capability, flow properties and setting times.
5. The paste of claim 1, further comprising a therapeutic
substance.
6. The paste of claim 1, wherein the fluid is selected from the
group consisting of water, saline, non-phosphate buffer solutions,
serum and tissue culture medium.
7. The paste of claim 5, wherein said therapeutic substance is
selected from the group consisting of growth factors, antibiotics,
anti-cancer agents, and analgesics.
8. The paste of claim 1, further comprising a crystallization
inhibitor selected from the group consisting of carbonates,
pyrophosphate, and magnesium.
Description
[0001] This application is a continuation-in-part application of
co-pending application U.S. Ser. No. 08/650,764 filed May 20, 1996
entitled "Novel Bone Substitution Material and a Method of Its
Manufacture", which is a continuation-in-part application of
co-pending application U.S. Ser. No. 08/446,182 filed May 19, 1995
entitled "Synthesis of Reactive Amorphous Calcium Phosphates", both
of which are herein incorporated in its entirety by reference. This
application also is related to several co-pending applications
filed on even day herewith, entitled, "Bioresorbable Ceramic
Composites", "Delivery Vehicle", "Cell Seeding of Ceramic
Compositions" and "Orthopedic and Dental Ceramic Implants", each of
which is incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to hard tissue implant materials
containing poorly crystalline apatitic calcium phosphate useful as
human or animal implantable bioceramics and for other purposes. The
invention further relates to bioresorbable composites, cell therapy
and therapeutic substance delivery devices useful in human and
animal therapeutics.
BACKGROUND OF THE INVENTION
[0003] Calcium phosphates are the principal constituent of hard
tissues (bone, cartilage, tooth enamel and dentine). Calcium
phosphates generally occur in apatitic form when found in
biological tissues. For instance, the composition of bone mineral
may be represented by the following formula:
Ca.sub.8.3(PO.sub.4).sub.4.3 (HPO.sub.4, CO.sub.3).sub.1.7 (OH,
CO.sub.3).sub.0.3
[0004] Unlike the ideal stoichiometric crystalline hydroxyapatite
(Ca.sub.10(PO.sub.4).sub.6(OH).sub.2), or stoichiometric apatites
in general (Ca.sub.5(PO.sub.4).sub.3X), which have a calcium to
phosphate ratio (Ca/P) of 1.67, bone mineral is a
non-stoichiometric apatite. Its non-stoichiometry is primarily due
to the presence of divalent ions, such as CO.sub.3.sup.2- and
HPO.sub.4.sup.2-, which are substituted for the trivalent
PO.sub.4.sup.3- ions. Substitution by HPO.sub.4.sup.2- and
CO.sub.3.sup.2- ions produces a change of the Ca/P ratio, resulting
in Ca/P ratio which may vary between 1.50 to 1.70, depending on the
age and bony site. Generally, the Ca/P ratio increases during aging
of bone, suggesting that the amount of carbonate species increases
for older bones. Naturally-occurring bone mineral is made of
nanometer-sized, poorly-crystalline calcium phosphate with apatitic
structure. The poorly crystalline apatitic calcium phosphate of
bone is distinguished from the more crystalline hydroxyapatites and
non-stoichiometric hydroxyapatites by its distinctive XRD pattern
as shown in FIG. 7. It is the Ca/P ratio in conjunction with
nanocrystalline size and the poorly-crystalline nature that yields
the specific solubility properties of the bone minerals. And
because bone tissues undergo constant tissue repair regulated by
mineral-resorbing cells (Osteoclasts) and mineral-producing cells
(Osteoblasts), solubility behavior of minerals is important in
maintaining a delicate metabolic balance between these cell
activities.
[0005] Synthetic bone graft material made to closely resemble
natural bone minerals can be a useful replacement for natural bone.
Acceptable synthetic bone can avoid the problem of availability and
harvesting of autologous bone (patient's own bone) and the risks
and complications associated with allograft bone (bone from a
cadaver), such as risks of viral transmission. An ideal synthetic
bone graft should possess a minimum of the following four
properties: (1) it should be chemically biocompatible; (2) it
should provide some degree of structural integrity in order to keep
the graft in place and intact until the patient's own bone heals
around it; (3) it should be resorbable so that the patient's own
bone ultimately replaces the graft; and, (4) because it may be
necessary to incorporate cells and/or biomolecules into the
synthetic bone material, it is desirable that the process used to
form the material employ low temperatures and chemically mild
conditions. Similar criteria are also important for other hard
tissue grafts (e.g. cartilage).
[0006] These criteria may be met by a material in which parameters,
such as Ca/P ratios, crystal size, crystallinity, porosity,
density, thermal stability and material purity are controlled.
While there have been considerable efforts to synthesize a ceramic
material for the use as implants, synthetic hydroxyapatites have
most often been used because their chemical formulae are very
similar to the naturally occurring mineral in bone. LeGeros R. Z.,
in Calcium Phosphates in Oral Biology and Medicine, Karger Pub.
Co., New York, 1991 teaches highly crystalline forms of
hydroxyapatite produced by solution precipitation followed by
sintering at high temperatures (800-1200.degree. C.). High
temperature treatment yields highly stoichiometric hydroxyapatite
with crystal sizes on the order of several microns with a Ca/P of
1.67. Such highly crystalline hydroxyapatite is essentially
non-resorbable in vivo. It is not replaced by living bone tissue
and remains intact in the patient for an undesirably extended
period of time.
[0007] A number of other approaches to the production of bone
substitute material have employed hydroxyapatite produced by a
solid-state acid-base reaction of primarily crystalline calcium
phosphate reactants. These hydroxyapatite bone substitute materials
are sometimes poorly-reacted, inhomogeneous, and have significant
crystalline hydroxyapatite content.
[0008] Constantz in U.S. Pat. No. 4,880,610 reports on the
preparation of calcium phosphate minerals by the reaction of a
highly concentrated phosphoric acid with a calcium source in the
presence of a base and hydroxyapatite crystals. The resultant
product is a polycrystalline material containing a crystalline form
of hydroxyapatite minerals. Likewise, U.S. Pat. No. 5,053,212 to
Constantz et al. discloses the use of a powdered acid source to
improve the workability and mixability of the acid/base mixture;
however, a mixed-phase calcium phosphate material similar to that
of US 4,880,610 is reported. Recently, Constantz et al. reported in
Science (Vol. 267, pp. 1796-9 (Mar. 24, 1995)) the formation of a
carbonated apatite from the reaction of monocalcium phosphate
monohydrate, Beta-tricalcium phosphate, Alpha-tricalcium phosphate,
and calcium carbonate in a sodium phosphate solution, to provide a
calcium phosphate material which is still substantially more
crystalline in character than naturally occurring bone
minerals.
[0009] Similarly, Brown et al. in U.S. Reissue Pat. No. 33,221
report on the reaction of crystalline tetracalcium phosphate (Ca/P
of 2.0) with acidic calcium phosphates. Liu et al. in U.S. Pat. No.
5,149,368 disclose the reaction of crystalline calcium phosphate
salts with an acidic citrate.
[0010] A number of calcium phosphate bone fillers and cements have
been described as "resorbable." Generally, these are compounds
comprising or derived from tricalcium phosphate, tetracalcium
phosphate or hydroxyapatite. At best these materials may be
considered only weakly resorbable. Of these, the tricalcium
phosphate compounds have been demonstrated to be the most
resorbable and after many years of study they are still not widely
used in clinical settings. The tricalcium phosphates are known to
have lengthy and somewhat unpredictable resorption profiles,
generally requiring in excess of one year for resorption.
Furthermore, unless steps are taken to produce extremely porous or
channeled samples, the tricalcium phosphates are not replaced by
bone. Recently it has been concluded that the "biodegradation of
TCP, which is higher than that of Hap [hydroxyapatite] is not
sufficient" (Berger et al., Biomaterials, 16:1241 (1995)).
Tetracalcium phosphate and hydroxyapatite derived compounds are
also only weakly resorbable. Tetracalcium phosphate fillers
generally exhibit partial resorption over long periods of time such
as 80% resorption after 30 months (Horioglu et al., Soc. for
Biomaterials, March 18-22, pg 198 (1995)). Approximately 30% of
microcrystalline hydroxyapatite implanted into the frontal sinus
remained after 18 months in cats.
[0011] All of these references disclose a chemical reaction
resulting in crystalline form of hydroxyapatite solids that has
been obtained by reacting crystalline solids of calcium phosphate.
There has been little reported on the use of amorphous calcium
phosphates (Ca/P of approximately 1.5) as one of the reactants
because the amorphous calcium phosphates are the least understood
solids among the calcium phosphates and amorphous calcium phosphate
has traditionally been considered to be a relatively inert and
non-reactive solid.
[0012] Amorphous calcium phosphate material has been used as a
direct precursor to the formation of a highly crystalline
hydroxyapatite compounds under generally high temperature
treatments. Such a highly crystalline material is inappropriate for
synthetic bone because it is highly insoluble under physiological
conditions. Chow et al. in U.S. Pat. No. 5,525,148 report the
testing of ACP precursors in a number of reaction schemes but
states that slurries of a variety of crystalline calcium phosphates
including ACP either alone or in mixtures do not produce a setting
cement or act as an effective remineralizing agent.
[0013] Brown et al. in U.S. Reissue Pat. No. 33,221 report on the
formation of crystalline hydroxyapatite for dental cement by
reacting an amorphous phase specifically restricted to tetracalcium
phosphate (Ca/P of 2.0) with at least one of the more acidic
calcium phosphates. Further, Brown et al., does not disclose the
preparation or the properties of such a tetracalcium phosphate in
amorphous state. Tung in U.S. Pat. No. 5,037,639 discloses the use
and application of standard amorphous calcium phosphate paste for
the remineralization of teeth. Tung proposes the use of standard
inert amorphous calcium phosphate mixed with and delivered through
as a chewing gum, mouth rinse or toothpaste, which upon entering
oral fluids converts to crystalline fluoride containing
hydroxyapatite which is useful to remineralize tooth enamel.
Simkiss in PCT/GB93/01519 describes the use of inhibitors, such as
Mg ions or pyrophosphate, mixed with amorphous calcium phosphate
and implanted into living tissues. Upon leaching of, for example Mg
ions, into surrounding bodily fluids, the amorphous
calcium-magnesium phosphate converts into a more crystalline
form.
[0014] There remains a need to develop new synthetic materials that
more closely mimic the properties of naturally-occurring minerals
in hard tissue. In particular, there remains a need to provide
synthetic bone materials which are completely bioresorbable, which
can be formed at low temperatures and are poorly-crystalline, with
nanometer-sized crystals.
SUMMARY OF THE INVENTION
[0015] The present invention provides a bioactive ceramic material
that is biocompatible, bioresorbable and workable for long period
of time at room temperature. The bioactive ceramic material may be
formed at low temperatures, is readily formable and/or injectable,
and yet can harden to high strength upon further reaction. The
bioactive ceramic material contains a poorly crystalline apatitic
calcium phosphate solids with Ca/P ratios comparable to naturally
occurring bone minerals and having stiffness and fracture toughness
similar to natural bone. The bioactive ceramic composite material
is strongly bioresorbable and its biosorbability and reactivity can
be adjusted to meet the demands of the particular therapy and/or
implant site. The material may be prepared as bone plates, bone
screws and other fixtures and medical devices, including
veterinarian applications, which are strongly bioresorbable and/or
ossifying.
[0016] These and other features of the invention are accomplished
by a self-hardening bioceramic composition, including a hydrated
precursor of a calcium phosphate and an aqueous-based liquid in an
amount sufficient to hydrate the calcium phosphate to form a paste
or putty, characterized in that hardening of the hydrated precursor
is associated with an endothermic reaction. Alternatively, a
self-hardening bioceramic composition, includes a hydrated
precursor of an amorphous calcium phosphate and an aqueous-based
liquid in an amount sufficient to hydrate the calcium phosphate to
form a paste or putty, characterized in that hardening of the
hydrated precursor occurs in more than ten minutes.
[0017] In another aspect of the invention, a bioceramic composition
is provided including a poorly crystalline calcium phosphate
prepared by promoting the hardening of a hydrated precursor
comprising an amorphous calcium phosphate and an aqueous-based
liquid in an amount sufficient to hydrate the amorphous calcium
phosphate to form a paste or putty, whereby hardening is associated
with an endothermic reaction and the conversion of the amorphous
calcium phosphate into the poorly crystalline calcium
phosphate.
[0018] The bioceramic composition of the invention may be prepared
by mixing in any order, (a) an amorphous calcium phosphate, (b) a
promoter, and (c) an aqueous-based liquid in an amount sufficient
to form a paste or putty, whereby the paste or putty is converted
into a poorly crystalline apatitic calcium phosphate and said
conversion is associated with hardening of the paste in an
endothermic reaction.
Definitions
[0019] "Amorphous"--By "amorphous" as that term is used here, it is
meant a material with significant amorphous character. Significant
amorphous character contemplates greater than 75% amorphous
content, preferably greater than 90% amorphous content, and is
characterized by a broad, featureless X-ray diffraction pattern. It
is recognized that a small degree of crystallinity may exist in the
material. However, for the amorphous precursor materials of the
present invention, it is preferable that the degree of
crystallinity be less than that desired in the product
material.
[0020] "Bioactive"--"Bioactive" refers to a material that induces
hard tissue formation in and about the implant. When implanted in
soft tissue, the bioactivity may also require the presence of a
growth or trophic factor, or the seeding of the implant with a hard
tissue forming cell type.
[0021] "Bioconipatible"--The term "biocompatible", as used herein,
means that the material does not elicit a substantial detrimental
response in the host. There is always concern, when a foreign
object is introduced into a living body, that the object will
induce an immune reaction, such as an inflammatory response that
will have negative effects on the host. For example, although
hydroxyapatite is generally considered to be "biocompatible",
significant inflammation and tissue necrosis have been observed
when crystalline hydroxyapatite microcarriers are inserted
intramuscularly in animals (see, for example, Ijntema et al., Int.
J. Pharm 112:215 (1994))).
[0022] "Bioresorbable"--"Bioresorbable" refers to the ability of a
material to be resorbed in vivo. "Full" resorption means that no
significant extracellular fragments remain. The resorption process
involves elimination of the original implant materials through the
action of body fluids, enzymes or cells. Resorbed calcium phosphate
may, for example, be redeposited as bone mineral, or by being
otherwise reutilized within the body, or excreted. "Strongly
bioresorbable", as that term is used herein, means that at least
80% of the total mass of material implanted intramuscularly or
subcutaneously is resorbed within one year. In preferred
embodiments of the invention, the strongly resorbing PCA calcium
phosphate is characterized in that, when at least 1 g (preferably
1-5 g) of PCA material is implanted at a subcutaneous or
intramuscular site, at least 80% of the material is resorbed w/in
one year. In more preferred embodiments, the material will be
resorbed within nine months, six months, three months, and ideally
one month. Furthermore, particularly preferred materials are
characterized in that they can be fully resorbed in the stated time
periods. For the purpose of this disclosure, "weakly" resorbable
means that less than 80% of the starting material is resorbed after
one year.
[0023] "Hardening"--"Hardening" refers to the process by which the
hydrated precursor is transformed into a hardened PCA material. The
PCA material is considered to be "hardened" when it is a
substantially non-formable solid. Such a hardened PCA material has
minimal compressibility and tends to undergo plastic as opposed to
elastic deformation.
[0024] "Hydrated precursor"--The term "hydrated precursor", as used
herein, refers to the paste or putty formed by hydration of the dry
PCA precursors in the presence of a limited amount of aqueous
solution (i.e., less than approximately 1 mL aqueous solution/1 g
precursor powder). The hydrated precursor may comprise both
reactants and products, in various combinations, depending on the
extent to which the conversion has progressed. Both the
"injectable" and "formable" PCA precursor pastes described herein
are hydrated precursors. Preferred "injectable" hydrated precursors
have a consistency appropriate for delivery through an 18 gauge
needle.
[0025] "Poorly crystalline apatitic calcium phosphate", "PCA
calcium phosphate" and "PCA material", as those terms are used
herein, describe a synthetic poorly crystalline apatitic calcium
phosphate. The PCA material is not necessarily restricted to a
single calcium phosphate phase provided it has the characteristic
XRD and FTIR pattern. A PCA calcium phosphate has substantially the
same X-ray diffraction spectrum as bone. The spectrum is generally
characterized by only two broad peaks in the region of
20-35.degree. with one centered at 26.degree. and the other
centered at 32.degree.. It is further characterized by FTIR peaks
at 563 cm.sup.-1, 1034 cm.sup.-1, 1638 cm.sup.-1 and 3432 cm.sup.-1
(.+-.2 cm.sup.-1). Sharp shoulders are observed at 603 cm.sup.-1
and 875 cm.sup.-1, with a doublet having maxima at 1422 cm.sup.-1
and 1457 cm.sup.-1.
[0026] "Promoter"--The term "promoter", as used herein, describes a
material or treatment that promotes hardening of a hydrated
precursor and may enhance the ACP to PCA calcium phosphate
conversion. Some promoters participate in the conversion and are
incorporated into the product PCA material; others, known as
"passive" promoters, do not participate.
[0027] "Reactive"--"Reactive" is used herein to refer to the
ability of an amorphous calcium phosphate when mixed with liquid to
form a hydrated precursor to undergo conversion to the PCA material
of the present invention in the presence of a promoter in
association with hardening of the precursor materials. Preferred
ACPs are characterized by an ability to convert completely, an
ability to convert quickly with hardening, an ability to undergo
conversion with otherwise inert compounds and/or an ability to
convert into a substantially homogeneous PCA material. Where the
ACP is reacted with a second calcium phosphate, the "conversion"
can encompass conversion of both the ACP and the second calcium
phosphate. The degree of hardening and the kinetics of the
hardening process are also important elements of reactivity. Some
ACPs are more reactive than others. An ACP is considered "highly
reactive" if it undergoes conversion and hardening to a PCA
material in the presence of a weak promoter, such as dicalcium
phosphate dihydrate ("DCPD") with a grain size distribution
containing a significant fraction of grains greater than 100 .mu.m.
Preferred highly reactive ACPs produce a hardened PCA material in
the presence of weakly promoting DCPD and water at 37.degree. C. in
less than twelve hours, with hardening being substantially complete
in about one to five hours, and ideally 10-30 minutes.
BRIEF DESCRIPTION OF THE DRAWING
[0028] FIG. 1 is a high-resolution transmission electron micrograph
of the reactive amorphous calcium phosphate illustrating the
nanometer-sized grains in clusters with relatively unclear
boundaries and partially immersed in shapeless form (arrows);
[0029] FIG. 2 is an energy-dispersive electron microprobe spectrum
of the reactive amorphous calcium phosphate of the present
invention after the vacuum heating procedure which yielded Ca/P to
be 1.58;
[0030] FIG. 3 is a solubility curve of a poorly crystalline
apatitic calcium phosphate product derived from amorphous calcium
phosphate of the present invention, as compared with a crystalline
hydroxyapatite. Note the relative higher solubility of the material
of the present invention versus a more crystalline form of
hydroxyapatite, as measured by the amount of calcium ions released
into solution at 37.degree. C.;
[0031] FIG. 4 are X-ray diffraction patterns of (a) reactive
amorphous calcium phosphate; and (b) dicalcium diphosphate used in
a reaction to form a bone substitute material of the invention;
[0032] FIGS. 5a-d are X-ray diffraction patterns tracking the
progress of the reaction of a mixture of reactive amorphous calcium
phosphate and dicalcium diphosphate to form a PCA material of the
present invention;
[0033] FIG. 6 is infrared spectra of (a) dicalcium phosphate
dihydrate, (b) the activated ACP of the invention, and (c) the PCA
material of the present invention;
[0034] FIG. 7 is an X-ray diffraction pattern of naturally
occurring bone;
[0035] FIG. 8 is a bar graph displaying particle size distribution
for various formulations described in Example 10;
[0036] FIG. 9 presents photomicrographs of tibial defects either
untreated (9a) or treated (9b) with a delivery vehicle of the
present invention; in FIG. 9a, the small arrows indicate one edge
of the defect; the large arrowhead is at the yet unbridged defect;
in FIG. 9b, large arrowheads denote one edge of the defect; and in
both Figures, magnification is 4.times., bone is decalcified, and
slides are treated with hematoxylin and eosin;
[0037] FIG. 10 is a photomicrograph of canine trabecular bone grown
into a defect treated with the drug delivery vehicle of the present
invention (magnification 10.times.; decalcified; hematoxylin and
eosin);
[0038] FIG. 11 is a photomicrograph of a canine cortical bone
defect that was treated with the drug delivery vehicle of the
present invention (magnification 4.times.; undecalcified, Light
Green Basic Fuchsin);
[0039] FIG. 12 presents photomicrographs of untreated (FIG. 12a)
and treated (FIG. 12b) rabbit tibia defects 4 weeks after surgery
(magnification 4.times.; decalcified; Masson's Trichrome);
[0040] FIG. 13 is an X-ray diffraction patterns of PCA calcium
phosphate prepared form Al.sub.2O.sub.3 passive promoter, in which
Al.sub.2O.sub.3 peaks are indicated by lines;
[0041] FIG. 14 is an X-ray diffraction pattern of PCA calcium
phosphate prepared as described in Example 1-2;
[0042] FIG. 15 is and X-ray diffraction pattern of PCA calcium
phosphate prepared as described in Example 14;
[0043] FIG. 16 is a DSC plot of the reaction of reactive ACP with
DCPD showing endothermic nature of the reaction;
[0044] FIG. 17 is infrared spectra of the amorphous calcium
phosphate material before heat treatment (FIG. 17a) and after heat
treatment (FIG. 17b); and
[0045] FIG. 18 is a full width XRD of the PCA calcium phosphate of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0046] The present invention is directed to biocompatible ceramic
compositions adapted for use in the repair and growth promotion of
hard tissue including the fabrication of resorbable orthopedic and
dental fixtures. The compositions comprise a biocompatible and
highly bioresorbable poorly crystalline apatitic calcium phosphate
(PCA calcium phosphate) sometimes combined with a suitable
biocompatible matrix or additive. The PCA calcium phosphate has
utility in dental, orthopedic, drug delivery, cell therapy and
other therapeutic applications.
[0047] The inventive composition may be applied as a bone cement to
the bone-contacting surfaces of prosthetic devices. It may be
applied directly to bone defects as a filler, where it is capable
of promoting the growth of new bone tissue. The composition may
similarly be applied for repair, growth or production of
cartilaginous tissue. Alternatively, the composition may be used to
fabricate fixtures or devices such as screws and plates, which
under appropriate circumstances will be resorbed and replaced by
bone or cartilage. The composition may also be used free standing
in soft tissue. When a pharmaceutically active agent is added to
the composition, it serves as a drug delivery device, and release
of the agent may occur over an extended time period after
implantation as the composition slowly biodegrades.
[0048] The invention also provides methods for promoting the
conversion of ACP to PCA calcium phosphate, in a controlled
fashion, in the form of a paste or putty which hardens
predictably.
[0049] The PCA calcium phosphate bioceramic of the invention is
generally calcium deficient with a calcium to phosphate ratio of
less than 1.5 as compared to the ideal stoichiometric value of
approximately 1.67 for hydroxyapatite. They are further
characterized by their biological bioresorbability and minimal
crystallinity. They may be rapidly bioresorbable and possess high
porosity and/or low density or slowly bioresorbable and possess
decreased porosity and/or high density. Their crystalline character
is substantially the same as natural bone without the higher degree
of crystallinity seen in the bone substitute materials known to the
art. The inventive PCA calcium phosphate also is biocompatible,
that is, there is no significant detrimental reaction (e.g.,
inflammation or fibrosis) induced in the host by the implanted
material. Materials which induce a medically acceptable level of
inflammation or fibrosis are considered biocompatible. The PCA
calcium phosphate may be used in a moist precursor form (i.e.,
hydrated precursor) and applied as a cement directly to a surgical
site such as a fracture, or it may be hardened ex vivo and
subsequently implanted.
[0050] The resorbability of the inventive PCA calcium phosphate is
attributable to a combination of density, porosity, chemical
composition and crystalline structure. Low crystallinity in
apatites is associated with somewhat increased solubility in
aqueous systems compared to other more crystalline species, and
thus the low crystallinity and/or presence of stable amorphous
apatitic domains in the inventive PCA calcium phosphate is believed
to be associated with its resorbability in biological systems.
Porosity facilitates both the penetration of cells and cell
processes into the bioceramic matrix and the diffusion of
substances to and from the matrix interior. Accordingly, PCA
calcium phosphate compositions of lower porosity resorb more slowly
in vivo than those of high porosity. In one embodiment, the use of
controlled particle size reactants leads to a PCA calcium phosphate
material of controlled porosity. Other methods of promoting
porosity may be employed, such as chemical or physical etching and
leaching.
[0051] The inventive PCA calcium phosphates may be manufactured
with a variety of resorption rates ranging from slow resorption
times of greater than one year (typical of weakly resorbing
hydroxyapatites bone fillers and bone substitutes known to the art)
to resorption rates as fast as several grams, e.g., 1-5 g, in 1 to
2 months. Thus depending upon the density, porosity, reactants
used, final crystallinity of the reaction product, and the amount
of crystallization inhibitors used, formulations can be prepared in
which a one gram device will fully resorb in any desired time
period--from 2 weeks to 1, 3 or 6 months to 1, 2 or three years. A
strongly resorbable PCA calcium phosphate of the instant invention
possesses an in vivo resorption rate in which 80% or more of at
least one gram (preferably 1-5 g) of starting material is resorbed
within one year, preferably within 6 months, more preferably in
less than 3 months, and most preferable within 1-2 months.
[0052] For the production of new bone in load bearing situations it
has been found that preparations which are fully resorbed and
replaced by bone in about six to eight weeks lead to histologically
normal bone by 12 weeks. In some load bearing situations it may be
desirable to have resorption occur more slowly. Additionally, when
hard tissue is being prepared ectopically or the shape of an
existing hard tissue is to be augmented, it may be desirable to
employ more slowly resorbing PCA calcium phosphate.
[0053] Adjustment of the density or porosity of the resultant PCA
calcium phosphate or the use of reaction parameters which affect
the speed and hardness of setting are all useful approaches to
varying the in vivo resorption time of the inventive PCA calcium
phosphate. These parameters may be adjusted alone or in combination
as required by specific applications.
[0054] Slow resorption (greater than three months) is favored by
the use of high density, low porosity PCA calcium phosphate and/or
rapid reaction and hardening times. Fast resorption (three or less
months) is favored by the use of low density, high porosity PCA
calcium phosphate, and/or slow reaction and setting times. Guidance
for adjustment of rate and completeness of reaction to form the PCA
calcium phosphate are given elsewhere herein. The following
describes the production of preferred PCA calcium phosphate
precursors which lead to a hardened PCA calcium phosphate cements
of differing resorbability kinetics in vivo.
[0055] A rapidly resorbing PCA calcium phosphate is obtained by
conversion of the highly reactive ACP of Example 5 using a DCPD
with a grain size distribution having a considerable content of
grain sizes greater that 100 .mu.m (e.g. corresponding to
distribution B1 in Table 3) as a promoter. The powders are prepared
as a hydrated precursor as described in Example 8.
[0056] A slowly resorbing PCA calcium phosphate is obtained by
conversion of the highly reactive ACP of Example 5 using DCPD with
a grain size distribution having a minimal content of grain sizes
greater than 100 .mu.m (e.g. corresponding to distribution B3 in
Table 3) as a promoter. The powders are prepared as a hydrated
precursor as described in Example 9.
[0057] The inventive PCA calcium phosphate undergoes ossification.
Ossification refers to the replacement of the implanted synthetic
calcium phosphate with bone which histologically is similar or
identical to natural bone. Ossification of the inventive PCA
calcium phosphate tends to occur in stages with more unorganized
bone appearing prior to the establishment of more natural appearing
tissue. The inventive PCA calcium phosphate is different from
previous bone fillers and cements because bone formation does not
occur only at the outer edge of the implant, but initiates
simultaneously throughout the implant, presumably in association
with the resorptive process. Within two to three weeks following
implantation of the PCA material into a load bearing region, such
as the tibia or radius, preliminary ossification is observed by the
formation of small foci of mineralized osteoid formation
(spicules). By four weeks, the spicules have given way to lacy
appearing thin cancellous trabecular bone and thin cortical bone.
At six weeks, ordered normal or thicker than normal compact
cortical bone with lacunae-containing osteocytes is observed. At
time points after six weeks, final remodelling occurs so that by
twelve weeks the newly ossified bone is indistinguishable from
native bone.
[0058] Thus, ossification in the presence of PCA calcium phosphate
generally reaches completion and appears to occur more rapidly than
normal bone growth. This rapid rate of ossification suggests the
inventive PCA calcium phosphate enhances bone healing. New bone is
observed as early as two weeks and may reach the fully
histologically organized state within six weeks, but in any case by
3-6 months. In sheep segmental defect fracture models employing
implants of up to 3 gms of hydrated precursor, bone having 100% of
the strength of non-fractured bone was found within three months.
In the presence of trophic or growth factors such as bone
morphogenic proteins this process may be accelerated.
[0059] In preferred embodiments, in order to optimize ossification,
devices, pastes and putties of the invention may be seeded with
bone forming cells. This is most easily accomplished by placing the
device (containing PCA calcium phosphate or a hydrated precursor
thereto) in contact with a source of the patient's own bone forming
cells. Such cells may be found in bone-associated blood or fluids,
including exogenous fluids which have been in contact with bone or
bone materials or regions, including the periosteum, cortical bone,
cancellous bone or marrow. They are also present in tissue
including cortical or cancellous bone, bone marrow or periosteum.
In the case of devices such as screws and pins, the introduction of
which into bone is accompanied by bleeding, no further seeding is
required. For plates, which oppose only cortical bone, induction of
a periosteal lesion which will contact the device is recommended.
In yet other embodiments, it will be useful to surgically prepare a
seating within the bone by removing a portion of cortical bone at
the implant site. Other steps may also be taken to augment
ossification, including introduction of bone forming cells
harvested from the patient into the graft, or incorporation of
trophic factors or bone growth inducing proteins into, or onto the
device. Non-autologous bone cells are also within the scope of the
invention if the desired amount of bone regeneration occurs prior
to host rejection of the bone forming cells. Thus, cells or tissues
obtained from primary sources, cell lines or cell banks may all be
useful in certain embodiments. Similar considerations apply for
cartilage formation and healing and the seeding of the inventive
PCA calcium phosphate with chondrocytes and/or other cartilage
forming cells.
[0060] Due to the nature of the reaction used to produce preferred
formulations of the inventive PCA calcium phosphate, the ease of
use as an implant material in a surgical setting is significantly
improved over other bone substitute materials known to the art.
Specifically, the reaction is initiated outside the body and
proceeds slowly at room temperature thereby minimizing the
possibility that the material will "set up" and become unusable
prior to application to the surgical site. The reaction accelerates
significantly at body temperature and the material hardens in
place. Furthermore, the consistency and formability of the
inventive PCA calcium phosphate as well as the reaction speed may
be varied according to the therapeutic need, by modifying a few
simple parameters.
[0061] Preparation of a PCA Calcium Phosphate. Many amorphous
calcium phosphates tend to spontaneously convert to a more
crystalline form over time. Hydroxyapatite is a thermodynamically
favored form of calcium phosphate and is often the product of such
conversion. The instant invention has recognized the value of a
controlled conversion of an ACP to a more crystalline form (e.g.
PCA calcium phosphate) without significant further crystallization,
particularly when the conversion is performed in the presence of a
limited amount of water and is accompanied by a hardening reaction.
The instant invention provides reactions which lead to the
formation of PCA calcium phosphate. These reactions advantageously
may be initiated outside of the body, using a precursor having a
paste or putty consistency and may be significantly accelerated at
37.degree. C. leading to a hardened calcium phosphate product. In
some embodiments, the hardened PCA calcium phosphate alone has a
durometer and bulk modulus similar to traditional blackboard chalk.
In some instances, hardened PCA material will be associated with
the presence of unreacted precursors, promoters, and/or
supplemental materials, side products and by-products.
[0062] According to the method of the invention, a paste- or
putty-like hydrated precursor is formed by addition of water to a
calcium phosphate precursor. The hydrated precursor is then heated
to about 37.degree. C., thereby initiating a substantially net
endothermic reaction which is characterized by hardening of the
paste or putty, as indicated by the differential scanning
calorimeter (DSC) data shown in FIG. 16. In preferred embodiments,
the PCA calcium phosphate material is produced from a hydrated
precursor by conversion of a reactive amorphous calcium phosphate
to PCA calcium phosphate in the presence of a promoter. Promoting
the conversion of ACP in a paste form to well crystallized
hydroxyapatite, accompanied by hardening of the paste via an
endothermic reaction is also considered to be within the scope of
the invention
[0063] An endothermically setting bone cement provides several
important advantages over calcium phosphate bone cements and
fillers known in the art. Because the reaction does not give off
heat there is no danger of heat related damage to cells and tissues
in the implant area. Additionally, the endothermic nature of the
reaction means reaction progress can be controlled by regulating
the amount of heat available to support the reaction. The hydrated
precursor reacts minimally at room temperature and below. This
means that many of the handling problems associated with surgical
cements and fillers known to the art are avoided.
[0064] In preferred embodiments, the reactants are mixed outside of
the body, yielding a hydrated PCA calcium phosphate precursor
material suitable for application to a surgical site. The reaction
generally is completed after application to the surgical site,
although in some embodiments the reaction is completed ex vivo. The
PCA calcium phosphate reactions of the invention generally lead to
hardening of the hydrated precursor in less than five hours,
substantially hardening in about one to five hours under
physiological conditions, and preferably in about 10-30 minutes. In
a preferred embodiment, the reaction is initiated by adding
physiological saline to a mixture of two dry components to form a
thick paste which hardens in association with an endothermic
reaction at 37.degree. C. in about a half an hour. Other aqueous
agents such as but not limited to, water, buffer solutions, serum
or tissue culture medium may be used in place of distilled
water.
[0065] Under any reaction scheme it is important that the ACP
retains significant amorphous character prior to conversion.
Specifically, the overall crystallinity within the starting ACP
cannot exceed that desired in the end product. Thus certain
reaction schemes may require stabilization of the amorphous nature
of the ACP throughout the reaction period. Examples of suitable
such inhibitors of crystal formation known to the art include
carbonate, pyrophosphate, and magnesium. Additional guidance for
the use of inhibitors of crystallization may be found in Elliot,
Structure and Chemistry of the Apatites and other Calcium
Orthophosphates, Elsevier, The Netherlands, 1994, herein
incorporated by reference.
[0066] Types of Promoters. The purpose of the promoter is to
promote the hardening of the hydrates precursor and preferably to
accelerate the conversion of ACP to a PCA calcium phosphate. Any
material or method which serves this purpose is considered to be
within the scope of the reaction. This includes the limited case
where hardening occurs in the absence of conversion, that is when a
PCA calcium phosphate precursor is used as the starting
material.
[0067] With respect to the conversion of ACP, a promoter may
promote the overall reaction or any intermediate reactions involved
in the conversion or hardening process. In this regard preferred
promoters will reduce the activation energy for one or more
specific steps in the conversion or hardening process.
[0068] The promoter used to convert a reactive ACP to the inventive
PCA calcium phosphate may itself be converted to PCA calcium
phosphate calcium phosphate or otherwise participate in a chemical
or physical reaction during the conversion process. Such promoters
are referred to herein as "participating" promoters.
[0069] Alternatively a promoter may remain substantially unchanged
during the reactive ACP conversion serving essentially to catalyze
or to initiate or enhance PCA nucleation and hardening. These
promoters are referred to as "passive" promoters.
[0070] Promotion of the hardening and conversion of a reactive ACP
to PCA calcium phosphate through the use of other means such as the
use of heat, pressure, reactive gases, solvents, ionic solutions,
or radiochemistry is also considered within the scope of the
invention. Such promoting means are termed reaction enhancing or
"enhancing" promoters.
[0071] Promoters may have different abilities or strengths in the
promotion of the production of a hardened PCA calcium phosphate
from ACP. Likewise, not all ACPs are equally reactive. Thus weak
promoters will not always be effective in reacting with ACPs with
low reactivity. In such circumstances stronger promoters will be
preferred. Promoter strength may conveniently be tested by
comparing the reactivity of a given promoter with the preferred
carbonated ACP of the invention in both its heat activated highly
reactive form as well as its non heat activated form using the
method described in Example 8. The use of hand mixing of reactants
is particularly suited for identification of highly reactive
promoters. Less reactive promoters may benefit from mixing in an
automated mill as described in Example 9. By use of these methods
DCPD with the grain size distribution of B1 in example 10 was
demonstrated to be a weak promoter, where as grain sizes in the
range of <100 .mu.m were found to be strongly reaction
promoting.
[0072] In addition to the guidance given above for the matching of
a particular promoter to a given ACP, such matching may be done
empirically by mixing a given ACP with a selected promoter in the
presence of about 1.0 mL water/g powder and heating the mixture at
37.degree. C. in a moist environment. A suitable promoter exhibits
PCA calcium phosphate formation and paste hardening under these
conditions.
[0073] The method of preparation of the promoter and/or the ACP
will affect the ease by which the hydrated precursor is converted
into the PCA material. As noted above, the method of mixing the
powdered reactants prior to addition of liquid affects the
reactivity of the system. Thus, hand mixing using a mortar and
pestle does not result in as reactive a system as a prolonged
machine grinding of the reactant powders. Therefore when comparing
promoters, it is important to used standardized preparation
conditions.
[0074] It is hypothesized that the conversion of ACP to the
reactive PCA calcium phosphate is a surface catalyzed phenomenon.
If so, it may be desirable to produce a particular promoter with a
reproducible surface area. Thus, to control reaction
reproducibility it is advantageous to provide a promoter with a
known grain size distribution. Standard sieving techniques are
suitable for selection of specific grain sizes.
[0075] Many calcium- or phosphate-containing compounds may be used
as participating promoters in the hardening reaction. A calcium
phosphate promoter, may be of any crystalline structure and should
be chosen so as to be reactive with ACP either directly or through
the use of enhancing promoters. Preferred participant promoters are
those which tend themselves to undergo conversion to hydroxyapatite
through an intermediate PCA calcium phosphate calcium phosphate
phase.
[0076] Appropriate participating calcium phosphate promoters
include neutral, basic and acidic calcium phosphates, preferably
apatitic phosphates, which provide the appropriate stoichiometry
for reaction to obtain an apatitic calcium phosphate. Suitable
calcium phosphate promoters include, but are in no way limited to,
calcium metaphosphate, dicalcium phosphate dihydrate, monetite,
heptacalcium phosphate, tricalcium phosphates, calcium
pyrophosphate dihydrate, Hydroxyapatite, poorly crystalline
apatitic calcium phosphate, tetracalcium phosphate, calcium
pyrophosphate, octacalcium phosphate, and a second ACP. Other
sources of phosphate or calcium, such as by way of example only,
CaO, CaCO.sub.3, calcium acetate, and H.sub.3PO.sub.4, may be mixed
to form a final product to yield a desired Ca/P ratio close to
natural bone. It may be desirable to provide the second component
in the amorphous or poorly crystalline state, as well.
[0077] In a preferred embodiment, DCPD is used as a participating
promoter with a grain size less than 200 .mu.m, in more preferred
embodiments with an average grain size of <95 .mu.m, and in most
preferred embodiments with an average grain size of about 35-45
.mu.m and a grain size maximum of less than about 110 .mu.m.
[0078] In those cases where amorphous calcium phosphate is used as
the sole precursor to produce the inventive PCA calcium phosphate
it is important to control the natural tendency of the ACP to
convert to highly crystalline hydroxyapatite. On the other hand,
the rate of conversion and hardening should be fast enough to have
surgical utility. One approach is to combine a precursor ACP
containing an inhibitor of crystal formation (e.g. the ACP of
Example 5) with an ACP that does not contain an inhibitor of
crystal formation (e.g., a promoter). The reactants may be mixed in
a dry state, with the appropriate particulate size and an excess of
the inhibitor-containing ACP. The reactants can then be exposed to
crystal-forming conditions such as the addition of water, followed
by an elevation in temperature, such as that which occurs following
introduction into the body, to convert the reactants to the PCA
calcium phosphate of the invention. Unless steps are taken to
further promote this reaction, the use of ACP as a promoter alone
leads to a PCA calcium phosphate that does not tend to harden
exceptionally well.
[0079] It is an interesting and unexpected feature of the inventive
reaction that along with ACP, a participating promoter may likewise
be converted to PCA calcium phosphate. This has been demonstrated
experimentally for both DCPD and stoichiometric hydroxyapatite.
Thus the conversion of a crystalline calcium phosphate to a less
crystalline state in a substantially endothermic reaction has been
shown for the first time.
[0080] While the conversion of ACP to PCA calcium phosphate has
been demonstrated herein above, it is recognized that alternative
materials may also be converted to a PCA calcium phosphate. Thus
the production of a hydrated precursor paste from a crystalline
calcium phosphate (including PCA calcium phosphate) in the presence
of a limited amount of water in association with a net endothermic
reaction at 37.degree. C. and accompanied by paste hardening is
considered within the scope of the invention. A preferred
embodiment of this approach features a PCA calcium phosphate and a
DCPD as reactants to produce a PCA calcium phosphate bioceramic
[0081] Hydroxyapatite is a thermodynamically favored form of
calcium phosphate. It is therefore also within the scope of the
invention to promote the conversion of the reactive ACP into a PCA
calcium phosphate in association with hardening of a hydrated
precursor, through the use of promoters which themselves do not
convert to PCA calcium phosphate (or hydroxyapatite). Suitable such
promoters are termed "passive" and include, but are not limited to
nucleation causing substances and catalysts. Particularly suitable
in this regard are substances which provide reactive surfaces which
weakly promote apatitic crystallization to produce a poorly
crystalline apatitic calcium phosphate.
[0082] In one aspect, the invention features the use of passive
promoters which are of limited solubility or insoluble in the
aqueous liquid used to hydrate the ACP. Suitable promoters include,
but are not limited to, metals, metal oxides, ceramics, silicates,
sugars, salts, or polymeric particulate. For many applications
preferred promoters will be themselves biodegradable. In general
these substances are provided in granular form with a grain size in
the range of 1 to 500 .mu.m, preferably 1 to 300 .mu.m, and most
preferably 1 to 200 .mu.m. The actual grain size used may be varied
to improve the reaction promoting characteristics of the particular
substance.
[0083] Table 2 of Example 3 reports the effect of a variety of
passive promoters in the conversion of ACP to PCA calcium phosphate
in the presence of a limited volume of water. Generally the
promoter is present in an amount less than or equal to the ACP, and
specifically in the range of about 1:1 to about 5:1 ACP:promoter.
An amount of water (here, weight=volume, since density of water is
one) approximately equal to the total weight of the two dry
components is used to prepares a paste. Actual proportions of ACP,
promoter and water can be conveniently determined by mixing the
components in varying amounts and selecting the formulation which
leads to a hardened PCA calcium phosphate at 37.degree. C. in the
desired amount of time. Preferred passive promoters include but are
not limited to granular forms of SiO.sub.2, mica, Al.sub.2O.sub.3,
poly(L-lactide) (PLLA), polyglycolide (PGA), and
poly(lactide-co-glycolid- e (PLGA) copolymers.
[0084] Lastly, suitable enhancing promoters include, but are not
limited to, water, heat, salts and additional calcium phosphate
sources. In general these substances act to enhance the reactivity
of ACP with a second calcium phosphate thereby promoting the
conversion of ACP to PCA calcium phosphate. Conversion reactions
may include acid/base, displacement, substitution, and hydrolysis
reactions.
[0085] The inventive reaction permits one to design and modify the
chemical composition of the resultant product, thereby providing a
further mode of controlling bioactivity of the final product.
Because the amorphous calcium phosphate tends to react completely
with the other solids, the Ca/P of the resultant solid will be
determined by the total calcium and phosphates present as starting
materials. This permits reliable manufacture of PCA calcium
phosphate products simply by selection of the relative proportions
of the starting amorphous and secondary calcium phosphates. It is
generally desirable to maintain a calcium to phosphate ratio of
about 1.1-1.9, preferably less than 1.5, and most preferably about
1.4.
[0086] A particularly useful approach is to form the precursor
paste into the approximate shape or size and then harden the
material in vitro in a moist environment at 37.degree. C. If
desired, the hardened material may then be precisely milled or
machined to the desired shape prior to use in the surgical setting.
In those cases where storage of the hardened material is desired,
it may be useful to enhance the stability of the inventive PCA
calcium phosphate. In such cases, exposure of the pre-formed object
to inhibitors of hydroxyapatite crystallization may be useful.
Inhibitors may be added to the aqueous medium used to prepare the
inventive PCA calcium phosphate calcium phosphate. Alternatively,
the finished material or objects made from it may be exposed to an
inhibitory substance. Suitable such inhibitors include but are not
limited to magnesium, carbonate, pyrophosphate, poly L-glutamate,
polyacrylate, phosvitin, casein, and protein-polysaccharides.
Guidance for the use of such compounds can be found in Termine et
al. Arch. Biochem. Biophys. 140:318-325 (1970) incorporated herein
by reference. Storage at 4.degree. C. or preferably colder
temperatures such as -20.degree. C., or -75.degree. C. will also
retard crystallization.
[0087] In the embodiments described above, the paste or putty is
hardened at 37.degree. C. Hardening at 37.degree. C. is important
for in vivo application of the hydrated precursor; however, the
reaction proceeds at both higher and lower temperatures. This
reactivity range may be taken advantage of when the paste or putty
is to be hardened outside the body. In such cases, higher
temperatures may be employed to further accelerate the hardening
process. In this regard temperatures less than about 48.degree. C.
are preferred.
[0088] For in vitro hardening the use of a moist environment is
useful (although not critical) because the reaction tends to
consume water. In addition it is desirable to avoid evaporative
water loss of the sample while it is hardening. Thus, use of a
reaction chamber with a high ambient humidity is preferred
(>80%, preferably 100% humidity). Alternatively the reaction and
hardening process can often be performed under water.
[0089] The PCA calcium phosphate materials and composites of the
invention are porous. Air dried samples can generally absorb water
to an extent of 20% or more of their total volume. In many
embodiments amounts of water greater than 30% of the total sample
volume may be absorbed and in some preferred embodiments, water in
amounts of greater than 40% preferably greater than 50% of the
sample volume may be absorbed.
[0090] Any approach affecting the porosity of the hardened sample
may be employed, although preferred approaches include the use of
controlled compression molding for ex vivo fabrication and the use
of specific promoter grain sizes for either ex vivo or in vivo
hardening. The reaction may be performed in a chamber or mold to
any pressure up to at least five tons.
[0091] In establishing new formulations of the inventive material
it will be useful to know the nature and extent of the reaction. A
number of tests for the identification of reaction products and
reaction completeness may be used.
[0092] Hardness may be determined by simple inspection or manually
probing the reaction product. The use of quantitative measures
employing load cells and force transducers is however preferred.
Hardness alone does not necessarily confirm conversion, although
the inventive reactions have been designed so that hardening is
accompanied by conversion.
[0093] The X-ray spectra of the inventive PCA calcium phosphate is
presented in FIG. 18. As can be seen from the figure the spectrum
is characterized by broad peaks at approximately 2.theta.=26 and
32. An additional broad shoulder occurs at approximately
2.theta.=29 and another may be present at approximately
2.theta.=33.6. Absent from the spectra are any additional sharp
peaks or sharp shoulders characteristic of crystalline
hydroxyapatite occurring in the range of 2.theta.=27-34. In
particular there are no sharp peaks or shoulders corresponding to
Miller's Indices of 210, 112, or 300 for hydroxyapatite.
[0094] FTIR spectrum is characterized by peaks at 563 cm.sup.-1,
1034 cm.sup.-1, 1638 cm.sup.-1 and 3432 cm.sup.-1 (.+-.2
cm.sup.-1). Sharp shoulders are observed at 603 cm.sup.-1 and 875
cm.sup.-1, with a doublet having maxima at 1422 cm.sup.-1 and 1457
cm.sup.-1 (see, FIG. 6c).
[0095] For some embodiments it may be desirable to actually to have
the presence of some unreacted crystalline calcium phosphate
present following conversion (e.g. DCPD or hydroxyapatite). In such
circumstances, the quantities of second calcium phosphate may be
adjusted relative to the quantity of ACP present. Alternatively,
reactions using a weaker promoter or less reactive ACP may also
result in some unreacted starting materials. Mixtures of PCA
calcium phosphate and DCPD, or PCA calcium phosphate and
hydroxyapatite or PCA calcium phosphate and other reactants are
within the scope of the invention. In some limited cases, the use
of PCA calcium phosphate itself provided it has a significant
amorphous character) in place of ACP is possible.
[0096] An implantable bioceramic material may be prepared in
precursor form as a paste or putty by addition of a fluid to the
precursor materials and mixing, The precursor materials may include
an ACP, a promoter and additional supplementary materials if
required (in some cases some or all of these constituents may be
partially pre-hydrated). The mixing of the components may occur in
any convenient order. The components may be mixed and/or physically
ground prior to the addition of fluid. Alternatively fluid may be
added to a single dry component, and then additional dry components
added to complete the paste.
[0097] A wide variety of proportions of reactants may be used, in
most cases the absolute ratio of constituents will depend on the
circumstances of the intended use. For systems employing only an
ACP and a participating promoter the reactants will generally be
used in equal amounts by weight. Water will also be added in a
weight approximately equal to the combined weight of the other dry
reactants.
[0098] In a preferred embodiment, a DCPD with grain size
distribution similar to distribution B3 from Example 10 and a
highly reactive carbonated ACP from Example 5 with an ACP:DCPD
ratio of 0.5 g:0.5 g may be combined with water in amounts ranging
from 0.7 to 1.3 mL.
[0099] In the case of reactions involving passive promoters and ACP
alone, it has been found that ACP:promoter proportions in the range
of about 5:1 to 1:1 work well. For a total weight of reactants of 1
gram, 0.5 to 1.5 mL water may be used.
[0100] Empirical determination of appropriate amounts of reactants
and water may be made by (a) establishing ratios of dry components
and water that lead to the formation of a workable paste or putty;
(b) selecting those formulations which lead to hardening in a
suitable amount of time (most often 20 to 60 minutes) at 37.degree.
C.; and/or (c) testing the performance of the selected formulations
in a suitable model system (e.g. in vivo subcutaneous resorption or
in vitro tissue culture resorption models)
[0101] In some preferred embodiments (e.g., Examples 8-10), the
reaction occurs slowly at room temperature and is almost
undetectable below 18 or 19.degree. C. (see DSC example). The
reaction is accelerated at higher temperatures, and particularly at
body temperature. This property is particularly useful in a
surgical situation, since the hydrated precursor paste formed by
mixing reactants with a limited volume of water remains injectable
and/or formable for a considerable period of time (up to several
hours) while held at room temperature, provided care is taken to
prevent evaporative moisture loss. Thus, at room temperature in air
(ca. 22.degree. C.) the paste hardens after a time greater than one
hour and remains formable and/or injectable for longer than 10
minutes, and preferably longer than one hour and most preferably
longer than three hours. Following injection at the implant site
(ca. 37.degree. C.), the paste hardens in less than about an hour,
preferably in about 10-30 minutes. When held at 4.degree. C. the
paste is not hard even after several days, provided care has been
taken to prevent evaporative moisture loss. Alternatively, once the
material has been implanted, hardening can be accelerated by
application of heat to the implant. Heat may be applied through the
use of lasers, ultrasound, and the like, or by other means
including the use of pharmaceutical to locally raise or lower the
body temperature.
[0102] Depending upon the amount of fluid added, the mixture of an
ACP and a promoter results in a hydrated precursor mixture with
varying consistency. By selecting the appropriate amount of liquid
to be added to the reactants, the viscosity of the precursor paste
may be adjusted according to need. The paste may be prepared either
with an injectable or a formable consistency or it may be prepared
with just enough liquid to be both injectable and formable.
[0103] Injectable paste is generally prepared by mixture of the
reactants in an amount of water or buffer sufficient to produce the
desired consistency for injection. Most often this will be as thick
as possible while still being able to be passed through a 16-18
gauge syringe. For some formulations requiring injection directly
into solid tissue (e.g. into cortical bone of an osteoporosis
patient) thinner consistencies (e.g., 1.5 mL H.sub.2O/g dry
precursors) may be desired. Because of the low crystallinity of the
component solids in the paste, the material has markedly improved
flow characteristics over prior art compositions. Flow
characteristics of the resultant paste are toothpaste-like while
prior art materials inherit a granular or oat meal-like
consistency. The paste may be prepared before use, up to a period
of several hours if held at room temperature and evaporative water
loss is minimized. Even when steps are taken to minimize
evaporation, holding at room temperature is sometimes accompanied
by drying out of the hydrated materials. In such instances, a small
amount of water may be added and mixed to restore the desired
consistency. The storage time may be extended by maintaining the
paste at reduced temperatures in the range of 1-10.degree. C. in
the refrigerator provided steps are taken to minimize evaporative
water loss.
[0104] In another preferred embodiment, a formable paste or putty
may be prepared, which can be introduced into the implant site. The
formable precursor is generally prepared by mixture of the dry
reactants in an amount of water or buffer sufficient to produce the
desired consistency for forming. Most often this will be as thick
as possible while still being formable by hand, although thinner
more flowable consistencies may be desirable for many applications.
In many embodiments the preferred consistency will be similar to
that of clay or glazing compound. The hydrated material may be
prepared before use, up to a period of several hours if held at
room temperature or below and evaporative water loss is minimized.
The storage time may be extended by maintaining the hydrated
material at reduced temperatures in the range of 1-10.degree. C. in
the refrigerator provided steps are taken to minimize evaporative
water loss.
[0105] Application to the implant site will be performed according
to the nature of the specific indication and the preferences of the
surgeon. Similar considerations apply for cartilaginous implants as
for bone. Injection techniques will be employed to deliver the
inventive PCA calcium phosphate precursors directly into hard
tissue (e.g. for osteoporosis patients) or into small fractures.
For larger fractures putty-like consistencies will be preferred and
will be implanted by hand or with a spatula or the like.
Reconstruction will often use putty like forms but in some
instances it will be more advantageous to pre-form, harden, and
shape the material ex-vivo and implant a hardened form. Exposure or
mixing of the material with blood or body fluids is acceptable and
in many cases will be preferred as a method to promote osteo- or
chondrogenesis. Implantation into soft tissues may employ any of
the above approaches.
[0106] Formation of the reactive amorphous calcium phosphate. In
preferred embodiments an ACP is converted in the presence of a
promoter and water to PCA calcium phosphate. The use of an
amorphous calcium phosphate, which can react quickly and completely
to a product PCA calcium phosphate without significant further
crystallization, provides a novel route to a highly resorbable
calcium phosphate, with a variety of medical uses. The promoters of
the instant invention promote conversion and hardening either by
direct participation as a reactant along with ACP, or passively by
serving as catalysts, nucleators or reaction enhancing agents, or
in a combination of modes.
[0107] Not all ACPs have the same reactivity with a given promoter,
and their reactivity is generally compared relative to their
reactivity with a DCPD of grain distribution similar to B1 in Table
3. Examples 10 and 11 describe a variety of ACPs which have been
tested for reactivity with such a DCPD. Use of a stronger DCPD
promoter with a smaller grain size facilitates the reaction with
weakly-reactive or otherwise un reactive ACPs. Generally less
reactive ACPs will require the use of stronger promoters and in
some cases combinations of promoters.
[0108] In a preferred embodiment, a highly reactive ACP is
employed. Hydrated precursors comprising this ACP are capable of
undergoing hardening and conversion either in the presence of a
strong promoter such as a DCPD with small grain size (e.g. <63
.mu.m) or in the presence of a relatively weak promoter such as a
DCPD sample comprising a substantial amount of grains greater than
100 .mu.m (e.g. distribution B1). One highly reactive ACP is a
carbonated ACP which has been activated by heat treatment for
approximately one hour at 460.degree. C.
[0109] The invention also provides a test for identifying suitable
reactive precursors for the inventive PCA calcium phosphate. The
test comprises combining an amorphous calcium phosphate, DCPD, and
water, producing a hydrated PCA calcium phosphate precursor
substance and demonstrating its ability to harden in about 10 to 60
minutes at or around body temperature. Reactants found to produce
hardened PCA calcium phosphate in this test may then be placed
intramuscularly in a test animal and checked for biological
resorbability. One hundred milligrams (100 mg), and preferably
three hundred milligrams (300 mg), of PCA calcium phosphate thus
prepared will resorb in less than 12 months, preferably less than 6
months and most preferably in less than 2 months in a rat muscle.
Further 80% of one gram placed intramuscularly will be resorbed in
the same time frame. Alternatively, at least 2 g placed
subcutaneously will be fully resorbed in rat in less than 12
months, preferably less than 6 months and most preferably in less
than 2 months in. For the identification of less reactive forms of
ACP it is preferred to use a weak DCPD promoter. Similar tests may
also be established using other participant or passive
promoters.
[0110] The method of the present invention permits initial
formation of amorphous calcium phosphate particles of less than
1000 .ANG., preferably 200-500 .ANG., and most preferably 300
.ANG., the further growth of which are curtailed by rapid
precipitation of the product from solution. In FIG. 1, a
high-resolution transmission electron micrograph is shown to
illustrate the morphological characteristics and the angstrom-scale
nature of the preferred reactive amorphous calcium phosphate of the
invention. Note the unclear boundaries separating the globule-like
clusters, lacking clear edges and surfaces, in contrast to
crystalline material.
[0111] During reaction of calcium and phosphate ion sources to form
an amorphous calcium phosphate, a third ion may be introduced in
the solution so that these ions are incorporated in the amorphous
precipitate structure instead of trivalent PO.sub.4.sup.3-
group(s). Because some PO.sub.4.sup.3- is replaced by the third
ion, the overall PO.sub.4.sup.3--decreases, thus increasing the
Ca/P ratio of the amorphous precipitate (as compared to standard
amorphous calcium phosphate) and modifying the valence or charge
state of the calcium phosphate. The amorphous solids then may be
rapidly freeze-dried to preserve the chemical and physical
properties of the material. The amorphous solids then may be
treated under specific conditions selected to promote removal of at
least some of the third ion. In the case of carbonate, specific
temperature and pressure conditions lead to the reduction of total
carbon, presumably as gaseous carbon dioxide from the amorphous
solid, while maintaining the amorphicity.
[0112] The source of the enhanced reactivity is not completely
understood; however, it is believed to be associated with the
degree of amorphicity (lack of crystallinity) and, in some
embodiments, site vacancies in the material, as created by the
process of the present invention. Site vacancies as envisioned
herein refer to the lack of one pair of an ion pair (e.g.
CO.sub.3.sup.2-) missing from CaCO.sub.3 in a material containing
many ion pairs. The presence of site vacancies may provide reactive
sites for subsequent reaction. This stoichiometric imbalance may be
responsible for the increased reactivity of the amorphous calcium
phosphate
[0113] The reactive ACP is a substantially amorphous solid with a
higher Ca/P ratio than is typically found in amorphous calcium
phosphates, which has generally been reported in the past to be
about 1.50.
[0114] The amorphous state is induced by controlling the rate and
duration of the precipitation process. The amorphous hydroxyapatite
of the present invention is precipitated from solution under
conditions where initial precipitation is rapid. Rapid crystal or
grain growth enhances the number of defects within each grain,
thereby increasing solubility. At the extreme end of the spectrum,
crystal or grain growth is so rapid and defect density is so
significant that an amorphous calcium phosphate results. Amorphous
calcium phosphate is gel-like and includes solid solutions with
variable compositions. These gels have no long range structure, but
are homogeneous when measured on an Angstrom scale. Under
physiological conditions, these amorphous compounds have high
solubilities, high formation rates and high rates of conversion to
PCA calcium phosphate.
[0115] The amorphous calcium phosphate solids produced by this
method retain their amorphous nature sufficiently long enough to be
introduced into the final reaction as substantially amorphous
solids.
[0116] In one embodiment of the present invention, a solution is
prepared which contains calcium and phosphate ions and a third ion
in a concentration, at a pH and at a temperature which will promote
the rapid nucleation and precipitation of calcium phosphate. When
precipitation is sufficiently rapid, an amorphous gel-like calcium
phosphate is formed. Because the thermodynamically favored
crystalline form of hydroxyapatite is enhanced by reducing the rate
of reaction, certain processing steps of increasing the rate of
reaction may be taken to ensure that an amorphous compound is
obtained. The following factors, among others, are to be considered
when designing a solution for the rapid precipitation of the
amorphous calcium phosphate of the present invention.
[0117] Preferred conditions: Rapid mixture of calcium and phosphate
sources to increase the rate of reaction. The rate of reaction is
increased to favor non-stable phases as a product. Allowing more
reaction time for each of the ions to juxtapose correctly to form a
solid will result in a more thermodynamically favorable crystalline
and stable structure.
[0118] Preferred calcium and phosphate sources: The use of highly
concentrated or near supersaturation solutions ensures that a more
rapid reaction will occur.
[0119] Preferred temperature: Although the reaction can be carried
out at room temperature, temperatures of near boiling point to
increase the concentration of one reactant is a possible means of
increasing the rate of reaction.
[0120] In one embodiment, an aqueous solution of calcium ions,
phosphate ions and carbonate ions are mixed together rapidly to
obtain a carbonate containing amorphous calcium phosphate solid.
The relative concentrations of the ions are selected to give a
precipitate having the desired Ca/P ratio. The carbonate ion
substitutes for a phosphate ion in the amorphous calcium phosphate.
The carbonated amorphous calcium phosphate may be obtained by
precipitation from an aqueous carbonate solution. Suitable aqueous
carbonate solutions include, by way of example only, bicarbonate
solution, sodium carbonate solution, potassium carbonate solution.
It is further contemplated as within the scope of the invention to
use non-aqueous solutions.
[0121] Use of a carbonated material is desirable because it permits
manipulation of the Ca/P ratio by substitution of PO.sub.4.sup.3-
by CO.sub.3.sup.2-. Additionally, the presence of CO.sub.3.sup.2-
is known to retard the development of crystallinity in amorphous
calcium phosphate. It is recognized, however, that other ions or a
mixture of ions may be suitable in place of or in addition to
carbonate ion in modifying the Ca/P ratio and in introduction of
reactive site vacancies into the amorphous calcium phosphate, such
as by way of example only, nitrate, nitrite, acetate, Mg.sup.+2 and
P.sub.2O.sub.7.sup.4- ions.
[0122] The amorphous calcium phosphate precipitate may be collected
and filtered prior to activation. It is preferred to perform this
step in a cold room or at sub-ambient temperatures so as to
preserve the amorphous state of the precipitate collected.
Collection may typically may be carried out by any conventional
means, including, but in no way limited to gravity filtration,
vacuum filtration or centrifugation. The collected precipitate is
gelatinous and is washed more than once with distilled water.
[0123] The washed precipitate is then dried under any conditions
which maintain the amorphous character of the material.
Lyophilization is a suitable, but not exclusive, technique. Upon
freezing, the precipitate while kept frozen, is dried to remove the
bulk of the entrained liquid. This procedure may be accomplished by
placing the frozen precipitate into a vacuum chamber for a given
period of time. Freeze-drying typically occurs at liquid nitrogen
temperatures for a time in the range of 12-78 hrs, preferably about
24 hours, and under a vacuum in the range of 10.sup.-1-10.sup.-4,
preferably 10.sup.-2, torr. A preferred method includes
lyophilization because the cryogenic temperatures typically used in
lyophilization inhibit further crystallization of the material. As
a result, the amorphous calcium phosphate obtained thereby is an
extremely fine free flowing powder.
[0124] The dried ACP may then be activated to a highly reactive
ACP. In a preferred embodiment, where carbonate is present in the
ACP, the ACP powder is heated to drive off remaining free water,
water of hydration, and to remove carbon, presumably through the
decomposition of CO.sub.3.sup.2- into CO.sub.2 and oxygen. The
heating step is carried out at a temperature of less than
500.degree. C. but more than 425.degree. C., so as to prevent
conversion of the amorphous calcium phosphate into crystalline
hydroxyapatite. Heating is preferably carried out at a temperature
in the range of 450-460.degree. C. for 1 to 6 hours preferably for
50 to 90 minutes.
[0125] Atmospheric pressure is used for convenience in most of the
embodiments for production of ACP described herein. However, the
use of vacuum with appropriate temperatures is considered to be
within the scope of the invention.
[0126] To produce a highly reactive ACP it is desirable to maintain
the amorphous property of the material throughout the entire ACP
synthesis. If significant crystallinity in its entirety (single
crystalline regions) or even in local domains (microcrystalline
regions) is introduced during the process or in the final product,
the solid has been found to become less reactive. The resultant
highly reactive calcium phosphate is amorphous in nature and has a
calcium to phosphorous ratio in the range of 1.55 to 1.65. In a
preferred embodiment, the amorphous calcium phosphate has a Ca/P
ratio of about 1.58.
[0127] Low crystallinity and site vacancies (porosity and/or
stoichiometric changes) may account for the observed higher
reactivity of the amorphous calcium phosphate of the present
invention. This is supported by the following observations: a.) A
carbonate-containing amorphous calcium phosphate which has been
heated to 525.degree. C. is observed to have an increased
crystalline content and to have a corresponding decrease in
reactivity. b.) Amorphous calcium phosphate that is heated to only
400.degree. C. retains its amorphous characteristic, but exhibits a
decreased reactivity. c.) Non-carbonated ACPs heated to 460.degree.
C. have been studied using the DCPD reaction (as described in
example 8) and while reactive with a strong DCPD promoter were not
reactive with a weak DCPD promoter.
[0128] These observations suggest that both amorphicity and
decreased carbon content (vacant reactive sites) are a factor in
reactivity. This is not intended to be in any way an exclusive
explanation for the basis of reactivity. Other basis for the
observed reactivity are considered to be within the scope of the
invention.
[0129] The resulting amorphous calcium phosphate powder is a highly
reactive amorphous calcium phosphate material with a Ca/P ratio of
between 1.1-1.9, preferably about 1.55 to 1.65, and most preferably
about 1.58. FIGS. 17a and 17b illustrate the infrared spectra of
the amorphous calcium phosphate after lyophilization process (FIG.
17a) and after the subsequent heat treatment at 450.degree. C. for
1 hr (FIG. 17b). Infrared peaks illustrating presence of local
chemical groups in the material show that the presence of H--O--H
(at approximately 3,400 cm-1 and 1640 cm-1) and CO.sub.3.sup.2-
group (at 1420-1450 cm-1) are significantly reduced after heat
treatment. However, the x-ray diffraction patterns in FIG. 4a of
heat activated ACP demonstrate that the amorphous state is retained
after heating and lyophilization. The XRD pattern is characterized
by broad peaks and undefined background with absence of sharp peaks
between 2.theta.=20 to 35 or at any diffraction angles that
correspond to known crystalline calcium phosphates.
[0130] The Ca/P measurement performed using wave length-dispersive
X-ray analysis on an electron micro-probe of the same material
after heat treatment yields Ca/P to be 1.58 (FIG. 2).
[0131] These characterizations demonstrate that although there is a
change in the local moiety of certain groups in the amorphous
calcium phosphate solids, the overall amorphicity is maintained
throughout the process.
[0132] The preparation of the PCA calcium phosphate calcium
phosphate as a composite material may sometimes be desirable in
order to provide an implant with different properties than the
inventive PCA calcium phosphates. Furthermore, the consistency,
formability and hardness of the PCA calcium phosphate calcium
phosphate, as well as the reaction speed, may be varied according
to the therapeutic need by selection of the appropriate
supplementary materials from which to prepare the implantable
bioceramic composite material of the invention.
[0133] Composites may be prepared by combining the PCA calcium
phosphate calcium phosphate of the invention with a selected
supplementary material. The PCA calcium phosphate calcium phosphate
phase may serve as a reinforcing material, a matrix or both. The
PCA calcium phosphate calcium phosphate of the invention in it's
initial paste form in preferred embodiments typically maintains a
pH of about 6-7 and is therefore compatible with a wide range of
additives without deleterious effect. The supplementary material is
selected based upon its compatibility with calcium phosphate and
its ability to impart properties (biological, chemical or
mechanical) to the composite, which are desirable for a particular
therapeutic purpose. For example, the supplementary material may be
selected to improve tensile strength and hardness, increase
fracture toughness, alter elasticity, provide imaging capability,
and/or alter flow properties and setting times of the bone
substitute material. The supplementary materials are desirably
biocompatible, that is, there is no detrimental reaction induced by
the material when introduced into the host.
[0134] The supplementary material may be added to the PCA calcium
phosphate calcium phosphate in varying amounts and in a variety of
physical forms, dependent upon the anticipated therapeutic use. The
supplementary material may be in the form of sponges (porous
structure), meshes, films, fibers, gels, filaments or particles,
including micro- and nanoparticles. The supplementary material
itself may be a composite. The supplementary material may be used
as a particulate or liquid additive or doping agent which is mixed
intimately with the resorbable PCA calcium phosphate calcium
phosphate. The supplementary material may serve as a matrix for the
PCA calcium phosphate calcium phosphate which is embedded or
dispersed within the matrix. Alternatively, the PCA calcium
phosphate calcium phosphate may serve as a matrix for the
supplementary material, which is dispersed therein. The
supplementary material may be applied as a coating onto a PCA
calcium phosphate calcium phosphate body, for example, as a
post-fabrication coating to retard resorption time or otherwise
affect the bioceramic material properties. Due to the high porosity
and water absorption characteristics of the inventive PCA calcium
phosphates, solid PCA calcium phosphate may be impregnated with
water soluble polymers by simple immersion in the aqueous polymer
solution. Lastly, the supplementary material may be coated with PCA
calcium phosphate calcium phosphate.
[0135] In most instances, the supplementary material will be
biocompatible and in many instances, it is desirable that the
supplementary material also be bioresorbable. In many preferred
embodiments, the supplementary material will have an affinity for
calcium, phosphate or calcium phosphates which will enhance the
strength of the hydroxyapatite/suppleme- ntary material interface.
The affinity may be specific or mediated through non-specific ionic
interactions. Suitable bioerodible polymers for use as a matrix in
the composite include, but are not limited to, collagen, glycogen,
chitin, celluloses, starch, keratins, silk, nucleic acids,
demineralized bone matrix, derivativized hyaluronic acid,
polyanhydrides, polyorthoesters, polyglycolic acid, polylactic
acid, and copolymers thereof. In particular, polyesters of
a-hydroxycarboxylic acids, such as poly(L-lactide) (PLLA),
poly(D,L-lactide) (PDLLA), polyglycolide (PGA),
poly(lactide-co-glycolide (PLGA), poly(D,L-lactide-co-trimethylene
carbonate), and polyhydroxybutyrate (PHB), and polyanhydrides, such
as poly(anhydride-co-imide) and co-polymers thereof are known to
bioerode and are suitable for use in the present invention. In
addition, bioactive glass compositions, such as compositions
including SiO.sub.2, Na.sub.2O, CaO, P.sub.2O.sub.5,
Al.sub.2O.sub.3 and/or CaF.sub.2, may be used in combination with
the poorly crystalline hydroxyapatite of the invention. Other
useful bioerodible polymers may include polysaccharides, peptides
and fatty acids.
[0136] Bioerodible polymers are advantageously used in the
preparation of resorbable hardware, such as pins, screws, plates
and anchors for implantation at a bone site. In preferred
resorbable hardware embodiments, the supplementary material itself
is resorbable and is added to the PCA calcium phosphate calcium
phosphate in particulate or fiber form at volume fractions of 1-50%
and preferably, 1-20 wt %. In some preferred embodiments, the
resorbable fiber is in the form of whiskers which interact with
calcium phosphates according to the principles of composite design
and fabrication known in the art. Such hardware may be formed by
pressing a powder particulate mixture of the poorly crystalline
hydroxyapatite and polymer. In one embodiment, a PCA calcium
phosphate calcium phosphate matrix is reinforced with PLLA fibers,
using PLLA fibers similar to those described by Tormala et al.,
Clin. Mater. 10:29-34 (1992) for the fabrication of biodegradable
self-reinforcing composites.
[0137] The resorbable nature of the inventive PCA calcium phosphate
as well as its ability to benignly interact with and adsorb
proteins, nucleic acids, and other substances make it an ideal
substance for use as an implantable depot for use in the delivery
of therapeutic substances to the body. In general, the main
requirement is that the agent to be delivered remains active in the
presence of the vehicle during fabrication and/or loading, or be
capable of subsequently being activated or reactivated. The
stability and/or compatibility of a particular agent with the
inventive material, as well as fabrication strategies, may be
tested empirically in vitro. Some representative classes of useful
biological agents include organic molecules, proteins, peptides,
nucleic acids, nucleoproteins polysaccharides, glycoproteins,
lipoproteins, and synthetic and biologically engineered analogs
thereof.
[0138] In one aspect of the invention, bone regenerative proteins
(BRP) are incorporated into the inventive PCA calcium phosphate.
BRPs have been demonstrated to increase the rate of bone growth and
accelerate bone healing. A bone graft including the inventive PCA
calcium phosphate and BRP is expected to promote bone healing even
more rapidly than a bone graft using the hydroxyapatite of the
present invention alone. The efficacy of BRP is further enhanced by
controlling PCA calcium phosphate resorption such that it dissolves
at a rate that delivers BRP, calcium, and phosphorus at the optimum
dosage for bone growth. Such a method of incorporating BRP would
include, but not limited to, mixing a buffer solution containing
BRP with its optimum pH that would maintain protein activity,
instead of distilled water. Exemplary BRPs include, but are in no
way limited to, Transforming Growth Factor-Beta, Cell-Attachment
Factors, Endothelial Growth Factors, and Bone Morphogenetic
Proteins. Such BRPs are currently being developed by Genetics
Institute, Cambridge, Mass.; Genentech, Palo Alto, Calif.; and
Creative Biomolecules, Hopkinton, Mass.
[0139] In another embodiment of the invention, it is contemplated
to incorporate antibiotics or its agents into the amorphous calcium
phosphate and its mixture. From a clinical sense, one of the major
implication arising from a bone-graft surgery is a need to control
the post-operative inflammation or infection. A bone graft
including the inventive PCA calcium phosphate and antibiotic(s) is
expected to reduce the chances of local infection at the surgery
site, contributing to infection-free, thus faster bone healing
process. The efficacy of antibiotics is further enhanced by
controlling their release from the PCA calcium phosphate delivery
vehicle by regulating the resorption rate such that it dissolves at
a rate that delivers antibiotic peptides or its active component at
the most effective dosage to the tissue repair site. Exemplary
antibiotics include, but are in no way limited to, Penicillin,
Chlortetracycline hydrochloride (Aureomycine), Chloramphenicol and
Oxytetracycline (Terramycine). Both antibiotics, mostly
polypeptides, and bone regenerating proteins may be intermixed with
the PCA calcium phosphate material of the present invention, to
locally deliver all or most of the necessary components in
facilitating optimum condition for bone tissue repair.
[0140] Non resorbable apatitic bone fillers and cements may also be
prepared by the methods of the current invention by promoting the
conversion of ACP to a more crystalline state than PCA calcium
phosphate. In general use of more hydroxyapatite stoichiometric
Ca/P ratios, decrease use of crystallization inhibitors, and
crystallization promoting conditions such as elevated temperatures
will tend to drive the conversion to a more crystalline
product.
[0141] The invention is further exemplified with reference to the
following examples, which are presented for the purpose of
illustration only and are not to be considered as limiting of the
invention.
EXAMPLE 1
[0142] Production of PCA Calcium Phosphate Using an ACP and
Participating Promoters. This example demonstrates the hardening
properties and PCA calcium phosphate formation from ACP using a
number of different participating promoters. Highly reactive ACP
was prepared according to Example 5.
[0143] The nanocrystalline hydroxyapatite of samples 1-1, 1-2 and
1-3 were prepared without inhibitors of crystallization as follows:
218 g of disodium hydrogen orthophosphate
(Na.sub.2HPO.sub.4.12H.sub.2O) were dissolved in 1200 mL of
solution of distilled water. For carbonated PCA calcium phosphate
of samples 1-1 and 1-2, 80 g of NaHCO.sub.3 were also added to this
solution. 70 g of calcium nitrate [Ca(NO.sub.3).sub.2.4H.su- b.2O]
were dissolved in 500 mL of distilled water. The calcium solution
was quickly poured into the phosphate solution at room temperature
with constant stirring. Precipitation was immediate and
substantially complete. The pH of the precipitate was adjusted to
7.4 by the addition of sodium hydroxide solution in order to avoid
the formation of acidic calcium phosphates. The precipitate was
immediately separated from the solution by filtration through a
Buchner filter (with a total surface about 0.1 sq.m), and was
washed by about 3 liters of distilled water. A gel cake of low
crystallinity calcium phosphate was obtained on the filter paper. A
portion of the gel cake was immediately lyophilized for samples 1-2
and 1-3.
[0144] For sample 1-1 the gel cake was treated as follows: After
filtration and washing, an appropriate amount of distilled water (5
to 80 weight %) was added to the gel precipitate. The gel was
homogenized by whipping energetically for a few minutes. It was
then cast into polytetrafluoroethylene (PTFE) molds (diameter 60
mm; height 2 mm), and sonicated for a few minutes in order to
release the air bubbles trapped in the gel.
[0145] The molds were dried in a chamber at controlled temperature
(5 to 37.degree. C.) and humidity (10 to 95% RH). The samples
shrank slowly on drying and released most of their water. The rate
of drying and the shrinkage of the samples depended on the initial
water content. The material hardened on drying and became glassy.
It contained about 10% of residual water.
[0146] The remaining hydroxyapatites and calcium sources were used
as is from commercial sources.
1TABLE 1 ACP Conversion Using Participating Promoters incubation
extent of PCA* PCA* by sample participating promoter at 37 .degree.
C. hardening by FTIR XRD 1-1 carbonated nanocrystalline 30 min
starting to set yes ND hydroxyapatite, air dried 2 hrs hard 1-2
carbonated nanocrystalline 30 min hard yes yes hydroxyapatite,
lyophilized 2 hrs hard 1-3 non-carbonated nanocrystalline 30 min
starting to set yes ND hydroxyapatite, lyophilized 2 hrs hard 1-4
Aldrich hydroxyapatite 30 min hard yes yes gram size <15-30.mu.m
1-5 Clarkson hydroxyapatite 30 min starting to set yes ND grain
size>250.mu.m 1-6 Monetite - non calcinated 30 min soft yes ND
grain size 15 hrs starting to set 1-7 CaCO.sub.3 30 min starting to
set yes ND 15 hrs 1-8 Ca(OH).sub.2 30 min soft yes and ND 1 5hrs
starting to set Ca(OH).sub.2 1-9 Ca(CH.sub.3COO).sub.2 30 min soft
yes ND 15 hrs soft *PCA-poorly crystalline apatitic calcium
phosphate ND-analysis not done
[0147] ACP was mixed with the specific promoter at a ratio (wt/wt)
of about 50:50 (see Table 1) for 5 minutes in a SPEX laboratory
mill. Approximately 0.8 mL H.sub.2O/g dry powders were added to the
dry precursor mixture and mixed to a paste. The mixture was then
shaped into a ball, wrapped in moist tissue paper and heated to
37.degree. C. for at least 30 minutes. After 30 minutes and at
various time points thereafter the paste was monitored for
hardness. FIGS. 14 and 15 are representative XRD from reactions 1-2
and 14. The use of two different grain size hydroxyapatites as
participating promoters yielded similar results as with different
grain size DCPDs (see Example 10) That is, the larger grain size
hydroxyapatite hardened more slowly and less completely than the
smaller grain size hydroxyapatite.
EXAMPLE 2
[0148] This example demonstrates the use of a neutral apatitic
calcium phosphate as a promoter for the conversion of ACP to the
inventive PCA calcium phosphate to promote bone growth in vivo.
Stoichiometric hydroxyapatite is mixed with reactive ACP as
described in Example 1-4. Hydrated precursor paste is applied to
animal subjects as described in Examples 15, 16 or 19. Bone healing
and biocompatibility is monitored as described at the time points
indicated.
EXAMPLE 3
[0149] This example demonstrates the production of PCA calcium
phosphate from ACP using a number of different passive
promoters.
[0150] Highly reactive ACP was prepared according to Example 5. ACP
was mixed with the specific promoter at a ratio (wt/wt) of about
5:1 or 1:1 (see Table 2) for 5 minutes in a SPEX laboratory mill.
Water (0.75-0.85 mL) was added and mixed to form a putty. The
mixture was then formed into a ball, wrapped in moist tissue paper
and heated to 37.degree. C. for at least 30 minutes. After 30
minutes and at various time points thereafter the paste was
monitored for hardness. FIG. 13 is a representative XRD from sample
24 employing an alumina promoter. In this figure the alumina peaks
can be seen superimposed over the standard PCA calcium phosphate
profile.
2TABLE 2 ACP Conversion Using Passive Promoters Passive Incubation
Promoter time Extent of PCA* by PCA* by study # (ACP:promoter) at
37 .degree. C. Hardening FTIR XRD 2-1 SiC).sub.2 (5:1) 30 min soft
yes yes 3 hrs very hard 2-2 Mica (5:1) 30 min soft yes yes 12 hrs
very hard 2-3 A1.sub.2O.sub.3 (1:1) 30 min soft yes yes 12 hrs very
hard 2-4 A1.sub.2O.sub.3 (5:1) 30 min soft yes yes 12 hrs very hard
*PCA-poorly crystalline apatitac hydroxyapatite
EXAMPLE 4
[0151] This example demonstrates the use of a scanning differential
calorimeter (DSC) to monitor temperature sensitivity and the net
endothermic nature of a preferred embodiment reaction employing
activated ACP and DCPD precursors.
[0152] The dry precursor mixture containing equal weights of ACP
and DCPD was prepared as described in Example 9. Water (0.05 mL),
prechilled to approximately 4.degree. C., was added to 47.27 mg of
the dry precursor mixture and immediately placed into the
calorimeter. The DSC (Perkin Elmer 7 series thermal analysis
system) was set to a starting temperature of 0.degree. C. with a
scan rate of 5.degree. C./min. The results are shown in FIG. 16.
The plot represents a monitoring of the first 7 minutes of
reactivity and shows essentially no heat flow between 0.0.degree.
C. and approximately 20.degree. C., at which point onset of
endothermic heat flow occurs. The heat flow properties indicate
that at 37.degree. C. the reaction is essentially endothermic, and
under the conditions used, the reaction occurs only very slowly if
at all at temperatures below about 20.degree. C. Thus, the net
reactivity in the system, that is, the sum of endothermic and
exothermic heat flow of the system, is endothermic.
EXAMPLE 5
[0153] This example describes the step-by-step preparation and
methods for the synthesis of a highly reactive amorphous calcium
phosphate of the present invention.
[0154] The inert carbonated amorphous calcium phosphate was then
prepared at room temperature by the rapid addition of solution B
(43 g Ca(NO.sub.3).sub.2.4H.sub.2O (calcium nitrate tetrahydrate)
and 1 g MgCl.sub.2.6H.sub.2O in 0.5 l of distilled water) to
rapidly stirring solution A (55 g Na.sub.2HPO.sub.4.7H.sub.2O
(sodium phosphate), 50 g NaOH (sodium hydroxide), 30 g NaHCO.sub.3,
(sodium bicarbonate) and 2 g Na.sub.4P.sub.2O.sub.7.10H.sub.2O, in
1.3 l of distilled water). The precipitate of gel-like amorphous
calcium phosphate thus formed was immediately filtered using filter
paper (0.05 sq. m) with medium filter speed and a vacuum pressure
of about 10.sup.-2 torr. The material formed a thin cake and was
washed with approximately 4 liters of distilled water by adding
water into the filtration funnel. The washed material was then
collected using spatula and immersed into a liquid nitrogen in a
2.5 L container. Following the formation of hard frozen pieces, the
container was transferred into a vacuum chamber for 24 hrs
(10.sup.-1-10.sup.-2 torr), until a fine and dry powder was
obtained.
[0155] Although the procedure described above may be performed at
room temperature, the entire process preferably takes place below
ambient temperature (4-5.degree. C.), so as to further prevent the
amorphous state from converting into more stable crystalline
form.
[0156] An infrared spectrum of the inert amorphous material at this
point in process is shown in FIG. 17a. This spectrum contains peaks
characteristic of P--O groups (570 and 1040 cm.sup.-1),
CO.sub.3.sup.2- group (1,420.sup.-1, 450 cm.sup.-1) with a
relatively large O--H group peak (.about.3,550 cm.sup.-1). The
X-ray diffraction pattern of the same material demonstrates the
amorphous nature of the material as indicated by absence of any
sharp peaks in the 2.theta.=20 to 35 range.
[0157] The amorphous material described above was then activated to
the highly reactive form by heating for 60 minutes at 450.degree.
C. (.+-.3.degree. C.). The IR of the heated material is shown in
FIG. 17b. This spectrum shows a reduction of particular O--H and
CO.sub.3.sup.2--groups, indicating a significant reduction of
H.sub.2O and CO.sub.3.sup.2- as CO.sub.2 and H.sub.2O. In similarly
prepared samples the carbon content was observed to drop
approximately 60% with a total carbonate ratio decreasing from
1.56% to 0.5%. Note, however, that the amorphous nature of the
material was not lost during this process, as demonstrated by the
x-ray diffraction pattern shown in FIG. 6a. The Ca/P ratio
measurement of this material after the heat treatment was
determined to be 1.575, using a method of quantitative electron
microprobe analysis. The overall morphological and ultrastructural
properties of the amorphous material was confirmed by transmission
electron microscopy as shown in FIG. 1. Note the "amorphous"
appearance of the material with absence of sharp edges separating
each granules with certain portion of the material to exhibit
shapeless form (arrows).
EXAMPLE 6
[0158] ACP was synthesized as described in Example 5 above, with
the exception that solutions A and B were prepared in the following
way: Solution A was prepared at room temperature by the rapid
dissolution of 90.68 g of Ca(NO.sub.3).sub.2.4H.sub.2O in 1.2 liter
of carbonated distilled H.sub.2O. Solution B was prepared by
dissolving 40.57 g of K.sub.2HPO.sub.4 in 1.53 liters of distilled
H.sub.20, containing 24 ml of 45 vol. % KOH solution. Chemical and
physical properties of the product amorphous calcium phosphate
resulting from this procedure were similar to those of the material
prepared accordingly for Example 5.
EXAMPLE 7
[0159] ACP was synthesized as described in Example 5 above, with
the exception that solutions A and B were prepared in the following
way: Solution A was prepared at room temperature by the rapid
dissolution of 10.58 g of Ca(NO.sub.3).sub.2.6 H.sub.2O in 0.15
liters of carbonated distilled H.sub.2O at pH greater than 9.0, as
adjusted by NaOH. Solution B was prepared by dissolving 7.8 g of
(NH.sub.4).sub.2HPO.sub.4 in 0.35 liters of distilled H.sub.2O.
EXAMPLE 8
[0160] This example describes the preparation of PCA calcium
phosphate of the invention with manual mixing of the dry
reactants.
[0161] Dicalcium phosphate dihydrate (DCPD) was prepared at room
temperature by the rapid addition of solution B (17.1 g
Ca(NO.sub.3).sub.2.4H.sub.2O (calcium nitrate tetrahydrate) in 250
mL distilled water) to solution A (10 g H.sub.9N.sub.2O.sub.4P
(diammonium hydrogen phosphate) in 500 mL distilled water at a pH
of 4.6-4.8) with constant stirring. Immediately thereafter, the
sample was filtered using filter paper (0.05 sq. m) with medium
filter speed and a vacuum pressure of about 10-2 torr. The material
formed a thin cake which was washed with about 2 liters of
distilled water and then dried at room temperature for 24-72
hrs.
[0162] The reactive amorphous calcium phosphate material prepared
from Example 5 was physically dry-mixed with dicalcium phosphate
dihydrate (CaHPO.sub.4.2H.sub.2O) at 50:50 wt. % using a mortar and
pestle for 3-5 min. Water (1 mL/g of mixed material) was then added
to the powder mixture to yield a paste-like consistency. The amount
of H.sub.2O added varied, depending on whether a thick or thin
paste was desired. The hydrated precursor material was then wrapped
loosely in moist tissue paper and heated to 37.degree. C. At this
temperature the paste hardened into a solid mass by means of a
substantially endothermic reaction. The hardening process could be
delayed for several hours by refrigerating the sample at 4.degree.
C. The hardened material was composed of PCA calcium phosphate with
an inherent solubility property that exceeded reported solubilities
for a synthetic hydroxyapatite material. This is demonstrated in
FIG. 3, where the concentration of calcium ions released into a
controlled pH buffer solution over 24 hrs at 37 .degree. C., was
significantly higher for the PCA calcium phosphate material of the
present invention (curve 50) than the standard crystalline
hydroxyapatite material (curve 52).
EXAMPLE 9
[0163] This example describes the preparation of the inventive PCA
calcium phosphate using automated mixing of the dry precursors.
[0164] The dry ACP and DCPD precursors were prepared as described
in Example 8. Instead of mixing with a mortar and pestle, the ACP
and DCPD were mixed using a SPEX 8510 laboratory mill with a SPEX
8505 alumina ceramic grinding chamber for 2 min. Preparation of the
hydrated precursor was accomplished by adding from 0.7 to 1.5 mL of
water per gram of mixed dry precursors.
EXAMPLE 10
[0165] This example demonstrates the preparation of PCA calcium
phosphate using DCPDs of specific grain size distributions.
[0166] DCPD was prepared as described in Example 8. The dry
material was ground for 5 minutes in a SPEX 8510 laboratory mill
with a SPEX 8505 alumina ceramic grinding chamber. Following
grinding, the material was serially sieved through a Tyler test
sieve shaker to produce DCPD with 8 different grain size
distributions as indicated in Table 3 and shown in FIG. 8.
3TABLE 3 DCPD Grain Size Distribution Extent of Grain Size
hardening at Sample Distribution 30 min, 37.degree. C. 10-1 <25
.mu.m hard 10-2 25-35 .mu.m hard 10-3 35-53 .mu.m hard 10-4 53-63
.mu.m hard 10-5 distribution hard B3 (FIG. 8) 10-6 106-125 .mu.m
not fully hardened 10-7 distribution not fully B2 (FIG. 8) hardened
10-8 unsieved not fully distribution hardened B1 (FIG. 8)
[0167] It has been found that the preliminary grinding of DCPD
prior to sieving can be replaced by a brief hand grinding using a
mortar and pestle without substantially changing the results.
[0168] The reactive amorphous calcium phosphate material prepared
from Example 5 was physically dry-mixed 1:1 (wt/wt) with each of
the DCPD samples from Table 3 for 10 minutes using a SPEX 8510
laboratory mill with a SPEX 8505 alumina ceramic grinding chamber.
Water (0.8-1.0 mL/g of dry mix) was then added to each powder
mixture to yield a hydrated PCA calcium phosphate precursor with a
paste-like consistency. Six of the eight samples indicated in Table
3 hardened well in 30 minutes at 37.degree. C. Samples 10-6, 10-7
and 10-8 did not harden as quickly or as firmly as the other
samples. Each of these samples had significantly higher percentages
of >100 .mu.m particles than the other samples. It is concluded
from these observations that the use of smaller grain size DCPD
leads to more rapid and complete hardening than larger grain size
DCPD.
EXAMPLE 11
[0169] This example describes two preferred embodiments of the
instant invention.
[0170] (a) Reactive amorphous calcium phosphate material prepared
according to Example 5 was physically dry-mixed with DCPD with a
particle size distribution of B3 of FIG. 3 at 50:50 wt. % using a
SPEX 8510 laboratory mill for 2 min with a SPEX 8505 alumina
ceramic grinding chamber, followed by sieving to a size of less
than 150 .mu.m. Water (0.8 mL/g of mixed material) was then added
to the powder mixture to from the hydrated precursor.
[0171] (b) This preferred embodiment was prepared as in (a) with
the exception that samples were dry mixed and subsequently ground
for 10 minutes.
EXAMPLE 12
[0172] This example describes alternative methods for preparing
hydrated PCA calcium phosphate precursor.
[0173] (a) Reactive ACP and DCPD were prepared as described in
Example 9 with the exception that the dry precursors were not
mixed. Water (0.8 mL) was added to ACP (0.5 g) and mixed thoroughly
to homogeneity with a spatula to form a paste. DCPD (0.5 g) was
then added to the paste and the paste was mixed for approximately 2
min. The resultant paste was placed into a moist environment at 37
.degree. C. for 30 min.
[0174] (b) Reactive ACP and DCPD were prepared as described in
Example 8. Water (0.8 mL) was added to DCPD (0.5 g) and mixed
thoroughly to homogeneity with a spatula to form a paste. ACP (0.5
g) was then added to the paste and the paste was mixed for an
additional 2 min. The resultant paste was placed into a moist
environment at 37.degree. C. for 30 min.
[0175] In both instances, the paste hardened after 30 minutes,
indicating a successful reaction.
EXAMPLE 13
[0176] This example describes hardness testing of a PCA calcium
phosphate calcium phosphate.
[0177] PCA calcium phosphate calcium phosphate was prepared
according to Example 9 to form a paste. The paste was placed into a
6 (dia.).times.10 (depth) mm hollow teflon tube submersed in
37.degree. C. water for 30 minutes. The hardened PCA calcium
phosphate was then removed from the tube and placed in water at
37.degree. C. for 1 hour and then, while still moist, placed
vertically on an Instron 4206 having a dual 10 kg/15 ton load cell.
Compressibility was determined using a crush test. Approximately,
200-250 N were required to bring the sample to failure. This force
corresponds to a compressive strength of 7-9 MPa.
EXAMPLE 14
[0178] These examples demonstrate the effect of fluid volume on the
consistency and reactivity of injectable paste to be used in the
formation of bone substitute material. Each of the pastes were
prepared as described in Example 8, above, and the consistency and
rate of reaction at room temperature and 37.degree. C. were
determined. Observations are reported in Table 4.
4TABLE 4 Formability, Injectability and Reactivity of Hydrate
Precursor. water hardening time at volume various temps. Example
No. (mL) formability injectability (4.degree. C./RT/37.degree. C.)
14-1 0.7 - - -/-/- crumbles 14-2 0.8* +++ + >60 min/>60 min/
easily formed 30 min paste 14-3 0.9* ++ ++ >60 min/>60 min/
toothpaste 30 min 14-4 1.0 + +++ >60 min/>60 min/ liquid 30
min toothpaste *Under some circumstances (e.g., evaporation) these
samples may dry out somewhat over a period of one hour at room
temperature. In such cases, additional water may be added to
restore the original consistency.
EXAMPLE 15
[0179] Implantation and Resorption of PCA Calcium Phosphate in a
Subcutaneous Site. This example demonstrates the resorption of the
inventive PCA calcium phosphate when implanted subcutaneously into
rats. It also demonstrates a useful screening procedure to test
resorption characteristics of new formulations of bioceramic
implant materials and composites.
[0180] Eighty male and eighty female Sprague-Dawley rats were each
implanted with 4 ml (24 gm) of the inventive PCA (prepared
according to Example 8) into the dorsal subcutis (>10.times. the
amount considered maximal in humans on a per kg basis). Control
animals were treated with an equal volume of saline. Operation
procedures are described in Example 16. The rats were sacrificed
according to the schedule presented below in Table 5; the implant
site was examined as described in Example 16.
5TABLE 5 Sacrifice Schedule Sacrifice PCA calcium Timepoint
phosphate implant 1 week 5 m/5 f 2 weeks 5 m/5 f 1 month 5 m/5 f 3
months 5 m/5 f 1 year 20 m/20 f
[0181] Blood for clinical pathology analyses was collected via
retroorbital sinus or cardiac puncture (all by the same method)
while the animals were under CO.sub.2 anesthesia. Blood samples
were collected from each group of animals prior to scheduled
sacrifice. Clinical observations of the animals for general health
and well-being were performed at least weekly until 3 months, and
then monthly.
[0182] At 1 week PCA material was present at the implant site and
was found associated with moderate to marked granulomas presumable
associated with the resorption process. At week two a small amount
of PCA material was still present at the implant site and
associated granulomas were mild to moderate. By week four most
tissue appeared normal with a few mild granulomas persisting at the
implant site. At week twelve no evidence of the implant
remained.
EXAMPLE 16
[0183] Implantation and Resorption of PCA Calcium Phosphate in an
Intramuscular Site. This example describes the preparation of PCA
calcium phosphates that have varied in vivo resorption times as a
result of varied grinding times.
[0184] Individual dry precursors, ACP and DCPD were prepared as
described in Example 8. Several different formulations of DCPD and
ACP were then prepared by i) grinding DCPD for 15 sec, 30 sec, 1
min, 2.5 min, or 5 min in a SPEX grinder; ii) combining the ground
DCPD 1:1 with ACP; and iii) grinding the mixture for an additional
15 sec, 30 sec, 1 min, 2.5 min, or 5 min, respectively. Total
grinding times for the different preparations were therefore 30
sec, 1 min, 2 min ("Type 2" powders), 5 min, and 10 min ("Type 10"
powders).
[0185] The PCA calcium phosphate, sterilized in powder form by
approximately 2.5 Mrad of gamma irradiation, was prepared as
described in Example 4 by taking the material in powder form and
mixing with sterile water or saline and forming it into
approximately 1 cm disks 2 mm thick and incubated for a minimum of
30 minutes at 37.degree. C. Disks were implanted into adult male
New Zealand White Rabbits immediately following fabrication.
[0186] Animals were assigned to dose groups which contained 3 males
for a total of 15 animals. The implants were assigned to the
rabbits randomly. 10-15 minutes prior to the surgery, the animal
was premedicated with xylazine (10 mg/kg, i.m.). The animal was
then given ketamine (50 mg/kg, i.m.). The dorsal surface of the
animal was clipped free of hair and washed with a betadine surgical
solution and alcohol. Before the surgery the animal was monitored
to be sure that is was properly anesthetized. To do this, pressure
was applied to the foot pad. When there was no response, the animal
was properly anesthetized. Throughout the procedure, the animal was
monitored for whisker twitching and the toe-pinch reflect, which
indicated that the animal was not waking up.
[0187] Using aseptic technique and a scalpel blade, an incision 1-2
cm in length was made in the skin over the m. longissimus lumborum
(which lies along both sides of the spine). When the incision was
made, the underlying fascia and muscle was also cut to allow the
sample to pas into the muscle. The sample disk was placed directly
into the muscle, being sure that the entire implant was embedded in
the muscle. The muscle was closed with a single absorbable suture
and the skin was stitched closed subcutaneously. Metal skin staples
were used to close the external skin surface incision. Five samples
were placed on each side in this manner. Each sample was placed at
the end of the incision and they were approximately 1 cm apart from
each other (see diagram). The samples were in the form of 7 mm by 2
mm disks weighing approximately 150 mg. The animals were monitored
and were given buprenorphine (0.02-0.05 mg/kg, s.q) upon awakening.
The analgesic was administered 2 times per day for three days after
surgery.
[0188] The animals were radiographed immediately after the surgery
and for every two weeks thereafter. The radiographs were compared
to track the resorption of the materials. A standardized method was
used for the radiographs to minimize any variation between
timepoints.
[0189] After euthanasia, implant sites were first evaluated by
gross examination. In those sites with visible implants, the
implants appeared as grey to yellow solid discs. In those sites
where the implant had been resorbed, areas of red to tan
discoloration of the muscle were observed.
[0190] Muscle tissue, with the implants, was removed, being careful
not to disturb the implants. The tissues and the identifying marks
were placed into labeled jars filled with 10% neutral buffered
formalin. All implant sites were processed and evaluated
microscopically. Observations included focal fibrosis, focal
granulomatous inflammation, and appearance of the implant (in some
cases). Fibrosis was primarily seen as fibrocytes and collagen.
Animals with gross resorption had fibrosis and minimal to moderate
granulomatous focal inflammation. Granulomatous inflammation was
seen as focal aggregates of macrophages and giant cells, often with
intracytoplasmic crystals, and occasional heterophils and
lymphocytes. Inflammation around the non-resorbed implants was
primarily minimal to mild fibrosis and/or granulomatous
inflammation, both of which are within the acceptable range for
intramuscular implants.
[0191] At four weeks, the pellets made from PCA calcium phosphate
implants that had been prepared by grinding for 30 seconds, 1
minute, or 2 minutes were fully resorbed. Those that had been
prepared by grinding for 5 minutes or 10 minutes were not fully
resorbed.
EXAMPLE 17
[0192] Reactive amorphous calcium phosphate material is prepared as
Example 5 and is dry-mixed with other calcium phosphate compounds,
according to the method described in Example 8 with the following
modification. Instead of DCPD, the following calcium phosphate
compounds are used, including, but not limited to:
Ca(PO.sub.3).sub.2 (calcium metaphosphates),
Ca,(P.sub.5O.sub.16).sub.2 (heptacalcium phosphate),
Ca.sub.2P.sub.2O.sub.7 (calcium pyrophosphate),
Ca.sub.3(PO.sub.4).sub.2 (tricalcium phosphates). The dry-mixture
ratio is properly calculated to be between Ca/P ratios of 1.5-1.70,
depending on the molar Ca/P ratio of the compound mixed with the
reactive amorphous calcium. The PCA calcium phosphate identity of
the resulting material is then confirmed through the use of XRD and
FTIR.
EXAMPLE 18
[0193] This example follows the conversion reaction occurring in
association with the hardening of the hydrated precursor using
X-ray diffraction and Fourier transform infrared spectrometry.
[0194] Hydrated precursor was prepared as described in Example 9.
The reaction mixture was placed in a moist environment at 37
.degree. C. and examined by X-ray diffraction spectrometry at
different times. FIGS. 5a-d are the X-ray diffraction spectra of
the reaction product between DCPD and the reactive amorphous
calcium phosphate as described in Example 5. X-ray scan conditions
are (a) copper anode, (b) .lambda.=1.4540598, and (c) a scan range
20-35.degree. at a step of 0.02.degree. and step interval of 2
seconds. FIG. 6 shows the infrared spectra of dicalcium phosphate
dihydrate (FIG. 6a), the activated ACP of the invention (FIG. 6b),
and the poorly crystalline hydroxyapatite of the present invention
(FIG. 6c).
[0195] Samples shown in FIGS. 5a-5d were incubated for 0, 20 min,
75 min and 5 hours, respectively. The samples were removed at the
noted time and lyophilized to preserve chemical characteristics.
FIG. 5a, taken at the start of the reaction, represents a
combination of peaks attributable to the starting ACP and dicalcium
diphosphate (see, FIG. 4 for component XRD patterns). The sharp
peaks at ca. 20.25.degree., 23.5.degree., 29.5.degree.,
30.75.degree. and 34.2.degree. for crystalline dicalcium
diphosphate are readily observed. With increasing reaction time,
the sharp crystalline peaks subside and wide (amorphous) peaks
appear centered at 2.theta.=26.degree., 28.5.degree., 32.0.degree.
and 33.0.degree.. It is interesting to note that there is no change
in the spectra after 75 minutes of reaction, indicating that the
reaction essentially complete in little more than one hour. The
X-ray diffraction pattern of the bone substitute material of the
invention (FIG. 5d) can be compared to that of naturally occurring
bone, shown in FIG. 7. The two spectra are nearly identical.
EXAMPLE 19
[0196] Implantation and Resorption of PCA Calcium Phosphate in a
Bony Site
[0197] The purpose of this study was to assay resorption and
ossification of PCA calcium phosphate in a bony implant site. The
method is also useful for testing the resorption and ossification
properties of PCA calcium phosphate formulations and composites of
the invention.
[0198] The test article used was a PCA calcium phosphate
formulation prepared as described in Example 4. The ACP and DCPD
were mixed in the specified proportions and ground for 1 minute, 30
seconds in the SPEX grinder equipment.
[0199] Adult (>5 month old) NZW male rabbits were held in
quarantine and acclimatized for a minimum of 10 days prior to the
initiation of the study. Animals were individually housed in
suspended stainless steel cages. Wood shavings were used in
dropping pans under the cages. Prior to initiation of the study,
animals were assigned to groups or treatments randomly and were
identified by a numbered ear tattoo and by a corresponding cage
card. All animals had single defects placed in one tibia.
Timepoints for evaluations were 2, 4, and 8 weeks (2 animals at
each timepoint). Surgery was performed under full anesthesia and
aseptic surgical conditions.
[0200] After obtaining adequate anesthesia (e.g., ketamine/xylazine
to effect), using aseptic technique, an incision was made over the
lateral proximal tibia. The soft tissue was deflected away and the
bone exposed. Using an approximately 5 mm trephine in a low speed
dental handpiece with irrigation (0.9% physiologic saline) as
needed, a .about.5.5 mm diameter hole was cut through the cortical
portion of the bone. The bony disk was dissected free from the
cortex and the site was prepared for implantation. The hydrated
precursor material in paste form was placed into the defect.
Defects in control animals were left untreated. The soft tissues
were then closed in layers. One sample per animal was prepared
using this method.
[0201] Clinical observations of the animals' general health and
well-being, with special regard to their ambulatory abilities, were
made at least weekly. All animals appeared to be in good health. At
the end of the study the animals were euthanized with an overdose
of anesthetic and the implant site collected. Radiographs of the
tibiae were made at scheduled intervals including after surgery and
at the time of necropsy.
[0202] The implantation sites were fixed in formalin and stained
with either hematoxylin and eosin, Masson's trichrome, or Von Kossa
stained slides from decalcified samples. Undecalcified histological
samples were also prepared and stained with light green basic
fuschin. Slides were microscopically evaluated by a board certified
veterinary pathologist (ACVP) with experience in laboratory animal
pathology. Subjective observations were made of bone morphology,
and presence or absence of organized bone and of detectable PCA
calcium phosphate material was noted.
[0203] Histological results indicated some mineralization at 2
weeks. By 4-6 weeks, animals receiving implants had normal
trabecular bone at the implant site with no evidence of remaining
PCA calcium phosphate. The untreated controls had not fully healed
in that they had less than full ingrowth and/or had
non-cortical-type bone. FIGS. 9a and 9b are photomicrographs of
untreated and treated tibia defects, respectively, 2 weeks after
surgery. As can be seen, bone to the right of the defect edge in
the untreated sample (FIG. 9a) is thin trabecular bone; new bone to
the right of the defect edge in the treated sample (FIG. 9b) is
thick trabecular bone.
EXAMPLE 20
[0204] This example demonstrates the difference in resorption time
between two precursor formulations with different DCPD grain size
distributions. PCA calcium phosphate precursor material is prepared
according to example 10. Two precursor mixes are prepared. Sample A
corresponds to sample 10-6 and sample B corresponds to a 2:4:3:1
mix of samples 10-1, 10-2, 10-3 and 10-4. Hydrated precursor pastes
of the two samples are tested in rodents in the subcutaneous test
of example 15. Resorption is monitored at various time points.
EXAMPLE 21
[0205] This example demonstrates the difference in promoting
activity of DCPD of two different grain size distributions in the
conversion of both highly reactive and reactive ACPs
[0206] ACP was prepared as in Example 5, with the exception that
for some of the samples the final heat activation step was omitted.
Two samples of DCPD with grain size distributions corresponding to
B1 & B3 of example 10 were prepared. The ACPs and DCPDs are
then mixed for 5 minutes, either by hand or in the SPEX grinder.
Hardening characteristics are then determined. It is clear that
machine milled samples exhibited superior hardening properties over
hand ground samples. It is also clear that the samples with a
smaller particle size (B3) exhibited superior hardening properties
over larger grained samples (B1).
6TABLE 6 Reactions Using Different Strength Promoters HARDENING @
ACP DCPD GRINDING 30 min heated B3 mortar ++ non-heated B3 &
(not done) heated B1 pestle +/- non-heated B1 - heated B3 SPEX +++
non-heated B3 5-10 min +++ heated B1 + non-heated B1 (not done)
EXAMPLE 22
[0207] This example determines the specific surface area and
porosity of a PCA calcium phosphate material.
[0208] ACP was prepared according to Example 5. Samples from before
and after the final heat activation step were compared for their
reactivity in an in vitro hardening assay with unsieved DCPD (as
described in example 8). Specific surface area and average porosity
were also measured. Results are tabularized in Table 7 below.
7TABLE 7 Specific surface Area and Porosity of the Inventive ACPs
specific surface Average DCPD sample area (sq.m./g) Porosity
(.ANG.) Reactivity Pre heating 120.5 130 - After heating 76.8 129
+
EXAMPLE 23
[0209] This example describes the conversion of ACP to PCA calcium
phosphate in the absence of a promoter and demonstrates the failure
of the newly formed PCA calcium phosphate to harden. Likewise,
promoter DCPD fails to harden or convert on its own.
[0210] DCPD and a variety of ACPs and other calcium phosphates were
mixed with water and tested for their ability to harden at
37.degree. C. Table 8 summarizes these results, as well as
identification of the reaction products, if any, following the test
period. Under no circumstances was hardening observed up to 3 days.
It was concluded that while conversion of ACP to PCA calcium
phosphate may occur, the presence of a promoter is desired to
achieve setting and hardening
8TABLE 8 ACP conversion in the absence of a promoter H.sub.2O
Harden- ACP (g) Incubation ing FTIR XRD ACP (Example 5) 0.8 30 min
soft ACP AGP 12 hrs soft PCA* PCA* DCPD (Example 8) 0.7 30 min soft
DCPD ND 38-53 .mu.m 12 hrs soft DCPD ACP (Example 7) 1.5 30 min
soft PCA* ND not heat activated 12 hrs soft HA ACP (Example 5) 1.5
30 min soft ACP ND non-carbonated ACP (Example 6) 1.5 30 min soft
ACP ND not heat activated ACP (Example 5) 1.5 30 min soft PCA* ND
non-carbonated; heat activated *PCA = poorly crystalline apatitic
calcium phosphate ND = analysis not done
EXAMPLE 24
[0211] Different Hydrating Agents Effects on Hardening and Final
Product.
[0212] A hydrated precursor (ACP and DCPD) was prepared as
described in Examples 8, 9, or 10, with the exception that a
variety of hydration media were used. Samples were then tested for
hardness and completeness of reaction at various time points. In
all cases, 1 g of the mixed precursors were hydrated with 0.75-1.0
mL of hydration medium to produce a paste. Table 9 summarizes the
results and demonstrates that a variety of aqueous based liquids,
and in particularly physiologically acceptable media, may be used
in the preparation of PCA calcium phosphate.
9TABLE 9 Effect of Hydrating Agents Hydration Medium Incubation
Time Hardening Tris 30 min hard 0.9 M NaCl 30 min hard MEM 30 min
hard MOPS 30 min hard HEPES 30 min hard BUFFERALL 30 min hard PBS
30 min hard
EXAMPLE 25
[0213] ACP was prepared as described in Example 5, with the
exception that the heating the ACP to 450.degree. C. was carried
out for either 1 hour or 6 hours. Following heating the ACP was
prepared for reaction with DCPD as described in Example 8. Hydrated
PCA calcium phosphate precursor prepared with ACP heated for 6
hours was found not to harden after 2 hrs at 37.degree. C.
EXAMPLE 26
[0214] The porosity of a hardened sample of PCA calcium phosphate
prepared according to Example 10-5 was determined.
[0215] A hardened sample of PCA calcium phosphate (1 g) was weighed
immediately after removal from the moist incubator, and then air
dried at room temperature for 12 hrs. The dried sample was
carefully weighed and then the volume was calculated. The sample
was placed into a 20 mL sample of water. After 1 minute the
approximate displacement volume was noted. The dried sample was
found to absorb up to 50-60% of its dry weight in H.sub.2O. These
results are interpreted to mean that the sample is up to 50-60%
porous. Density was approximated at 1.65 g/cm.sup.3.
EXAMPLE 27
[0216] This example demonstrates the use of a resorbable polymer to
promote the conversion of ACP to PCA calcium phosphate.
[0217] Granular PLLA is prepared and sieved to a size of 100 .mu.m.
The powder thus obtained is mixed with the ACP (5:1 ACP:PLLA) of
Example 9 and ground for 5 minutes in a SPEX laboratory mill. Water
is added to 1 g of the mixture to form a workable paste. The paste
is shaped into a ball and is heated to 37.degree. C. in a moist
environment for 1 hour. The hardened sample is analyzed using FTIR
and XRD.
EXAMPLE 28
[0218] This example investigates the hardening characteristics of
the hydrated precursor at sub-ambient temperatures.
[0219] Hydrated precursor was prepared with water as described in
Example 9 and then tightly sealed to avoid evaporative loss either
in parafilm or in an aluminum tube. The samples were then held for
up to 1 hr, 24 hrs and 6 days. At the indicated time points, the
hydrated sample was removed from refrigeration placed in a moist
environment at 37.degree. C. In all instances the samples hardened
within 30 minutes.
EXAMPLE 29
[0220] This example demonstrates the efficacy of the inventive PCA
calcium phosphate in promoting the healing in a large animal model,
of a full segmental defect in a weight bearing limb.
[0221] Hydrated precursors Type 2 and Type 10 were prepared and
treated immediately prior to surgery as described in Example
16.
[0222] Animals fasted for 24 hours prior to anesthesia, during this
time interval water was available ad libitum. Ketamin
(Aescoket.RTM., 10 mg/kg i.m.) and atropine (1.5 mg i.m.) was
administered as a pre-medication about 15 minutes before fully
anesthetizing the animals. Etomidate (Hypnomidaat.RTM., 0.3 mg/kg
i.v.) was used as the anesthetic. After intubation, anesthesia was
maintained with an O.sub.2/N.sub.2O-mixture (1:1, vol/vol)
supplemented with 2% isoflurane.
[0223] Surgery was performed asceptically under full anesthesia.
After shaving and iodinating the skin, an incision was made over
the anteromedial side of the tibia. The muscles were bluntly
dissected and the tibial shaft was prepared free of tissue to as
great an extent as possible. After reaming the medullary cavity, an
intramedullary nail (diameter 8 mm) was inserted via a hole in the
anterior tibial plateau. The inserted nail was locked with two
proximal and two distal bolts. A 20 mm osteoperiostal segmental
defect was then created in the mid-shaft of the tibia with the aid
of a thread saw and an oscillating saw.
[0224] The defect was filled according to the treatment group. In
one group, autologous bone was harvested from the ipsilateral iliac
crest and placed into the defect. In the other group, approximately
2-4 g of the hydrated PCA calcium phosphate precursor (type 2 or
type 10) was applied by hand to fill the defect. The soft tissues
and the skin were closed in layers with resorbable suture
material.
[0225] The animals received post operative lincomycin/spectinomycin
(Vualin Plus.RTM., 5 mg/10 mg per kg per day) for 3 days by
intramuscular injection. The animals were kept outside in the
meadow as soon as full weight bearing of the operated limb was
possible. Animals were sacrificed prior to explanation of the
tibiae as follows: As a premedication ketamin (Aescoket.RTM., 500
mg i.m.) and xylazin (Rompun.RTM., 40 mg i.m.) were given. Then 0.5
mg fentanylcitrate (Fentanyl.RTM.), 10 mg etomidate
(Hypnomidate.RTM.), 4 mg pancuronium bromide (Pavulon.RTM.), and
1.4 gram potassium chloride were administered intravenously.
[0226] Animals receiving the inventive PCA calcium phosphate
demonstrated complete healing at three months. The test bones were
then dissected from the animal and tested for strength. Preliminary
results indicated that the inventive PCA calcium phosphate was
resorbed and ossified to produce bone equal to or better than
autologous implants in less than three months.
EXAMPLE 30
[0227] The purpose of this study was to evaluate resorption,
ossification and biocompatibility of two formulations of the
inventive PCA calcium phosphate in canine mandibular sites.
Prehardened PCA calcium phosphate was implanted in a canine
mandibular onlay model which additionally may be used as an
augmentation model.
[0228] The test article was PCA calcium phosphate in two
formulations, corresponding to Types 2 and 10 described in Example
18. The PCA calcium phosphate was pre-hardened in a moist
environment at approx. 40.degree. C. immediately prior to
implantation. The control implants were 3 mm.times.4 mm cylinders
of silicone and porous hydroxyapatite, respectively.
[0229] Two adult female hound-type dogs (20 to 25 kg) were used in
the study. Both dogs received two control implants (1 of each) on
the right side of the mandible and one each of the Type 2 and Type
10 PCA calcium phosphate formulations on the left (opposite)
side.
[0230] Implantation was performed under full anesthesia and aseptic
surgical conditions. The animals were premedicated with
tranquilizers and atropine-type agents and induced with
barbiturates. The animal's vital signs (temperature, heart rate,
respiratory rate) were monitored before and throughout the
procedure. The animals were tested for proper anesthetic depth by
toe pinch and corneal stimulus. After obtaining adequate
anesthesia, using aseptic technique, an incision was made in the
skin over the midlateral ventral surface of the mandible and
proximal neck (over the mandible lower edge). The soft tissue was
deflected away and the bone was exposed. The periosteum over the
outer mandibular surface was elevated and the bone surface was
roughened with a burr or drill until it was rough and bloody in a
shape to accept the cylindrical implants. The control articles and
pre-hardened PCA calcium phosphate were placed into the defects.
Two samples per animal per side were onlaid onto each outer
mandible surface using this method (two experimental PCA calcium
phosphate samples and two controls). The samples were placed about
1 cm to insure that they do not appose each other. The periosteum
was closed first using 3.0 vicryl. The soft tissues were then
closed in layers with 3-0 vicryl absorbable suture. The skin was
closed with simple interrupted sutures of 5-0 nylon. The animals
were allowed to heal for scheduled periods of time. One dog was
sacrificed at 3 weeks and the other at 3 months and the test sites
were removed for histology. All animals were euthanized and
identifying marks were collected.
[0231] The implantation sites were prepared as undecalcified
sections. Sections were evaluated for biointegration,
biodegradation, and biocompatibility.
[0232] The results were as follows: At all time points excellent
biocompatibility was observed. No giant cells and minimal
macrophage were observed. There was only minimal reaction layer of
only a few cells thickness at the base of the PCA calcium phosphate
implants. This is significantly better than was observed for either
of the controls.
[0233] At three weeks, the majority of the Type 2 material was
resorbed. At twelve weeks, the Type 2 was completely resorbed to
the surface of the original bone. Additionally the bone in the
socket was not fully differentiated.
[0234] The Type 10 samples demonstrated osseointegration with new
bone ingrowth and cell migration into the implant. The implant
itself was approximately 10% resorbed after twelve weeks.
[0235] The silicon control implant, which is not resorbable,
displayed a mild to moderate foreign body reaction. Voids were
unfilled at three weeks, but by twelve weeks were filled with
fibrous tissue. The hydroxyapatite control implant showed no signs
of resorption or osseointegration within the first twelve
weeks.
[0236] This experiment confirms the excellent biocompatibility of
the inventive PCA calcium phosphate. Additionally, a difference in
resorption time between the two PCA formulations was observed, with
a prolonged resorption time course for the sample in which the
precursors were mixed/ground for a longer period of time (Type
B).
[0237] The results also point out the slower resorption and
ossification properties observed in the non-load bearing mandible
implant site as compared to the rapidly ossifying load bearing
applications of Example 29. Finally, the results demonstrate the
need for slowly resorbing PCAs for proper osseointegration in
augmentation plastic surgery.
EXAMPLE 31
[0238] This example demonstrates the effect of maintaining the
hydrated precursor uncovered at room temperature.
[0239] The dry precursor was prepared as described in Example
11(b). The dry precursor was mixed with the indicated amount of
water and tested for hardening and injectability through a 16 gauge
needle after standing uncovered at room temperature for various
time periods. The results are reported in Table 10.
10TABLE 10 Paste Injectability after Standing at Room Temperature.
injectability water standing room for 16 hardening; Sample added
mixing time temp. gauge 130 wt (g) (mL) time (s) (min) (.degree.
C.) needle min/37 .degree. C. 1 0.8 20 10 25 v. good v. good 1 0.8
20 20 24 v. good v. good 1 0.8 20 30 25 v. good v. good 1 0.8 20 40
25 good v. good 1 0.8 20 50 24 poor v. good 5 4.2 40 10 24 v. good
v. good 5 4.2 40 20 25 v. good v. good 5 4.2 40 30 25 good v. good
5 4.2 40 40 25 poor v. good
[0240] These results demonstrate that a one gram sample may be
stable as an injectable paste at ambient conditions for up to 45
minutes and that a 5 gram sample may be stable as an injectable
paste for up to 30 minutes at ambient conditions (in air,
25.degree. C.).
Other Embodiments
[0241] It will be understood that the foregoing is merely
descriptive of certain preferred embodiments of the invention and
is not intended to be limiting thereof. The following claims cover
all of the generic and specific features of the invention herein
described in the text and accompanying drawings.
* * * * *