Patent 6,953,594

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 and 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. A composite material is provided including a strongly bioresorbable, poorly crystalline apatitic calcium phosphate composite and a supplementary material. The supplementary material is in intimate contact with the hydroxyapatite material in an amount effective to impart a selected characteristic to the composite. The supplemental material may be biocompatible, bioresorbable or non-resorbable. A method for treating a bone defect also is provided by identifying a bone site suitable for receiving an implant, and introducing a strongly resorbable, poorly crystalline apatitic calcium phosphate at the implant site, whereby bone is formed at the implant site. The implant site may be a variety of sites, such as a tooth socket, non-union bone, bone prosthesis, an osteoporotic bone, an intervertebral space, an alveolar ridge or a bone fracture.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

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.

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.

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.

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% (preferably 95-99% and more preferably >99%) 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.

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.

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.

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.

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 μm (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.

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 μm (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.

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 remodeling occurs so that by twelve weeks the newly ossified bone is indistinguishable from native bone.

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.

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.

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.

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° 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.

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° 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

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.

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° 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 saline.

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 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.

Types of Promoters, The purpose of the promoter is to promote the hardening of the hydrated 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.

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.

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.

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.

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.

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 μm were found to be strongly reaction promoting.

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° C. in a moist environment. A suitable promoter exhibits PCA calcium phosphate formation and paste hardening under these conditions.

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.

It is hypothesized that the hardening with the associated 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. Specific surface area of the ACP and promoter powders can be controlled to control the reaction condition and final PCA material properties. 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. Specific surface area has been shown to be correlated to the compressive strength, and possibly the porosity and resorbability, of the PCA material.

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 participating promoters are those which tend themselves to undergo conversion to hydroxyapatite through an intermediate PCA calcium phosphate phase.

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, CaCO₃, calcium acetate, and H₃PO₄, 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.

In a preferred embodiment, DCPD is used as a participating promoter with a grain size less than 200 μm, in more preferred embodiments with an average grain size of <95 μm, and in most preferred embodiments with an average grain size of about 35-45 μm and a grain size maximum of less than about 110 μm.

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.

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.

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° 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

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.

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 μm, preferably 1 to 300 μm, and most preferably 1 to 200 μm. The actual grain size used may be varied to improve the reaction promoting characteristics of the particular substance.

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 prepare 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° C. in the desired amount of time. Preferred passive promoters include but are not limited to granular forms of SiO₂, mica, Al₂O₃, poly(L-lactide) (PLLA), polyglycolide (PGA), and poly(lactide-co-glycolide) (PLGA) copolymers.

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.

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.

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° 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° C. or preferably colder temperatures such as -20° C. or -75° C. will also retard crystallization.

In the embodiments described above, the paste or putty is hardened at 37° C. Hardening at 37° 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° C. are preferred.

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.

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.

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.

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.

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.

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 2θ=26 and 32. An additional broad shoulder occurs at approximately 2θ=29 and another may be present at approximately 20θ=33.6. Absent from the spectra are any additional sharp peaks or sharp shoulders characteristic of more crystalline apatites occurring in the range of 2θ=27-34. In particular there are no sharp peaks or shoulders corresponding to Miller's Indices of 210, 112, or 300 for hydroxyapatite.

FTIR spectrum is characterized by peaks at 563 cm^-1, 1034 cm^-1, 1638 cm^-1and 3432 cm^-1(±2 cm^-1). Sharp shoulders are observed at 603 cm^-1and 875 cm^-1, with a doublet having maxima at 1422 cm^-1and 1457 cm^-1 (see, FIG. 6c).

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.

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.

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.

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.

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.

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° 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).

In some preferred embodiments (e.g., Examples 8-10), the reaction occurs slowly at room temperature and is almost undetectable below 18 or 19° 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° 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° C.), the paste hardens in less than about an hour, preferably in about 10-30 minutes. When held at 4° 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 pharmaceuticals to locally raise or lower the body temperature.

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.

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 H₂O/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° C. in the refrigerator provided steps are taken to minimize evaporative water loss.

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° C. in the refrigerator provided steps are taken to minimize evaporative water loss.

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.

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.

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.

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 μm) or in the presence of a relatively weak promoter such as a DCPD sample comprising a substantial amount of grains greater than 100 μm (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° C.

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. 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.

The method of the present invention permits initial formation of amorphous calcium phosphate particles of less than 1000 Å, preferably 200-500 Å, and most preferably 300 Å, 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.

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 PO₄^3- group(s). Because some PO₄^3- is replaced by the third ion, the overall PO₄^3--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.

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. CO₃^2-) missing from CaCO₃ 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

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.

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.

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.

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.

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.

Preferred calcium and phosphate sources: The use of highly concentrated or near supersaturation solutions ensures that a more rapid reaction will occur.

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.

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.

Use of a carbonated material is desirable because it permits manipulation of the Ca/P ratio by substitution of PO₄^3- by CO₃^2-. Additionally, the presence of CO₃^2- 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⁺²and P₂O₇^4- ions.

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.

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^-1-10^-4, preferably 10^-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.

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 CO₃^2- into CO₂ and oxygen. The heating step is carried out at a temperature of less than 500° C. but more than 425° 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° C. for 1 to 6 hours preferably for 50 to 90 minutes.

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.

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.

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° 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° C. retains its amorphous characteristic, but exhibits a decreased reactivity. c.) Non-carbonated ACPs heated to 460° 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.

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.

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° 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 CO₃^2- 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 2θ=20 to 35 or at any diffraction angles that correspond to known crystalline calcium phosphates.

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).

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.

The PCA calcium phosphate of the invention may be used in a variety of formulations and in a variety of applications, some of which are described hereinbelow.

Composite Materials.

A strongly bioresorbable ceramic composition may be used in the repair and growth promotion of bone tissue (a bone substitute composite). The composition comprises a biocompatible and strongly bioresorbable poorly crystalline apatitic (PCA) calcium phosphate combined with a suitable biocompatible matrix or additive.

In one aspect, the invention provides for a strongly bioresorbable composite comprising a bioresorbable, PCA calcium phosphate and additional bioresorbable supplementary materials which can be prepared under mild conditions at room or body temperature (e.g., 20-40° C.). The composite may be applied to bone-contacting surfaces of prosthetic devices, for use as a bone cement. It may be applied directly to bone defects as a filler, where it is capable of promoting the growth of new bone tissue. Alternatively, the composite may be used to fabricate fixtures or devices such as screws and plates, which under appropriate circumstances will be resorbed and replaced by bone. The composite may also be used free standing in non-osseous tissue. When a pharmaceutically active component is added to the composite, it serves as a drug delivery device. Release of the agent may occur over a long period of time after implantation as the composite slowly biodegrades. See, related co-pending application U.S. Ser. No. 08/729,342 entitled "Delivery Vehicle", herein incorporated by reference.

The current invention employs a strongly bioresorbable and ossifying PCA calcium phosphate useful as an implantable bioceramic for the treatment of bone disorders and injuries and other biological applications requiring resorbable calcium phosphate. "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. The composites disclosed herein may undergo resorption (i.e., at least 80%, preferably 95-99% and most preferably >99%) of the total mass (at least 1 g and preferably 1-5 grams) of the implanted PCA material preferably within one year, more preferably within 9 months or 6 months, more preferably in less than 3 months, and most preferable within 1 month.

The PCA calcium phosphate of the invention is characterized by its biological resorbability and substantial absence of crystallinity, as is discussed hereinabove. Its crystalline character is substantially the same as natural bone, as compared to 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, no significant detrimental reaction (e.g., inflammation or fibrosis) is induced in the host by the implanted composite material. Materials which induce a medically acceptable level of inflammation or fibrosis are considered biocompatible. In addition, the material is also bioactive, in that apposition of new bone at the host/composite interface occurs.

In an important aspect of the invention, the ease of use of the inventive implantable bioceramic material in a surgical or manufacturing setting is significantly improved over other bone substitute composite materials known in the art. Specifically, the setting reaction associated with the formation of PCA calcium phosphate may be initiated outside the body and proceeds slowly at room temperature thereby minimizing any possibility that the material will "set up" prior to heating (e.g. prior to application to the surgical site or in the manufacturing incubation). Setting accelerates significantly at about 37° C. causing the material to harden. 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.

By formulating the PCA material as a composite, mechanical properties of the material may be improved. In some formulations, the hardened PCA calcium phosphate alone is brittle and has a durometer and bulk modulus similar to traditional blackboard chalk. The preparation of PCA calcium phosphate as a composite material is desirable in order to alter the mechanical properties for some medical uses. Furthermore, the consistency, formability and hardness of the PCA 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.

Preparation of the implantable bioceramic composite. The composite material of the present invention is prepared by combining the PCA calcium phosphate of the invention with a selected supplementary material. The PCA calcium phosphate may serve as the reinforcing material, the matrix or both. The PCA calcium phosphate of the invention in it's initial paste form (i.e., as a hydrated precursor) 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 PCA calcium phosphate.

The supplementary material may be added to the PCA calcium phosphate in varying amounts and in a variety of physical forms, dependent upon the anticipated therapeutic use. By way of example only, 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 intimately mixed with the resorbable PCA calcium phosphate. The supplementary material may serve as a matrix for the PCA calcium phosphate, which is embedded or dispersed within the matrix. Alternatively, the PCA 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 body, for example, as a post-fabrication coating to retard resorption time or otherwise affect the bioceramic material properties. Lastly, the supplementary material may be coated with PCA calcium phosphate.

The supplementary materials are desirably biocompatible, that is, there is no detrimental reaction induced by the material when introduced into the host. 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 PCA calcium phosphate/supplementary material interface. The affinity may be specific or mediated through non-specific ionic interactions. By way of example only, 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 α-hydroxycaboxylic acids, such as poly(L-lactide) (PLLA), polyp,L-lactide) (PDLLA), polyglycolide (PGA), poly(oactide-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 SiO₂, Na₂O, CaO, P₂O₅, Al₂O₃ and/or CaF₂, may be used in combination with the PCA calcium phosphate of the invention. Other useful bioerodible polymers may include polysaccharides, peptides and fatty acids.

Bioerodible polymers are advantageously used in the preparation of bioresorbable hardware, such as but not limited to intermedulary nails, pins, screws, plates and anchors for implantation at a bone site. In preferred bioresorbable hardware embodiments, the supplementary material itself is bioresorbable and is added to the PCA calcium phosphate in particulate or fiber form at volume fractions of 1-50% and preferably, 1-20 wt %. In some preferred embodiments, the bioresorbable 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 PCA calcium phosphate and polymer. In one embodiment, a PCA calcium phosphate matrix is reinforced with PLLA fibers, using PLLA fibers similar to those described by Törmälä et al., which is incorporated herein by reference, for the fabrication of biodegradable self-reinforcing composites (Clin. Mater. 10:29-34 (1992)).

The implantable bioceramic composite may be prepared as a paste by addition of a fluid, such as water or a physiological fluid, to a mixture of a PCA calcium phosphate and a supplemental material. Alternatively, a mixture of the supplementary material with hydrated precursor powders to the PCA calcium phosphate can be prepared as a paste or putty. In cases where the supplementary material is to be dispersed within or reacted with a PCA calcium phosphate matrix, water may be added to one of the precursor calcium phosphates to form a hydrated precursor paste, the resulting paste is mixed with the supplementary material, and the second calcium phosphate source is then added. Alternatively, the calcium phosphate sources which make up the PCA calcium phosphate precursor powder may be premixed, water may then be added and then the supplementary material is added. In those cases where it is desirable to have the supplementary material serve as the matrix, the fully hardened PCA calcium phosphate will be prepared in the desired form which will most often be of controlled particle size, and added directly to the matrix forming reaction (e.g., to gelling collagen). These materials may then be introduced into molds or be otherwise formed into the desired shapes and hardened at temperatures ranging from about 35-100° C. A particularly useful approach is to form the composite precursor paste into the approximate shape or size and then harden the material in a moist environment at 37° C. The hardened composite may then be precisely milled or machined to the desired shape for use in the surgical setting. The amount of particular PCA calcium phosphate to be incorporated into the supplemental material matrix will most often be determined empirically by testing the physical properties of the hardened composite according to the standards known to the art.

In preferred embodiments, the reactants are mixed outside of the body, yielding a formable composite material comprising a hydrated precursor material having a physical integrity suitable for application to a surgical site. Conversion to the PCA material generally is complete after application to the surgical site. The supplemental materials will generally be in final form when added to the PCA calcium phosphate or hydrated precursor paste, although the use of polymer monomers and precursors, added to the paste is considered within the scope of the invention. In a preferred embodiment, the conversion reaction is initiated by adding distilled water to a mixture of the dry precursor components to form a thick hydrated precursor in the form of a paste or putty. Other aqueous agents such as buffers, saline, serum or tissue culture medium may be used in place of distilled water. In other preferred embodiments, sufficient water may be added to the precursor powders to form a paste which is readily injectable with an 18 gauge needle. Bioceramic composite materials of the invention generally harden in less than five hours and substantially harden in about one to five hours under physiological conditions, and preferably in about 10-30 minutes. Most often the resulting bioresorbable PCA calcium phosphate will be 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.

The invention also provides a test for identifying suitable reactive PCA calcium phosphate and reactive precursors for use in the composites of the invention. Specifically, precursors are combined, are hydrated with a limited amount of water (so that a paste or putty is formed), and are allowed to harden into a PCA material. Desirable precursors are capable of hardening in a moist environment, at or around body temperature in less than 5 hours and preferably within 10-30 minutes. Components which harden in this way may then be placed intramuscularly or subcutaneously in a test animal and checked for biological resorbability. Desirable materials are those that, when implanted as a 1-5 g pellet, are at least 80% (preferably 95-99% and most preferably >99%) resorbed within one year. Generally, it is easier to test resorption of gram quantities of material in subcutaneous sites.

Medical devices prepared from the inventive composites using all bioresorbable supplementary materials will themselves be resorbable and in preferred embodiments, strongly bioresorbable. The composites used in these devices may be designed to impart the desired mechanical properties to the devices making them useful in the surgical setting (e.g., orthopedic pins and screws). Following placement in the host, the devices will gradually be replaced by bone i.e., ossification of the bone site occurs. This is in contrast to merely biocompatible materials where the device promotes apposition of bone at its surface, but does not resorb so as to ossify the implant site. While resorption time in vivo will generally depend upon the actual identity of the supplementary material, as well as the graft size and location, for those composites with less than 20% vol/vol supplementary material, resorption of the PCA calcium phosphate and ossification at the implant site will be generally complete in less than six months and most often in about one month. In some cases, the resorbable supplemental material will still be present embedded in the newly formed bone, thus being resorbed over a longer time course than the PCA calcium phosphate. The use of resorbable hardware obviates the need for a subsequent surgical procedure to remove the device.

The resorbability of the implantable bioceramic composite material of the instant invention is attributable in part to the porosity, crystallinity and chemical composition of its component materials. The bioceramic composite material of the invention comprises a poorly crystalline apatitic calcium phosphate, substantially similar to that found in natural bone. Lack of 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 is believed to promote its resorbability in biological systems, Porosity facilitates both the penetration of cells and cell processes into the bone substitute material matrix and the diffusion of substances to and from the matrix interior. Accordingly, low porosity composite materials resorb more slowly in vivo than those of high porosity.

In preferred embodiments, in order to optimize ossification, the devices and objects may be seeded with bone forming cells. This is most easily accomplished by placing the device in contact with a source of the patient's own bone forming cells. Such cells may be found in bone-associated tissue, blood or fluids, including exogenous fluids which have been in contact with bone or bone materials or regions, including the periosteum, cancellous bone or marrow. In the case of devices such as screws and pins, the introduction of which into bone is accompanied by breach of the periosteum and/or 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. Use of non-autologous bone cells is 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. See, U.S. application Ser. No. 08/729,354 entitled, "Cell Seeding in Ceramic Compositions" incorporated herein by reference.

Bioresorbable polymers may also be used in the preparation of bone glues or putties for use in load bearing situations. Supplementary materials may be added to the composite to increase compressibility and load-bearing properties of the bone glue. In particular, carbon fibers or other reinforcing fibers may be added to the composite. In the production of fiber-reinforced bone substitute glues, it may be advantageous to plasma etch the fibers to improve the quality and strength of the calcium phosphate/fiber interface. PCA calcium phosphate may also be hardened at 37° C., pulverized or otherwise fragmented, and mixed with known binders such as bone glues cements, fillers, plasters, epoxies, other calcium phosphates, or gels such as, but not limited to, calcium sulfate, tricalcium phosphate, tetracalcium phosphate, alginate, collagen, or commercially available products such as Endobone (Merck), Hapset (Lifecore Biomedical), SRS (Norian), Bonesource (Leibinger), Collograft (Zimmer), Osteograf (CereMed), and Simplex (Howmedica). For applications where hardened PCA calcium phosphate will be dispersed within the binder substance, most often the binder will be prepared by methods known to the art and mixed with the particulate PCA calcium phosphate in approximately equal volumes, although actual proportions will be varied in ways known to the art to produce compositions of desired consistency, workability and adherence.

In yet another embodiment, braided sutures, typically prepared from polyester, may be coated with the PCA calcium phosphate of the invention, to improve their biocompatibility. Coated sutures may be prepared by dipping the suture into a slurry containing finely divided particulate PCA calcium phosphate. The affinity of the suture for the PCA calcium phosphate coating may be improved by surface treating either the suture, the PCA calcium phosphate particle or both. Surface treatments include plasma etching and/or chemical grafting.

In other embodiments, a composite is provided comprising PCA calcium phosphate and a non-resorbable or poorly resorbable material. Suitable non-erodible or poorly erodible materials include dextrans, polyethylene, polymethylmethacrylate (PMMA), carbon fibers, polyvinyl alcohol (PVA), poly(ethylene terephthalate)polyamide, bioglasses, and those compounds listed previously for use in bone glues or putties. In one embodiment, carbon fibers may be used to reinforce the PCA calcium phosphate. In such applications, fibers lengths of 0.05 μm -20 cm and fiber content typically in the range of 0.01-50 vol % are used depending upon the intended use.

Another use is to permanently imbed useful objects, such as a pin or reinforcing mesh, into bone itself. The object serves as an anchor for the stable attachment to natural bone. This is particularly useful in the attachment of ligaments and tendons to bone. Objects comprising bioresorbable and ossifying or dental prosthesis seating PCA calcium phosphate and a suitable non-resorbable hardware may be placed into a bone and further secured with additional PCA calcium phosphate material or composite material in a bone glue formulation. The hardware then becomes embedded into the bone following reossification of the PCA calcium phosphate.

Calcium phosphates, including hydroxyapatites, tricalcium phosphate and tetracalcium phosphate, may be used as the non-resorbable supplementary materials of the inventive composites, in particular to maintain biocompatibility of the composite. In these embodiments, the calcium phosphates are most likely to be non-resorbable and to be pre-hardened in a particulate, fiber-like or other pre-formed shape. These solid calcium phosphate additives may further be compressed, sintered or otherwise modified prior to mixture with the PCA calcium phosphate.

In yet another embodiment of the invention, a composition is prepared by intimately mixing the PCA calcium phosphate with an additive which alters the resorption properties, setting time and/or flow characteristics of the composite. For example, silicone oil or other lubricating polymers or liquids may be added to the composite to improve the flow characteristics of the composite for delivery to the host by syringe. The lubricant is preferably biocompatible and capable of rapid leaching from the bone substitute material composite following solidification of the PCA calcium phosphate in vivo. Suitable lubricants include, by way of example only, polymer waxes, lipids, surfactants and fatty acids. Lubricants may be used in a concentration of about 0.1 to about 30 wt %.

In yet another embodiment of the invention, the composite contains a PCA calcium phosphate and a radiographic supplemental material for imaging the implant in vivo. Suitable electron dense materials include materials known in the art, such as titanium and barium oxide, in clinically relevant concentrations.

In a preferred embodiment, a bioceramic material may be prepared with a Young's Modulus similar to bone by preparing a polyethylene composite containing the resorbable PCA calcium phosphate of the invention. In other preferred embodiments, a resorbable polymer such as poly(L-lactide) or collagen may be used to prepare a composite with similar properties as normal bone. In another preferred embodiment, the particulate PCA calcium phosphate is pressed into a desired shaped and the pressed body is impregnated with the supplementary material. In yet another preferred embodiment, hydrated precursor materials of the PCA calcium phosphate are mixed with the supplementary material and the conversion to the bioceramic material is initiated in the presence of the supplementary material. Generally, the inventive PCA calcium phosphate will be present in the composite at a volume fraction of less than 0.7 and preferably less than 0.5.

The composition of the invention may be prepared in any conventional manner useful in the preparation of composite materials, including but not limited to blending, mixing, alloying, laminating, filament winding and pultruding. A variety of strategies for the design and fabrication of polymer/inorganic composites, fibers and matrix resins and other reinforcement technologies are useful and will be known in the art. Guidance regarding the preparation of HA/polyethylene composites can be found in Bonfield in Introduction of Bioceramics at pp. 299-303 and the references therein, all incorporated by reference. Additional guidance may be obtained from the following sources, incorporated herein by reference: Jang, Advanced Polymer Composites: Principles and Applications, ASTM Int'l, Materials Park, Ohio (1994); Opila et al. Eds., Polymer/Inorganic Interfaces, Materials Research Soc., Pittsburgh, Pa. (1993); Saifullin, Physical Chemistry on Inorganic, Polymeric and Composite Materials, Ellis Horwood, N.Y. (1992); Ducheyne et al. in Introduction to Bioceramics, Hench and Wilson, Eds. World Scientific Publishing, N.J. pp 281-298 (1993); and Törmälä, Clin. Materials 10:29-34 (1992).

The bioceramic composite material may also be prepared with varying degrees of porosity. In one embodiment, the use of a dry mixture of controlled particle size reactants leads to a porous composite material. Other methods of promoting porosity, such as chemical or physical etching and leaching, may be employed.

In yet another embodiment, a mixture of the PCA calcium phosphate and a polymeric supplemental material may be extruded by conventional polymer extrusion techniques to form tubes, fibers and other shapes. For extrusion purposes, the supplemental material is preferably an organic polymer. In some situations, where increased tensile strength and modulus and stiffness are desired, the composite may be extruded or otherwise mechanically deformed to align polymer chains to increase composite strength. The composite may also be hardened under pressure and/or heat to provide a composite that is more dense, tougher and resorbs at a slower rate in vivo. In general, conditions which cause rapid conversion of the PCA calcium phosphate to the more crystalline HA should be avoided.

In some embodiments, it may be desirable to modify the surface of the PCA calcium phosphate and/or the supplemental material in order to improve the interface between the two materials and/or to improve the affinity of pharmaceutically active agents, e.g., proteins, to the composite. For example, the inventive calcium phosphate may be grafted with moieties which show affinity for proteins and other organic molecules. Alternatively, the composite may be subjected to surface treatments, such as plasma etching to improve interfaces between the two phases as is known in the art.

For some embodiments in which the composite material is prepared and hardened in advance of its surgical use, and where storage is desired, it may be desirable to enhance the stability of the poorly crystalline state of the composite. In such cases, exposure of the pre-formed composite to crystallization inhibitors may be useful. Inhibitors may be added to the aqueous medium used to prepare the inventive PCA calcium phosphate, or the finished composite or objects made from it may be exposed to inhibitory substance subsequent to fabrication. Suitable such inhibitors include, but are not limited to, magnesium ion, carbonate ion, poly(L-glutamate), polyacrylate, phosvitin, casein, and protein-polysaccharides. Guidance for the use of such compounds can be found in LeGeros in Monographs in Oral Science Vol. 15 pp 84-107; LeGeros Prog. Crystal Growth Charact. 4:1-45 (19810; and Termine et al. Arch. Biochem. Biophys. 140:318-325 (1970), incorporated herein by reference.

The inventive composite may also be used as a drug delivery system by incorporation of a biologically active material into the composite. Further details are found in co-pending application U.S. Ser. No. 08/72,342 entitled "Drug Delivery Device", which is herein incorporated by reference.

Orthopedic and Dental Devices.

The strongly bioresorbable ceramic material may be used in the repair and growth promotion of bony tissue, i.e., as a bone substitute material. In one aspect, an orthopedic or dental implant is introduced into an implant site and is demonstrated to exhibit strong bioresorbability, excellent reossification and bone ingrowth of both cortical and trabecular bone at the implant site. The orthopedic or dental implant of the present invention is comprised of a synthetic, strongly bioresorbable poorly crystalline apatitic calcium phosphate material. In preferred embodiments, it is the material described in co-pending applications U.S. Ser. No. 08/650,764 and/or U.S. Ser. No. 08/446,182, now issued as U.S. Pat. No. 5,650,176, and/or the application entitled "Method and Products Related to the Physical Conversion of Reactive Amorphous Calcium Phosphate", U.S. Ser. No. 08/729,344, each of which is incorporated herein by reference.

The current invention employs a strongly bioresorbable and reossifying PCA calcium phosphate as an implantable bioceramic for the treatment of bone and dental disorders and injuries and other biological applications. The implant is useful in a variety of treatments. By way of example and in no way limiting of the invention, the ceramic material may be applied to bone-contacting surfaces of prosthetic devices, for use as a bone cement. It may be applied directly to bone defects as a filler, where it is capable of promoting the growth of new bone tissue. It may be applied to a tooth socket to avoid problems associated with tooth extraction such as dry socket and/or to provide a fixed substrate on which to anchor a replacement tooth. Alternatively, the PCA material may be used to fabricate fixtures or devices such as screws and plates, which will be resorbed and replaced by bone. When a pharmaceutically active component is added to the composite, such as growth factors or antibiotic, it serves as a drug delivery device. Release of the agent may occur over a long period of time after implantation as the PCA material slowly biodegrades. See, related co-pending application U.S. Ser. No. 08/729,342 entitled "Delivery Vehicle" which is hereby incorporated by reference.

An implant prepared using the inventive PCA material is strongly bioresorbable, that is, at least 80% (preferably 95-99% and most preferably >99%) of the mass of the implanted PCA material is resorbed within one year of implantation. By modifying the characteristics of the PCA material, i.e., porosity, composition, crystallinity, etc., the resorption profile may be modified so that at least one gram (preferably 1-5 grams) of the PCA material is at least 80% resorbed within 12 months, 9 months, 6 months, 3 months or ideally, 1 month, from implantation.

In addition the implant prepared from the inventive PCA calcium phosphate strongly promotes ingrowth of new bone into the implant site. Many current bone implant materials, e.g., bioresorbable organic polymer, merely promote bone apposition at the implant surface. In contrast, the implant of the present invention promotes the growth of new bone within the implant itself. Growth of both trabecular bone and cortical bone (outer bone layer) has been demonstrated to occur. Significant ingrowth occurs within days of implantation. Substantially the entire implant site has been subsumed by new bone within six months, and ideally within one month, of implantation. Weight-bearing bones tend to regenerate bone more rapidly than non-load bearing bones. Thus, ingrowth for the latter may occur somewhat more slowly.

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 remodeling occurs so that by twelve weeks the newly ossified bone is indistinguishable from native bone.

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.

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, endosteum 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.

The implant also prevents deleterious reactions from occurring within the bone gap. For example, fibrous tissue often forms at bone defect sites, which impairs the ingrowth of bone. The implant of the invention is biocompatible and has been demonstrated to reduce the incidence of fibrotic growth at bone defects.

The orthopedic or dental implant of the present invention may be implanted in a patient in a paste or putty form (i.e., as a hydrated precursor). Since the inventive reaction that produces the PCA material can be initiated outside the body, and proceeds slowly at room temperature, the possibility that the material will "set up" prior to application to the surgical site and become unusable is minimized. The reaction accelerates significantly at body temperature and the material hardens in place. This feature is particularly useful in the surgical setting, where custom fitting of the device to the implant location is typically required. Alternatively, the inventive orthopedic or dental implant may be pre-hardened outside the body and implanted at a later time. This approach is useful in those situations where custom shapes are not essential, and where production of large numbers of implants is desired.

Method of application of the implant to bony sites. The implant of the invention may be prepared outside the body in a variety of forms and introduced into the patient at the implant site using methods appropriate to the form of the implant and nature of the malady.

In one embodiment, the implant may be prepared as an injectable paste. A liquid is added to precursor powders to form an injectable hydrated precursor which is capable of in vivo conversion into a bioresorbable PCA calcium phosphate, as described hereinabove. The precise amount of liquid will vary dependent upon the desired consistency of the paste and the nature of the precursor powders used to prepare the PCA material. Typically, about 0.75-1.1 ml liquid per gram powder is used. The paste is desirably injected into the implant site by syringe, preferably using a sixteen or an eighteen gauge syringe. In some embodiments, it may be desirable to prepare the paste ahead of time and to store the paste in the syringe at sub-ambient temperatures until needed. In some embodiments, injection by syringe into a body cavity or intermedullary space may be aided by the use of vacuum to aid in displacing fluids or gases. Most often a vacuum may be applied by insertion of a second needle in the vicinity of the intended injection site. A gentle vacuum may then be applied through the second needle. Application of the implant by injection is particularly desirable for situations in which the material is used as a bone cement to join and hold bone fragments in place or to improve adhesion of, for example, a hip prosthesis. Implantation in a non-open surgical setting is also desirable.

In another embodiment, the implant may be prepared as a formable putty. A liquid is added to precursor powders to form a putty-like hydrated precursor which is capable of in vivo conversion into a bioresorbable PCA calcium phosphate. The precise amount of liquid will vary dependent upon the desired consistency of the putty and the nature of the precursor powders used to prepare the PCA material. Typically, less than about 1.0 ml liquid per gram powder is used. The hydrated precursor putty may be prepared and molded to approximate the implant shape. The putty may then be pressed into place to fill a gap in the bone, tooth socket or other site. Use of a bone putty may be particularly desirable in repair of bone defects in non-union bone or other situations where the gap to be filled is large and requires a degree of mechanical integrity in the implant material to both fill the gap and retain its shape.

In yet another embodiment, dry precursor powders may be applied directly to a bone defect. Hydration and conversion of the precursor into the PCA material occurs at the bone defect site by direct exposure to blood or other physiological fluids. Such application may be particularly desirably where the bone defect is accompanied by excessive bleeding. The hydroscopic nature of the precursor powders serves to absorb body fluids, provide a physical barrier to protect the wound site and to provide a bone substitute material which promotes bone in growth at the defect site.

In still yet another embodiment of the invention, the implant may be prepared from a prehardened PCA calcium phosphate which has been shaped into the desired form. This may be accomplished by preparing a hydrated precursor as a putty or paste as described above, injecting or pressing the hydrated precursor into a mold, and allowing the precursor material to convert and harden into the PCA calcium phosphate. Alternatively, the PCA calcium phosphate may be prepared as a solid block or other such geometry and shaped into the desired object using drills or other such shaping tools known in the art. This method is particularly desirable for production of resorbable objects such as anchors for tooth implants, spacers for cervical fusion, resorbable screws and plates, and slowly resorbable shapes for augmentation.

Orthopedic and dental implants. The implants described hereinabove may be useful in the treatment of a variety of orthopedic and dental disorders. The materials used in the preparation of the implant are desirably sterile and may be sterilized using conventional techniques, including by not limited to gamma irradiation, filtration, and ethylene oxide.

Healing of bone fractures and defects. PCA calcium phosphate may be used to join two or more bone pieces together and/or to improve healing of bone fractures by filling the gap left by the fracture, or space caused by compressive damage as a result of the fracture.

In the situation involving non-union bone fractures, the implant can be used to stabilize the bone defect because the implant hardens in place in vivo. The implant of the invention is especially advantageous in that the bone gap can be filled without open surgery. To this end, the bone defect site may be observed by x-ray to ensure proper positioning of the injection needle. The implant may then be directly injected into the defect site. X-ray or MRI visualization may be used, if desired, to confirm placement. FIG. 19 is a pictorial illustration of application of the implant to a tibial defect in which the implant material is injected into the bone defect. When the gap is particularly large, it may be desirable to first immobilize or "fix" the defect and then fill the gap with implant material. The defect may be fixed using conventional fixation devices, such as titanium screws, pins and plates. In preferred embodiments, the defect is fixed with screws or plates prepared from hardened PCA material and/or composites thereof, which are themselves bioresorbable and hence allow complete bone ingrowth at the defect site and require no post surgical treatment to remove the hardware.

In the situation where the bone has been crushed or fragmented, the bone fragments may be reassembled and the implant material may be used to hold them in place while a bone matrix regrows at the fracture site. FIG. 20 is a pictorial illustration of a fragmented bone which has been reassembled. Hydrated precursor paste is injected around the bone fragments which are held rigid once the paste converts into PCA calcium phosphate. Bone regrowth occurs to regenerate bone tissue and imbed original bone fragments in new bone matrix.

The implant may also be used to heal compression fractions, such as compression of the tibia. The cortical bone surface can be re-aligned and fixed in place using mechanical fixation and the implant can be used to fill the void created by the compressive destruction of the bone.

In yet another embodiment of the invention, the PCA calcium phosphate implant may be used to secure pins, screws and other more complicated prosthesis devices which are used to hold bone in place. By immobilizing the fracture using hardware and embedding the hardware in PCA paste, potential voids are filled, thereby expediting new bone formation around the screw. In addition, the implant acts to distribute the force of the screw across a greater surface area, thereby reducing the likelihood of pull out or early bone resorption. This approach is used most often in repair of broken hip bones, where a hip prosthesis is used to reinforce the weight-bearing femoral neck of the femur.

Where it is desired to minimize surgical intervention, it is preferred to use the PCA material as a paste and to introduce the implant by syringe into the bone defect. Of course, where minimal intervention is not an issue, i.e., during open surgery, the implant may be used as a putty. Indeed, this may be preferred in some circumstances as the added formability of the PCA putty gives the physician increased control over the final shape of the implant device and improves implant conformity with neighboring bone surfaces.

Treatment of osteoporosis. As bones age, they lose mass, thereby becoming more porous and brittle. PCA implant material may be used to promote bone growth and to densify the bone. FIG. 21 includes a pictorial illustration of a normal bone 60 having a regular and dense network of trabecular bone. FIG. 21 also illustrates osteoporotic bone 62 in which significant bone mass has been lost. Osteoporotic bone may be treated with reossifying PCA material of the invention to densify the bone and protect against bone fracture and failure. Bone strength density may be improved by injecting hydrated precursor paste into the bone interior. The precursor serves to improve bone in several ways. Firstly, the hydrated precursor hardens into PCA calcium phosphate which is strong and serves to reinforce the already brittle bone. Secondly, the PCA calcium phosphate is a biocompatible matrix accommodating and stimulating new bone growth, so that as it bioerodes, new bone is formed to replace it. Thirdly, the eroding PCA calcium phosphate is a source of bioavailable calcium for osteoblasts to use in the formation of new bone.

The implant may be particularly effective in preventing the collapse of vertebrae. FIG. 22 is a pictorial illustration of a portion of a spinal column including vertebrae 70, 71, 72 and discs 73, 74. Vertebra 70 is healthy, and exhibits dense trabecular bone matrix. Vertebra 72 is an osteoporotic vertebra which has been crushed due to increased porosity and reduced bone density. Vertebra 71 is an osteoporotic vertebra undergoing implantation of PCA calcium phosphate to strengthen bone and regenerate bone mass.

Spinal and cervical fusion. As a general rule, when discs and vertebral bodies are removed for the treatment of degenerative disease, trauma or tumor, they need to be replaced with a structural graft to maintain the patient's cervical alignment. Bone graft is usually placed as a spacer between vertebrae to facilitate fusion of vertebral bodies and to restore height. Conventional spacers, some of which are known as "cages" are made from titanium or autologous or allograft bone. However each of these prior art devices have disadvantages. Autologous bone may not always be available, allograft bone carries the risk of infection and pathogen exposure, and titanium is not resorbed by the body and either remains or must be surgically removed.

To overcome these disadvantages of the prior art implants, PCA calcium phosphate may be used as a spacer in cervical fusion procedures. The PCA calcium phosphate is prepared as a disk or shim. The PCA calcium phosphate disk may be used as a hardened, slowly resorbing spacer for the fusion of adjacent vertebrae. In preferred embodiments, the spacer is in the form of a hollow ring. The center of the ring may be filled with a PCA calcium phosphate formulated for rapid bioresorbability and bone ingrowth.

Spinal fusion is also done across lateral processes. See, for example, Sandhu et al, Spine, vol 20: 2669-2682, 1995.

Prostheses. Prostheses for joint replacement, particularly hip replacement are widely used and can substantially improve the quality of life for the patients receiving them. However, current cementing techniques are unable to prevent all "micromotion" and gaps between the prosthesis and the natural bone receiving the implant, resulting in increased incidence of loosening and failure of the joint replacement over time with concomitant pain or discomfort to the patient.

FIG. 23 is a pictorial illustration of a hip prosthesis being secured firmly into natural bone using PCA calcium phosphate as a bone cement. Thus, the hip ball and socket may be positioned in the natural bone in spaces prepared to received them. Once positioned, the hydrated precursor paste may be injected around the prosthesis to fill gaps between the bone wall and the prosthesis and to firmly cement the prosthesis to the patient's own bone. Alternatively, the bone surface may be coated with the hydrated precursor and the prosthesis may be inserted into position in the PCA material-coated bone. The hydrated precursor hardens and sets to thereby firmly anchor the prosthesis into place. In both scenarios, the PCA material slowly bioresorbs and is replaced by natural bone; thus, gaps and micromotion associated with the prosthetic device are minimalized.

In another embodiment, the prosthesis may be coated with the PCA material. Thus, a hydrated precursor may be applied to the surface of the prosthesis outside the body and is allowed to harden and convert to PCA calcium phosphate. The coating facilitates acceptance by the host of the prosthesis and promotes bone growth on the prosthesis surface.

The present implant material may also be used as an in vivo treatment of previously implanted prosthesis devices which have formed cysts at the prosthesis-bone interface. The cyst may be removed by conventional techniques, but this procedure often leaves large gaps adjacent to the prosthesis. These gaps may be filled by injection of the implant material of the invention into the gap.

Replacement material for autologous bone implants. For various reasons, the PCA material may not be preferred for use as an implant and the patient's own bone is preferred (e.g. autologous bone harvested from the patients own iliac crest). This is often the case in the treatment of bone cancer. However, the PCA material may be used at the bone removal site to rapidly promote bone regrowth at the bone harvesting site to prevent cosmetic deficiencies or create new bone for future use.

Reconstructive plastic surgery. Prehardened PCA calcium phosphate in the desired shape may be attached using hydrated precursor paste. Alternatively, a hydrated precursor paste may be formed and shaped in vivo and secured in place using hydrated precursor paste. Where synthetic bone graft is medically inappropriate, the patient's own bone may be harvested and secured at the implant site using a hydrated precursor paste or putty. As described previously, the precursor is converted into PCA calcium phosphate which is gradually resorbed and which promotes new bone growth within the implant site in preferred embodiment existing periosteum is drawn over the implant surface prior to closure.

Periodontal defects. PCA calcium phosphate may be used as an implant in teeth sockets to avoid the problems associated with teeth extraction, such as dry socket, infection and fibrous growth. FIG. 24 is a pictorial illustration of a tooth socket receiving an implant by injection. The implant converts into PCA calcium phosphate and is replaced with new bone within six months, preferably within six weeks and ideally as fast as three weeks. The new bone provides an enhanced surface in which to implant dental prosthesis (replacement teeth).

Alveolar ridge defects. When, through trauma, congenital abnormalities or illness, bone loss occurs to the jaw section containing the teeth sockets (alveolar ridge), rebuilding of the ridge may be needed before dental prosthetic implantation can occur. The management of alveolar ridge deficiencies poses a challenge because the magnitude of the osseous defects are often greater than that resulting from tooth extraction and may require replacement (or regrowth) of a significant amount of bony tissue.

The conventional procedure may call for nasal floor elevation, bone grafting and bone regeneration. Bone generation prior to dental prosthesis implantation has the advantage of providing a greater bone mass for implantation and hence improved implant alignment and strength. However, the process typically occurs in staged intervals because of the length of time conventional bone regeneration requires in order to develop bone of sufficient strength to handle the bone implant. Thus, a two step technique has the disadvantage of long healing time before implant placement (ca. nine months) and poor bone quality of the regenerated tissue. See, C. M. Misch and C. E. Misch Implant Dentistry 4(4): 261 (1995).

The implant of the present invention allows the build-up of the alveolar ridge and dental implantation to occur over a much shorter time and often in a single step. The implant is introduced as a paste or putty to the ridge site, where it sets up and hardens in situ. Within hours or even minutes, the implant is sufficiently hard to accept the dental implant. Within six months, the implant will gradually regenerate natural bone, thereby bonding the dental implant into a hard bony site. Thus, ridge augmentation and dental prosthesis implantation may occur at the same time or within days of one another. The dental implant may also be introduced into the hydrated precursor prior to hardening. The inventive PCA calcium phosphate may also be injected into the implant in conjunction with traditional methods to increase bone ingrowth to and around the dental implant.

Likewise, the implant of the present invention may be used to augment the ridge alongside the nasal cavity, where natural bone may be too thin to accept the prosthesis. Thus, as pictorially illustrated in FIG. 25, a hole may be drilled into the alveolar ridge adjacent to the sinus cavity, the sinus sac may be raised and implant material is introduced into the site by injection and allowed to harden. With hardening and ossification, the ridge is ready for dental prosthetic implantation.

Use as a hemostatic agent. The dry precursor powder may also be applied dry as a hemostatic or absorptive agent. Once in contact with body fluids the material hydrates and then hardens in place similarly to the hydrated precursors prepared ex vivo. This property is particularly useful to control bleeding in both hard and soft tissues alike. In one application the material is applied to the opening following spinal taps or spinal surgery to form a patch and prevent CSF leaks.

Cranial repair. Cranial repair has presented a particular problem due to the slow healing of bone involved in reconstructive surgery of the cranium. The inventive PCA material may be used both to repair and to stimulate the growth of cranial bone. Additionally, growth factors or osteogenic cells may be included in the implant to further stimulate healing.

Cartilage growth. PCA calcium phosphate implants may be used to promote new cartilage growth. Cartilage forming cells (e.g. primary chondrocytes, or chondrocytic cell lines) may be used with the implant. The implant provides a matrix for cell growth and proliferation as well as a connecting means to other tissue surfaces (e.g. bone or cartilage).

To this end, the cartilage sac is ruptured and PCA calcium phosphate is injected into the cartilage site. The PCA material desirably contains chondrocytes which promote cartilage growth. For further information of cell seeding of tissue matrices, see "Cell Seeding of Ceramic Compositions", U.S. Ser. No. 08/729,354, which is hereby incorporated by reference.

Bone distraction. The PCA calcium phosphate of the invention is useful in a procedure known as bone distraction. Bone distraction is an orthopedic operation that ultimately lengthens the bone. In this procedure, the bone is cut and gradually parted using orthopedic clamps. PCA material may be transdermally injected or surgically implanted to fill the bone gap produced by expansion of the bone. When used in this application, PCA calcium phosphate promotes bone growth and repair. Because the PCA material serves as both scaffold and calcium source, the distraction rate may be significantly accelerated over previous methods. The maximum distraction rate using current practices is limited to approximately 1 mm/day. Using PCA calcium phosphate, the distraction rate can be increased to greater than that, in some instances up to 2-5 mm/day.

Temporary bony structures. The implant of the invention may be used in other than bony sites in the body. For example, an implant may be prepared from PCA calcium phosphate to be used as a protective structure for various organs of the body. According to the invention, the PCA material may be used to support, shield or frame sensitive organs. By way of example, the PCA material could be prepared outside the body as a hardened vascular stent in the treatment of heart disease or as a gastrointestinal stent in the treatment of Krone's disease. Alternatively, the implant can be used to provide temporary support for sutured or stapled repairs, bypasses, or organ or tissue transplants and implants. The hydrated precursor may be placed around the structure in need of support, where it hardens in place providing support or mechanical protection until resorption occurs.

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.

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 PCA calcium phosphate 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.

In another embodiment of the invention, it is contemplated to incorporate antibiotics or other 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.

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 cry stallization piomoting conditions such as elevated temperatures will tend to drive the conversion to a more crystalline product.

Solid PCA Calcium Phosphate Devices.

In another application of the invention, solid PCA calcium phosphate compositions are prepared, either in vivo or ex vivo. The first method of making a solid is to compress the unreacted precursors of the PCA material. ACP converts to PCA calcium phosphate once the pellet has been exposed to an aqueous environment (e.g. in vivo implantation). The second method of production involves compressing already converted PCA granules into a desired shape. The material can also be formed by any other pellet fabrication method known in the pharmaceutical industry. Once the shape has been fabricated, it can be modified in the following ways: A coating can be added to the shaped material. Therapeutic substances are absorbed to the solid material. There can also be further modification of the shape and texture of the pellet. Sterile pellets may be prepared through the use of presterile components or by terminally sterilizing the pellet. All variations to the solid PCA calcium phosphate are considered within the scope of the present invention.

Methods of Pellet Production. In one embodiment, compressing the unreacted precursors of the PCA material produces the prehardened pellet. The first component is an amorphous calcium phosphate. The second component is the promoter. The preferred promoter is dicalcium phosphate dihydrate (DCPD). In other cases, the promoter may be other calcium phosphates such as crystalline HA. The two components are compressed and molded into the desired shape by any suitable method. Preferred embodiments of compression and molding include hand-held presses and hydraulic presses as described in examples 32 and 33. The pressure of the compression is dependent on what characteristics are desirable for the pellet. For instance, lower pressures are favorable for a pellet that is quickly resorbable. Other methods of pellet fabrication known in the pharmaceutical industry are also acceptable. The compressed object of desired shape most preferably reacts endothermically at 37° C. in vivo to form PCA calcium phosphate. A conversion of ACP in the presence of a promoter occurs under these conditions during this reaction to form PCA calcium phosphate.

In another embodiment, PCA calcium phosphate is formed in vitro. An amorphous calcium phosphate in the presence of a promoter and a limited volume of aqueous medium is converted to poorly crystalline apatitic calcium phosphate. In the most preferred embodiment, the PCA material is hardened at 37° C. Once the PCA material is a solid, it is lyopholized. The dry material is then ground for a specified amount of time in a grinding chamber. Other methods of grinding, such as a mortar and pestle, are also acceptable. The powder is then formed into a pellet or other desired shape by the methods described above.

PCA material may also be prepared by combining an amorphous calcium phosphate with a promoter and a biologically suitable aqueous medium. At this time, the PCA material, as a paste or putty consistency, is molded by any suitable method into the desired form. Once the material is molded, it is then hardened most preferably at 37° C. A range of temperatures below and above 37° C. is also acceptable. Once the molded object is a solid, it is then lyopholized. The object is lyopholized because the presence of water in the pellet may cause the material to be more unstable and have a tendency to become more crystalline.

Once the PCA material is produced, it is formed and hardened, and then lyopholized as described previously. In some instances, it may be unstable and tend to become more crystalline and eventually converts to hydroxyapatite. The prepared solid PCA calcium phosphate can then be stored either wet or dry. Stability issues surrounding the storage of PCA material include temperature, lyopholization, the use of inhibitors, and whether the material is wet or dry. Lyophilization improves the stability of the PCA material because the presence of water is cause for the conversion reaction. Lower temperatures will enable the PCA material to be more stable when compared to the stability at room temperature or in vivo. Ideal conditions include dry storage of pellets at room temperature with no exposure to moisture. The PCA material can also be stored in an aqueous medium for up to 30 days, at room temperature, and pH=7. FTIR and XRD analysis may be conducted on the PCA material to monitor the stability of the PCA material during the storage period. The presence of peaks at 563 cm-1, 1034 cm-1, 1638 cm-1, and 3432 cm-1 (FTIR) should remain unchanged.

Medical Uses of Pellets. The solid PCA calcium phosphate material can be used in many different applications, depending on the details of the situation. The first application applies to orthopedic implants. Pellets, plates, screws, granules, bone void fillers and other forms are appropriate for orthopedic applications. The pellets, plates, and screws can be of various shapes and sizes.

Bone void fillers are gently packed into voids in the bone, which are—surgically created defects or defects created from traumatic injury, tumors or other diseases. Sand grain granules (1-2 mm) of PCA calcium phosphate can also be used in additional hard tissue sites. The granules are particularly useful in alveolar ridge repair and hairline fractures. However, other applications include, but are not limited to tibial fractures, maxillo and cranial indications, extraction socket voids, and later spinal fusion. A significant advantage to granules is that they can be arranged to fit into small areas where bone regeneration is needed. Also, the sand size granules are used to anchor prosthetics since they can shift and settle into the areas where implanted and serve to hold the various medical devices in their proper locations. In addition, the pellets can be mixed with PCA paste for implantation purposes. The use of these solid resorbable implants also eliminates the need for metal implants in the body.

A second application for solid PCA calcium phosphate is to provide support matrices for living tissues. These matrices can be used to promote cell growth, cell transplant and cell therapy. By supplying the appropriate cells onto the support matrix of prehardened PCA material, the cells are effectively delivered to the desired implait site. Cells may be seeded into the PCA in vitro or in vivo depending on what is appropriate for the given indication. The use of living cells in the body promotes self-healing through tissue regeneration.

The porosity of the solid PCA material implants is an important aspect for the ingrowth of cells to regenerate bony tissues. Since the support matrices are comprised of PCA material they are also fully resorbable in the body, therefore the implanted matrix initiates cell growth while it is being resorbed.

Yet, another, third application for the solid PCA material is as a delivery vehicle. Solid PCA calcium phosphate can be used in association with antibiotics, vaccines, bone morphogenetic proteins or other medicinally useful substances. Each biologically active agent can be added in the precursor stage of fabrication, or after the conversion reaction has taken place. The pellet may also be dipped or otherwise coated with the factor to be delivered.

Variations of Use. Prehardened PCA calcium phosphate can be altered to accommodate other variations for a bone substitute material. The first alternative is to use PCA material in a composite. Substances such as binders, polymers, fillers, and coatings as well as others are added to the PCA material to change the physical and/or mechanical properties of the material. Binders and polymers added to PCA to alter its mechanical and resorptive characteristics. Fillers will allow the PCA calcium phosphate to be shaped into pellet form with lower compressive forces. Binders and fillers also add strength, bulk, and adhesion to the PCA material. After the addition of a filler or binder, it may not be necessary to compress the material into a pellet form since the filler or binder may provide enough scaffolds to form a solid matrix. Coatings on the PCA material provide a buffer to the material and protect the inner surface from moisture exposure, which would eventually cause a conversion from ACP to PCA.

In the preferred embodiment, the PCA materials are replaced by bone following implantation. Replacement of the solid PCA by tissues other than bone may be induced through the seeding of the PCA with stem cells or committed stem cells or precursors to other tissues such as cartilage. In addition, characteristics of the implant site will also dictate the replacement tissue (e.g., reduced oxygen leads to chondrogenesis).

Additionally, a third alternative variation of the production method of PCA calcium phosphate is to vary the promoter. The promoter takes on different roles depending on the desired result

Claim 1 of 14 Claims

1. A self-hardening calcium phosphate composite, comprising:

an amorphous calcium phosphate;

a second calcium phosphate having a calcium to phosphorous atomic ratio (Ca:P) of less than or equal to 1.67, wherein the amorphous calcium phosphate and the second calcium phosphate in combination have a calcium to phosphorous atomic ratio in the range of 1.1 to 1.9;

a supplemental material, said supplemental material comprising demineralized bone matrix; and

a carrier fluid in an amount sufficient to form a paste or putty.

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