CA1120961A - Whitlockite ceramic - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A novel, polycrystalline whitlockite ceramic in either pore-free or porous form, in particular, a poly-crystalline ceramic which is translucent, isotropic and sub-stantially pore-free, said ceramic comprising whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 per cent by weight sulfite ion and having an average crystallite size in the approximate range 0.3 to 3 microns and a density greater than about 99 per cent of the theore-tical density of .beta.-whitlockite, said ceramics being useful as biological implant materials. The preparation of said ceramics as well as related non-ceramic whitlockite are dis-closed.

Description

6~ .

This invention deals with ceramics having paxticular utility in the area of orthopedics.
Much current research in th~ field o~ biomaterials is focused on the preparation o~ biocompatible ceramics which can be used as a ~ubstitute for bone.
The calcium phosphates, e.g. whitlockite ~trical-cium phosphate Ca3(PO4)2], which closely resemble biological hard tissue in chemical composition are of particular interest and have been the subject of numerous investigations. Previous attempts to produce whitlockite in macroform hav~ generally in~
~olved powder preparation and compaction and sintering under pre~ure. The products produced have usually been porous and lacking the strength charact~riskics required of many orthop~dic implant devic~s.
Belgian Patent 831,944 di~closes a strong, dense, non-porous polycrystalline sintered ceramic comprising a mixture o~ whitlockite and hydroxylapatite useful a~ a dental and surgi-cal implant material and which is prepared by precipitating th~
calcium phosphat~ from aqueous solution and sint~ring the re-sulting product at 1000C.-1350C.
J.C. Heughebaert et al. I Bull. Soc. Chim. France, 2923-2924 (1970), disclose a proce~s for preparing whitlockite which comprlses rapidly mixing stoichiometric amounts of cal-cium and phosphate ions and separating the re~ulting precipitate 25 as it is formed.

- 2 In accordance with the present invention there is provided a translucent, isotropic, substantially por~-free r polycrystalline ceramic comprising whitlockit2 containing within the crystal lattice thexeof about 0.1 to 2.2. percent by weight sul~ate ion and having an average crystallite size in the approximate rang~ 0.3 to 3 microns and a density greater than about 98 percent of the theoretical density of ~-whitlockite.
This ceramic is use~ul as a strong resorbable dental and surgi-cal prosthetic material.
The invention also deals with a polycrystalline foamed ceramic body comprising whitlockite containing within th~ crystal lattice thereof about 0.1 to 2.2 percent by weight sul~at~ ion and having an average crystallite size in the approximate range 0.3 to 3 microns, a total por~ volume in the approximate range 20 to 80 percent, said pore volum~ comprising a substantially uniorm distribution of open pores having a pore si~e diameter in the approximate range 50 to 300 microns. This ceramic is useful as a resorbabl~ dental and surgical prosthetic material.
The invention also deals with a process ~or producing a translucent, isotropic, substantially pore-free, polycrystal-line ceramic comprising whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulate ion and having an average crystallite siz~ in the ap-proximate range 0.3 to 3 microns and a density greater than about 98 percent o the theoretical density of ~ whitlockite which comprise~ reacting calcium ion with phosphat~ ion in a molar ratio in the approximate range 1 2~1.5 to 1 in aqueous m~dium at a pH of about 10 to 12 to produce a gelatinous precipi tate o~ calcium phosphate having a molar ratio o~ calcium to

3~2~6~l phosphorus in the approximate range 1.50-1.53 to 1, separating the gelatinous precipitate from the solution, washing the precipitate free of soluble salts with wator, homog~neously suspending the washed precipitate in 1 to 3 percent (w/w) aqueous ammonium sulfate in the amount of about 10 to 20 ml.
per gram of theoretically expect~d whitlockite ceramic, sapa-rating the precipitate from the am~onium sulfate solution, drying the precipitate and heating the dried precipitate at a tempera-ture in the approximate range 1000C. to 1350C. :Eor about O.S
to 4 hours.
~ he in~ention also d~als with a proce~ for producing a polycrystalline foamed ceramic body comprising whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulfate lon and having an average crystallite size in the approximate range 0.3 to 3 microns, a total por~
volume in the approximate range 20 to 80 percent, said pore volume comprising a sub~tantially uniform distribution of open pore~ having a pore ~ize dlameter in the approximate range 50 to 300 microns which compri~es reacting calcium ion with phosphate ion in a molar ratio of about 1.2-1.5 to 1 in aqueous medium at a pH of about 10-12 to produc~ a gelatinous precipitate of calcium phosphate having a calcium to phosphorus molar ratio of about 1.50-1.53 to 1, ~eparating the gelatinous precipitate from the solution, wa~hing the precipitate free of soluble salts with water, homogeneously su~pending the wash~d precipitate in 1 to 3 percent (w~w) aqueou~ ammonium sul~ate in the amount of about 10 ko 20 ml. per gram of theoretically oxpacted whitlocki te ceramic, sepa-rating the precipitate from the ammonium sulfate solution, mixing the precipitat~ with about 0.5 to 10 percenk by weight of a blowing agent and 0.5 to 10 percent by weight of a foam stabil-~Z6~9~
izer, heating the resulting mixture at about 70C. to 90C. until de~omposition of the blowing agent and drying o~ the resultant foam are substantially complete, and then heating the dried foam at about 1000C. to 1350~C. until S volatilization o the foam stabilizer and sintering of the resulting product are substantially compl~te.
The invention also relates to a procass for producing non-ceramic crystalline whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulfate ion which comprises reacting calcium ion with phosphate ion in a molar ratio in the approximate range 1.2-1.5 to 1 in aqueous medium at a pH of about 10 to 12 to produce a gelatinous precipi-tate of calcium phosphate having a molar ratio of calcium to phosphorus in the approximate range 1.50-1.53 to 1, separating the gelatinous precipitate from the solution, washing the pre~
cipitate free of soluble salts with water, homogeneously suspen-ding the washed precipitate in 1 to 3 percent (w~w~ aqueous a~nonium sulfate in the amount of about 10 to 20 ml. per gram of theoretically expected whitlockite ceramic, separating the precipitate from th~ ammonium sulfate solution, drying the preclpi-tate and heating the dried precipitate at a temperature in the approximate range 725C. to 900C. for about 0.5 to 4 hours.
For filling a void in a living bone one can fill the void with either th biocompatible, ~ubstantially pore-free, poly-crystalline ceramic or the biocompatible polycrystalline foamedceramic ~ody of the invention. Similarly one can prosthetically reconstruct a defective living bone by implanting an artificial bone prosthesi~ in the form of either the biocompatibl~ substan-t~ally pore-free, polycrystalline ceramic or the biocompatible 0 polycrystalline foam~d ceramic body of the invention.

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Whitlockite exl ts in two crystalline modiflcatlons,the alpha rorm, a metastable state occurrlng at hl~h tempera-tures, and the beta form, the thermodynamically stable s~ate.
Unless otherwise indlcated the term "whltlockite" as u~ed hereln ls lntended to comprehend either the alpha or the beta ~orm or any ~ixture of these.
The novel~ translucent/ isotropic~ substantlally pore-~ree~ polycrystalline ceramic o~ this invention co~prises whitlockite containing within the cry~tal lattice thereof about 0.1 to 2.2 percent by weight ~ulfate ion and characterlzed by an average cry~tallite slze ~n the approximate range 0.3 to ~ microns and a density greater than about 98 percent of the theoretical density of ~-whitlockite. Thi~ whltlockite cera-mlc is further characterlzed by a compres~lon strength in the approxi~.ate range 90,000 to 1~0,000 psi and a tens~le strength ir. the appro~imate ran~e 10,000 to ~0,0~0 p9i.
In ~iew of the known dependence of com?ression and tensile strength~ on the shape, dimensions and surface ch~rac-teristics ol the materi~l tested it will be appreciated that the sub~tantially fully dense ceramic whitlockite pro~ided by ~his lnvention, when suitably labricated, has compression and tensile strengths substantially greater than 1~,000 and 3' psi respectively.
The ceramic whitlocklte Or the present ln~ention can be fabricated ln any desired form or shape employi~g conven~
tlonal mode~ o~ Labrlcation such as moldlng, castlng, ~ach1ning, milllng and the llke. In such ~nner the ceramic can be pro-~ Z~6~
duced as, ~or example, a flat sheet or any desired thickness, a cyllnder, a cone, a sphere, granules, powder, etc.
I~ addition to havin~ the above-de~cribed propertie~, the whitlockite ceramic of thi~ inventlon i~ al~o completely ~iocompatlble and resorbable, and thererore eminently sultable as a sur~ical implant material, especially for bone reconstruc-tion a~d repair. Thus a defect or ~o.Ld in a bone is rilled with the ceramic either a~ a shaped body or ln particulate fo~m. As the ceramlc ls ~lowly reso~bed, it i replaced by new biological hard tissue.
The hlg~ strength characteristics o~ the above-de~cribed ceramic, which are due to its unlque microstructur~, l.e. to i~ small crystalllte size and substantially complete non-porosity, are of prime importance in prosthetic materlalY
15 which are implanted at sites sub~ect to stress such as load-; bearing bones. However, in certain applications, circulation of body fluids, tissue ingrowth and stlmulation o~ new bone rormation, such as would be optimally promoted by a porous i~p~.ar,t ~aterial, are paramount. ~or quch use, as described in detail herelnbelow, a ceram~c whitlockite can b2 afforded ~y this inventlon essentlall~ 2S a ceramlc foam having a total pore volume of about 2Q to 80 percen~ said pore volume com-prising a sub tantially unlform distribution of open pores (pores connected to an exterior surface) with an a~erage po~e ~ize dlameter of about 50 to 300 microns. This material is of particular utllity in the repair of periodontal leslons or re-construction of faclal bone and constitutes a further composl-tlon aspect o~ the present inventlon. It will, of course, be appreciated that the lntroduction or pores lnto the ceramic 7 _ ~6~
whitloc~lte wlll e~ect a reductlon ln compres~lon and ten~lle ~tren~th~ Ne~ertheless, due to the 3mall cr~s~allite slze and the absence Or appreciable flne matrix poro~ity~ the porous ceramic retains substantial ~echanical strength.
A~ described in detail herelnbelow, the preparatlon of the novel whitlockite ceramlc of thi~ inventlon requlre~
the inltial prec~pitation of the appropriate calcium phosphate ~rom aqueous ~ediu~. The interaction of calcium ion wlth phosphate lon in aqueou~ medium i~ a complex and incompletely 1~ under~tood proccs~ usually lnvolvlng a number of equllibr,um reactionq proceeding at varying rates and producing different products [E2nes et al~, Na~ure 208, 365 (1965) and Bett et al.
J. Amer. Chem. Soc. ~ ~ 5535 (1967)]. As might be expected, the results o~ such i~teractions are pro~oundly af~ected by stoichiometry, i.e. the molar ratio o~ calcium to phosphoru~
(Ca/P), reaction timç, ~emperature and pH. It is generally believed that calcium and pnosphate ions lnitially combine to form an insoluble calcium-deficient apatite naving a calcium-to-phosphorus ratio of about 1~5, the correct stoichiometry ~or whitlockite. However, the apatite crystal lattice appears to be the most stable configuration in the calcium phosphate sy~tem and, in the presence of sufficient excess calcium lon, the inltial precipltate undergoes slow transfor~ation to hydroxy-lapatite wlth a calc~um-to~phosp~loru~ ratio o~ 1.67 ~Eanes et al., supra~. An interme~iate calclum-to phosphorus ratio~
i.e. between 1.5 and 1.67 a~fords ~ mixture Or whitlockite and hydroxylapatite when the preclpitate i~ heated ~elglan Pakent 831,944~ It thus becomes apparent that in order to ob-taln whitlockite, a calcium-to~phosphoru~ ratlo or 1.~ must be 8~

malntained. The apparently simple expedlent Or reacting cal-clum and phosphate ion ln a molar ratlo o~ 1.5 to 1 was found ine~ective in producing pure whitlocklte and lnstead arforded a mixture of whltlocklte and hydroxylapatite. In fact, eve~
5 reduclng the calc ium ion to phosphate ion ratio to l . 2 to ulti~ately produced a mlxture o~ whitlocklte and hydroxylapa-tite. Reactlng calcium ion wlth phosphate ion in the ~hit~
lockite stolchlometry, l.e. Ca~P = 1.5, and immediately iso-lating the initially formed precipitate ~J~Co ~eughebaert and Go ~ontel~ ~ull. ~oc. ChimO France, 292~-2924 ~1970) ], thereby presumably preventing further equllibration, was partially successful in producing pure whitlockite. However, this pro-cedure was ~ound not readily reproducible and is unsuited to large scale commercial production.
It has now been discovered that the addition o~ a ~m~ll amount or sulfate ion to the calcium phosphate precipi-tate followed by collecting and heating,resultæ in complete conversion of the latter to whitlockite conta~ning no detect-able ~race of hydroxylapatite. Moreover, the sintering of the whitlocklte so-produced affords a high-quality ceramic having superior physical and mechanical properties and which i~ eminently suitable as a biological implant material.
Thus, the process o~ this invention for preparlng ~ubstantially pure whitlockite (containing within its crystal lattlce about 0.1 to 2.2 percent by weight sulfate ion) as a ~trong, tran~lucent, isotroplc, sub~tantially pore-rree, poly-crystalline ceramic comprises reactlng calcium ion ~ith pho~phate ion in a molar ratlo o~ about 1.2-1.5 to 1 ln aque-ou~ medlum at a pH Or about 10-12 to produce a gelatlnous pre-3 ~LZ~36~Lclpitate o~ calclum phosphate having a molar rat-lo Or calc~um to phosphorus in the approxi~ate range 1.5~ 53 to 1, 3epara-ting sald gelatlnous preclpltate ~rom the solutlon, washlng sald preclpitate ~ree Or soluble salts with water, homogene~
ou~ly suspend~ng the ~ashed preclpitzte ln approxir~tely 1 to 3 percent aqueous ammonium sul~ate in the amount Or about 10 to 2~ ml. per gram of expected whitlocklte, separatlng the precipitate ~rom khe ammonium ~ulfate solutlon, drying sald precipitate and heating the dried precipitate in the approxl-~te range 1000C. to 1350C. ror about 0.5 to 4 hours~
Thu8, whltloc~ite ls precipitated from aaueous me-dlum by reacting calclum ion with phosphate iOn at a pH of about 10~-12. Any calcium - or pho~phate - containing compou~d~
which provide calcium and phosphate ions in aqueous medium are 3uikable provided that the respective counter ions o~ said compounds are ea~lly separated from the whitlockite product, are not themselves lncorporated in the whitlockite lattice, or do not otherwise inter~ere with the precipitation or ~so~
lakion of the whitlockite. Compounds which pro~ide calc um ion are, ror example, calcium nitrate9 calcium hydroxide, cal-cium acetate and the l-lke. Phosphate lon can be proYided by dlammonlum hydrogen phosphate, am~onlum phosphate, pnosphoric acid and the like. In the present method, calcium n~trate and diammonium hydrogen phosphate are the pre~erred sources or calcium and phosphate ions respectlvely.
Firsk, calcium nitrate and dlam~onium hydrogen phos-phate in a molar ratio of about 1.2-1.5 to 1 are interacted in a~ueous solutlon ak a pH of ~bout 10-12 to produce a gela-tlnous preclpitate of calclum phosphate. Temperature ls not 6~
crltical and the preci~ltatlon can be carrled out from a`vout O~C. to 100C., but iq prererably carrled out at about room temperature. The gelatinou~ preclpltate thus obtalned i~
separated rrom the solutlon by suitable mean~, ~or example by centri~ugation and decantatlon of the supernatant. The re~idual mineral sludge can be washed ~ree of any remalning soluble salts by suspending ln distilled water, centrifuging and decanting the supernatant. Although not e~ential to the pro-ce~s, the latter step appears to minimize cracking during sub-sequent sintering. The residual product can then be suspendedin 2 mlnlmum amount o~ distilled water and stored for future use. Conversion to crystalline whitlockite is ef~ected by ~irst homogeneously suspending the mineral sludge in 1 to 3 percent (w/w) aaueous ammonium sulfate. Ordinarily, 10 to 20 ml. o~ 1 to 3 percent (w/w) aqueous a~onium sulrate per gram Or theoretically expected whitlockite ceramic is employed. The solid is then ~eparated from the solution by centrl~ugation and ~acuum filtration. The gelatinous product thus collected contains a large amount o~ occluded water, much of which can be removed by pres~ing. The result~ng wet clay~ e material i~ cut or shaped into a convenient form or, alternativeiy cast in a suitable mold. A shrinkage of approximately 25 percent occurs when the wet product ls dried and a further shrin~age o~ about 25 percent takes place during the sintering herein-arter described, and this should of course, be taken into ac-~ount when ~haping or molding the material. ~le wet product i~ slowly heated up to the sinterlng temper~ture of 1000C.
to 1~50C. at which polnt all re~ining water will have been driven of~. Maintaining the temperature at 1000C. to 1350C.

~11--for approxll~tely 0.5 to 4 hour~ ~111 then erfect the sinteri.
and substantially m~xlmum den~l~icatlon o~ the product. Or-dlnarlly, it 1~ convenient to i301ate the drled product prlor to sintering. Thus, the wet product ls dried at about 90C.
to 900C. ~or approximately ~ to 24 hours or unti} the water content thereo~ has ~een reduced to about 6 percent. It is generally preferred to use drying conditions of approximately 9~C. to 95C. ~or about 15 hours or until the water conte~t has been reduced to about 6 percent. The whitlocklte obt2ined ln this ~nner ls brittle and porous, but nas considerable mechan~cal strength. Some separation or cracking of the clay-like materlal may occur on drylng especially if a thic~ fil-ter cake is used. Separation or cracklng during drying can be minimized or prevented by adding to the suspension o~ freshly precipitated calcium phosphate ~mall amounts of conventional binders known in the ceramics art such as polyethylene glycol or polyvinyl alcohol.
It is usually convenient at this stage to ~urther cut or shape the dried ~h~tlockite into rou~hly the form de~
sired as the end product, taking lnto accoun~ the shrln'~age mentioned above which occurs on sintering~
The bodies of whitlocklte prior to sintering should be uniform and free o~ defects. The pre~ence of crac~ or fissures can cause the pieces to fracture durln~ the sintering proces~. The products are then slntered at about 1000C. to 1~50C. for approximately 0~5 to 4 hours~ Sintering is pre-rerably e~fected at 1150C. to 1~0C. for approxlmately 1 hour. There are thus produced articles Or the hard, dense ceramic as described herelnabove. These articles can then be :l~LZ~1~6~
poll3hed or Ir~chlned uslng conventlonal technlques.
The ceramic initlally produced upon heatlr.g at about lO~O~C. comprlse~ a mlxture Or the metastable ~-whltlockite and the ther~odynamlcally stable ~-whitloc~ite.
I~ de~ired, the ceramic can be equllibrated by heatlng below the sintering temperature~ for example, at about 9~0C. ~or approxlmately 4 hours to afrord sub~tantially pure ~-whitlockite ceramic.
It i~ important ln the chemical process described above that the calcium to phosphorus ratio of the i~ola~ed precipitate correspond as closely as possible to the theore-~lcal value for whitloc~ite, i.e. Ca ~ - 1.50, in order to minimlze the hydroxylapaklte content of said precipitate an~
thereby minlmize the amount o~ ammonium ~ulfate required to produce the substantially pure whitloc~ite Or this invention Thu~ the calcium to phosphorus ratio of the precipitate ls ~ubstantlally ~reater than about 1.5~, exposure to 1 to , percent aqueous ammonium sulfate is lnadequate to produce pure whitlocklte and affords lnstead a mix~ure of whitlocklte 20 and hydroxylapatike. A calcium phosphate preclpitate having a calclum to phosphorus ratio greater than 1.53 can be con-verted completely to whitlockite by employing a larger ar.ount of an~onlum sulfate. In ~act~ a preclpitate Or pure h~droxy-lapatite (C2/P = 1 67) can be converted to whitlockite by using sufficiently large quantities of ammonlum ~ul~ate. Hc-~-ever~ the whltlockite produced therebv is contaminated wlth ~lgnl~lcant amounts o~ calcium sulfate and lacks the superlor physical and mechanical properties OI' the whitlocklte ceramic a~rorded by the present invention. Accordlngly, in order to :~Z~9~il en~ure that the calcium to pho~phoru~ ratio does not exceed about 1.53) the calclum and pho~phate salts are mlxed 1n a molar ratio Or 1.5 to l or less, preferably l.2-l~4 to l.
The calcium phosphate precipltate so-produced has a calclum to phosphorus ratio ~f about l.50 to l.53~ and following treatment with 1 to ~ percent aqueous ar~onium Yulrate ~about lO-~0 ml. per gram of theoretically expected whitlocklte) ultlmately affords the substantially pure whitlockite o~ thls ~nventionO
It i~ crltical, in the c~.emical process descrlbed a~ove to prepare the whitloc~ite as a gelatinous precipitate from aqueous solution for it i~ only in thls cohesive gela-tinous stake that whitlocklte can be shaped or molded and then dried and sintered to produce a ceramic body. Dry, particu-l~ late o~ granular whitlockite cannot be reconstltuted ~nto this cohesive ~elatinou~ state~ If, ~or example) powdered whitlockite is suspended in water and ~iltered there is o~-talned a non-coheslve, particulate ~i,lter cake which simply dries and crumbles and cannot be shaped, molded or converted lnto a ceramic body. Moreover, although powdered whitlocklte ~an be mechanically compressed into a shaped body~ such as a tablet, when sintered, the product obtained is highly ~orous, opaque and does not possess the high-strength character1stics Or the lnstantly clalmed ceramic.
The process of this lnvention ror preparlng the roamed ceramic body described hereinabove comprises reacting çal~iu.~ ion wlth pho~phate ion in a molar rat10 Or about 1.2-l~5 to l in aqueous medium at a pH Or about lO-12 to produce a gelatinous preclpitate o~ calcium phosphate having a calclum , -14-to pho~phorus molar ratio of about 1.50~1.53 to 1, ~eparating the ~elatinous precipitate rrom the solutlon, washlng the pre-cipitate ~ree o~ soluble ~alt3 wlth water~ homogeneously ~us-pendlng the wa~hed precipltate in approxi~ately 1 to ~ percent (w/w) aqueous ammonlum sulfate in the amount of about 10 to 20 ml. per gram of theoretically expected whitlockite ceram~c, separating the precipitate from the ammonium sulfate solutlon, mlxing the precipitate with about 0.5 to 1~ percent by weight of a blowing agent and about 0.5 to 10 percent by weight o~ a ~oam stabilizer, heating the resultlng mixture at about 70C.
to 90C. until decomposition of the blowing agent and drying o~ the resultant foam are substantially complete, znd then heating the dried foam at about 10~C. to 1~50C. until vola-tilization or the ~oam stabilizer and sintering of the resul-ting product are sub~tantially complete.
The calcium phosphate p.ecipitate can be mixed with any conventional and readily available foam stabllizer, e.g.
albumen, polyvinyl alcohol or polyethylene glycol, and any conventional blo~ing a&ents such as azodicarbonamide, hydrogen 2~ peroxide o.r ammonlum carbonate. Upon heating the mixture, the blow~ing agent releases g2seous decomposition products which are tr2pped by the ~oam s~abilizer thereby creating a ~table ~oam. The latter is dried and ultimately sintered to produce a porous ceramic.
A-ternatively, the blowing agent can be omitted and the foam created mec-nanically by whlpping air into the mixture.
It i~ also possible to o~lt the ~oam stabilizer and to employ as pore ~orming agents flbrous organlc materials such as tarch, collagen and cellulose or volatile organic compounds ~-~z~
such as naphthalene.
Thu~, the cer~mlc Or this lnventlon can be convenl-ently obtained in a porou~ ~orm a~ f ol . ows:
Calcium phosphate i~ precipltated, wa~hed rree ~r soluble salts and washed with aqueous a~unonium sulrate as de~cribed above. The resulting mineral sludge is mixed with about 10 to 100 m~.~ preferably about 15 to 20 mg., of spray-dried egg white per gram of theoretically expected whitlockite ceramlc and at least an equal amount~ i.e. about 10 to 2~0 rr~ ., pre~erably 15 to 30 ~ of azodicarbonamide. Addltional water can be added lf desired to ad~ust the consistency of the mix-ture to permit e~ficient ~tirring and transfer of 'he latter without introduction o~ large air bubbles therein. The spray-dried egg white is difficult to wet and there~ore somewhat difficult to homogeneously mix with the whitlockite sludge.
Accordingl~, it i9 advanta~eous, though not necessary, to re-constitute the dried egg white prior to lts additlon to the whitlockite sludge. This ls conveniently achieved by thorou~hly mixing with about 10 times its ~eight of water. The resultin~
reconstituted egg white can then be easlly and homogeneously mlxed with the whitloc~ite slud~e. The resul~ing mixture is then dried by heating at about 70~C~ to 90C. for approximately 8 to 20 hours. If desired~ the mixture can be poured into a ~uitable mold prior to drying. Ordinarily, the mixture is covered loosely ln order to prevent drying out before deco~po-sitlon o~ the azodicarbonam~de ls completeO Alternatlvely, drylng can be carrled out ln a hlgh-hurnidity chamber. The dried product is ~lnally sintered by heatlng at a temperature in the approxlmate range 1000C. to 1350C. ror about 0.5 to 2 hour~, preferably at 1050C. to 1150C. ror 1 hou~ At this point any residual foamlng agent or fo2m ~tabllizer will have been volatillzed and the whitlocklte will have under~one substantially co~,plete sinterlng. The re~ulting porou~ cera-mic body can be ~urther cut or machined lnto a~y desired shape.
As noted above for the rully den~e materialJ theporous material can be produced either as pure ~-whitlocklte or as a mixture of the latter and a-whltlockite.
A~ noted hereinabove, the ceramic a~orded by the present invention, ln either the pore-free or porous form, is useful as a resorbable biological implant material. The rate of resorption is partially dependent on the crystalline phase o~ the whitlocklte ceramlc, the a form re~orbing rnore rapidly than the ~ form. Accordingly, it is possible to control the rate Or resorption o~ a whitlockite ceramic implant device by varying the ratio o~ ~-to-~-whitlockite therein.
The process Or ~his invention ~or preparing non-ceramic crystalllne whitlockite containing within the crystal lattlce thereor about 0.1 to 2.2 percent by weight sul~ate ion comprises the above-described steps for producing the dried calcium phosphate precipita~e and then heating the latter to at least about 725C., the temperature at which the inif i-ally obta ned preclpitate under~oes a phase transformat1on to crystalllne whltlockite as ind~cated by dirferential thermal analysis and X-ray dif~raction~ but below about lOOGC., the temperature at which whitlockite begins to sinter. This pro-ces~ a~rords a simpliried, reliable and economical method of producing substanti211y pure (i.e. 97.2-99.9 percent pure) whltlockiteO -17-The lnvention ls further lllustrated by ~he follo~lr~
examples wlthout, however, being limlted thereto.

Example 1 An aqueous solution containlng 0.24 mole of diam-monium hydrogen phosphate (2~5 ml. of a 1.02M ~olution) was brought to pH 11 with 150 ml~ of concentrated aqueous ammonia.
An additlonal 600 ml, o~ water was added to dissolve precipl-tated a~monlum phosphate. The resulting solution was added dropwise over 0.~ hour to a stirred solutlon containin~ 356 ml.
o~ l.OlM aqueous calclum nitra~e (0.36 mole)(Ca/P - 1.5) di~
luted with 350 ml. o~ water and previously ad~usted to pH 11 with 15 ml. of concentrated aqueous ammonia. Stlrring was contlnued for a ~hort time after the addition was complete ar.d then the resulting suspension was allowed to stand overnight at room temperature. The mixture was rapidly stirred for abou~ 20 min. to regenerate a homogeneou~ susFension. Or.e quarter Or the suspension ~as removed, centrifuged and t~e supernatant decanted. The residue was suspended in 200 ml.
of 5 percent ~w/w) aqueous ammonium sulrate~ The suspen~ion was centrifuged and the supernatant decanted. The residue was ~lltered and dried at room temperature overnignt. The resultirg dried m~terlal was heated 1 hr. at 900Cu to give a product shown by conventlonal X-ray diffractlon analysis to be 10~ per-cent ~-whitlockite (As given here and hereina~ter~ percentage composltlon determlned by X-ray dif~ractlon is, o~ course, wlthin the llmlts Or a~ccuracy Or the di~rractometer, i.e.
2 percent).

~v~
The product had a calclum to phosphorus ratio (Ca/P ) Or 1~53 ~0.03 as ~hown by standard elemental analysis.

Example 2 Four 250-ml. aliquots were withdrawn from a well 5 stlrred, homogeneous suspension prepared by reacting 264.1 g.
(2 moles) of diammonium hydro~en phosphate wlth 3 . 4 1. or o.88M aqueous calcium nitrate (3 moles)(Ca/P _ 1.5) e~sentlally as described 1n Example 1. The fcur allquots were treated respectiYely wlth 100 g. o~ 0.5, 1,2 and 3 percent (w/w) aque ou~ ammonium sulrate~ The allquots were then shake~ to inaure homogeneit~, the ~olids collected by flltratlon, pressed dry under ~acuum and then dried overnight at gooc. A sample of each o~ the solids so-produced was sintered at llOO~C. for one hour. The products resulting from ~he aliquots ~reated with 1, 2 and 3 percent aqueous ammonium sulfate respectively were shown by conventional X-ray di~raction analysis to be 100 percent ~-whitlockite. The m~terial which had been tr~ated with Q.5 percent aqueous ammonium sulfate contained approx~-m~tely 8~ percent ~-whi~lockite and 11 percent hydroxylapatite.
Standard elemental analysis o~ the materlal resultin~ from treatment with 1 percent aqueous ammonium sulfate 1ndicated Ca/P = 1~54 +0.03 and a ~ulfate content of o.67 percent by weight.
A 5-~iter ali~uot o~ the above ~uspension was centri-ruged and the supernatant decanted. The residue was suspended with thorough agitation in 1250 ml. o~ 1 percent (w~w) aqueous ammonium sulrate. The suspension wa~ de~assed by stirrin~
gently under reduced pressure ~or 2 hour~ and the solids were ~2~
collected by ~lltratlon and dried at 8~C. to give 96 g, Or product which was then heated, at 800C~ for 2 hour~ and at 1100C. for 1 hour. Sinterlng was completed by heating 1 hour at 1150C~ followed by equlllbration ak 900C. ror 4 hours.
A sample was fractured and the ~racture surface was thermally etched by heatlng at 1100Co for 1.5 hour~. The etched sample wa~ then mounted wlth silver paste on a scan-ning electron microscope sample holder, coated with a thin layer o~ gold and observed in an AMR 1000 scanning electron microscope. The sample had an a~erage ~rain size of 0.424 micron and contalned no pores. A sample which had been sin-tered at 1200~C. for 1 hour and then equillbrated at 900C.
for 4 hours had an average grain size of o.48~ micron and was al~o pore-~ree.
The densities of the samples ~intered at 1150C.
and 1200C. as determined by the standard liquid displacement method? were ~.04 g./cm~ and 3.o6 g./cm3 respectively.

Example 3 A l-llter allquot of a well-~tirred, homogeneous suspension prepared by reacting 264.1 g. (2 moles) of diam~onlu~
hydrogen phosphate wlth ~4 1. of 0~881~ aqueous calcium nitrate (3 moles)(Ca/P = 1.5) essentially a~ descrlbed in Example 1 was centrifu~ed at 2000 rpm for 10 min. and the supernatant decanted. The residue was treated wlth 250 ml. of 1 percent (w/w) aqueous ammonium sulfate~ shaken thoroughly5 degassed for 1 hr.~ ~iltered and dried overnight at 95C. to give 19.6 g.
Or white solld. Slntering at 1100C. ~or 1 hr> af~orded a ~trong, whlte3 translucent ceramic prod~ct. Standard elemental .

~ IL20~
anal~si~ o~ thP latter indlcated Ca/~ = 1. 55 (+ O. 03 ) and conventlonal X-ray diffraction analysls showed the product tc be 100 percent ~-whltlockite. ~ive polished cylindr~cal plugs

4.6 mm. in diameter and 1.64 mm. in helght were prepared and tested for compre~slon stren~th employing conventional tech-nlque~. The avçra~e compre3sion ~trength wa~ found to be 95,gO0 +4900 psi.
~ nother sample prepared essentially as the above-described W2S shown by X-ray dl~fraction to be 100 percent ~ whitlockite ~nd had Ca/P = 1.52 (tO~03) and an ~Yerage com-pression strength o~ gg,600 +1~,200 psi.

Example 4 An aqueous solution containing 0.5 mole of diammonium hyd~ogen phosphate (174 mlO o~ a 2.8~M solution) was d~luted to 750 ml. with distllled water~ brought to pH 11 with 600 ml.
o~ concentrated aqueous ~mmonia and lurther diluted to 2500 ml.
with wa~er to give a cle2r solution. The latter was adde~ in a ~ine stream over 15 min. to a stirred solution containing 40~ ml. o~ 1.735M aqueous calcium nitrate (0.7 mole)(Ca/P = 1.4) 20 ' diluted to 1250 ml. with distilled water and previously adlust-ed to pH 11 wlth 20 ml. of concentrated aqueous a~onia. The resulting suspension was stirred overnight at room temperature.
A 100 ml~ aliquot of the homogeneous suspension was withdrawn and centri~uged. The supernatant wa~ decanted and the residue was suspended wlth thorough agitatlon in 25 ml. o~ 1 percent ~w/w) aqueou~ a~onium sulfate. The product was col~ected by rlltration and dried overnlght at 6~C. The resulting whlte solid was sintered 1 hr. at 1150C. afrording a ceramlc product ~hown by ccnventlonal X-ray dirrraction a~alysis to be 100 percent whitlocklte, approximately 96 percent o~ ~h1ch ~as in the ~ form and 4 percent in the a form. Standard elemental analysis showed Ca/P = 1.48 (~0.0~).
Another aliquot of the suspen~ion was worked up as above described and the resulting procluct was sintered by heating slowly to 1125C.) maintaining that temperature for 1 hour and then cooling and maintaining the sample at 900C.
rOr 4 hour5. The re~ulting ceramic had Ca/P - lo ~1 and a sul-~ate content of 1.2 percent by weight. X-ray dirfraction indicated the product to be 100 percent ~-whltlockite. Crack-~ree sample~ of this ~terial were pollshed to 600 grit SiC
and sub~ected to the standard 3 point bend test. The samples displayed an average tensile strength of 19, &oo -~s7oo psi .
The density of this m2terial a~ determined by the standard liquid displacement method was ~ound to be 3~050 ~0.002 g/cm3' ExamDle 5 An aqueous solution containing 0.5 mole of diam~o-nium hydrogen phosphate (174 ml. of a 2.88~1 soluticn) was di-luted to 750 ml. with dlstilled water, adJusted to pH 11 with 600 ml~ o~ concentrated ammonia and ~urther diluted to 2~00 ml.
Nith d1~tilled water to glve a clear solution. Thi5 solution was added over 0.5 hrO to a stirred aqueous solution con-tainlng o.6 mole Or calcium nitrate (346 ml. o~ a 1.735M
solution)(Ca/P = 1.2) diluted to 1250 ml. with distilled water and previously adjusted to pH 11 with 20 ml. o~ concen-trated ammonia and the re~ulting mixture stlrred overnl~ht.
A~ter standing several days the mlxture was stlrred 1 hr. to a.. :b 96~L
glve a homc~eneous susper.slon. A 5~0-ml. aliquot was wlt~dr?~n, centrifuged, 'he su~ernatant decanted and the re~idue sus~ended with thorough agltation in 125 ml. of 2 percent (w/w) aqueous ammonium sulfate 'Fhe product was collected by ~lltration, dried at 50C. overnight and then sintered at 1150C. for 1 hr.
followed by equlllbration at goooc. for 4 hours to give 7.2 g.
of wh~te, translucent ceramic consisting of 100 percent Q-whitlockite as shown by X-ray diffraction and having Ca/P =
1.50 and a sulfate content of 0.10 percent by weight as indi-cated by elemental analysis7 Exa~Dle 6 A solution containing 264.1 g. (2 moles) of diam-monium hydrogen phosphate ln sufficient distllled water to ~i~e a total volu~e of 5.4 1. was brought to pH 11 with 3.0 1.
1~ of concentrated aqueous ammonia. The resultlng precipitate was dissolved by diluting with distilled water to a volume of 10~1. The resulting solution was added in a rine stream to a stirred solution containing 1499 ml. o~ 1.7~5M aqueous c21c~ um nitrate diluted to 5.4 1. wlth dlstllled water ancl previously ad~usted to pH 11 with 90 mlO of concentrated amrionia. Af~er the addition was complete, the reaction mlxture wa~ stirred an additional 5 hours and then allowed to stand overnight at room temperature. The supernatant was decanted and the re-malning suspenslon was centrifuged. The supernatant was de-canted and the residual sludge was washed twice by suspendingin distilled water, centrifuging and decanting the ~u~ernatant.
The washed sludge was then suspended ln 1500 ml. of distilled water. The suspenslon was rap1dly stirred to insure homo~eneit;
~23 and a 175 ml. aliquot was drawn or~r The aliquot was centrl-ru&ed, the supernatant dec2nted and the re~idue suspended ~n 100 ml. of 1 percent (w/w~ aqueous an~nonlum sulfate. The re-sulting suspenslon was centri~uged and the superna~ant was decanted. The re~idual sludge was mlxed with 180 mg. of spray-dried egg white(previously reconstituted by thoroughly mixing with 10 ml. of distilled water) rollowed by 180 ~.
of azodicarbonamide. The resultlng mixture was stirred vigor-ousl~ for about 0.25 hr. and then poured into cube-shaped molds~ loosely covered and drled at 8~C. overnight. The re-sulting dried ~oam was sintered at 1050C. for 1 hr. to give a porous whitlockite cera,nic having an average pore size of about 100 microns.

Example 7 Followin~ a procedure similar to that described in Example 6 and sintering samples of the resulting dried whit-lockite foam for 1 hr. at 1050C., 1100C. and 112~C. affor~ed three porous whitlockite ceramic bodies having bulk densities of 1.41~ 1.6 and 1.72 ~/c~ respectively, and apparent poro-sities of 54.7, 52.0 and ~5~1 percent respectively~ as deter-mined by standard ASTM methods.
The biocompatibillty of the whitlockite ceramic a~forded by the present inventlon was confirmed by implantin~
in voids ~n the ~emurs of live dogs, plugs and granules of porous whitlockite ceramic prepared in accordance with the above-described procedures. The irnplant sites were character-ized by norrnal healing and the absence o~ an~ evidence of in-~lar~tion or foreign body response, and resorption of the implant material was nearly complete at two months and re-placement -thereof hy new dense bone was evident. At seven months, remodeling of the new dense bone which filled the spaces formerly occupied by the whitlockite ceramic and of S the hone surrounding the original implant site was more advanced than at two months.
The term "crystalline" as used herein is an adjective use~ to characterize a substance or body having the properties of a crystal, i.e., a regular arrangement of atoms in a space lattice. "Polycrystalline" therefore characterizes a substance or body comprising a plurality of crystals. On the other hand, the term "crystallite"
is a nun which identifies a single entity, namely, a single grain in a polycrystalline body. A crystallite can be further defined as a microscopic, imperfectly formed crystal, or alternatively, a microscopic body formed in the early stages of crystallization.

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A translucent, isotropic, substantially pore-free polycrystalline ceramic comprising whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulfate ion and characterized by an average crystal-lite size in the approximate range 0.3 to 3 microns and a density greater than about 98 percent of the theoretical den-sity of .beta.-whitlockite.

2. A ceramic according to claim 1 wherein the whitlockite is .beta.-whitlockite.

3. A ceramic according to claim 1 shaped or formed substantially into a flat sheet.

4. A ceramic according to claim 1 shaped or formed substantially into a cylindrical rod.

5. A strong, dense, resorbable artificial bone pros-thesis in the form of a polycrystalline ceramic according to claim lo

6. A polycrystalline foamed ceramic body comprising whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulfate ion and charac-terized by an average crystalline size in the approximate range 0.3 to 3 microns, a total pore volume in the approxi-mate range 20 to 80 percent, said pore volume comprising a substantially uniform distribution of open pores having a pore size diameter in the approximate range 50 to 300 microns.

7. A foamed ceramic body according to claim 6 wherein the whitlockite is .beta.-whitlockite.

8. A foamed ceramic body according to claim 6 shaped or formed substarltiallY into a flat sheet.

9. A foamed ceramic body according to claim 6 shaped or formed substantially into a cylindrical rod.

10. A resorbable artificial bone prosthesis in the form of a polycrystalline ceramic according to claim 6.

11. A process for producing a ceramic according to claim 1 which comprises reacting calcium ion with phosphate ion in a molar ratio in the approximate range 1.2-1.5 to 1 in aqueous medium at a pH of about 10 to 12 to produce a gelatinous pre-cipitate of calcium phosphate having a molar ratio of calcium to phosphorus in the approximate range 1.50-1.53 to 1, se-parating said gelatinous precipitate from the solution, washing said precipitate free of soluble salts with water, homogeneously suspending the washed precipitate in 1 to 3 percent (w/w) aqueous ammonium sulfate in the amount of about 10 to 20 ml.
per gram of theoretically expected whitlockite ceramic, separa-ting the precipitate from the ammonium sulfate solution, drying said precipitate and heating the dried precipitate in the ap-proximate range 1000°C. to 1350°C. for about 0.5 to 4 hours.

12. A process according to claim 11 wherein the dried precipitate is heated at about 1150°C. to 1200°C. for approxi-mately 1 hour.

13. A process according to claim 12 wherein the product is subsequently heated at about 500°C. for ap-proximately 4 hours whereby the resulting whitlockite ceramic consists essentially of .beta.-whitlockite.

14. A process for producing a polycrystalline foamed ceramic body according to claim 6 which comprises reacting calcium ion with phosphate ion in 2 molar ratio of about 1.2-1.5 to 1 in aqueous medium at a pH of about 10-12 to produce a gelatinous precipitate of calcium phosphate having a calcium to phosphorus molar ratio of about 1.50-1.53 to 1, separating said gelatinous precipitate from the solution, washing said precipitate free of soluble salts with water, homogeneously suspending the washed precipitate in 1 to 3 percent (w/w) aqueous ammonium sulfate in the amount of about 10 to 20 ml.
per gram of theoretically expected whitlockite ceramic, se-parating the precipitate from the ammonium sulfate solution, mixing the precipitate with about 0.5 to 10 percent by weight of a blowing agent and about 0.5 to 10 percent by weight of a foam stabilizer, heating the resulting mixture at about 70°C. to 90°C. until decomposition of the blowing agent and drying of the resultant foam are substantially complete, and then heating the dried foam at about 1000°C. to 1350°C. until volatilization of the foam stabilizer and sintering of the resulting product are substantially complete.

15. A process according to claim 14 wherein the blowing agent is azodicarbonamide and the foam stabilizer is egg albumen

16. A process according to claim 15 wherein the mixture of the calcium phosphate precipitate, azodicarbonamide and egg albumen is dried at about 70 C. to 90°C. for approximately 8 to 20 hours and the resulting dried foam is heated at about 1050°C. to 1150°C. for approximately 1 hour.

17. A process for producing non-ceramic, crystalline whitlockite containing within the crystal lattice thereof about 0.1 to 2.2 percent by weight sulfate ion which comprises reacting calcium ion with phosphate ion in a molar ratio in the approximate range 1.2-1.5 to 1 in aqueous medium at a pH
of about 10 to 12 to produce a gelatinous precipitate of calcium phosphate having a molar ratio of calcium to phosphorus in the approximate range 1.50-1,53 to 1, separating said gelatinous precipitate from the solution, washing said pre-cipitate free of soluble salts with water, homogeneously suspending the washed precipitate in 1 to 3 percent (w/w) aqueous ammonium sulfate in the amount of about 10 to 20 ml.
per gram of theoretically expected whitlockite ceramic, sepa-rating the precipitate from the ammonium sulfate solution, drying said precipitate and heating the dried precipitate in the approximate range 725°C. to 900°C. for about 0.5 to 4 hours.