WO1994023944A1

WO1994023944A1 - Fluorapatite thermally engineered dental restoratives

Info

Publication number: WO1994023944A1
Application number: PCT/US1994/003580
Authority: WO
Inventors: Martin J. Sladek
Original assignee: Dentalyt, Inc.
Priority date: 1993-04-09
Filing date: 1994-03-31
Publication date: 1994-10-27

Abstract

Dental restorative compositions comprising fluorapatite crystals (3) are disclosed. In one aspect, dental restorative ceramics are provided which comprise a matrix of sintered fluorapatite crystals (1), the crystals (3) having an average fluoride content of about 3.70 % to about 3.80 % by weight. In antoher aspect, dental restorative composites are provided which comprise, in mixture, fluorapatite crystals (3) having an average fluoride content of about 3.70 % to about 3.80 % by weight and a photo-curable, organic resin. Methods of preparing such dental restoratives and a kit for their use are also disclosed.

Description

FLUORAPATITE THERMALLY ENGINEERED DENTAL RESTORATIVES

BACKGROUND OF THE INVENTION

This invention relates to dental restoratives, and more particularly, to dental restoratives which contain stoichiometrically pure fluorapatite crystals. Naturally occurring tooth enamel consists essentially of densified hydroxyapatite crystals having a crystal length of approximately 1 micron. Hydroxyapatite has the stoichiometric formula:

Ca₁₀(PO )₆(OH)₂ (1)

Dental restoratives, including crowns, inlays, dental cementors, and the like, are presently made largely from organic polymers and/or silica compounds which do not match the thermal, chemical, or mechanical properties of tooth enamel. Such conventional dental restoratives suffer from a number of drawbacks. For a particular restorative, these may include susceptibility to abrasion or chemical attack, poor assimilation with, and weak affinity toward, tooth enamel, and an unaesthetic appearance (e.g., where the restorative is clearly identifiable as a "false tooth").

Synthetic apatites, including fluorapatite, have been studied for use in dental restoratives because of their close chemical and structural similarity to tooth enamel. However, the results of these efforts have met with mixed results. Dental restoratives incorporating synthetic apatites have frequently not performed well due to inadequate mechanical properties. While apatite crystals found in natural tooth enamel are approximately 1 micron in length, synthetic apatite crystals which have been employed for use in dental restoratives typically have an average maximum length of anywhere from 20 to 300 microns. Moreover, restoratives composed of such apatites are not as thermally or mechanically stable as enamel, and fail to approximate the physical properties of enamel, such as its refractive index, translucency, and modulus of elasticity.

A need has existed for improved dental restoratives useful in a wide variety of direct and indirect applications which more closely simulate the makeup and physical properties of tooth enamel.

SUMMARY OF THE INVENTION

Among the several objects of the invention, therefore, may be noted the provision of dental restoratives which include stoichiometrically pure fluorapatite crystals; the provision of compositions which have wide applicability to the various forms of restoratives, such as crowns, inlays, dentin simulators and cementors ; the provision of dental restorative compositions which more closely match the chemical composition and physical properties of tooth enamel than do conventional restorative compositions; the provision of dental restorative ceramics which advantageously combine qualities of porcelain and composite; and the provision of dental restorative compositions which provide a time-released fluoride treatment. Further objects of the invention include the provision of a method for the manufacture of dental restorative ceramics and composites which include stoichiometrically pure fluorapatite crystals; the provision of such methods which incorporate photo-curable organic resins and/or fluoride ions substantially throughout the dental restorative; the provision of a method which allows for machining the restorative using a dental CAD/CAM system; and the provision of a method for in situ sintering of the dental restorative ceramic using a laser.

Briefly, therefore, the present invention is directed to a novel dental restorative. The dental restorative comprises a restorative amount of dental material comprising fluorapatite crystals having from about 3.70% to about 3.80% fluoride by weight. In further embodiments of the invention, a kit for restoration of teeth comprising the above dental restorative material in combination with a binder for affixing the dental restorative material, and a dental restorative ceramic comprising a matrix of sintered fluorapatite crystals having an average fluoride content from about 3.70% to about 3.80% by weight, are also provided.

Additionally, the present invention is directed to a process for the preparation of a dental restorative. The process includes the steps of hydrolyzing dibasic calcium phosphate in an aqueous reaction medium under hydrothermal conditions to form hydroxyapatite, which is then reacted with potassium fluoride in an aqueous reaction medium in the presence of a buffer composition under hydrothermal conditions to form fluorapatite crystals having from about 3.70% to about 3.80% fluoride by weight. A mass containing a restorative amount of dental material is then formed, including the fluorapatite crystals produced, and the mass is shaped into a predetermined form of a dental restorative.

In another aspect of the invention, a process for the preparation of a dental restorative ceramic is provided which allows for in situ sintering of the ceramic. In this process, fluorapatite crystals produced as discussed above are wetted with a sufficient amount of a saturated solution of aqueous calcium fluoride to form a moldable mass. This mass is applied to a dental surface to form a dental restorative. The dental restorative is contacted with a laser beam to sinter the restorative in situ, and thereby form a dental restorative ceramic. Other objects and features will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram illustrating the structural orientation of the atoms comprising a representative fluorapatite crystal. Fig. 2 is a diagrammatic depiction of fluorapatite crystals sintered into a matrix for a dental restorative ceramic in accordance with the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the invention, novel dental restoratives are provided which contain stoichiometrically pure fluorapatite crystals. These dental restoratives, available in the form of ceramics and composites, have utility for a variety of direct and indirect applications, including crowns, inlays, dentin simulators and cementors. It has been discovered that by utilizing hydrothermal reaction conditions, preferably in conjunction with agitation of the reaction medium during the synthesis of the fluorapatite crystals, that stoichiometrically pure fluorapatite crystals having a crystal length in the micron to sub-micron range are produced which provide the essential material for novel compositions which more closely approximate the size, chemical makeup and physical properties of natural tooth enamel. These stoichiometrically pure fluorapatite crystals may be sintered to form ceramics or mixed with organic resins to form composites, which are shaped to form the aforementioned dental restoratives.

Fluorapatite has the stoichiometric formula: Ca₁₀(PO-)₆F₂ (2)

An illustrative diagram depicting the relative orientation of the atoms comprising a representative fluorapatite crystal is shown at Figure 1. It is evident from a comparison of Formula 2 with Formula 1 that the chemical composition of fluorapatite and the hydroxyapatite component of natural tooth enamel are essentially the same, except that the hydroxide ions contained in hydroxyapatite have been replaced by fluoride ions in fluorapatite. It is believed, however, that fluorapatite is preferable to hydroxyapatite as a dental restorative material because, inter alia, the fluoride ion causes less crystal strain than the hydroxide ion, thus enhancing structural stability. Fluorapatite crystals comprise bipyramidal, isometric, hexagonal prisms, and belong to the space lattice group P63/111 (hexagonal elementary cell).

For use in dental restoratives in accordance with the instant invention, it is essential that the fluorapatite crystals have a fluoride content which achieves the theoretical value for stoichiometrically pure fluorapatite. Stoichiometrically pure fluorapatite is defined for purposes of this invention as having an average crystal fluoride content, by weight, of between about 3.70% and about 3.80%. Preferably, the fluorapatite crystals prepared for use in the dental restorative compositions of this invention have an average fluoride content, by weight, of between about 3.75% and about 3.78%. In accordance with the invention, a dental restorative is produced by utilizing a restorative amount of a dental material which includes stoichiometrically pure fluorapatite crystals. Preferably, these fluorapatite crystals comprise at least 10% of the volume of the dental restorative. The specific amount of dental material required and the concentration of the fluorapatite used will vary depending on the type of dental restorative under consideration, e.g., crown or inlay, ceramic or composite. For the dental restorative ceramic of this invention, sintered, stoichiometrically pure fluorapatite crystals are used to form a matrix for the ceramic. Preferably, this polycrystalline sintered ceramic has an apparent density of between about 90% and about 95%. The balance of the ceramic is comprised of pores which are dispersed throughout the crystal matrix, located between the prisms of the fluorapatite crystals. Preferably, these pores have a diameter ranging from about 0.5 to about 0.7 μm. Sintering causes the individual crystals to join together in a random interlocking of crystal particles to produce a reinforced framework much stronger than a single crystal of a size comparable to the framework. An illustration of a sintered matrix of fluorapatite crystals showing interlocking crystal particles and interprismatic pores (not drawn to scale) is depicted by Figure 2.

As shown at Fig. 2, a portion of a sintered matrix of fluorapatite crystals is depicted generally by the reference numeral 1. This matrix is constituted of a plurality of individual fluorapatite crystals 3 separated from each other by interprismatic pores 5 and randomly joined at interlocking points of fusion 7 caused by the sintering process. The individual crystals 3 are comprised of two bases 9, six facets 11, and two truncated pyramids 13 to form the needle-shaped, bipyramidal hexagonal prism - form of the apatite crystal habit.

It is desirable that the fluorapatite crystals approximate, as nearly as feasible, the average length of crystals found in tooth enamel. This reduction in synthetic crystal length is believed to result in greater tensile strength, compressive strength and modulus of elasticity, and to increase the physical similarity of fluorapatite to tooth enamel for such characteristics as material translucency and refractive index, thereby enhancing the aesthetic appeal of the restorative. Thus, in a preferred embodiment, the sintered fluorapatite crystals have an average length of less than about 15 microns, and most preferably, less than about 5 microns.

In a particularly preferred embodiment, the pores of the dental restorative ceramic are infused with a photo-curable organic resin, such as an acrylate, or an acrylate adduct. Examples of such photo-curable organic resins include bisphenol A-glycidyl methacrylate (Bis-GMA) adduct, bisphenol A-glycidyl methacrylate (Bis-GMA), ethoxylated bisphenol A dimethacrylate, urethane dimethacrylate, urethane dimethacrylate, and benzyl acrylate. Of these, Bis-GMA adduct is most preferred. The organic resin infused into the dental restorative ceramic is dispersed throughout the crystal matrix, substantially filling the interprismatic pores. The combination of sintered crystals and organic resin serves to strengthen the ceramic and to provide resiliency to the dental restorative, thereby providing the advantages attributable to both porcelain and composite. In another preferred embodiment, the dental restorative ceramic is infused with an aqueous alkaline earth metal fluoride, and preferably CaF₂. The fluoride ions infused into the ceramic are adsorbed onto the surface of the crystals in an amount sufficient to inhibit dental decay. The fluoride ions are slowly released from the restorative over time to serve as anti-cariogenic agents in the area around the dental restorative.

For restorations on dark stained teeth, feldspathic staining or shading materials which are well-known in the art, e.g., low fusing metallic oxides such as FeO or CuO, may be added to the external surface of the restoration. This staining material is preferably fired in an oven at the manufacturer's recommended temperature. In another embodiment of the dental restorative of this invention, the stoichiometrically pure fluorapatite crystals and one or more of the photo-curable organic resins discussed above are mixed together to form a dental restorative composite. Preferably, Bis-GMA adduct is used as the organic resin. In this embodiment, the fluorapatite crystals comprise between about 10 and about 95% of the total volume of the composite. The makeup of the composite is selected to reflect the general composition of the dental component for which it serves as a replacement. Thus, where an enamel replacement such as a crown or inlay is desired, the fluorapatite content of the composite is preferably between about 90 and about 95%. If a dentin simulator is being manufactured, one or more inorganic coloring agents is added to aid in simulating the physical appearance of dentin and, together, the fluorapatite crystals and coloring agents preferably comprise between about 50 and about 70% of the total volume of the composite. The amount and nature of inorganic coloring agents vary according to the desired shade. When a cementor is the desired product, it is preferable that the bulk of the composite consists of the organic resin and only about 10 to about 25% of the composite consists of fluorapatite crystals.

A kit for the restoration of teeth is provided by combining a dental restorative material of the types described above, and in particular, a dental restorative ceramic or composite, together with a binder for affixing the dental restorative material to a dental surface. The binder may be any conventional binder, but preferably is a cementor composed of from about 75% to about 90% of an organic resin and from about 10% to about 25% of fluorapatite crystals. Other materials which facilitate the restoration of teeth, such as various cavity- retardant agents, may optionally be included in the kit. The invention also relates to a method of preparing the dental restorative compositions described above. Stoichiometrically pure fluorapatite crystals which are used in both the ceramics and the composites of the invention are prepared in two phases. In the first phase (hydrolysis phase), dibasic calcium phosphate is hydrolyzed in an aqueous reaction medium under hydrothermal conditions to form hydroxyapatite. In the second phase (substitution phase), the hydroxyapatite formed in the first phase is reacted with potassium fluoride in the presence of a phosphate buffer under hydrothermal conditions to form the stoichiometrically pure fluorapatite crystals. Preferably, the aqueous reaction medium is agitated during both reaction phases in order to minimize crystal growth. In a preferred embodiment, positive agitation is supplied by two six-blade downward thrust impellers, one located at the bottom of the reaction vessel to keep particles in suspension, and one located at the base of the vessel's gas-liquid vortex to entrain gases into the liquid phase. The impellers are operated at stirring speeds of between about 200 and about 1000 rpm, preferably substantially continuously from before the warm up and until after the cool down of the reaction medium, thereby producing crystals having an average length of less than about 15 microns, and preferably less than about 5 microns. As used herein the term "hydrothermal conditions" refers to conducting the reaction in a hydrothermal reaction vessel applying sufficient heat and pressure to bring the intended reaction to completion. In a preferred embodiment of the process of the invention, the vessel used is a nickel alloy, split-ring closure reactor with magnetic drive, 230 volt variable speed motor, 2000 psi gage, serpentine internal cooling, aluminum block electric heater with internal cooling coil and microprocessor based Proportional-Integral-Derivative (PID) system controller (e.g., Reactor Series 4555, Parr Instrument Co., Moline, Illinois). In the hydrolysis phase, the reaction vessel is heated to between about 275°C and 325°C and placed under pressure of from about 1000 psi to about 1500 psi until complete hydrolysis has occurred (approximately 12 hours). In the substitution phase, the reaction vessel is heated to between about 375°C and 425°C and placed under pressure of from about 1000 psi to about 1500 psi until complete substitution has occurred (approximately 72 hours). Conducting the hydrolysis and substitution reactions at higher temperatures and/or pressures will reduce the reaction time, while lowering them causes the reactions to proceed at slower rates.

In the first phase, a high purity dibasic calcium phosphate is dissolved into distilled water and is hydrolyzed to form hydroxyapatite. The pH of the solution falls from about 7 at the start of the reaction to about 2.0 to 2.5 at complete hydrolysis. The equation for the hydrolysis reaction is:

300°C

(3) 10 CaHP0₄ + 2 H₂0 > Ca₁₀(P0₄)_fi(OH), +

1250 PSI 4H⁺ + 4 H^PO_j (pH -2.5)

The hydroxyapatite-containing solution produced is then vacuum filtered and the resultant solid is washed with distilled water and dried.

In the substitution phase the dried crystalline product of the hydrolysis phase and potassium fluoride are placed in a buffer system (preferably IN, Na₂HP0/_ : IN, NaH₂P0_|, 3:2) in the hydrothermal reaction vessel. Complete substitution occurs within 72 hours at the above described temperature and pressure in a closed system reactor vessel. The equation for the substitution reaction is:

Ca₁₀ ( PO₄ ) ₆ ( OH ) ₂ + 2 NaH₂P0₄ + 3 Na₂HP0₄ + 2 KF

400°C

(4) > Ca₁₀(PO₄)₆F₂ +

1250 PSI 2 KOH + 2 NaH_jPO^ + 3 Na₂HP0₄ (pH 6.5)

Complete substitution occurs with the ratio of two fluorides added per unsubstituted apatite molecule. The reaction proceeds favorably when 0.3 grams of reactants are used per one milliliter of buffer system. At completion of the reaction (confirmed by establishing that the pH has reached 6.5), the solution is vacuum filtered and the resultant solid is washed, rinsed and dried. Hydrothermally synthesized fluorapatite crystals are thus prepared which form the essential component of the dental restoratives discussed above. The crystals are needle-shaped and achieve the theoretical value for stoichiometrically pure fluorapatite having a fluoride content between about 3.70 and 3.80%, and preferably between about 3.75 and 3.78%. The molar Ca/P ratio of the fluorapatite crystals is approximately 1.67. Impurities such as the cations Fe, Cu, and Mg are preferably minimized to an assay range below 0.005%. Clumps or microaggregates are removed, e.g., by running the crystals through a mesh leaving particulate fluorapatite crystals.

In a preferred embodiment, the crystals are initially run through a coarse mesh (e.g., 19 μm) to remove large clumps, and then through a fine mesh (e.g., 5 μm) to extract any microaggregates. The stoichiometrically pure fluorapatite crystals produced may be used either to produce a dental restorative ceramic or composite. To prepare a dental restorative ceramic, particulate fluorapatite crystals are wetted with sufficient amounts of water to form a moldable mass. The mass is formed into a desired shape, e.g., on a dental refractory model. In one embodiment, most of the water is removed by condensing/vibrating and evaporation, and the shaped mass is sintered by heating in an oven at a temperature between about 1100 and about 1300 °C for about 15 to 30 minutes, producing a dental restorative ceramic comprised of a matrix of sintered, stoichiometrically pure fluorapatite crystals. Preferably, the ceramic is dried and sintered to an apparent density between about 90 and about 95%, with the balance comprised of pores dispersed throughout the crystalline matrix between and among the prisms of the crystals .

For restorations on dark stained teeth a feldspathic staining/shading material may be applied and thermally fused to the external surface of the restoration at about 870°C to about 1065°C. After all custom shading and staining treatments and before cementation, the restoration may be placed under vacuum in a saturated solution of aqueous CaF₂ for a time sufficient (approximately 2-20 mins . ) to fully adsorb a caries-inhibiting amount of fluoride ions onto the surface of the fluorapatite crystals, which are de- adsorbed slowly over time. The restoration may also be placed under vacuum in a container of unpolymerized, photo-curable organic resin, of the types described previously, in order to infuse the resin into the interprismatic pores located throughout the matrix of crystals. Preferably, the ceramic is treated with both the CaF₂ to provide a time-release fluoride treatment and with the organic resin to improve the strength and resiliency of the restorative.

In a second embodiment, the ceramic is shaped by utilizing a dental computer assisted design-computer assisted machining (CAD/CAM) system for machining the restorative to the desired dimensions. Examples of commercially available CAD/CAM systems include the Cerec system by Siemens Medical Systems, Inc. of Charlotte, N.C. and the Sopha system by Sopha Bioconcept, Inc. of Los Angeles, CA. In this procedure, the wetted mass of fluorapatite crystals is eased into a negative impression dental refractory mold of such dimensions and block form to be able to accommodate the dental CAD/CAM system. When the mold is full, the excess water is removed and the mass, now in block form, is sintered in an oven as described previously. The sintered block is finished for use in the dental CAD/CAM system for machining. Custom shading, fluoride treatment, and/or infusion with an organic resin may then be undertaken as described above. In a third embodiment of the process, a restorative ceramic is produced using a laser to sinter the fluorapatite crystals in situ. In this embodiment, the fluorapatite crystals are wetted with a saturated solution of aqueous CaF to form a workable, moldable mass which is applied directly to a dental surface and formed into the shape of a dental restorative for the purpose of sealing fissures or restoring incipient decay voids after organic debris and demineralized tooth structure have been removed. The calcium from the CaF solution serves to maintain the Ca/P ratio in enamel when the tooth surface undergoes laser treatment and the fluoride is adsorbed on enamel crystals for a caries-inhibiting effect, as described above. The dental restorative is then contacted with a laser beam to fuse the restorative to the tooth and to effect in situ sintering of the dental restorative, resulting in the formation of a dental restorative ceramic. Preferably, the laser beam has a wavelength of about 10.6 microns. Application of approximately 10 to 20 Joules/cm in the area treated is required to achieve in situ sintering by laser. A carbon dioxide laser is preferred to YAG lasers for this procedure. Stoichiometrically pure fluorapatite crystals may also be utilized as an inorganic filler for the photo-curable, organic resins disclosed above in order to prepare a dental restorative composite. The dental restorative composite comprises from about 10% to about 95% stoichiometrically pure fluorapatite crystals, depending on the purpose for which the restorative is to be adapted. To form a replacement for enamel, such as a crown or inlay, a mix is prepared containing between about 90 and about 95% by volume stoichiometrically pure fluorapatite crystals and between about 5 and about 10% by volume organic resin. This mix is then placed in the tooth to be restored and photocured in situ by exposure to an appropriate light source.

Indirect fabrication of composite restorations may also be provided by two methods, CAD/CAM preparation or Laboratory preparation. In the first method, the composite is shaped in block form in order to accommodate a dental CAD/CAM system and photocured to complete polymerization before machining. In the second method, the Laboratory preparation, the restoration is fabricated on a dental stone duplication of the teeth prepared for restoring, rather than in situ. The dental stone duplication of the prepared teeth is obtained by taking a negative impression of a quadrant of the mouth and placing the mixture for the dental stone into the negative impression until it is set and hardened. The duplicated quadrant, or working model, is then used for the placement of the composite and cured with a UV/visible light source. The restoration is then removed from the working model. In both methods, the intracoronal surface is applied with cementor, placed into the tooth and cured.

If a dentin simulator is to be prepared, the stoichiometrically pure fluorapatite crystals are combined with coloring agents to serve as the inorganic filler for the photo-curable organic resin. The dentin simulator is prepared in a paste system containing between about 50 and about 70% by volume inorganic filler and between about 50 and about 30% organic resin. This material may be provided in several different shades to match the shade of dentin lost due to decay or removal. It is applied to the area of lost dentin and bonded by photocure.

A cementor is prepared using between about 10 and about 25% stoichiometrically pure fluorapatite crystals as inorganic filler for the organic resin. The cementor is provided to facilitate the bonding of the indirect ceramic and composite restoratives described herein to the tooth.

The following examples illustrate the invention.

EXAMPLE 1

Preparation of Hydroxyapatite. 8.3 mg of high purity, precipitated dibasic calcium phosphate (General Electric brand, GE Cat. # 111-30-28; 7740-H (9/1986)), per milliliter of distilled water is placed into a nickel alloy, split-ring closure reactor hydrothermal reactor vessel with magnetic drive, 230 volt variable speed motor, 2000 psi gage, serpentine internal cooling, aluminum block electric heater with internal cooling coil and microprocessor based PID system controller (Reactor Series 4555, Parr Instrument Co., Moline, IL) and hydrolyzed at 300 °C, 1250 psi for 12 hours with positive agitation at 600 rpm throughout. The final pH of the solution is measured and verified at pH 2.2. The solution is then vacuum filtered and the stable solid phase unsubstituted apatite is washed with distilled water, rinsed with absolute alcohol, and dried for 15 hours at 90 °C.

EXAMPLE 2 Preparation of Stoichiometrically Pure Fluorapatite. The dried crystalline product of Example 1 is placed in a sodium phosphate buffer system (IN, Na₂HP0₄: IN, NaH₂P0 , 3:2) with potassium fluoride at a ratio of 0.2g (0.002 mole) hydroxyapatite to O.lg (0.002 mole) potassium fluoride per milliliter of buffer system, within the reactor vessel described in Example 1. Complete substitution occurs within 72 hours at 400°C and 1250 psi in the closed system reactor vessel. The reaction medium is positively agitated at 600 rpm throughout the reaction cycle. Upon completion, the pH of the solution is measured and verified at 6.5. The solution is vacuum filtered and the resultant solid is washed with distilled water, rinsed with absolute alcohol, and dried at 90 °C for 15 hours. The resulting crystalline product ,is stoichiometrically pure fluorapatite having a fluoride content of 3.77% by weight, and an average crystal length of 1.0 μm. The product is first run through a triangular wire weave, 635 mesh (19 μm) , and then a monofilament nylon mesh (5 μm) (Cross Wire Co., Belmar, N.J. ) to remove clumps and microaggregates. EXAMPLE 3 Preparation of a Ceramic Crown. A sufficient amount of the product of Example 2 to form a crown is wetted with water to form a moldable mass. The mass is then shaped into the form of a crown on a dental refractory model. Most of the water is removed by condensing/vibrating and evaporation. Thereafter, the material is sintered in an oven at 1200 °C for 25 minutes. A polycrystalline sintered ceramic is produced having a density of 92%, with interprismatic pores dispersed throughout a matrix of crystals. The sintered restoration is cooled to room temperature and is adjusted, refined and polished with successively fine grits. The crown is custom shaded to match the patient's teeth, using a conventional feldspathic staining material (Ceramco II fine grain stains, Ceramco Inc., Burlington, N.J.) and fired in an oven at the manufacturer's recommended temperature. Before cementation, the crown is placed in a saturated solution of aqueous calcium fluoride (General Electric Cat. # 111-30-3; 7740-F(9/86) ) for 5 minutes under vacuum and then removed. The water is allowed to evaporate from the interprismatic pores, leaving a cavity-inhibiting residue of fluoride ions adsorbed to the surface of the fluorapatite crystals. The crown is then placed in a container of unpolymerized, Bis-GMA adduct under vacuum for an additional 15 minutes, to infuse resin into the interprismatic pores of the crystalline matrix of the crown. The crown is removed from the resin container, excess resin is eliminated, and the crown is cemented into place according to conventional procedures.

EXAMPLE 4 Preparation of a Ceramic Inlay Using a Dental CAD/CAM System. 7 grams of the product of Example 2 are wetted with water to form a moldable mass. Incremental portions are eased into a negative impression dental refractory mold in block form. When the mold is full, the excess water is removed by condensing/vibrating and evaporation. The mass, now in block form, is sintered in an oven at 1200 °C for 45 minutes. A polycrystalline sintered ceramic block having a density of 90% is produced. The sintered block is cooled to room temperature, removed from the refractory mold and finished for use in a Sopha Dental CAD/CAM system, (Sopha Bioconcept, Inc., Los Angeles), and machined into the shape of a dental inlay.

EXAMPLE 5

Preparation of a Laser Sintered Dental Restorative

Ceramic. The product of Example 2 is wetted with a saturated solution of aqueous CaF₂ to form a workable, moldable mass which is applied to the surface of a tooth after removal of organic debris and demineralized tooth structure through vaporization by a laser beam. The mass is placed into the cavity to be filled and the filling is fused to the tooth enamel and sintered in situ. A laser

•* beam wavelength of 10.6 microns at 15 J/cπr is used for both vaporization and sintering.

EXAMPLE 6 Preparation of a Direct Dental Restorative Composite. Fluorapatite is prepared according to the method described in Example 2 and used as an inorganic filler for Bis-GMA adduct. A composite mix is prepared by mixing 90% by volume fluorapatite and 10% by volume Bis-GMA adduct. The composite mix is placed in the tooth to be restored and is photocured in situ.

EXAMPLE 7 Preparation of an Indirect Dental Restorative Composite Using a CAD/CAM System. A composite mix is prepared according to the method described in Example 6 and then shaped into block form to accommodate a Dental CAD/CAM system. The composite is photocured to complete polymerization for machining.

EXAMPLE 8

Preparation of an Indirect Dental Restorative Composite Using a Laboratory Preparation. A composite mix is prepared according to the method described in Example 6. A dental stone duplication of the prepared teeth is then obtained by taking a negative impression of a quadrant of the mouth and then placing material for the dental stone into the negative impression. The dental stone is allowed to set and harden. The duplicated quadrant is used as a working model for the placement of the composite and cured with ultraviolet light. The composite mix is shaped into the form of a dental restoration using the working model, then removed from the working model, coated with a cementor applied to its intracoronal surface, placed into position in the tooth for bonding, and then cured.

EXAMPLE 9 Preparation of a Dentin Simulator. A dentin simulator is prepared in a paste system containing 60% filler-- fluorapatite (prepared according to the method described in Example 2) and a metallic oxide coloring agent — and 40% ethoxylated bisphenol A dimethacrylate. The composite is applied to the area of lost dentin and bonded by photocure.

EXAMPLE 10 Preparation of a Dental Cementor. A dental cementor is prepared by combining 20% fluorapatite, prepared according to the method described in Example 2, and 80% urethane dimethacrylate. The cementor is applied to the restoration, the restoration is placed into the area restored and then bonded to the tooth by photocuring.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above compositions and methods, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A dental restorative comprising a restorative amount of dental material comprising fluorapatite crystals, said fluorapatite crystals having an average fluoride content from about 3.70% to about 3.80% by weight.

2. A dental restorative as set forth in claim 1 wherein the dental material is comprised of at least about 10% by volume of the fluorapatite crystals.

3. A dental restorative as set forth in claim 1 wherein the restorative comprises a composite, which composite comprises, in mixture, the fluorapatite crystals and a photo-curable organic resin.

4. A dental restorative composite as set forth in claim 3 wherein the resin is selected from the group consisting of bisphenol A-glycidyl methacrylate adduct, bisphenol A-glycidyl methacrylate, ethoxylated bisphenol A dimethacrylate, urethane dimethacrylate, or benzyl acrylate.

5. A dental restorative composite as set forth in claim 3 wherein the composite is adapted for use as a cementor and the fluorapatite crystals comprise between about 10 and about 25% of the composite by volume.

6. A dental restorative composite as set forth in claim 3 wherein the composite is adapted for use as a dentin simulator, the composite further comprises one or more inorganic coloring agents, and together the fluorapatite crystals and the coloring agents comprise between about 50 and about 70% of the composite by volume.

7. A dental restorative comprising a restorative amount of dental material comprising fluorapatite crystals, said fluorapatite crystals having an average fluoride content from about 3.70% to about 3.80% by weight and belonging to the space lattice group P63/m (hexagonal elementary cell).

8. A dental restorative ceramic comprising a matrix of sintered fluorapatite crystals, said fluorapatite crystals having an average fluoride content from about 3.70% to 3.80% by weight and belong to the space lattice group P63/m (hexagonal elementary cell).

9. A dental restorative ceramic as set forth in claim 8 wherein the ceramic contains pores dispersed throughout the matrix of crystals.

10. A dental restorative ceramic as set forth in claim 8 further comprising a caries-inhibiting amount of fluoride ions adsorbed onto the crystals contained in the ceramic.

11. A dental restorative ceramic as set forth in claim 9 wherein the pores are substantially filled with a photo-curable organic resin.

12. A dental restorative ceramic as set forth in claim 11 wherein the resin is selected from the group consisting of bisphenol A-glycidyl methacrylate adduct, bisphenol A-glycidyl methacrylate, ethoxylated bisphenol A dimethacrylate, urethane dimethacrylate or benzyl acrylate.

13. A dental restorative ceramic as set forth in claim 8 wherein the sintered fluorapatite crystals are sintered by heating at a temperature of at least about 1100°C.

14. A kit for restoration of teeth comprising a) a dental restorative material comprising fluorapatite crystals, said fluorapatite crystals having an average fluoride content from about 3.70% to about 3.80% by weight and belonging to the space lattice group P63/m (hexagonal elementary cell) and b) a binder for affixing said dental restorative material.

15. A process for the preparation of a dental restorative comprising the steps of: hydrolyzing dibasic calcium phosphate in aqueous reaction medium under hydrothermal conditions to form hydroxyapatite, reacting potassium fluoride with the hydroxyapatite in aqueous reaction medium in the presence of a buffer composition under hydrothermal conditions to form fluorapatite crystals having an average fluoride content from about 3.70% to about 3.80% by weight, forming a mass containing a restorative amount of dental material including said fluorapatite crystals, and shaping the mass into a predetermined form of a dental restorative.

16. A process for the preparation of a dental restorative as set forth in claim 15 wherein the process further comprises drying and sintering the dental restorative to form a dental restorative ceramic comprising a matrix of sintered fluorapatite crystals having pores dispersed throughout the matrix of crystals.

17. A process for the preparation of a dental restorative ceramic as set forth in claim 16 further comprising infusing the ceramic with a photo-curable, organic resin.

18. A process for the preparation of a dental restorative ceramic as set forth in claim 16 further comprising the step of immersing the dental restorative ceramic in a saturated solution of aqueous alkaline earth metal fluoride under vacuum for a time sufficient to adsorb a caries-inhibiting amount of fluoride ions onto the crystals contained in the ceramic.

19. A process for the preparation of a dental restorative ceramic as set forth in claim 16 in which, after sintering, the dental restorative ceramic is machined using a dental CAD/CAM system.

20. A process for the preparation of a dental restorative composite as set forth in claim 15 wherein the mass is applied to a dental surface and photo-cured in situ.