Infused ceramic network for fabricating odontoforms and dental restorations

Abstract: The invention relates to a process for producing a ceramic network material from a ceramic suspension by casting said suspension in a tooth-shaped mold. The suspension includes dispersed alumina particles in a medium containing deionized water with a pH in a range of about 4 to 5 and a concentration of polyvinyl alcohol in a range of about 0.5 to 1% by weight. The process includes the steps of slip casting the suspension in a plaster tooth mold and drying the molded suspension to draw the water from the suspension. The dried suspension then is fired in a furnace at a temperature in a range of about 1000 to 1400 °C to form a ceramic network. After the dried suspension has been fired and excess water and polyvinyl alcohol has been removed from the suspension, lanthanum aluminosilicate glass is infused to a coronal portion of the ceramic network to form a glass layer with a thickness in a range of about 1 to 2 mm within the ceramic network. A monomer, such as a monomer solution, then is infused to at least the portion of the ceramic network within the glass layer. Alternatively, the ceramic network may be infused with a metal alloy to form a layer on the coronal portion or with one or more monomers.

Description: The invention relates to ceramic network materials used to produce odontoforms which replicate the physical and mechanical properties of natural teeth. Further, this invention relates to fabricating dental restorations and, in particular, to an interpenetrating network material which may be used as a restorative material. This restorative material may be used to fabricate restorations, such as inlays, onlays, crowns, and bridges, by centrifugal or pressure casting in mastercast molds or by machining blocks of the restorative material. Referring to Fig. 1 , natural tooth enamel 10 is a hard, vitreous substance that covers the outer portions on a tooth crown 1. Mature enamel is highly mineralized, containing about 96 to 98% inorganic substance. Its most abundant mineral component, i.e., about 90%, is hydroxyapatite, which is crystalline. Hardness is an important enamel property because enamel 10 provides a protective covering for the softer underlying dentin 12. Enamel 10 also serves as a masticatory surface on which food is crushed, ground, and chewed. Specifically, hardness is a measure of the ability of tooth enamel to withstand deformation by indentation or scraping, and mature enamel demonstrates a Knoop hardness number (KHN, i.e., the ratio of a given load to an area of indentation expressed in Kg/mm 2 ) in a range of about 200 to 500 KHN. Measurement of tooth enamel's microhardness is valuable not only in assessing its plastic properties in relation to masticatory forces, but also as a consideration in the selection of restorative materials. Generally, the density of enamel 10 decreases from its surface to the dentinoenamel junction 11. Further, the thickness of enamel 10 may vary throughout different regions of the tooth and from one type of tooth to another. For example, over the cusps of molars, enamel may attain an average thickness of about 2.6 mm; over the cusps of premolars, it may be about 2.3 mm; and over the incisal edge of incisors, it may be about 2 mm thick. Enamel 10 may become progressively thinner towards the cervical regions, and its thickness decreases even further towards the cementoenamel junction 13. Enamel coverage usually terminates at cementoenamel junction 13. Because enamel is semitranslucent, the color of enamel depends in part upon its thickness. For this reason, enamel may assume the various colors of its underlying structures. Thus, when enamel is thicker, and consequently more opaque, it may appear grayish or bluish white reflecting more of its inherent coloration. When enamel is relatively thin, however, it may be yellow-white in appearance, reflecting the underlying generally-yellowish dentin. Regardless of its thickness or color, enamel is hard in order to function properly as a masticatory surface, yet hardness alone may not be sufficient to enable enamel to withstand the pressure brought to bear on each tooth during mastication. To the contrary, enamel's hardness, as reflected by its brittleness, may represent a structural weakness. In fact, natural enamel is prone to splitting and chipping. For the most part, however, enamel does withstand masticatory pressures, and this may be due in part to the cushioning effect of the underlying dentin. Enamel may possess a relatively high elastic modulus, e.g., about 131 GPa (19 x 10 6 PSI) which may suggest that it is extremely brittle, and a relatively low tensile strength, e.g., about 75.8 MPa (11,000 PSI), which may suggest that it has a rigid structure. Dentin 12, on the other hand, is a highly compressive tissue having compressive strength of about 275.8 MPa (40,000 PSI) and acts as a cushion for overlying enamel 10. Thus, in addition to the properties discussed above, the ability of a tooth to withstand masticatory forces may be related to the physical and mechanical interrelationship between enamel 10 and dentin 12. As demonstrated by Fig. 1 , dentin 12 may constitute the largest single component of tooth structure, extending almost the entire length of the tooth. Dentin 12 is covered by enamel 10 on the crown 1 and by cementum 14 on root 2. The internal surface of dentin 12 forms the walls of a pulp cavity 16 which primarily contains pulpal tissue 18. Further, the walls of pulp cavity 16 may closely conform to the outline of the external surface of the dentin 12. Dentin and bone in general are natural ceramic-composites. Chemically, dentin 12 is composed of organic and inorganic matter. As noted above, the inorganic matter includes calcium phosphate in the form of hydroxyapatite (Ca 10 (PO 4 ) 6 (OH 2 )). The organic matter is primarily collagenous material. Particles of hydroxyapatite, the ceramic, are bonded together by collagenous material, an organic polymer. The hardness of dentin 12 is less than that of enamel, but generally greater than either cementum or other bone. Further, microhardness values may vary in the different layers (not shown) of dentin 12. The highest microhardness values of intact dentin 12 generally occur in the area about 450 µm from the dentinoenamel junction 11, with an average KHN of about 70. The lowest microhardness value, e.g., about 20 KHN, may be found in the innermost layer of dentin at distances of about 100 µm from the pulp. Moreover, with advancing age, the microhardness of dentin 12 tends to increase. A comparison of representative microhardness values of dentin, enamel, and restorative materials is shown in Table I. Comparison of Microhardness Values Materials KHN Dentin 65 Enamel 300 Silicate cement 70 Gold (pure) 32 Amalgam 90 Although dentin may be considered a hard structure, it also has elastic properties which may be important for the support of the relatively brittle and non-resilient enamel, as described above. Dentin may have a modulus of elasticity of about 11.51 GPa (1.67 x 10 6 PSI). Moreover, the tensile strength of dentin, e.g., about 41.4 MPa (6,000 PSI), is less than that of compact bone. Restorative materials generally are tested on three criteria: sufficient hardness, adequate fit, and acceptable aesthetics, e.g., color match. Of these three, however, aesthetics often are the deciding factor in the choice of restorative materials. Composite resins and ceramics are in widespread use due in part to their ability to match the color of a patient's natural teeth. Composite resins may be composed of a glass in a polymer matrix. This combination, however, may result in rapid wear of the restoration, as the softer polymer is lost, and the glass filler pulls out of the remaining polymer. High wear rates are associated with rapid loss of restorative strength. Restorative ceramics may also be problematic. Despite relatively high hardness, ceramic materials also are generally brittle, such that they may withstand only minimal deformation without failing. Thus, wear in current composite resin materials and catastrophic fracture of ceramic restorations are significant limitations of currently available restorative materials. According to its abstract, US-A-3 423 828 discloses an artificial tooth capable of being united to a synthetic resin denture base comprising a major proportion of dental porcelain particles, the interstices between the particles being filled with a synthetic dental plastic. The dental plastic and the dental porcelain particles are united by a reactive organic silicon bonding agent present as a coating on the porcelain particles. According to its abstract, US-A-4 772 436 discloses a method for manufacturing a dental prosthesis from a shaped, slightly solid phase fritted metal oxide infrastructure whose pores are impregnated with a glass by molding a tooth model from a linearly expanding molding mass and contacting the model with a slip of metal oxide particles, which particles are agglomerated onto the model. The agglomerated metal oxide particles forming an infrastructure which is shaped and baked to effect dehydration and withdrawal of the model from the infrastructure before the fritting and glass impregnation of the infrastructure. Dental students generally learn to restore teeth in the laboratory before working on patients' teeth. Extracted teeth are in limited supply, however, so students often use odontoforms for practice. Further, because of the scarcity of suitable patients for examination purposes, students may be tested for graduation or licensure, in part, on their ability to cut and prepare odontoforms for restorations. Present odontoforms may consist of molded white plastic. Such plastic teeth, however, bear little or no resemblance to natural teeth with respect to mechanical properties, e.g., hardness or elastic modulus, or physical properties, e.g., how the tooth structure feels when being cut with a diamond bur in a dental handpiece. Thus, odontoforms currently used are inadequate to represent and teach the natural tooth structure for purposes of education or examination. Thus, a need has arisen for an odontoform which more closely possesses the physical and mechanical properties of natural teeth to adequately prepare students for dental practice on patients and for use in board examinations to qualify such students as dentists. Further, a need has arisen for a restorative material which combines the beneficial restorative qualities of polymers with those of ceramics, and yet avoids disadvantages of polymers. An embodiment of the invention is a process for producing a ceramic network material from a ceramic suspension. The suspension includes dispersed ceramic particles in a medium containing water and a dispersant. The process includes the steps of casting the suspension in a mold and drying the molded suspension to draw the water from the suspension. The dried suspension then is fired to produce a ceramic network, and a metal alloy is infused to at least a portion of the ceramic network. Other objects, advantages, and features will be apparent when the detailed description of the invention and the drawings are considered. For a more complete understanding of the present invention and the technical advantages thereof, reference is made to the following description taken in conjunction with accompanying drawings, in which: Fig. 1 is a cross-sectional view of a natural tooth. Fig. 2 is a flow chart depicting the steps of a process upon which the invention is based. The embodiment shown does not form part of the invention. Fig. 3 is a cross-sectional view of an odontoform manufactured by a process disclosed herein. Fig. 4 is a flow chart depicting the steps of another process upon which the invention is based. The embodiment shown does not form part of the invention. In a first embodiment, the invention involves the creation of an interpenetrating network material of ceramic, and metal alloy with improved resistance to wear. Referring to Fig. 2 , an embodiment of the process includes the colloidal casting of a suspension of ceramic particles in a mold. This process is based on colloidal processing of materials. Colloidal processing involves the intimate mixing of two substances, one of which may be referred to as the dispersion phase or colloid, e.g., the ceramic particles, and is uniformly distributed in a finely divided state throughout a second substance. The second substance may be refereed to as the dispersion or dispersing medium e.g., deionized water. The dispersion medium may be a solid, liquid, or gas, and the mixture of the substances is known as a colloidal suspension. The ceramic particles in the suspension may be metal oxides, such as alumina and zirconia, or feldspathic glasses, such as sodium and potassium feldspar and nepheline glasses. The mechanical properties, as well as the color, of the suspension may be altered by the choice of ceramic particles. An alumina suspension may be created in deionized water with a pH in the range of about 2 to 6, e.g., about 4 to 5. An acid, such as nitric or citric acid, may be added to the deionized water to lower pH. A dispersant is added to the deionized water to promote a substantially uniform dispersion of the ceramic particles throughout the suspension. Suitable dispersants include acids, such as benzoic acids and acrylic acids; alcohols, such as polyvinyl alcohol; organic polymers, such as polyvinyl pryollidone and cellulose, including cellulose esters; silicates, such as sodium silicate; and derivatives of these. Benzoic acids further may include p-hydroxy and p-amino benzoic acid, and acrylic acids may include methacrylic acids. If the dispersant is a polyvinyl alcohol, it may be in solution with water at a concentration of about 0.5 to 2% by weight. The suspension is mixed, e.g., sonicated, to further promote dispersion of the ceramic particles. For example, an ultrasonicator, such as the Heat Systems Ultrasonicator, may be used, and the suspension may be sonicated at a 25% power level for a period of between about 5 and 20 minutes. In step 20 of Fig. 2 , a precipitant may be added to the suspension to remove excess water and dispersant before casting. For example, if the dispersant is polyvinyl alcohol, a non-solvent, such as acetone, may be added to precipitate a more concentrated ceramic suspension. Alternatively, a precipitant may be added to the suspension after casting. Regardless, when the precipitant is added to the suspension, the precipitate is a coacervate, i.e., an aggregate of colloidal droplets bound together by the force of electrostatic attraction. The alumina suspension (colloidal) may be cast in a rubber mold of a tooth as described in step 21A. The rubber mold may be made from silicone, polysulfide, or polyether-based rubbers. Rubber molds are desirable because they are easily deformed to remove the dried suspension. The base of such a tooth mold may be exposed, and a slab of a calcium sulfate hemihydrate based material, e.g., a gypsum slab, may be set on the mold in contact with the suspension. According to step 22A, the calcium sulfate material draws water from and dries the suspension, thereby, solidifying the alumina mass. Preferably, the suspension is allowed to dry in this manner for about 24 hours before being removed from the mold. Alternatively, the suspension may be cast in a plaster mold, i.e. , slip cast, as described in step 21B. A calcium sulfate hemihydrate based material, such as gypsum, is a common ingredient in plaster for use in fabricating such molds, and water is drawn from the suspension into the plaster, according to step 22B. It may be necessary to break such plaster molds in order to remove the dried suspension. Nevertheless, such molds are inexpensively and easily fabricated, and the plaster may be reusable or disposable. After the dried suspension is removed from the mold, as indicated in step 23, the dried alumina suspension is fired in a furnace at a temperature in a range of about 1000 to 1400 °C, according to step 24. In order to avoid thermal shock, the dried suspension may be placed in a cold furnace and heated gradually to the firing temperature. For example, the temperature of the furnace may be raised at a rate of about 2 to 15°C per minute until the desired firing temperature is reached. Referring to step 25A, in embodiments which do not form a part of the claimed invention, the coronal portion of the fired suspension is then infused with a glass, such as lanthanum aluminosilicate glass or borosilicate glass, or a combination thereof, for about 0.5 to 1 hours at about 1100°C. This achieves a glass layer depth of about 1 to 2 mm. Alternatively, as indicated in step 25B, the coronal portion of the fired suspension may be infused with a monomer, such as a light or heat-cured monomer. Such monomers, e.g., a mixture of triethylene glycoldimethacrylate (TEGDMA) and 2,2bis[4(2-hydroxy-3 methacryloyloxy-propyloxy)-phenyl]propane (BIS-GMA), may have a hardness in a range of about 40 to at least about 60 KHN and a flexural strength in a range of about 50 to 80 MPa. For example, a heat-cured monomer may be cured in a low heat furnace at a temperature in a range of about 40 to 75 °C for about 24 hours. Moreover, camphoroquinone and benzoyl peroxide may be added to the monomer to effect light curing. Such a light-cured monomer may be cured with light in the visible blue spectrum for about 15 minutes. Suitable monomers include acrylic monomers, such as hydroxy ethyl methacrylate (HEMA) ,TEGDMA, and BIS-GMA. Other suitable monomers include urethane dimethacrylate (UDM), biphenyldimethacrylate (BPDM), n-tolyglycine-glycidyhnethacrylate (NTGE), and polyethylene glycol dimethacrylate (PEG-DMA). Such monomers may be used alone, or two or more monomers may be used in combination. Further, colloidal silica may be added to the monomer to alter its index of refraction. By altering the monomer's index of refraction, the color of the odontoform or restoration may be adjusted. Moreover, acetone may be added to individual monomers (or monomer solutions) to alter their viscosity and control the rate of infusion. In accordance with the claimed invention, the ceramic network is infused with a metal alloy to form a metal alloy layer. A suitable metal alloy has a melting point at least about 50 °C less than the ceramic network. For example, if the ceramic network includes alumina, a metal alloy including at least about 60% by weight of gold and at least about 5 to 10% by weight palladium or platinum, or both. Such a gold infused ceramic may be highly ductile, and appropriate adjustment of the ceramic network may produce a metal-ceramic material which retains many of the properties of the infused metal. These properties may include increased flexion and burnish ability and resistance to fracture. As described in step 26, the fired suspension, at least the portion within the layer of a metal alloy, may be infused with a light or heat-cured monomer, or both. As discussed above, the monomer used for the infusion may be selected to change the mechanical properties of the portion to which it is infused. Moreover, the type of monomer may be varied, so that the coronal portion is hard, e.g., to a hardness of about 40 to at least about 60 KHN, and the internal portion is softer. Alternatively, if the fired suspension is to be used as part of a dental restoration, the entire fired suspension may be infused with the metal alloy, and/or a monomer. Excess glass, monomer, or metal alloy may be removed from the surface of the infused suspension, by using, for example, air abrasion or grinding, as described in step 27. If air abrasion is used to remove the excess glass or monomer from a fired suspension which contains alumina particles, alumina particles with a mean diameter of about 50 µm are preferred as the abrasive. Such alumina particles are preferred as the abrasive because they minimize the contamination of the fired and infused suspension. Referring to Fig. 3, a cross-sectional view of an odontoform 30 manufactured according to a process based on the invention is shown. Odontoform 30 is cast in a tooth-shaped mold (not shown). As discussed above, the mold may be a rubber, e.g., silicone; a plaster; or a split-metal mold. A glass layer 32 is infused in a coronal portion of odontoform 30, which corresponds to the portion of a natural tooth which is exposed above healthy gingiva and extends to a depth of about 1 to 2 mm below the surface of the tooth crown 31. Suitable glasses may include lanthanum aluminosilicate glass, borosilicate glass, or a combination of these glasses, or other low fusing temperature glasses. Glass layer 32 has a depth of about 1 to 2 mm and may be infused uniformly over the coronal portion of odontoform 30. Alternatively, as discussed above, a monomer may be infused to the coronal portion of odontoform 30. Such a monomer is selected to achieve an increased hardness in the coronal portion, but less than that obtained in glass layer 32. In addition, at least a first portion 34 of odontoform 30, including a second portion 36 within glass layer 32, may be infused with a monomer, such that first portion 34 has physical and mechanical properties that are different from those of glass layer 32 and that generally are similar to those of natural dentin. For example, the glass layer may have a hardness in a range of about 300 to 600 KHN, an elastic modulus in a range of about 103.4 - 172.4 GPa (15 to 25 x 10 6 PSI). In addition, the glass layer may have a flexural strength of about 240 MPa. Further, a pulp cavity (not shown) may be hollowed out in first and second portions 34 and 36. For example, the pulp cavity may be filled with a gel, an acrylic, or another material chosen because of its similarity to the physical and mechanical properties and color of pulp. In addition, caries found in natural teeth may be simulated on such an odontoform by acid etching or machining of the glass or monomer or metal alloy layer. Conventional dental composite resins produce a restorative material with only about 60 volume % of ceramic filler. The process of the invention, however, may produce a restorative material within a range of about 65-90 volume % of ceramic filler. A restoration formed on a metal supporting structure may be used without further modification, except for minor adjustments to the biting surface and internal surface too improve occlusion. Thus, the process may be used to fabricate dental restorations directly. When the suspension has been molded, e.g., pressure molded, into blocks, however, ceramic machining systems may be used to produce onlays, inlays, crowns, and bridges. Blocks of the fired and infused suspension may be used to manufacture implants for dentistry and medicine. Dental implants for tooth replacement and medical implants for bone or joint replacement may be fabricated from blocks of a metal alloy infused suspension. Such materials are particularly useful in Computer Aided Design-Computer Aided Machining (CAD-CAM) systems, as well as copy milling systems, such as Mikrona Corporation's CELAY® copy milling system. A process of this invention may produce restorative materials by the infusion of a dried ceramic suspension with a metal alloy. Traditional composite materials possess discreet and isolated phases dispersed in a network. A material having interpenetrating network involves the intertwining of two different types of materials. This intertwined network produces a single material with improved mechanical properties. This material may be produced by filling the pores of an open pore network of one material with a second material. When the pores of the first material are continuous throughout, capillary action may be used to draw the second material into the pores to create an interpenetrating network. The infused suspension produces an improved restorative material which resists pull out of the filler because the ceramic particles are connected to each other and intertwined with metal alloy. Wear may be reduced due to the high filler content, as well as the interpenetrating network of the monomer and ceramic filler. The material also may withstand flexing forces better than all-ceramic restorations because of the flexibility of the monomer (or metal alloy). Other embodiments of the invention will be apparent to the skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Although a detailed description and examples of the present invention has been provided above, it is to be understood these are exemplary and that the scope of the invention is not be limited thereby, but is to be determined by the claims which follow.

Claims: A process for producing a ceramic network material from a ceramic suspension, wherein said suspension includes dispersed ceramic particles in a medium containing water and a dispersant comprising the steps of: casting said suspension in a mold; drying said molded suspension to draw said water from said suspension; firing said dried suspension to form a ceramic network; and infusing a metal alloy to at least a portion of said ceramic network. The process of claim 1 which comprises: infusing a monomer to at least a portion of said ceramic network. The process of claim 2, wherein said monomer is an acrylic monomer. The process of claim 3, wherein said monomer comprises at least one monomer selected from the group of acrylic monomers consisting of hydroxy ethyl methacrylate (HEMA), triethylene glycol dimethacrylate (TEGDMA), 2,2-bis[4(2-hydroxy-3 Methacryloyloxy-propyloxy)-phenyl] propane (BIS-GMA), urethane dimethacrylate (UDM), biphenyldimethacrylate (BPDM), n-tolyglycineglycidylmethacrylate (NTGE), and polyethylene glycol dimethacrylate (PEG-DMA). The process of claim 2, wherein said monomer is selected from the group consisting of heat-cured monomers and light-cured monomers. The process of claim 1, wherein said mold is tooth-shaped. The process of claim 1, further wherein said water is acidic and has a pH in a range of 2 to 6. The process of claim 1, wherein said mold is made from rubber and when said suspension is cast in said mold, a base of said mold is exposed, such that the step of drying said suspension comprises placing a slab of calcium sulfate in contact with said base, thereby drawing said water from said suspension in said slab. The process of claim 1, wherein said mold is a plaster mold, such that when said suspension is cast in said mold, said water is drawn from said suspension in said plaster mold, thereby drying said suspension. The process of claim 1, wherein said dispersed ceramic particles are selected from the group consisting of metal oxides and feldspathic glasses. The process of claim 1, wherein said suspension is fired in a furnace at a temperature in a range of 600 to 1400°C. The process of claim 1, wherein said dispersant is selected from the group consisting of acids, alcohols, organic polymers, silicates, and derivatives. The process of claim 1, further comprising the step of adding a precipitant to said suspension to remove excess water and dispersant from said suspension. The process of claim 13, wherein said precipitant is selected from the group consisting of acetone and citric acid. The process of claim 1, wherein the metal alloy is selected from the group consisting essentially of gold, palladium and platinum such that said metal alloy includes at least 60% by weight of gold and at least 5 to 10% by weight palladium or platinum or both. An odontoform produced by the process of claim 6.

Inventors: GIORDANO RUSSELL A, GIORDANO, RUSSELL A.
Applicants: UNIV BOSTON, TRUSTEES OF BOSTON UNIVERSITY
Application year: 1995
Main classification: A61K6
IPC codes: A61C5, C04B41, A61K6, A61C13, C04B35