Aesthetic dental materials

The invention provides for a material comprising(a) a hardenable resin; and (b) a filler comprising (i) clusters of nano-sized particles, the clusters comprising non-beavy metal oxide particles and heavy metal oxides, and being not fully densified particles and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof. The material is suitable for use as dental materials.

 

What is claimed is:

1. A material comprising:

(a) a hardenable resin; and

(b) a filler comprising (i) clusters of nano-sized particles, said clusters comprising non-heavy metal oxide particles and heavy metal oxides and being not fully densified and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof,

wherein said material is a dental material.

2. The material of claim 1, wherein said non-heavy metal oxide particles are selected from the group consisting of silica, titanium dioxide, aluminum oxide, and combinations thereof.

3. The material of claim 1, wherein said heavy metal oxide comprises a heavy metal having an atomic number greater than 30.

4. The material of claim 1, wherein said heavy metal oxide is selected from the group consisting of zirconium oxide, cerium oxide, tin oxide, yttrium oxide, strontium oxide, barium oxide, lanthanum oxide, zinc oxide, ytterbium oxide, bismuth oxide, and combinations thereof.

5. The material of claim 1, wherein said clusters have an average diameter of less than about 1 micrometer.

6. The material of claim 1, wherein said non-agglomerated nano-sized particles have an average diameter of less than about 100 nanometers.

7. The material of claim 1, wherein said filler comprises at least about 60% by weight of said clusters and at most about 40% by weight of said nano-sized particles, based on the total filler.

8. The material of claim 1, wherein said hardenable resin is selected from the group consisting of acrylates, methacrylates, epoxies, and combinations thereof.

9. The material of claim 1, wherein said material is selected from the group consisting of dental resotratives, dental adhesives, dental mill blanks, dental cements, dental prostheses, orthodontic devices and adhesives, dental casting materials, and dental coatings.

10. The material of claim 1, wherein the material, after hardening, has a visual opacity of less than about 0.35 as measured on a MacBeth transmission densitometer Model TD-903.

11. A method of making a dental material comprising:

providing a hardenable resin;

providing a filler comprising (i) clusters of nano-sized particles, said clusters comprising non-heavy metal oxide particles and heavy metal oxides and being not fully densified and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof;

surface treating said filler to yield surface-treated filler particles; and

mixing said surface treated filler particles with said hardenable resin.

12. The method of claim 11, wherein said non-heavy metal oxide particles are selected from the group consisting of silica, titanium dioxide, aluminum oxide, and combinations thereof.

13. The method of claim 11, wherein said heavy metal oxide comprises a heavy metal having an atomic number greater than 30.

14. The method of claim 11, wherein said heavy metal oxide is selected from the group consisting of zirconium oxide, cerium oxide, tin oxide, yttrium oxide, strontium oxide, barium oxide, lanthanum oxide, zinc oxide, ytterbium oxide, bismuth oxide, and combinations thereof.

15. The method of claim 11, wherein said clusters have an average diameter of less than about 1 micrometer.

16. The method of claim 11, wherein said non-agglomerated nano-sized particles have an average diameter of less than about 100 nanometers.

17. The method of claim 11, wherein said filler comprises at least about 60% by weight of said clusters and at most about 40% by weight of said nano-sized particles, based on the total filler.

18. The method of claim 11, wherein said hardenable resin is selected from the group consisting of acrylates, methacrylates, epoxies, and combinations thereof.

19. The method of claim 11, wherein said dental material is selected from the group consisting of dental resotratives, dental adhesives, dental mill blanks, dental cements, dental prostheses, orthodontic devices and adhesives, dental casting materials, and dental coatings.

20. The method of claim 11, wherein said dental material, after hardening, has a visual opacity of less than about 0.35 as measured on a MacBeth transmission densitometer Model TD-903.

21. A method of using a dental material comprising:

placing the material near or on a tooth surface;

changing the topography of the material; and

hardening the material, wherein the dental material comprises a hardenable resin and a filler comprising (i) clusters of nano-sized particles, said clusters comprising non-heavy metal oxide particles and heavy metal oxides and being not fully densified and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof.

22. The method of claim 21, wherein placing, changing, and hardening are performed sequentially.

23. The method of claim 21 further comprising finishing the surface of the hardened material.

24. The method of claim 21 wherein the hardened material forms a dental article selected from the group consisting of dental mill blanks, dental prostheses, orthodontic devices, artificial crowns, anterior fillings, posterior fillings, and cavity liners.

25. A dental article preparable by a method comprising:

hardening a dental material comprising a hardenable resin and a filler comprising (i) clusters of nano-sized particles, said clusters comprising non-heavy metal oxide particles and heavy metal oxides and being not fully densified and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof; and

fabricating a dental article selected from the group consisting of dental mill blanks, dental prostheses, orthodontic devices, artificial crowns, anterior fillings, posterior fillings, and cavity liners.

TECHNICAL FIELD

The present invention relates to fillers useful for aesthetic dental materials. In particular, the filler is a combination of nano-sized particles and clusters of nano-sized particles, where the former, due to their size and shape, reside in the interstitial spaces between the clusters.

BACKGROUND

Dental materials have special requirements. For health reasons, dental materials should be suitable for use in the oral environment. In certain applications, strength and durability of a dental material is important to ensure satisfactory performance. For example, for dental work at locations where mastication forces are generally great, high strength and durability is desirable. In other applications, an aesthetic character (e.g., luster and translucency) is desired. This is often the case where dental work is performed at locations where a tooth repair or restoration can be seen from a relatively short distance.

Strength in a dental material is typically achieved by adding fillers. Generally, a dental material has greater mechanical strength when it contains fillers having an average diameter greater than 0.4 to 0.6 micrometers. A disadvantage to these dental materials, however, is their tendency to lack luster and aesthetic character. Another disadvantage of composites with such average particle size is that with repeated toothbrushing (a requirement for oral hygiene), the hardened resin can wear away leaving a dull, unaesthetic surface. The worn surface can be a site for subsequent plaque accumulation.

Some skilled in the art have investigated using a combination of different average particle size fillers to improve the aesthetic character of the dental material.

For example, U.S. Pat. No. 5,936,006 (Rheinberger et al.) discloses a filled and polymerizable dental material characterized in that it contains a sol of amphorous SiO.sub.2 particles in a liquid, organic dispersion agent, the SiO.sub.2 particles being organically surface modified, having an average size of 10 to 100 nm and being non-agglomerated. The sol is referred to as "silica organosol (a)." The SiO.sub.2 particles of the silica organosol (a) are organically modified at the surface. The dental material can also contain at least one polymerizable organic binder (b), and can contain conventional inorganic or organic particle-shaped fillers (c).

WO 00/03688 provides a dental enamel material having an opacity less than 50 percent and localized wear volume loss less than 0.025 mm.sup.3. The material comprises a polymerizable matrix forming liquid having a first refractive index (N.sub.D) and inorganic filler particles having a second N.sub.D. The first N.sub.D is within 5 percent of the second N.sub.D. The filler comprises particles of (a) low median particle size between 0.1 and 1.0 micrometers and (b) high median particle size between 1 and 10 micrometers. Preferred filler material is radiopaque dental glass such as barium aluminum-borosilicate glass, barium aluminofluorosilicate glass, and mixtures thereof. It is stated that the dental enamel material exhibits similar or improved physical characteristic when compared to known dental composites. Such physical characteristics include, among other things, opacity improvements, diametral tensile strength, polymerization shrinkage, and wear.

WO 99/65453 provides a dental composite comprising a resin base and a structural filler of ground particles having an average particle size between 0.05 and 0.5 micrometer. It is explained that because the structural filler particles are ground, they are non-spherical, providing increased adhesion of the resin to the structural filler. This increased adhesion is said to enhance the overall strength of the composite. The dental composite is said to provide the luster and translucency required for cosmetic applications. The structural filler is ground, typically by agitator milling, to the preferred mean particle size. This grinding-method is distinguished from the sol-gel process, in that the grinding method results in non-spherical particles.

Although the foregoing technology may provide useful dental materials, other compositions are sought.

SUMMARY

The present invention provides a dental material having a unique combination of filler particles: nano-sized particles and clusters of nano-sized particles (often referred to as "clusters" for convenience). It has been discovered that this combination provides a syngeristic effect that results in enhanced performance, as shown by the properties, of the inventive material. It is currently believed, as shown by transmission electron microscopy, that the smaller nano-sized particles fill the interstitial spaces between the larger clusters thereby minimizing voids in the composite dental material.

In brief summary, the inventive material comprises (a) a hardenable resin; and (b) a filler comprising (i) clusters of nano-sized particles, the clusters comprising non-heavy metal oxide particles and heavy metal oxides, and being not fully densified particles and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-beavy metal oxide particles, heavy metal oxide particles, and combinations thereof. Preferably, the inventive material comprises a filler comprising at least about 60% by weight of the clusters and at most about 40% by weight of the nano-sized particles, based on the total filler. The material is a dental material.

In brief summary, the inventive material comprises (a) a hardenable resin; and (b) fillers comprising (i) clusters of nano-sized particles, the clusters comprising non-heavy metal oxide particles and heavy metal oxides, and being not fully densified particles; (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof. The material is a dental material.

A method of making the inventive dental material comprises the acts of: (a) providing a hardenable resin; (b) providing a powder of filler particles comprising (i)-clusters of nano-sized particles, the clusters comprising non-heavy metal oxide particles and heavy metal oxides, and being not fully densified and (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof, (c) surface treating the filler particles to yield surface-treated filler particles; and (d) mixing the surface treated filler particles with the hardenable resin.

A method of making the inventive dental material comprises the acts of: (a) providing a hardenable resin; (b) providing a powder of filler particles comprising (i) clusters of nano-sized particles, the clusters comprising non-heavy metal oxide particles and heavy metal oxides, and being not fully densified; (ii) non-agglomerated nano-sized particles selected from the group consisting of non-heavy metal oxide particles, heavy metal oxide particles, and combinations thereof, (c) surface treating the filler particles to yield surface-treated filler particles; and (d) mixing the surface treated filler particles with the hardenable resin.

The clusters provide, among other things, strength while the nano-sized particles provide, among other properties, aesthetic quality, polishability, and wear resistance to the inventive material. Although the clusters and the nano-sized particles are structurally different types of fillers, they can have similar chemical constituents. Depending on the composition of the clusters and resin, one can add nano-sized non-heavy metal oxide particles, and/or heavy metal oxide particles to optimize the visual opacity and other properties of the inventive material. In this way, the present invention allows for flexibility in matching the refractive index of the components to minimize visual opacity. As a result, the inventive material exhibits excellent aesthetic quality while providing desirable properties. Such properties include, but are not limited to, good tensile strength, compressive strength, polishability, wear or abrasion resistance, luster, and low shrinkage after cure. Although low visual opacity is typically a desired property, it is not necessary for posterior applications.

The inventive material can be used in dental applications. Such applications include dental adhesives, artificial crowns, anterior or posterior fillings, casting materials, cavity liners, cements, coating compositions, mill blanks, orthodontic devices and adhesives, restoratives, prostheses, and sealants. The materials can be placed in the mouth and cured or hardened in situ. Alternatively, it may be fabricated into a prosthesis outside the mouth and subsequently adhered in place in the mouth.

DEFINITIONS

As used herein,

"Aesthetic quality" of a material, particularly a dental material, tends to be a subjective characteristic and yet a well-understood property in the dental industry. It can be quantified by visual opacity and/or polishability.

"Hardenable" is descriptive of a material that can be cured or solidified e.g., by heating to remove solvent, heating to cause polymerization, chemical crosslinking, radiation-induced polymerization or crosslinking, or the like;

"Non-heavy metal oxide" means any oxide of elements other than those of heavy metals.

"Polishability" is a property that can influence a material's aesthetic quality. Polishability is a measure of the retained gloss, i.e., polish retention and luster of a material after repeated abrasive contact, such as after tooth brushing.

"Visual opacity" is a property that can influence a material's aesthetic quality. In general, a low the visual opacity value is desirable.

DETAILED DESCRIPTION OF THE INVENTION

The components of the inventive material include fillers dispersed in a hardenable resin. Based on the total weight of the inventive material, the filler can be present at any amount, preferably at least 60%, more preferably at least 70%, and most preferably at least 75% by weight. Each component of the inventive material is discussed below in detail. As used in this document, the term "about" is presumed to preceed every recitation of a numeric value.

Filler Particles

The present invention provides for a combination of filler particles: clusters of nano-sized particles and non-agglomerated nano-sized particles. The clusters contains non-heavy metal oxide particles and heavy metal oxide. The nano-sized particles, on the other hand, generally tend to be discrete particles, and they can be non-heavy metal oxide particles and/or heavy metal oxide particles. The average diameter of the clusters, on a volumetric basis, is less than 10 micrometer, preferably less than 1 micrometer. The average diameter of the nano-sized particles, preferably based on TEM, is less than 200 nm, preferably less than 100 nm, more preferably less than 50 nm, and most preferably less than 20 nm. Each type of filler particles are discussed below in detail, first, the clusters, followed by the nano-sized particles.

The clusters are substantially amorphous in structure. The term "cluster" refers to the nature of the association among the non-heavy metal oxide particles in the cluster. Typically, the non-heavy metal oxide particles are associated by relatively weak intermolecular forces that cause them to clump together, i.e., to aggregate, even when dispersed in a hardenable resin. The heavy metal oxides can be present in various forms (as described in detail below). But, when they are present in the cluster as particles, the heavy metal oxide particles may display a similar association to each other and to the non-heavy metal oxide particles. By "substantially amorphous," it is meant that the clusters are essentially free of crystalline structure. The crystallinity of a material can be determined by a procedure that provides a Crystallinity Index (CI). A value of 1.0 on the CI indicates a fully crystalline structure, and a value near zero indicates predominantly amorphous phase. The clusters preferably have a CI of less than 0.2, more preferably less than 0.1, most preferably less than 0.05, according to x-ray diffraction methods.

Unlike conventional filler particles, the clusters are not fully densified. The phrase "fully densified," describes a particle that is near theoretical density, having substantially no open porosity detectable by standard analytical techniques. One useful technique is the Brunauer-Emmet-Tell (BET) method, which is described by S. J. Gregg and K. W. S. Sing in Adsorption, Surface Area, and Porosity, (Academic Press, London, 1982). The BET method uses nitrogen adsorption to determine the surface area of the particles, thereby giving an indication of porosity. Such measurements may be made on a QUANTASORB apparatus made by Quantachrome Corp., Syossett, N.Y. Density measurements may be made using an air, helium or water pycnometer.

The clusters can be made using a process that includes heat treatment. The clusters gain surface area after heat treatment. The ratio of the surface area after heat treatment compared to the surface area before heat treatment is preferably greater than 50%, more preferably greater than 80%.

Now turning to the chemical constituents of the clusters, they comprise non-heavy to metal oxide particles and heavy metal oxides.

First, the non-heavy metal oxide particles have an average diameter of less than 100 nanometer (nm), preferably less than 50 nm. These dimensions are preferably based on a TEM method, where one analyzes a population of particles to determine the average particle diameter. Such particles are preferably substantially spherical and substantially non-porous.

Suitable and useful non-heavy metal oxide particles include, e.g., silica, calcium phosphate, titanium oxide, feldspar, aluminum oxide, and the like. Silica, including its various forms such as fumed silica, colloidal silica, or aggregated silica particles, is preferred. Although the silica is preferably essentially pure, it may contain small amounts of stabilizing ion such as ammonium and alkaline metal ions.

Silica particles are preferably obtained from an aqueous silica colloidal dispersion (i.e., a silica sol or aquasol). In the silica sol, typically 1 to 50 weight percent is colloidal silica. Useful silica sols are those supplied as an aqueous dispersion of amorphous silica (such as the Nalco colloidal silicas made by Nalco Chemical Co., Naperville, Ill.) and those low in sodium concentration and can be acidified with a suitable acid (e.g., LUDOX colloidal silica made by E. I. DuPont de Nemours & Co., Wilmington, Del.). The silica particles in the sol have an average particle diameter of 5 to 100 nm, preferably 10 to 50 nm, more preferably 12 to 40 nm. Useful silica sols include NALCO 1040, 1042, 1050, and 1060, all commercially available from Nalco Chemical Co.

The second chemical constituent in the clusters is the heavy metal oxide, which imparts radiopacity to the inventive material. The term "radiopacity" describes the ability of the inventive material, after hardened, to be distinguished from tooth structure using standard dental X-ray equipment. The desired degree of radiopacity can be varied, depending upon the particular application and the expectations of the practitioner evaluating the X-ray film.

Useful heavy metal oxides contain metals having an atomic number greater than 28, preferably greater than 30, more preferably greater than 30 but less than 72. The heavy metal oxide should be chosen such that undesirable colors or shades are not imparted to the hardened resin in which it is dispersed. For example, iron and cobalt would not be favored, as they impart dark and contrasting colors to the tooth color. Suitable metal oxides are the oxides of yttrium, strontium, barium, zirconium, hafnium, niobium, tantalum, tungsten, bismuth, molybdenum, tin, zinc, lanthanide elements (i.e., elements having atomic numbers ranging from 57 to 71, inclusive), cerium, and combinations thereof. Preferred heavy metal oxides are oxides of lanthanum, zinc, tin, zirconium, yttrium, ytterbium, barium, strontium, cerium, and combinations thereof.

The heavy metal oxide component, as well as other additives, may exist in the inventive materials various forms. For example, they can be particles on the surface of the non-heavy metal oxide, particles amongst the non-heavy metal oxide or as a coating on at least a portion of the surface of a non-heavy metal oxide. Alternatively, the heavy metal oxide may be present in the non-heavy metal oxide particle as a solid solution (e.g., continuous glass) or a precipitate (a second phase).

Preferably, the heavy metal oxide is provided in the form of particles. When in particle form, the heavy metal oxide particles have an average diameter of less than 100 nm, preferably less than 50 nm, more preferably less than 10 nm. The heavy metal oxide particles may be aggregated. If so, the aggregated particles are less than 100 nm, and preferably are less than 50 nm in average diameter.

The heavy metal oxide is made from a precursor. Useful precusors can be organic or inorganic acid or water soluble salts, such as the heavy metal salts of aliphatic mono- or dicarboxylic acids (e.g. formic, acetic, oxalic, citric, tartaric, and lactic acids). Preferred precursors contain zirconium. Useful inorganic zirconium compounds include zirconium oxynitrate, zirconium acetate, and zirconium oxychloride. See U.S. Pat. No. 3,709,706, column 4, line 61, to column 5, line 5, for further details on zirconia sources that can be used in this invention. Zirconyl acetate compounds are preferred, particularly zirconyl acetate from MEI (Magnesium Elektron, Flemington, N.J.).

The clusters are prepared from a suitable non-heavy metal oxide sol and one or more heavy metal oxide precursor, which may be salts, sols, solutions, or nano-sized particles. Of these, sols are preferred. The term "sol" means a stable dispersion of colloidal solid particles within a liquid. The solid particles are typically denser than the surrounding liquid and small enough so that the dispersion forces are greater than the gravitational force. The particles are of a sufficiently small so that they generally do not refract visible light. Judicious choice of the precursor sols leads to desired degree of visual opacity and strength. The choice of the sol may depend on the following properties. The average size of the individual particles is preferably less than 100 nm in diameter. The acidity or the pH of the sol should preferably be below 6 and more preferably below 4. And, the sol should be free of impurities that cause undue aggregation (during the filler preparation process) of the individual discrete particles, during the subsequent steps, such as spray drying or calcining. If the starting sol is basic, it should be acidified, e.g., by adding nitric or other suitable acid to decrease the pH. Choosing a basic starting sol is less desirable because it requires an additional step and may lead to the introduction of undesired impurities. Typical impurities that are preferably avoided are metal salts, particularly salts of alkaline metals, e.g., sodium.

Prior to mixing the non-heavy metal oxide sol with the heavy metal oxide precursor, the pH of the non-heavy metal oxide sol is preferably reduced to provide an acidic solution having a pH of 1.5 to 4.0.

Once pH adjusted, the non-heavy metal sol and heavy metal oxide precursors are mixed together at a molar ratio to match the refractive index of the hardenable resin. This matching of refractive index imparts a low visual opacity. Preferably, the molar ratio ranges of non-heavy metal oxide ("non-HMO") to heavy metal oxide ("HMO"), expressed as non-HMO:HMO, is 0.5:1 to 10:1, more preferably 3:1 to 9:1, and most preferable 4:1 to 7:1. In a preferred embodiment where the clusters contain silica and zirconium, the method of preparation starts with a mixture of silica sol and zirconyl acetate, at a 5.5:1 molar ratio.

During mixing of the non-heavy metal oxide sol with the heavy metal oxide precursor, vigorous agitation is preferably used. After thorough mixing, the solution is dried to remove water and other volatile components to yield intermediate particles. Drying can be accomplished in various ways, including, e.g., tray drying, fluidized bed drying, and spray drying. In the preferred method where zirconyl acetate is used as the heavy metal precursor, spray drying is used with a 120.degree. C. outlet temperature. Such a process removes water and acetic acid.

The intermediate particles are preferably made up of small, substantially spherical particles and hollow spheres. They are calcined to further remove residual organics to yield clusters. The calcining step increases the brittleness of the particles. In general, brittle particles tend to be easier to reduce in size. During calcining, the soak temperature is preferably 200.degree. C. to 800.degree. C., more preferably 300.degree. C. to 600.degree. C. Soaking is performed for 0.5 to 8 hours, depending on the amount of material being calcined. It is preferred that the soak time of the calcine step be such that a plateaued surface area is obtained. It is preferred that the time and temperature be chosen such that the resulting clusters are white in color, free from black, grey, or amber colored particles, as determined by visual inspection.

The calcined material is then preferably milled to a median particle size of less than 5 microns, preferably less than 2 microns (on a volumetric basis), as determined using a Sedigraph 5100 (Micrometrics, Norcross, Ga.). The particle size determination is performed by first obtaining the specific density of the filler using an Accupyc 1330 Pycometer (Micrometrics, Norcross, Ga.) described in the Test Methods. Milling can be accomplished by various methods including for example, stirred milling, vibratory milling, fluid energy milling, jet milling and ball milling. Ball milling is the preferred method.

The second type of filler particles is the nano-sized particles, which can be non-heavy metal oxide particles and/or heavy metal oxide particles. Suitable and useful materials for non-heavy metal oxide nano-sized particles and heavy-metal oxide particles can, and preferably are, the ones described above for the clus