Dental composition containing discrete nanoparticles

Details Dental composition containing discrete nanoparticles Dental composition containing discrete nanoparticles

2001-05-16

2003-07-15

Yoon, Tae H. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S113000, C523S116000, C523S118000, C524S430000, C524S444000, C433S202100, C433S226000

Reexamination Certificate

active

06593395

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally related to a composite resin material used for dental restoration, and more particularly to a universal composite resin material suitable for all dental restorations incorporating a uniformly dispersed nanometer sized discrete particulate filler which provides high strength, improved wear resistance and gloss retention in clinical use.
BACKGROUND OF THE INVENTION
In dentistry, practitioners use a variety of restorative materials in order to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a resin matrix. These materials may be dispersion reinforced, particulate reinforced, or hybrid composites.
Dispersion reinforced composites include a reinforcing filler of, for example, fumed silica having a mean particle size of about 0.05 &mgr;m or less, with a filler loading of about 30%-45% by volume. Because of the small particle size and high surface area of the filler, the filler loading into the resin is limited by the ability of the resin to wet the filler. Consequently, the filler loading is limited to about 45% by volume. Due to the low loading, the filler particles are not substantially in contact with one another. Thus, the primary reinforcing mechanism of such dispersion reinforced composites is by dislocation of flaws in the matrix around the filler. In dispersion reinforced materials, the strength of the resin matrix contributes significantly to the total strength of the composite. In dentistry, dispersion reinforced composite resins or microfills are typically used for cosmetic restorations due to their ability to retain surface luster. Typically, these microfill resins use free radical-polymerizable resins such as methacrylate monomers, which, after polymerization, are much weaker than the dispersed filler. Despite the dispersion reinforcement, microfill resins are structurally weak, limiting their use to low stress restorations.
One example of a dispersion reinforced composite is HELIOMOLAR®, which is a dental composite including fumed silica particles on the order of 0.05 &mgr;m mean particle size and rare earth fluoride particle on the order of less than 0.2 &mgr;m mean particle size. HELIOMOLAR® is a radiopaque microfill-type composite. The rare earth fluoride particles contribute to both flexural strength and radiopacity.
Particulate reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 &mgr;m and a filler loading of about 60% by volume. At these high filler loadings, the filler particles begin to contact one another and contribute substantially to the reinforcing mechanism due to the interaction of the particles with one another and to interruption of flaws by the particles themselves. These particulate reinforced composite resins are stronger than microfill resins. As with the dispersion reinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate reinforced composites has a greater impact on the total strength of the composite. Therefore, particulate reinforced composites are typically used for stress bearing restorations.
Another class of dental composites, known as hybrid composites, include the features and advantages of dispersion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 &mgr;m or greater with a microfiller having an average particle size of about 0.05 &mgr;m or less. HERCULITE® XRV (Kerr Corp.) is one such example. HERCULITE® is considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 &mgr;m and a filler loading of 57.5% by volume. The filler is produced by a wet milling process that produces fine particles that are substantially contaminant free. About 10% of this filler exceeds 1.50 &mgr;m in average particle size. In clinical use, the surface of HERCULITE® turns to a semi-glossy matte finish over time. Because of this, the restoration may become distinguishable from normal tooth structure when dry, which is not desirable for a cosmetic restoration.
Another class of composites, flowable composites, typically have a volume fraction of structural filler of about 10% to about 30% by volume. These flowable composites are mainly used in low viscosity applications to obtain good adaptation and to prevent the formation of gaps during the filling of a cavity.
Various methods of forming submicron particles, such as precipitation or sol gel methods, are available to produce particulate reinforcing fillers for hybrid composites. However, these methods do not restrict the particle size to at or below the wavelength of light to produce a stable glossy surface. In U.S. Pat. No. 6,121,344, which is incorporated by reference herein in its entirety, a resin-containing dental composite is described including a structural filler of ground particles having an average particle size of between about 0.05 &mgr;m and about 0.5 &mgr;m that has the high strength required for load-bearing restorations, yet maintains a glossy appearance in clinical use required for cosmetic restorations. Because the structural filler particles are ground, the particles are nonspherical, providing increased adhesion of the resin to the structural filler, thereby further enhancing the overall strength of the composite. Through the use of structural filler particles that are ground and that have an average particle size less than the wavelength of light, that is less than about 0.50 &mgr;m, the dental composite exhibits the luster and translucency required for cosmetic restorations. Specifically, because the structural filler size is less than the wavelength of visible light, the surface of a dental restoration will reflect more light in some directions than in others even after wear of the composite by brushing. The visible light waves do not substantially interact with the structural filler particles protruding out of the surface of the composite, and therefore, haze is reduced and the luster of the surface is maintained even after substantial brushing. The particles are still large enough to reinforce the composite by the particulate reinforcement mechanism, so the restorations are also stress bearing. The number of larger particles, above 0.5 &mgr;m in diameter, are also minimized to help produce the stable glossy surface.
In U.S. Pat. No. 6,121,344, fumed silica microfill particles having an average particle size less than about 0.05 &mgr;m are added, preferably between about 1% by weight and about 15% by weight of the composite. The microfill particles contribute to dispersion reinforcement, fill the interstices between the larger structural filler particles reducing occluded volume, and provide a large surface area to be wetted by the resin to increase strength. The fumed silica microfill particles also contribute to the flow properties of the uncured resin. Fumed silicas are produced by hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. During this process, silicon dioxide molecules condense to form particles of size usually less than 50 nm. The particles then attach to each other and sinter together. Due to the nature of the flame process, a three-dimensional chain aggregate with a length of 200-300 nm forms. Further mechanical entanglement occurs upon cooling to give agglomerates. Attractive interactions between the surface silanol groups of the particles give thixotropic properties to liquids in which these fumed silicas are suspended. The fumed silicas are hydrophobically treated to make them compatible with resins employed, but still substantial interactions result from attractive interactions of the residual silanol groups that are not reacted. The particle-particle interaction prevents homogenous dispersion of the microfiller in the resin matrix and increases the viscosity of the suspension, which cor

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