Prepolymerized filler in dental restorative composite

(54) PREPOLYMERIZED FILLER IN DENTAL
RESTORATIVE COMPOSITE

(75) Inventors: Christos Angeletakis, Orange, CA
(US); Minh-Dang Son Nguyen,
Orange, CA (US); Alvin I.
Kobashigawa, Fountain Valley, CA
(US)

(73) Assignee: Kerr Corporation, Orange, CA (US)

( * ) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.S.C. 154(b) by 388 days.

This patent is subject to a terminal disclaimer.

(21) Appl. No.: 10/234,594

(22) Filed: Sep. 4, 2002

(65) Prior Publication Data

US 2003/0032693 Al Feb. 13, 2003

Related U.S. Application Data

(63) Continuation-in-part of application No. 09/859,106, filed on May 16, 2001, now Pat. No. 6,593,395.

(51) Int. CI.7 A61K 6/083; C08K 3/22;

C08K 3/34; A61C 13/08

(52) U.S. CI 523/115; 523/113; 523/116;

523/118; 523/217; 106/35; 433/228.1

(58) Field of Search 523/115, 113,

523/116, 118, 217; 106/35; 433/228.1

(56) References Cited

U.S. PATENT DOCUMENTS

4,389,497 A 6/1983 Schmitt et al 523/116

4,503,169 A 3/1985 Randkiev 523/117

4,781,940 A 11/1988 Denton, Jr 427/2

5,192,815 A 3/1993 Okada et al 523/115

5,460,701 A 10/1995 Parker et al 204/164

5,514,349 A 5/1996 Parker et al 422/186.21

5,609,675 A 3/1997 Noritake et al 106/35

5,610,712 A 3/1997 Schmitz et al 356/335

5,788,738 A 8/1998 Pirzada et al 75/331

5,851,507 A 12/1998 Pirzada et al 423/659

5,874,684 A 2/1999 Parker et al 75/228

5,936,006 A 8/1999 Rheinberger et al 523/116

5,979,805 A 11/1999 Angeletakis 241/21

5,984,997 A 11/1999 Bickmore et al 75/343

6,010,085 A 1/2000 Angeletakis 241/21

6,020,395 A 2/2000 Angeletakis 523/116

6,098,906 A 8/2000 Angeletakis 241/21

6,121,344 A 9/2000 Angeletakis et al 523/116

6,127,450 A 10/2000 Angeletakis 523/116

6,194,481 Bl 2/2001 Furman et al 522/77

6,196,843 Bl 3/2001 Kawaguchi et al 433/212.1

6,232,367 Bl 5/2001 Kobashigawa et al 523/116

6,300,390 Bl 10/2001 Angeletakis 523/116

6,306,927 Bl 10/2001 Blackwell et al 523/116

6,359,090 Bl 3/2002 Angeletakis 526/277

6,572,693 Bl * 6/2003 Wu et al 106/35

6,593,395 B2 * 7/2003 Angeletakis et al 523/115

FOREIGN PATENT DOCUMENTS

EP 0368657 5/1990

EP 1 050 291 A2 11/2000

EP 1 234 567 A2 8/2002

WO WO 99/65453 12/1999

WO WO 00/61073 10/2000

WO WO 01/26611 4/2001

WO WO 01/30304 5/2001

WO WO 01/30305 5/2001

WO WO 01/30306 5/2001

WO WO 01/30307 5/2001

* cited by examiner

Primary Examiner—Tae H. Yoon

(74) Attorney, Agent, or Firm—-Wood, Herron & Evans, LLP

(57) ABSTRACT

A dental composite having high filler loading and low shrinkage thereby providing good strength and marginal integrity. Through the use of prepolymerized filler particles in combination with a structural filler and a nanofiller, the composite exhibits very low shrinkage and is useful in stress bearing restorations and in cosmetic restorations. The structural filler used is ground to a mean particle size of less than 0.5 fim and the nanofiller has discrete particles of a mean particle size less than 100 nm to improve handling and mechanical characteristics. The prepolymerized filler particles also allow for less viscosity and better handling characteristics of the composite.

49 Claims, 1 Drawing Sheet

1

PREPOLYMERIZED FILLER IN DENTAL
RESTORATIVE COMPOSITE

RELATED APPLICATIONS

The present application is a continuation-in-part of application Ser. No. 09/859,106, filed May 16, 2001, now U.S. Pat. No. 6,593,395 which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to composite resin materials for use in dental restorations, and more particularly to a universal composite resin material incorporating a structural filler, a prepolymerized filler, and a discrete nanoparticle sized filler to provide a composite of high strength, improved loading and reduced shrinkage in clinical use.

BACKGROUND OF THE INVENTION

In dentistry, practitioners use a variety of restorative materials, for example 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 having a mean particle size of about 0.05 fim or less, with a filler loading of about 30%-45% by volume. However, loading of the dispersion-reinforcing filler into the resin is limited by the ability of the resin to wet the filler due to the small particle size and high surface area of this type of reinforcing 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.

Microfills generally use prepolymerized filler material for dispersion reinforcement, as described in U.S. Pat. Nos. 4,389,497,4,781,940 and 6,020,395. Generally, prepolymerized filler is produced by mixing finely divided silica with a polymerizable monomer, heat polymerizing the mixture in bulk, and pulverizing or grounding the mixture down to the desired agglomerate size to give a filler material comprising splintered polymerized particles. Polymerized particle sizes are large, generally greater than 1 fim in diameter, allowing for better reinforcement but a less stable, less glossy surface. The polymerized particles or prepolymerized filler material is then mixed with a polymerizable monomer, typically an acrylate or methacrylate-based resin, and an additional filler material, such as colloidal or fumed silica, to form the final microfill dental composite.

Particulate reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 fim and a filler loading of about 60% by volume.

2

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

5 themselves. These particulate reinforced composite resins are stronger than microfill resins. As with the dispersionreinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate reinforced composites has a greater impact on the total

10 strength of the composite, such that the particulate reinforced composites have been used for stress bearing restorations.

Another class of dental composites, known as hybrid composites, include the features and advantages of disper

15 sion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 fim or greater with a microfiller having an average particle size of about 0.05 fim or less. HERCULITE® XRV (Kerr Corp.) is one such example. HERCU

20 LITE® has been considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 fim 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

25 this filler exceeds 1.50 fim 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.

30 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 (about 0.5 fim) to produce

35 a stable glossy surface. U.S. Pat. No. 6,121,344, which is incorporated by reference herein in its entirety, describes a resin-containing dental composite including a structural filler of ground particles having an average particle size of between about 0.05 fim and about 0.5 fim that has the high

40 strength required for load-bearing 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 ground

45 structural filler particles having an average particle size less than the wavelength of light, the dental composite exhibits the luster and translucency required for cosmetic restorations.

In U.S. Pat. No. 6,121,344, fumed silica having an aver

50 age particle size less than about 0.05 fim 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

55 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 silica is produced by hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. During this

60 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 entangle

65 ment occurs upon cooling to give agglomerates. Attractive interactions between 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 it compatible with resins employed, however, treatment is usually not complete and residual unreacted silanol groups typically remain, resulting in substantial interactions of these groups with other reactive 5 groups in the composite. The particle-particle interaction prevents homogenous dispersion of the microfiller in the resin matrix and increases the viscosity of the suspension, which correspondingly decreases the workability of the composite paste. This places a limitation on the practical 10 filler loading in fumed silica microfilled restorative composites. A high filler loading is desirable in dental restorations because the high loading provides a paste with improved handling properties over a paste with low filler loading. Moreover, higher loading gives a composite that 15 experiences lower shrinkage upon curing and has a coefficient of thermal expansion better matching that of a natural tooth.

Resin shrinkage upon polymerization, however, is a problem that has faced composites of the prior art incorporating 20 dispersion reinforced, particulate reinforced and hybrid filler materials. The resin matrix shrinks upon polymerization during the curing process. Polymerization shrinkage, both axial and volumetric, generally results from the conversion of the carbon-carbon double bonds of low molecular weight 25 monomers in the polymeric composite to corresponding carbon-carbon single bonds of crosslinked polymers during the curing reaction. Such shrinkage tends to cause gap formation between the restorative composite and the tooth, leading to microleakage, secondary caries and decreased 3Q longevity of the repair.

There is thus a need to develop a hybrid dental restorative composite that has a physical make-up to afford high filler loading, appropriate viscosity for good workability of the composite paste, and lower shrinkage during polymeriza- 3J tion.

SUMMARY OF THE INVENTION

The present invention provides resin-containing dental composites that have high filler loading and reduced shrinkage while being mechanically strong and effective for stress 40 bearing and cosmetic restorations without exhibiting the drawbacks of the prior art dental composites. To this end, a resin-containing dental composite is provided which includes a structural filler of ground particles having an average particle size of between about 0.05 fim and about 0.5 45 fim, a prepolymerized filler, and a nanofiller having discrete, non-agglomerated particles of mean particle size less than about 100 nm. Also provided is a method for making a dental composite paste and a method for restoring a tooth using said paste. 50

The structural filler comprises between about 10% and about 70% by volume of the composite, and advantageously between about 20% and about 60% by volume, and because the structural filler is ground, the particles are non-spherical, providing increased adhesion of the resin to the structural 5J filler thereby enhancing the overall strength of the composite. Further, by virtue of the particles having a mean particle size less than the wavelength of light, that is less than about 0.5 fim, the structural filler contributes to the luster and translucency of the composite required for cosmetic restorations. In an exemplary embodiment, the ground structure 60 filler contains less than 50% by volume of particles above 0.5 fim in diameter.

The discrete, non-agglomerated nanofiller particles comprise at least about 0.01% by volume of the composite, and advantageously between about 1% and about 15% by 65 volume, and contribute to dispersion reinforcement, fill the interstices between the larger structural filler particles reduc

ing occluded volume, and provide a large surface area to be wetted by the resin to increase strength. In addition, particleparticle interactions are minimized, thereby allowing for high filler loading and lower shrinkage upon curing.

The prepolymerized filler comprises at least about 1% by volume of the composite, and advantageously about 1% to about 60% by volume, and allows for an increased total filler loading in the composite by virtue of the low surface area of the prepolymerized filler relative to the other fillers. The prepolymerized filler particles are prepared by mixing an inorganic filler with an organic polymerizable resin and curing the mixture. The cured mixture is then ground to a desired size, for example a bimodal particle size distribution including a fine particle fraction of mean particle size in the range of about 1 fim to about 3 fim and a coarse particle fraction of mean particle size in the range of about 30 fim to about 70 fim, at least about 95% of the particles of size less than about 100 fim. The ground bimodal prepolymerized filler may be further air classified to separate the fine particle fraction from the coarse particle fraction to provide the prepolymerized filler with a coarse particle size distribution including a mean particle size in the range of about 30 fim to about 70 fim, at least about 90% of the particles of size less than about 100 fim, and at least about 85% of the particles of a size greater than about 10 fim. The prepolymerized filler enhances the polydispersity of the composite paste, thereby enabling the higher filler loading and improving the handling properties of the paste. The non-spherical surface of the ground particles provides additional adhesion between the prepolymerized filler and the resin, thereby further strengthening the cured composite. Further, a reduction in shrinkage upon curing of the composite is provided by virtue of inclusion of the prepolymerized filler, thereby increasing the tooth-composite marginal integrity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1 and 2 are particle size distributions for two exemplary prepolymerized fillers used in composites of the present invention, each including an unclassified bimodal filler and a classified coarse filler.

DETAILED DESCRIPTION OF THE
INVENTION

The present invention provides a dental restorative composite comprising a ground structural filler having a mean particle size between about 0.05 fim and about 0.50 fim, a prepolymerized filler, and a discrete nanofiller having a mean particle size less than about 100 nm in a curable resin, such as a resin containing polymerizable methacrylate monomers. Generally, the resinous composite is cured by mixing two paste components containing a catalyst and an accelerator, respectively, or by a photopolymerization process wherein the resins are exposed to actinic radiation, such as blue visible light. Photopolymerizable resins containing monomers other than methacrylates, such as cationically photocurable oxiranes, for example, may be used in the present invention as persons of ordinary skill in the art would appreciate. The resinous dental composite is generally applied to one or more teeth by the dental practitioner and cured by exposure to visible light. The composite cures with a low shrinkage, such as about 2% or less, which is a significant improvement over prior hybrid composites.

To provide the ground structural filler having a mean particle size of less than 0.5 fim, an extensive comminution step is required. Comminution may be performed in an agitator mill, and advantageously an agitator mill designed