State of the Art and Organic Components of Current Composite Restoratives
Composites have been used as direct restorative materials to replace missing biological tissue for more than 40 years. The term “composite” subsumes materials, which are composed of at least two different phases. In the case of resin composites, these include the polymerizable organic matrix and fillers (Scheme 21). The organic matrix of a photopolymerizable composite restorative, to which the filler particles can be covalently bonded, is based on a mixture of polymerizable monomers or oligomers, the photoinitiator system, and further additives such as stabilizers, colorants, or optical brighteners. Nearly all current composite restoratives are hybrids, that is, they contain at least two different fillers, for example, of different shape, chemical composition, or size. The fillers are surface-modified with suitable coupling agents to improve the linkage between the fillers and the organic matrix. Resin composites are used for a variety of applications in dentistry, including direct restorations and indirect restorations, such as inlays or onlays, crowns, bridges, veneers as well as cements, fissure sealants, or cavity liners. The different types of dental composite materials are distinguished mainly by their viscosity, the type of fillers used, and the applied type of resin (Scheme 22).94,95 Packable composites exhibit a higher viscosity, are condensable like amalgam, and enable the formation of tight interproximal contacts. Packable composites were developed to limit the polymerization shrinkage, wear, and fracture of the restoration within the body and at the margins of the restorations. Compared to the universal composite restoratives, which can be placed with a common dental syringe, flowable composites show a lower viscosity. Thus, flowable composites can be used to penetrate small pits and grooves in teeth avoiding the need to remove additional tooth structure. Fluoride containing, flowable composites are ideally suited as preventive sealants, especially for children. The viscosity or consistency of a composite can be reduced by the reduction of the filler content, the increase of the average filler particle size, and the reduction of the viscosity of the organic resin. Because of the major influence of the fillers on the physical properties, the frequently used classification of dental composites is based on the type and the particle size of the reinforcing fillers. Subsequently, resin composites can be classified into fiber-reinforced composites (FRC) and particulate filler composites. FRCs, which can be divided based on the fiber length in long-fiber and short-fiber composites or according to the fiber orientation in composites with continuous unidirectional and bidirectional fibers. In clinical dentistry, FRC technology may solve some problems associated with a metal alloy substructure, such as corrosion or toxicity.96 However, FRCs are not used as direct restoratives, which are generally based on particulate fillers. According to the particle size, the particulate fillers can be divided into macrofillers (10–100 µm, used in the first composite restoratives), midifillers (1–10 µm), minifillers (0.2–1 µm), and nanofillers. Microfill composites contain fillers with a primary particle size between 5 and 100 nm, which have to be called nanofillers instead of microfillers.95 To enhance the handling properties and achieve a higher filler load, heterogeneous microfill composites may contain prepolymerized resin fillers that are based on a homogeneous microfill material. Microfill composites are generally weak due to their relatively low filler content. They were developed to satisfy the need for esthetic, more polishable composites. Therefore, the presently used universal composites, which can be used for most anterior and posterior applications, are hybrids based on a mixture of minifillers and nanofillers and produce an optimal combination of adequate mechanical strength with sufficient polishability. According to the particle size of the fillers, the hybrids can be further distinguished in microhybrids and nanohybrids. Microhybrids are based on a mixture of minifillers, whereas nanohybrids contain only nanoscale particles or a dominant proportion of nanoparticles.
Restorative composites have to meet extensive requirements, which include physical, chemical, processing, toxicological, and clinical requirements. Excellent properties of composite restoratives can be obtained by the optimum choice of the fillers, their suitable surface modification, and various components in the organic matrix. Thereby, each component influences the composite properties in a different manner. The curing rate, light sensitivity, polymerization shrinkage, color stability, water uptake, or storage stability are mainly influenced by the organic matrix (Scheme 23). Typically filler-determined properties are wear resistance, hardness, radiopacity, gloss, polishability, or the release of fluoride ions. Moreover, the flexural strength, flexural modulus of elasticity, consistency, translucency, thermal expansion coefficient, water solubility, or biocompatibility is dependent on the chemical composition of both the organic matrix and the fillers. Finally, for the long-term stability or durability of the composites, the interphase between the matrix and the fillers is very important. The clinical success of a light-cured direct composite restoration also depends strongly on additional factors, such as an optimal preparation or design of the cavity, a good bonding to the adhesive layer, the optimum placing in layers/increments, and sufficient curing of the composite.
Most of the currently used commercial direct composite restoratives or composite cements are based on a mixture of conventional dimethacrylates.97,98 The most frequently used dimethacrylates are BisGMA and UDMA (Scheme 6), which show a relatively low polymerization shrinkage of 6.1 and 6.7 vol %, a high reactivity in the free-radical polymerization, and form polymer networks with excellent mechanical properties. Because of their high viscosity, they have to be diluted with dimethacrylates of lower viscosity, such as TEGDMA (Scheme 6). Because of the relative high water solubility and high cytotoxicity of TEGDMA,7 more hydrophobic diluents, such as decanediol dimethacrylate (D3MA) or bis-(3-methacryloyloxymethyl)tricyclo-[5.2.1.02,6]decane (TCDMA, Scheme 24) are also used. The selection of appropriate dimethacrylate cross-linkers for the formulation of a composite strongly influences the reactivity, viscosity, and polymerization shrinkage of the composite paste as well as the mechanical properties, water uptake, and swelling of the cured composite. Alternatives to BisGMA or UDMA have been used. The ethoxylated (BisEMA), propoxylated bisphenol A dimethacrylate (BisPMA), and 1,3-bis[1-(2-methacryloyloxypropoxycarbonylamino)-1-methyl-ethyl]benzene (TMXDMA) are three examples shown in Scheme 24. In this context, it can also be mentioned that the partially aromatic urethane dimethacrylate TMXDMA showed similar reactivity, flexural strength, flexural modulus of elasticity, polymerization shrinkage, and water sorption to materials based on BisGMA.99 Trimethacrylates such as trimethylolpropane trimethacrylate (TMPTMA) are rarely used in composite restorative materials, as they entail low double bond conversion. In contrast to this, monomethacrylates improve the double bond conversion; however, they may result in an increase of residual monomer, a decrease of cross-linking density, and a corresponding decrease of the modulus of elasticity of the cured materials.
Beside the conventional dimethacrylates, acidic carboxylic groups containing dimethacrylates are use in so-called compomers. The term is a combination of the terms composite and glassionomer. These water-free, single-component, light-cured composites are based on carboxylic groups containing dimethacrylates, for example, TBCDMA or CTCDMA (Scheme 25), which can be easily synthesized by the reaction of HEMA with the anhydride of the corresponding polyfunctional carboxylic acid, such as butane-1,2,3,4-tetracarboxylic acid dianhydride (TBCDMA) or 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohex-3-ene-1,2-dicarboxylic acid anhydride (CTCDMA). The compomers are reinforced with strontium or barium alumofluorosilicate glass particles. In the presence of water, the acid groups ionize and undergo an acid–base reaction with the basic glass components and potentially show anticariogenic properties due to fluoride release. Nevertheless, conventional hybrid composites show better physical properties compared to compomers. Therefore, compomers are mainly used in cervical class V restorations or for the restoration of decayed deciduous teeth. Another group of acidic monomers containing composites are self-adhesive composite resin cements. Presently, there are more than 15 self-adhesive composite resin cements on the market, which are based on a mixture of usual dimethacrylate cross-linkers and acid-functionalized monomers.98 For this purpose, not only the strongly acidic adhesive monomers, such as the phosphate methacrylates MDP, GDMP, or PENTA-P (Scheme 5) but also the less acidic 4-methacryloyloxyethyl trimellitic anhydride are used. These self-adhesive composite cements adhere to the tooth structure without the application of a separate adhesive or etchant and have the major benefit to simplify the adhesive restorative therapy. Until now, clinical evaluations are few and short term, so long-term conclusions about the performance of these cements in the dental practice are not yet possible.
A few restorative composites are on the market, which are based on organic–inorganic hybrid materials. These materials are called “ormocers” (organically modified ceramics) and the resins are polycondensates prepared by hydrolytic condensation of suitable polymerizable trialkoxysilanes (Scheme 25).100 These hybrid materials may show interesting new properties in relation to each individual component and can be fine-tuned by adjusting the composition. The main reason for incorporating inorganic units in the organic matrix is to increase the biocompatibility and wear resistance of the dental composites. The first commercialized ormocer restorative composite was based on the silane UDMASi, which was prepared by the reaction of GDMA and 2-isocyanatopropyltriethoxysilane. For the composite production, the silane was hydrolytically condensed, resulting in a liquid UDMASi-polycondensate, the ormocer. The viscosity of the UDMASi-polycondensate was very high and therefore during the composite manufacturing a certain amount of a common dimethacrylate diluent had to be added. Also in case of the polycondensates of 1,3-bis(methacryloyl)-2-(3-trimethoxysilyl)propoxy)propane, TEGDMA, which shows a relatively low biocompatibility, was used as diluent.101 In contrast to this, we prepared dimethacrylate-diluent-free composites with an ormocer, which was a polycondensate of the amide dimethacrylate trimethoxysilane ADMASi (Scheme 25).102,103 It was found that the viscosity of the obtained polycondensates was predominantly influenced by the kind of the organic spacer between the polymerizable groups and the silicon atom. The formed structure depended both on the type of catalyst used for silane hydrolysis/condensation and the bulkiness of the organic substituent at the silicon atom. Based on the low cytotoxicity of the synthesized ormocer, the prepared restorative composite showed good biocompatibility.
The fourth type of resins in presently used dental resin composites contains cationically ring-opening polymerizable cycloaliphatic epoxides. The resin is a mixture of the diepoxides bis[2-(7-oxabicyclo[4.1.0]hept-3-yl)ethyl]methylphenylsilane 1 (BECEPSi, Scheme 26) and the tetrafunctional cyclosiloxane epoxide 2,4,6,8-tetrakis[2-(7-oxabicyclo[4.1.0]hept-3-yl)ethyl]-2,4,6,8-tetramethyl-1,3,5,7-tetraoxa-2,4,6,8-tetrasilacyclooctane (TECTMSi). In addition, it was found that the spiro monomer 3,9-diethyl-3,9-bis(trimethylsilylpropoxymethyl)-1,5,7,11-tetraoxaspiro[5.5]undecane (TOSU) may significantly reduce the polymerization stress of these epoxide mixture.104–107 For the main monomer TECTMSi, the term “silorane” was introduced to represent hybrid monomer systems that contain both siloxane and oxirane structural units. The main benefits of these resins are the very low polymerization shrinkage, for example, 2.0 vol % in case of TECTMSi, the good biocompatibility, the very low water solubility of the monomers, and low water sorption of the formed polymer networks. Thus, not only the cytotoxicity rating of TECTMSi is as good or better compared, for example, to BisGMA, but it also appears to be nonmutagenic.108,109 Furthermore, it should be mentioned that polymerizable silsesquioxanes, which were synthesized by hydrosilylation first of 4-vinyl-cyclohexene epoxide and then of di(propylene glycol)allyl ether methacrylate with octakis(dimethylsiloxy)silsequioxane, were proposed as useful components for low-shrinkage composites.110 Although the silorane-based restorative composite was introduced in the dental market in 2007, it could not attain broad market acceptance until now. A number of reasons are responsible for this situation. The establishment of the mechanical properties during the light-curing of the silorane composite takes more time and the exothermic effect is twice as high compared to dimethacrylate-based composites with similar filler load. Furthermore, siloxane-based epoxides have a significantly lower refractive index compared to the commonly used radiopaque fillers, which affects the transparency and thereby the attainable curing depth of the composite. Because of the increased exothermic effect and the limited transparency, the very attractive application of a restorative composite in thick layers or in bulk is difficult to achieve. Moreover, the storage stability of the cycloaliphatic epoxide-based composites is significantly shorter compared to that of dimethacrylate-based composites. Finally, results of the long-term performance of silorane composites are not available and the composites have to be used with a special enamel–dentin adhesive. The adhesive system was a methacrylate-based, two-step self-etch system and resulted in a high degree of double bond conversion in the formed hybrid layer.111
Dental composite restoratives based on difunctional or polyfunctional methacrylates (conventional, acid-modified, or inorganic polycondensates) harden by free-radical photopolymerization. As mentioned in the previous chapter, the most widely used photoinitiator is the mixture of CQ with tertiary amines, for example, EMBO.29 In contrast to adhesives, special topics for photocured composites are the depth of cure and the bleaching behavior, particularly for highly esthetic composite applications. The depth of cure of a resin-based composite strongly depends on the transparency and the shade of the composite; darker shades may require extended irradiation. Additionally, the depth of cure is influenced by the concentration, the absorption behavior, and reactivity of the applied photoinitiator system. Further parameters are the irradiation time, light intensity, emission spectrum of the curing light, and the distance of the light probe to the restoration. The transparency of a resin composite again is mainly influenced by the refractive index of the used monomers and fillers, the particle size, shape and content of each filler, and the color and the content of pigments. In case of nanosized fillers, the agglomeration of the nanoparticles may significantly impair the transparency of the composite. A method to improve the depth of cure, bleaching behaviour, and color stability of the composite restoratives is the combination of CQ-amine with other photoinitiators, for example, the acylphosphine oxide APO, the bisacylphosphine oxide BAPO (Scheme 9), or 1-phenyl-propandione-1,2 (PPD). In the silorane composite, a mixture of CQ, EMBO, and a diphenyl-iodonium salt is used as photoinitiator.103
Fillers in Current Composite Restoratives
The particulate fillers used in dental composites directly influence properties such as radiopacity, abrasion resistance, intrinsic surface roughness, flexural modulus, coefficient of thermal expansion, and translucency. Composite restoratives require filler materials that can adjust the mechanical properties, color, translucency, and so forth, of teeth. The fillers are characterized by their shape, morphology, average particle size and corresponding surface, particle size distribution, chemical composition, density, and refractive index. As mentioned before, the particulate fillers can be classified according to the particle size into macrofillers, midifillers, minifillers, and nanofillers (Scheme 23). In principle, the particle size can be realized by either a top-down manufacturing approach, for example, a conventional milling process of glass, or bottom-up approach, for example, new sol-gel routes or flame-spray pyrolysis (FSP) starting from molecular precursors. The size of the filler particles incorporated in the resin matrix of commercial composites has continuously decreased over the last 10–15 years from the traditional composites to the nanohybrids. Many modern dental composites use fillers listed in Table 1. The scanning electron microscopic (SEM) picture of some fillers, which we use in our current direct composite restoratives, is shown in Fig. 3. Typical midifillers and minifillers are purely inorganic, usually splinter-shaped, and prepared by grinding larger particles of radiopaque glass, quartz, or ceramics into smaller particles. Today, they have an average particle size between 0.2 and 5.0 µm and a specific surface between 2 and 25 m2 g−1. The glass fillers are made from melts of different mixtures of SiO2, Al2O3, B2O3, BaO, SrO, CaO, Na2O, and, if necessary, additional compounds. Thereby, elements such as barium or strontium contribute to a high radiopacity (300–700% Al) and a high refractive index (1.50–1.55) of the glass fillers. Radiopacity, the visibility of a filling in X-rays, is an important clinical requirement for direct composite restoratives. This requirement can be very efficiently fulfilled by the use of ytterbium trifluoride (YbF3), which we widely use as radiopaque filler in our composites. A benefit of this filler is that with YbF3 radiopaque composites can be prepared, which additionally release fluoride ions. Moreover, fluorosilicate glasses or sparingly soluble fluoride salts are also added to composites for fluoride release. Because of the milling process, the ground glasses have a relatively broad particle size distribution. The abrasion resistance of the cured composite is strongly influenced by the particle size and form of the fillers. In the case of ground glasses, the mean particle size is less crucial than the maximum particle size.
Figure 3. SEM pictures of different fillers used in current dental composites: a: Ba-silicate glass (0.4 µm), b: Ba-silicate glass (0.7 µm), c: spherical Zr/Si-oxide, d: pyrogenic silica, e: ytterbium trifluoride, and f: prepolymer filler.
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Table 1. Types of Fillers and Filler Size Used in Currently Dental Composites
Pyrogenic silica is an important representative of nanofillers. The nanoparticles are frequently spherical in shape. The most commonly used pyrogenic silica shows a primary particle size of 40 nm and a specific surface of about 50 m2 g−1. Because of the large specific surface area of the nanofiller particles, they have a pronounced thickening effect. Often, the primary particles of nanofillers are agglomerated and thus the thickening effect is reduced or can be reduced by special thermal treatments. However, the formed agglomerates influence the transparency of the composite. Another possibility to attain maximum inorganic filler loading with nanofillers is to incorporate the nanofillers, for example, pyrogenic silica, into an organic resin matrix, heat-cure the mixture, and mill the formed composite material to the desired particle size of 10–100 µm (Table 1). With these larger prepolymerized filler particles, a higher filler content can be achieved, which reduces the polymerization shrinkage. Furthermore the density of the prepolymerized filler particles is significantly lower than that of conventional pure inorganic fillers. Therefore, composites containing prepolymerized fillers exhibit less polymerization shrinkage than composites composed of inorganic filler with the same wt % content of monomers.
Tailor-made nanofillers can be prepared via the sol-gel route, starting from tetraalkyl orthosilicates or metal alkoxides such as titanium(IV) and zirconium(IV) ethoxide or mixtures thereof. Sol-gel nanofillers mostly consist of spherical primary particles and particles with an average particle size less than approximately 40 nm and of very narrow particle size distribution. The particle size distribution of such nanoparticle fillers is frequently monodisperse. However, metal alkoxides are relatively expensive, their hydrolytic condensation is accompanied by large volume shrinkage, and the sol-gel process is difficult to scale-up. Therefore, one interesting alternative approach to sol-gel or pyrogenic nanofillers is the use of so-called organosols.100 Organosols are stable colloidal dispersions of inorganic nanoparticles in organic media (solvents or monomers). Organosols may contain discrete almost nonagglomerated nanoparticles with a very narrow particle size distribution. For example, transparent organosols of silica particles with a size of 10–30 nm are commercially available, which are prepared starting from silicic acid in water under special conditions. The water of the resulting aqueous silica sol is subsequently replaced by water-soluble solvents, such as alcohols. Then the solvent can be replaced by monomers, for example, HEMA or dimethacrylates. Before the silica nanoparticles are incorporated into the polymerizable monomers, they are thoroughly surface silanized with a polymerizable silane, for example, 3-methacryloyloxypropyltrimethoxysilan (MTMSi) to stabilize the particles in the dispersion and to obtain a covalent bond between the filler and the matrix after curing the composite. Thus, we prepared different dental dimethacrylate monomer-based organosols and investigated their viscosity and transparency as a function of varying amounts of silica nanofillers with a particle size of 13 nm in diameter. Compared to dispersions of the usual pyrogenic silica, the silica organosols showed a significantly lower viscosity and a higher transparency. It was found that the capacity of the filler load is strongly dependent on the initial viscosity of the dimethacrylate, but the hydrophilicity and polarity of the monomer as well as an optimal surface modification of the SiO2 nanoparticles were also important. For example, in the case of TEGDMA, SiO2 organosols can be filled with about 50 wt % of nonagglomerated silica nanoparticles without showing an enormous increase in viscosity or any pseudoplastic behavior (Fig. 4). In contrast to this, a dispersion of pyrogenic silica in TEGDMA, which contains agglomerated SiO2 nanoparticles of the same primary particle size (12 nm), showed a significant increase of viscosity at a SiO2-content of only few wt %. Compared to TEGDMA, Bis-EMA is less polar and shows a higher viscosity. Therefore, Bis-EMA-based organosols exhibited a significant increase in viscosity already at a lower SiO2-content of about 30 wt %. Silica sols also enable the synthesis of tailor-made agglomerates. One example is the so-called nanoclusters, which were synthesized from a silica sol and a zirconyl salt.112 The formed zirconia-silica nanoparticles are not clusters in the sense of a chemical definition but are rather a specific kind of agglomerates. Nevertheless, the dental nanocomposites based on them showed high translucency, high polish, and polish retention similar to microfill composites while physical properties and wear resistance were equivalent to commercial hybrid composites. Silica nanofillers, which are prepared by usual pyrolysis of SiCl4 in the H2/O2-flame, contain mainly agglomerates or aggregates of nanoparticles and produce composites with limited transparency. Using the versatile FSP, we synthesized radiopaque nonporous mixed-oxide nanoparticles, for example, ytterbium oxide mixed silica (6–14 nm in diameter) or Ta2O5-containing SiO2 particles (6–10 nm).113,114 By controlling the FSP parameters, such as the precursor mixture, the solvent composition, or the liquid-to-gas ratio in the feed of the methane flame, it was possible to adjust both the composition, size of particles, and the refractive index of the nanofillers. Based on the synthesized nanoparticles, transparent nanocomposites with a high radiopacity were prepared. Finally, it should be mentioned that glass-ceramics with a wide range of compositions can also be prepared by sol-gel technique. For example, a ceramic filler with the composition SiO2:60-P2O5:3-Al2O3:14-CaO:6-Na2O:7-K2O:10 (wt %) was manufactured for dental application from an aqueous mixture of the precursors nitric acid, tetraethoxysilane, triethyl phosphate, aluminum nitrate nonahydrate, potassium nitrate, sodium nitrate, and calcium nitrate tetrahydrate.115
Figure 4. Shear viscosity of dispersed SiO2 particles (12 nm primary particle size) measured at a shear rate of 1 s−1 at 20°C: silica organosol in TEGDMA (♦) or Bis-EMA (▴). Dispersion of pyrogenic silica: TEGDMA (▪) or Bis-EMA (•).
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Before the composite paste preparation, the surface of the fillers has to be modified with a suitable coupling agent to ensure a strong and durable bond between the formed polymer network matrix and the filler particles. The filler treatment is very important for the long-term durability of the cured composite, in particular, for the mechanical properties, for example, tensile strength or fracture toughness, fatigue resistance, and wear resistance. In case of metal oxide nanoparticles, which typically show a large specific surface, the surface treatment also may result in a significant reduction of the thickening effect of the nanoparticles, which is caused by the attractive interaction of the large number of free MeOH groups (Me: Si, Zr, or Al) on the surface. Most of the usual dental fillers (Table 1) contain silica, which can be efficiently surface-treated with functionalized silanes, such as the already mentioned MTMSi. Furthermore, polymerizable dihydrogen phosphates or phosphonates can be efficiently used as coupling agent for ZrO2 or other metal oxides.116
The physical properties of dimethacrylate-based composite filling materials such as flexural strength, flexural modulus, or polymerization shrinkage mainly depend on the used filler types, particle size, and filler load (Table 2).95,97,117 Most of the current commercial composites meet the required flexural strength (≥80 MPa) for light-curing direct filling resins according to the ISO Standard 4049.118 Flowable microfill and nanohybrid composites show the poorest mechanical properties. Nevertheless, a direct comparison of the composite performance in a clinical situation based on selected properties is not possible. The improvement of the performance of composite restoratives using new components involves the following issues:94,95,119,120 (1) reduction of polymerization shrinkage and the contraction stress. (2) Enhancement of wear resistance, strength, and fracture toughness. (3) Improvement of the handling (less stickiness to instruments and less technique sensitivity) and shortening of the processing time (shorter curing, larger increments, or bulk placement). (4) Improvement of biocompatibility (low cytotoxicity and less elution of components) and the incorporation of antibacterial agents and pH controlling components.
Table 2. Correlation Between Composite Type, Filler Load, and Physical Properties of Dimethacrylate-Based Composites