Update on Dental Nanocomposites

Matrix

Most commercial dental composites use Bis-GMA monomer as their organic matrix. Other base monomers used in current commercial composites include triethyleneglycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), ethoxylated bisphenol-A-dimethacrylate (Bis-EMA), decanediol dimethacrylate (D₃MA), bis(methacryloyloxymethyl) tricyclodecane, and urethane tetramethacrylate (UTMA). The most commonly used organic matrix, Bis-GMA, has a very high viscosity due to the hydrogen bonding interactions that occur between the hydroxyl groups and the monomer molecules. Therefore, Bis-GMA must be diluted with more fluid monomers to provide the proper viscosity for use in dental composites (Ferracane, 1995). TEGDMA, which is less viscous and has excellent copolymerization characteristics, is frequently used as the diluent monomer for UDMA and Bis-GMA-based composites to provide a fluid resin that can be maximally filled with inorganic particles. TEGDMA increases vinyl double-bond conversion (Reed et al., 1997). Since UDMA and Bis-EMA have higher molecular weights and fewer double bonds per unit of weight, they generally have less shrinkage than TEGDMA. Therefore, TEGDMA has been replaced by UDMA and Bis-EMA in several products to reduce shrinkage, aging, and the negative effects of environmental factors such as moisture, acid, and temperature changes (Yap et al., 2000).

Fillers

Modern composite systems contain fillers such as quartz, colloidal silica, and silica glass containing barium, strontium, and zirconium. These fillers increase strength and modulus of elasticity and reduce polymerization shrinkage, the coefficient of thermal expansion, and water absorption.

Silane Coupling Agents

The formation of a strong covalent bond between inorganic fillers and the organic matrix is essential for obtaining good mechanical properties in dental composites. Bonding of these two phases is achieved by coating the fillers with a silane coupling agent that has functional groups to link the filler and the matrix chemically. A typical coupling agent is 3-methacryloxypropyltrimethoxysilane (MPTS). One end of the molecule can be bonded to the hydroxyl groups of silica particles, and the other end is capable of copolymerizing into the polymer matrix.

Visible-light Initiator

Light-activated composite resins undergo polymerization by irradiation via a blue-light-curing unit in the wavelength range of 410-500 nm. Light in this region is most effectively absorbed by an α-diketone photoinitiator. Camphoquinone (CQ) is a commonly used visible-light initiator, and ethyl-4-(N,N-dimethylamino)benzoate (4EDMAB) is a commonly used co-initiator. CQ creates an excited state that reacts with an amine-reducing agent such as N,N-dimethylaminoethyl methacrylate (DMAEMA) or ethyl p-dimethylaminobenzoate (DMAB) to produce free radicals that initiate the cross-linking polymerization. The absorption spectrum of CQ lies in the 450- to 500-nm-wavelength range, with peak absorption at 470 nm (Lee et al., 1993).

Modification of Nanofillers

Nanofillers can be prepared by various techniques, such as flame pyrolysis, flame spray pyrolysis, and sol-gel processes. Because extremely small filler particles have dimensions below the wavelength of visible light (0.4-0.8 μm), they are unable to scatter or absorb visible light. Thus, nanofillers are usually invisible and offer the advantage of optical property improvement (Mitra et al., 2003). Additionally, nanofillers are capable of increasing the overall filler level due to their small particle sizes. More fillers can be accommodated if smaller particles are used for particle packing. Theoretically, with the use of nanofillers, filler levels could be as high as 90-95% by weight. However, the increase in nanofillers also increases the surface area of the filler particles, which limits the total amount of filler particles because of the wettability of the fillers. Since polymerization shrinkage is mainly due to the resin matrix, the increase in filler level results in a lower amount of resin in nanocomposites and will also significantly reduce polymerization shrinkage and dramatically improve the physical properties of nanocomposites.

Nanofillers in Commercial Nanocomposites

There are several products of nanocomposites on the market. Three nanocomposites with different compositions and nanosized fillers have been chosen as typical examples.

The first is Filtek Supreme (3M ESPE, St. Paul, MN, USA), which contains nanometric particles (nanomers) and nanoclusters (NCs). Nanomers are monodispersed, non-agglomerated, and non-aggregated silica particles of 20 and 75 nm in diameter. Nanocluster fillers are loosely bound agglomerates of nanosized particles (Fig. 1). Two types of NCs were synthesized. The first type consisted of zirconia-silica particles synthesized from a colloidal solution of silica and zirconyl salt. The primary particle size of the NC filler ranged from 2 to 20 nm, and the average size of the spheroidal agglomerated particles was less than 0.6 μm. The second type of NC filler was synthesized from 75-nm primary particles of silica, and the average size of the agglomerated particles was 0.6 μm. The silica particles were treated with 3-methacryloxypropyltrimethoxysilane (MPTS), as a coupling agent, which contains a silica ester functional group on one end for bonding to the inorganic surface and a methacrylate group on the other end to make the filler compatible with the resin before curing to prevent any agglomeration or aggregation (Mitra et al., 2003). The filler content of Supreme is about 58-60% by volume and 78.5% by weight.

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Figure 1.

Nanomers and nanoclusters in Filtek Supreme nanocomposite. Scale bar = 0.6 µm.

A second commercial product, Premise (Kerr/Sybron, Orange, CA, USA), is a nanohybrid composed of 3 different types of filler components: nonagglomerated “discrete” silica nanoparticles, prepolymerized fillers (PPF), and barium glass fillers (Fig. 2). The non-agglomerated discrete silica nanoparticles are spheroidal and 20 nm in size. The prepolymerized fillers (PPF) are about 30-50 μm in size, and the barium glass filler has an average particle size of 0.4 μm. The technique for incorporating barium glass fillers into the resin matrix in the nanocomposites uses the same technology as the microhybrid resin Point 4 (Kerr/Sybron, Orange, CA, USA). Compositionally, Point 4 is based on Bis-GMA resin matrix filled to 57.2% by volume (76% by weight) with barium aluminoborosilicate glass and fumed silicon dioxide filler particles. The filler particles have an average size of 0.4 μm (hence the name "Point 4"), and 90% of the particles are smaller than 0.8 μm. The manufacturer claims that by incorporating a "polymerizable dispersant" it has been able to: (1) increase the percentage filler level and (2) use filler particles with an average size smaller than that used in traditional hybrid resin composites. This provides the material with excellent esthetics and strength. Thus, the nanocomposite Premise uses a similar technique for incorporating barium glass fillers into the resin matrix, with a “trimodel” approach to provide an optimal combination of 3 different filler components: silica nanoparticles, PPF, and barium glass fillers. This combination of 3 fillers allows for increased filler loading of 69% by volume and 84% by weight. The discrete unassociated nanoparticles that are well-dispersed in the matrix on a nanoscale level allow for increased filler loading and reduced viscosity of nanocomposites, and thus result in increased hardness, abrasion resistance, fracture resistance, and polishability and in reduced polymerization shrinkage (reported to be 1.4% to 1.6% by volume) and shrinkage stress (Bauer et al., 2003). As the interparticle dimension decreases, the load-bearing stress on the resin is reduced, inhibiting crack formation and propagation. The spheroidal shape of the nanoparticles provides smooth and rounded edges, distributing stress more uniformly throughout the composite resin (Terry, 2004a).

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Figure 2.

Three different types of filler components: nonagglomerated “discrete” silica nanoparticles, prepolymerized fillers (PPF), and barium glass fillers in Premise nanocomposite. Scale bar = 0.4 µm.

The third commercial nanocomposite, Ceram-X (Dentsply DeTrey, Konstanz, Germany), is an ormocer-based, nano-ceramic composite (Schirrmeister et al., 2006). Ceram-X contains glass fillers (1.1-1.5 μm), but differs from conventional hybrid composites in two important features: Methacrylate-modified silicon-dioxide-containing nanofiller (10 nm) substitutes for the microfiller that is typically used in hybrid composites (agglomerates of silicon dioxide particles). According to the manufacturer’s data, filler concentration is 57% by volume and 76% by weight.

Furthermore, most of the conventional resin matrix is replaced by a matrix full of highly dispersed methacrylate-modified polysiloxane particles (2-3 nm). According to the manufacturer’s information, these nano-ceramic particles are inorganic–organic hybrid particles. Both nano-ceramic particles and nanofillers have methacrylate groups available for polymerization (Fig. 3).

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Figure 3.

Nanofillers in Ceram-X nanocomposite.

A summary of these 3 commercial nanocomposites is shown in Table 1.

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Table 1.

Comparison of Commercial Nanocomposites

Brand Name	Type	Filler Composition, vol% (wt%)	Manufacturer
Filtek Supreme	Nanofilled	58-60 vol% (78.5 wt%)—combination of aggregated zirconia-silica/silica cluster filler with primary particle size of 2-20 nm and 75 nm, and non-agglomerated/non-aggregated 20-nm and 75-nm silica fillers	3M ESPE, St. Paul, MN, USA
Premise	Nanohybrid	69 vol% (84 wt%)—polymerized resin fillers (PPRF) (30-50 μm), barium glass (0.4 μm), silica nanoparticles (20 nm)	Kerr/Sybron, Orange, CA, USA
Ceram-X	Nanohybrid	57% vol% (76 wt%)—contains glass fillers (1.1–1.5 μm) with methacrylate-modified silicon-dioxide-containing nanofiller (10 nm)	Dentsply DeTrey, Konstanz, Germany

Reinforced Fillers

TiO₂ Nanoparticles

To improve the mechanical properties of composites, investigators modified the surfaces of TiO₂ nanoparticles (< 20 nm) with organosilane allytriethoxysilane (ATES), and the nanoparticles were blended with resin monomers. The particles were then manually added into Z100 dental-resin-based composites (3M, ESPE) at different weight ratios, depending on the sample group. The mixture was then thoroughly blended (Xia et al., 2008). Two groups of specimens were used as controls for comparison, one with the same amount of unmodified TiO₂ nanoparticles, and one without any TiO₂ nanoparticles. It was found that surface modification by the organosilane ATES influenced the dispersion and linkage of TiO₂ nanoparticles within a resin matrix. Adding the modified TiO₂ nanoparticles improved the microhardness and flexure strength of the composites.

Caries-prevention Fillers

Epoxy-Polyol Matrix

In general, resins shrink less by ring-opening polymerization (ROP), because of the increase in excluded free-volume associated with the ring-opening process (Tilbrook et al., 2000). Epoxy-polyols were shown to have significant advantages over dimethacrylates, including lower polymer shrinkage, no oxygen inhibition layer, higher strength, and equivalent hardness, as well as acceptable glass transition temperatures (Tilbrook et al., 2000). However, the mass increase of the epoxy-polyol materials at 37^oC is almost double that of the conventional dimethacrylate matrices, and cracking was observed in some samples during hydration. The properties of epoxy-polyol matrices can be controlled by varying their formulation. The key parameter in modifying their properties is the molar ratio of epoxy groups to polyol groups (R), which, for a sample mixture of one epoxide resin and one polyol, is calculated according to the equation

where W_epoxy and W_polyol, RMM_epoxy and RMM_polyol, and f _epoxy and f _polyol are the weights (in grams), the relative molecular masses, and the functionalities of the epoxy resin and polyol, respectively. By varying R, it is possible to vary the properties and cure characteristics of the resin system. It is generally accepted that decreasing R increases the cure speed. This increase in reactivity is usually accompanied by an increase in flexibility and toughness (Tilbrook et al., 2000). It was concluded that R should be between the limits of 4 and 8 to ensure that the matrix has a balance of acceptable properties (Tilbrook et al., 2000).

Epoxy Functionalized Cyclic Siloxane—Silorane™

Silorane™ (Fig. 4) is an epoxy functionalized cyclic siloxane whose name is derived from the combination of its chemical building blocks siloxanes and oxiranes. The Silorane™ material is a cationic ring-opening monomer system, with the target profile of a low-shrinkage, high-reactive, biocompatible composite (Schweikl et al., 2002, 2004) developed by 3M ESPE (Seefeld, Germany). The network of Silorane is generated by the cationic ring-opening polymerization of the cycloaliphatic oxirane moieties. The cationic cure starts with the initiation process of an acidic cation which opens the oxirane ring and generates a new acidic center, a carbocation. After the addition to an oxirane monomer, the epoxy ring is opened to form a chain or, in the case of two- or multifunctional monomers, a network. Nanocomposites containing Silorane and fillers exhibit low shrinkage and comparable mechanical properties (Weinmann et al., 2005).

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Figure 4.

Silorane monomer.

Epoxy Resin ERL4221

In our previous study (Chen et al., 2006), we developed a low-shrinkage, high-strength nanocomposite by using a 4-epoxycyclohexylmethyl-(3,4-epoxy) cyclohexane carboxylate (ERL4221) (Fig. 5) matrix with 55% of 70- to 100-nm nanosilica fillers through ring-opening polymerization. In our design, we used γ-glycidoxypropyl trimethoxysilane (GPS) to modify the surfaces of the silica nanoparticles. The nanocomposite was shown to exhibit low polymerization shrinkage strain, which was only a quarter of that of currently used methacrylate-based composites. It also exhibited a low thermal expansion coefficient comparable with that of the methacrylate-based composites. The strong interfacial interactions between the resin and fillers at nanoscales were demonstrated by the observed high strength and high thermal stability of the nanocomposite. A transmission electron microscope (TEM) study of the nanocomposite showed no aggregation of nanoparticles (Fig. 6).

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Figure 5.

Chemical structure of ERL4221.

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Figure 6.

TEM of epoxy-resin-based nanocomposite with GPS for silanization demonstrated no aggregation of silica nanoparticles. Scale bar = 25 nm.

Silsesquioxane (SSQ)

Soh et al. (2007a) found that the hardness and modulus of nanocomposites with different wt% silsesquioxane (SSQ) ratios decreased when increased amounts of SSQ monomers were added, indicating that the incorporation of SSQ monomers into the control generally helped to reduce both rigidity and polymerization shrinkage. The results demonstrated that, in the correct formulation, SSQ materials have great potential to be used as low-shrinkage composites. In other experiments, nanocomposites based on multifunctional SSQ with reduced shrinkage have also been reported (Soh et al., 2007b). For multifunctional SSQ-based nanocomposites, SSQ with various amounts of methacrylates and/or epoxide groups were prepared via Pt-catalyzed hydrosilylation of 8 equivalents of di(propylene glycol) allyl ether methacrylate and/or propargyl methacrylate, and 4-vinyl-cyclohexene epoxide combinations with (HMe₂SiOSiO_1.5)_8. The results also demonstrated that these materials have significantly lower shrinkage during curing than traditional monomers used for dental composite applications.

Bioactive Poly(methyl methacrylate)/SiO₂-CaO Nanocomposites with Dimethyldiethoxysilane (DMDES)

Lee and Rhee (2009) developed a bioactive poly(methyl methacrylate)/SiO₂-CaO nanocomposite using either dimethyldiethoxysilane (DMDES) or tetraethoxysilane (TEOS), which could produce 2 and 4 siloxane linkages, respectively, after a sol-gel reaction. In their study, methylmethacrylate was co-polymerized with 3-(trimethoxysilyl) propyl methacrylate and then co-condensed with either DMDES (specimen D) or TEOS (specimen T), respectively, with calcium nitrate tetrahydrate under acidic conditions. The results demonstrated that the fracture toughness of specimen D was higher than that of specimen T and did not lose apatite-forming ability. They suggested that these results were due to the decrease of siloxane linkage numbers and the introduction of alkyl groups into the silica structure in specimen D. The covalently bonded siloxane linkages may produce hard and brittle fracture behavior in the nanocomposite, while the alkyl groups help to make the silica a linear chain structure. This nanocomposite can be applied to the filler materials for bone cement and dental composite resin because of its good bioactivity and improved mechanical properties.

Silanization with MPTS and OTMS

Nanoparticles often form microscopic aggregations during processing. To eliminate this, the silica fillers are often coated with MPTS (Fig. 7a) to deter particle aggregation and promote interfacial adhesion by allowing the particle surface to copolymerize with the matrix polymer through covalent and H-bonding (Wilson et al., 2005). By contrast, n-octyltrimethoxysilane (OTMS) (Fig. 7b) is a non-reactive aliphatic silane which does not react with the resin matrix, but interacts through weak von der Waals forces. Dual-silanization of silica particles with blends of reactive MPTS and non-reactive OTMS offers several potential advantages compared with silanization with MPTS only. These advantages include improved uncured paste-handling characteristics, higher double-bond conversion during photopolymerization (Wilson et al., 2005), improved durability in the aqueous oral environment, and lower polymerization stress through non-bonded nanofiller particles (Condon and Ferracane, 2002). Wilson et al. (2007) also found that the covalent bonding and H-bonding of MPTS-rich nanoparticles with the matrix are necessary for preparing well-dispersed nanocomposites and interphases containing equal masses of MPTS and OTMS, to yield composites with optimal properties. The effects of the interface are more prominent in nanocomposites than in conventional composites containing microsized fillers, because nanocomposites and their clusters have a much larger surface area per unit mass, and the film derived from organosilane can influence dispersion and improve bonding between inorganic nanoparticles and the resin matrix (Mohsen and Craig, 1995).

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Figure 7.

Chemical structure of silanes.

Silanization with UDMS, MPTS, and OTMS

Karabela and Sideridou (2008) compared the absorption characteristics of a water or ethanol/water solution (75 vol%/25 vol%) by dental nanocomposites consisting of a Bis-GMA/TEGDMA (50/50, wt/wt) matrix and silica nanoparticles (Aerosil OX50), silanized with various silanes. The silanes used were: 3-[(1,3(2)-dimethacryloyloxypropyl)-2(3)-oxycarbonylamido] propyltriethoxysilane (UDMS), which is a urethane dimethacrylate silane (Fig. 7c); 3-methacryloxypropyltrimethoxysilane (MPTS); octyltrimethoxysilane (OTMS); a blend of UDMS/OTMS (50/50, wt/wt); and a blend of MPTS/OTMS (50/50, wt/wt). In their studies, the silane structure used for the silanization of nanosilica was found to affect the absorption behavior of an water or ethanol/water solution (37^oC) by the composites. The composite containing the UDMS with the hydrophilic urethane group showed the highest amount of absorbed water. The composite with the OTMS, which does not contain a methacrylate moiety and cannot react with the dimethacrylate monomers, showed the highest solubility in both water and ethanol/water. In all composites, the amount of absorbed ethanol/water solution was much higher than that of water. The OTMS-composite absorbed the highest amount of ethanol/water, and the MPTS-composite absorbed the lowest amount. Therefore, it was concluded that the silane structure used for the silanization of nanosilica has an effect on the solvent absorption and solubility of composites (Karabela and Sideridou, 2008).

Silanization with GPS

As described in the matrix section of epoxy resin ERL4221, GPS (Fig. 7d) has been used in epoxy-resin-based nanocomposites (Chen et al., 2006). With GPS as a coupling agent, the nanosilica particles were well-dispersed in the epoxy resin without aggregation (Fig. 6). It was also found that a high concentration of SiO₂ nanoparticles without the coupling agent GPS has a higher tendency to aggregate into a macrosize cluster that scatters light and reduces curing efficiency.

Silanization with ATES

Organosilane allytriethoxysilane (ATES) (Fig. 7e) was used in the TiO₂ nanoparticle-reinforced nanocomposite developed by Xia et al. (2008), who found that surface modification by the organosilane ATES increases the dispersion and linkage of TiO₂ nanoparticles within the resin matrix.

Silanization for Improving Fracture Toughness

Chan et al. (2007) proposed that the near-tip fracture processes in nanocomposites involve several sequences of fracture events, including (1) particle bridging, (2) debonding at the poles of the particle/matrix interface, and (3) crack deflection around the particles. The fracture processes of crack deflection and interface cracking were modeled for investigation of their role in the fracture toughness of the nanocomposites with various levels of particle fillers. Nanosized particles appear to enhance fracture toughness of nanocomposites in two ways: (1) The large surface-to-volume ratio improves interface bonding and, consequently, interface toughness; and (2) the high strength of the nanosized particles helps to prevent particle fracture during interface cracking. With nanosized particles, the interface toughness can be increased to higher levels without the risk of causing particle fracture, and thus allows a higher fracture toughness value to be attained in nanocomposites. By using analytical and finite-element methods to model the observed sequences of fracture events, these investigators found that silanization and nanoparticle loading improved the fracture toughness of dental nanocomposites through a combination of enhanced interface toughness by silanization, crack deflection, and crack bridging (Chan et al., 2007).

Consideration of Light-curing Modes

Nanocomposites may present higher degradation in the oral environment than hybrid composites (da Silva et al., 2008). A soft-start light-activation mode may increase the solubility of resin composites. However, Ilie et al. (2005) suggested that the soft-start polymerization concept is still valid, even with high-power light-emitting diode (LED) curing units. In their study, it was found that a soft-cure polymerization resulted in reduced shrinkage stress while simultaneously keeping the degree of cure and mechanical properties constant. Atai and Motevasselian (2009) reported that the Filtek Supreme (R) nanocomposite showed less increase in temperature and a lower degree of conversion in comparison with the hybrid composite. It was also found that the LED curing unit induced considerable total and irradiation temperature increase without any improvement in the degree of conversion. Ramp-curing mode (soft-start mode) showed lower temperature rise and delayed gel point and was found to be more effective than the quartz-tungsten-halogen (QTH) standard mode and LED units. The study showed that the LED curing units have no advantage over conventional QTH units in terms of temperature rise and degree of polymerization conversion. Gritsch et al. (2008) compared the role of light parameters on nanohybrid composite curing. In their study, 2 nanohybrid resins were cured by 2 LED devices and by 1 QTH device with different combinations of energy density and power density. The results indicated that, above a certain energy density threshold, the power density may not significantly influence the polymerization kinetics.

Consideration of Mechanical Properties

Mitra et al. (2003) reported that compressive and diametral tensile strengths and the fracture resistance of nanocomposites were equivalent to or higher than those of the other commercial composites that they tested. The nanocomposites also showed better polish retention than the hybrids and microhybrids tested after extended brushing periods. Watanabe et al. (2008) demonstrated that the fracture toughness values of hybrid and nanoparticle resin composites are significantly higher than those of micro-filled resin composites. However, Yesil et al. (2008) reported that nanocomposites did not significantly improve wear resistance or the amount of opposing cusp wear when compared with the traditional materials tested. Curtis et al. (2008) found that water uptake and mechanical properties of composites were influenced by the size and morphology of the reinforcing particulate phase.

Curtis et al. (2009) investigated 7 commercially available resin-based composites. After being pre-loaded, the specimens were stored either in a lightproof container or a water-bath for 24 hrs prior to being tested. They found that the “nanocluster” system provided a distinct reinforcing mechanism compared with the microhybrid, microfill, or nanohybrid resin-based composites, resulting in significant improvements to the strength and reliability, regardless of environmental storage or testing conditions. They suggested that the agglomerated nanoparticles produced an interconnected network where the interstices were infiltrated with the silane coupling agent, producing an interpenetrating phase composite (IPC) structure. In a “wet” testing environment, hydrolysis and polymerization within the nanocluster silane phase could modify stress transfer both to and within the cluster particles, producing an enhanced capacity to tolerate local stresses and cluster deformation (Curtis et al., 2009). However, such improvements in performance may be compromised over time by hydrolytic degradation of the silane (Curtis et al., 2008). There are several commercial nanocomposites that have good strengths after 1 day of immersion in water. However, their strength can decrease by more than 50% after just a couple of months’ immersion (Curtis et al., 2008). Therefore, the strength durability is also an important issue, especially in ion-releasing composites. Xu et al. (2006) found that most composites did not show a significant decrease in strength after 1 day’s immersion. Only the chemically cured nano-DCPA-whisker composites with HEMA at DCPA:Whisker = 1:2 had a significant strength loss. After 56-day immersion, the chemically cured DCPA-whisker composites (with and without HEMA) showed significant strength loss (approximately 20-30%). In another study, Xu et al. (2009) demonstrated that, after immersion in solution at pH of 7.4, 6, and 4, for 28 days, the TTCP-whisker composite, similar to a commercial hybrid composite, did not show reduced strength, in contrast to a significant strength loss for a releasing control material.

Ilie and Hickel (2009) analyzed the mechanical behavior of the silorane-based composite in comparison with that of 6 homologous clinically successful methacrylate-based composites. They found that the silorane-based composite was comparable with clinically successful methacrylate-based composite materials, encouraging the clinical use of the new composite material.

According to the kinetic model for the shrinkage-strain rates of dental resin composites developed by Atai and Watts (2006), there is a linear correlation between the shrinkage-strain (and shrinkage-strain rate) and filler-volume fraction. The filler fraction did not affect the degree-of-conversion of the composites. It was suggested that only a relatively high filler-surface area, as may be obtained with nanofillers, will affect the network-forming kinetics of the resin matrix.

Ozel et al. (2008) investigated the cervical microleakage and internal voids of nanocomposites, comparing them with a hybrid composite in Class II restorations with the margins located coronal and apical to the cement-enamel junction (CEJ). The results indicated that the location of the gingival margin affects the microleakage of nanocomposites, but has no significant effect on the internal voids.