In the last decade, growing demands by patients for mercury-free, esthetic restorations have markedly increased the use of direct, light-activated resin composites in restorative dentistry .
A major drawback of current composite-based resins is that they contract or shrink during conversion from monomer to polymer. The resin matrix of all composite- based resin restorative materials shrinks volumetrically approximately 10 percent during polymerization . This polymerization shrinkage stresses the adhesive between the tooth and the restorative material, frequently resulting in failure of this bond and marginal infiltration .
These problems have activated producers to find solutions and to make either the material or the technique easier to apply and faster to use. The effect of lowering viscosity to improve adaptation of the composite and to improve ease of placement has been shown to be important .
Compared with conventional composite resins, these new composite resins with low viscosity boost few advantages: easily applicable to cavities with complicated forms, time required for the filling procedure is short, and excellent in cavity sealing [5, 6].
1.1. Flowable Composites
One of the methods used for decreasing composite viscosity is the development of flowable resin composites. Flowable resin composites; being less viscous material improve the wettability by flowing onto all prepared surfaces creating an intimate union with the micro structural defects in the floor and the walls of the cavity preparation. Moreover, they act as a flexible intermediate layer that helps to relieve stresses during polymerization shrinkage of the restorative resin. These characteristics and a syringe delivery system make them an ideal choice for the use as a liner [7, 8]. Flowable composites achieve their lower viscosity primarily by a reduction in reinforcing filler content and changes in the matrix chemistry [9, 10]. However, it is well known that a decrease in filler content will affect various properties of a hardened composite resin, such as the mechanical strength and curing shrinkage . Against this background, it is important to investigate how to lower the viscosity of a composite resin without decreasing its filler content . Thus far, the most uncomplicated method of decreasing the viscosity of composites is to lower the viscosity of the monomer mixture itself.
1.2. Lowering Monomer Viscosity
The base monomer most widely used commercially is BisGMA (bisphenol A diglycidil dimethacrylate; MW=512 g/mol. Despite its high intrinsic reactivity, the presence of hydroxyl groups on the backbone and the π-π interactions given by the aromatic rings increase the initial viscosity (η=1,200 Pa) to a point that the homopolymer typically does not reach high conversion . For that reason, and also to improve handling characteristics and allow incorporation of higher inorganic filler contents, BisGMA is usually combined with low free viscosity monomers like TEGDMA (triethylene glycol dimethacrylate; MW=286 g/mol, η=0.01 Pa. However, addition of TEGDMA increases water sorption and polymerization shrinkage [14, 15]. To overcome these effects, studies have been directed toward developing low viscosity, more hydrophobic Bis-GMA analogs such as the hydroxyl-free propoxylated Bis-GMA (CH3Bis-GMA) and propoxylated fluorinated Bis-GMA (CF3Bis-GMA), as replacements for TEGDMA in Bis-GMA mixtures [16, 17]. Also, in an attempt to improve properties of methacrylate resins, aldehyde-propanal (propionaldehyde) or diketone-diacetyl (2,3-butanedione) have been added as potential crosslinking agents with appreciable success [18, 19].
1.3. Heating Composite
Many polymer resins exhibit lower viscosity when they are heated. The theoretical basis for this behavior is that thermal energy forces the composite monomers or oligomers further apart, allowing them to slide by each other more readily. Studies have shown that heating general polymers and resin composites lowers viscosity and thereby improves adaptation .
1.4. Sonic Vibration
A new method of restoration relies on more sophisticated instruments that condense the material by vibration. Such devices have been created by several producers and operate according to the same principle: sonic vibration .The principle of this technique assumes that vibration lowers the viscosity of the resin, allowing the material to flow and easily adapt to the cavity walls without air pores, in a similar way as a flowable composite. Thus, a condensable material with increased viscosity can be used similar to a flowable composite, without the disadvantage of high polymerization shrinkage and poor mechanical properties .
Our review aimed to disclose that if decreasing viscosity of resin composites through the four mentioned methods has an impact on the performance of the restoration by improving its properties.
Today, improvements in formulations, optimization of properties and the development of new techniques for placement have made the restoration of direct composite more reliable and predictable. Producers tried to decrease the viscosity of dental composite to make either the material or the technique easier to apply and faster to use. Several solutions have been proposed to decrease composite viscosity.
3.1. Flowable Composites
The first way discussed in this review was; using flowable composite Table (1) [22-35]. The use of flowable resin composite as an intermediate layer liner when studied for occlusal, cervical and proximal restorations, showed different results. Some authors found this application improves the marginal seal, Olmez et al., 2004  concluded that; a composite lining in a Class II resin composite with margins below the cemento enamel junction may reduce marginal microleakage and voids in the interface and the total number of voids in the restoration. Sadeghi et al., 2009  also concluded that; a layer of flowable materials at the gingival floor of Class II composite restorations may be recommended to improve the marginal seal of a restoration. The same idea was supported by Simi et al., 2011  who concluded that; both resin-modified and flowable composite liners under nano composite restorations result in comparable reduction of micro leakage.
In contrast; others failed to show any benefits from using flowable composite as a liner. Tredwin et al., 2004  found use of a flowable composite liner against cementum/dentin was associated with increased micro leakage. Lindberg et al., 2005  and Pecie et al., 2013  found that the use of flowable resin composite did not rinfluence the interfacial adaptation. Van dijken et al., 2011  also concluded that; the use of flowable resin composite as an intermediate layer did not result in improved effectiveness of the Class II restorations.
Using liner under occlusal restoration was studied by Efes et al., 2006  who found that; the clinical performance of occlusal restorations did not benefit from the additional use of the flowable composite.
Using flowable composite as a liner in class V cavities was studied by Arslan et al., 2013  who stated that; micro leakage is not affected by the application of either conventional or new-generation flowable composite resin as an intermediate material between composite resin and dental substrates. Also, Loguercio et al., 2005  stated that; the use of Filtek Flow as a liner under Filtek Z250 restorations did not improve the clinical performance of class V restorations after 6 and 12 months of evaluation.
When performance of flowable resin materials in non-carious cervical lesions was studied; acceptable clinical performance (except for the retention rates of the Dyract Flow restorations) in non-carious cervical lesions was stated by Celik et al., 2007  with no significant differences were found between the flowable and microhybrid resin materials (p > 0.05). The same conclusion was recorded by Kubo et al., 2010  who found that: both types of resin flowable and hybrid resin composite in conjunction with S3 Bond demonstrated an acceptable clinical performance up to 3 years when applied to non-carious cervical lesions with no significant differences.
Clinical efficacy of two flowable composite resins used to restore occlusal caries lesions was investigated by Gallo et al., 2010  who observed that marginal discoloration, polishability and marginal adaptation significantly worsened at 36 months and he suggested that they should be limited to small restorations such as preventive resin restorations having isthmus widths of one-quarter or less of intercuspal distance. Bonilla et al., 2012  reached the same conclusion when they studied placing flowable composite as minimally invasive occlusal restorations. Their results showed that a conventional microhybrid composite material, leaked significantly less than all the flowable composite groups. Tiny microscopic bubbles were seen within many of the flowable composite specimens, as were a few voids.
3.2. Lowering Monomer Viscosity
The second discussed way for decreasing composite resin viscosity was decreasing monomer mixture viscosity (Table 2) [36-42]. Most of the composite resins widely used in restorative dentistry contain the highly viscous monomer 2,2-bis [4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl] propane (Bis-GMA) and low-viscosity monomers, used as diluents, in order to achieve high filler loading. In particular, triethylene glycol dimethacrylate (TEGDMA) has been extensively used for such purpose.
Studies have been directed toward developing low viscosity, hydroxyl free, more hydrophobic Bis-GMA analogs. Okamura et al., 2006  evaluated the dental application possibility of producing, experimental composite resins of low-viscosity monomer mixtures using low-viscosity monomer mixtures of newly developed polyfunctional acrylates. The viscosity of composite pastes with high filler content was markedly lower than that of Bis-GMA based composites. Compressive strengths of composite resins produced using the new monomer mixtures were similar to that of composite resin produced using Bis-GMA monomer mixture. In terms of setting shrinkage, the composites consisting of new monomer mixtures exhibited significantly smaller shrinkage than the Bis-GMA based composites, and decreased with increase in filler content. Such small setting shrinkage might be attributed to the relatively large volume of polyfunctional monomers and the relatively small intermolecular distance.
Pereira et al., 2005  investigated the influence of new diluent agents, diluent ratio and filler content, on relevant mechanical properties of several novel composite resins containing Bis-GMA as resin matrices, and to compare these with the properties of composites based on TEGDMA, a conventionally used diluent. Two Bis-GMA analogues were synthesized combining three monomer mixtures (Bis-GMA/TEGDMA, Bis-GMA/CH3 Bis-GMA and Bis-GMA/CF3 Bis-GMA). Materials with CH3 Bis-GMA showed an enhanced micro hardness VHN. Mean Flexural strength (FS) was higher for matrices containing TEGDMA. Overall, dilution favored FS and VHN but not modulus of elasticity ME.
Adding additives to the Bis-GMA monomer that decreased viscosity was studied by Prakki et al.,2009  where two additives, propionaldehyde/aldehyde or 2,3-butanedione/diketone, was added. Degree of conversion (DC %), flexural strength (FS), modulus of elasticity (E), modulus of resilience (R) and diametral tensile strength (DTS) were determined. Incorporation of additives led to an increase in DC%, FS and E for Bis-GMA/TEGDMA and Bis-GMA/CH (3) Bis-GMA systems. R-values for all systems were unaffected by addition of additives. They had no significant effect on DC% or mechanical properties of Bis-GMA/CF (3) Bis-GMA. Same conclusion reached by the same authors  when they tested wear, roughness and hardness as affected by the two additives and concluded that; incorporation of additives led to improved W and H values for bis-GMA/TEGDMA and bis-GMA/CH3bis-GMA systems, additives had no significant effect on the W and H changes of bis-GMA/CF3bis-GMA, also concluded that; Bis-GMA/TEGDMA, bis-GMA/CH3bis-GMA copolymers with additives became smoother after abrasion test.
The same conclusions was reached by Denis et al., 2012  who evaluated the physical, rheological, and mechanical properties of Bis-GMA diluted with CH3bis-GMA containing 0, 2, 8, 16 and 24 mol% propionaldehyde. It has been reported that the viscosity of propionaldehyde is 3 × 10–5 Pa.s and its incorporation into comonomers significantly lowers the viscosity of CH3bis-GMA-based resins. The present findings revealed that the incorporation of propionaldehyde into the experimental resins gradually increased % DC as the mol % of the additive increased.
Prakki et al., 2012  tested the effect of additives on the water sorption characteristics of Bis-GMA based copolymers and composites containing TEGDMA, CH3Bis-GMA or CF3Bis-GMA. Water sorption and desorption were evaluated in a desorption-sorption-desorption cycle. Water uptake (%WU), water desorption (%WD), equilibrium solubility (ES; µg/mm³), swelling (f) and volume increase (%V) were calculated using appropriate equations. All resins with additives had increased %WU and ES. TEGDMA-containing systems presented higher %WU, %WD, ES, f and %V values, followed by resins based on CH3Bis-GMA and CF3Bis-GMA. Aldehyde and diketone led to increases in the water sorption characteristics of experimental resins.
Charton et al., 2007  showed different results about monomer viscosity when he investigated Influence of glass transition temperature Tg, viscosity and chemical structure of monomers on shrinkage stress in light-cured dimethacrylate-based dental resins. The large differences in stress values for the pairs with the same viscosity, showed that; it is not the viscosity in itself which has a dominating influence on stresses (via the DC). They concluded that; whatever the viscosity, the UEDMA-based matrices developed higher shrinkage stresses than the Bis-GMA homologues.
3.3. Heating Composite
The third discussed method in this review was preheating composite before photo activation (Table 3) [43-54]. Chair side warming of composite resins before photo polymerization is one of the recent trends in their application.
Daronch et al., 2005  concluded that; pre-heating composite prior to photo activation provides greater conversion requiring reduced light exposure than with room-temperature composite. Different results were reached by Lohbauer et al., 2009  and Fróes-Salgado et al., 2010 .
The first one concluded that; pre-heating of resin composites does not increase degree of conversion over time. It can be clinically beneficial, due to a superior marginal adaptation. The second one reached the same conclusion when he studied the effect of composite pre-polymerization temperature and energy density on the marginal adaptation (MA), degree of conversion (DC), flexural strength (FS), and polymer cross-linking (PCL) of a resin composite. He concluded that: Pre-heating the composite prior to light polymerization did not alter the mechanical properties and monomer conversion of the composite, but provided enhanced composite adaptation to cavity walls.
Deb et al., 2011  evaluated if pre warming of composites would influence the flow and enhance marginal adaptation and, he stated that; pre-warming of the composites studied enhanced flow as observed by measuring film thickness and did not significantly affect other properties.
Micro leakage as affected by preheating was discussed with two different points of view. The first one correlated composite preheating to the reduction in micro leakage in class II composite. Wagner et al., 2008 45and Aksu et al., 2004  supported this view, the first one compared micro leakage in Class II composite restorations prepared using: preheated resin composite, and unheated composite and found no statistical differences among materials at the occlusal margin. However, at the cervical margin, the preheated samples showed statistically lower micro leakage than the controls and all other treatments. The second study also stated that; preheating of the composite investigated resulted in significantly less micro leakage at the cervical margin compared to the control or the use of the corresponding flowable resin. The other point of view by Dos Santos et al., 2011  who found that; the decrease in micro leakage in Class II cavities restored with preheated dental composite happened when a QTH with low irradiance was used but, not improved when high-irradiance LED was used. Karaarslan et al., 2012  also tested the micro leakage as affected by preheating composite and found no significant differences among the preheated groups.
Mechanical properties and polymerization shrinkage for preheated composite were tested with varied results. Walter et al., 2009  studied whether temperature affects the polymerization shrinkage of composite resin and concluded that: Preheating composite to relatively high temperature 54c or 88 c to increase its flow and adaptation causes an increase in volumetric shrinkage. This result was opposed by Tantbirojn et al., 2011  who evaluated the effect of composite preheating and light-curing duration on hardness and postgel shrinkage and stated that; preheating of the composites only slightly increased hardness values and did not negatively affect postgel shrinkage.
Lucey et al., 2010  tested the pre-cured viscosity and post-cured surface hardness for the preheated composite and stated that; pre-heating resin composite reduces its pre-cured viscosity and enhances its subsequent surface hardness. This conclusion was supported by Nada et al., 2011  who found that the effect of pre polymerization warming on composites' mechanical properties, is material dependent; pre warming significantly improved surface hardness and bulk properties of the composites; however, this improvement was significant in only some of the tested materials.
3.4. Sonic Vibration
The last discussed method was sonic vibration (Table 4) [55-59]. Up to now, little independent studies assessed the vibration technique, also some data offered by the producers of such devices being available.
Microleakage was investigated by two studies the first by Eunice et al., 2012  who stated that; the sonic system has no effect concerning microleakage. The second study by Poggio et al., 2013  on contrast proved that sonic fill composites showed the lowest microleakage values when compared with other groups tested micro leakage in "deep" Class II composite restorations with gingival cavosurface margin below the CEJ.
Alrahlah et al., 2014  studied post-cure depth of cure of bulk fill resin composites through using Vickers hardness profiles (VHN), they stated that SonicFill exhibited the highest VHN, also sonic fill and Tetric EvoCeram bulk fill had the greatest depth of cure among the composites examined. Ilie et al., 2013  also studied the mechanical performance of seven bulk-fill RBCs and he stated that; the significant highest flexural strengths were measured for SonicFill.
Yapp et al., 2011  found that; sonic fill adequately cured at the maximum recommended depth when cured with Demi curing light set to yield a saw tooth when used according to manufacturer’s direction for use.
CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.