The esthetic appearance of the surface of a resin composite restoration is also a direct reflection of the particle size. Esthetic restorations require biomaterials to have optical properties similar to those of tooth structure. Because resin composite does not have hydroxyapatite crystals, enamel rods, and dentinal tubules, the composite restoration must create an illusion based on the way light is reflected, refracted, transmitted, and absorbed by dentin and enamel microstructures. Recreating a natural anatomical surface requires a similar orientation of enamel and dentin. Newer formulations of resin composites possess optical properties that render the tooth polychromatic. In addition, the filler particle sizes and distribution can influence the color and esthetics of a restoration through a phenomenon called the double-layer effect, also known as the chameleon effect or blending effect.18–20 This mechanism applies to the relationship between natural tooth structure and esthetic materials. It occurs when a composite material is placed as a restoration and diffused light enters from the surrounding hard dental tissues; when emitted from the restoration, the shade is altered by absorbing color from the tooth and the adjacent teeth. This color alteration depends on the scattering and absorption coefficients of the surrounding hard dental tissues and restorative material, which can produce an undetectable color match by blending with tooth color.21 Furthermore, the surface quality of the composite restoration is influenced by the composition and the filler characteristics of the composite.22,23 Newer formulations of nanocomposites have altered filler components with finer filler size, shape, orientation, and concentration, improving not only their physical and mechanical properties but also their optical characteristics (Fig 1-1). These universal resin composite systems allow the composite to be polished to a higher degree, which can influence color integration between the material and the tooth structure.
Current Developments in Nanotechnology with Resin Composite
Nanotechnology, or nanoscience,24 refers to the research and development of an applied science at the atomic, molecular, or macromolecular level, also known as molecular engineering/manufacturing. The prefix nano- is defined as a unit of measurement in which the characteristic dimension is one-billionth of a unit.25 Although the nanoscale is small in size, its potential is vast. There has been significant advancement in the world of small. Small has become a common research theme for building nanomotors, nanorobots, nanocircuits, and nanoparticles. Recent advances by scientists and engineers in manipulating matter at this small magnitude indicate potential applications of this nanoscience in every arena of our economy, including telecommunications, aerospace, computers, textiles, homeland security, microelectronics, biomedicine, and dentistry.25
In dentistry, nanotechnology24 may provide resin composites with filler particles that are dramatically smaller in size and that can be formulated in higher concentrations and polymerized into the resin system with molecules designed to be compatible with polymers and provide unique characteristics (physical, mechanical, and optical). In addition, optimizing the adhesion of restorative biomaterials to the mineralized hard tissues of the tooth is a decisive factor for enhancing the mechanical strength, marginal adaptation, and seal of the adhesive restoration, as well as improving its reliability and longevity. Currently, the particle sizes of many of the conventional composites are so dissimilar to the structural sizes of the hydroxyapatite crystal, dentinal tubule, and enamel rod that a potential exists for compromises in adhesion between the macroscopic (40 nm to 0.7 μm) restorative material and the nanoscopic (1 nm to 10 nm) tooth structure.26 However, nanotechnology has the potential to improve this continuity between the tooth structure and the nanosized filler particle and to provide a more stable and natural interface between the mineralized hard tissues of the tooth and these advanced restorative biomaterials.
Flowable composite materials have been evaluated in numerous studies1–5,7,27–45 since their inception. Some of the more recent studies39,42,43 indicate that the clinical performance of specifically tested flowable resin composites is similar to or better than that of specifically tested universal resin composites. Attar et al28 showed that different flowable composites possessed a wide range of mechanical and physical properties. Earlier studies by Gallo et al29 on specific flowable resin composites suggested that these materials should be limited to small- and moderate-sized restorations with isthmus widths of one-fourth or less of the intercuspal distance.36 However, Torres et al43 reported that, after 2 years of clinical service, no significant differences were found between Class II restorations restored with GrandioSO (VOCO) conventional nanocomposites and those restored with GrandioSO Heavy Flow (VOCO) flowable hybrid nanocomposites. A study by Karaman et al39 showed similar clinical performances over 24 months in restorations of noncarious cervical lesions restored with conventional nanocomposites (Grandio, VOCO) and those restored with flowable material (Grandio Flow, VOCO). A more recent study by Sumino et al42 indicated that the flowable materials G-aenial Universal Flo, G-aenial Flo, and Clearfil Majesty Flow (Kuraray) had significantly greater flexural strength and a higher elastic modulus than the corresponding conventional nanocomposite materials, Kalore (GC America) and Clearfil Majesty Esthetic (Kuraray). The wear and mechanical properties of these specific flowable materials suggested an improved clinical performance compared with that of the universal composites. Several in vitro studies conducted at GC Research and Development comparing specific flowable material properties of several conventional composites found results similar to those of Sumino et al. Of the next-generation flowable systems studied, G-aenial Universal Flo and Clearfil Majesty ES Flow (Kuraray) showed superior gloss retention and similar wear resistance to the conventional nanocomposites tested, which included Filtek Supreme Ultra