Smart Monochromatic Composite: A Literature Review

Over the previous years of the 20th and 21st centuries, there has been a progression in the field of bonded esthetic restorations. At the present time, the inclination of dentists toward the smart monochromatic shade of composite is flourishing owing to the fact that it decreases the requirement of a range of composite shades, curtails the waste of unconsumed composite shades, lessens chair side interval, abolishes the shade selection, and decreases dependency on shade-selecting methods. Smart monochromatic composite is known to obtain the color of the adjacent tooth structure in which it is placed. Therefore, the current literature elucidates the several features of innovative and revolutionary monochromatic composites including color stability, mechanical and optical properties, and shade-matching capability that could have a positive impact potentially over other resin composites.


Introduction
In the last century, restorative dentistry has presented much development in resin composite including adhesive technology and techniques. Previously, acrylic resins were used as a restorative material; however they had several disadvantages: such as poor abrasion resistance, low color stability, higher shrinkage, and poor peripheral seal [1]. Consequently, R. Bowen introduced polymeric restorations that were reinforced with quartz fller that recognized as "resin composite." Composite is a three-dimensional compound which consists of two or more chemically dissimilar materials with excellent properties than those of an individual component. Resin composite presents extremely conservative and esthetic restorations to an individual owing to signifcant progression along with its compatible use for the last couple of decades. Formerly, composites were suggested as a restorative material merely for anterior teeth; however at present, fllers combined with acid etching and its good compatibility to tooth structure made it worthwhile for both anterior and posterior restorations [2]. Presently, resin composites are recommended as an inexpensive and esthetic substitute to other direct and indirect restorations in consequence of its optimization of formulations, up gradation of properties, and innovative methods for application [3].  (3 M) and Adaptic (Dentsply Sirona)}. Tey are composed of large fllers with typical particle sizes ranging from 0 to 5 μm with rough surface texture. Wearing of occlusal contact area with deposition of plaque occurs due to hardness of fller particles. Teir physical and mechanical properties are superior over unflled acrylic resins. Tey are recommended in pressurebearing areas, for instance, Classes I and II and large size cavities of Classes III and IV [4]. [5]. Tey have a polished and smooth surface texture because of small particle size that enables resin composite to resist against plaque, debris, and stain. Tey carried inferior mechanical properties owing to higher matrix content, lower color stability, and increased marginal breakdown. Tey are indicated as restoration of anterior teeth and cervical lesions [5].

Hybrid Composites.
In the 1990s, hybrid composites were introduced. As refected by name, they are composed of organic part which is reinforced by an inorganic phase [6]. Tey were difcult to polish because diferent sizes of glasses were used in their composition, with particle size of <2 μm and comprise 0.04 μm-sized fumed silica as well. Tey exhibited admirable polishing and texturing properties, better abrasion and wear resistance, and reduced polymerization shrinkage. Tey presented higher surface smoothness and better strength recommended for both anterior and posterior restorations. Consequently, in the era of 2000, innovative formulations were introduced with improved esthetic properties. Tis was the frst-time that variations in shades have been permitted to emulate the natural tooth structure.

Nanoflled and Nanohybrid Composites.
After the year of 2000, nanoflled and nanohybrid composites were developed as Tetric EvoCeram (Ivoclar Vivadent) and Filtek Supreme Plus (3 M) with typical particle size ranging from 5 to 75 nm and nanocluster fllers with particle size ranging from 5 to 20 nm that were less than that of microflled composites [6,7]. Tey exhibited better physical properties similar to the original hybrid resin composite and restorations with a smoother surface texture and polish [8].
Classifcation of the composite on the basis of particle size and structure is shown in Figure 1.

Bulk-Fill
Composites. By the 2010s, bulk-fll composites were introduced which got approval by many dental practitioners due to less signifcant polymerization shrinkage with a better depth of cure up to 4 mm [8]. Te frst fowable bulk-fll composite was recognized as SureFil SDR Flow (Dentsply Sirona) that was applied as a base beneath restorations. Newer bulk-fll agents such as Tetric EvoCeram Bulk-Fill (Ivoclar Vivadent) and Estelite Bulk Flow (Tokuyama Dental America) do not need any additional layer of composite as a crowning. Tey revealed greater strength and better esthetics, but some of them were translucent that showed their advantages and disadvantages reliant on the restorations [9].
One research revealed that pigments in food or drinks or habits such as smoking cause extrinsic or intrinsic staining of composites. Tus, bleaching techniques are applied to have desired esthetic. Bleaching can eliminate developed stains on composites and can reproduce their original shade; however, it cannot modify the shade of composite restorations to a brighter color. Because of this reason, bleaching is commonly suggested before restoration of an anterior composite so that composite restoration is harmonized to the original and brighter tooth shade [10].

Smart Monochromatic Composite.
Smart monochromatic composite is a leading shade-matching composite that gained more acceptance in recent times. It possesses distinctive characteristics that are based on "smart chromatic technology." It has the capability to capture the structural color of its surrounding tooth that is controlled by the size of its fller particles [11]. It has no extra dyes or pigments, whereas fllers itself produce red-to-yellow structural color that matches the surrounding tooth color. Color is the light wavelength that enters into our eyes. Human teeth come into the range of red-to-yellow color [11]. Smart monochromatic composite is a one-shade material that is specifed to match entirely 16 VITA Classical shades (VITA North America, Yorba Linda, CA). It has another shade that is opaque, termed as Blocker to represent the color of dentine in translucent areas like restorations in class IV cavities.
Smart monochromatic composite has a distinctive feature that helps clinicians not be confused by many shades. It presents a rapid and easy method that makes striking and functionally esthetic restorations. Smart monochromatic composite has been recognized to possibly save time in the clinic to get rid of the requirement of shade selection. In this composite, material has homogeneously sized spherical-shaped fller particles. It adjusts the light that is transmitted all along the red-to-yellow area of the color scale and shows matching the color of neighboring teeth of patients [11].
Te main characteristics of smart monochromatic composite include better polishing capability, superior fexural and compressive strength, easy handling, clinically satisfactory outcomes, and resistant to ambient light. It carries minimal wear of composite and opposing tooth structure. Smart monochromatic composite is available in the form of opaque-white paste that allows the material more visible to clinicians during manipulation and placement. Te material is evenly mixed with adjacent teeth prior to application of light source during curing. A chamfered margin is preferred to get better marginal seal [12].
A single shade is only required to match color in Classes I and II restoration in posterior teeth. In case of extensive Class III and Class IV restorations of anterior teeth, a blocking agent could be applied as 0.5 mm thin coat prior to insertion of the smart monochromatic composite. Particularly, in case of discoloration, it camoufages the internal portion of the crown. Additionally, it also reduces the shadematching interference [11]. To the best of our knowledge, various brands of smart monochromatic composites available in the market to date are summarized in Table 1.

Color-Generating Phenomena.
Munsell sphere shows the wide-ranging perceptible color space ( Figure 2). Te natural color teeth range is relatively restricted and distributed in A narrow range of red to yellow from A1 to D4, indicating variable grades of darkness, lightness, and saturation [13]. Teeth color matching is related to two colorproducing phenomena:

Chemical Color.
Chemical color is the common form of perceptible color that results as material particles refect specifc wavelengths. Chemical color is produced by adding dyes and pigments that most commonly present in various composites.  International Journal of Dentistry Currently, most composites need many shades to mimic every dental shade. Tey rely on red and yellow colorants that are added to the resin material to match tooth shades [14].

Structural Color.
Structural color occurs while diferent light wavelengths are augmented or declined by the material structure itself showing colors other than what the material may actually be. It rarely exists, and the efects can be spectacular, for instance, colors in nature, from morphs of butterfies to peacocks, in addition to soap bubble flm and surfaces of compact disc. Smart monochromatic composite uses structural color mechanism in composite dentistry, without dyes or pigments. Te ideal match from A1 to D4 and beyond is produced by the spherical fllers itself that create red-to-yellow structural color that blends with the color of the neighboring dentition [11]. Smart monochromatic composite is composed of 260 nm spherical fllers that are the exact dimension and outline needed to create red-to-yellow color when available light crosses through the composite.

Recommendations for the Smart Monochromatic Composite
(i) Direct restorations in both posterior and anterior teeth.
(iii) Diastema closure or closure of space between any teeth. (iv) Composite and porcelain repair [11].

Key Characteristics of the Smart Monochromatic Composite
2.4.1. Shade-Matching Ability. Harmonizing the shade of resin composite with anterior teeth is a difcult task that is experienced by the dental practitioners regularly. Te color of underlying dentine has an efect on tooth shade [15]. Normally, various factors make the color matching challenging for dental practitioners. Color matching depends on diferent chromatic features that are associated with resin composite and teeth such as hue, chroma, and value; opalescence, translucency, and fuorescence; light difusion and transmission; and surface texture properties [16,17]. It is imperative for restorative material to imitate the natural tooth with all chromatic characteristics along with the color stability to have ideal esthetics [18]. Over the past decades, modifcations have been executed to enhance the esthetic properties of resin composite restorative materials. Recently, single-shade structurally colored universal composites are directly applicable in most cases. Tey have the capability to change color according to the adjacent dentition. Consequently, they can enhance the appearance of the restoration esthetically as well as reduce the dependency on many shade-matching methods [19] and shade guide tabs [20,21]. It was revealed that smart monochromatic material had no pigments and dyes, so its color properties are dependent solely on the physical properties of light. It has excellent color-matching ability for all shades [22,23].

Esthetic Properties and Efect of Bleaching on Surface
Roughness. Resin composites are most accepted esthetic restorative materials that are used in dentistry due to their excellent optical properties, sufcient strength, and inherent bonding to tooth structure [24]. Another signifcant characteristic is a good surface texture without porosities that makes the restoration clinically successful because rough surfaces of restorations encourage plaque deposition, staining, and gingival irritation which eventually develops secondary caries [25]. Surface properties of composite restorations are afected by the oral environment and usual dietary habits that have a negative impact on the strength of composite restoration. In addition, few dental procedures, for instance, tooth bleaching, have a negative efect on resin composite flling materials, which is an easy and noninvasive method for tooth whitening based primarily on oxidation by hydrogen peroxide or one of its precursors [26]. Tooth bleaching causes undesirable alteration in resin composites as compared to other tooth-colored restorative materials because of the existence of organic matrix component. Bleaching agents that are used in bleaching treatment had International Journal of Dentistry peroxides that can provoke deterioration of the organic matrix complex of resin composites and cause surface roughness [27].

Surface Texture and Color Stability.
Surface smoothness and color stability are necessary for resin composite to be clinically successful. Multiple factors can afect the color stability of restorative material such as absorption of water, extent of polymerization, dietary habits of an individual, and surface irregularity of the restoration [28]. It is stated that the color sensitivity of material has a direct impact on polishing and fnishing steps in addition to components of material [29]. Rough surface of the restoration becomes discolored by the efect of external factors such as cofee, tea, or red wine [30,31]. Consequently, it is evidently supported that the smoothness of restoration enhances its esthetic appearance and success of restorative material, whereas surface roughness increases the probability of plaque deposition, secondary caries, and staining of the restoration [32]. Furthermore, it has also been found in the literature that the fnishing and polishing system comprising diamond particles provides the least color diference on single-shade composite restorations [33,34]. Te color stability of a resin composite relates to organic matrix, magnitude of fller particles, polymerization depth, and coloring agents [35]. Similarly, another research by Kowalska et al. reported that the chemical variations in resin components like fraction of oligomers and monomers, proportion or kind of activators, initiators and inhibitors and oxidation of nonreactive carbon-carbon double bonds may have an infuence on color stability [36]. Existence of micro cracks and micro voids at the merging point between the fller and the resin matrix are more susceptible areas for staining. Te surface roughness due to wear and chemical damage can also have a negative impact on surface shine followed by an extrinsic staining [37].

Optical Properties.
Resin composites are extensively used in restorative dentistry. Optical and structural synchronization of the composite material into the tooth structure and with the neighboring dentition is a signifcant element for the patient's satisfaction along with acceptance with the dental esthetic restorations. Multiple-layered techniques [38] with resin-based composites of diverse opacity and colors have been experienced to imitate the physical appearance of teeth [39,40]. So far, this multilayering restorative treatment entails an accurate shade selection along with technically higher skills that frequently raise the working time and cost as well [41]. Terefore, in order to alleviate the treatment intricacy and to improve efciency, the word "chameleon efect" (blending efect) illustrates the capability of material to attain a shade identical to adjacent tooth structure [42]. Tis ability of a material has facilitated the recognition of innovative dental composites that makes determination of shades easy. Initially, approach was the so-called "group-shaded" composites that involved an extremely limited shade variety wherein every shade covered a suggested group of VITA classical shades [43]. Currently, the perception of "singleshade" or "one-shade "resin composites was established to explain resin-based composites intended to esthetically imitate every shades with single nominal shade. Tese resin composites formulated on this broad shade-matching conception, apparently merge fawlessly into the neighboring dentition [41]. Perceived color is determined by the wavelengths refected from an object [43]. In esthetic restorative materials, for instance, ceramics and resin-based composites, this wavelength refects as a result of the presence of pigments added as constituents. On the other hand, novel technological methods have introduced the single-shaded resin composites that do not contain pigment, and their optical features are relied upon structural color, a "smart chromatic technology" where the resin-based composite reacts to light waves at a specifed frequency by accurately refecting a particular wavelength within the tooth shade space [44]. Similarly, some research studies [21,45,46] demonstrated that the main benefts of OMNICHROMA are based on an enhanced color adjustment potential [47]. One more skill, used to develop Venus Pearl One and Venus Diamond One (Kulzer) that is relied upon "the adaptive light matching" idea, is where the restoration color is attained by absorbing the wavelengths refected by the adjacent tooth color [48]. Likewise, another study by Brewer et al. proposed that the color stability and optical properties of restorative materials are greatly afected by the changes in the dimensions of fller particles and their composition [49].
Concerning composite translucency, Essentia Universal (GC Europe), Filtek Universal (3 M Oral Care), OMNI-CHROMA (Tokuyama Dental America, Inc.), SimpliShade Universal Composite (Kerr Corp), and TPH Spectra ST (Dentsply Sirona) are existing in one translucency instead of multiple enamel, dentin, and body shades that have been used to emulate the optical properties of diferent regions of tooth [46].

Masking Ability of Single-Shade Composite.
Tough the person eye can perceive the change in color variation, but it is a challenging task to get shade harmonizing towards surrounding tooth structure, particularly in cases of Class III and Class IV restorations or in severely discolored tooth structure where there is no or limited surrounding tooth structure residues [50]. Terefore, in those cases, one-shade resin composite with better opacity is applied as a blocking/masking agent in a thin coat prior to application of smart monochromatic material. Tis mask assists in camoufaging the inner stained part of the tooth structure and prevents the shade-matching interference due to discoloration. In case of limited surrounding dentition such as large class III and IV restorations, this blocker is placed over the lingual side to lessen shade-matching interference. Additionally, it is valuable in THE reconstruction of an extremely opaque tooth [51].

Mechanical Properties and Curing
Depth. Dental restorative materials faced multiple types of stresses such as compressive, tensile, and shear that refect the mechanical International Journal of Dentistry properties of dental flling material [52]. Mechanical properties of dental resin composites can be assessed by determining diferent aspects including fatigue, hardness, strength, elastic modulus, fracture toughness, edge strength (chipping), and tooth wear [53].
Optimal properties can be achieved by the adequate curing of dental resin composite restorations, whereas inadequate curing causes restoration failure [54]. An insufciently cured dental resin composite restoration exhibited lower physical and mechanical properties [55]. One of the factors that afect polymerization of dental resin composites is the wavelength of the dental curing light [56].

Wear Resistance and Less Polymerization Shrinkage.
Wear resistance is essential for posterior teeth restoration. Occlusal and proximal wear of class II cavities causes failure of posterior composites. It is reported that incidence of failure for both Classes I and II restorations has been predicted to be 40%-50% [57]. High wear resistance for composites leads to improve their longevity, color permanency, and their function; on the other hand, low wear resistance may cause tooth relocation, temporomandibular joint complaints, muscular infammation, and periodontal infections [58,59]. Wear of composite is afected by the type and size of fller particles, volumetric ratio, organic matrix nature, and coupling agent. Te physical and mechanical properties of dental composites can be improved by modifying new monomers, fller particle size, content change, and fller surface modifcation [60,61]. In earlier studies, it was predicted that the wear of older resin composite was about 50-75 µm annually; however, innovative composites have less signifcant wear which was about 10-20 µm annually [62].
Estelite sigma quick composite comprises uniform silica-zirconia supra-nano spherical-shaped fllers, size of 100-1000 nm with an average size of 200 nm having good wear resistance [63]. Similarly, OMNICHROMA shows an excellent equilibrium among volume loss of the resin composite and human tooth wear. OMNICHROMA is a resin composite that is less prone to damage opposite teeth [44].

Radiopacity.
Te composition and content of the inorganic fller of composites determine their radiopacity. Te radiopacity of a resin rises with the content of high atomic number of fllers. Tough, fllers having huge amount of greater atomic number elements apt to have large refractive indices. Te radiopacity of smart monochromatic material is moderate and appropriate for diagnostic purposes [64].

Limitations
(i) Long-term color stability of smart monochromatic composite in oral cavity is questionable [14]. (ii) Te infuence of aging on the physical properties of smart monochromatic composite is promising [65].
(iii) Te color-matching ability of smart monochromatic composite is excellent with lighter tooth shade while it is not very good with darker tooth shade [66].

Future Perspective
It could be interesting in the future to test the present material in terms of fexural strength and hardness [67,68].
Additionally, the mineral deposition should be assessed in order to gain more knowledge about this interesting recently introduced smart monochromatic composite [69].

Conclusion
Over the past decades of enhancing esthetic restorative materials, there have been many remarkable composite restorations recognized by clinicians worldwide. Nonetheless, smart monochromatic composites are innovative resin composites which presented promising results and most stimulating advancements in the recent times. Tey are easy to apply, having higher mechanical properties, good wear resistance, and better optical properties along with color stability than those of conventional resin composites that ofering admirable esthetics. Further research studies are needed along with follow-up of cases in order to get promising prospects. Additionally, extra experimental trials are also required to document their long-lasting use.

Data Availability
Te data supporting this literature review are from previously reported studies and datasets, which have been cited. Te processed data are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.