Understanding Color: How Materials Get Their Looks

Understanding Color: How Materials Get Their Looks. Image: © belahoche - stock.adobe.com

Esthetics is key in restorative dentistry. Although mechanical function is paramount in successful restorations, the primary concern for many patients is esthetics. As important as texture and contour are, color is perhaps the most critical aspect in achieving impeccable esthetics.

“In the past, esthetics of dental restoration were not as achievable as they are today,” says Amelia Triplett, junior research scientist at products and materials manufacturer BISCO. “Esthetics took a back seat to the important mechanical properties required of a dental restoration. Now, science and technology have advanced enough to ensure the strength [required] of a restorative material using chemistry, and we are able to have greater focus on the esthetics.”

With the strength issues handled, esthetics can take center stage. There are strong materials easily customizable for color and opacity, allowing for more natural-looking restorations. In addition to these chemical advancements, Triplett says, clinicians now have the tools of color science and technology, such as spectrophotometry and light control, to aid in the shading process.

“We’ve got the physical strength requirements taken care of,” Triplett says. “Now the artistry of restorative dentistry can shine through.”

A big component of this artistry is color. Although we were all introduced to the color wheel in kindergarten, color theory in dentistry is a little more complex than the old childhood mnemonic ROYGBIV.

How Color Works

At its most basic level, light is the key to color. To make a color, light is reflected from a surface or filtered through something translucent. For example, according to subtractive color theory, blue only appears that way because it absorbs the colors that are not blue (red and green) but reflects blue light.

According to the Munsell color system, color is described by hue, value, and chroma. Developed in the early 1900s by Albert H. Munsell, the system was the first to separate color into the 3 dimensions and apply a numeric value to colors.

In this context, hue is defined as “the dominant range of wavelengths in the visible spectrum that yields the perceived color, even though the exact wavelength of the perceived color may not be present.”1 In plain language, the hue is the color. Value is the quantity of light that is returned from a material; essentially, the lightness of the color, or how dark the color falls on a black-to-white scale. Chroma refers to the saturation or intensity of the hue and is inversely linked to value.

Although hue, value, and chroma play a large role in coloration, the opacity of a material also affects the appearance of color. Materials that are transparent allow light to pass through with minimal interference, simplifying color matching. In comparison, opaque materials reflect and absorb light.

However, things get complicated when it comes to translucency. Translucent or semitranslucent materials (such as a tooth’s surface) scatter, absorb, and transmit light, affecting how the material interacts with light. This makes it much more difficult to determine shade. For example, in crowns, higher translucency means less light is reflected, lowering the crown’s value.

“Depending on the thickness of the material, translucency allows the underlying color to show through in varying degrees,” says Shotaro Matsubara, CEO of Tokuyama Dental America. “It will affect the value and chroma of the final shade.”

In short, the more translucent the material, the less light is scattered within it, and the more its color will be affected by the surrounding colors.

“Translucent materials will take on the color of the background, as the light reflected from more colorful/opaque background objects will be able to pass right though a translucent object easily,” Triplett says. “The thickness in which the material is used also has a more pronounced effect on the perceived color of translucent materials than it does on the perceived color of opaque materials.”

Fluorescence also plays a role in restoration color. Fluorescence is the emission of light from a material that has absorbed light, most often emitted in a longer wavelength. In natural teeth, fluorescence usually occurs in dentin, as it is highly organic. To mimic this, fluorescent powders are often added to restorations to bolster the amount of light returned, masking discolorations and decreasing the chroma—particularly beneficial in high-value shades where it can increase value without hurting translucency.¹

Triplett notes that dental materials are also affected by molecules within that interact with visible light wavelengths. The color of a material can be affected by the light in which it is viewed, because if the light does not have a specific wavelength band, the material cannot reflect it.²

Regarding dentistry, Triplett says, one of the molecules involved in light-curable materials is camphorquinone. This material absorbs the near-UV visible light from curing lights and initiates free radical polymerization.

“Because camphorquinone absorbs near-UV visible light, which to us appears bluish in color, camphorquinone reflects the other wavelengths of light, resulting in this chemical appearing yellow to us,” Triplett explains. “If camphorquinone is not optimized in a dental restorative material, it can possibly cause an unesthetic yellow color.”

When it comes to color in dental materials, colorants (pigments or dyes responsible for the chromatic reflection of light with the material) are employed to create certain shades. The chemical composition of a particular colorant absorbs certain sections of the visible spectrum.² 

“Most dental materials get their color from adding chemical color—pigments and dyes to achieve the various shades and opacity needed to blend with natural dentition,” Matsubara says.

These pigments and dyes, Triplett clarifies, are used in small amounts labeled Generally Recognized as Safe or approved under the Federal Food, Drug, & Cosmetic Act (FD&C) or by the United States Food and Drug Administration. They include titanium dioxide and FD&C Red No. 40. However, in recent years, alternatives to colorants have emerged.

“The assumption that we have to use chemical color, pigments, and dyes to achieve shade matching in resin composites has really had a paradigm shift in the [past] few years,” Matsubara says. “In 2019, an alternative option launched with the release of OMNICHROMA. The resin composite relies on structural color—the interaction between structure and light to create color—to shade match to surrounding tooth structure.”

This concept of structural color was a novel development for resin composites. A 1-shade universal composite, OMNICHROMA employs supranano spherical fillers that can shade match any color from A1 to D4. In one study, conventional A2-shade resin composites (composed of 5-50–nm fused silica fillers and 100-nm spherical fillers) were compared with structural color composites. Researchers determined that the structural color composite (comprised of 260-nm spherical fillers) exhibited lower color differences between natural tooth and restored areas and a higher color adjustment potential.³

“When OMNICHROMA was first launched, there was a bit of hesitation since the concept of structural color was never considered viable for resin composites,” Matsubara says. “Structural color is a natural way to see color and has been studied for practical application in many industries due to its long-term stability and environmentally friendly properties.”

Another unique benefit of OMNICHROMA is its ability to shade match post bleaching, or as teeth change color over time. “This is a big benefit for esthetic dentistry, because patients who need anterior restorative work no longer have to decide immediately [whether] they plan to whiten their teeth,” Matsubara explains. “OMNICHROMA gives them the freedom to decide when and how much they want to brighten their smiles in the future.”

What to Consider

When it comes to choosing a material, there are several things to keep in mind that can affect color, particularly in the long run. “A hydrophilic material will take on water over time, which can affect the perceived color of the restoration. Additionally, a high monomer percentage in a material’s composition can lead to higher water sorption. It is important to ensure these materials are properly cured, which can increase a material’s hydrophobicity,” Triplett says.

This is of particular importance with materials that have lower depths of cure, such as highly opaque materials. To lower the chances of marginal discoloration, clinicians should choose materials with minimal shrinkage during curing. Additionally, Triplett says, materials should be polishable to help prevent plaque accumulation and bacterial adhesion.

“In addition to water sorption, exposure to UV light causes degradation of dental materials over time and this degradation can cause color change,” she states. “Proper curing can help decrease water sorption, as well as choosing less hydrophilic materials for restoration. When it comes to slowing UV degradation of the material, a chemical called a UV stabilizer can be included in formulation.”

Whichever material clinicians choose, they cannot forget one important thing: Color is all about light. Light quality is one of the most important factors when identifying shades. Everything can affect the quality of light, including season, weather conditions, time of day, or whether the light is natural or artificial.

“Dentists should keep in mind under which kind of light they perform shade matching,” Triplett says. “Restorative dental materials have the potential to look slightly different under different light sources. It is recommended to perform shade selection using natural light or by employing a D65 daylight lamp, if possible.”

Light comes into play before the material ever makes it to the dentist’s chair.

“Materials get their esthetics through careful and meticulous optimization of each component of the material’s formulation,” Triplett says. “This includes the photoinitiators, monomers, inorganic fillers, and tints. We make sure to study how light interacts with each of our products with spectrophotometry, intense light irradiation, and measurement of refractive indices.”

Once a material has made it out of development and into the chair (and the patient’s mouth), the challenges do not end. Over time, dental materials are subjected to all the stresses faced by natural teeth, including the patient’s lifestyle, eating and drinking habits, and oral hygiene.

“The best way a clinician can protect his patient from these factors is to have a strong, reliable bonding, finishing, and polishing protocol,” Matsubara says. “This will limit any lifestyle factors that may impact the restorative in the future. Here, a single-shade composite that adapts to the changing tooth color will be handy.”

The bottom line when selecting esthetic materials is that clinicians find something that meets their clinical needs and satisfies the patient. “Oftentimes we are swept away by the possibilities when we look at esthetic material and how many options we have to achieve these amazing restorations if we buy a complex composite system that has every shade under the sun,” Matsubara says. “If you have patients that necessitate a huge system, then yes, it makes sense. If you don’t use a wide range of shades in your general restorations, you probably don’t need to spend the money for a complex kit. We have the single-shade system option, and a reduced shade system—a very user-friendly esthetic composite kit, Estelite Omega, in collaboration with Newton Fahl Jr, DDS, MS. He helped us design a system that would achieve high esthetics with only 11 well-crafted shades.”

The options are out there, the science is advancing, and the color of restorative materials is constantly improving, ensuring that restorative dentistry becomes more esthetic by the day.

“We love the science,” Triplett says. “[Although] we employ traditional dental material chemicals in our products, we are also always aiming to improve our technology and studying new chemistries to apply to future products...The dentists we connect with are an integral part of our product development in R&D. It is always a pleasure to discuss the science behind dental materials.”

References

1. Sikri VK. Color: implications in dentistry. J Conserv Dent. 2010;13(4):249-255. doi:10.4103/0972-0707.73381

2. Brewer JD, Wee A, Seghi R. Advances in color matching. Dent Clin North Am. 2004;48(2):v, 341-358. doi:10.1016/j.cden.2004.01.004

3. Kobayashi S, Nakajima M, Furusawa K, Tichy A, Hosaka K, Tagami J. Color adjustment potential of single-shade resin composite to various-shade human teeth: effect of structural color phenomenon. Dent Mater J. 2021;40(4):1033-1040. doi:10.4012/dmj.2020-364