Cronicon · Cronicon OPEN ACCESS EC DENTAL SCIENCE Review Article Contemporary Understanding of Colors in Aesthetic Dentistry: Review Nagy Abdulsamee1* and Passant Nagi2 1Consultant

CroniconO P E N A C C E S S EC DENTAL SCIENCEEC DENTAL SCIENCE

Review Article

Contemporary Understanding of Colors in Aesthetic Dentistry: Review

Nagy Abdulsamee1* and Passant Nagi2

1Consultant Prosthodontics and Head of Dental Biomaterials, Faculty of Oral and Dental Medicine and Surgery, Modern University for Tech-nology and Information, Egypt2Lecturer of Pediatric Dentistry, Faculty of Oral and Dental Medicine and Surgery, Cairo University, Egypt

Citation: Nagy Abdulsamee and Passant Nagi. “Contemporary Understanding of Colors in Aesthetic Dentistry: Review”. EC Dental Science 19.2 (2020): 01-18.

*Corresponding Author: Nagy Abdulsamee, Consultant Prosthodontics and Head of Dental Biomaterials, Faculty of Oral and Dental Medicine and Surgery, Modern University for Technology and Information, Egypt.

Received: November 18, 2019; Published: January 11, 2020

AbstractOne of the most challenging procedures in restorative dentistry is to closely matching natural teeth with an artificial restoration.

Color and shape of natural teeth vary greatly providing more information about the background and personality of our patients. The final goal of dentistry is to restore patient’s unique characteristics or to replace them with alternatives. Learning the language of color and light characteristics provides the ability to assess and properly communicate information to laboratory technician. This help the technician duplicating what has been distinguished, understood, and communicated to ceramic, composite filing or artificial teeth in the shade matching of natural dentition.

Anterior restorations that gave esthetic satisfaction to both dentist and patient must acquire as much optical properties as natural dentition; color, gloss, opalescence, fluorescence, all these physical optical properties are mandatory to consider. Better understand-ing and some strategies for best assessing of what happens when incident light interacts with teeth surface will be given to each individual of the dental profession helping them in duplicating the best of restoration color and aesthetics.

Keywords: Illuminant; Observer; Objects; Color; Hue; Chroma; Value; Colorimeter; Metamerism; Opalescence; Counter-Opalescence; Fluorescence; Gloss

Introduction

One of the ideal requirements of dental restorative materials is that they should match the soft and hard natural oral tissues in color and appearance. The primary objectives of the restorative dentist are to restore the damaged or missing tooth structures into normal functional and aesthetic properties. For many patients especially nowadays, restoring dental esthetics is of priority. The restorative mate-rial used, anatomical form, surface texture, translucency and color are important factors affecting the aesthetics of a dental restoration. This means that considering the patterns of reflection and absorption of the light is not an easy task to restore natural tooth appearance accurately [1,2].

Dentistry is defined science and art. Knowledge of the underlying scientific principles of color is essential for the dentist and the technician because esthetic dentistry imposes several demands on their artistic abilities. For improvement of esthetics by making the restoration appear natural and attractive needs an idea about color mixing. Before evaluating and selecting the proper color shade for the



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restoration, the fundamental principles of color and light, the radiation spectrum and the optical characteristics of the object should be understood [3].

For a vivid and extremely natural restorations a better understanding of the inherent characteristics of the dental tissues to incident light allow a more artistic restorative approach where the light can be manipulated in each restoration [4,5]. Knowledge of the underlying scientific principles of color and other optical effects is essential for the dentist and technician which would affect their artistic abilities for aesthetic dentistry. Learning color language and light characteristics will improve the ability to assess and properly communicate color information to our laboratory technician to duplicate in ceramic what has been communicated in the shade matching process of the natural dentition [6]. The aim of this work is to present a simple and effective understanding of color fundamental principles implicated in dentistry which will be reflected upon helping the practitioners to obtain imperceptible final restorations.

Important factors affecting esthetic appearances are color, translucency, gloss, fluorescence, opalescence and counter-opalescence. Each of these factors, as perceived by an observer such as a dentist, technician, or patient, is influenced by the illuminant (light source), the object, and the interaction of the observer. Color is not only a static attribute inherent to objects but also is a brain response to an electromagnetic stimulus sensitive to our eyes. For a color to be felt there should be presence and interaction of three fundamental and interdependent factors: the illuminant, the object and the observer (Figure 1). Illuminant sends radiations to be reflected or transmitted by the object to observer’s eye. The eye is responsible for capturing and transformation of this physical energy into chemical impulses to be interpreted in optical center in the brain [2,7].

Figure 1: The color triplet.

Light source or illuminant

The illuminant may be a natural or artificial light source, which, according to its origin, can change the perceived color of an object. A phenomenon known as metamerism will result where a white sheet of paper seems bluer under fluorescent light and more yellowish under an incandescent light bulb, returning to its original color in the presence of daylight [1]. Electromagnetic radiation of light can be detected by the human eye. The eye is sensitive to wavelengths from approximately 400 nm (violet) to 700 nm (dark red). In order for an object to be visible, either it must emit light or it must reflect or transmit light falling upon it from an external source. Objects of dental interest generally transmit light. The incident light is usually polychromatic (mixed light of various wavelengths) that is a mixture of the various wavelengths, commonly known as “white” light. The reaction of an object to the incident light is to selectively absorb, transmit


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and/or scatter certain wavelengths [8]. The spectral distribution of the transmitted or reflected light will resemble that of the incident light although certain wavelengths will be reduced in magnitude but still able to stimulate photosensitive retinal cells, triggering the pro-cess of color perception [2,7].

The quality of light source is the most influential factor when determining tooth shade. For accurate color comparison, natural light occurring around mid-day, is the ideal light source. The time of the day, month and weather conditions affect the color of sunlight. Light reflected from an object will be changed by changing the light source and perceiving a different color too [9]. Natural light or white light contains all the colors of the visible spectrum such as sun light [10]. Sunlight should be considered the first choice in chromatic proce-dures because it plays a key role in the science of colors. It is highly recommended to use one of sun light alternative lamps that simulate solar lighting in ideal weather conditions if sun light is not available. There are several alternative light sources according to international commission of illuminating (CIE, Commission Internationale de L’eclairage) and each one has its color temperature described in Kelvin degrees (K) [11]. However, the most commonly alternative light sources found in supermarkets and specialized stores having tempera-ture of 2856 K (standard A) and with a temperature of 6500 K (standard D) can be considered basic sources [12,13]. The use of artificial lighting that approximates standard daylight is ideal for shade matching due to the absence of ideal conditions. To measure the capacity to reproduce standard daylight for color matching the light source, color temperature, spectral reflectance curves and color rendering index (CRI over 90) are all used. Dental unit usually used Incandescent bulbs emitting lights high in the red-yellow spectrum and low at the blue end. For more accurate color reproduction regular cool white fluorescent lights which are high in the green-yellow spectrum and color-corrected fluorescent lights are also available [9]. Incandescent bulbs are now replaced by the full-spectrum light-emitting diodes (LEDs). The shade-matching ability is better with a light-correcting source than under natural light [7].

To eliminate the variability of different light sources, full-spectrum LEDs showing a color spectrum similar to mid-day light “the Op-tilume Trueshade device” is used. To create optimum diffuse daylight use diffusion lenses over the LEDs to mix the three (RGB) colors of light emitted by the individual color. The clinician can more accurately assess the true color by setting the LEDs at a 45-degree angle to minimize spectral reflectance or glare [14]. A unique feature of Optilume Trueshade device has a unique property of reducing the inten-sity of the light source while maintaining the color temperature. For better perception of surface details, such as topography, ridges and enamel striations a lower-intensity light should be used [3]. Incandescent illuminants “standard A” should not be used alone in chromatic procedure because it presents yellow chromatic trend with low spectral amplitude. The illuminant “standard D” presents a bluish chro-matic trend, higher spectral amplitude and ultraviolet (UV) wavelength included, being called fluorescent and the light spectrum emitted by them closely resembles natural daylight and, therefore, should be considered the first option when you can’t use it [2,15].

Importance of illuminant in dentistry

The appearance of an object depends on the type of the light by which the object is viewed. Daylight, incandescent lamps and fluo-rescent lamps are all common sources of light in the dental operatory. Objects that appear to be color matched under one type of light may appear very different under another light source. This phenomenon is called metamerism (Figure 2) [8]. The quality and intensity of light are factors that must be controlled when matching colors in dental restorations. Because the light spectrum of incandescent lamps, fluorescent lamps, and the sun differ from each other, a color match between a restorative material and tooth structure in one lighting condition might not match in another [16].

By selecting a shade and confirming it under different lighting conditions, such as natural daylight and fluorescent light the problem of metamerism can be avoided [17]. Whenever possible, shade matching should be done in conditions where most of the patient’s activi-ties will occur [16] and by the use of isomeric pair e.g. the effect of metamerism between shade tabs and veneering material is avoided, because the shade tabs are made of the same ceramic as the veneering ceramic i.e. isomeric pairs [18].


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Figure 2: Example of metamerism: the apple changes color depending on the light source used to illumin.

The observer

Perception of color

Without light, color does not exist. It is the interaction of light with the object that allows the perception of color. If the light interacts with an object, some of the light is absorbed by an object. The wavelength that are not absorbed (i.e. those that are reflected, transmitted, or emitted directly to the eye) are perceived by receptor cells. The human eye and brain, which enable color vision, form an amazing and complex system. The visual system of a person with normal color vision can identify millions of different colors [19].

Visible light enters the eye through the transparent area of the cornea and is focused by the crystalline lens on the retina. The retina is composed of two types of specialized photosensitive cells and is the receptor system for vision. These specialized receptor cells are called rods and cones and they contain photosensitive pigments [2]. The more numerous of the two photoreceptors are the rods, which are sensitive to low levels of light. The rods are primarily responsible for our peripheral vision and are unable to detect color. In low levels of light, rods help us see objects in gray scale; as the light becomes brighter, the rods become inactive. On the other hand, the cones operate in bright light and provide high acuity color vision. Both photoreceptors transform light into chemical energies that stimulate millions of nerve endings. The neural signals are transported by the optic nerve to the brain, where color is interpreted. There are three types of cones in the retina that are sensitive to different wavelengths of light: blue, green, and red. The blue cones are most responsive to short wavelengths. The green and red cones are most responsive to medium and longer wavelengths, respectively, with some overlap [20].

An image is focused on the retina when light enters the eye through the cornea and lens. The iris controls the amount of light entering the eye as it dilates or constricts depending on the level of illumination. Variation of light intensity is adjusted by the retinal rods and cons. Differences in color discrimination among observers with normal color vision is the responsibility of the area around the fovea centralis which has a mixture of sensors [9]. The area of the retinal field stimulated by light affects the accuracy of color perception. In dim light the pupil widens stimulating sensors that are less accurate but in high illumination the reverse takes place. Light intensity is a critical factor in color perception and shade matching depending on the diameter of the pupil. Successive contrast, simultaneous contrast and color constancy are the three important features that reflect color matching [21].



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Afterimage and visual distortion afterimages (Eye fatigue)

Alterations in perceptions are caused by physiologic effects of the cone receptors with normal function. The spreading effect that occurs when light is removed from the retina; the receptors continue for a short time to be active and send a signal to the brain is an ex-ample of afterimage that commonly affects clinicians [22]. The eyes will flick back and forth between two adjacent areas of different color, involuntarily if presented simultaneously and the color seen for each will be a combination of two colors. It is important for the clinician to decide within seconds, when holding a shade guide close to a tooth, because the two will immediately begin to appear more and more alike [23].

Fatigue of the cone receptors lead to a negative afterimage due to decreasing their sensitivity to further stimulation. The red receptors in the roaming eyes become fatigued, while the blue and green receptors remain fresh and can be fully stimulated, if strong red lipstick re-mains next to the tooth being evaluated yielding a perception of the tooth that is too blue-green. To avoid the negative afterimage, give the eyes a break with neutral gray backgrounds like Kulzer’s small intraoral gray cardboard, Pensler shields screen background color glare; or 18% reflective gray cards are the photographic industry standard achromatic background [24]. Do not use blue backgrounds because they cause afterimages and will bias perception to the complementary color, orange. Others recommend the use of a blue background to make the eyes more sensitive to yellow-orange, but this selectively fatigues one type of cone and does not make the others any more sensitive [25]. An excellent background for photographic evaluation of hue and chroma is the 18% reflective gray card [26].

The color of object

The interactions an object with the incident light will produce its color. These interactions include reflection, refraction, absorption (and fluorescence), and/or transmission. The illuminant affects the color of any object. Colorants (pigments or dyes) are responsible for chromatic reflection of light results from the colorants (pigments or dyes) present in that object. By reflecting that part of the spectrum of light incident upon a material and absorbs the other parts of the light spectrum, it gains its reflective color, e.g. blue surface absorbs all colors except blue one, white surface reflects all incident wavelengths, black object absorbs all wavelengths. Colored objects appear black when viewed under lights that do not contain their specific wavelengths, e.g. red object appears black under blue light. The chemical composition of a colorant selectively absorbs one part of the visible spectrum more than another. When a particular wavelength segment of light is reflected and enters the eye, the sensation of color is produced. Color can be denied as a psychophysical sensation provoked in the eye by the visible light and interpreted by the brain [21].

Types of color

Additive color theory

The additive primary colors (Figure 3a) are red, green and blue (RGB). The additive secondary colors (cyan, magenta and yellow) result from mixing one of these additive primary colors with equal amounts of another one. Additive color is commonly associated with phenomena such as TV viewing, flames, florescence, phosphorescence etc. (light emitting sources) i.e. the eye is stimulated directly by waves coming from the light emitting source (Figure 4A). Two colors are said complementary if their addition results in white color, e.g. cyan + red and yellow + blue.

Figure 3: a- Additive and b- subtractive colors.


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Figure 4: A) Additive color perception, B) Subtractive color perception.

The subtractive color theory

The primary subtractive colors are cyan, magenta and yellow (Figure 3b). Color is more commonly perceived by a subtractive process where an object is viewed by a process of reflection and absorption of incident visible light. It occurs when the eye is stimulated indirectly by a synthesis of the visible light emitted by the light source and the reflected light of the illuminated object (Figure 4B). Effectively the illuminated object through the contained pigments may remove (subtract) certain wavelengths, depending on the relative intensities of the emitted and absorbed light, hence subtractive color. The additive secondary colors are the subtractive primaries-Cyan, Magenta and Yellow- and vice versa for the subtractive secondaries (Figure 3).

Mixing of cyan and magenta (two subtractive primary colors) will form blue color which is complimentary to their third subtractive primary color, yellow. Black color results from mixing of the three subtractive primary colors (Figure 5) [27].

Figure 5: In subtractive system, black results mixing of the three primary colors.


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Measurement of object’s color

Munsell color system (visual) and the CIE color system (instrumental) are the most commonly used methods for measuring the color of restorative dental materials.

Visual method

Many color order systems are available, but for a variety of reasons, including worldwide recognition, consistency, flexibility and sim-plicity, the Munsell color order system, devised by American artist Alfred H. Munsell in 1898, is the system of choice for color matching in dentistry. The color tree is a representation of the tridimensional organization of colors within the Munsell system (Figure 6) [19]. It forms the basis for the classification of colored objects in the three parameters hue, value, and chroma). It is important to grasp the con-cepts of hue, value, and chroma to fully understand dental shade matching [28].

Figure 6: Munsell color system.

Quantitatively color is measured by three parameters according to Munsell: a) Hue describes the dominant color of an object, e.g. red, orange, yellow, etc. The hues are arranged on a circle (Figure 7a), b) Value (lightness or brightness) corresponds with the lightness or darkness of the color (Figure 7b). It forms the vertical axis of the cylinder system (Munsell’s system) and ranges from white (10/) to black (0/). It is the most important color factor in tooth color matching because a tooth of low value appears gray and non-vital. Teeth and other objects can be separated into lighter shades (higher value) and darker shades (lower value), and c) Chroma measures the strength or intensity of the color (Figure 7c). It forms the radial axis of Munsell’s system with a minimum of 1 and a maximum of 18 [20].


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Figure 7: Color parameters according to Munsell.

A large set of color tabs is used to determine the color. Value (lightness) is determined first by the selection of a tab that most nearly corresponds with the lightness or darkness of the color. Chroma is determined next with tabs that are close to the measured value but are of increasing saturation of color. Determination of the hue is done lastly by matching with color tabs of the value and the chroma already chosen. measuring of the hue in increments of 2.5 for each of the 10 color families (red, R; yellow-red, YR; yellow, Y; green-yellow, GY; green, G; blue-green, BG; blue, B; purple-blue, PB; purple, P; red-purple, RP) is performed easily on a scale from 2.5 to 10. The color of the attached gingiva of a healthy patient has been measured using Munsell color system as 5R/ 6/4 to indicate a hue of 5R, a value of 6, and a chroma of 4.

The difference in two similar colors can be calculated using Munsell color system according the formula derived by Nickerson: I = (C / 5) (2 Δ H) + 6 Δ V + 3 Δ C

where C is the average chroma and ΔH, ΔV, and ΔC are differences in hue, value, and chroma of the two colors. For example, if the color of attached gingiva of a patient with periodontal disease was 2.5R 5/6, the color difference, I, between the diseased tissue and the afore-mentioned healthy tissue (5R 6/4) would be as follows:

I = (5 / 5) (2) (2.5) + (6) (1) + (3) (2) = 17

A trained observer can detect a color difference, I, equal to 5 [16].

Instrumental technique

Advantages of instrumental color over visual color determination are instrumental readings are objective, can be quantified and are more rapidly obtained. In an attempt to overcome problems with visual shade matching in dentistry, spectrophotometers and colorim-eters have been used with modifications. The color space, in instrumental system consists of three coordinates: L*, a* and b* (Figure 8) [29]. The L* refers to the lightness coordinate which ranges from 0 for perfect black to 100 for perfect white, and the a* and b* are the chromaticity coordinates in the red-green axis and yellow-blue axis, respectively. Positive values of a* reflect the red color range whereas



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negative values indicate the green color range. The same for b* positive values indicate the yellow color range while negative values indi-cate the blue color range.

Figure 8: CIE L*a*b* color arrangement.

Using instrument (CIEL) the differences between two colors can be determined from a color difference formula: Δ Eab * (L * a * b *) = [(Δ L *) 2 + (Δ a *) 2 + (Δ b *) 2] ½

where L*, a*, and b* depend on the tristimulus values of the specimen and of a perfectly white object. Under standardized conditions a value of ΔE* of 1 can be observed visually by half of the observers. A value of 3.3 for ΔE* is clinically perceptible [16].

Computerized shade-matching systems have appeared on the market over the last decade (Figure 9). When compared with Munsell or CIE color measuring systems, it offers better accuracy, improved efficiency, and esthetic benefits to patient, dentist, and technician. These systems are based on technology imported from the painting industry to analyze the color of the natural teeth; calculate the exact ratio of hue, chroma, and value for a multitude of points on the tooth surface; and display this information on the dentist’s computer screen. It illuminates the tooth surface associated with reading the shade tabs and greatly facilitates the communication of information to the lab technician. It encourages the fabrication of predictably accurate, highly esthetic restorations because of improved flow of information. From the lab’s perspective, the frequency of remakes is reduced.

Computerized shade-matching systems include hardware and software that identifies the variously colored, translucent, reflective and characterized areas of a tooth, and are available in a variety of formats. The provided information gives a computerized shade map, one that offers more detail than traditional shade matching and communication. More advantages for the computerized shade matching are elimination of the subjectivity and frequent perception errors associated with traditional shade taking, improving the accuracy and pre-dictability of restorative procedures, particularly those difficult situations in which a single anterior tooth is being replaced (Figure 10). Furthermore, accurate shades can be taken anywhere at any time because there is no requirement for standardizing the light environment of the dental operatory [27].


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Figure 9: VITA Easy shade intraoral dental spectrophotometer in use.

Figure 10: A, MHT SpectroShade Micro. B and C, Screenshots. It calculates the numeric difference between natural tooth and the selected color in terms of brightness, chroma, and hue.

Translucency

The gradient between transparency and opacity is called translucency where human teeth have a varying degree of it. The color of an object can be modified by the intensity and shade of the pigment or coloring agent and by its translucency or its opacity. Hard and soft tissues vary in their degree of opacity. Most exhibit some translucency. Translucency is a midway between transparency and opacity


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(Figure 11) and differentiated from them as it allows passage of some light and scatters or reflects the rest and an object cannot be clearly seen through them, e.g. tooth enamel and porcelain. Translucency is important property in dentistry as it gives lighter color appearance. A more translucent material will show more effect of the backing on the color and appearance. Transparency allows passage of all light through if and an object can be clearly seen through them, e.g. glass. Opposite to transparency opacity prevents passage of light through it and an object cannot be seen at all through them, e.g. dentine and opaque porcelains [16].

Figure 11: Opacity, translucency, and transparency.

Opalescence

The optical property that is called opalescence gets its name because it was first observed in opal stones (Figure 12) [30]. Opal is a natural stone composed of aqueous disilicate that breaks transilluminated light into its component spectrum by refraction making opal acting like prisms and refract different wavelengths to varying degrees [6]. Opalescent materials, such as dental enamel, are able to scatter shorter wavelengths of light. Under transmitted light, they appear brown/yellow, whereas shades of blue are perceptible under reflected light (Figure 13). Enamel opalescent effects make the tooth brighter and give it optical depth and vitality. To produce highly esthetic res-torations that truly mimic the natural appearance of the tooth, materials with opalescent properties should be used.

Figure 12: Opal stone observed under direct reflected light (left) and transmitted light. Note the blue shade under reflected light and the orange shade under transmitted light.


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Figure 13: Demonstration of opalescence in a ceramic restoration. The tooth appears brown under transmitted light and blue under reflected light.

Depending on transmission or reflection of light at the incisal edge, the incisal third may display an opalescent pattern with a distinct line of reflection described as the incisal halo (Figure 14) [20]. Enamel of all natural teeth presents opalescence (Figure 15). This opal-escence is better seen in the upper central incisors, in the form of a blue band, located near to the incisal edge called opalescent halo [31,32]. Another optical phenomenon called counter-opalescence responsible for orange appearance that can be observed in the region of the mamelons tip of anterior teeth appears beside the opalescent halo [32,33]. These optical phenomena are attributed to the enamel as it is highly mineralized tissue where these minerals act as a filter and has the ability to forward longer wavelengths while at the same time reflect the shorter waves. Because of this reflection, incisal enamel can be viewed as having a bluish white color (milky appearance). When longer waves that were transmitted through enamel and reach dentine they are reflected back giving enamel an orange color (an effect known as counter-opalescence). The opalescence was considered by some authors as a chromatic scale due to its great aesthetic importance [33].

Figure 14: The incisal halo: distinct line of opalescent reflection at incisal edge.



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Figure 15: Central incisor slices observed under reflected and transmitted light in black background. Under reflected light (a) we could see bluish shades on enamel. Under transmitted light (b) enamel shows an orange color.

The ceramist can produce opalescence and counter-opalescence effects by using different opalescent glazes. Some have a bluish color and others have an orange color, depending on the addition of pigments to the ceramic powder. Opalescence of ceramics has been respon-sible to solve aesthetic problems related to value and translucency, making possible to produce unnoticeable restorations [34,35]. For correct reproduction of opalescence with ceramic systems involves careful observation by the clinician who recorded it as information and will forwarded it to the technician who performs the restoration to mimic natural teeth (Figure 16) [2].

Figure 16: A) Clinical photograph taken in natural light of two extracted teeth (b and c) and one all-ceramic tooth replica (a), B) The same teeth shown (A) under reflected light show opalescence. The properties of the ceramic tooth are similar to those of the natural teeth, and C) The same teeth shown in (A) and (B) under transmitted light show counter-opalescence. The ceramic tooth exhibits the

same translucency as the natural teeth.

Fluorescence

Fluorescence by definition is the absorption of light by a substance and its emission spontaneously at the same time at a longer wave-length. Such a substance emits more visible light than it receives, making it appear brighter than a non-fluorescent substance which, at best, can only reflect the visible light that is falling on it [30,36,37]. Fluorescent light is emitted from sound human teeth when excited by ultraviolet radiation (365 nm), the fluorescence being polychromatic with the greatest intensity in the blue region (450 nm) of the spectrum (Figure 17) [2]. Normal dentitions had their auto-fluorescence property due to mainly dentin that contained amino acids as tryptophan that found at the collagen fibers substrates which made dentin fluoresce three times more than enamel (Figure 18) [38].


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Figure 17: a) Natural extracted teeth under natural daylight conditions, and b) The same teeth shown in Figure 16-A exposed to UV light appear to glow due to their natural fluorescence..

Figure 18: Fluorescence of central incisor slices under daylight (A) and black light (B). Because it is associated with the amount of organic matter, note that under black light (B) it presents three times greater intensity in dentin than in enamel.

Mechanism of fluorescence

A material will fluoresce when its molecular system absorbed some type of energy from an incident invisible light and some of its elec-trons got excited to a higher quantum level then after a lag period, the excited electrons returned to the equilibrium state and lost their energy in the form of photons usually with lower energy and longer wave length than the absorbed one (Figure 19).

Figure 19: Mechanism of fluorescence.


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Fluorescence occurred when an orbital electron of a molecule, atom or nanostructure relaxed to its ground state by emitting a photon of light after being excited to a higher energy level by a type of energy [39].

Significance of florescence in dentistry

1. Diagnosis of early caries: early carious dental tissues whether it was enamel or dentin cannot be diagnosed under normal light-ing (Figure 20a) but appeared black or dark brown under UV light illumination (Figure 20b) because they had been losing their fluorescence [40,41].

2. Fluorescence makes a definite contribution to the brightness and vital appearance of human teeth e.g. a person with ceramic/composite restorations that do not contain fluorescing agent appear to be missing teeth when viewed in UV light. Therefore, any artificially substituting material to them should contain fluorescing materials. Differences in fluorescence properties of com-posite resins can be better observed when compared to a natural tooth (Figure 21). Some anterior restorative materials contain fluorescing agents (rare earths excluding uranium) to reproduce the natural appearance of tooth structure [16,42]. Ceramic crowns are made from fluorescent powder to increase the quantity of light returned back to the viewer, block out discolorations, and decrease chroma [43]. Porcelain consists of agents that cause the restoration to become fluorescent. Fluorescence adds to the natural look of a restoration and minimizes the metameric effect [2]. Composite resins fluoresced because of the luminescent (fluorophores) incorporated in, luminescent elements such as europium, cerium, and ytterbium (rare earths) oxides. Composite fluorescence meant to be very similar to that of tooth structure was obtained through the elements belonging to groups (III, IV and V) in the periodic table. This fluorescence, however, presented as highly dependent on the type of the material which they were incorporated in. Uranium oxide used as a fluorescent for years, but its use abandoned because it released radiations [40].

Figure 20: The same tooth seen by a) normal light and b) ultra violet light.

Figure 21: Schematic representation of the importance on the composite resin surface in fluorescence of restorations. Restoration A and restoration B show fluorescence, while restoration C and D do not.


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Gloss

It is the optical property that produces a lustrous appearance. It is an attribute of visual appearance that originates from the geometri-cal distribution of light reflected by surfaces. The term gloss is used to describe the relative amount of mirror-like (specular) reflection from object’s surface. Reflection from a smooth, mirror-like surface produces clear well-defined image. Metals are usually distinguished by stronger specular reflection than that from other materials, smooth surfaces will appear glossier than rough ones [44].

Importance of gloss in dentistry

1) It is an important appearance of dental materials. Differences in gloss between restorations or between restorations and teeth are easily detectable even if colors are matched.

2) In a restorative material, high gloss also lightens the color appearance. When white light shines on a solid, some of the light is di-rectly reflected from the surface and remains white light. This light mixes with the light reflected from material bulk and dilutes the color. As a result, an extremely rough surface appears darker (low value) and less chromatic (greyer) than a smooth one [6]. Reflection from smooth, mirror-like surface results in the production of a clear well defined image. The more wavelengths will return to your eyes in case of highly reflective surface with more additive combination of wavelengths to cause brightness to that surfaces (hue change). Restorations with such surfaces will appear closer to the viewer and looks like to “jump out at him”. Objects appear farther away by lowering their value. Dentists can use this to their artistic advantage [6]. The most important fac-tors, affecting how light interplays with the tooth surface, are the surface texture and luster. If an object has roughened surface texture it will not yield a well-defined image because of the scattered light reflection as well as the individual wavelengths will all bend differently yielding a substantially different spectrum returning to the eye [45].

Conclusion

1) Contemporary Understanding of Color basics will help each individual of the dental profession in the assessment of restoration color.

2) The natural teeth must be considered as a model dynamic mosaic affected by interacting different variables that result in a unique aesthetics.

3) The restorative materials should present optical properties that are similar to those of the dental structure to obtain optimal esthetic results.

4) For proper tooth shade selection when dental practitioners use traditional shade-matching techniques, several variables should be considered to obtain efficient results and satisfy themselves and their patients.

Disclosure Statement

The authors have no financial interest in the materials or devices reviewed, and declare no conflict of interest.

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Volume 19 Issue 2 February 2020©All rights reserved by Nagy Abdulsamee and Passant Nagi.


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https://www.researchgate.net/publication/313635170_Esthetic_color_training_in_dentistry



Cronicon · Cronicon OPEN ACCESS EC DENTAL SCIENCE Review Article Contemporary Understanding of Colors in Aesthetic Dentistry: Review Nagy Abdulsamee1* and Passant Nagi2 1Consultant

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