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Color and shade matching in dentistry.

Introduction

An understanding of the nature of light and how the eye perceives and the brain interprets light as color is important for successful esthetic restorations, particularly when metal-ceramic or all-ceramic restorations are being made [1].

Nature of Light

Electromagnetic waves are everywhere and light is only a small part of them. Light is basically photons and mostly moves as waves. White light that is seen by human eye is called as "visible" light (380 nm to 780 nm) [2]. Waves called radio, microwave, infrared, ultraviolet (UV), x-rays, and gamma rays cannot be seen by the human eye, and therefore called the "invisible" spectrum. Together, the visible and invisible spectrums make up the electromagnetic spectrum (Fig 1).

[FIGURE 1 OMITTED]

The light is not really white; the white we see is a combination of all the colors of the rainbow--Red-Orange-Yellow-Green-Blue-Indigo-Violet. When white light is made to pass through a crystal prism, as was done by Sir Issac Newton in 1676, it is bent, and each wavelength changes direction by a different amount and the individual colors of the visible spectrum are seen [3]. There are three things that can happen to a light wave. It can be reflected, absorbed, or transmitted. If all of the light is reflected, the object appears white. If the light is entirely absorbed, the object appears black.

Color

Color is a property of light. Objects have no color of their own; they just reflect a particular wavelength from the color spectrum. For example a blue object absorbs all of the wavelengths, except for blue. The remaining wavelengths enter our eyes and this is what we see.

Description of Color

The most popular method for describing color is the Munsell system. The three attributes of color in this system are called Hue, Chroma and Value.

Hue: It is defined as the particular variety of a color. The Hue of an object can be red, green, yellow, and so on.

Chroma: The intensity or saturation of a hue is called Chroma.

Value: The relative darkness or lightness of a color is called Value.

There are two types of color, additive and subtractive [4].

Additive Color: These are the color obtained by emitted light and are associated with television and computer displays. The primary additive colors are Red, Blue and Green and the secondary additive colors are Cyan, Yellow and Magenta. When additive primary colors are combined they produce White.

Subtractive Color: These are the colors associated with reflected light and are used in pigments for making paints, inks, fabrics etc. The primary subtractive colors are Red, Yellow, and Blue and the secondary subtractive colors are Green, Violet and Orange. When subtractive primary colors are combined they produce Black.

Color Perception

Eyes can't see alone. Our eyes and brain have to work together to make a sense of light and color. Light goes through the pupil and splashes on the rods and cones of the retina. There, the light causes a chemical reaction. The optic nerve connects eyes to the brain. It understands the chemical reaction and carries a message to the brain. There the color perception takes place.

Eyes: The initial process occurs in the retina of the eye. The retina contains millions of cells called photoreceptors that are sensitive to light. There are two types of photoreceptors, some shaped like rods and some like cones. These photoreceptors process light into nerve impulses and pass them along to the cortex of the brain via the optic nerve. 120 million RODS in the outer edges of the retina help eyes adjust when one enters a dark room. They are good for detecting motion and for seeing in low light-levels. At low light levels, the rods of the human eye are more dominant than the cones and color perception is lost. As the brightness becomes more intense, color appears to change (BEZOLD-BRUCKE EFFECT) [5]. There are 6 million CONES in each eyeball which are sensitive to colour. There are three types of cone cells, each sensitive to the long, medium or short wavelength of light (red, blue and green color respectively).

Color Blindness

Color blindness is the inability to distinguish the differences between certain colors. This condition results from an absence of color-sensitive pigment in the cone cells of the retina. Humans are born color blind because the cones do not begin functioning until a baby is about four months old. One male in twenty suffers from some form of color blindness, but only one in several hundred females are color blind. Color blindness is usually inherited, that is, a genetic defect. Not all color deficiency is due to heredity. Aging, certain medications, and retinal or optic nerve disease may interfere with normal color vision.

Shade Selection

Quality of Light: Energy distribution of a light has definite effects on the type of color being perceived. The clinician should try and use a source of light that contains full spectrum of rays without the dominance of any wavelength; because when an object is viewed under lights dominating in particular wavelengths (color bands), that specific color becomes dominant to the observer. There are three types of light sources [1]:

1. Incandescent Light: Emits high concentration of yellow waves. It is not suitable for shade matching. It has low Color Rendering Index (CRI).

2. Fluorescent Light: Emits high concentration of blue waves. It is not suitable for shade matching. It has CRI of 50-80.

3. Natural Daylight: Northern daylight is considered the best because it is closest to emitting the full spectrum of white light. It is used as the standard by which to judge other light sources. It has CRI close to 100.

Most dental offices are fitted with incandescent and fluorescent lights [1].

Color Rendering Index: Northern daylight, which can be close to full-spectrum white light and often, is used as the "normal" standard for judging light from other sources. It has a color rendering index (CRI) close to 100. The color rendering index, on a scale of 1 to 100, indicates how well a particular light source renders color as compared to a specific standard source.

Another light source reference standard is color temperature, which is related to the color of a standard black body when heated. Color temperature is reported in degrees Kelvin (K), or absolute (0[degrees]K = -273[degrees]C). Northern daylight has an average color temperature of around 6500[degrees]K, but this varies with the time of day, cloud cover, humidity, and pollution.

Although daylight is often used as the standard against which other light sources are compared, never use direct sunlight to take tooth shade. The distribution of light waves from the sun depends on the time of day and on humidity and pollution. Morning and evening incident light has shortened blue and green waves scattered and only the longer waves penetrate the atmosphere. Therefore daylight at dawn and dusk is rich in yellow and orange but is lacking in blues and greens. Northern daylight around the noon hour on a bright day is considered ideal, because the incident daylight is most balanced within the Visible Light Spectrum.

Metamerism: Another aspect of lighting is the subject of metamerism. Two objects may appear to be identical colors under a certain kind of light, yet under another kind of light they may appear totally different. This is called metamerism. The problem of metamerism can be avoided by selecting a shade and confirming it under different lighting conditions (e.g., natural daylight and fluorescent light).

Key optical properties of teeth [6]

1. Fluorescence is the absorption of light by a material and the spontaneous emission of light in a longer wavelength. Teeth are fluorescent because they emit visible light when exposed to Ultra-Violet light.

2. Opalescence is the ability of a translucent material to appear blue in reflected light and red-orange in transmitted light. The opalescent effect is based on the behavior of translucency of natural teeth.

3. Translucency is transmission and diffusion of light through an object so that definite image beyond the object cannot be seen.

Guidelines for Shade Selection

1. Teeth to be matched should be cleaned of all debris and stains. Prophylaxis should be done before shade selection.

2. Brightly colored lipstick/makeup should be removed (7) (strong red lipstick next to the tooth will fatigue the red receptors while the blue and green receptors remain fresh and fully stimulated. This makes the tooth that looks blue-green) and bright clothing should be draped with gray napkin. The operatory walls should be painted gray.

[FIGURE 2 OMITTED]

3. Patient should be viewed at eye level and at arms length, so the most sensitive part of the retina will be used [7].

4. Shade comparisons should be made under different lighting conditions. Initial shade may be taken under a color corrected fluorescent light and then confirmed in natural daylight (taking patient to an operatory window).

5. Shade comparisons should be made at the beginning of a patient's visit. Teeth increase in value when they are dry because of desiccation.

6. Shade comparisons should be made quickly (5 seconds), with shade tabs placed just under the lip and adjacent to the teeth to be matched.

Hue sensitivity: After 5 seconds of staring at a tooth or a shade guide, the eye accommodates and becomes biased. If one stares at any color for longer than 5 seconds and then stares away at a white surface, or closes one's eyes, the image appears, but in the complementary (color opposite to each other in a color wheel) (Fig.2) hue. This phenomenon is known as hue sensitivity which badly effects shade selection.

7. Look at a gray walls or patient's napkin between each shade evaluation.

Types of shade guide [8]

The most popular shade guides currently used for dental shade matching are:

--Vita Classic (Vita Zahnfabrik, Bad Sackingen, Germany)

--Vitapan 3D-Master (Vita Zahnfabrik, Bad Sackingen, Germany)

--Chromascop (Ivoclar--Vivadent, Schaan, Liechtenstein)

--Custom or specific chroma and value guides

Vita Classic Shade Guide: It is a very popular shade guide and has been in use since 1960's [9]. Tabs of similar hue are grouped into letter groups like:

A (hue of red-yellow)--A1, A2, A3, A3.5, A4

B (hue of yellow)--B1, B2, B3, B4

C (hue of gray)--C1, C2, C3, C4

D (hue of red-yellow-gray)--D2, D3, D4

Chroma is designated with numerical values 1, 2, 3 and 4.

Sequence for shade matching while using Vita Classic shade guide

Step 1: Hue Selection

Operator should select hue closest to that of natural tooth. Use area of tooth highest in chroma for hue selection.

Step 2: Chroma Selection

Once the Hue selection has been made, for example B. Chroma is selected from gradations within the B tabs B1, B2, B3, and B4. Several comparisons should be made. Avoid retinal fatigue. Rest eyes between comparisons (look at gray walls).

Step 3: Value Selection

Use of second, value oriented shade guide is recommended.

Value oriented shade guide: B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4,

Final value is selected by using a second shade guide whose samples are arranged with light shade tabs first and dark shade tabs last. Value is most easily determined by observing the guide and teeth to be matched at a distance, standing slightly away from the chair and squinting. Squinting reduces the amount of light that reaches the retina. Therefore stimulation of the cones is reduced while rods become more sensitive to the increasingly achromatic conditions. The dentist should concentrate on which disappears first--the tooth or the shade tab. The one the fades first has the lower value.

Step 4: Final Check / Revision

Following value selection, tabs selected for hue and chroma may not coincide with shade tab selected for value. If the Value of shade tab is lower than natural teeth then select new shade tab with higher value because one cannot increase value of restoration with extrinsic staining because it will only increase opacity of the restoration. If the Value of shade tab is higher than natural teeth then select new shade tab with lower value or bridge the difference with intrinsic or extrinsic staining.

Vitapan 3D-Master: In the early 1990s, Vita introduced the 3D-Master shade guide with the aim of accurately assessing shade according to the three components of colour: hue, value and chroma. Unlike the majority of dental shade guides, the 3D-Master attempts a three-dimensional analysis of tooth color. The tabs are arranged systematically and logically, rather than randomly as in the Classic guide. The tabs are grouped into five categories, sequentially numbered, with an increasing value (1, 2, 3, 4 and 5). All tabs within a value group have the same brightness. In a given value group, the chroma increases from top to bottom. All groups, with the exception of 1 and 5, are designated three letters, L, M and R, corresponding to varying hue. For example 2M2 corresponds to the second value group, the M hue subgroup and a 2-chroma level. For an intermediate tooth shade, a combination of two tabs is used for the final color prescription.

Chromascop: The Chromascop uses numbers to distinguish hue, e.g., 100 (white), 200 (yellow), 300 (orange), 400 (grey) and 500 (brown). Chroma is indicated by another set of numbers, 10 are high value with low chroma, while 40 is low value with high chroma. A conversion chart is available to convert Chromascop shade tabs to the Vita Classic shades.

Custom guides: Finally, if the tooth color fails to concur with any of the shade guide tabs, porcelains can be used to fabricate a custom shade tab. If extremes of value and chroma are required, specific detailed chroma and value guides are available. These may be necessary with aged teeth of deep chroma, or youthful teeth with high values.

Conclusion

An understanding of the science of color and color perception is crucial to the success in the ever expanding field of esthetic restorative dentistry. Limitations in materials and techniques may make a perfect shade selection impossible. Shade selection should be approached in a methodical and organized manner. This will enable the practitioner to make the best choice and communicate it accurately to the laboratory.

References

[1.] Rosenstiel SF, Land MF, Fujimoto J, Contemporary Fixed Prosthodontics: Color science, esthetics and shade selection, 3rd edition. Mosby, St Louis, Chicago, p.592-608 (1995).

[2.] Christopher CK Ho, Shade selection, Australasian Dental Practice: September/October, p.116-119 (2007).

[3.] Ubassy G., Shape and color--The key to successful ceramic restorations: Basic terms of the phenomenon of color, Quintessence publishing co. inc. Chicago, p.17-24, (1993).

[4.] Patrick W Naylor, Introduction to metal ceramic technology: Adjusting and finishing metal ceramic restoration, 1st edition, Quintessence publishing co. inc. Chicago, p.145-170 (1992).

[5.] Anusavice K.J., Phillip's Science of Dental Materials: Physical properties of dental materials, 11th edition, Elsevier, St. Louis, Missouri, p. 46-52 (2003).

[6.] Chu S.J., Fundamentals of color--shade matching and communication in esthetic dentistry: Elements affecting color, Quintessence publishing co. inc. Chicago, p. 19-50 (2004)

[7.] Shillingburg HT, Hobo S, Whitesett LD, Jacobi R, BrackettsSE, Fundamentals of Fixed Prosthodontics: Esthetic considerations, 3rd edition. Quintessence publishing co. Inc. Chicago, p. 419-432 (1997)

[8.] Ahmad I, Protocols for predictable aesthetic dental restorations: Color and shade analysis, Blackwell Munksgaard, Oxford, UK, pp. 77-97 (2006)

[9.] Marcucci B., A shade selection technique, J. Prosthet. Dent. 89, 518-21 (2003).

Mohammed Shammas *, Rama Krishna Alla

Faculty of Dentistry, Al-Fateh University of Medical Sciences, Tripoli, Libya

* Corresponding author: (shammasm@rediffmail.com) Dr. Mohammed Shammas

Received 18 July 2011; Accepted 18 August 2011; Available online 8 September 2011

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Author:Shammas, Mohammed; Alla, Rama Krishna
Publication:Trends in Biomaterials and Artificial Organs
Article Type:Report
Geographic Code:6LIBY
Date:Oct 1, 2011
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