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CLARK SAID, "Color, like form, has three dimensions, but they are not in
general use. Many of us have not been taught neither names, nor the scales of
their measurement. In other words, we as dentists are not educationally equipped
to approach a color problem."
This statement is, unfortunately, still true. Dentists have had little or no
training in vision physiology or color science. A 1967 survey revealed that 23
out of 112 dental schools had some sort of color education in their curriculum.
A survey conducted the following year, reported that 3 of 115 dental schools
taught a formal course in color. Thirty years later, comprehensive color
training continues to be a missing part in the dental school curriculum. If any
training at all is given in dental school, it is cursory or simplistic and
usually consists of presenting an incomplete explanation of three abstract
concepts of the Monsell Notation:4 hue, value and chroma.
Albert Einstein is reported to have said, "If you cannot explain it
simply, then you do not understand it well enough." This appears to be the
problem in educating dental students regarding color matching. In a time of
growing interest in cosmetic dentistry, there is a need for adequate training
and communication for better and more lifelike results.
The increase in newer types of ceramic restorations and the improving quality
of esthetics means the dentist of the 21st century must be trained to detect
differences in color and shades in individual teeth, select a shade that
reflects the color and exact shade, transmit this information to a dental
technician, and then be able to make any necessary adjustments to the
restoration. However, there are a number of factors that stand in the way of
properly selecting a color match. Subjective faults range from differences in
color perception to ocular fatigue and lack of education regarding the basic
principles of color. Metamerism may occur if proper lighting is not used during
shade selection in the dental office and laboratory Finally, existing shade
guides are limited and require a more extensive range of shades.
Therefore, it is no surprise that color matching for crowns and dentures can
be a frustrating and discouraging experience for the dentist, technician and
patient. The breakdown in communication over matters ranging from shade guide to
laboratory prescriptions must be addressed. This study will review the problems
of color matching and will attempt to guide the reader toward enhancing his or
her techniques regarding shade selection and communication with the ceramist.
Tooth vs. Porcelain
Prior to shade matching, the dentist must have an understanding that the
human tooth and dental porcelain transmit light waves differently It is their
physical composition that determines the differences in light-wave transmission,
absorption, reflection, refraction, scattering and surface gloss. The manner in
which light strikes an object determines the total appearance of the material.
Transparent materials allow for the passage of light with little change.
Translucent materials scatter, transmit and absorb light. Opaque materials
reflect and absorb; however they do not transmit. Surface characteristics, such
as gloss, curvature and texture, will affect the degree of light diffusion when
striking the particular object.
A vital tooth is both naturally translucent and transparent. Enamel rods are
transparent and therefore refract and reflect light. Light that strikes the
incisal edges of an anterior tooth passes through with maximum transmission
because of a high degree of translucency.7
Porcelain, however, is a heterogeneous material. It contains transparent
properties and metallic oxides that act as opacifiers. These porcelains modify
light by absorption, transmission and reflection. Absorption is largely
responsible for color. It occurs when light passes through the layers of the
porcelain. Scattering occurs when light encounters interfaces between the
materials (i.e., pigments and glass). The smaller the pigment size, the less
light that is absorbed, resulting in less detectable color. The larger the
pigment size, the more reflection that occurs as light scatters at the particle
surfaces. Scattering light is necessary in dental porcelains to simulate the
prismatic effect of enamel. Yet, one must keep in mind that too much dispersed
reflection through internal scattering will create an unnatural looking
prosthesis.
Light Sources
One of the questions asked when selecting a shade is, what light source
should be used? Shade determination should be performed under color corrected
fluorescent lighting, which contains a balance of the entire visible spectrum.
The operatory should be lit using a luminous ceiling with translucent diffusing
panels that are simple to maintain. Clean watt saver lamps having a color
temperature of 4200K or higher is advocated. An illumination level of 200 to 250
foot candles is highly recommended. Shade selection should not be made using
daylight, because daylight is subject to constant changes.
One must also be concerned with the phenomenon of metamerism, which occurs
when the color of two objects looks identical when observed under one light
source but different under other light conditions.8 Metamerism occurs only when
two objects have different wavelength distribution and therefore reflect
different spectra.
The color of the operatory can also affect shade selection. Colors should be
kept at a low saturation level. Walls and cabinets should be glossy enough to
maintain brightness without causing a glare. It is recommended that the color of
the walls and ceiling be white or off-white.
The dentist should be concerned with "blue fatigue:' this occurs when
the eye is unable to differentiate between the various shades of blue. However,
blue fatigue increases sensitivity to yellow therefore, to improve shade
selection in the yellow range, the operator should stare at a blue card or
patient napkin between shade comparisons.
It has been suggested that dentists use natural north daylight for shade
matching. Many dental offices have been designed to face the north to enhance
the selection process. However, daylight is not at a constant throughout the day
and therefore must not be used as the only light source for shade matching.
The Problem with Shade Guides
Shade guides have become the standard for selecting shade, yet there have
been many errors associated with the use of commercial shade guides. Problems
that may arise include the following:
1. Porcelains do not match the shade guides that they are being compared to.
2. Shade variations occur between different die lots of porcelain from the
same manufacturer.
3. Shade guide tabs are 4-5 mm thick compared to the thin 1.5 mm piece of
porcelain used for the restoration.
4. Shade guides are not always made with fluorescent porcelain, which causes
inconsistencies in color matching.
5. It is difficult to predict the final shade after the layering of opaque,
dentin and enamel.
6. Guide tabs lack a metal backing when using porcelain-fused to-metal
restorations.
7. Shade tabs are condensed differently than porcelain used for final
restorations.
Now that the reader understands the potential problems that arise when
selecting shade, it is imperative that the dentist have a proper education in
color. However, we must assume that not every dentist will seek out the proper
courses. The latter portion of this article will be a review of numerous methods
fox enhancing laboratory communication between the dentist and the dental
technician, to assure the success of proper shade matching. The dentist must
then decide for himself or herself how much information is enough to guarantee
the replication of the restored teeth.
Shade Selection Guidelines
The dentist must have a working knowledge of the basic principles of color.
This allows for accurate shade selection. Munsell described the three dimensions
of color as hue, value and chroma.
Hue is the property of color that is determined by wavelength, which
distinguishes one color from another. Value is a quantity of brightness. It is a
qualitative term related to lightness or blackness of color and not the quantity
of the color gray Chroma is the saturation of color.
Matching the proper shade is not carried out just by holding up a guide tab
to the tooth in question. There are a number of methods that can be employed to
intensify the shade selection. They are as follows:
1. If patient is wearing bright clothing, drape him or her with a neutral
colored cover.
2. Have patient remove lipstick or other make-up.
3. Clean the teeth and remove all stains and debris.
4. Have patient's mouth at dentist's eye level.
5. Determine shade at the beginning of the appointment to avoid ocular
fatigue.
6. Shade comparisons should be performed at five-second intervals so as not
to fatigue the cone cells of the retina.
7. Obtain value levels by squinting.
8. Compare shade under varying conditions (i.e., wet vs. dry lips; retracted
lip vs. pulled down lip).
9. Use the canine as a reference for shade because of the highest chroma of
the dominant hue of the teeth."
10. If unable to precisely match shade, select a shade of lower chroma and
higher value.
11. Grind off the necks of the shade tabs because they tend to be darker than
the rest of the shade tab.
Custom Shade Guides
To properly start the shade matching process the dentist should acquire a
custom shade guide. This guide is the beginning of improving communication with
the laboratory Each custom shade guide should include the ceramists metal,
porcelain, staining kits, equipment and techniques. It should also contain
pointers as to what to look for when selecting a shade. The technician needs to
send a chart along with the guide for jotting down any additional information
that will allow for a better understanding of the particular shade. The dentist
may choose to create a luster tab and send it to the laboratory with the
prescription. The technician will then have a visual aid for what he must
fabricate. Numerous techniques regarding custom shade guides have been noted in
the literature.
Laboratory Prescriptions
With few exceptions, laboratory work authorizations do not request enough
information from the dentist. This could be because there is not enough space on
the prescription to record it. It is, therefore, important to use a laboratory
that fully understands the need for shade matching. Each laboratory prescription
should contain enough space to record clinical information about each ceramic
component of the restoration-for example, different shades of porcelain and
opaque, and where to place them on the tooth. The authorization should include
numerous diagrams of the tooth so that the dentist can draw helpful notes on
them (i.e., shade, translucency, staining, glaze and surface texture). And the
dentist should be in contact with the ceramist to ensure that the technician
fully understands what the dentist is requesting. It is through these methods
that the dentist builds a relationship with the technician. Ceramists will
usually return the quality that the dentist sends to them.15
Models
Along with the laboratory prescription, the technician should have a set of
study models to use as a guide. Preoperative models give the ceramist
information about occlusion, tooth alignment, position of soft tissue, diastema,
surface texture, wear facets, and more.16 A diagnostic wax-up will aid in the
occlusion and form of the restoration. Matching shade is obviously only part of
the task of replicating the natural tooth.
Photography
The macro lens has become an important tool in communicating with the
laboratory. It has been said that, "The photograph is the cosmetic
dentist's radiograph."17 A magnification of either 1: I or 2: I enables the
technician to evaluate the color of a particular tooth, see craze lines, stains,
surface texture and luster. Multiple pictures should be taken at different
angles and under different light sources. The patient's occlusion should also be
photographed.
Along with photographs of the teeth being worked on, it is best to include
pictures of the patient's smile. These photographs can tell the technician about
the patient, his or her age, personality and character.
It is a good idea to photograph the prosthesis at the try-in stage. If color
adjustments are necessary, the technician will have a visual aid to help make
the proper corrections. Written instructions alone are not enough information
for the technician. They leave a tremendous amount of room for interpretation.18
Computers
Computers have become a valuable communications tool for the dentist and
laboratory Cosmetic imaging can take the place of photographs. The dentist can
take a picture with an intraoral camera and send it over the Internet to the
laboratory. Film developing is eliminated, saving time and money. The ceramist
can use these images to fabricate the proper prosthesis.
E-mail can be sent between dentist and laboratory to facilitate discussions
of shade matching. The ceramist may decide to offer suggestions to the dentist
based on the original images sent over the computer. E-mail can be used if the
dentist is busy with a patient and cannot come to the telephone.
With the addition of modems and capture boards, the dentist and ceramist can
engage in real-time (15-second delay) consultations.19 This has the potential to
allow for making immediate decisions about the restoration and will decrease the
number of office visits the patient has to make.
Colorimeter
Colorimetry is "the branch of color science concerned with numerically
specifying the perceived color of an object as well as differences in perceived
color between two objects judged to be different."20 Photometric analysis
techniques of porcelain surfaces may enhance our abilities to evaluate and
design an esthetic restoration. Computer matching has been widely used in paint,
plastics, ink and textiles. However, color matching and reproduction in
dentistry appear to be more difficult than in the other industrial fields. There
are many reasons for these difficulties, including the following:
1. Teeth are semitranslucent.
2. The size of teeth and the prosthesis are small.
3. Surrounding components of the tooth are complex.
4. Tooth surface is irregular.
5. A prosthesis may be composed of a number of materials.
6. There are many variables in fabricating a prosthesis.
Because of these errors, the accuracy of color measuring devices is subject
to great variability Therefore, the practical and routine use of measuring shade
with these types of instruments is of little value to the dentist.22 However,
research is being conducted continuously to create a computerized shade matching
program. This program may change the way color matching is performed and may
allow for more accurate shade selection and communication.
Conclusion
The introduction of advanced materials and techniques
has and will continue to increase the degree of success in shade communication.
As stated previously, "How much information is enough?" This decision
is left up to the dentist. It depends on the dentist's philosophies in
practicing dentistry and how much pride he or she takes in his or her work.
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Shade-Matching
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