Masters Technique Level: Shade Matching the Single Central
Dec. 4, 2010, 6:29 a.m.
James F Fondriest, DDS560 Oakwood Avenue, Suite 200Lake Forest, IL 60045
There are very few things our client patients can evaluate our talents on clinically.They do not really know if our margins fit or the occlusion we have designed for them is adequate or appropriate.They do know however if our restorations match.Therefore it is important for the dentist/technician team to develop their skills to allow for the faithful reproduction of nature in our restorations.Developing our skills so that we can communicate exactly what is seen in the mouth will allow the laboratory technician to produce a more lifelike replica.This paper suggests a protocol for the evaluation of the most important parameters of a match and for the photographic communication of these parameters between the laboratory and the dentist practitioner.
IntroductionAccurately reproducing the unique characteristics of a single maxillary central so that the artificial replacement is perceived as “natural” can be the biggest challenge in restorative dentistry.This challenge comes at several levels.First, the practitioner needs to have an understanding of what factors go into achieving a good match and some basic knowledge of the nomenclature of light science in order to communicate what is seen.Secondly, the practitioner is responsible for creating a protocol to accurately assess what is happening when light hits the surface of the tooth to create its visual appearance.There are ways to increase what we see visually or photographically in the mouth lessening metamerism, afterimages, and other visual distortions.Thirdly, the practitioner needs to develop written, graphic, and photographic communication devices that are more comprehensive and less confusing.For example, we have no common dental standard for communicating the degree of translucency, hypocalcification of enamel, or varying degrees of surface luster.If we had to describe in great detail what we see in the teeth in Figure 1 without photography, how many words would it take to deliver a non-confusing synopsis?And lastly, the laboratory needs to develop itself along with the practitioner partner because all of the levels of communication conveyed to the lab must also be recognized and understood in the photographs.
How easy would it be to describe these teeth in written form?
It is important to realize that matching the hue and chroma is fifth or sixth in importance on the list of things to match when constructing a prosthetic replacement 1.You have to be fairly close to someone to detect subtle differences in hue; yet surface morphology, value, and opacity disparities can be seen from four or five feet away or more.Disparate tooth silhouettes or perimeter shapes of the teeth can be seen from even 10 feet away. The order of importance while matching a single maxillary central is:1.silhouette or perimeter shape2.
A high percentage of light that hits at 900 will return to the viewer while light that hits a surface at an oblique angle will be deflected away from the viewer.An observer only sees an object when light comes from that object.Surfaces that are perpendicular to us send the most light back to us.
surface morphology and texture3.value4.translucency or opacity5.chroma6.hueShape, Morphology, and TextureThe appearance of teeth is mostly determined by how light interacts with its curved and varied surface.The perimeter shape and the morphology of the buccal surface have the greatest affect on the appearance of teeth because they determine how the majority of light is reflected. An observer only sees an object when light comes from that object.Surfaces that are smooth and perpendicular to us send more light back to us (see Figure 2).The reflective surfaces of the tooth will not return significant light to our eyes if they are not perpendicular to our eyes even if they are highly polished.Figure 3 shows an example of in this case sand which is uniform in color.The sand can look very different depending on its contour and angle of illumination.
Appearance varies depending on surface contour and the angle of illumination
The textures of a maxillary central can be divided into three subcategories, vertical, horizontal, and localized.Vertical textures tend to be manifestations of the three developmental lobes.The horizontal textures are initially created by the laying down of enamel layer upon layer. The end of each layer leaves a line on the enamel surface called the striae of retzius 2.These striae run roughly parallel to each other and are called perichymata.As the years go by, the surface of the tooth wears and the striae eventually disappear. Different sections of the tooth calcify with different levels of mineralization and hardness.In time these dissimilarly hardened areas can wear unevenly forming much larger and wider spaced horizontal undulations.The localized group of surface textures is a catchall for characterizations such as orange peel, stippling, cracks, craze lines, chips, developmental defects, etc.
There is a double reflection and absorption of light in concavities causing diminution of light coming out of these areas while light is reflected more in bulging and curved areas
Reflection from a smooth surface results in the production of a clear well defined image.This is called specular reflection.A specular reflection returns a high percentage of direct non-diffused light, and if strongly illuminated, will be brighter and stand out.Most teeth have irregular surfaces with convexities and concavities.The convexities (Figure 4) tend to wear and become smooth with specular reflective characteristics.Concavities tend to collect light by reflecting inwards and tend to be unpolished, thus diffusing the light and less returns to the viewers eyes.The visual impact of a tooth comes from the specular highlights that reflect off the heights of contour and give the tooth its visual shape and perceived length and width dimensions 3.Smoothing the texture of the buccal surface will make teeth appear lighter and brighter and therefore a significant determinate of value.The more reflective the surface, the more wavelengths return to your eyes and the additive combination of more wavelengths yields whiter light (hue, chroma, value, opacity all change).If we were to smooth out the wind rippled sand in our Figure 3, it would become brighter.Brighter objects appear closer to the viewer.This is the reason why a restoration that is too light appears to “jump out at you”.Lowering the value makes objects appear farther away.Silhouette and surface morphology can best be documented with photography.Photographic protocols will be described later in this paper.Value is the next most important parameter of a match.Value Equals BrightnessValue or brightness is the sum total amount of light that returns from the target tooth (contralateral maxillary central incisor) to the eyes.The brightness of teeth is mainly determined by the saturation or intensity of hue (chroma) and the surface reflectivity as discussed above but is also appreciably affected by the optical characteristics common to translucent bodies.These optical characteristics in order of importance include opacity, opalescence, fluorescence, and optical density. Human teeth are characterized by varying degrees of opacity.Translucency and opacity can be defined as the measurement of the gradient between transparent and opaque.Value is affected by the opacity of the various layers of the tooth.As the opacity goes up, more light is scattered instead of being trans-illuminated.Reducing the surface luster of a piece of clear window glass by wet sanding or etching will produce a frosty white look.As light hits the surface of the etched glass, it scatters or bends irregularly.This scattering of the light at the surface causes an increase in opacity (Figures 5,6).The light does not travel through and away from the surface but rather is reflected.As the glass becomes less translucent, the value goes up.The net effect is more light returns to the viewer as the luster goes down.
A smooth polished surface produces a well-defined image and can be more translucent 4
A roughened surface diffuses light 4
Polishing the rough glaze off of a porcelain restoration is a subtle way to lower value by making the porcelain clearer and more translucent 5.Super polished surfaces can appear bright due to the crisp specular reflection but they also have more translucency because the light isn’t scattered or bent at the surface.It is important to note that surface texture and not luster determines specular reflection.With our window glass example, although the surface luster has been roughened the glass remains flat and has low texture so it will remain a specular reflector.Opalescence can be described as a phenomenon where a material appears to be one color when you observe light reflected from it and looks another color when you see light transmitted through it 6.A natural opal is an aqueous di-silicate that breaks trans-illuminated light down into its component spectrum by refraction. Opals act like prisms and refract (bend) different wavelengths to varying degrees.The shorter wavelengths bend more and have a higher critical angle needed to escape the optically dense enamel than the reds and yellows. The hydroxyappetite crystals of enamel also act as prisms.When illuminated, enamel will trans-illuminate the reds and scatter the blues within its body.This is why enamel not backed by red-yellow reflecting dentin such as at the incisal edge or interproximally, will appear bluish even though it is intrinsically colorless 7-9.The opalescent effects of enamel brighten the tooth and give it optical depth and vitality 10.The easiest way to evaluate enamel thickness is to look for the opalescent blue areas.Documentation of the translucent enamel is best done photographically due to dentistry’s lack of words to describe levels of opacity.Fluorescence by definition is the absorption of light by a material and the spontaneous emission of light with a longer wavelength 11.Fluorescence in a natural tooth primarily occurs in the dentin due to the higher amount of organic material present 2,7,8,12.The more non-visible UV light the dentin absorbs, the more it fluoresces increasing the value 7.We live in a world of UV light.UV light can have a dramatic affect on the brightness of teeth and our restorations.The dentist practitioner cannot measure fluorescence easily but the porcelains used in the restoration should have fluorescent qualities or the value will be too low in sunlight or other high UV situations.When light enters enamel, it gets bounced around the enamel like a fiber-optic cable. If you illuminate one side of a tooth with a curing light, the entire crown is lighted.Similar to a fiber-optic cable, enamel is an optically dense material bordered on either side by air or dentin, both with significantly lower optical densities. Normally, increasing opacity or reflectivity increases value.By increasing the optical density of dental ceramics, the fiber-optic properties of natural enamel can be replicated and the prosthetic crown can be bright and translucent at the same time.It is with the translucent enamel layer that the ceramist achieves color depth and the illusion of a vital natural tooth.Measurement of optical density is also difficult and not necessary but porcelains that have higher optical densities tend to be more lifelike.Chroma and HueEvery opaque object that you see around you is receiving light or is receiving the three primary color ranges of red, green, and blue-violet in some ratio.Some of these objects reflect all of the light they receive and others absorb it almost totally 7.Most “opaque” objects absorb partially and reflect the rest.The dominant wavelength/s reflected back to your eye is the perceived color of the object.White objects reflect almost all visible light rays.Black objects absorb most of the light so nothing is reflected back to your eyes.Hue is the quality that distinguishes one family of colors from another.Hue is specified as the dominant range of wavelengths in the visible spectrum that yields the perceived color.Chroma is the saturation, intensity, or strength of the hue.Shade Assessment SystemsShade tabs from any vendor are helpful if your laboratory uses that guide or you can share the tab while the case is completed.The Vita ClassicÔ shade guide at this time is used by approximately 90% of practitioners.This guide unfortunately represents a minority of the natural and unnaturally brightened teeth to be matched.Hopefully our not too distant future will bring a non-proprietary universal full spectrum guide that the dental material manufacturing industry will adopt. There are currently better shade tab systems on the market that cover more of the hue and value spectrums such as the VitapanÔ shade guide however, this system is tied to a proprietary porcelain system.The current mechanical shade assessing systems based on colorimeters, spectrophotometers, or camera sensors (CCD or CMOS) will not rival the results achieved when the practitioner / technician team utilize well drawn shade maps and quality multi-image photography 13.Measuring Low Light ValueValue is best evaluated in low or subdued light.When the Vita ClassicÔ Shade guide is arranged by order of value (order suggested by the manufacturer) and evaluated in good light, the order might be considered suspect.Some of the darker appearing tabs seem to be in the middle.If viewed in subdued light, the amount of light you might have with an ominous thunderstorm with dark clouds, the order seems perfect.The discrepancy occurs because of color confusion.In better light the color perceiving cones in our eyes are stimulated and the color in the tabs becomes more evident.The colors confuse your ability to assign value intensities.In lower light, the cones do not fire and only the rods are activated.The rods in our eyes are sensitive to lightness/darkness or gray scale.Rods are very sensitive even with small amounts of light. The rods are what we use in night vision.The cones only become activated with higher light levels.If you think about it, you don’t see colors at night when you are driving except for colored lights.In summary, you want ambient light levels to be low enough where colors are not that obvious so you are only using the rods in your eyes to assess the brightness of the target tooth.In the past some authors have suggested squinting as a way to assess value 14,15. Instruct the laboratory to confirm in low light the overall value of the final restoration.Low light value is always the first thing done in the restorative procedure.By doing low light value first, your pupils have not been closed down by the glare of the bright dental unit light 16.Also the tooth has not become dehydrated.Dehydration increases opacity of the enamel.Light no longer can go from hydroxyappetite crystal to crystal.Intra-operative dehydration causes significant changes in value, translucency, chroma, and hue.Less translucency causes more reflection so the tooth is brighter 12.Chroma being inversely related to value is dropped and the hue becomes more the color of the light source, which is assumed to be white. Once the low light value is taken select hue and chroma tabs.
Selecting shade tabs
Create a neutral colored environment.Complimentary color afterimages of any bright color will occur in milliseconds.The ideal background when assessing color is neutral gray 17,18.Neutral gray has no complimentary color and is restful to the cones.This is more critical with aged teeth that have a glossy surface that reflects the shade of any color placed in close proximity 18-21.The color of the walls in the operatories and lab can alter color perception and should be subdued.In a blue room you see more orange than is actually present since the complement of blue is orange.Use a gray bib to cover the patient’s clothes 22 and remove or cover any lipstick.Try to limit the amount of red tissue background by cropping it out with intra-oral gray backgrounds such as Pensler ShieldsÔ (order # 50009211) by Kulzer.These disposable cardboard backgrounds can be shaped easily to match the arch form.If positioned too far behind the teeth and out of focal distance, the gray will darken to black which increases glare 7, 23.The most important circumstance of selecting hue is the lighting condition.Due to the variability of daylight, blinds should be used with a color corrected light that approaches 5500K, a CRI (color rendering index) of 93, with the proper luminosity, for the practitioner and the lab.Viewing teeth under diffuse illumination will minimize the distortion of reflected light.Reflection from the specular surfaces of a tooth reveals more of the color of the illuminating light than the color of the tooth 24.The average recommended luminosity for dental shade matching is 150 ft-candles 15,19,25-28.To have 150ft-candles intensity in the operatory at the level of the dental chair, eight to ten four foot fluorescent bulbs would be needed in a 10x10 ft room with 8-foot ceilings 19,27.The diffusion panels covering your fluorescent bulbs are important also because they can screen out wavelengths.As they age, the panels change what wavelengths they absorb. The best diffusers are those that don’t filter out any wavelengths of the spectrum, preferably the egg-crate type. Using ten color corrected bulbs on the ceiling will yield more light in the operatory than what would be considered comfortable.There are portable high quality light units such as the Videntä light which are ideal.First impressions are the best due to eye fatigue.Don’t stare at the teeth for more than 5 seconds to prevent hue accommodation 15.Miller has suggested using a Vita ClassicÔ shade guide arranged by hue with the A and B hues at opposite ends and C and D in the middle.C and D have hues in between A and B 29 on the linear rainbow (chroma and value are manipulated to yield different looks).When choosing the hue family, use the A-4 and B-4 or A-2 and B-2 tabs which facilitate the process of elimination by using tabs with the greatest hue spreads 15.The chroma is very low for shades A1 and B1.It can frequently be very difficult to distinguish the proper hue family using these tabs.When choosing the hue with a shade tab, look to the mid-buccal of the tooth.Differences between the shade tab and the natural colors of the teeth increase near the root.Compared to the Vita ClassicÔ shade guide, natural teeth exhibit increased redness and lower translucency at the cervical 20,30.If in doubt as to the hue family, choose the A family 31,32.Most natural teeth have more red than the B family.Perhaps as much as 80% of natural teeth are a closer match to the A hue family 31.Hold the shade tab incisal edge to the incisal edges of the teeth.This effectively isolates the shade tabs from the teeth so they don’t reflect onto each other 20,21 and itreduces afterimages.Most humans have eye dominance and one eye will preferentially perceive shade 16.It is wise to hold the shade guide on both sides of the tooth at each vector 21.In addition, difficulties can arise where the tooth being examined differs considerably in size from the specimen on the shade guide.A variation in color perception can occur with the relatively larger area appearing brighter and more vivid than the smaller 33.Shade mapping
In dental ceramics, we try to imitate the appearance of the tooth as a sum of all its visual dimensions.Even though you intend to provide excellent photographic images for your technician, it is extremely valuable for you to provide a written graphic with your interpretation of these dimensions in the drawing.If no shade tab matches what you see, then consider customizing a tab by applying surface stains.Caution must be used with this technique because the lab is encouraged to duplicate this surface staining which will increase metamerism in the final restoration 34,35.Shade-map all that you see in full page three-dimensional drawings or printed photographs of the target tooth and other proximal teeth.Utilize several views (e.g. 900 straight buccal, 1350 angle from the buccal incisal, and straight incisal/occlusal).Break the labial face of the crown into zones.Note the low light value from gingival to incisal, map the base hues and which chroma stop in what areas of the buccal surface.A chroma stop is an arbitrary measurement of hue saturation and is designated by the number of the Vita ClassicÔ shade guide. Do not hesitate to alter these chroma stop designations.For example, it is acceptable to note an area to be A-2.5 or A-3.75 though there are no tabs that have these chroma intensities.The surface anatomy must be described.The pre-op models will help duplicate these contours.Although the luster and texture can be better determined photographically, describe it on the prescription form and add the age of the patient.Describe surface texture and luster as heavy, moderate, and light therefore giving different combinations of surface characteristics.Because these surface features determine the character of light reflection and affect the amount of light that enters the tooth, the surface morphology of a crown should be designed to simulate the light transmission and reflectance pattern of adjacent teeth 4,30,36-41.When the practitioner is mapping the translucency of the target tooth, he/she looks for the opalescent blue areas.You will see them better when using a black background, which limits the reds reflecting from the back of the mouth and re-adding to the blues to yield white light again 42,43.When drawing proximal translucence, ask the patient to turn from right to left, which allows a better analysis.This reevaluation at different angles is called vectoring 16,39,44,45.The practitioner and technician should build a collection of shade guides and tabs that can be shared between the team.There are proprietary guides that have tabs that represent different levels of enamel opacity, frost, occlusal staining, etc.
Photographic Documentation Protocols
Silhouette and Surface Morphology
Black background preferred but not mandatory
Camera lens should be oriented perpendicular to the surface being evaluated
Dry and clean surface of tooth
Vector for gingival, mid-buccal, and incisal thirds
Surface morphology is best captured photographically with a dual point or circumferential flash.These types of flash mechanisms maximize the reflections.
Developing expert photographic skills is very worthwhile as the better images yield more information.The practitioner is responsible for creating an environment and protocol to assess what is happening when light hits the surface of the tooth to create the visual appearance of the tooth.There are ways to increase what we see photographically in the mouth 43,46.There are fairly simple choreographed images that serve to communicate the more important parameters for matching.Communicating with photography will always be better than with written or verbal descriptions. Almost all quality levels of images are better than nothing.That alone should encourage more photographic documentation.
Shape, surface morphology, translucency, chroma, and hue can all be documented very well using three choreographed photographs:
I) Silhouette and Surface Morphology(Figure 7)
When you position the lens and flash over the surface of the tooth, the light will reflect like a mirror off the perpendicular surfaces back to the camera.All surfaces not perpendicular will reflect the light away from the camera which highlights the texture variations.
Translucency·Clean teeth·Black background·Close down aperture which allows discernment of layers and depth ·Set flash on Manual (turn off TTL flash), and slightly underexpose by incrementally adjusting F stop which allows us to see into the tooth better by lessening further the surface reflections.·Angle lens >300 from perpendicular so reflections do not return to camera.
II) Translucency(Figure 8)Camera flash reflections are very helpful when evaluating textures.However, as these reflections come off of the tooth they obscure our view below the surface.We want to minimize the flash reflections in our photograph when evaluating translucency.The target tooth can be wetted for translucency, hue, and chroma evaluation to limit the influence of surface morphology.By angling the lens away from perpendicular to the target tooth surface and taking the shot either from above or below (>300) we limit reflections.Ring flashes tend to surround the exposure field and yield more reflections.More angulation may be necessary with a ring flash.III) Chroma and Hue(Figure 9)Visual distortions dramatically affect our ability to color render 7.The two main distortions in dental circumstances are the spreading effect and the negative afterimage.Simply stated, the spreading effect occurs whenever two dissimilarly colored objects are placed next to each other.Because our eyes don’t stared fixedly at an object but rather continually roam the visual field, the color of each object is mixed with the other and the objects soon (within seconds) appear more alike.If some distance is placed between the tooth and the mid-buccal part of the shade guide, a better assessment can be made.Some advocate grinding off the incisal edge of the VitaÔ Classic shade tabs but they do help provide that little visual separation which lessens the spreading effect 7,21,23,48.Orient the shade tabs so that the incisal edge of the tab co-approximates the incisal edge of the tooth.
Chroma and Hue
Clean tooth surface but saliva can be left on surface.The water tends to flatten surface and lessens reflections caused by surface textures.
·Use 18% reflective gray card background·Take images >300 from perpendicular to surface or reflections will obscure proper evaluation.·Use three shade tabs in photo arranged incisal edge to incisal edge.Tabs should be parallel and equidistant to teeth from lens.Center tab is considered ideal match with other tabs one chroma stop up or down.·Chroma evaluation is easier with slight underexposure created by manual flash settings rather then going TTL and adjusting the compensation settings.
Contrast is caused by a difference between the brightness or color of an object and its immediate background.Object forms with high contrast are easier to pick out than objects with low contrast.While some contrast is helpful to our visual system, excessive contrast causes glare.An extremely bright object against a dark background causes discomfort and can interfere with our color perception 7,23.This interference is generically called glare.This glare reduces our ability to perceive visual information.With dental photography, the use of a black background increases impact, but it will cause glare.This is counter-productive when matching hue and chroma due to the increase in glare and it will mask shade mismatches.Negative afterimages are caused due to fatigue of the cones in our eyes.We tend to see afterimages in the form of complimentary colors.Background reds in the mouth create the perception of more blue than is actually present.An achromatic background is quite valuable in hue assessment. The 18% reflective gray card is the photographic industry standard achromatic background.A gray card creates less glare and fewer afterimages.
Bracketing in manual mode
Varying the film/sensor exposure can be accomplished several ways and is called bracketing.One type of bracketing is the incremental adjustment of the lens aperture by fractions of an F-stop while taking multiple exposures.When you close down the aperture or lens opening, less light will make it to your film or digital sensors.Bracketing your F-stops is beneficial in documentation photography because you often see different things at different settings.Closing down your F-stop will decrease the influence of surface reflections but not decrease your ability to see the opalescence.It also increases the depth of field.Closing down the F-stop will increase your ability to see the layers within the tooth, which is helpful for determining translucency, hue and chroma 47.The camera has to be set to manual with a constant shutter speed and flash.The TTL flash setting cannot be used with this technique.Send all of your exposures to the laboratory but you will begin to notice that the slightly underexposed images deliver more information.
A sequential protocol for selecting value, shade tabs, shade mapping, and then photographing for shape and surface morphology, translucency, and finally chroma and hue has been described.This protocol should be performed before performing any restorative procedures and even prior to