the optical characteristics of natural teeth

7/23/2019 The Optical Characteristics of Natural Teeth

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The Optical Characteristics of Natural TeethUnderstanding the properties of light and color will help communicate shade

prescriptions to the laboratory for optimal results.

By James Fondriest, DDSMatching one or two artificial restorations to a highly characterized natural dentition

can be a challenging procedure for the dentisttechnician team. !he capability toappraise and fully share the appropriate information from the operatory to a distant

laboratory can be greatly enhanced by learning the language of color and the opticalcharacteristics specific to teeth. "t is difficult to duplicate nature in ceramics if the

clinician cannot describe or fully illustrate what he or she sees in the shade#matchingprocess. !his article should gi$e the reader a better understanding of how light

interacts with a natural tooth, and to gi$e the dentisttechnician team the nomenclatureto best describe it.

HueSynonyms for hue are color, cast, shade, tint, or tone. %ue is specified as the dominantrange of wa$elengths in the $isible spectrum that yields a percei$ed color &Figure '(.

Because hue is a biologic and psychological interpretation of the combination of thewa$elengths reflected bac) from an ob*ect to the obser$er, the e+act wa$elength of the

percei$ed color may not e$en be present.'!he lower the intensity of the hue, theharder it is to distinguish."n this age of highly bleached teeth, where little color

e+ists, it is becoming less important to document the hue.

Chroma-hroma is the measure of how much color is present.hen brown pigment is placed

into white powdered porcelain, the mi+ture ta)es on a brown hue. /ach time morebrown pigment is added, the strength or saturation &chroma( of the brown colorincreases but the mi+ture is still the same brown hue. 0s the powder becomes more

saturated with brown pigment, the mi+ture appears dar)er, so the increase in chromahas a corresponding change in $alue. 0s chroma increases, the $alue decreases1 they

are in$ersely related.2

!he chroma of a natural tooth comes mainly from the dentin, and the thic)ness and

opacity of the o$erlying enamel determines how much chromatic influence the dentinhas.3hen the enamel is thin at the gingi$al third but thic) incisally, a chroma

gradient is created. "ncreasing opacity of the enamel, as seen with dehydration and

bleaching, can e+aggerate the chroma gradient.

Value!he words $alue and brightness are synonymous. 4alue is the sum total amount oflight that is returned from an illuminated ob*ect. 5owering $alue means diminished

light returns from the illuminated ob*ect1 thus, more light is absorbed, scatteredelsewhere, or transmitted through and away from the ob*ect. Using porcelain with less

chroma and less translucency can increase a dental restoration6s brightness.!he characteristics of brightness and translucency wor) counter to each other. 0s

tooth bleaching becomes e$en more popular, dentistry will continue to search for

porcelain that is both bright and translucent. -hoosing porcelains with higheropalescence,7optical density, and fluorescence will manifest brighter 8ualities.


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Translucency"n the dental conte+t, translucency can be described as the gradient betweentransparent and opa8ue. /namel and dentin ha$e $arying degrees of translucency.

0reas within a tooth or a restoration with higher translucency will ha$e a lower $aluebecause light transilluminates through and away from the $iewer. hen e$aluating

enamel translucency, the obser$er will often focus on the opalescent blue areas&Figure (. !ranslucent enamel displays the characteristic of opalescence. 9palescence

causes tooth enamel to reflect blue light bac) to the obser$er. Blue light tends tobendrefract more or to scatter within the enamel body. 5onger red#yellowwa$elengths do not bend as much in the enamel1 therefore, a higher percentage will

trans#illuminate the tooth. !he blue accumulates within the enamel, gi$ing it a bluishappearance from a frontal $iew e$en though it is intrinsically colorless. :#;

Opalescence!his optical phenomenon is named after the appearance of opals.


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critical angle to escape, once the light enters the fiber at a low angle $irtually parallelto its walls, it will tra$el down the fiber without lea)ing out. By increasing the optical

density of the ceramics used to build the enamel layer, the fiber#optic properties ofnatural enamel can be replicated, resulting in a prosthetic crown that is brighter and

more translucent at the same time.

Optical Anisotrophy!he opalescent effects of enamel brighten the tooth and gi$e it optical depth and

$itality.''!he property of opalescence will cause a tooth to appear to be one colorwhen light is reflected from it and another color when light is transmitted throughit.7!he more opalescent a tooth or porcelain is, the more anisotropic light 8ualities it

has. 9ptical anisotropy is the change of $isual appearance depending on the angle of$iew or the angle of illumination. 0n e+ample of this can be seen in the change of

width, chroma, and hue of the incisal halo depending on the angles of illumination&Figure 3(.

0 percentage of the red#yellows fail to escape the lingual surface of the tooth and canbe trapped by the fiber#optic properties of the enamel. !his increases the anistropy of

the enamel. !hese longer wa$elengths will tra$el around the circumference of thetooth and can manifest themsel$es as an e$er#changing light show at the incisal edge.

!he anisotrophic properties of natural teeth will yield infinite opalescent opticaleffects depending on ambient lighting conditions. hat appears to be a good match in

static chairside photographs ta)en with a macro flash ne+t to the lens and at the sameperpendicular buccal $iew will not necessarily match in the real world.

MetamerismMetamerism is the unfortunate characteristic of restorations matching well in

operatory lighting or in photographs but then displaying differently when the patientsmiles in other light conditions.'erhaps you ha$e matched clothing under onelighting type and were shoc)ed to find the mismatch under different lighting. 9ne

ob*ect may ha$e the ability to reflect more blue than another. %owe$er, if there is noblue range in the lighting source, they will appear the same1 then, when $iewed under

a light source containing blue, the differences will appear. !he percei$ed colordepends on the nature of the light source illuminating the ob*ect and what

wa$elengths are reflected. !he closer the sum is of the reflecting wa$elengths of thetwo materials to be matched, the more successful the color match will be.'2Using

opa8ue surface stains to correct mismatches will increase metamerism. henreconstructing the dentin and enamel layers with dental porcelain, selecting porcelains

with the same optical properties will minimize metamerism.

FluorescenceFluorescence might be categorized as a type of reflection by opa8ue material.Fluorescence by definition is the absorption of light by a substance and the

spontaneous re#emission of lower#energy light of a longer wa$elength. '3Fluorescencein a natural tooth occurs primarily in the dentin because of the higher amount of

organic material present.C,'3#':0mbient, non#$isible, near#ultra$iolet &U4( light isabsorbed and then fluoresced bac) as $isible light primarily in the blue end of the

spectrum. hy this is noteworthy in dentistry is that the more the dentin fluoresces,the higher the $alue of the tooth.'C!o increase the fluorescence in stac)ed dental


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porcelains, opa8ue metallic#o+ide powders with fluorescent 8ualities can be added toincrease the 8uantity of light returned bac) to the $iewer, to bloc) out discolorations,

and to decrease chroma.'CUnfortunately, these powders will reduce the percei$edtranslucency if not )ept to the deeper dentin layers or to the coping. !he porcelains

commonly used today all $ary in their natural fluorescent 8ualities. irconium

e+hibits little or no fluorescence.!he brightness of natural teeth changes significantly when illuminated with U4 light,which can ha$e a dramatic affect on the le$el of percei$ed $itality e+hibited by

porcelain restorations. "magine how a crown made of porcelain with low fluorescent8ualities will stand out in bright sunlight.

i!ht "ources and Color #enderin!"f a wa$elength is not part of the ambient light spectrum, it is not there to reflect off

the tooth. !o properly assess chroma and hue, a full#spectrum light source is needed.Dental unit lights are commonly used for color rendering. Most are incandescent

lights that emit light high in the red#yellow spectrum and low at the blue end & Figure7(. !he ambient light 8uality of the operatory must be maintained with artificial

lighting &natural light conditions $ary(. !here are ceiling fluorescent bulbs that ha$efull color content and render color more accurately. !he 8uality of ambient light is

commonly measured by the color temperature and the color#rendering inde+ &->"(,which measures the percentage of the near#U4 and $isible light spectrum that a light

emits. 0ny light source o$er ;' is ade8uate for dentistry. Euality S5> macro#flashsystems ha$e e+cellent ->"s greater than ;2, and are the best way to document color

to the laboratory. "deally, both the dentist and the laboratory technician should ha$ebalanced, full#spectrum lighting conditions. -olor temperature is an inade8uate

measure of light 8uality for shade rendering."urface Morpholo!y0ll teeth ha$e surfaces with morphological $ariations. Surface morphology affects

how light will reflect. hen light hits a surface perpendicular to your eyes, asignificant portion of that light can reflect bac) to your eyes. 0nterior teeth ha$e

surfaces perpendicular to a $iewer1 thus, these reflections ha$e a significant influenceon appearance. !he @percei$edA shape, length, and width of an anterior tooth is

significantly influenced by the specular reflections coming off the heights of contourof the buccal surfaces.'CDocumenting the surface morphology of posterior teeth is of

less importance.

"urface Te$ture and uster!he surface te+tures of ma+illary incisors can be described as $ertical, horizontal, and$aried.'4ertical surface te+tures are primarily composed of the heights of contour of

the marginal ridges and the de$elopmental lobes. Fine, trans$erse, wa$eli)e groo$escalled perichymata or the striae of retzius':,';create most of the horizontal te+tures.

!hese horizontal undulations ne$er cross each other, and they go circumferentially&Figure :(. !hese horizontal te+tures are formed on top of $ertical te+tures, meaning

the horizontal patterns follow into the conca$ities formed by the $ertical te+tures butthe $ertical te+tures are not affected by the horizontal te+tures. !he $aried te+tures are

crac)s, chips, and other surface aberrations or patterns such as @orange peelA & FigureC(.


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0t eruption, teeth ha$e their roughest surface te+ture. 0 roughened surface te+turewill not yield as well defined an image and will scatter the light. =ith age, these

surface features gradually wear. 0s the wear process continues into the later years oflife, usually all signs of the peri)ymata are lost and e$en the definition of the

de$elopmental lobes is obliterated &Figure (.

5uster is often described as surface polish. >educing the surface luster of windowglass by sand blasting will produce a frosty white loo). 0s light hits the surface of theroughened glass surface, it scatters or bends irregularly. !his scattering of the light at

the surface causes an increase in opacity. !he light does not pass the surface but isreflected, causing an increase in brightness &Figure ;through Figure ''(. 0s the glass

becomes less translucent, the $alue goes up. !he net effect is that more light returns tothe $iewer as the lusterpolish goes down. olishing a porcelain restoration is a subtle

way to lower $alue by ma)ing the porcelain clearer and more translucent.';,'

ConclusionMatching a natural tooth with an artificial restoration will always remain an artisticchallenge. !he dentisttechnician team is responsible for artfully re#creating the

natural tooth anatomy, alignment, and wear patterns of the lost tooth structure whilealso matching the optical parameters that generate the o$erall $isual appearance of the

tooth. Beyond talent, the ultimate li)elihood of faithfully re#creating nature is limitedonly by how complete the communication process is. Gnowing the language of color

as well as understanding how the optical properties of the human dentition aredisplayed will impro$e the outcomes of our restorati$e endea$ors.

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the optical characteristics of natural teeth

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