Colour and Shade Matching in Restorative Dentistry - Copy.pptx

Color and Shade Matching in Restorative Dentistry  Colour plays a critical role in the success or failure of esthetic dental restorations. Color and Perception  The Color Triplet (Observer Situation)  The human perception of colour results from the interaction of three elements:...

Color and Shade Matching in Restorative Dentistry  Colour plays a critical role in the success or failure of esthetic dental restorations. Color and Perception  The Color Triplet (Observer Situation)  The human perception of colour results from the interaction of three elements: light source, object and observer  Because all three of these elements can be modified, any change in one element will affect the final perception of colour.  The light source is a visible form of electromagnetic (EM) radiation that illuminates the object.  When light strikes the object (tooth) a proportion of the energy is absorbed, transmitted or reflected.  Colour perception is dependent upon the subjective ability of the human visual system to combine and interpret the physical interactions of light and object.  The quantity of reflected light reaching the observer’s eyes stimulates a subjective sensation in the brain that we experience as colour.  In other words, the perception of colour ultimately resides in the brain and not merely in the property of the object.  For this reason, colour can be defined as a psychophysical sensation provoked in the eye by the visible light and interpreted by the brain. Colour Vision  Rods and Cones:  The visual system of a person with normal colour vision can identify millions of different colours.  At the innermost retinal layer of the eye are two types of specialized neurons that function as photoreceptors, called rods and cones.  I. Rods:  i. The more numerous of the two photoreceptors are the rods, which are sensitive to low levels of light.  ii. The rods are primarily responsible for our peripheral vision and are unable to detect colour.  iii. In low levels of light, rods help us see objects in gray scale; as the light becomes brighter the rods become inactive.  II. Cones:  i. The cones on the other hand operate in bright light and provide high-acuity colour 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 colour is interpreted.  ii. There are three types of cones in the retina which 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. Colour Deficiency  Colour vision deficiency is a weakness or absence in one or more of the three types of cones.  Individuals with these colour deficiencies still see colour; however, their colour vision is distorted.  i- The most common colour deficiency in the population is an individual with a partial green defect known as deuteranomaly.  ii- Other deficiencies are protanomaly, which is caused by a reduced sensitivity to red light.  iii- Tritanomaly, which is someone lacking blue vision.  Colour-blind individuals lack all three types of cones; this condition is called monochromacy or achromatopsia.  Colour deficiency poses a challenge for the clinician when performing visual shade matching.  Popular general tests to check colour vision are the Ishihara test and the Farnsworth–Munsell test. Colour Dimensions  There are numerous systems and theories for arranging colour in some orderly fashion (i.e. colour spaces).  The most popular system for visual colour matching in dentistry is based on the three-dimensional model devised by American artist Alfred H. Munsell in 1898.  Munsell’s colour system forms the basis for the classification of coloured objects in the three dimensions: hue, value and chroma.  Hue  Hue enables the distinction and differentiation among different colours.  This colour dimension is the attribute by which an object is judged to appear red, orange, yellow, green, blue, purple etc. These are the ‘pure’ colours found on a basic colour wheel or a simple box of crayons.  These hues, which appear on the visual spectrum, are placed on a continuous, circular scale.  Value  The dimension of value refers to the lightness of a colour.  It is the achromatic vertical scale from black to white representing all shades of gray.  It is usually communicated in terms of lighter or darker. A tooth that appears lighter or ‘brighter’ as a result of bleaching would display an increase in value.  Chroma  Chroma is related to variation in strength of the same colour.  The further away from the achromatic vertical axis, the higher the chroma (stronger, more intense).  The closer the colour is to the achromatic (value) axis, the lower the chroma (paler, weaker).  Chroma is often described as more chromatic or less chromatic.  A tooth with a redder and/or yellower appearance at the cervical region, as commonly seen on a canine, can be described as more chromatic at that region.  As the chroma increases, the hue becomes more specific. Munsell colour system Other Optical Properties  Translucency  Translucency is the degree to which an object scatters light upon transmission, resulting in an appearance between complete opacity and complete transparency.  Complete opacity will obscure the substrate beneath it by blocking the passage of light, while a completely transparent object will transmit light without scattering and will clearly show the substrate beneath it.  Iridescence  Iridescence is a rainbow-like effect caused by the diffraction of light that changes according to the angle from which it is viewed or the angle of incidence of the light source.  Iridescence occurs when light is diffracted from a thin layer that lies between two mediums of different refractive index (e.g. air and water), as in a soap bubble or a thin film of oil on water.  Teeth do not display the property of iridescence, which is often confused with opalescence.  Opalescence  Opalescence is a milky iridescence that resembles the internal play of colours of an opal.  In a natural tooth, opalescence is caused by light scattering between two phases of enamel that have different indexes of refraction.  Short wavelengths of light are reflected displaying a blue hue, whereas longer light wavelengths, such as the orange and red, are transmitted through the tooth.  Gloss  Gloss is an attribute of visual appearance that originates from the geometrical distribution of light reflected by surfaces.  Particularly, gloss is a term used to describe the relative amount of mirror-like (specular) reflection from the surface of an object.  Metals are usually distinguished by stronger specular reflection than that from other materials, and smooth surfaces will appear glossier than rough ones.  Fluorescence  Fluorescence is a form of luminescence, that is, a form of light emission by a substance as the result of some external stimuli.  Following the excitation by light, usually ultraviolet (UV), a fluorescent substance will re-emit some of the absorbed energy in the form of longer wavelengths.  When the luminescence continues after the source of excitation has been removed, the ‘after-glow’ is referred to as phosphorescence. Surround Effects and Blending  Colour not only is defined by its colour dimensions and optical properties, but also depends on the surroundings in which the object appears, the adaptation of our eyes and our recent visual experience.  Chromatic Induction  When two objects of the same colour are surrounded by different coloured backgrounds, an illusory sensation of colour can be created without direct stimulation of the corresponding cones.  The two objects can be of the same colour when viewed in isolation, but when each is combined with different surrounds, the objects can have a perceived colour difference in relation to each other.  This is what colour scientists call chromatic induction.  Contrast and Assimilation  Chromatic induction can generate either a contrast effect or an assimilation effect.  When the object’s colour shifts towards the complementary colour of its surroundings, this is known as simultaneous colour contrast.  When the opposite occurs, that is, the perceived difference between the object’s colour and its surrounding is reduced, this is known as chromatic assimilation.  Blending Effect  Clinically, the perceptual phenomenon of assimilation occurs when a restorative material (object) takes on the colour of the tooth (background/surround) and appears more similar combined than when viewed in isolation.  In the dental literature, this visual blending of tooth and material is known as blending effect (BE).  The BE is commonly (and incorrectly) referred to as ‘chameleon effect’.  Complementary Afterimage  Similar to contrast and assimilation effects, our eyes can adapt to a recent visual experience and provoke illusionary perceptions of colour.  When we stare at a solid colour for approximately 30 s or more, our photoreceptors become sensitized (retinal fatigue) and can create illusionary images of the complementary colour; this is known as complementary afterimage.  If one concentrates on a solid colour red target, for example, the red cones gradually respond less strongly to that reflected red signal.  If one switches his or her gaze to a solid white target, now all colours are reflected to the retina and cones will send a strong green signal and a strong blue signal, but a weak red signal. - One will see a cyan colour afterimage, cyan being the complementary colour of red Colour and Appearance of Teeth and Dental Materials  Tooth Colour and Appearance  How various reflections and transmissions of light integrate to generate perceived colours in human teeth is a complex process that is not entirely understood.  The polychromatic appearance of the tooth originates from the relative interactions of light signals and perceptions.  The quantification of tooth colour has been reported in the past by measuring the three regions of the tooth; cervical, middle and incisal.  Describing tooth regions can help understand how they are related to overall tooth colour and appearance.  However, this approach is not necessarily sufficient to capture the polychromatic  Dentin  In general, the colour of the tooth is not uniform.  Dentin contributes significantly to tooth colour.  This is particularly noticeable in the cervical region, where only a thin layer of enamel exists.  This region is typically the most chromatic, with the chroma progressively decreasing through the middle to the incisal third, displaying hues ranging from yellow to red.  Dentin is also the primary source of tooth fluorescence.  Enamel  In a healthy unworn tooth, the incisal third is typically all enamel.  If the enamel were isolated from the dentin, it would appear primarily achromatic like transparent or white frosted glass (Fig. 17.9).  The translucency and value of the enamel can vary depending on many factors such as its thickness and age. Thick enamel generally appears higher in value relative to thin enamel.  High-value white patterns, or white spots, may also demonstrate hypomineralized regions within the enamel.  For anterior teeth, the enamel gets thinner towards the incisal and can appear gray to bluish against the dark background of the oral cavity.  Depending on the 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 (Fig. 17.10).