REVIEW ARTICLE
Review of Translucency Determinations and Applications to Dental Materials WILLIAM M. JOHNSTON, PhD
ABSTRACT Statement of Problem: There are several ways to measure the translucency of materials, as various basic colorimetric concepts have been used to describe the translucency of natural and esthetic dental materials. There are currently no guidelines regarding which method to use to describe the translucency of these materials. Purposes of Study: Two initial purposes of this study are to review the initial developments of the major translucency measurement methods first in the color science literature and then in the dentistry literature, and to review in the dentistry literature the recent uses of the various translucency measurement methods in light of the objectives of the presented research. Material and Methods: A major color science textbook was reviewed to obtain the background information and selected references regarding the original methods of opacity measurement and the original references regarding the development and use of a translucency parameter were also reviewed. Then the recent dentistry literature was reviewed to describe the uses of the various methods of opacity or translucency determinations for various dental materials. Results: The three major methods of translucency measurement were found to be contrast ratio, transmittance, and translucency parameter, with the contrast ratio and transmittance methods each having the possibility of being either luminous or spectral. Translucency measurements were mainly used to describe dental resin composites and dental ceramic materials, but prosthetic elastomers, fiber posts, orthodontic brackets, natural tooth dentine and enamel, and combinations of materials were also studied using at least one of these methods. Conclusions: The more-developed use of models that relate the thickness of the translucent materials to the translucency measurement of interest is encouraged. Also, care must be taken when comparing previously generated translucency measurements with any newly generated data because technical details of the thickness and the backings used in previous research must be matched or adjustments must be made to make any newly generated data comparable with published values.
CLINICAL SIGNIFICANCE The method of specifying the translucency of esthetic dental materials may be based on clinical appearance requirements of the patient, on technical demands of optimizing the setting of underlying material, or on both. The method or methods used to describe translucency may provide clinically relevant information in order for the clinician to select the optimum material to satisfy these requirements. (J Esthet Restor Dent 26:217–223, 2014)
INTRODUCTION Although transparency is of concern for components of the human eye, transparent materials do not otherwise
appear within the oral and maxillofacial complex, and it is easily observed that some natural enamel is translucent. Much recent emphasis has been placed on the translucency on natural and restorative dental
Professor, Division of General Dentistry and Materials Science, College of Dentistry, The Ohio State University, Columbus, OH, USA
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materials.1 As will be clarified, measurement of translucency was addressed in terms of its opposite, opacity, when discussed in developmental color science literature where the concepts of translucency or transparency needed little distinction as they were each opposite from opacity. This review will therefore approach measurements of opacity as measurements of translucency with such measurement correlated inversely. Opacity was also referred to as the hiding power in the textbook by Judd and Wyszecki,2 where the chapter on the physics and psychophysics of colorant layers included the descriptions of three important aspects of color science related to translucency: gloss, opacity or hiding power, and Kubelka–Munk analysis. In this chapter, the importance of reflections where there is an optical discontinuity due to a change in the index of refraction was emphasized, and theoretical descriptions of the reflectance and transmittance of light-scattering materials provided useful relationships between important optical aspects related to translucency and the thickness of the translucent material. This review will first describe developments of measurements of translucency or its inverse opacity as applied first to industrial materials and then to dental materials. Then, more recent work regarding applications of the various translucency determination methods to dental materials will be reviewed in order to overview the applications of these methods for the various contemporary materials and combinations of materials. Suggestions will then be made regarding the application of color science to descriptions of translucency of esthetic materials.
DEVELOPMENTS OF DETERMINATIONS OF TRANSLUCENCY OR OPACITY The hiding power or opacity of materials was described by its contrast ratio (CR)2,3,4 for industrial applications, and the opacity of a dental material was described in 1937 by Paffenbarger and Judd5 and then refined in 1979 for “aesthetic dental filling materials.”6
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Subsequently, more modern direct composite materials were described by the CR based on luminous reflectance7 and then the knowledge regarding the translucency of dental composite materials was advanced by several other researchers as will at least partly be noted below. As most commonly used and as used by these cited authors, CR was defined as the ratio of the luminous reflectance of a translucent material on a black backing to the luminous reflectance of the same material on a white backing. In this definition, it is important to note that luminous reflectance is the Y tristimulus value in reflectance as defined by the CIE.8 From this definition, it is obvious that when the two luminous reflectance values are identical, the material is completely masking the backings and CR is one, which is as high as possible for this measure. When the material is completely transparent, the luminous reflectance values are the values of the backings, respectively, and in this case, CR has the lower limit of the ratio of the luminous reflectance of the black backing to that of the white backing. CR was therefore well established as an opacity measure, not to be confused with an opalescence measure, which has been given the acronym OP.9 Four important concepts should be noted from the definition of CR. First, the thickness of the transparent material will significantly affect CR, as the thickness of the material will affect its reflectance on each of these backings.2 Second, the reflectance values of the backings will affect the calculated CR, as the reflectance of the backing and the optical continuity between the translucent material and the backing each affect the reflectance of the material on that backing.2 Third, because luminous reflectance is a function of the illuminant and the observer used for the luminous reflectance calculations,7 each of these parameters must be described when using CR based on luminous reflectance values. And fourth, although CR was commonly based on luminous reflectance values, it is possible to calculate CR as a function of wavelength because the reflectance on each backing can easily be described as a function of wavelength.2,10 As noted first in the color literature,2,3 the thickness required to produce a certain CR can be calculated
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when the relationship between thickness and CR is known. This method then offers the possibility of establishing a critical CR for a given application and then solving for the thickness required to obtain this critical value of translucency.
Unfortunately, several publications have not properly identified the source of the translucency measurement method used in the research. Authors are encouraged to cite correctly.
It was also common in the color science literature to use transmittance2,11 for opacity measures. Just as for CR, the thickness to produce a certain transmittance could also be calculated because the relationship2,9 between thickness and transmittance is known. Further, luminous transmittance is based on CIE colorimetry7 and luminous transmittance descriptions must therefore include the illuminant and the observer used for the calculation. Finally, transmittance may also be spectral2,9 so transmittance can be expressed at wavelengths of interest when this wavelength or wavelength range has a special meaning with regard to the material system being studied, as well noted in the case of the light-curing of an adhesive layer under an esthetic material.12
RECENT ADVANCES IN KNOWLEDGE ABOUT TRANSLUCENT DENTAL MATERIALS
The translucency parameter (TP) was provided as a direct measure of translucency first for maxillofacial elastomer materials13 where this parameter was defined as the color difference found for the material at a specified thickness, where the color difference was between the material when in optical contact with ideal black and white backings. It was then described as corresponding directly to common visual assessments of translucency when applied to changes in esthetic restorative materials during and after curing.14 Again, several important issues should be noted based on the definition of the TP. TP is also based on CIE colorimetry7 and therefore the illumination, the observer, and the color difference formula used for the color difference calculations must also be presented.15 Also, the thickness of the translucent material will significantly affect the color difference, as the thickness of the material will affect its color on each of these backings.2 And again, the reflectance values of the backings will affect the calculated or observed colors, as the color of the backing and the optical continuity between the translucent material and the backing each affect the color of the translucent material on that backing.2,7,9
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This review will now concentrate on the dental literature published during and after 2008, although much development research was performed prior to this time. Differences in brands and shades of dental composite materials using TP determinations were reported.15–19 Polymers from conventional and industrial polymerizations were shown to widely vary in translucency using wavelength-specific transmittance determinations.20 Comparison of direct and indirect composite materials9,21,22 also showed the change in translucency by curing using TP measurements. Further, this change by curing was demonstrated when investigating curing depth23 using both TP- and wavelength-specific transmittance measurements. Polymerization kinetics was studied24 using only wavelength-specific transmittance measurements. Effect of resin matrix composition showing the influence of bisphenol A glycidyl methacrylate concentration was demonstrated using both spectral transmittance measurements.25 Using straight-line light transmittance calculated from the peak gain at an angle of zero degrees, and the transmitted light diffusion property calculated as the diffusion factor, combinations of composite materials were studied with26 and without27 TP determinations. Thickness effects were well noted15,28–31 for dental composite materials. Staining effects were verified using transmittance32 and TP33 determinations. The effect of filler34–36 in dental composite materials was found using TP determinations, as was the effect of thermocycling.37 The method of TP in combination with wavelength-specific CR was used to show the changes in translucency with wavelength.38 The combination of TP and luminous transmittance measurements39,40 was used to show differences in dental composite materials, and such differences were also found41,42 using a
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combination of TP and CR determinations. TP determinations were also used to show that the translucency of translucent composites is different from that of tooth enamel.43
composites62 and among dental porcelain with different shades of cement.63 TP determinations were used to show differences among zirconia ceramics and human and bovine dentin.64
Studies on the translucency of dental ceramic materials demonstrated differences in CR among shades of zirconia-based ceramics44 and the effect of thickness on CR was also demonstrated for different brands of these ceramics.45 Differences among these ceramic materials were shown using direct transmission measurements.46 Differences due to sintering conditions of these ceramics was shown using total transmission47 and diffuse transmittance measurements,48 and the effect of veneering was further demonstrated using total luminous transmittance.49 The curing of underlying cement was the major topic of interest in determining the translucency of these ceramics using wavelength-specific transmittance.50 TP determinations were used to show differences due to thickness of various ceramic materials.51 Both CR and transmittance determinations were used to determine that CR was insensitive to transmission differences below 50% transmission for a wide variety of ceramic materials.52 TP-, CR-, and wavelength-specific transmittance determinations were used to show differences among shades of dental ceramic materials,53 whereas only TP and CR determinations were used to show the significant correlation between TP and CR.54 TP determinations of ceramic materials were solely used to determine significant effects of the illuminant,55 surface texture,56 and surface-finishing technique.57 Using ceramic materials, TP values measured by the spectroradiometer and the spectrophotometer were found to be significantly different but highly correlated.58 A significant correlation between the translucency of ceramic materials and polymerization efficiency of underlying luting composites was found using TP determinations.59 A translucency perception threshold of human subjects was determined using CR determinations on dental porcelain materials.60 A newly defined “relative translucency” was used to demonstrate translucency differences in ceramic core materials.61 With regard to combinations with ceramic materials, TP determinations were used to show differences among combinations of ceramic and core build-up
Regarding natural dental materials, TP determinations were used to demonstrate the significant effect of bleaching on human enamel.65 Data recommended for use as reference in the development of esthetic restorative materials and clinical shade matching were presented using both CR and TP determinations.66 Ground 400-μm sections of root dentin stained with 1% methylene blue were visually assessed for a relationship between root translucency and the subject’s age.67 Vital natural unrestored teeth were analyzed by a digital imaging and shade analysis device, which was used to indicate areas of visually noticeable tooth translucency.68
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Regarding other various dental materials, transmittance determinations were used to find significant differences in fiber posts69 and orthodontic brackets.70 TP determinations were used to show differences in polymer-based paint-on temporary coating materials.71 TP determinations were also used to show aging effects of resin-modified glass ionomer cement72 and prosthetic elastomers.73
DISCUSSION AND CONCLUSIONS Methods of determining the translucency of natural and esthetic restorative dental materials have seen significant advances, but technical details might limit the comparisons of previously published works to any newly generated research. Calculations based on CIE colorimetry7 must include descriptions of the illuminant and the observer used for the calculations; these calculations include luminous transmittance, luminous reflectance, and color. All of the mainly used determinations of translucency, i.e., CR, transmittance, and TP, each require that a thickness be specified and the specification of this thickness is usually dependent on clinical opinions of importance. What has not been well developed is the
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use theoretical relationships between each of these translucency methods and thickness, as has been proposed for materials in general using either reflectance or transmittance,2,9 and more specifically for the reflectance of pigmented maxillofacial elastomer,74 dental porcelain,75 dental composite materials,76 and natural human enamel and dentine.77 The more general relationships2,9 allow for the calculation of any one of these translucency measures and would also allow for any set of different backings to be used in the calculations involving CR or TP because adjustments could be made to allow for ideal black and white backings and for any thickness. Also, if these relationships involving reflectance would be developed for restorative materials of interest, the use of perception and acceptability thresholds78 would provide values that may be used to calculate thickness thresholds based on the TP, as a threshold thickness could be determined for any set of different backings as might be found in the clinical setting. Using comparisons at established thickness values on backings also demands that the values of the backings must be either set at specified values or adjusted to established values for these backings. The use of wavelength-specific translucency determinations is desirable when the interest is in chemical systems influenced by wavelength-specific illumination, such as the case of curing of an adhesive through another material.11,49 Otherwise, spectral data are difficult to interpret without using established colorimetric methods, as can be provided by CIE colorimetry.7
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DISCLOSURE STATEMENT 16.
The author has no financial interest in the materials or devices reviewed, and declares no conflict of interest.
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65. Ma X, Jiang T, Sun L, et al. Effects of tooth bleaching on the color and translucency properties of enamel. Am J Dent 2009;22:324–8. 66. Yu B, Ahn JS, Lee YK. Measurement of translucency of tooth enamel and dentin. Acta Odontol Scand 2009;67:57–64. 67. Singhal A, Ramesh V, Balamurali P. A comparative analysis of root dentin transparency with known age. J Forensic Dent Sci 2010;2:18–21. 68. Bayindir F, Gozalo-Diaz D, Kim-Pusateri S, Wee AG. Incisal translucency of vital natural unrestored teeth: a clinical study. J Esthet Restor Dent 2012;24: 335–43. 69. Goracci C, Corciolani G, Vichi A, Ferrari M. Light-transmitting ability of marketed fiber posts. J Dent Res 2008;87:1122–6. 70. Lopes Filho H, Maia LE, Araújo MV, Ruellas AC. Influence of optical properties of esthetic brackets (color, translucence, and fluorescence) on visual perception. Am J Orthod Dentofacial Orthop 2012;141:460–7. 71. Takenaka S, Wakamatsu R, Ozoe Y, et al. Translucency and color change of tooth-colored temporary coating materials. Am J Dent 2009;22:361–5. 72. Lee YK, Yu B, Zhao GF, Lim JI. Effects of aging and HEMA content on the translucency, fluorescence, and opalescence properties of experimental HEMA-added glass ionomers. Dent Mater J 2010;29(1):9–14. PMID: 20379006. 73. Polyzois GL, Eleni PN, Krokida MK. Optical properties of pigmented polydimethylsiloxane prosthetic elastomers: effect of “outdoor” and “indoor” accelerating aging. J Craniofac Surg 2011;22:1574–8. 74. Hu X, Gilbert AB, Johnston WM. Interfacial corrections of maxillofacial elastomers for Kubelka–Munk theory using non-contact measurements. Dent Mater 2009;25:1163–8. 75. Johnston WM, O’Brien WJ, Tien TY. The determination of optical absorption and scattering in translucent porcelain. Color Res Appl 1986;11:125–30. 76. Mikhail SS, Azer SS, Johnston WM. Accuracy of Kubelka–Munk reflectance theory for dental resin composite material. Dent Mater 2012;28:729–35. 77. Ragain RC, Johnston WM. Accuracy of Kubelka–Munk reflectance theory applied to human dentin and enamel. J Dent Res 2001;80:449–52. 78. Khashayar G, Bain PA, Salari S, et al. Perceptibility and acceptability thresholds for colour differences in dentistry. J Dent 2014;42:637–44. Reprint requests: William M. Johnston, PhD, The Ohio State University College of Dentistry, 305 W. 12th Ave., Columbus, OH 43210, USA; Tel.: 614-292-0955; email: [email protected]
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ABSTRACT Statement of Problem: There are several ways to measure the translucency of materials, as various basic colorimetric concepts have been used to describe the translucency of natural and esthetic dental materials. There are currently no guidelines regarding which method to use to describe the translucency of these materials. Purposes of Study: Two initial purposes of this study are to review the initial developments of the major translucency measurement methods first in the color science literature and then in the dentistry literature, and to review in the dentistry literature the recent uses of the various translucency measurement methods in light of the objectives of the presented research. Material and Methods: A major color science textbook was reviewed to obtain the background information and selected references regarding the original methods of opacity measurement and the original references regarding the development and use of a translucency parameter were also reviewed. Then the recent dentistry literature was reviewed to describe the uses of the various methods of opacity or translucency determinations for various dental materials. Results: The three major methods of translucency measurement were found to be contrast ratio, transmittance, and translucency parameter, with the contrast ratio and transmittance methods each having the possibility of being either luminous or spectral. Translucency measurements were mainly used to describe dental resin composites and dental ceramic materials, but prosthetic elastomers, fiber posts, orthodontic brackets, natural tooth dentine and enamel, and combinations of materials were also studied using at least one of these methods. Conclusions: The more-developed use of models that relate the thickness of the translucent materials to the translucency measurement of interest is encouraged. Also, care must be taken when comparing previously generated translucency measurements with any newly generated data because technical details of the thickness and the backings used in previous research must be matched or adjustments must be made to make any newly generated data comparable with published values.
CLINICAL SIGNIFICANCE The method of specifying the translucency of esthetic dental materials may be based on clinical appearance requirements of the patient, on technical demands of optimizing the setting of underlying material, or on both. The method or methods used to describe translucency may provide clinically relevant information in order for the clinician to select the optimum material to satisfy these requirements. (J Esthet Restor Dent 26:217–223, 2014)
INTRODUCTION Although transparency is of concern for components of the human eye, transparent materials do not otherwise
appear within the oral and maxillofacial complex, and it is easily observed that some natural enamel is translucent. Much recent emphasis has been placed on the translucency on natural and restorative dental
Professor, Division of General Dentistry and Materials Science, College of Dentistry, The Ohio State University, Columbus, OH, USA
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materials.1 As will be clarified, measurement of translucency was addressed in terms of its opposite, opacity, when discussed in developmental color science literature where the concepts of translucency or transparency needed little distinction as they were each opposite from opacity. This review will therefore approach measurements of opacity as measurements of translucency with such measurement correlated inversely. Opacity was also referred to as the hiding power in the textbook by Judd and Wyszecki,2 where the chapter on the physics and psychophysics of colorant layers included the descriptions of three important aspects of color science related to translucency: gloss, opacity or hiding power, and Kubelka–Munk analysis. In this chapter, the importance of reflections where there is an optical discontinuity due to a change in the index of refraction was emphasized, and theoretical descriptions of the reflectance and transmittance of light-scattering materials provided useful relationships between important optical aspects related to translucency and the thickness of the translucent material. This review will first describe developments of measurements of translucency or its inverse opacity as applied first to industrial materials and then to dental materials. Then, more recent work regarding applications of the various translucency determination methods to dental materials will be reviewed in order to overview the applications of these methods for the various contemporary materials and combinations of materials. Suggestions will then be made regarding the application of color science to descriptions of translucency of esthetic materials.
DEVELOPMENTS OF DETERMINATIONS OF TRANSLUCENCY OR OPACITY The hiding power or opacity of materials was described by its contrast ratio (CR)2,3,4 for industrial applications, and the opacity of a dental material was described in 1937 by Paffenbarger and Judd5 and then refined in 1979 for “aesthetic dental filling materials.”6
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Subsequently, more modern direct composite materials were described by the CR based on luminous reflectance7 and then the knowledge regarding the translucency of dental composite materials was advanced by several other researchers as will at least partly be noted below. As most commonly used and as used by these cited authors, CR was defined as the ratio of the luminous reflectance of a translucent material on a black backing to the luminous reflectance of the same material on a white backing. In this definition, it is important to note that luminous reflectance is the Y tristimulus value in reflectance as defined by the CIE.8 From this definition, it is obvious that when the two luminous reflectance values are identical, the material is completely masking the backings and CR is one, which is as high as possible for this measure. When the material is completely transparent, the luminous reflectance values are the values of the backings, respectively, and in this case, CR has the lower limit of the ratio of the luminous reflectance of the black backing to that of the white backing. CR was therefore well established as an opacity measure, not to be confused with an opalescence measure, which has been given the acronym OP.9 Four important concepts should be noted from the definition of CR. First, the thickness of the transparent material will significantly affect CR, as the thickness of the material will affect its reflectance on each of these backings.2 Second, the reflectance values of the backings will affect the calculated CR, as the reflectance of the backing and the optical continuity between the translucent material and the backing each affect the reflectance of the material on that backing.2 Third, because luminous reflectance is a function of the illuminant and the observer used for the luminous reflectance calculations,7 each of these parameters must be described when using CR based on luminous reflectance values. And fourth, although CR was commonly based on luminous reflectance values, it is possible to calculate CR as a function of wavelength because the reflectance on each backing can easily be described as a function of wavelength.2,10 As noted first in the color literature,2,3 the thickness required to produce a certain CR can be calculated
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when the relationship between thickness and CR is known. This method then offers the possibility of establishing a critical CR for a given application and then solving for the thickness required to obtain this critical value of translucency.
Unfortunately, several publications have not properly identified the source of the translucency measurement method used in the research. Authors are encouraged to cite correctly.
It was also common in the color science literature to use transmittance2,11 for opacity measures. Just as for CR, the thickness to produce a certain transmittance could also be calculated because the relationship2,9 between thickness and transmittance is known. Further, luminous transmittance is based on CIE colorimetry7 and luminous transmittance descriptions must therefore include the illuminant and the observer used for the calculation. Finally, transmittance may also be spectral2,9 so transmittance can be expressed at wavelengths of interest when this wavelength or wavelength range has a special meaning with regard to the material system being studied, as well noted in the case of the light-curing of an adhesive layer under an esthetic material.12
RECENT ADVANCES IN KNOWLEDGE ABOUT TRANSLUCENT DENTAL MATERIALS
The translucency parameter (TP) was provided as a direct measure of translucency first for maxillofacial elastomer materials13 where this parameter was defined as the color difference found for the material at a specified thickness, where the color difference was between the material when in optical contact with ideal black and white backings. It was then described as corresponding directly to common visual assessments of translucency when applied to changes in esthetic restorative materials during and after curing.14 Again, several important issues should be noted based on the definition of the TP. TP is also based on CIE colorimetry7 and therefore the illumination, the observer, and the color difference formula used for the color difference calculations must also be presented.15 Also, the thickness of the translucent material will significantly affect the color difference, as the thickness of the material will affect its color on each of these backings.2 And again, the reflectance values of the backings will affect the calculated or observed colors, as the color of the backing and the optical continuity between the translucent material and the backing each affect the color of the translucent material on that backing.2,7,9
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This review will now concentrate on the dental literature published during and after 2008, although much development research was performed prior to this time. Differences in brands and shades of dental composite materials using TP determinations were reported.15–19 Polymers from conventional and industrial polymerizations were shown to widely vary in translucency using wavelength-specific transmittance determinations.20 Comparison of direct and indirect composite materials9,21,22 also showed the change in translucency by curing using TP measurements. Further, this change by curing was demonstrated when investigating curing depth23 using both TP- and wavelength-specific transmittance measurements. Polymerization kinetics was studied24 using only wavelength-specific transmittance measurements. Effect of resin matrix composition showing the influence of bisphenol A glycidyl methacrylate concentration was demonstrated using both spectral transmittance measurements.25 Using straight-line light transmittance calculated from the peak gain at an angle of zero degrees, and the transmitted light diffusion property calculated as the diffusion factor, combinations of composite materials were studied with26 and without27 TP determinations. Thickness effects were well noted15,28–31 for dental composite materials. Staining effects were verified using transmittance32 and TP33 determinations. The effect of filler34–36 in dental composite materials was found using TP determinations, as was the effect of thermocycling.37 The method of TP in combination with wavelength-specific CR was used to show the changes in translucency with wavelength.38 The combination of TP and luminous transmittance measurements39,40 was used to show differences in dental composite materials, and such differences were also found41,42 using a
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combination of TP and CR determinations. TP determinations were also used to show that the translucency of translucent composites is different from that of tooth enamel.43
composites62 and among dental porcelain with different shades of cement.63 TP determinations were used to show differences among zirconia ceramics and human and bovine dentin.64
Studies on the translucency of dental ceramic materials demonstrated differences in CR among shades of zirconia-based ceramics44 and the effect of thickness on CR was also demonstrated for different brands of these ceramics.45 Differences among these ceramic materials were shown using direct transmission measurements.46 Differences due to sintering conditions of these ceramics was shown using total transmission47 and diffuse transmittance measurements,48 and the effect of veneering was further demonstrated using total luminous transmittance.49 The curing of underlying cement was the major topic of interest in determining the translucency of these ceramics using wavelength-specific transmittance.50 TP determinations were used to show differences due to thickness of various ceramic materials.51 Both CR and transmittance determinations were used to determine that CR was insensitive to transmission differences below 50% transmission for a wide variety of ceramic materials.52 TP-, CR-, and wavelength-specific transmittance determinations were used to show differences among shades of dental ceramic materials,53 whereas only TP and CR determinations were used to show the significant correlation between TP and CR.54 TP determinations of ceramic materials were solely used to determine significant effects of the illuminant,55 surface texture,56 and surface-finishing technique.57 Using ceramic materials, TP values measured by the spectroradiometer and the spectrophotometer were found to be significantly different but highly correlated.58 A significant correlation between the translucency of ceramic materials and polymerization efficiency of underlying luting composites was found using TP determinations.59 A translucency perception threshold of human subjects was determined using CR determinations on dental porcelain materials.60 A newly defined “relative translucency” was used to demonstrate translucency differences in ceramic core materials.61 With regard to combinations with ceramic materials, TP determinations were used to show differences among combinations of ceramic and core build-up
Regarding natural dental materials, TP determinations were used to demonstrate the significant effect of bleaching on human enamel.65 Data recommended for use as reference in the development of esthetic restorative materials and clinical shade matching were presented using both CR and TP determinations.66 Ground 400-μm sections of root dentin stained with 1% methylene blue were visually assessed for a relationship between root translucency and the subject’s age.67 Vital natural unrestored teeth were analyzed by a digital imaging and shade analysis device, which was used to indicate areas of visually noticeable tooth translucency.68
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Regarding other various dental materials, transmittance determinations were used to find significant differences in fiber posts69 and orthodontic brackets.70 TP determinations were used to show differences in polymer-based paint-on temporary coating materials.71 TP determinations were also used to show aging effects of resin-modified glass ionomer cement72 and prosthetic elastomers.73
DISCUSSION AND CONCLUSIONS Methods of determining the translucency of natural and esthetic restorative dental materials have seen significant advances, but technical details might limit the comparisons of previously published works to any newly generated research. Calculations based on CIE colorimetry7 must include descriptions of the illuminant and the observer used for the calculations; these calculations include luminous transmittance, luminous reflectance, and color. All of the mainly used determinations of translucency, i.e., CR, transmittance, and TP, each require that a thickness be specified and the specification of this thickness is usually dependent on clinical opinions of importance. What has not been well developed is the
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use theoretical relationships between each of these translucency methods and thickness, as has been proposed for materials in general using either reflectance or transmittance,2,9 and more specifically for the reflectance of pigmented maxillofacial elastomer,74 dental porcelain,75 dental composite materials,76 and natural human enamel and dentine.77 The more general relationships2,9 allow for the calculation of any one of these translucency measures and would also allow for any set of different backings to be used in the calculations involving CR or TP because adjustments could be made to allow for ideal black and white backings and for any thickness. Also, if these relationships involving reflectance would be developed for restorative materials of interest, the use of perception and acceptability thresholds78 would provide values that may be used to calculate thickness thresholds based on the TP, as a threshold thickness could be determined for any set of different backings as might be found in the clinical setting. Using comparisons at established thickness values on backings also demands that the values of the backings must be either set at specified values or adjusted to established values for these backings. The use of wavelength-specific translucency determinations is desirable when the interest is in chemical systems influenced by wavelength-specific illumination, such as the case of curing of an adhesive through another material.11,49 Otherwise, spectral data are difficult to interpret without using established colorimetric methods, as can be provided by CIE colorimetry.7
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DISCLOSURE STATEMENT 16.
The author has no financial interest in the materials or devices reviewed, and declares no conflict of interest.
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65. Ma X, Jiang T, Sun L, et al. Effects of tooth bleaching on the color and translucency properties of enamel. Am J Dent 2009;22:324–8. 66. Yu B, Ahn JS, Lee YK. Measurement of translucency of tooth enamel and dentin. Acta Odontol Scand 2009;67:57–64. 67. Singhal A, Ramesh V, Balamurali P. A comparative analysis of root dentin transparency with known age. J Forensic Dent Sci 2010;2:18–21. 68. Bayindir F, Gozalo-Diaz D, Kim-Pusateri S, Wee AG. Incisal translucency of vital natural unrestored teeth: a clinical study. J Esthet Restor Dent 2012;24: 335–43. 69. Goracci C, Corciolani G, Vichi A, Ferrari M. Light-transmitting ability of marketed fiber posts. J Dent Res 2008;87:1122–6. 70. Lopes Filho H, Maia LE, Araújo MV, Ruellas AC. Influence of optical properties of esthetic brackets (color, translucence, and fluorescence) on visual perception. Am J Orthod Dentofacial Orthop 2012;141:460–7. 71. Takenaka S, Wakamatsu R, Ozoe Y, et al. Translucency and color change of tooth-colored temporary coating materials. Am J Dent 2009;22:361–5. 72. Lee YK, Yu B, Zhao GF, Lim JI. Effects of aging and HEMA content on the translucency, fluorescence, and opalescence properties of experimental HEMA-added glass ionomers. Dent Mater J 2010;29(1):9–14. PMID: 20379006. 73. Polyzois GL, Eleni PN, Krokida MK. Optical properties of pigmented polydimethylsiloxane prosthetic elastomers: effect of “outdoor” and “indoor” accelerating aging. J Craniofac Surg 2011;22:1574–8. 74. Hu X, Gilbert AB, Johnston WM. Interfacial corrections of maxillofacial elastomers for Kubelka–Munk theory using non-contact measurements. Dent Mater 2009;25:1163–8. 75. Johnston WM, O’Brien WJ, Tien TY. The determination of optical absorption and scattering in translucent porcelain. Color Res Appl 1986;11:125–30. 76. Mikhail SS, Azer SS, Johnston WM. Accuracy of Kubelka–Munk reflectance theory for dental resin composite material. Dent Mater 2012;28:729–35. 77. Ragain RC, Johnston WM. Accuracy of Kubelka–Munk reflectance theory applied to human dentin and enamel. J Dent Res 2001;80:449–52. 78. Khashayar G, Bain PA, Salari S, et al. Perceptibility and acceptability thresholds for colour differences in dentistry. J Dent 2014;42:637–44. Reprint requests: William M. Johnston, PhD, The Ohio State University College of Dentistry, 305 W. 12th Ave., Columbus, OH 43210, USA; Tel.: 614-292-0955; email: [email protected]
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