The objective was to compare absolute translucency with a frequently used surrogate measure of relative translucency (contrast ratio) from 14 all-ceramic materials having a wide range of translucencies.
Materials and methods
Standardized disks were fabricated from fourteen ceramics, varying both thickness (0.3 mm, 0.5 mm, 1.0 mm, and 1.5 mm) and chroma (high and low). Absolute translucency (percent transmission) was measured using a spectroradiometer with an integrating sphere. Relative translucency was measured using a spectrophotometer as contrast ratio (contrast ratio; ratio of L * values recorded on black and white backgrounds). Non-linear regression was used to compare measurements of absolute versus relative values for each of the spectrophotometers.
Contrast ratio was not able to characterize translucency across the range of materials studied and became less sensitive with increasing opacity. A non-linear correlation was found between percent transmission and contrast ratio down to 50% transmission ( r 2 = 0.97) and contrast ratio was insensitive to transmission differences below 50% transmission.
Contrast ratio is not a direct measure of translucency and cannot be used below 50% transmission.
Choices for specific ceramic systems are driven largely by the translucency of the dentition to be matched. For example, older dentitions exhibiting high translucency (low “value” in Munsell color space) are most easily matched with low value ceramics and more opaque dentitions matched with higher value ceramics. In this clinical assessment it is the core ceramic that controls Munsell value. Veneering ceramics are used to match spots of internal color and to mimic incisal characteristics. It is the goal of dental ceramics to imitate a tooth’s color and vitality by recreating an appropriate mix of light absorption and scattering. Thus, information regarding relative translucency of ceramics is of clinical interest.
Although many authors agree that translucency is a key characteristic for choosing an all-ceramic system, there is limited quantitative literature available . Heffernan et al. is the most recent study to compare quantitatively the translucency of various full-coverage, all-ceramic restorations . Yet this study measured the contrast ratio of core ceramics and found no difference between the light transmitted through a cast metal or a zirconia disk. Based on visual examination, full-coverage, all-ceramic restorations with zirconia cores appear to transmit more light than with metal-ceramics yet in this study the contrast ratio was 1.00 for both materials indicating equivalently opaque specimens . Such a finding raises questions about the value of contrast ratios when comparing ceramics and forms the basis of this study.
The primary objective of this research project was to compare absolute translucency with a frequently used surrogate measure of relative translucency (contrast ratio) among a commercially important set of materials used primarily for all-ceramic restorations. Materials were chosen both to provide relative translucency data by material, thickness and chroma for clinicians (in a separate paper) as well as to have a wide range of translucencies for the comparison of contrast ratio with translucency.
Compared to absolute translucency, contrast ratio is relatively easy to measure. Absolute translucency necessitates use of a dual beam, integrating sphere radiometer or spectrophotometer able to capture all of the light transmitted through a specimen in comparison to the intensity of light from a split beam. By comparison, contrast ratios can be measured with any device capable of providing standard radiation intensity and detection, as is defined as follows:
CR = L B L W
where L B is the luminance flux (reflectance) with the specimen on a black background and L W is the luminance flux (reflectance) with the specimen on a white background.
How contrast ratio came to be used in dental research as a measure of translucency or opacity is a relevant question; it appears to derive from an American Society for Testing and Materials (ASTM) standard used to measure opacity in papers . This protocol was then adopted by the American Dental Association in Specification No. 27 for direct filling resins . In both standards opacity was qualitatively observed compared to standards. In 1978 Powers extended the contrast ratio quantitatively to the examination of parameters affecting the color of direct restorative resins, basing their protocol on the ASTM standard. Further quantitative development of contrast ratio was provided by Crisp et al. in 1979 with reference back to earlier work at the National Bureau of Standards involving the application of Kubelka–Munk theory. This work of Crisp et al., also addressed the issue of most white backings having a reflectivity of 80% while standard tests called for a reflectivity of 70%.
Contrast ratio has been used to compare commercial glass ionomer cements , dental ceramics and direct restorative materials . An interesting extension of the contrast ratio approach was used to define a translucency parameter (TP) based on measures of L *, a *, and b * parameters on white and dark backgrounds, with TP then defined as:
TP = ( ( L B − L W ) 2 + ( a B − a W ) 2 + ( b B − b W ) 2 ) 1 / 2