Translucency of human teeth and dental restorative materials and its clinical relevance

A Comparative Study of Light Transmission by Various Dental Restorative Materials and the Tooth Structure

CLINICAL RELEVANCE

Light transmission through dental materials and tooth structure has direct clinical implication on such factors as selecting an appropriate curing technique during a restorative process.

SUMMARY

Effects of the Thickness Ratio of Zirconia-Lithium Disilicate Bilayered Ceramics on the Translucency and Flexural Strength

PURPOSE

To determine the effects of the total thickness and core/veneer thickness ratio of bilayered ceramics on their three-point flexural strength and translucency.

MATERIALS AND METHODS

Two groups of specimens were prepared with two different total thicknesses, 1.0 mm and 0.6 mm. These groups were divided into five and three subgroups with ten specimens each, respectively, having different core/veneer thickness ratios. Lithium disilicate was pressed on the zirconia surface using the "lost-wax technique." The translucency and three-point flexural strength were measured, and a one-way analysis of variance test (p < 0.05) was performed to determine whether the translucency and three-point flexural strength were affected by the total thickness and core/veneer thickness ratio of the specimens.

RESULTS

For the same total thickness, translucency decreased with a increase in the zirconia core proportion (p < 0.001). For the same core/veneer ratio, the translucency decreased with an increase in the total thickness (p < 0.001). If the zirconia thickness was less than half of the total thickness, the bilayered ceramic specimens containing zirconia cores and monolithic lithium disilicate specimens exhibited a similar translucency. The three-point flexural strength increased with the increasing proportion of zirconia; however, it did not change significantly with the total thickness. However, for the same total thickness, the flexural strengths of the bilayered ceramics were higher than those of the monolithic specimens.

CONCLUSIONS

At the same total thickness, increasing the zirconia core thickness decreased the translucency and increased the three-point flexural strength. Bilayered ceramic specimens with low zirconia ratios exhibited translucency similar to that of monolithic lithium disilicate ceramic but a higher flexural strength.

Translucency of Zirconia Ceramics before and after Artificial Aging

PURPOSE

The aging of zirconia ceramics (Y-TZP) is associated with tetragonal to monoclinic phase transformation. This change in microstructure may affect the optical properties of the ceramic. This study examines the effect of aging on the translucency of different zirconia materials.

MATERIALS AND METHODS

120 disc-shaped specimens were fabricated from four zirconia materials: Cercon ht white, BruxZir Solid Zirconia, Zenostar T0, Lava Plus (n = 30 per group). Accelerated aging was performed in a steam autoclave (134°C, 0.2 MPa, 5 hours). CIELab coordinates (L*, a*, b*) and luminous reflectance (Y) were measured with a spectrophotometer before and after aging. Contrast ratio (CR) and translucency parameter (TP) were calculated from the L*, a*, b*, and Y tristimulus values. The general linear model (Bonferroni adjusted) was used to compare both parameters before and after aging, as well as between the different zirconia materials (p ≤ 0.05).

RESULTS

CR and TP differed significantly before and after aging in all groups tested. Before aging, Zenostar T showed the highest and Lava Plus showed the lowest translucency. After aging, Cercon ht and Zenostar T showed the highest and BruxZir and Lava Plus the lowest translucency.

CONCLUSIONS

Aging reduced the translucency in all specimens tested. Furthermore, translucency differed between the zirconia brands tested. Nevertheless, the differences were below the detectability threshold of the human eye. The aging process can influence the translucency and thus the esthetic outcome of zirconia restorations; however, the changes in translucency were minimal and probably undetectable by the human eye.

Impact of 35% Hydrogen Peroxide on Color and Translucency Changes in Enamel and Dentin

Abstract This study evaluated the effects of tooth bleaching with high-concentration of hydrogen peroxide on alterations of translucency parameter (TP) and color of dentin and enamel. The crown of five human molars was sectioned into four slices parallel to buccal surface. The dentin of external slices containing buccal/ lingual enamel was fully removed with diamond bur; while these slices were used to assess alterations on enamel. Alterations on dentin were assessed into the center of internal slices. The color of specimens was measured over white and black backgrounds using a spectrophotometer (CieL*a*b) at baseline, allowing to calculate the TP by difference between the color measured over each background. Specimens were submitted to three 15-min applications of 35% hydrogen peroxide followed by their storage in water for one-week. Afterwards, the color measurements were repeated at both backgrounds. Color (∆L, ∆a, ∆b and ∆E) and translucency (∆TP) changes were calculated and data individually analyzed by T-test (α = 0.05). Influence of hard tissue and assessment time on each color parameter was also analyzed by 2-way repeated measure ANOVA (α = 0.05). Tooth bleaching resulted in increased lightness for the enamel, whereas no alteration on this parameter occurred for dentin. No difference between the tooth hard tissues was observed regards the other color parameters and ∆E. A slightly reduction on TP was observed only for the enamel. In conclusion, 35% hydrogen peroxide caused similar color and translucency changes on dentin and enamel.

Relative Translucency of a Multilayered Ultratranslucent Zirconia Material

AIM

The aim of this study was to compare the translucency parameter (TP) of ultratranslucent multilayered (UTML) zirconia according to thickness and layer level.

MATERIALS AND METHODS

Rectangles of UTML zirconia with four layers [dentin layer (DEL), first transitional layer (FTL), second transitional layer (STL), and enamel layer (ENL)] and four different thicknesses (0.4, 0.6, 0.8, and 1 mm) were milled from blanks. Digital images were taken in a dark studio against white and black backgrounds under simulated daylight illumination and international commission on illumination (CIE) Lab* color values recorded using Photoshop Creative Cloud software. The TP was computed and compared according to thickness and layer level using analysis of variance (ANOVA) followed by Bonferroni post hoc analysis for multiple comparisons. Significance was set at p < 0.05.

RESULTS

In each thickness, TP values were similar between any two layers. The significant effect of thickness on the TP was observed only in the first two layers. In the DEL, translucency was significantly greater at 0.4 mm than all other thicknesses. In the FTL, differences were significant between 0.4 and 0.8 mm and between 0.4 and 1 mm.

CONCLUSION

The investigated zirconia does not seem to show gradational changes in relative translucency from dentin to enamel levels regardless of the thickness used. Thickness affected the TP only in the first two layers with better translu-cency at 0.4 mm.

CLINICAL SIGNIFICANCE

Since relative translucency does not seem to be significantly different between layers, clinicians can modify the apicocoronal positioning of the UTML layers within the restoration according to the desired Chroma without any implications on the clinically perceived translucency. While the thickness of 0.4 mm may be suggested for anterior esthetic veneers because of its higher translucency, the other thicknesses of 0.6 to 1 mm can be used to mask colored abutments in full contour restorations.

[Comparison among three translucency parameters]

OBJECTIVE

This study aims to compare the three commonly used translucency parameters in prosthodontics: transmittance (T), contrast ratio (CR), and translucency parameter (TP).

METHODS

Six platelet specimens were composed of Vita enamel and dental porcelain. The initial thickness was 1.2 mm. The specimens were gradually ground to 1.0, 0.8, 0.6, 0.4, and 0.2 mm. T, color parameters, and reflection were measured by a spectrocolorimeter for each corresponding thickness. T, CR and TP were calculated and compared.

RESULTS

TP increased, whereas CR decreased, with decreasing thickness. Moreover, 
T increased with decreasing thickness, and exponential relationships were found. Two-way ANOVA showed statistical significance between T and thickness, except between T and the 1.2 mm and 1.0 mm enamel porcelain groups. No difference was found among the coefficient variations (CV) of T, CR and TP. Curve fitting indicated the existence of exponential relationships between T and CR and between T and TP. The values for goodness of fit with statistical significance were 0.951 and 0.939, respectively (P<0.05).

CONCLUSIONS

Under the experimental conditions, T, TP and CR achieved the same CV. T and TP, as well as T and CR, were found with exponential relationships. The value of CR and TP could not represent the translucency precisely, especially when comparing the changing ratios.

Translucency changes of direct esthetic restorative materials after curing, aging and treatment

The purpose of this article was to review the changes in translucency of direct esthetic restorative materials after curing, aging and treatment. As a criterion for the evaluation of clinical translucency changes, visual perceptibility threshold in translucency parameter difference (ΔTP) of 2 was used. Translucency changes after curing were perceivable depending on experimental methods and products (largest ΔTP in resin composites = 15.9). Translucency changes after aging were reported as either relatively stable or showed perceivable changes by aging protocols (largest ΔTP in resin composites = -3.8). Translucency changes after curing, aging and treatment were perceivable in several products and experimental methods. Therefore, shade matching of direct esthetic materials should be performed considering these instabilities of translucency in direct esthetic materials.

Criteria for clinical translucency evaluation of direct esthetic restorative materials

The purpose of this review was to suggest practical criteria for the clinical translucency evaluation of direct esthetic restorative materials, and to review the translucency with these criteria. For the evaluation of reported translucency values, measuring instrument and method, specimen thickness, background color, and illumination should be scrutinized. Translucency parameter (TP) of 15 to 19 could be regarded as the translucency of 1 mm thick human enamel. Visual perceptibility threshold for translucency difference in contrast ratio (ΔCR) of 0.07 could be transformed into ΔTP value of 2. Translucency differences between direct and indirect resin composites were perceivable (ΔTP > 2). Universal and corresponding flowable resin composites did not show perceivable translucency differences in most products. Translucency differed significantly by the product within each shade group, and by the shade group within each product. Translucency of human enamel and perceptibility threshold for translucency difference may be used as criteria for the clinical evaluation of translucency of esthetic restorative materials.

Translucency of Dental Ceramic, Post and Bracket

Translucency of dental ceramics, esthetic posts and orthodontic brackets was reviewed. Translucency parameter (TP) and contrast ratio (CR) are generally used for translucency evaluation. For the evaluation of translucency, two criteria such as the translucency of human teeth (TP = 15-19, 1 mm thick) and the visual perceptibility threshold for the translucency difference (∆CR > 0.07 or ∆TP > 2) were used. In ceramics, translucency differences were in the perceptible range depending on the type of material and the thickness. However, variations caused by the difference in the required thickness for each layer by the material and also by the measurement protocols should be considered. As to the translucency of esthetic posts, a significant difference was found among the post systems. Translucency was influenced by the bracket composition and brand, and the differences by the brand were visually perceptible.