Quantification of the amount of blue light passing through monolithic zirconia with respect to thickness and polymerization conditions

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Abstract

Statement of problem

Dual-polymerized luting composite resin cements would benefit from enhanced irradiance transmitted through a ceramic restoration. A quantification of the amount of transmitted light through translucent zirconia is lacking.

Purpose

The purpose of this study was to evaluate the amount of light (360 to 540 nm) passing through translucent and conventional zirconia and a glass ceramic with respect to material thickness and different polymerizing modes.

Material and methods

Six translucent and a conventional zirconia (negative control) and a glass ceramic (positive control) were considered. Ten specimens of each material and thickness (.5, 1, 1.5, 2, 2.5, 3 mm) were fabricated (n=480). Zirconia materials were sintered according to manufacturers’ instructions. The irradiance passing the different ceramics and thicknesses was measured with a violet-blue LED polymerizing unit in 3 polymerizing modes (plasma, high, and standard power mode) with a USB4000 Spectrometer. The polymerizing unit was placed directly on the specimen’s surface. Data were analyzed with one and multivariate analysis and the Pearson correlation analysis (α=.05).

Results

In all materials, the translucency and its rate decreased exponentially according to the specimen thickness. The highest influence on the measured irradiance passing through translucent zirconia was exerted by ceramic thickness (P<.05, partial eta squared [ηP²]=.998), closely followed by polymerizing mode (ηP²=.973), while the effect of the material (P=.03, ηP²=.06) and mean grain size (P=.029, ηP²=.027) was significant but low.

Conclusions

Zirconia was less translucent than the glass ceramic, but the translucency decreased more slowly with material thickness, thus approaching the translucency of glass ceramics at a specimen thicknesses of 2.5 to 3 mm.

Section snippets

Material and Methods

Four hundred eighty specimens were fabricated. The analyzed ceramics, their manufacturer, lot number, and chemical composition are presented in Table 1. Zirconia (12 mm wide × 12 mm long) and glass ceramic disks (10 mm wide × 10 mm long) were cut with a high performance cut-off machine (Secutom-50; Struers) in the following thicknesses: .5, 1, 1.5, 2, 2.5, and 3 mm. Zirconia specimens were sintered (LHT 02/16; Nabertherm GmbH) according to the manufacturers’ instructions (Table 2). A glass

Results

Ceramic thickness exerted the highest influence on the transmitted irradiance (P<.001, partial eta squared [ηP²]=.998), closely followed by polymerizing mode (P<.001, ηP²=.993) and ceramic (P<.001, ηP²=.873). The interaction effect of the binary and ternary combinations of the 3 independent parameters (ceramic, ceramic thickness, and polymerizing mode) was significant for all combinations (P<.001), while the effect strength in descending order was: ceramic thickness combined with polymerizing

Discussion

All null hypotheses were rejected. The translucency of zirconia is determined by a complex combination of different parameters, including residual porosity,7 grain size,8 primary particle size,9 additives such as alumina,12 and sintering.9 Confirming the literature data,5 the analyzed zirconia were all less translucent than the glass ceramic used as positive control. The comparison among the zirconia materials proved that the so-called translucent zirconia and traditional zirconia differed less

Conclusions

Within the limitations of this study, the following conclusions can be drawn:

  • 1.

    In less advantageous conditions (material thickness of 3 mm, standard power mode), the amount of light passing through specimens ranged from 48.4 mW/cm² to 102.6 mW/cm² for the materials in the following order: Prettau, InCoris TZI, Ceramill Zolid, DD Bio ZW, ZENOStar, VITA Mark II, GC ZR Disc CIP, and Ceramill ZI.

  • 2.

    The decrease in transmitted irradiance with increased specimen thickness in all materials is well

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