Strategies for Masking Metal Show-Through in Disilicate Dental Ceramics: A Systematic Evaluation Using Different Assessment Techniques

Abstract

Objectives

The aim of this study was to assess the masking effect of two lithium disilicate ceramics at various thicknesses, when bonded to opaqued and nonopaqued silvery metallic core buildups, using spectrophotometry and digital photocolorimetric analysis.

Materials and Methods

Sixty bis-acrylic blocks in A2 shade were fabricated to serve as control unrestored substrates, and 120 blocks were prepared (8.0 mm × 8.0 mm × 3.0 mm, W × L × H) and restored with amalgam. Sixty amalgam-restored blocks were treated with 0.65 mm (±0.10 mm) resin-based opaquer. Low-translucency (LT) lithium disilicate CAD/CAM blocks from two different manufacturers (Ivoclar IPS e.max CAD and GC Initial LiSi) were sectioned, crystallized (e.max), and polished, then divided into three subgroups according to their thickness (1.0, 1.5, and 2.0 mm, n = 30). The ceramic slices were treated and bonded to the three substrate types (control, opaqued amalgam, and nonopaqued amalgam) using a resin-luting cement. The masking effect of the resulting ceramic-substrate assemblies was investigated using a spectrophotometer and polarized digital photocolorimetric (PDPC) analysis. CIE L*a*b* coordinates were measured, and color difference analysis (ΔE ₀₀) was performed using the CIE ΔE ₀₀ formula. Acceptability and perceptibility thresholds were set at ΔE ₀₀ ≤ 1.8 and ΔE ₀₀ ≤ 0.8, respectively. The data were statistically analyzed using three-way and two-way ANOVA, followed by Tukey's test (α ≤ 0.05).

Results

Spectrophotometric analysis showed that, for nonopaqued amalgam substrate, IPS e.max achieved ΔE ₀₀ mean (SD) of 4.2(0.4), 2.8(0.3), and 2.1(0.2), for 1.0, 1.5, and 2.0 mm thicknesses, respectively. Initial LiSi achieved ΔE ₀₀ mean (SD) of 3.8 (1.1), 2.8(0.9), and 2.7(0.4) for 1.0, 1.5, and 2.0 mm thicknesses, respectively. When the substrate was opaqued amalgam, IPS e.max and LiSi both resulted in a mean ΔE ₀₀ of 2.3(0.7). As the thickness increased to 1.5 and 2.0 mm, IPS e.max achieved an acceptable (ΔE ₀₀ = 1.3(0.4)) and an excellent (ΔE ₀₀ = 0.8(0.3)) masking, respectively. Initial LiSi achieved a moderately unacceptable color difference (ΔE ₀₀ = 2.0(0.8)) at 1.5 mm and an acceptable masking (ΔE ₀₀ = 1.5(0.6)) at 2.0 mm. PDPC analysis presented similar trends for both ceramic types and substrates, except for LiSi on opaqued amalgam. Individual CIE L*a*b* color coordinates, however, showed significant differences between the two analysis methods.

Conclusions

The use of opaquer proved to be an effective strategy for masking silvery metallic core buildups in preparation for lithium disilicate-based restorations. IPS e.max CAD provided a superior masking effect compared to Initial LiSi when used in conjunction with a resin opaquer.

Clinical Significance

This study shows the potential of resin-based opaquers to mask metallic restorations when combined with lithium disilicate at different thicknesses. Providing knowledge in this aspect may emphasize conservative dental practices by preventing the replacement of metallic restorations and the accompanying unnecessary removal of tooth structure.