Effect of thermocycling and bleaching on the translucency and opalescence of monolithic CAD-CAM dental ceramics

Abstract

Statement of problem

An acceptable dental restoration needs to retain stable translucency and opalescence after years of service in the oral environment. Information about the optical properties of recently introduced computer-aided design and computer-aided manufacturing (CAD-CAM) monolithic ceramics and the effectiveness of bleaching on these materials has not yet been documented.

Purpose

The purpose of this in vitro study was to evaluate the effects of thermocycling and bleaching on the translucency and opalescence of monolithic ceramics, including zirconia, lithium disilicate (LDS), polymer infiltrated ceramic network (PICN), presintered zirconia reinforced lithium silicate (PS-ZLS), and PS-ZLS with additional sintering (PS-ZLS+AS).

Material and methods

A total of 50 plate-shaped specimens in A2 shade with 1 mm thickness were prepared (n=10). Each specimen underwent coffee thermocycling for a total of 20 000 cycles and a 20% carbamide peroxide bleaching for 14 days. The color coordinates on standard white and black backgrounds were measured for each specimen at 3 stages: initial, thermocycled, and bleached. The translucency parameter (TP) and opalescence parameter (OP) were calculated at each stage. A 2-way repeated measures ANOVA was conducted to assess the effects of material type and measurement stage, followed by Bonferroni-adjusted multiple comparisons for pairwise analyses (α=.05).

Results

Zirconia specimens showed the lowest TP, while the LDS group had the highest TP. The thermocycled PS-ZLS+AS group had significantly higher TP than the thermocycled PS-ZLS (P=.001). Thermocycling significantly decreased the TP of PS-ZLS (P=.018) and PICN (P=.012), while bleaching increased this parameter. The PS-ZLS and PS-ZLS+AS groups showed the highest OP, while the LDS and PICN groups had the lowest OP. Bleaching significantly decreased OP for the zirconia (P=.007), PICN (P=<.001), and PS-ZLS+AS (P=.01) groups.

Conclusions

Among the studied materials, zirconia and LDS showed the most stable TP and OP during thermocycling and bleaching. Bleaching can restore the translucency of PICN and PS-ZLS lost during thermocycling. Additional sintering for PS-ZLS material led to more stable translucency during thermocycling. The highest opalescence increases during thermocycling and reductions during bleaching were found in PS-ZLS, followed by PICN.