Heat Treatment Influences Monomer Conversion and Bond Strength of Indirect Composite Resin Restorations

Purpose: To assess the resin microtensile bond strength (MTBS) and the degree of conversion (DC) of indirect composite resin restorations polymerized with light and heat. Materials and Methods: Two direct (Filtek Z100 and Premise) and one indirect (Premise Indirect) composite resins were polymerized with a combination of light and heat (138°C for 20 min). For MTBS, 42 cylinders were fabricated (n = 7). After the surface treatment, cylinders were bonded to each other using adhesive resin (Optibond FL). Specimens were stored in water for 24 h. Another 15 cylinders (n = 5) were fabricated for determining degree of conversion using Fourier Transform Infrared Spectrometry immediately and at 24 h. The MTBS and the DC was submitted to two-way ANOVA. The interaction with existing data was explored with univariate ANOVA and two-way ANOVA. Tukey's HSD post-hoc test was used to detect pairwise differences (α = 0.05). Results: The MTBS to light and heat polymerized Z100 was 75.7 MPa, significantly higher than that to Premise (58.6 MPa) and Premise Indirect (63.9 MPa). The immediate DC for Z100, Premise, and Premise Indirect were 51.0%, 68.7%, and 61.8%, respectively. The DC at 24 h ranged from 53.4% (Z100) to 72.8% (Premise Indirect) and significantly increased for Premise Indirect only. Comparison with previously published data revealed that the heat treatment increased both MTBS and DC of Premise and Premise Indirect. Conclusion: Z100 showed better bond strength but lower DC. Heat treatment and a 24-h delay before delivery can benefit DC of Premise Indirect. The increase in DC of Premise and Premise Indirect did not affect their bond strength. Keywords: adhesion, bond strength, direct composite resin, dental materials, indirect composite resin, degree of conversion