Effects of thermocycling on color stability and mechanical properties of universal resin composites: an in vitro comparison

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Abstract
Objectives: To evaluate the effect of thermocycling on the optical and mechanical properties of universal resin composites, with emphasis on color stability, Knoop microhardness, flexural strength, flexural modulus, and filler characteristics. Methods: Four composite resins: Filtek Z250 XT (FZ), Vittra APS Unique (VT), Spectra Smart (SS), and Harmonize (HZ) were evaluated (n = 10 per group). Specimens were subjected to thermocycling (10,000 cycles; 5–55 °C). Color and Knoop microhardness were measured before and after aging (repeated measures), while flexural strength and modulus were determined using independent specimens. Color differences were calculated using the CIEDE2000 formula. SEM/EDS analyses were performed for filler characterization. Statistical significance was set at α = 0.05. Results: Thermocycling significantly affected all evaluated properties (p < 0.05), with a material-dependent response. All materials exhibited ΔE₀₀ values above clinically acceptable thresholds after aging. SS showed the lowest color variation, while VT presented the highest values. In terms of mechanical behavior, FZ and HZ exhibited greater reductions in flexural strength (> 50%) and microhardness (≈ 30–45%), whereas SS demonstrated the highest stability and VT showed intermediate performance. Additionally, FZ and HZ presented flexural strength values below 80 MPa after thermocycling. Conclusions: Thermocycling resulted in clinically relevant color changes in all evaluated universal resin composites, exceeding perceptibility and acceptability thresholds. Additionally, mechanical properties were adversely affected in a material-dependent manner, with reductions in flexural strength and microhardness suggesting decreased structural stability after aging. Clinical Significance: Universal resin composites may exhibit clinically relevant color changes and significant mechanical degradation after aging. Clinicians should consider that materials with greater reductions in flexural strength and microhardness may present reduced durability over time.

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