Effect of preheating on flexural strength, flexural modulus and microhardness of single-shade universal resin composites

The aim of this in-vitro study was to evaluate the effect of preheating on the flexural strength, flexural modulus and microhardness of several commercially available single-shade universal resin composites (SSURCs). In this study, six SSURCs - Omnichroma (OC), Clearfil Majesty Es-2 Universal (CMU), Charisma Diamond One (CDO), Vittra APS Unique (VAU), ONEshade (OS), Essentia Universal (EU) and one conventional composite (Filtek Z250 (Z250)) as a control group were evaluated. Each material was divided into 2 subgroups as non-preheated (NPH) and preheated (PH) groups. Flexural strength (FS), flexural modulus (FM) and microhardness (Vickers Hardness Number-VHN) results of the specimens were evaluated. Data normality was verified with the Shapiro-Wilk test. The effects of material and preheating on all tested properties were analyzed with two-way ANOVA, followed by Tukey’s HSD post-hoc test for multiple comparisons. Paired t-tests were used to compare NPH and PH conditions within each material (p < 0.05). Z250 and CDO generally demonstrated the highest values, while Omnichroma OC and CMU showed the lowest. Preheating significantly increased the FS of Z250, CDO, VAU and OS (p < 0.05), but did not affect OC, CMU or EU. Its effect on VHN was limited, only improving the top surface hardness of CDO, OC, and EU. The mechanical properties of SSURCs are material-specific. Preheating at 39 °C provides selective, non-universal benefits, primarily enhancing flexural properties of certain composites without consistently improving microhardness.