Comparison of microstrength changes of monochrome composite resins:An in vitro study

Recent developments in single-shade composite resins have enabled these materials to reflect the color of surrounding tooth tissues through light interaction, thereby eliminating the need for multi-shade selection. This feature reduces clinical application time and decreases material inventory load. Although studies on the optical performance of these materials are widespread, data regarding how their mechanical properties—particularly under different underlying substrate colors—may vary remain limited. The aim of this thesis study is to compare the microhardness values of single-shade composite resins with different compositions and to evaluate whether the color of the underlying tooth structure causes any change in these values. In this study, acrylic teeth were used as substrates in order to enable controlled variation of the underlying color and to ensure standardization during measurements. The samples were divided into three main groups according to the VITA Classic shade scale: A1 (n=80), A2 (n=80), and A3 (n=80) (Ardiş Color G2, Dentaş, Ankara, Türkiye). The samples were further divided into four groups based on the composite type to be evaluated; three single-shade composites with chameleon effect—Omnichroma (Tokuyama, Japan), Zenchroma (President Dental, Germany), and Charisma Diamond One (Kulzer, Germany)—were used, and the multi-shade composite resin G-aenial Anterior (GC, Japan) served as the control group. A total of 240 samples were assigned into 12 subgroups according to the combination of composite type and underlying shade. After polymerization, the prepared samples were stored in distilled water for 24 hours, and Vickers microhardness testing was performed at 300 gf for 15 seconds (Buehler MMT-3 digital testing device, Waukagen Lake Bluff, IL, USA). Three measurements were taken from each sample, and mean values were statistically analyzed (ANOVA, p<0.05). According to the findings of the study, a significant difference in microhardness was observed among the composite types (p<0.05). The highest microhardness value was recorded for Charisma Diamond One, whereas the lowest value was obtained for the multi-shade composite resin G-aenial Anterior. However, the underlying shade (A1, A2, A3) was found to have no significant effect on microhardness values (p>0.05). In conclusion, the primary determinant of the microhardness of single-shade composite resins appears to be the formulation characteristics of the material (monomer structure, filler morphology, cross-linking capacity), rather than the color of the underlying substrate.