Effect of Layer Thickness on Conversion Degree and Microhardness of Bulk-Fill Composites

Background: This study aimed to evaluate the effect of layer thickness on the conversion degree and microhardness of low- and high-viscosity bulk-fill composite resins. Methods: Six different bulk-fill composite resins, SDR (Dentsply), Sonic Fill (Kerr), X-tra Fil (Voco), Beautifil Bulk Restorative (Shofu), Beautifil Bulk Flowable (Shofu), and Filtek Bulk Fill Flowable (3M ESPE) were tested. Disc-shaped samples (7 mm diameter, 2 and 4 mm thickness, n=4 per group) were prepared and polymerized using an LED (Light-Emitting Diode) curing unit (Elipar S10, 3M ESPE). After 24 hours at 37°C, the degree of conversion was analyzed using Fourier Transform Infrared Spectroscopy (FTIR)- Attenuated Total Reflectance (ATR) (Perkin Elmer Spectrum Two), and microhardness was measured using a Knoop hardness tester (Buehler MMT-3 Digital Microhardness Tester). Data were analyzed using Shapiro-Wilk, ANOVA, Tukey’s HSD, and Pearson correlation with SPSS 26.0V (IBM SPSS Corp.; Armonk, NY, USA). Results: No significant difference in bottom/top surface hardness ratio was observed in 2 mm samples (P > .05). However, at 4 mm, a statistically significant difference was found among the composite groups (P= .003). SDR had the highest hardness ratio (0.87 ± 0.04), while Beautifil Bulk Restorative (BBR) had the lowest (0.68 ± 0.08). The highest degree of conversion at both thicknesses was in SDR (65.13 ± 2.00 at 2 mm, 62.90 ± 2.30 at 4 mm), whereas X-tra Fil had the lowest (41.25 ± 13.57 at 2 mm, 39.25 ± 6.75 at 4 mm). In BBR, conversion at 2 mm was significantly higher than at 4 mm (P=.004). Cite this article as: Kütük Ömeroğlu M, Ertaş E. Effect of layer thickness on conversion degree and microhardness of bulk-fill composites. Essent Dent. 2025; 4, 0024, doi: 10.5152/EssentDent.2025.25024.