Mechanical, Antibacterial, and Physico-Chemical Properties of Three Different Polymer-Based Direct Restorative Materials: An In Vitro Study

A novel resin-based bulk-fill restorative material (ST; Stela SDI, Bayswater, Victoria, Australia) has been recently introduced as a self-curing alternative to traditional light-cured composites. Promoted for its unlimited depth of cure, enhanced aesthetics, and unique primer composition, it aims to address challenges associated with amalgam and light-curing composites. Thus, the aim of this in vitro study was to investigate the performance of the new self-curing polymer-based restorative material, ST, compared to two conventional light-cured composites for direct restoration. The study evaluated compressive strength with and without aging, antibacterial activity, mineral deposition in contact with Phosphate-Buffered Saline (PBS) and artificial saliva, porosity, and wettability of ST (Tetric EvoCeram (TE; Ivoclar Vivadent, Schaan, Liechtenstein) and Clearfil Majesty ES-2 (CM; Kuraray Noritake Dental, Tokyo, Japan)). The data was statistically analyzed (α = 0.05) through one-way and two-way analysis of variance (ANOVA). ST demonstrated significantly higher compressive strength than TE and CM at baseline and after aging (<i>p</i> < 0.001), while aging significantly reduced compressive strength across all materials (<i>p</i> < 0.001). Fracture mode analysis revealed brittle fractures for TE and CM, whereas ST fractured in multiple smaller fragments. CM showed the highest void volume and diameter, significantly differing from ST and TE (<i>p</i> < 0.001). Scanning electron microscopy (SEM) analysis revealed cubical-like crystalline formations on ST's surface after 28 days of immersion in PBS and saliva, indicating some level of bioactivity, whereas no changes were observed for TE and CM. Wettability testing showed ST had the lowest contact angle (12.24° ± 2.1°) compared to TE (62.78° ± 4.68°) and CM (64.64° ± 3.72°) (<i>p</i> < 0.001). Antibacterial activity testing displayed a significant decrease in bacterial growth for CM compared to ST (<i>p</i> = 0.001) and TE (<i>p</i> = 0.002); however, ST and TE showed no significant differences (<i>p</i> = 0.950). To conclude, ST Automix demonstrated promising results across several key parameters, making it a potential candidate for long-lasting restorative applications. Future studies should explore its long-term clinical performance and investigate formulations that enhance its antibacterial properties. Moreover, the bond strength of these materials to dentin and the cytotoxicity should be evaluated.