Effect of erosive solutions and thermal cycling on the surface properties of universal injectable and regular consistency resin composites

Injectable resin composites, such as G-aenial Universal and Bulk Injectable, offer high flowability and easy application for minimally invasive repair of eroded or chipped teeth. However, limited in vitro data exist regarding their resistance to erosive and thermal aging under neutral and acidic oral conditions. Disc-shaped specimens (10 mm × 2 mm) were prepared from two injectable composites—G-aenial Universal Injectable and G-aenial Bulk Injectable—and two conventional composites—3 M Filtek Z350 XT Universal Restorative and 3 M Filtek One Bulkfill. Seventy-two discs per material were subdivided into three aging protocols (n = 24 each): (1) deionized water immersion (pH 7), (2) HCl immersion (pH 1.2) followed by thermal cycling, or (3) citric acid immersion (pH 3) followed by thermal cycling. Each subgroup was tested for surface roughness, Vickers microhardness, and two-body wear resistance (n = 8). Surface roughness was measured via contact profilometry; selected specimens underwent atomic force microscope (AFM) and scanning electron microscope (SEM) imaging. Microhardness was assessed using a digital Vickers tester, and wear resistance was simulated with a programmable wear device measuring weight and volume loss. Two-way ANOVA, post-hoc, and Pearson’s correlation analyses were performed (p < 0.05). Erosive and thermal aging led to significant increases in surface roughness and reductions in microhardness across most materials, with no significant change in two-body wear resistance. Under the HCl + thermal cycling condition, G-aenial Bulk Injectable demonstrated the highest roughness and lowest hardness, whereas Filtek Z350 XT Universal showed the lowest roughness and highest hardness. Following citric acid exposure, no significant differences in roughness or hardness were identified, although G-aenial Bulk Injectable incurred the greatest weight loss. According to this in vitro study, G-aenial Universal Injectable is best suited for neutral pH oral conditions, while Filtek Z350 XT Universal provides superior performance under acidic challenge. Incremental composites generally preserve surface quality better than bulk-fill materials. The more severe degradation resulting from gastric-level acid compared to citric acid illustrates the value of selecting restorative composites based on anticipated oral environmental exposures.