The Influence of Filler Morphology and Loading Level on the Properties of Light-Curing Dental Composites

<b>Background/Objectives:</b> Light-curing dental resin composites remain limited by high polymerization shrinkage, inadequate wear resistance, and elevated water sorption. The combined influence of filler shape, size, and loading level on mechanical performance and hydrolytic stability remains insufficiently understood. This study aimed to systematically investigate the effects of filler morphology and particle size distribution on the key properties of dental composites. <b>Methods:</b> Spherical silica (SiO₂) nanoparticles (D50 = 0.50 μm) were synthesized via the Stöber method, while irregular aluminosilicate glass was used in coarse (D50 = 3.71 μm) and fine (D50 = 1.98 μm) fractions. Three composite groups were formulated: Group 1 (72 wt.% filler with 0-30% SiO₂), Group 2 (maximum filler loading 76-80 wt.% with 10-30% SiO₂), and Group 3 (74.5 wt.% filler with varying coarse/fine glass ratios). Flexural strength, flexural modulus, Vickers microhardness, depth of cure, water sorption, and solubility were evaluated according to ISO 4049:2019. <b>Results:</b> Incorporation of spherical SiO₂ nanoparticles significantly reduced composite viscosity, enabling maximum filler loading to increase from 72 to 80 wt.%. All composites exceeded ISO requirements for flexural strength (80.54-118.11 MPa), depth of cure (3.01-5.65 mm), water sorption (14.61-22.87 μg/mm³), and solubility (1.20-5.90 μg/mm³). The highest flexural strength (118.11 ± 10.54 MPa) and modulus (9.26 ± 1.12 GPa) were achieved at 78 wt.% filler loading. Bimodal glass systems (50/50 ratio) demonstrated optimal mechanical properties, while higher fine fractions reduced strength. <b>Conclusions:</b> Spherical SiO₂ nanoparticles effectively reduce viscosity and enable higher filler loading. The optimal balance between filler loading, particle shape, and size distribution should be tailored to clinical requirements, with high-strength formulations suited for posterior restorations and bimodal formulations for universal applications.