Introduction: Composite materials are the most used materials for dental restorations due to their physical and chemical properties, aesthetic value, and satisfactory clinical performance. They consist of an organic resin matrix, an inorganic reinforcing phase, and coupling agents. Dental composites are divided into direct (intraoral) composites, which polymerize inside the oral cavity, and indirect (extraoral) composites, which cure under controlled laboratory conditions, influencing their mechanical properties. Purpose: The aim of this study was to compare the mechanical properties and microstructure of intraoral and extraoral composite materials used in dental prosthetics. The main objectives were to characterize the microstructure of selected composites, measure their density, and determine flexural strength, elastic modulus, and strain at maximum load. Methods: The theoretical part was based on a review of existing literature using a descriptive method. For the experimental part, specimens of three composites (4 × 4 × 20 mm) were prepared: G-aenial Universal Flo (intraoral), VITA Vionic Dent Disc multiColor and VITA VM LC (both extraoral). Microstructure was analysed using light and scanning electron microscopy (SEM), chemical composition was determined by EDS analysis, density was measured using a pycnometer, and flexural properties were evaluated by a three-point bending test according to EN ISO 178:2003. Results: SEM analysis revealed a heterogeneous two-phase microstructure with local microporosity in G-aenial Universal Flo, a homogeneous two-phase microstructure in VITA VM LC, and a three-phase structure with fairly uniform phase distribution in VITA Vionic Dent Disc multiColor. EDS analysis confirmed the presence of inorganic fillers (SiO₂, Al, Sr) in all composites. The highest density was recorded for G-aenial Universal Flo (1.7363 g/cm³) and the lowest for VITA Vionic Dent Disc multiColor (1.2668 g/cm³). Flexural testing showed that VITA VM LC achieved the highest average flexural strength (107.59 ± 5.41) MPa, followed by G-aenial Universal Flo (102.65 ± 34.15) MPa, and VITA Vionic Dent Disc multiColor (74.42 ± 6.30) MPa. The highest elastic modulus was recorded for G-aenial Universal Flo (3268.20 ± 165.89) MPa. Discussion and conclusion: The study demonstrated that mechanical properties of composites are directly related to their microstructure, filler type, and quality of bonding between the polymer matrix and the reinforcing phase. VITA VM LC proved to be the most mechanically reliable material, with a homogeneous microstructure and flexural strength consistent with manufacturer specifications (≈ 110 MPa). G-aenial Universal Flo achieved high stiffness despite microporosity, indicating an efficient filler system. VITA Vionic Dent Disc multiColor is suitable for aesthetically oriented restorations with lower mechanical demands. The results confirm that composite selection should be based on understanding the microstructure and intended clinical application.
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