Mechanical properties of 3D-printed and milled composite resins for definitive restorations: An in vitro comparison of initial strength and fatigue behavior

Objective: To evaluate the flexural strength and fatigue behavior of a novel 3D-printed composite resin for definitive restorations. Materials and Methods: Fifty disc-shaped specimens were manufactured from each of a nanohybrid composite resin (NHC), polymer-infiltrated ceramic network (PICN), and 3D-printed composite resin (3D) with CAD-CAM technology. Biaxial flexural strength (sigma(in)) (n = 30 per group) and biaxial flexural fatigue strength (sigma(ff)) (n = 20 per group) were measured using piston-on-three-balls method, employing a staircase approach of 10(5) cycles. Weibull statistics, relative-strength degradation calculations, and fractography were performed. The results were analyzed with 1-way ANOVA and Games-Howell post hoc test (alpha = 0.05). Results: Significant differences in sigma(in) and sigma(ff) among the groups (p < 0.001) were detected. The NHC group provided the highest mean +/- standard deviation sigma(in) and sigma(ff) (237.3 +/- 31.6 MPa and 141.3 +/- 3.8 MPa), followed by the PICN (140.3 +/- 12.9 MPa and 73.5 +/- 9.9 MPa) and the 3D (83.6 +/- 18.5 MPa and 37.4 +/- 23.8 MPa) groups. The 3D group exhibited significantly lower Weibull modulus (m = 4.7) and up to 15% higher relative strength degradation with areas of nonhomogeneous microstructure as possible fracture origins. Conclusions: The 3D-printed composite resin exhibited the lowest mechanical properties, where areas of nonhomogeneous microstructure developed during the mixing procedure served as potential fracture origins. Clinical Significance: The clinical indications of the investigated novel 3D-printed composite resin should be limited to long-term provisional restorations. A cautious procedure for mixing the components is crucial before the 3D-printing process, since nonhomogeneous areas developed during the mixing could act as fracture origins.