A comparative study between 3D-printed and CAD-milled dental resins through nanoindentation tests

The main objective in contemporary prosthetic dentistry is to identify restorative materials with biomechanical behavior as close as possible to that of natural enamel. Advances in additive manufacturing (AM) are transforming dental care by automating processes traditionally performed by hand. Among AM technologies, 3D printing stands out for the creation of dental prostheses because it not only allows for precise geometries but also enables control over the mechanical properties of the materials used. In parallel with AM, the production method based on computer-aided design and manufacturing (CAD-CAM) systems has introduced a range of materials that offer greater precision, durability, and aesthetics for prostheses. Recently, materials with various chemical compositions, such as polymer-infiltrated ceramic networks, have been used in CAD-CAM systems. This kind of materials combines the beneficial characteristics of dental ceramics and composite resins, providing an optimal balance for dental applications. In this scenario, the mechanical characteristics of dental resins play a crucial role. Therefore, in this study, an indentation test protocol was designed to mechanically characterize four hybrid composites that will be used as dental prostheses: two 3D printed via stereolithography and two produced with CAD-CAM technology through a milling process. Two glass-ceramic materials were also analyzed as controls which, unlike the resins, do not contain any polymer components. Additionally, this study evaluated the effect of a thermal aging process that simulates the oral environment on the mechanical performance of the samples. Preliminary tests were conducted on 3D-printed resins to set the maximum load in order to reduce the indentation size effect and to evaluate the influence of indentation rate on the calculated mechanical parameters (nanoindentation hardness and modulus). Based on these preliminary tests the protocol was established. This protocol includes an indentation rate of 450 mN/min and a maximum load of 300 mN, resulting in a constant strain rate of 0.01 s-1. The testing campaign revealed that the mechanical parameters increase as the polymer content of the samples decrease, covering a range of approximately [0.4 – 8.8] GPa for the hardness and [5.7 – 103.1] GPa for the modulus. Following the thermal aging process, a decrease in these parameters was observed in all analyzed samples, with a percentage decrease in the range of [4.7% – 29.3%] for the hardness and [1.5% – 18%] for the modulus. Notably, a greater decrease was observed in the 3D-printed resins compared to those produced using CAD-CAM technology.