# Influence of Printing Angulation on the Flexural Strength of 3D Printed Resins: An In Vitro Study
## 1 Introduction  
According to ISO/ASTM 52900:2023, additive manufacturing is the process of joining materials to make parts from 3D model data, usually layer by layer, as opposed to subtractive manufacturing and formative manufacturing methodologies [1]. Three-dimensional printing has been used in prosthodntics to produce master casts, patterns for fixed dental prostheses, interim restoration, removable dentures, and custom trays [2]. Today, since the development of new materials, the integration of 3D printing techniques has made it possible to apply additive manufacturing to the fabrication of definitive restorations, reducing time and costs [3]. In fact, 3D printable materials have a lower cost compared to milled ones, lower production time, and lower waste of materials [4]. Different factors can influence the quality and characteristics of the printed restorations, such as resin type [5; 6], 3D printer [7; 8; 9], printing parameters [10; 11], and post-curing procedures [12]. In fact, all these parameters can influence accuracy in terms of the internal fit of the restorations [13], surface roughness [14], and mechanical properties [15; 16]. The printing orientation is still a controversial aspect in the literature [17; 18; 19; 20; 21]. In fact, even if printing at a 90\({}^{\circ}\) printing orientation allows an optimization of the printing procedure, reducing printing time and allowing the production of multiple objects compared to the 0\({}^{\circ}\) printing orientation, it seems to lead to the production of restorations with inferior mechanical properties [17; 18; 21]. Different studies reported the highest flexural strength at 0\({}^{\circ}\)compared to 45\({}^{\circ}\) or 90\({}^{\circ}\)[17; 18; 19], while two studies demonstrated higher values when the specimens were printed vertically (90\({}^{\circ}\)) [12; 20]. [PERSON] et al. [21] reported that the three-point bending test induces stresses in the specimens, which can cause the low-strength interfaces between the 2D layers to delaminate before the 2D layers fracture. The observed failure between the layers at the vertical printed samples tends to be more catastrophic because of fractures than the horizontal printed ones, but the most important factor is played by the different printable materials. The mechanical performance of resin composites is also related to their chemical formulation. The characteristics of the co-monomers involved will determine the hydrophilicity, mobility, and kinetic parameters [22; 23]. Additionally, flexural strength can be strongly affected by the reinforcement of 3D-printed resin by the addition of fillers. It was reported that nanofillers improved the strength of 3D-printed resin according to their type, size, and concentration [24; 25]. In particular, the addition of glass silica and zirconia nanoparticles to 3D printable resins seems to enhance the mechanical properties of these materials. The wear resistance is enhanced by the addition of glass silica nanoparticles that create a stronger microstructure [26; 27]. The risk of wear, chipping, or fractures over time is lowered by the reinforcement, resulting in more durable dental restorations. The enhanced properties obtained with the addition of nanoparticles are a promising approach for improving the mechanical properties of dental resins [28]. On the other hand, the high concentration of fillers and nanofillers might have adverse effects on flexural strength due to the clustering of the particles, which clinically might be the starting point of a line of fracture [29]. Several studies evaluated crown and bridge 3D printed restorations in both in vivo and in vitro conditions [30; 31; 32]. However, recently, an increasing number of 3D-printed resins have been presented in the market for crown and bridge restorations with promising results. The flexural strength analysis of these new materials can be fundamental to better understanding their clinical indications before their wide clinical use.