Computer-aided design/computer-assisted manufacture (CAD/CAM) technology introduced a paradigm shift in the manufacture of indirect dental restorations, reducing time and cost for the patients [1], [2]. Recently, the additive manufacturing, sometimes referred as 3D printing or rapid prototyping, has gained popularity mostly for constructing polymer and metal objects. Such technology allows the production of large complex designs and multiple parts at one time, reducing material waste [3]. However, there is still much to improve in mechanical properties and esthetic appearance of 3D printing dental materials, which restricts their clinical success and, therefore, limiting their potential growth as restorative materials [4].

Commercially available 3D printing polymers were primarily intended for temporary restorations. However, new resin-based materials were introduced for long-term use offering greater mechanical properties [5]. In this sense, it has been shown that the printing layer [6], post-printing treatment conditions [6], [7], [8], [9], [10], aging in water [11] and artificial saliva [12], and the chemical composition [13] can influence the mechanical properties of 3D-printed resins. In addition, the filler content of 3D printing resins may affect the flexural strength, elastic moduli, surface hardness [14] and wear behavior [15]. Further, the layering strategy determines (an)isotropic characteristics in the printed structure [16]. Thus, building orientation, also referred as printing angle or printing direction, may influence the mechanical properties of a 3D-printed object [4], [14], [16], [17], [18], [19], [20], [21].

Mechanical failure occurs when the generated stress becomes greater than the strength of the material [22]. The flexural strength (σf) and the elastic or Young modulus (E) have been used to assist on the mechanical behavior characterization of polymeric dental materials [4], [11], [23], [24], [25]. However, measuring material strength alone is not necessarily an indicative for the cause of structural failure or clinical longevity [26], [27]. In this respect, fractographic analysis is of particular interest as it allows critical flaw determination and component strength predictions, which are often associated with materials mechanical properties [28], [29]. Moreover, surface characteristics and topography of 3D-printed resins may vary with printing orientation [20], [30] and they influence the microbial adhesion [20], which is of relevance in restorative dentistry.

Therefore, the objective of this study was to evaluate the influence of printing orientation on flexural strength and elastic modulus of different 3D printing resins intended for dental restorations, testing the hypothesis that the evaluated mechanical properties are influenced by the building orientation of 3D-printed restorative resin structures.