## 1 Introduction  
Vital tooth behaves like an empty and laminated structure and the cusp morphology allows receiving functional loads, distributing them evenly without causing any damage [1].  
However, endodontically treated teeth present a different biomechanical behavior at different levels [2]. When pulp is removed the protective feedback is lost [3], increasing the risk of fracture [4], due to the significant loss of the dental structure [5] during endodontic access to the pulp chamber and caries removal [6; 7]. The absence of pulp tissue causes irreversible alterations in dentin, [6] reducing its wettability and the collagen content [8], affecting the Young's elastic modulus of dentin and its proportional limit, in other words, the proportional limit determines the greatest stress that is directly proportional to strain The proportional limit is the point on a stress-strain curve where the linear, elastic deformation region transitions into a non-linear, plastic deformation region.  
The use of chemical solutions during root canal treatment can reduce the mineral content of dentin, rendering it weaker [2].  
All changes that occur in endodontically treated posterior teeth can induce it to fracture during chewing [9] causing a greater cusp deflection [10], increasing according to the size of the restoration; where the teeth will be subjected to greater stress.  
The mechanical behavior of upper premolars is fully different from molars by anatomical reasons, bicuspids not multicuspids, internal fracture resistance and architecture. Upper premolars require extra coronal support to be restored [11] since they are more likely than molars to be subjected to lateral forces during chewing due to their smaller diameter [12]. However, [PERSON] et al. reported by means of a finite element analysis that the resistance of upper premolars restored through fiber post with mesio-occlusal-distal (MOD) cavity allow a positive distribution of occlusal forces; preventing dangerous stress concentration [13]. In addition, if the tooth has been restored, the type of fracture will not only depend on whether the tooth has been previously endodontically treated or not but also on the mechanical properties of the restorative material [14].  
To improve fracture resistance of endodontic teeth, new materials with greater physical properties have been tried [15] and also new techniques. Many kind of investigations were carried out not only depending on destructive test but including virtual and real simulations by means of CAD-FEM analyses [16; 17] With the latest advances in adhesive restorations, a concept of minimal intervention in dentistry has been introduced to preserve the dental structure as much as possible [18], enhancing its mechanical properties and its ability to adhere to the tooth [19]. Although composite resins (CR) core, glass fiber reinforced CR, fiber reinforced composite (FRC) posts and elastics FRC posts have been used in clinic, we want to analyze their mechanical behavior because we have not found publications where they compare. The aim of this work was to analyze and compare the fracture resistance of endodontically single-rooted first upper premolar teeth restored with glass FRC posts, FRC reinforced elastic posts, conventional composite resin and glass fiber reinforced composite resins as root canal treatment restorations, with a null hypothesis (H0), which states that the restoring materials tested has no statistically significant effect on the fracture resistance of endodontically treated tooth.