## Materials and Methods
### Experimental Procedure  
The freshly extracted teeth were stored in 1% tymol (Braun(r), Melsungen, Germany) before being used (1 month maximum) at room temperature and randomly (Epidat 4.1, Galicia, Spain) distributed into the following study groups (in order to standardize the results only compare a selection of specific commercial products): A. Conventional glass FRC posts (Fiber Post(r) 0.8 mm, GC Europe, Leuven, Belgium), restored with a dual-curve CR core material (Gradia Core(r), GC Europe, Leuven, Belgium) (\(n=10\)); B. Elastic FRC posts (EverStick Post(r) 0.9 mm, GC Europe, Leuven, Belgium), restored with a dual-curve CR core material (Gradia Core(r), GC Europe, Leuven, Belgium) (\(n=10\)); C. Conventional FRC posts (Fiber Post(r) 0.8 mm, GC Europe, Leuven, Belgium), restored with a FRCresin (Everx X Posterior(r), GC Europe, Leuven, Belgium) (\(n=10\)); D. Elastic FRC posts (EverStick Post(r) 0.9 mm, GC Europe, Leuven, Belgium), restored with a FRC resin (Everx X Posterior(r), GC Europe, Leuven, Belgium) (\(n=10\)); E. mesio-oclusal-dist (MOD) cavity directly restored with a dual-cure CR core material (Gradia Core(r), GC Europe, Leuven, Belgium) (\(n=10\)); F. MOD cavity directly restored with a FRC resin (Everx X Posterior(r), GC Europe, Leuven, Belgium) (\(n=10\)); G. intact teeth without MOD cavity, nor endodontic treatment nor restorations (\(n=10\)) (Figure 1).  
The preparation of the samples, except for the control group comprising, began with the preparation of a MOD cavity. This cavity was performed with a diamond bur (Ref. 882 314 012, Komet Medical, Lemgo, Germany) in a high-speed handpiece and under constant irrigation on the occlusal surface of each tooth with the following characteristics: a depth of 4 mm at the occlusal cavity and 1.5 mm at the proximal cavities. A vestibule-palatal width of 2/3 (mean 3.7 mm) of the intercuspal distance (mean 5.5 mm), a mesio-distal width of the proximal cavities of 3 mm [22]. Subsequently, the endodontic access cavity was performed to allow a straight access to the root canal system and a chamber opening of 5 mm of depth (Figure 2).  
Figure 1: Schematic representation of experimental groups.  
Figure 2: (**A,B**) Representation of the cavity preparation used in this work. In blue, endodontic access cavity.  
The working length of the root canal was established using a direct method, by subtracting 1 mm from the actual root length determined by introducing a 10/.02 K-file (Dentsply Maillefer, Ballaigues, Switzerland) until it was visible through the apical foramen. Canal instrumentation was performed using an R25 rotary file (Reciproc(r), VDW, Munich, Germany) and irrigated with 5 mL of 5.25% sodium hypochlorite (NaOCl) (Clorox, Oakland, CA, USA), 5 mL of 17% EDTA (SmearClear(r), SybronEndo, CA, USA), 5 mL of 5.25% NaOCl (Clorox, Oakland) and sterile saline solution (Braun(r), Melsungen, Germany) using an endodontic needle (Miraject Endo Luer(r), Hager & Werken, Duisburg, Germany) with a diameter of 0.3 mm inserted 1 mm into the working length. Afterwards, the root canal system was dried with sterile paper points (Dentsply Maillefer, Ballaigues, Switzerland) and finally, each root canal system was sealed using a warm gutta-percha system (Calamus(r), Dentsply Maillefer, Ballaigues, Switzerland) and an epoxy-amine resin-based sealer (AH Plus(r), Dentsply DeTrey, Konstanz, Germany) until the cement-enamel junction and the endodontic access cavity was temporarily sealed with Cavit(r) (3M ESPE, Saint Paul, MN, USA). The teeth were embedded into an epoxy resin models (Ref.: 20-8130-128. EpoxiCure(r), Buehler, IL, USA) that simulate periodontal ligament to absorb some of the mechanical loading (Flexural strength (DIN 53452) 50 N/mm\({}^{2}\), e-modulus (DIN 53452) 3900 N/mm\({}^{2}\)) and then they were stored in an incubator (mco-18 aic, Sanyo, Moriguchi, Osaka, Japan) for 1 week (37 \({}^{\circ}\)C, 100% relative humidity). Elastic Modulus (GPa) of different teeth's components are described in Table 1.