## 2 Materials and Methods  
The current study used a cross-sectional retrospective research approach. A total of 100 patients received post and core treatment after endodontic therapy in 2016-2017 from final year undergraduate dental students at Jazan University in Saudi Arabia. The same undergraduate students had already been trained to execute the post and core basic procedures on typodont teeth as part of their preclinical training in fixed prosthodonics. Prior to the commencement of the study, an ethical clearance with the approval reference number REC42/1/051 was received. The sample size was estimated using R statistical software, version 3.3.1 (R Core Team 2016; R Foundation for Statistical Computing, Vienna, Austria). The optimal sample size was determined using a one-way analysis of variance power calculation for more than two groups. The findings indicated that a sample size of 75 endodontically treated teeth would be sufficient to reject the null-hypothesis with a power of 90% and a two-sided significance level of 5%. According to the following inclusion criteria, subjects were included in the study:  
1. Patients who had received post and core treatment after root canal therapy by undergraduate students. 2. After the post-and-core therapy, digital periapical radiographs had been acquired and were included in the patient records. Subjects were disqualified from the research based on the following criteria:  
1. Post and core treatment executed by specialists or interns. 2. Any variable that could affect the result of root-canal therapy. For example, a broken instrument or file in the root canal, or overextended or underextended root canal fillings, calcified canals, or root fractures. 3. Any distortions in the radiographs. Final year undergraduate students performed the post and core treatment on the endodontically treated teeth with compromised tooth structure. Diagnostic preparation was performed to evaluate the remaining tooth structure and ferrule. An intraoral periapical radiograph was made to evaluate the post space preparation. Depending on the shape, diameter, length, and curvature of the root canal, [PERSON] drills and peeso reamers (MANI, Utsunomiya, Japan, size #1 to #6) were used in the sequence to prepare the post space. In order to maintain the periapical seal, the root canal was prepared in such a way that the remaining 5 mm of gutta-percha was left to maintain the apical seal. An intraoral periapical radiograph was made to confirm the post space preparation and remaining gutta-percha. Based on the diameter and length of the prepared post space, the length and diameter of the prefabricated glass fiber post (3M RelyX, St. Paul, MN, USA) was selected. A universal twist drill, supplied by the manufacturer, corresponding to the selected prefabricated post was used to shape the canal before cementation. The entire procedure was carried out under rubber dam isolation. The prefabricated glass fiber post was cemented with self-adhesive resin cement (3M RelyX Unicem 2 Automix, St. Paul, MN, USA). Custom cast posts were fabricated using an indirect method. The separating media was applied to the prepared post space with paper points. The plastic posts (Spee-Dee plastic pins, Pulpdetent corporation, Watertown, MA, USA) were used to make the post space impression using pattern resin (Pattern Resin LS, GC America Inc., Alsip, IL, USA). After the initial set, the pattern was moved up and down to assure that it came out passively. After that, the core component of the crown was shaped with the pattern resin according to the shape of the future abutment. The manufacturer's instructions were followed while spruing, investing, and casting. The retrieved castings were evaluated intraorally for their fit and minor adjustments were performed. The metal cast posts were cemented with zinc phosphate cement (Primedent, St. Paul, MN, USA). The canal may be recontaminated in various ways such as contact between the oral bacterial flora and root canal tubule inlets.