Introduction. The effect of reformatory reduction on seal and retention of the cemented post with zinc phosphate cement considered in four different times.Methods. After tooth preparing, supply of molding post and cement, the reformatory reduction performed on samples except control group. The starting times are as follows: 1. Immediately after cementing; 2. Six minutes after cementing; 3. One hour after cementing; And 4. 24 hour after cementing. Thensamples are kept in normal saline, in 37c,moisture 100%. In order to close test qualificauon to normal condition of mouth, after reduction completion, the samples go under the termocyclic operation. Then dye penetration, sample incision and amount of leakage, considered by reflective microscope.Results. The dye penetration"s average in different groups has has meaningful difference. The group which has no meaningful difference with control group, is that on which reformatory reduction has performed 24 hour after post core cementing. The others have meaningful difference with control group. There is no meaningful difference between two consecutive groups on which reformatory reduction performed in different times.Discussion. We recommend cememted post-core after final setting of cement

Introduction: Root canal-treated teeth are more prone to failure in functional forces. Post and core procedures are often indicated in these cases. On the other hand, there have been interests in the use of all-ceramic restorations with improved properties in posterior teeth reconstruction. This study aimed to investigate the fracture strength of four different post and core systems in root canal-treated premolars with all-ceramic crowns. Materials and Methods: A total of 48 mandibular premolars were selected and subjected to root canal standard treatment. Afterward, they were divided into four groups of Ni-Cr post and cores casting, glass fiber post/composite core, non-precious gold post and cores casting, and custom-made milled zirconia posts and cores. Each specimen received all-ceramic crowns with zirconia copings and was then loaded by the Zwick device. The amount of fracture resistance by the specimens up to the breaking point was measured and the modes of failure were analyzed in this study. The data were analyzed using analysis of variance and Tukey's honest significance test (α =0. 05). Results: The mean fracture strengths of the four groups were 780. 59± 270, 70± 323, 963. 24± 915. 307, and 435. 34± 220 N, respectively. There were differences between the core and post groups regarding this difference (P=0. 000). Conclusion: According to the results, out of the post-and-core procedures used for the reconstruction of prosthetic root canal-treated premolars with all-ceramic crown, the custom-milled zirconia had lower fracture strength, and non-restorable fractures were more prevalent in these posts.

Purpose: There is a wide clinical application of Duralay acrylic resin pattern for direct post and core fabrication.As the effect of time elapse, desinfection solution and preservation places is not studied together, this study was designed to evaluate the interaction of these variables on dimensions (length, coronal and apical diameters) of duralay acrylic resin patterns.Methods & Materias: In this in vitro and quasi experimental study 36 duralay patterns fabricated in simulation method were used. Then patterns were categorized in one of the 3 groups of glutaraldehyde, sodium hypochloride and control. Each group of 12 patterns was divided in the two groups of preservation places (wet and dry). Patterns were measured in time intervals of 0, 1, 3, 6, 9, 24 hours. The 3 dimensions of length and apical and coronal diameter were measured with digital micrometer with 10 microten accuracy.Results: Statistical analysis demonstrated that coronal and apical diameters differ significantly by the effect of time elapse and disinfection solutions and preservation places (P<0.005). Coronal diameter demonstrated significant difference as the etfect of time elapse. (P<0.005) Significant differences were not noted between wet ordry preservation places. Glutaraldehyde caused contraction and sodium hypochloride caused expansion of duralay patterns. The mechanism of this different effect was not understood.Conclusion: It was observed that linear dimensional change of duralay acrylic patterns didn't follow a linear correlation formula for prediction of duralay changes and the best time for casting. Different effects of desinfection solution on casting and cementation stages must be evaluated to achieve better evidence and clinical relevance.

Introduction: most endodontically treated teeth need post and core restoration to do acceptable percentage of their function but selection of proper post and core system can be difficult and ambiguous .this study compared the effect of cast post and core and fiberglass post and core system on the fracture resistance and fracture pattern of crowned endodontically treated teeth.Material and method: A total of 30 recently extracted human canines with their crown removed were endodontically treated. Two groups of 15 specimens were formed. Teeth were restored with cast post core in the 1st group and with fiberglass post in the 2ndgroup. All posts were cemented with panavia F2 adhesive system. All fiber posts were restored with composite core. Cemented with zinc phosphate cement. Each specimen embedded in acrylic resin and then scored in universal load-testing machine (Instron). A compressive load was applied at a 45 angle to the long axis of the tooth until fracture at a crosshead speed of 1mm/min. Result: the Fracture resistance of 2 groups according to the (t) test analysis didn't have any significant difference. The mean fracture resistance of the teeth restored with cast-post system were 1489.33 ± 1045.34 and facture resistance of the teeth restored with fiber glass were 1195.93± 624.12 teeth restored with cast post exhibited higher fracture resistance than the other group. Fracture that occur in teeth restored with fiberglass post and composite core usually are restorable and %80 of fracture that occur in teeth restored with cast post and core are not restorable.Discussion: considering that fracture resistance of teeth in tow groups evaluated in this study were not significantly different and fracture of tooth restored with Fiber glass post were from a point that was more easily restorable, compare to cast post and core if seems reasonable to use Fiber glass post in appropriate cases, Although it is suggested to do and evaluate other clinical studies in long term.

Background: It is unclear which the material is a better choice for post and core rehabilitation systems in endodontically treated teeth. This study aimed to compare the fracture resistance of three different postcore systems. Materials and Methods: In this in vitro study, 33 extracted premolars were treated endodontically and divided into three groups, namely (A) the prefabricated fiberglass postcomposite core, (B) the Ni‑, Cr cast postcore, and (C) the polyetheretherketone (PEEK) postcore groups. After postcementation, the core was restored with Ni‑, Cr crown. By a universal testing machine, the fracture resistance of the posts was assessed. Furthermore, types of fractures were determined with radiography and confirmed objectively by cutting the acryl resin boxes. The data of the fracture resistance were analyzed using the one‑, way ANOVA and Tukey test, and the data of the fracture pattern were analyzed using Fisher’, s exact test (P < 0. 05). Results: The fracture resistance was significantly higher in Group B (Ni‑, Cr) than in Group C (PEEK) (P = 0. 001) and Group A (fiberglass) (P < 0. 001). Moreover, the fracture resistance was higher in Group C (PEEK) than in Group A (fiberglass) but was not statistically significant (P = 0. 405). The fracture pattern was different in all the groups based on the types of fracture (P = 0. 009) and the repairability of fracture (P = 0. 036). Conclusion: The present study showed that the fracture resistance was significantly higher in the Ni‑, Cr post than in the fiberglass and PEEK posts. Further, the fracture mode was more repairable in teeth restored with the PEEK post, as compared to the other posts.

'Post-truth' has become a buzzword for numerous current crises: the fragmentation of the media landscape, the ongoing debate about 'fake news', the loss of trust in science, etc. Although these crises take place in society, it is claimed that the roots of post-truth can be traced back to the history of philosophy. Occasionally, it is asserted that Karl Popper's critical rationalism gave rise to post-truth: His rejection of verificationism has limited truth claims in the realm of science. Given the absence of infallible evidence and certainty, critical rationalism calls for challenging scientific authority. I argue that post-truth is compatible with critical rationalism from an epistemological point of view, considering that both positions are critical of certainty. However, in critical rationalism, fallibilism, responsibility, and the idea of criticism are combined, and in this respect, it offers a possible way to overcome the problems that are associated with post-truth. This treatment of the problems of post-truth results from the recognition of moral responsibility to take action on the basis of a hypothesis that remains open to revision.

Background and Aim: Composite resin cores with pre-fabricated posts are commonly used to restore endodontically treated teeth. This study compared the sealing ability of Core Max II and Clearfil photocore by Panavia F 2.0 cement, with and without adhesive.Materials and Methods: Crowns of sixty recently extracted human second premolar teeth were cut 3 mm above the CEJ. After preparing proximal boxes and finishing root canal therapy, the teeth were randomly divided into four groups of fifteen teeth each. The Dentatus # 2 Long pins with approximately 8 mm length were cemented in canals. In the first two groups, Core Max II with adhesive (group 1) and without adhesive (group 2) was applied for cementation. Pins in the other two groups were cemented with Clearfil photocore composite and Panavia F 2.0 cement with adhesive (group3) and without adhesive (group 4). After 24 hours, specimens were thermally cycled for 500 cycles. Then the teeth were sealed with nail varnish beyond 1 mm of restoration margins and immersed in a 0.5% fuschin solution. Afterwards samples were embedded in epoxy resin, sectioned mesiodistally and observed under 20 x magnifications. The microleakage was recorded as the amount of dye penetration in the route connecting the proximal boxes to the end of the pin. ANOVA and Tukey tests served for statistical analysis.Results: The first (61.57±14.24) and fourth (12.37±13.85) groups had the highest and the lowest value of microleakage, respectively. Using adhesive for both cements reduced microleakage (P<0.05).Conclusion: Panavia cement had less microleakage compared to Core Max II. Key words: Dental Leakage - Post and Core Technique - Composite Resins.Corresponding Author: Dr. Moosavi H., Assistant Professor, Department of Operative Dentistry, School of Dentistry, Mashhad University of Medical Sciences. Mashhad, Iran.

Introduction: The purpose of this study was to investigate and compare the dimensional changes in the acrylic patterns that were stored in the different times and conditions of storage. Materials In this experimental study The sample number was 28 in 4 groups. The mixed paste & Methods: of acrylic samples were placed inside a metal mold and after setting time with the slightest of deformations it was removed. Using this method 28 samples were prepared for each material. The diameters of samples were then measured immediately using a digital caliper with a precision of 0. 01 mm. The samples were then divided into four quadruple groups and stored at 22° C room temperature, 22° Cat water, Dry 4° C environment and 4° C water. The dimensions of the four groups were measured at intervals of 1, 8, 24, and 48 hours after polymerization and being removed from the metal molds. The ANOVA test was used for data analysis (p value = 0. 05). Results: The findings of the current study showed that the dimensional changes of Duralay and GC acrylic resins at both 4 and 22° C in dry and water environments are decreasing over time (p value < 0. 001). This study indicated that the contraction rate of two types of acrylic resins, Duralay and GC, at two temperatures of 4 and 22° C, and dry and humid conditions and at various times were not different. Conclusion: Dimensional changes in of the two types of acrylic resin, Duralay and GC, During the lapse, the procedure is going to be decreased but have not seen any statistical difference between these type of acrylic resin in environment storage.