Fatigue failure load of prefabricated fiber reinforced post: The influence of the post diameter and fatigue test method
Introduction
Extensive caries injuries, fractures and failures in previous restorations can result in loss of tooth structure[1]. Depending on the degree of coronal destruction, an intraradicular post is required to retain the restorative material after endodontic treatment[2,3]. Different intraradicular posts can be used, as conventional cast metal posts, prefabricated metal, carbon, and glass fiber posts or milled computer aided design and computer-aided manufacturing (CAD-CAM) glass fiber, metal, or ceramic posts [[4], [5], [6], [7], [8]]. The choice of prefabricated posts has been cited in surveys published recently[9]. Prefabricated fiber reinforced composite posts (FRC post) such as those glass fiber reinforced are an alternative because of its similar mechanical properties (low elastic modulus) to the remaining tooth structure[1,2,10]. Results of 9 years of follow-up of glass FRC post and cast metal showed good and similar clinical performance[11]. However, the use of FRC posts may lead to a more homogeneous distribution of forces to the dental remnant, thus reducing the risk of catastrophic failures when compared to metallic posts or cast metal posts[12].
Distinct fiber post diameters can be clinically performed depending on the conditions encountered, mainly considering the enlarging of intracanal space[13]. From this standpoint, the distinct diameters might express different mechanical behavior since higher fiber post diameter increases the fracture resistance of this polymer. It is stated that FRC posts with a higher cross-sectional area can withstand a higher load per area unit compared to FRC posts with smaller diameters, thus an increase in the diameter also leads to increasing the polymer resistance to flexural loadings[14]. As a result, the load-bearing capacity under fatigue (cyclical intermittent loading) of fiber posts might be affected by its diameter, but this has not been evaluated in the literature yet.
Fatigue tests are indicated to assess the long-term behavior of FRC posts, as they better represent the cyclic stress that natural and restored teeth are exposed to during chewing[15,16]. Cyclical intermittent loading might lead to a failure/fracture which spreads from a defect or weak point in the structure over time[2]. The fatigue behavior can be evaluated through fatigue tests such as S–N curves; however, these tests are usually time-consuming and expensive, which often make their execution unfeasible. Therefore, information regarding materials’ fatigue behavior can be evaluated in a faster and still reliable way by accelerated fatigue tests. Despite the severe testing conditions, the accelerated fatigue parameters are selected so that failures occur faster, but with caution to maintain the failure modes observed in clinical conditions[17].
There are two commonly used accelerated fatigue methods to test dental materials: staircase and step stress tests. In the staircase method, the specimens receive progressively increasing loads (when surviving) or decreasing loads (when failing) for a predetermined number of cycles. Collins [18] states that at least 15 samples need to be tested after the first stair reversal to ensure test reliability. Meanwhile, in the step stress method the same specimen is subjected to increasing load levels (steps) for a fixed number of cycles in each load-step until failure occurs[18]. Although both methods are commonly used to evaluate the fatigue behavior of dental materials [19], no study comparing these methods in FRC posts is found in the literature.
Taking into consideration the aforementioned views, the present study aimed to compare the fatigue performance of FRC posts with different diameters by using two different fatigue testing methods of staircase and step stress tests. The study hypotheses were that: (1) FRC posts with a wider diameter would present better fatigue performance; and (2) fatigue performance would not be influenced by the fatigue method used.
Section snippets
Specimens preparation
A total of sixty-nine (N = 69) FRC posts (White Post DC, FGM, Joinville, SC, Brazil), composed of a glass fiber reinforced epoxy resin matrix, double-tapered (wider coronal diameters and narrower apical diameters) with a cylindrical coronal part and presenting 20 mm in length associated with different coronal diameters (1.4, 1.6, 2.0 mm) were used in this study.
The FRC posts were delimited at 14 mm in relation to their apical part with a digital caliper, fixed in a surveyor, and embedded in PVC
Results
Both tests showed that a wider diameter is correlated to a greater fatigue failure load of the FRC post (Table 3 – Step stress: DT1.4 < DT1.6 < DT2; Staircase: DT1.4 ≤ DT1.6 ≤ DT2) and a greater the number of cycles required for failure in the step stress test (Table 3). Table 4 summarizes the FRC post survival rates in the step stress test for each load step and number of cycles for failure, while Fig. 1 shows the survival curves of the two parameters mentioned above. Fig. 2 shows the
Discussion
Regardless of the fatigue test method used, we found that the FRC post with a wider diameter induces better fatigue performance (greater fatigue failure load in both tests and greater number of cycles for failure in the step stress test). Thus, the proposed hypotheses were accepted. In addition, a similar failure pattern could be observed regardless from the fatigue test and the FRC post diameter.
Despite the differences in the methodology of each fatigue test employed in the present study, both
Conclusion
Fatigue performance is influenced by the FRC post diameter, showing that the wider the diameter, the better the fatigue performance. In addition, the fatigue test method (step stress or staircase) does not affect the fatigue performance of the studied FRC posts.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors declare no conflicts of interest and emphasize that this study was supported by the Franciscan University Scientific Initiation Scholarship Program (PROBIC/UFN). We especially thank FGM, Dentsply Sirona, Kulzer for donating the research materials, and Dental Cremer for donating resources for the acquisition of other research materials.
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