Root-canal treated teeth are more susceptible to root fracture than teeth without endodontic treatment [4, 29]. In this context, endodontic sealers are used to promote reinforcement of the remaining structure, adhering to the root canal surface, in an attempt to contribute to the long-term permanence of these teeth [3, 15]. Thus, the present study evaluated the fatigue behavior of teeth filled with two different endodontic sealers (Bio-C Sealer and AH Plus), with or without a main core material (gutta-percha). Indeed, it was observed that the endodontic access and preparation might induce damage to the root that can only be attenuated by the materials and filling techniques explored. In this sense, the null hypothesis was accepted since no significant difference was found among the experimental groups. However, the AP-GP group produced fatigue behavior similar to the group with no treatment (C-), which had the best fatigue performance.
In order to maintain a tridimensional seal within the root canal system, a filling material must adhere to the dentin walls [30]. Many studies have shown that epoxy resin-based sealers result in a high bond strength to the root canal dentin [19], penetrating into micro-irregularities and partially filling dentinal tubules [28], thus increasing mechanical retention and resistance to shear forces [30, 31]. In the present study, teeth filled with AH Plus (an epoxy resin-based sealer) and gutta-percha (AP-GP group) showed the best fatigue behavior, similar to teeth that did not receive any intervention (group C-). These results were also found in a previous study [15, 28], which demonstrated that AH-Plus in association with a gutta-percha point can enhance the fatigue resistance of root-canal treated teeth. Based on that, the filling procedure with AH Plus sealer and the single cone technique improved the mechanical performance of the teeth, producing a result similar to the observed in sound teeth.
Sealers containing calcium silicate represent an important alternative to filling procedures nowadays, especially due to their remarkable biocompatibility. Another feature that made calcium silicate-based sealers so popular is the potential ability to form a chemical bonding to dentin [32, 33]. Although the exact mechanism is still unclear, the nanoparticles present in these materials may allow it to flow into dentinal tubules forming interlocking bonds and establishing a mineral infiltration zone with the posterior formation of hydroxyapatite [33]. Recent studies have demonstrated that Bio-C Sealer showed better cytocompatibility, mineralization capacity [20], higher penetration and better adaptation to the dentinal tubules [34] compared to other filling materials. Moreover, it was shown that teeth filled with calcium silicate-based sealers presented higher values for fracture resistance when compared to those filled with AH Plus [3, 15]. These findings contradict those described in our study, where teeth filled with Bio-C Sealer presented a similar mechanical behavior to AP groups. When compared to non-prepared teeth, filling with Bio-C sealer showed significantly worse fatigue behavior. This may be explained by the different methodologies applied since most studies regarding mechanical behavior of root filled teeth use static load tests [3, 15, 35]. One of the highlights of the present study was the use of a cyclic fatigue test to assess the mechanical behavior.
Tooth fracture is a consequence of cyclic fatigue that happens in the oral cavity in response to the stress caused during mastication. In this situation, failure happens in a much lower load than the load capacity [36]. The cyclic fatigue test used here simulates, under controlled parameters such as number of cycles, frequency, and load, the intermittent loading movements observed in the mouth [28]. Thus, cyclic fatigue tests provide results replicating the clinical conditions and the cyclic nature of chewing. Studies using static loading only provide the maximum critical stress through the increased applied load, which does not allow predicting failures over time, since they do not simulate the stimuli of the oral cavity [28, 36].
Due to its potential ability to bond to root dentin, the manufacturers claim calcium silicate-based sealers perform successfully regardless of using a main core material [19]. This kind of obturation would create a single interface between the root filling material and root dentin, forming a “monoblock” along the root canal walls, avoiding the creation of gaps between gutta-percha points and sealer [19]. In that regard, previous studies have reported that root canal sealers showed higher push-out bond strength values when the obturation was performed in bulk than when it was associated with a master gutta-percha point [6, 19, 24]. Despite these favorable findings, herein, the teeth filled without gutta-percha (AP-B and BC-B groups) showed a similar mechanical behavior compared to those with gutta-percha (AP-GP and BC-GP groups). Also, it is fundamental to consider the early failures evidenced in the groups without gutta-percha (BC-B and AP-B), starting to fail in steps 225 N (25,000 cycles) and 250 N (35,000 cycles), respectively. In contrast, the AP-GP groups started to present failures at approximately the double of the load 300 N (55,000 cycles).
Those early failures present in groups filled without gutta-percha could be justified by a thicker layer of sealer, which can shrink during setting and dissolve over time [24]. These findings contradict the possible advantage of the obturation in bulk once the absence of a main core material may compromise the spreading of the sealer along the root canal walls, facilitating the appearance of bubbles in the obturation mass. When subjected to cyclic loads, the presence of empty spaces in the root canal filling could induce the occurrence of early failures.
Regarding the fracture mode, fracture below the cement-enamel junction was the most prevalent pattern. The main reason for this occurrence might be the angle of the applied load, a factor that can influence fracture strength [37]. Anterior teeth are loaded in an unfavorable way during chewing and biting since the forces applied to them are oblique (not directed to their long axes) and consequently more harmful [37, 38]. According to our findings, the type of sealer and the filling technique had no impact on the fracture mode.
In vitro studies present inherent limitations; thus, the results of our study should be carefully analyzed. Human extracted teeth were used and we must assume that some anatomical variability will exist in this case. Another consideration is that periodontal ligament simulation was not performed in the present study. Some authors suggest this reproduction would affect the stress distribution during the fracture strength test [39]. However, there is a tendency not to perform the simulation in studies regarding fatigue resistance [28, 35, 40]. According to Marchionatti et al. [41], it may not be necessary to reproduce the periodontal ligament in those tests since it does not affect the fatigue resistance of teeth. Additionally, using an elastomeric material between the roots and the PVC cylinders could favor the displacement of the specimen, due to the direction of load application, impairing the test accomplishment.