One of the goals of root canal treatment is sealing possible communications of the pulp space and the external surface of the tooth. If the mechanical resistance of the interface dentin-repairing material is low, the material will be easily dislodged from the cavity and the treatment will fail. ProRoot MTA White is considered the gold standard when testing calcium silicate-based cements. The aim of this study was to compare this first material with other recently developed calcium silicate-based cements in order to verify whether their new features have improved their mechanical behavior and they can stand as real alternative to MTA in terms of bond strength.
Most clinicians perform apical or coronal barriers of a random thickness between 3 and 5 mm, which are the recommended thicknesses for apexification, endodontic surgery or revitalization procedures. Several studies have evaluated the sealing ability of calcium silicate-based cements at different thicknesses and have established that a 3 to 5mm thick plug is able to produce an acceptable seal. [22, 31] Nevertheless, the resistance to dislodgement that these different thicknesses achieve has not been tested as much as the sealing ability.
The pushout test is a widely accepted method to assess the bond strength of dental materials to dentin, due to its reliability and reproducibility. [26, 32, 33] Some authors recommend the use of dentin sections for push-out tests with thicknesses not greater than 1.5mm, in order to avoid an overestimation of the bond strength due to an increased friction area. [25, 34] However, several studies have tested POBS of calcium silicate-based cements using samples with thicker samples. [24, 27] In the present study 3 and 5mm slices were used in order to obtain a more accurate simulation of the clinical situation of apical or coronal plugs, and specifically, to verify whether the thickness of the plug may influence the bond strength achieved by the different materials.
The results of the present study suggest that the POBS of ProRoot MTA may not significantly change with thickness, while in the case of TF and BD an increase in the thickness of the barrier results in an improvement of their resistance. Thus, for these new materials the thickness of the plug built with them should be taken into account when mechanical performance is a critical requirement for the clinical application in which they are used.
At 3mm thickness, no statistically significant difference was observed in the POBS values of BD and PMTA, and both of them obtained significantly higher values than TF. These findings are in agreement with those of Stefaneli Marques et al., who found no differences (at a thickness of 2 mm) between BD and PMTA. [35] Likewise, a recent study found similar resistance to dislodgement results with both Biodentine and ProRoot MTA, which demonstrated to be significantly higher than those of Endosequence (commercialized as Totalfill in Europe) [36] On the contrary, Kadić et al obtained better results with TF than with BD. [37] The contradictory results may be due to differences in the methodologies used in the different studies, such as the different times that the materials were allowed to set before the test was performed, or the specific presentation (putty or paste) of the TF used in each research. Hence, high quality clinical trials are urgently needed, for a more reliable comparison of the clinical performance of the different materials and consistencies.
In addition to the usual inferential statistical analysis of the results of mechanical test, Weilbull analysis was used to better compare the predictability of the tested materials, as this analysis is a useful tool for the study of the reliability of materials. Weibull distribution depends on the shape and scale parameters. The shape parameter, or Weibull modulus (m), is the slope of the line. A higher slope represents a lower variability of the feature tested. Thus, a large Weibull modulus is a highly desirable property for a dental material as it guarantees more uniform performance, and therefore a higher reliability. [28] The Weibul scale parameter (σ0) indicates in this particular study, the resistance value at which 63.2% of the tested samples fracture. Therefore. the higher this value, the more resistant to displacement is the material. [29]
TF showed the weakest POBS in plugs 3mm and 5mm thick and also lower predictability. This could be explained by the consistency in which the material is packaged and commercialized. The premixed consistency could cause less control over defects that may persist in the mass of the material, as suggested by Toia et al. [38]
Increasing the thickness of PMTA did not result in a significant improvement of its POBS values. However, it did affect the predictability of this material as the m parameter was higher at 3mm (3.09) than at 5mm (2.28). This could be explained by the higher probability of defects both in the bulk of the material, due to the high porosity of MTA [39, 40] and also by the presence of gaps in the dentin-material interface. [41] The specific need for an intrinsic water supply for optimal setting of PMTA could be responsible for the presence of defects, as increasing the thickness of the cavity makes it more difficult for moisture to reach the deeper layers. [37] Thus, the greater the thickness, the greater the probability of imperfections. In contrast, at a thickness of 5mm, this parameter was higher for BD than for TF and PMTA, meaning that BD is more predictable than the other two materials. This could be caused by the low porosity and high homogeneity shown by BD [37, 39], probably due to the. insoluble polymers included in its composition, which help maintain a balance between the water content and the consistency of the preparation, to obtain a more homogeneous and dense cement. [16] Also, BD has smaller particle size and shows more tags in the interface with dentin than PMTA, which may account for the higher bond strength results. [24]