## 4 Discussion
In view of the fact that the decrease of microhardness and modulus of elasticity of material is similar in contact with external environment and with thinner root canal walls, we can assume that the water permeability of thinner root canal wall can be as high as it can have deleterious effect in early phase of MTA setting. In other words, water can more easily penetrate from surrounding tissue through the canal wall into the root canal and influence the setting process of MTA. It might be due to wash-out of material, altered water-powder ratios, or carbonation of hydrated MTA. From our study, we are not able to determine if it is due to different proportional representation of phases (such as elevated outer product in comparison to inner product of calcium silicate hydrate gel) or due to reduced mechanical properties of particular phases. To assess this influence, it would be necessary to couple measurement of microhardness with backscatter scanning electron microscope. From our study, we can conclude that the reduced mechanical properties of material are not caused by reduced frequency of clincker grain. The variability in study designs of published papers makes the comparability of the results impossible. Despite endeavors for standardization of teeth specimen by sectioning and preparation of a root canal [8], it is highly unlikely to have a standardized root canal wall thickness because of their dissimilarity in shape and structure. In addition, human teeth are available only in a limited number. In this respect, the use of standardized samples can reduce the variability between specimens and provide a more accurate basis for comparison and, in turn, a more accurate insight into the factors which influence the setting dynamics. To achieve the best possible standardization of root canal wall thickness, the bovine compact bone was chosen as it was used to study bond strength of adhesives [28] or for standardized in vitro model of fracture resistance [29]. The permeability coefficient of human dentin is 2.7 \(\times\) 10\({}^{-4}\) cm/min and that of bovine compact bone is 1.1 \(\times\) 10\({}^{-5}\) cm/min [30; 31]. The permeability of dentin is influenced by thickness of dentin, distance from dental pulp, localization of dentin [32], and by irrigation protocols used in endodontics, especially when ethylenediaminetetraacetic acid (EDTA) is used [33]. It was shown that the use of EDTA during endodontic treatment increases the permeability of root canal wall because of dissolution of smear layer [33]. This increased permeability may have detrimental effect on sealability of MTA. It was shown that the use of EDTA and removal of smear layer during endodontic treatment leads to higher apical microleakage [34]. It must be emphasized that EDTA and other smear layer removal agents has detrimental effect on MTA [35; 36]. Moreover, EDTA retained its calcium-complexing ability when mixed with sodium hypochlorite [37], so a copious amount of sodium hypochlorite has to be administered to wash out remnants of EDTA from the root canal system. The effect of increased permeability could be another explanation of increased leakage of apical plugs after orthograde root canal treatment in contrast to retrograde filling. Thicker layers of MTA (4 or 5 mm) provide a better seal compared to 2- or 3-mm layers [38; 39], but there is no difference between 3- and 5-mm thick retrofill [40]. The better seal can be achievednot only by thicker layer of MTA but maybe even less deteriorated material in coronal part of material where the root canal wall is thicker. We must be cautious about assessing these studies because most of them used nail polish on the root surface during setting to avoid lateral microleakage, which affects the permeability of radicular dentin [39; 40]. In recent studies, specimens were subjected to Vickers microhardness test only in horizontal plane and in specific distance from simulated apical foramen.