## 4 Discussion  
To the best of our knowledge, in this study, the influence of root canal wall thickness and distance from root canal wall inner surface on the mechanical properties (indentation hardness and elastic modulus) of MTA was studied for the first time. The in vitro results demonstrated deteriorated mechanical properties of MTA in the contact with internal surface of root canal wall and with external environment. The analysis of hardness and elastic modulus values revealed deterioration of mechanical properties of MTA in the closest vicinity to the root canal wall only in the specimen with the thinnest root canal wall thickness (microhardness decrease of 32%). The decrease in modulus of elasticity was most prominent in the material located closest to root canal wall of \"Thin\" group (27%) but was detectable even in \"Medium\" and \"Thick\" groups (both 10%). During detailed nanoindentation mapping of randomly chosen specimen, we observed decreased microhardness in \"Thin\" and \"Medium\" specimen, which reached up to 190 \(\upmu\)m from inner surface of root canal wall. This decrease was not present in the \"Thick\" specimen. A similar decrease pattern was observed for modulus of elasticity. The similar result can be observed in the material which is in contact with external environment. To sum it up, Study-I detected, in particular, a decrease of the modulus of elasticity in the locations in the proximity of root canal wall, while Study-II, based on more detailed measurement, specified this decrease as linear dependence on the distance from the center of the sample. Moreover, it turned out that the decrease gets more dramatic with thinner walls. Study-I also clearly showed the effect of the contact with the external environment, which causes a significant decrease of mechanical properties of the material, compared to its values measured in the inner parts of the samples. This strong effect overcame the moderate effect of the vertical position within the samples. This property was uncovered in Study-II, in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Thickness** & **LP** & **LC** & **RC** & **RP** \\ \hline
0.8 mm & 1 & 3 & 3 & 2 \\ \hline
1.2 mm & 5 & 5 & 5 & 4 \\ \hline
1.6 mm & 3 & 3 & 1 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Clicker grain distribution according to the thickness of the root canal wall (0.8, 1.2, 1.6 mm) and location within a sample (LP–left peripheral, LC–left center, RC–right center, RP–right peripheral). which measurements were located on a dense grid in the vertical center of the samples and which, therefore, allowed a more detailed insight in the structure of the samples. The microhardness and modulus of elasticity are decreased most probably by excessive amount of moisture. When the mixed material is in contact with additional moisture in early phase of setting, the water-powder ratio can be changed [25]. The higher water-powder ratios influence several properties of MTA, such as decreasing its bond strength [8] and diminishing its compressive strength [26]. The increased hydration of portland cement leads to higher carbonation of hydrated cement with escalated shrinkage and raised production of calcium carbonate [27]. The disruption of MTA setting was more significant for the thinner root canal walls, and this effect can reach up to 190 \(\upmu\)m in the material but is more prominent in the layers which are in close contact with inner surface of root canal wall. This effect is not so evident for the thickest root canal wall. In previous studies, it was mentioned that possible source of moisture can be deposition of water in dentine tubules. If the deposition of water in the root canal wall was the main source of additional moisture, the effect would be most distinct in the specimen with thicker root canal walls.