## 4 Discussion  
In this research, a 3D optical system based on the digital image correlation method was used for the evaluation of dimensional changes on the surface of a material during setting time. Strain is the quantity that is used to reflect deformation. Von Mises strain was chosen as it represents an index combining the principal strain at any given point at which strain occurring on the X-, Y- and Z-axis will cause failure [35]. This system is able to read a broad spectrum of micro-changes on the tested surfaces, it is simple to perform and it can be repeated. Following the full-field strain measurement, a 3D image is made of the whole field of deformation and the zones of greatest changes are clearly visible [36]. The time period during which the dimensional changes are analyzed is the setting time specified by the manufacturer's instructions. The setting time is defined as the length of time for a material to transition from a fluid state into a hardened state [37]. The goal of this research was to record dimensional changes during and immediately after setting. Considering that the microhardness [38] and bond strength to dentin [39] of MTA and Biodentin are similar, and that, according to the micro-CT findings in [40], MTA and Biodentin show a comparable result in terms of their internal adaptation on the dentin surface, then the degree of dimensional stability of materials could indicate the reliability of the adherent sealing of the chosen material. During the setting time, dimensional changes and deformation of the material occur [41], on which canal irrigation agents could have a negative impact [42]. In this research, the results showed a significant difference in deformation between all tested materials in the central and peripheral sections of the dentine discs. The highest deformation was measured in the Biodentine group and the lowest deformation was in the Well-Root PT group in both zones. Previous research on the dimensional changes of calcium silicate materials showed that particle size has an effect on the expansion of cements during the setting time. Well-Root PT has the smallest particle size (5 \(\upmu\)m to 30 \(\upmu\)m) and therefore less dimensional changes are expected [43]. White MTA also has a small particle size (73% of it is in the range of 6-10 \(\upmu\)m), and therefore has less hygroscopic expansion during the setting of the material [44; 45; 46]. Material expansion is related to water uptake [46; 47]. It does not occur in cements immersed in oil [46]. Additionally, 90% of specimens of dry MTA powder inside root canals had hardened simply by the absorption of water through the root surface within 72 h [48]. The lower values of von Mises strain of Well-Root PT could be explained by the fact that the material is premixed, injectable and its putty consistency significantly improves its handling characteristics. As a result of the better manipulation ability, a better adaptation of the material to the walls of the perforation cavity can be expected, which will have an effect on the sealing ability of the material. Well-Root PT material is based on tricalcium silicate and it includes zirconium oxide, calcium aluminosilicate and a thickening agent. The presence of aluminosilicate in the powder provides a more rapid setting compared to calcium silicate. There are few published data related to the properties of this material, but there are several studies concerning Well-Root ST sealer from the same manufacturer and based on the same basic components. Well-Root ST demonstrated less microleakage