## 4 Discussion
[43; 44; 45] investigated the impact of various factors on the accuracy of the quantitative measurement of the porosity volume in CSCs by micro-CT, such as voxel size, image-processing software, and the radiopacity of the material. According to the authors [45], smaller voxel sizes are preferable for evaluating both the thickness of materials and the volume of interfacial voids produced by them. Moreover, the immersion period of root-end filling materials in distilled water may affect their stability, as various CSCs demonstrated volumetric changes to different extents [43]. In a recent noteworthy study, [PERSON] et al. [45] found that the evaluated cements demonstrated a decrease in the void percentage when a greater voxel size was used. These discrepancies occurred since the micro-CT images are limited by the dimensions of the target object. If the resolution exceeds the dimensions of micropores or very thin structures, they may not be properly visualized. In the current study, the specimens were scanned at an isotropic resolution of 6 \(\upmu\)m, which provided greater clarity and contrast of images, thus increasing their quality and reliability. Even voids smaller than 5 \(\upmu\)m were detected inside the VOI by the defined threshold gray levels. On the other hand, a lower spatial resolution may induce errors in the calculation of the percent volume, creating an overestimation of some parameters. Therefore, consistent protocols are needed for micro-CT research evaluating endodontic porous biomaterials [44]. **Porosity** is an inherent characteristic of tricalcium silicate cements. It is an important parameter for characterizing the microstructure of this group of bioactive materials that corresponds to the volume of the void space, which may contain fluid or air, over the total volume of the material. According to accessibility, the pore system can be divided into two main categories: open and closed porosity [46]. External (interfacial) pores that have a continuous channel of communication to the outer surface (so-called blind or interconnected pores) constitute an open porosity fraction. A hydroxyapatite layer is formed during the hydration process of CSCs as a result of the precipitation of calcium and hydroxyl ions. It was suggested that this layer provides chemical bonding between the material and the dentin wall [29]. In fact, a pore structure develops during the cement hydration, thus forming micropores within the material and resulting in the formation of gaps or voids in the cement-to-dentin interface [46, 47]. **Open (interfacial) porosity** adversely affects the marginal adaptation of the root-end filling material and its sealing ability. It increases the permeability of the cement, thus allowing the tissue fluid percolation and the flow of microorganisms and their byproducts. Liquid sorption takes place as well. For this reason, the volume of the open pores is also referred to as the active or effective pore volume. It sheds light on the hermeticity of sealing, provided by the retrofilling at the interface [46, 48]. **Closed (internal) porosity**, or residual porosity, refers to micropores having no communication with the surrounding dentin walls. It occurs as a result of the spaces between the unhydrated cement particles. After the material hydrates, water fills these empty spaces. As the hydration reaction progresses, the hydration products fill these gaps and the porosity decreases; however, if excess water is used during mixing, it gradually evaporates and leaves unfilled voids. It was considered that the volume of micropores inside MTA does not affect the marginal adaptation because it represents a small fraction [35]. Moreover, the number of the internal pores decreases as the cement ages [46]. From another perspective, the porosity may be beneficial to the hydration procedure of CSCs and increase their capacity to release bioactive ions. According to [PERSON] et al.