## 4 Discussion  
Most likely, there are currently about fifty different kinds of calcium silicate-based cement on the market. Based on variations in their chemistry, a system of categorization for this group of materials has been suggested [14]. All the generations of CSCs are summarized in Table 2, along with their representatives. The survival rates following endodontic surgery range from 48% to 93% [15]. The prognosis and long-term success of this complex procedure depend on many factors, such as case selection, instrumentation technique, suitable equipment, the clinician's qualification, skill, and experience, the age of the patients, etc. Among them, the choice of the appropriate retrograde filling material plays a critical role [16; 17]. One of the most important requirements for the root-end filling material is the ability to ensure a hermetic seal of the apical area. Inadequate sealing of the retrograde cavity is considered the main reason for the failure of endodontic surgery [17; 18; 19]. Good marginal adaptation to dentin and internal porosity of calcium silicate cements are related to their sealing capacity and ability to resist bacteria penetration and prevent the percolation of tissue fluids [20; 21]. Therefore, the porosity characteristics of bioceramic materials are important determinants of healing outcomes of endodontic surgery [22; 23]. The present study assessed quantitatively all aspects of the porosity (interfacial, internal, and total) of a new MTA brand, namely Harvard MTA Universal, used as root-end filling material. Only a single piece of research was found, evaluating the marginal adaptation of this cement at the same application site through scanning electron microscopy (SEM). [PERSON] et al. [24] reported higher average gap size values with Harvard MTA (7.44 \(\mu\)m) at the interface than those observed in the MTA Flow group (3.95 \(\mu\)m). However, the presented statistical results were inconclusive regarding the significance of this difference. To the best of the authors' knowledge, this is the first experimental micro-CT study evaluating volumetrically the marginal adaptation and internal porosity of this innovative endodontic cement. Due to the lack of scientific evidence on **Harvard MTA**, the data obtained from the present study were compared with **similar micro-CT investigations on retrograde application** of other calcium silicate cements. Only a limited number of previous studies have used the same methodology to quantitatively assess the volume of interfacial gaps and inner porosity of selected CSCs applied as root-end filling materials. Some of them evaluated the loss of volume and density of tested CSCs exhibited after exposure to acidic or alkaline conditions, blood, or after retrieval of various intracanal medicaments, along with their marginal adaptation [25; 26; 27; 28]. Given the differences in the study designs, our results were compared only with those obtained in similar experimental conditions, i.e., after material immersion in distilled water or in normal saline only. In general, **Biodentine**, probably the most tested CS cement, is suggested to be superior to ProRoot MTA and other new bioceramic materials in terms of interfacial gap volume, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Generations** & **Main Characteristics** & **Representatives** \\ \hline
1 st Generation & MTA Original & ProRoot MTA \\ \hline
2 nd Generation & Modifications to MTA & MTA Angelus (gray and white form) \\ \hline \multirow{4}{*}{3 rd Generation} & Bioagermaics group & Bioaggregate, Biodentine, Ortho MTA, MTA BIO, EndoSequence BC (iRootSP), Aureoseal, Bio MTA+, \\  & (New formulations) & MTA Repair HP, Neo MTA Plus, Bio-C Repair, \\  & & BiOfactor MTA, MTA Bio-C Pulpo, MTA Bio Rep, \\  & & Endo-Eze MTA Flow, RootDent, Harvard MTA, CEM \\  & & cement, ALBO-MPCA, Trioxident, One-Fill PT, etc. \\ \hline
4 th Generation & Hybrid cements & TheraCal LC \\  & (Light-cured) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Classification of calcium silicate-based cements.