DISCUSSION

Endodontic treatment failures can be caused by microorganisms able to survive in the apical root canal system or outside the apical foramen. Bacteria can form an organized structure in a biofilm around the root. Conventional treatment cannot remove this persistent infection due to its external localization (PURICELLI et al., 2014).
Thus, if the endodontic retreatment by the conventional endodontic treatment is unsuccessful, endodontic periapical surgery is considered the last option before tooth extraction (SERRANO GIMENEZ M; TORRES; ESCODA 2015). This procedure has a success rate between 69% and 93% (PINTO et al., 2020). The European Society of Endodontology defines periapical surgery success  as the absence of pain, swelling, or other symptoms, satisfactory healing of soft tissues, and radiological evidence of repair of apical periodontitis, including restoration of the periodontal ligament. Occasionally, a radiolucent area such as a “scar” should be followed up to 4 years. (ESE, 2006).
Endodontic periapical surgery consists of two stages. The first is root preparation, which can be performed with drills or ultrasonic instruments, and root-end filling. Root preparation aims to remove the apical portion of the root contaminated with resistant microorganisms, and the second step aims to seal the apex of the remaining canal (LI H et al., 2021) to promote the elimination of apical periodontitis and prevent new contamination (HARGREAVES K; BERMAN L, 2015)
Among the factors that influence the prognosis of endodontic surgery are smoking, location, shape of the tooth, absence or presence of dentinal defects, interproximal bone level, and filling material used (PINTO D et al., 2020).  Knowing that the quality of the root canal filling material influences the success of surgical endodontic treatment, it is expected that an ideal root filling material is biocompatible, has dimensional stability, resistance to resorption is bactericidal and bacteriostatic, easy to handle, and an excellent sealing capacity (CHONG; FORD, 2005; PINTO et al., 2020; DEL FABBRO et al., 2016).
This study analyzed the presence of empty spaces (quantifying the volume and percentages of cavities filled with the root-end filling material) both inside the root-end filling material and at the interface between dentin and cement through computerized microtomography, providing a three-dimensional volumetric analysis. It also allows viewing the relationship between the root-end filling material interface with the dentin and filling material interface without sample destruction (GANDOLFI et al., 2013; ZASLANSKY et al., 2011; CAMILLERI et al., 2012)
The results showed that all materials tested (MTA Angelus, NeoMTA Plus, BioRoot RCS) failed after the set period. Empty spaces were present at the cement-dentin interface and in the center of the root-end filling material, as in figure 1. There was a statistical difference between NeoMTA Plus and MTA and BioRoot RCS (p<0.05) concerning the number of empty spaces, rejecting the null hypothesis. (Table 1). The flaws in the root-end filling material may allow the reinfection of the apex with residual bacteria from inside the canal, which may migrate to the periodontal ligament due to its flaws. (CAMILLERI et al., 2013). Although MTA and BioRoot RCS have different chemical compositions, setting time and particle size (CAMILLERI, 2015b) (DIMITROVA et al., 2015) and MTA has poor handling properties (SHETTY; HIREMATH; YELI, 2017), while the manipulation and insertion of BioRoot RCS in the root-end cavities was much easier than that of MTA, no statistical differences were found in the percentage of empty spaces of these materials after filling the root-end cavities. The distribution of samples through stratified randomization and the performance of micro-CT to measure the volume of root-end cavities made in bovine teeth ensured the comparability of the groups, which could also be one of the reasons for the absence of statistically significant differences between the materials. In addition, MTA is composed of hydrophilic powder particles that absorb water during powder hydration, causing the expansion of the material during the setting process, providing better interfacial adaptation than MTA Plus (SHETTY; HIREMATH; YELI, 2017) demonstrating good marginal adaptation in the dentin walls (KÜÇÜKKAYA; PARASHOS, 2018). NeoMTA Plus is a calcium silicate-based cement with adequate radiopacity according to ISO Standard 6876:2012 (root canal sealing materials), finer powder particles than MTA, prolonged setting time, and high-water release capacity. Calcium and hydroxyl ions (SIBONI et al., 2017) resulted in greater solubility and mass loss during time than MTA Angelus (QUINTANA et al., 2019; GANDOLFI et al., 2014), and this is directly associated with the voids found in the NeoMTA Plus group. During the setting process, the material was in a humid environment, in contact with cotton rolls moistened with water. Materials containing calcium silicate immersed in deionized water present more voids than materials in contact with biological-type saline solutions (GANDOLFI et al., 2011). Thus, when in contact with body fluids, calcium and hydroxyl ions from the materials combine with phosphate from the periapical fluids, precipitating a superficial layer of calcium phosphate to fill the empty spaces opened at the adaptation interface between the root-end filling material and dentin. (SIBONI et al. 2017)
It is also necessary to describe that other factor, such as viscosity, could influence the presence of empty spaces within the root-end filling materials analyzed. The lower the viscosity, the greater the penetration of the materials into the dentinal tubules and the prepared surface, and the better interfacial adaptation of the root-end filling material (KÜÇÜKKAYA; PARASHOS, 2018). The conditions of the cavity surfaces also influence interfacial adaptation. Although ultrasound works under irrigation, debris remains after preparation requiring additional irrigation, and when not performed, may leave debris inside the cavity (KÜÇÜKKAYA; PARASHOS, 2018).