## 3 Discussion
In cases A and B, the canal anatomy was easily interpreted by the periapical radiograph (single-rooted, straight, single-canal teeth) and the absence of suspected iatrogenic perforations did not justify performing a second-level radiologic examination according to the ESE guidelines, which recommend a CBCT examination when the additional information from reconstructed three-dimensional images can improve clinical management [28]. However, [PERSON] et al. [24] showed how CBCT could play a crucial role in identifying the presence of 13 apical fenestration in 13 cases: the use of CBCT after HA should show a higher presence of apical fenestration as the cause of HA, so reducing the role of density bone in the occurring of HA. In other words, it could be hypothesised that the lack of apex coated is at the origin of HA. This could justify the hypochlorite accident in the absence of conceptual and technical errors on the part of the operators (lack of preoperative radiographs, iatrogenic perforations, incorrect determination of the working length) [29]. When the apical foramen is in contact with the soft tissues of the mucosa, the apical resistance of the bone tissue is absent, so even small increases in pressure during irrigation, considered irrelevant by the operator, may be sufficient to cause a hypochlorite accident. Concerning the patients' clinical conditions, vital pulp and normal periapex were present in all reports. Although necrotic pulps and teeth associated with periapical lesions are risk factors in HAs [7], the presence of optimal preoperative conditions was not a sufficient element to prevent this kind of accident. Examined dental elements had straight root canals but different apical diameters (Report A: #60; Report B: #70; Report C: #35; Report D: #25). Furthermore, in the second case, the positive irrigation technique and open-ended needle, although calibrated, could have favoured the occurrence of the accident: the extrusion could have occurred through the distal lateral canal, present in the middle third and visible in the preoperative radiography (Figure 6). In the third and fourth reports, the operator did not feel blockage of the needle. Still, it is likely that the extrusion, as shown by the CBCT investigation, was facilitated by an apical fenestration (C) and a stripping and apical transportation (D). In the last reported three cases, the working length of the irrigation needle was not reported, and the accident occurred during the final washing when the shaping phase was completed. Therefore, we hypothesised that an operator's concentration reduction could have happened in the final steps of root canal treatment. About the timely management of HA, the accident was not immediately diagnosed in the first case. As reported by the patient, swelling and ecchymosis had arisen a few hours after the endodontic session. Since pain and swelling can worsen after a few minutes/hour [30], in the presence of symptoms such as pain and intracranial bleeding during canal treatment, it is judicious to exclude all possibility of doubt waiting a few minutes before discharging the patient; however, following the clinical course is highly recommended. All accidents reported in this study showed a low severity level: ecchymosis was limited to the face without significant neck involvement, and no permanent neurological and respiratory complications were reported, pain and swelling lasted for a short period, while signs of subcutaneous emphysema were not present. The complications described in the reports (ecchymosis, swelling and pain) belong to the first category of [PERSON]'s classification [31] and the second category of the classification illustrated by [PERSON] et al. [8]. Echymoses of the facial skin are a common sign of hypochlorite accidents; they may occur immediately or after a few hours and may be accompanied by tissue necrosis and sometimes by paresthesia. In cases of extensive ecchymosis, there should be damage to the blood vessels with extravasation of blood into the adjacent subcutaneous soft tissue [8].