## 4 Discussion
This procedure may not fully replicate the typical clinical scenario where intact coronal structures present additional challenges for the removal of remnants. The existing literature shows contrasting data regarding the amount of remnants measured through CBCT and Micro-CT analysis in canals filled with bioceramic sealers compared to epoxy resin-based sealers [11; 29; 30]. The quantity of remnants identified in this study shows significant differences depending on the type of radiographic analysis performed: two-dimensional examinations on periapical radiographs show no significant differences between the two groups, while the three-dimensional analysis on the CBCT images indicates some differences. The two-dimensional analysis indicates a slightly higher quantity of remnants in the GF group (30%) compared to in the CS group (20%) after RB40 instrumentation. These findings can be justified by the location of the remnants after retreatment. Periapical radiographs taken in the buccolingual direction may introduce bias in evaluating the amount of remnants present in the canal: there may be an underestimation when the remnants are disposed axially relative to the canal or an overestimation when the remnants remain longitudinal relative to the canal. Interestingly, the middle third of the canal of the GF group appeared as the section most influenced by the presence of remnants after retreatment with RB40. The instrumentation with RB50 led to greater remnant removal in the GF group compared to in the CS group. The nonsignificant but higher amount of remnants in the middle third of the canals obturated with the GF warm technique can be justified by the technique used in relation to the anatomy of the root. The use of a warm technique in an oval canal may lead to the spreading of gutta-percha in the oval sections of the canal, which are more difficult to remove during retreatment procedures. In contrast, remnant removal in the coronal third is facilitated by the use of Gates Glidden burs and the more accessible anatomy, and the apical third physiologically narrows, tending to reduce its oval section. The cold SC technique did not show a greater accumulation of remnants in the middle third. In this technique, remnants were equally distributed in the apical and middle thirds, possibly due to the cold nature of the technique, which reduces the spreading of gutta-percha. As a consequence, the percentage of remaining filling material was lower in the GF group when compared to the CS group after both R40 and R50 instrumentation phases (Table 6). On the other hand, the CBCT analysis showed that the GF group had a lower amount of remnant volumes after the use of RB40, with a markedly greater remnant reduction after the use of RB50. The CS group shows a reduction in the remnants between RB40 and RB50 of approximately 20%, while the GF group shows a reduction of approximately 50%. This trend can be explained by the different techniques used. Specifically, the single-cone technique requires more sealer compared to the carrier-based technique. The use of a greater amount of sealer during the retreatment procedures can make it more challenging to remove, which may account for the presence of a larger quantity of remnants. These data are confirmed by another in vitro study (conducted using SEM) that used the same sealer (NeoSealer Flo) but different instrumentation techniques (single-cone technique vs. the warm vertical compaction technique). Indeed, the study reported a greater amount of remnants with the single-cone technique, in agreement with our data [31]. The correlation analysis reported in Table 7 evidenced a significant positive association of the measured remnant areas and volumes in the SC group but a lower association in the GF group. This aspect deserves some consideration. A linear positive correlation should be expected when considering regular (conical) root canals.