Complete removal of the original filling materials in the root canal is essential for thorough root canal irrigation and disinfection during root canal retreatment. Previous studies have shown that many filling materials remain in the root canal after filling materials are removed from the root canal of incisors by nickel-titanium instruments [18, 19]. The additional use of organic solvents can soften gutta-percha, making it easier for the nickel-titanium instruments to be screwed into the filling materials, reducing the occurrence of instrument separation, dissolving the filling materials in irregular areas, and increasing the removal efficiency of the filling materials. However, their use increases the difficulty of root canal cleaning [20] and has certain carcinogenicity [21]. To the best of our knowledge, there is no report on the use of nickel-titanium rotary instruments combined with MNiTiH files during root canal retreatment. Therefore, the present study was designed to evaluate the efficiency of the strategy of gutta-percha removal by nickel-titanium rotary instruments combined with MNiTiH files.
The results showed that less volume and a smaller covered area of RFMs was found in the MNiTiH file groups compared to the chloroform groups. MNiTiH files are root canal retreatment instruments made of M-wire nickel-titanium alloy, which is characterized by a large taper and superelasticity. With a large taper, it can be screwed into the filling materials to have a chance to pull out the filling materials in large pieces. Even if the filling materials cannot be directly pulled out, the channel established step-by-step by screwing MNiTiH files into the gutta-percha can effectively prevent instrument separation when the PTN is used for repreparation in a crown-down manner. Traditional hand stainless steel Hedström files are too rigid and prone to instrument separation, especially in curved canals.
As mentioned before, chloroform is often used to soften gutta-percha to make it easier for instruments to enter filling materials. However, the effectiveness of chloroform in cleaning root canals has been suggested to be limited in previous studies [22, 23], which is consistent with our research. The total area covered by RFMs and the volume of RFMs at the middle third of the root canal in samples in the PTN-CL groups were significantly higher than those in samples in the PTN-MNiTiH file groups. This result may be explained by the fact that softened gutta-percha has more possibility to be squeezed into the irregular area of the root canal and covers the root canal surface in the form of the membrane [24] and thus increase the difficulties of root canal cleaning.
This study also found that there was no significant difference with respect to time to reach the working length and the time to complete root canal repreparation between samples in the PTN-MNiTiH file groups and the PTN-CL groups. However, the amount of extruded apical debris was reduced after the channel was established by MNiTiH files in samples in the different root canal morphology groups, which can effectively reduce the incidence of postoperative pain after root canal therapy [25, 26]. This finding was consistent with Pawar et al. [27], who found that spinning MNiTiH files into filling materials and pulling them out together reduced the possibility of extruding the debris out of the apical foreman.
Micro-CT is a noninvasive imaging technique with high resolution. This approach has been widely used in previous research on root canal retreatment, and the volume and its percentage of RFMs can be evaluated in three dimensions [5, 28]. Biofilm can adhere to RFMs and increase the difficulty in canal disinfection. Additionally, RFMs can cover the canal surface, affecting the efficacy of canal irrigation and intracanal medications on the canal wall. Therefore, the untreated area covered by RFMs has a more significant impact on root canal retreatment, increasing the risk of reinfection. At present, the commonly used method for measuring the untreated area covered by RFMs is the measurement after splitting the roots longitudinally to evaluate the canal walls microscopically or radiographs. Nevertheless, this method is destructive, not three-dimensional, and there may be a risk of underestimating root canal wall cleanliness. Currently, few studies have discussed the area of the root canal covered by RFMs. Theoretically, for RFMs with the same volume, the larger the covered root canal area is, the more dentin tubules will be blocked. Thus, the effect of root canal irrigation and medication will be compromised. In our study, another important finding was that the area covered by RFMs in the circular canal was smaller than that in the oval canal, which was in line with our expectations. Because the cross-section of root canal repreparation instruments was round and did not match the shape of oval root canals, RFMs in buccolingual areas of root canals could not be contacted. Therefore, more RFMs were found in these areas. The surface area of samples covered by RFMs in the PTN-MNiTiH file groups was less than that of samples in the PTN-CL groups. The observed difference may be attributed to the lack of solvents in the PTN-MNiTiH file groups. The filling materials did not form a sticky film covering the root canal surface, so the contact area was smaller.
In line with previous studies [5, 6], the results of this experiment also showed that no matter which method was used and no matter what root canal morphology was, the filling materials in the root canal could not be removed entirely, and most RFMs were located in the apical third [28, 29]. One possible reason for this result is that the anatomic variations are often most remarkable in the apical third; therefore, this finding underlines the debridement limitation of the currently available retreatment technology facing a complex root canal anatomy. These irregular areas cannot be contacted by the instrument, which highlights the limitation of the current instrument in that it cannot match complex root canal anatomical structures.
Samples in the round canal groups had less RFMs than samples in the oval canal groups, which is consistent with our expectations. This could be explained by the fact that PTN rotary files machine the root canal into a form with a round cross-section; therefore, substantial untouched areas may be left on the buccal and lingual sides of a flat root canal. It may be possible to further explore the removal efficiency of filling materials through some auxiliary methods, such as ultrasound instruments and apical enlargement.
Under the conditions of the present study, establishing a channel by screwing MNiTiH files into filling materials and pulling them out together may be a promising method for root canal retreatment, but further research remains to be carried out to confirm and validate these findings. Moreover, some additional measures can be used to further increase the removal effect on filling materials.