Several methods have been described in the literature for the removal of the old filling material. However, the variations between measurement procedures used and the different scenarios, make the standardization of such analysis challenging [16]. Therefore, most studies evaluate the amount of remaining gutta-percha [5,10,17,18] and do not take into account the time needed for retreatment, which is very important since these are long and tedious procedures in which patient and operator fatigue play a key role in the final result.
In addition, several articles that evaluate retreatment time do not provide sample size calculation or statistical power [3,6,7,19,20]; between 2018 and 2020, 33.9% of articles published in 3 endodontic journals missed a priori sample size calculation and determination of study power, therefore, this is a prevalent problem in the endodontic literature [21]. It is essential to summarize the information on the different techniques for retreatment, as well as the methodologies used to see which are more relevant and can be translated to daily clinical practice [16].
The results of this study showed that retreatment of plastic carrier obturators using the new exposed technique is faster than the conventional technique.
In a study in which retreatment with plastic core obturators using the XP-Endo Retreatment System (FKG, Dentaire, La-Chaux-de-Fonds, Switzerland) was evaluated, a mean working time of 5.92 minutes was obtained in decoronated teeth with straight canals, which facilitate retreatment and do not reproduce in vivo conditions [12]. Baratto Filho et al. carried out retreatment in a mean time of 5.24 minutes with the 0.04 ProFile system (DentsplyMaillefer), again in straight canals and without checking the quality of the seal after the initial obturation [22]. In a posterior study, the same system was used at 1500 rpm, obtaining an average time of 1.28 minutes, less than the technique proposed in this article; however, the security of the method was not analyzed in detail despite the fact that instrument fracture was observed in some specimens, and it was not specified if the retreatment time included the changes of instruments and irrigation [20]. On the other hand, two studies published in 2018 and 2020, presented methods in which NiTi rotary instruments were used obtaining times of 175 and 112 seconds respectively, however they stated that the time for irrigation and instrument changes had not been included, so these results do not reproduce the clinical conditions [3,23]. In neither of the aforementioned studies complete removal of the old filling material was achieved [3,12,20,22,23].
The new technique offered shorter retreatment times, in which irrigation between instruments and instrument changes were taken into account, and therefore the safety of the method was evaluated.
It was suggested that success in retreatment of carrier-based obturators is primarily determined by the ability to completely remove the carrier [4]. With the Direct technique (in which an instrument was introduced directly on the canal), there is an increased risk for plastic carrier fracture due to its low modulus of elasticity, complicating its subsequent retrieval and the ability to reach the apical constriction [3,7]; in addition, the incidence of instrument fracture and other complications such as perforations, apical transportation or ledge formation grows due to the increased torsional stress and cyclic fatigue that the plastic carrier exerts on the instruments, as well as the progressive narrowing of the canal, which results in a thin layer of gutta-percha between the wall and the core [2,3,7,20].
The fracture of the plastic core during retreatment was higher in Group 0, increasing the difficulty of removal of the old filling material and consequently the retreatment time. In contrast, there were no statistically significant differences between the two groups concerning instrument fracture, which could also have been affected by the fact that each instrument employed was discarded after one use; however, two instruments fractured in Group 0, whereas no fractures occurred in Group 1. Both instruments fractured in samples where the plastic carrier had previously broken.
On the other hand, apical transportation and ledge formation were also statistically higher in Group 0, and also occurred in samples where complete extraction of the plastic carrier was not achieved.
The presence of remaining material was not statistically significant between both groups.
Often, complete three-dimensional elimination of the old filling material is not possible [24,25], especially in presence of complex anatomies, and therefore meticulous disinfection and sealing of the prepared canal is essential to obtain optimal results with non-surgical retreatment [26,27]. In this study, lower premolars non decoronated, with curvature were selected because the greater difficulty of retreatment tested the efficacy and safety of the techniques analyzed, due to the less instrument contact with the surfaces of the canal and the increased stress they are subjected to [25,28–30].
A systematic review concluded that the different techniques used, either with hand files or with rotary instruments in continuous or reciprocating rotation, designed for endodontic retreatment or not, obtained similar results in terms of gutta-percha removal [24]. This was corroborated by other studies [9,10,18,30]. However, the use of hand files was associated with longer retreatment times compared to rotary instruments [24,30], whilst the number of perioperative complications were more frequent with the use of the last ones [24]. With the indirect technique, the prior withdrawal of the plastic core minimizes the perioperative complications that can occur with the use of rotary instruments and allows taking advantage of its speed, flexibility and efficiency [31].
Several methods have been described for the assessment of remaining material [16]. Conventional radiographs have been used for this purpose; however, their two-dimensional character and the possible distortions mean that they are not the most suitable imaging technique for this aim [16,32]. One solution to avoid distortions is to use the same parameters and to standardize the distance from the X-ray to the sensor [32]. Computed microtomography (micro-CT) or cone beam computed tomography (CBCT) provide accurate three-dimensional images [16,30], being the first one the most widely used today to quantify this variable [10,12,17,18]. However, micro-CT requires a long analysis time, is expensive and cannot be used in the clinic [34]. CBCT does allow its use in patients, however, its higher radiation dose compared to conventional periapical radiographs is not justified to evaluate this situation [34,35].
Another technique used to evaluate the remaining material is direct visualization, where the tooth is usually split longitudinally, thus loss of residual material and damage to the canal walls can occur [16,30], which could affect some of the variables analyzed in the study.
Periapical radiographs taken at two different angles, followed by longitudinal division of the tooth and binary (Yes/No) measurement for remaining of filling material were used in this study, being one of its major limitations. However, evaluate remaining material is not the primary aim of this study, as it has been shown previously that it is poorly affected by the retreatment techniques employed. Therefore, from a clinical point of view, conventional radiograph is the most common method and in this study, it was complemented with direct observation, minimizing the disadvantages of both methods. Although there were no statistically significant differences in terms of remaining material between the two techniques, this was greater in Group 0, and this is probably not due to the technique itself, but to the greater number of perioperative complications that occurred and that facilitate the retention of old filling material.