Less extrusion debris during the retreatment of curved canals using twisted files with higher rotational speeds: an ex vivo study

The aim of this study was to evaluate the use of TF for endodontic retreatment in terms of prevention of extrusion debris and in cases of severely curved mesiobuccal root canals. To do so, three different speeds of rotation were compared when using the TF and manual treatment of curved root canals. The results showed that each of the methods used produced extrusion debris, but that TF at 1500 rpm produced the smallest amount.

Reducing extrusion debris is important for effective endodontic retreatment as it reduces the likelihood of inflammation and postoperative pain. These results are consistent with previous retreatment studies that used various nickel titanium rotary instruments compared with conventional instruments, and demonstrated that rotary instruments produced less debris [12]. This is due, at least in part, to the up-and-down motion of the manual files, which is more likely to push debris toward the apical end [12].

Previous studies on retreatments have been performed on relatively straight root canals [11–13], because their low curvatures eliminated complications likely to arise in the instrumentation of severely curved root canals. However, it can be argued that the filling materials in straight root canals are more easily removed so that canal repreparation tends to extrude less debris, and this may not accurately represent the challenge of retreating curved canals. On the contrary, teeth with high degrees of curvature may give rise to different results [18]. Therefore, studying these devices in straight root canals only does not fully reflect the differences among various technologies. It has been shown that in mandibular first molars’ mesiobuccal root canals the mean curvature is 25° and the mean radius is 10.6 mm [15]. Thus, the present study investigated mesiobuccal root canals with severe curvature angles of 25–35° and a short radius of 10 mm in an attempt to simulate these challenging clinical conditions. To the best of the authors’ knowledge, there have been no peer-reviewed studies in which molars with severely curved roots were used to assess the amount of dentin debris extruded during retreatment.

In previous studies, the continuous wave of condensation technique [19] has been rarely used in curved canals. We used this method before retreatment to ensure filling and to avoid filling defects, to make that the results of the investigation are more reliable. The technique has become increasingly popular because it uses heat to produce a homogenous obturation that adapts well to the canal walls and replicates the prepared root-canal space [20, 21].

Unlike previous studies, a major factor in the present study was the use of different rotational speeds. High rotational speed not only increases efficiency but also probably produces more heat [22]. This may be because rotary reinstrumentation generates frictional heat during its contact with gutta-percha and dentin. In addition, rotational speed is associated with heat generation during the formation of martensite [23]. The resulting heat could plasticize the gutta-percha, making it into a single block, thus facilitating its removal and minimizing extruded debris [24]. In addition, plasticized gutta-percha cannot pass through the apical foramen [24], which could have contributed to the differences observed at high rotational speed. Furthermore, the Archimedes screw effect due to higher rotational speed should be taken into account [25]. On the other hand, the exact same methods for coronal introduction of gutta-percha was used among all groups, and eventual differences in pressure should be counterbalanced by the standard method and randomization of the samples. Nevertheless, the plasticization of gutta-percha due to heat during retreatment could be associated with thermal damage to the teeth [26], but heat damage was not examined in the present study. Additional studies are necessary to determine if heat due to rotational speed during retreatment is associated with significant heat damage.

Although the current ground-fluted rotary instruments that are commonly used have excellent performance, there are inevitably many drawbacks due to their design. They have difficulties negotiating a severe curvature, which may increase the risk of iatrogenic damage such as bleeding and perforation of the lateral wall of the root canal. Furthermore, they must be operated in strict accordance with the speed and torque instructions, otherwise there is a risk of unexpected instrument separation, especially in curved root canals [27]. Since most devices have similar low rotational speeds, and the difference is very small, comparisons between device speeds are almost meaningless. Rotational devices that can be used effectively in curved root canals and at higher speeds could improve retreatment.

TF, which are manufactured by twisting instead of grinding the nickel titanium alloy, have been shown to have excellent performance [8]. TF with R-phase technology gives a much higher level of flexibility and can perform side-cutting with great efficiency, while still successfully negotiating a complex curvature. Above all, TF can work at speeds up to 1500 rpm, underlining that using higher rotational speed will lead to less debris. The manufacturer suggests that using a speed of 500 rpm is likely to reduce waste and increase the service life of the instrument, but our experience of using it between 900 and 1500 rpm is also positive and more efficient. We therefore tested TF at three different speeds 500, 1000, and 1500 rpm to assess whether speed would influence the amount of apical debris. The results showed that TF at the highest rotational speed produced the least debris. The results of the experiment were consistent with our inference of the benefits of increased speed and temperature. Nevertheless, other instruments use different rotational speeds and additional studies are necessary to assess these instruments.

This study has some limitations. The clinical relevance of this ex vivo study should be interpreted with caution. In vivo, the apex would be surrounded by periradicular tissues that may serve as a natural barrier preventing debris extrusion. In an early (1977) in vivo study, Salzgeber and Brilliant [28] showed that vital tissues helped control the apical and lateral penetration of an irrigating solution. In addition to the apical control of extruded debris, other factors involved in retreatment require further evaluation. For instance, the heat produced by high speed rotation may be transmitted to the outer root. It has been shown that a 10°C rise may cause irreversible thermal damage to the supporting periodontal structures [29]. Whether the heat produced in this investigation could cause damage to the surrounding tissue needs further evaluation. Finally, apart the power tool nature of TF, there was intrinsic differences between the TF and the manual instrument, such as the cross section (round for TF and triangular for manual files), alloy, and the number of files used. Nevertheless, groups A, B, and C used the same instrument. Additional research will be necessary to evaluate the cleaning efficiency of the TF at higher speeds and to determine the optimal rotational speeds (and torque values) for the removal of gutta-percha.