One of the main goals of root canal disinfection is to eliminate microorganisms [76]. However, it is feared that the solubility of residual medicaments in tissue fluids might facilitate bacterial proliferation, because remnants might prevent sealers adaption and leave voids in the filling dentine interface [7, 77]. Several studies have focused on improving the removal of intracanal medicaments during endodontic treatment process, but none of the activation regimens can render the root canals completely free of dressing material [13, 66, 78]. Numerous studies have suggested the superiority of XPF and PUI over other instruments on intracanal medicaments removal [30, 79]. However, a consensus on which approach is better remains to be reached.
Agreements and disagreements with other studies or reviews
In recent years, studies on the cleaning effects of instruments in the root canal systems attract more and more attention [80–82]. However, the experimental conditions were difficult to standardise in the complex curved canals, because the range of length, radius, curvature, and isthmus of canals could affect the experimental results [83]. The clinical trials that compared the cleaning capacities of PUI and XPF in the curved canals were scarce and not related to our research topic [58]. Merely four studies focused on the effectiveness of calcium hydroxide removal using PUI in the curved canals, and they did not compare it with XPF [78, 83–85]. Moreover, two trials researched the cleaning efficacy of XPF instrument, but they did not focus on the intracanal medicaments removal effects [86, 87]. So far, the intracanal medicaments removal efficacy of PUI and XPF has not been compared in curved root canals, hence all trials included in the meta-analysis were conducted only in the single straight root canals.
The present quantitative synthesis illustrated a better effectiveness of PUI than XPF on intracanal medicaments removal in single straight root canals, which can be explained by the following reasons. The file size, working mechanism, and tip insertion position of PUI could benefit its effects in single straight root canals. First, tip size of the PUI file is matched to the intracanal diameters, and the final preparation size of the physiological foramen. Before the process of root canal disinfection, the single straight root canals were usually prepared up to size 40/0.04, which reached a balance among cleaning effectiveness, anatomical enlargement, and the apical accident risk [33, 75, 80]. However, XPF is a size 25, and nontapered instrument, thereby the size of XPF might be less matched to the foramen size than PUI [29]. Second, the working mechanism of the PUI depends on higher velocity and irrigant stream, along with a thermal effect and the transmission of energy [88, 89]. It contributes to the cleaning effects on the large straight canal space by flowing in the apical-to-coronal direction [90, 91]. A study published by Sabins et al. found that a working time of 30 seconds was sufficient for the PUI to exert a higher cleaning effect than SNI [92]. However, in the large canal space, one minute was probably not sufficient for the XPF to remove intracanal medicaments efficiently [30, 75]. Meantime, Kfir et al. and Wigler et al. also worried that the contact time between the XPF file and the groove in straight root canals was too short [30, 75]. Third, the situation of the tip placement would impact the irrigation effects in endodontic treatment [93]. Uzunoglu et al. reported that PUI might decrease the amount of irrigant extruded through the apex [94]. In teeth with open apex, Peeters et al. also revealed that use of PUI during final irrigation procedures barely resulted in apical extrusion of NaOCl in endodontic therapy [95]. One explanation is that with the insertion depth of the ultrasonic tip becoming deeper, the amounts of debris and irrigants extrusion would also increase [96]. Therefore, from the standpoint of efficiency and safety, the use of PUI was better than XPF in intracanal medicaments removal in single straight root canals.
However, the cleaning effectiveness of PUI might be reduced in the complex curved root canals. Amato et al. compared the ultrasonic action efficiency between straight and curved root canals, founding that the dental debris removal efficacy of PUI could be decreased in curved root canals [97]. One possible explanation was that touching the curved root canal wall would reduce the action of ultrasonic inevitably [97]. Compared with the PUI files, XPF could expand more flexibly and have better fracture resistance, making it better adapt to the irregular anatomy of curved canals [87]. In detail, the XPF could change to a unique spoon shape and adapt three-dimensionally to the root canal morphology at body temperature [90]. Meanwhile, the file has good resistance to fatigue and high stress, which is of particular importance for irrigation in curved root canals. Vaz-Garcia et al. concluded that XPF instruments performed better when compared its cyclic fatigue with that of the other anatomic finishing file, XP-Clean instruments [98]. However, ultrasonic tips might fracture during the endodontic shaping process [99]. In addition, Song et al. suggested that pre-curved files removed calcium hydroxide more effectively than the none-pre-curved files in curved root canals [84]. However, during the PUI procedure, pre-curving the ultrasonic file to fully adapt to the curved root canals is challenging. The tip of the PUI file unable to fully extend into the apical position because straight-line access is difficult to build [100]. Therefore, when the tip of the XPF file showed a unique spoon-shape and flexibly extended into the apical complex region in the curved root canal, the effects of intracanal medicaments removal might be higher than using the PUI file. [99]. Further studies are encouraged to investigate the efficacy of instruments on intracanal remnants removal in the curved root canal regions.
In addition, as the subgroup analyses showed, in teeth with single straight root canals, anatomical areas, irrigant protocols, and intracanal medicament time might influence the overall intracanal medicaments removal effectiveness.
Consistent with some previous studies [73, 74], PUI were superior to XPF for removing medicaments from the apical third regions of single straight root canals. A systematic review published by Yaylali et al. showed that PUI was superior over SNI and EA for removing calcium hydroxide from the root canal apical third area [101]. It is more difficult for the irrigation techniques to fully contact with the canal wall because the apical third regions have more lateral canals, apical ramifications, and isthmus than the coronal and middle third regions[102]. In addition to the complex anatomical factors, the phenomenon of vapor lock also prevented the irrigant solutions to penetrate into apical third regions [103]. PUI was effective in eliminating vapor lock during endodontic irrigation in the apical third of the root canal region [104]. Donnermeyer et al. also reported that PUI was significantly better than XPF in the removal of medicaments from the apical region [27].
In addition, the meta-analysis suggested that PUI performed more effectively than XPF when NaOCl was used as the only flushing agent. NaOCl and EDTA have been the most commonly used irrigating solutions, with the function of dissolving organic substances, killing microbes, and cooling files [105, 106]. Lee et al. reported that the combined use of ultrasound and NaOCl led to synergistic effects on reducing bacteria on steel and iceberg lettuce [107]. Therefore, using NaOCl as the irrigant might enhance the working efficacy of PUI. However, when NaOCl and EDTA were used in combination, the cleaning efficacy of the PUI and XPF was similar. Azimian et al. assessed the efficacy of XPF on the removal of smear layer and residual debris [108]. They suggested that the synergistic effect of XPF and EDTA was beneficial for better root canal cleaning effects [108]. With the assistance of EDTA, XPF could achieve similar cleaning effectiveness with PUI on the removal of intracanal medicaments. In addition, the concentrations of irrigants and operation temperature might also influence the cleaning effectiveness, but the specific mechanisms are still unclear [109].
Strengths and limitations
The present systematic review and meta-analysis had several advantages. First, two authors searched key databases independently using an adequate searching strategy. It provided a precise screen range and improved the possibility of generalising the outcomes [110]. Second, the detailed inclusion and exclusion criteria were defined beforehand. We focused on the process of intracanal medicaments removal and excluded articles comparing the efficacy of removing other substances in endodontic treatment, such as smear layer, bacteria, and organic tissue. It should be noted that these listed substances were cleaned in different periods of endodontic therapy. The files in different preparation or disinfection process might play different key roles [90]. Third, only RCTs and CCTs, which are regarded as the highest level of evidence, were included in our quantitative analysis [111]. Moreover, prior protocol registration and subgroup analysis were also the strengths of the current meta-analysis [112, 113]. Different root canal area, irrigation protocols, or intracanal medicament periods are potential factors affecting the heterogeneity. To our knowledge, the meta-analysis is the first study to quantitatively compare the effectiveness of XPF with that of PUI techniques on intracanal medicaments removal.
Several limitations exist in the present meta-analysis. First, language was limited to English during the literature screening process, and this might increase the possibility of reporting bias. When compared to non-English-language journals, English-language journals are more likely to publish positive results [91]. Second, because the case number of some studies was small, the statistical power might be reduced. Third, in most of the included trials, the description of RCT was too simple, and it would influence the quality of studies. Accordingly, improving the quality of related RCTs is advocated.