Anatomical complexities and high variabilities in C-shaped canal of the mandibular first premolar increase the difficulty in evaluating the instrumentation and obturation during the root canal treatment. The 64 mandibular first premolars were pre-screened before preparation using micro-CT. Based on root canal volume, surface area, and 3D C-shaped canal classification, the specimens were paired and randomly allocated into the PN and WG instrumentation groups to ensure a good uniform distribution of samples between the two groups and to reduce the risk of bias due to anatomical structures. There was no significant difference in the canal volume and surface area before and after preparation in the two instrumentation groups (p > 0.05). After the preparation, the teeth were divided into two obturation subgroups (n = 16) in each experimental group. Statistical analysis confirmed the homogeneity of baseline parameters between the groups and improved the internal validity of this study [42, 43].
The increase of canal surface area and volume, as well as the amount of removed dentine in the two experimental groups has no significant difference in present study. The X2 file in the PN system created a .06 taper in the apical 3 mm and ISO #25 diameter, while the Primary file in the WG system created a larger taper (.07 taper) and an equal diameter (0.25 mm) during instrumentation of the canal space. The tiny difference of taper in file design is not enough to cause the significantly different changes in canal volume and surface area, as well as dentine removal in the present study.
Previous studies on mandibular second molars have shown that the wall of the C-shaped root canal was thinner on the side near the radicular groove than on the remaining sides [44–46]. Gu et al. [47] measured the canal wall thickness at various levels in C-shaped root canals of mandibular first premolar teeth and confirmed that the above findings were also applicable in mandibular first premolar teeth.
The canal in mandibular first premolar characteristically bifurcates in the apical and middle regions to form C-shaped root canal structures [4]. The irregular morphology of C-shaped root canals is more likely to result in strip perforation during root canal preparation [36, 48]. Our previous study revealed that the further the distance from the cemento-enamel junction, the deeper root radicular groove, the smaller the radicular groove angle, and the thinner the mesial wall. As a result, mechanical preparation of the root canal, particularly the lingual root canal and the lower segment, should be executed carefully to prevent unexpected deviation and perforation.
In this study, the thickness of the mesial wall in the cross-section with C-shaped canal was measured before and after instrumentation. After cut by the two systems, the remain MT was less than 0.5 mm in 65.63% of the samples. The results supported the above-mentioned statements. Although the C-shaped morphology may occur in various levels along the root canal of mandibular first premolars, the distribution of sample size in each level is uneven. It also has been demonstrated in present study that C-shaped morphology more commonly occurred in the middle regions of the canal. Based on the fact, the degree of MT thinning between the two systems was compared in the M + 1, M, M-1, and AM levels. There was no significant difference between the two files on the degree of MT thinning, which might be accounted for the similar cross section design of the two instruments. It has been suggested that the isthmus of C-shaped canals should not be instrumented by files larger than ISO #25 to avoid perforating the canals [49]. In the present study, no perforation was observed using either X2 or Primary file in two experimental groups. However, the extreme value of MT after instrumentation was 0.14mm, which carried an extreme risk of perforation during the canal cleaning.
The two experimental groups left a similar percentage of untouched surface area in the C-shaped canals of mandibular first premolars. In the PN and WG group, the untouched surface area was 18.75% and 22.69%, respectively. The results are less than those reported in previous studies about the instrumentation of the C-shaped canals in mandibular molars with different NiTi systems [50–53]. Although the two type teeth all possessed C-shaped canal systems, quantitative divergence between the previously-reported data and the present data may be attributed to the difference in root canal volume of different tooth types. The C-shaped canal in mandibular first premolar is relatively narrower and smaller, and therefore larger proportion canal wall would be treated by the instruments during the shaping process.
Despite the innovations in designs, metallurgies, kinematics and thermal treatments, neither of the two NiTi systems were capable of completely touching the root canal walls. There were no significant differences between the mechanized preparation systems regarding the percentages of US. Several previous studies reported that untreated surface varied from 19.9–41.5% in curved mesial root canals of mandibular molars and 55.3% in distal root canals of mandibular molars after using Protaper Next [54–56]. Moreover, two recent studies on Waveone Gold shaping in oval-shaped canals reported that the percentage of unprepared area was 18.9–50.9% [56, 57]. Although it has been reported that greater taper files were able to touch more canal surface [58], the statement was not confirmed in the present study.
The percentage of US remained in the apical regions of the canal space was 35.25% in PN group and 64.34% in WG group, respectively. The values of US in apical region were higher than those in coronal region in the either groups. The similar results regarding the instrumentation of C-shaped canal in mandibular molars were reported by Zhao et al. [50]. High percentages of US in the apical regions may be attributed to the complexity of apical anatomy of C-shaped canals. Factors such as the severe curved canal, the existence of accessory canals, lateral canals, intercanal communications and apical deltas [59–61] are likely to hamper optimal debridement of the apical region in these complicated canal systems. These findings also support the viewpoint that the irrigation and intracanal medicament plays a key role in chemo-mechanical preparation to compensate for the deficiencies of mechanical debridement [58, 62].
Hard tissue debris, as an undesirable by-product of dentin removal during mechanical preparation [63], was usually packed into the irregular regions of the root canal system. Some studies have found that accumulated debris in canal would compromise the effectiveness of canal irrigation or medication [64] and also block the flow of filling material [65]. Moreover, the debris contains bacteria and serves as a nidus for root canal re-infection. Paqué et al. [64] has established a method to qualitatively and quantitatively analyze the remained dentine debris in root canal system during instrumentation.
In the present study, AHTD was identified in all specimens irrespective of the file system employed. There was no significant difference on AHTD in the total canal using two shaping techniques. However, the PN system produced a relatively lower percentage of AHTD in the apical regions of the canal space, when compared to the WG system. The result may be explained by the differences in movement kinematics and the tapers of the respective file systems.
The kinematics of a NiTi file system may be an influential factor in debris removal. With respect to this issue, data published in the literature are contradictory. Whereas reciprocating systems were reported to produce more debris accumulation than rotary systems in the mesial root canals of mandibular molars [39] and in the C-shaped canals of mandibular molars [50], this difference could not be validated in another study that used a similar experimental design [66]. The continuous forward motion of the rotary file enables constant exit of debris up the flute of the file, while reciprocating motion might push debris into recesses and isthmus areas [67].The PN rotary file had an off-centered rectangular cross-section with progressive and decreasing percent taper design, superior strength and novel asymmetric rotational motion [68, 69] for maximum debris removal [13]. These PN design features provide a wider chip space and a smaller tip taper (X1: size 17/.04 taper and X2: size 25/.06 taper), and when accompanied by rotary movement, improve debris removal from the canal system. The debris that remained in C-shaped canals of the mandibular premolars shaped by the WG system was less in the present study than the previous studies [39, 66]. The difference may be attributed to different tooth type and irrigation protocols.
Results on AHTD distribution in isolated regions were similar in the two groups. Data from the present study were consistent with those published in the literature by Zhao et al [50] who compared reciprocating and rotary techniques on a C-shaped canal in mandibular molars. The vol% was highest in apical region and lowest in coronal region. The complexity of apical anatomy in C-shaped canals and the difficulty for root canal irrigants in reaching the apical region [70] could lead to the aforementioned result.
According to the available findings and the aforementioned results in the present study, no instrumentation protocol has been able to render the complex areas in C-shaped canal free of debris [50, 66, 67].Therefore, a three-dimensional sealing of root canal is of critical importance for successful endodontic treatment when accumulated debris is present within the irregular areas.
Several methodologies are applied to evaluate the quality of root canal filling, including: two-dimensional radiographs [71], dye or alternative tracer leakage models [7], the stereomicroscopic evaluation of root cross-sections [21, 25], and micro-CT scanning [72, 73].The dye penetration cannot adequately simulate true clinical conditions. The air entrapment in voids along the root canal filling will hinder the fluid dye infiltration [74]. The loss of material during sectioning and discontinuous cross sections may affect the accurate evaluation on the percentage of void [11]. Reconstructing 3D model by micro-CT scanning and imaging allows accurate assessment of canal filling outcomes. Micro-CT has the potential to differentiate filling materials, voids, and tooth structures [75]. The methodology provides a clear information about the distribution of the root filling materials, the location and volumetric measurements of internal voids along the entire root canal system.
The majority of published papers on the filling quality of C-shaped canal have focused on studying mandibular molars. Due to the diversity of research methods, sample selection, and filling methods, the results on the effectiveness of C-shaped root canal filling are not completely consistent. Previous studies have demonstrated that the filling quality of C-shaped canals was unsatisfactory with the cold lateral compaction technique, the MicroSeal system, the thermos-plasticized gutta-percha obturation, and the core-carrier technique. For the significant difficulties in sample collection, the present study only evaluated the quality of obturation on the volume percentage of void after the use of SC and CWC in the C-shaped canals of mandibular first premolars.
The present results showed that more voids were observed in the apical 1-3mm regions in all subgroups. The present and previous studies all indicated that the apical third of C-shaped canals was filled less completely using the cold lateral compaction, the MicroSeal, the core carrier, the CWC, and the SC technique [22, 24, 25]. The poor filling quality of the apical part could be caused by the irregular anatomy of the C-shaped canal. The divergent areas in the apical region of C-shaped canals are commonly unshaped, which hinders the obturating material, including gutta-percha and sealer, from flowing into the abnormalities. Although softened warm gutta-percha has excellent adaptation in the coronal two-thirds of the canal, the insufficient extension is still the main reason of the unsatisfactory apical filling [76]. Moreover, the dentine plug in the canals also serves as a barrier for the high quality apical obturation [77].
The CW technique can provide better filling of canal irregularities and lateral canals [23], especially in the coronal regions [34]. The findings were comparable to the present study. The results may be accounted for the heated gutta-percha adapts more easily to the irregularities of the root canal during the backfilling procedure [78]. Moreover, the coronal gutta-percha receives forces more directly, optimizing filling material adaptation in this region.
iRoot SP has the superior flowability and the ability to slightly expand during setting [79]. As a bioceramic material, the sealer can produce hydroxyapatite, which precipitate within the calcium silicate hydrate phase and reinforces a bond between the dentinal wall and the sealer [80]. In addition, iRoot SP displayed potent antibacterial effect against E. faecalis [81], which might be a combination of high pH, hydrophilicity, and active calcium hydroxide release. The above-mentioned characteristics make iRoot SP a good choice as the sealer to be used in a SC technique. Regarding to the obturation quality of SC in various types canal, the studies have shown mixed results. Inan et al. [82] found the apical sealing ability of SC was comparable with that of lateral condensation and Thermafil techniques in lower premolars. Holmes et al. [26] found that the filling quality of the SC was inferior to that of cold lateral technique and superior to that of core carrier technique in the C-shaped canals. In the present study, the SC with iRoot SP sealer did not produce superior filling quality than the CWC with `AH plus sealer in the C-shaped canal of the mandibular first premolars. Other studies also suggested that the SC technique yielded similar obturation quality when compared with the continuous wave compaction technique in the curved mesial canals of the mandibular molars and the canals of the maxillary left central incisor 3D–printed replicas [26, 34, 83].
Aithough many studies has shown that the technique has questionable sealing ability, the simplicity is the main advantage of the SC which makes the faster canal obturation possible. Moreover, in a retrospective survey about the clinical outcome of SC technique with Bioceramic Sealer, the success rate of the initial treatment and retreatment was up to 90.9%. Therefore, the SC with iRoot SP sealer is still a viable option for the C-shaped canal obturation in mandibular first premolars.