Apically extruded debris, canal transportation, and shaping ability of nickel-titanium instruments on contracted endodontic cavities in molar teeth

Abstract

Purpose: Apically extruded debris, canal transportation and shaping ability were compared between contracted endodontic cavities (CECs) and traditional endodontic cavities (TECs) after instrumentation with XP-endo Shaper (XPS), ProTaper Gold (PTG), ProTaper for hand-use (HPT) and Hero Shaper.

Methods: The CECs or TECs groups were sub-divided into 24 groups according to root canal morphology and nickel-titanium (Ni-Ti) instruments. The weight of apically extruded debris was calculated using the Myers and Montgomery model. Pre- and postoperative images of teeth were scanned using micro-CT and the three-dimensional models were constructed and compared.

Results: Under CECs or TECs, XPS and PTG produced less apical debris and formed less canal transportation than HPT and Hero Shaper (P < 0.05). XPS group under CECs extruded less apical debris than that under TCEs for round canals with curvature of 20°-35° (P < 0.05). The centering ratios of four tested instruments were higher under TECs than those under CECs (P < 0.05). The HPT and Hero Shaper had more transportation under CECs than that under TCEs (P < 0.05). No statistical difference was found regarding shaping ability among all the groups.

Conclusion: Under CECs, XPS preserves the original root canal anatomy, meanwhile it produces less apical debris than the other instruments.

Introduction

Root canal therapy (RCT) is the most commonly used and effective method for treating pulp and periapical disease [1,2]. It consists of four stages: access cavity preparation, root canal preparation, root canal disinfection, and root canal obturation. Access cavity preparation is considered as a fundamental step in RCT, which is helpful for exposing the dental pulp cavity, clearing pathogenic microbes and infected pulp, establishing access to the root canal orifices, exploring the root canal, and forming the root canal preparation pathway.

Contracted endodontic cavities (CECs) and traditional endodontic cavities (TECs) are two possible choices for the access cavity in RCT. For TECs, with complete removal of the pulp chamber roof, it is possible to obtain nearly straight-line access to the apical foramen and maximum maintain initial canal curvature, reducing the difficulty of RCT [3,4,5]. The TECs cut an excessive amount of the hard tissue of the crown, which leaves the tooth structure biomechanically impaired and causes it to become brittle. For CECs, inspired by the concept of minimally invasive dentistry, it foregoes complete unroofing of the pulp chamber and emphasizes tooth structure preservation, including pericervical dentin that is important for the ferrule and improves fracture resistance [6,7,8,9,10]. However, it may increase the difficulty of root operating and filling due to the small operating space, insufficient visible light, and may also cause apical transportation, as well as ledge and instrument fracture [6]. Compared with TECs, CECs preserves a greater amount of dentin thickness, and some reports in the literature indicated that the CECs might improve flexural strength and reduce the risk of fracture resistance of the teeth with the RCT [11,12]. However, the effects of CECs on fracture resistance, cleaning microorganisms and maintaining root canal morphology of teeth after RCT are still controversial and further research is required to verify them.

With technical progress, advances in nickel-titanium (Ni-Ti) instruments might provide a chance to avoid the shortcomings of CECs [6]. Some advanced file systems are known as the XP-endo Shaper (XPS, FKG Dentaire SA, La Chex-de-Fonds, Switzerland) and ProTaper Gold (PTG, Dentsply Maillefer, Ballaigues, Switzerland). The XPS made by MaxWire alloy converts from martensitic phase at room temperature to austenitic phase at body temperature for the resistance to cycle fatigue and flexibility [13]. PTG maintains the geometric design and convex triangular cross-sectional shape of the ProTaper Universal rotary instrument systems and adds the advantage of the improved properties of gold wire for increased flexibility [14,15]. The ProTaper for hand-use (HPT, Dentsply Maillefer) was chosen here because it has a similar convex triangular cross-sectional design to PTG, while it retains the advantages of hand-use instruments [16]. Hero Shaper (Micro-Mega, Besancon, France) has a variable helical angle and an adapted pitch that increases with the taper to avoid the screwing effect of the instrument [17], while it was less flexible than the other three Ni-Ti instruments used in this study.

Advanced Ni-Ti instruments provide a better choice for endodontists to achieve the perfect preparation of root canal and meanwhile preserve the tooth structure under CECs. How to choose an appropriate Ni-Ti instrument for root canal preparation is still a critical problem under CECs. This study compared apically extruded debris, canal transportation and shaping ability of XPS, PTG, HPT and Hero Shaper instruments on different root canals under CECs and TECs. However, there have been no previous studies on this comparison. It was hypothesized that CECs could be used for endodontic treatment after choosing the appropriate Ni-Ti instrument. In contemporary endodontics, this article provides the clinical reasoning for root canal preparation under CECs and the possible benefits.

Materials and Methods

Tooth specimen selection and groups

After the research protocol was approved by the Ethics Committee of Affiliated Stomatological Hospital of Nanjing Medical University (PJ2018-026-001), 110 extracted human noncarious, mature, intact, mandibular molars were stored in a 0.9% saline solution at 4ºC until used. Teeth were examined under an operating microscope (Zeiss, Munich, Germany), and teeth with preexisting cracks, caries, and other decay were excluded from the samples.

To obtain an outline morphology of the original root canals, the specimens were scanned before instrumentation under a micro-computed tomographic (micro-CT) device (vivaCT80, SCANCO Medical AG, Bruttisellen, Switzerland) at 15.6 μm voxel size, with 250 ms integration time, 70 kVp energy, and 114 μA intensity. Preoperative three-dimensional (3D) morphometry models of the root canal systems were constructed (CTVol v.2.2.1, Bruker microCT, Bruker, Billerica, MA, USA) for qualitative evaluation of the canal configuration. 3D measurements (volume and surface area) based on a surface rendered volume model of the root canal in the 3D space extended from the cementoenamel junction level on the buccal aspect of the root to the apex [18]. The normality was determined using the Shapiro-Wilk test, and the degree of homogeneity was evaluated using one-way analysis of variance (ANOVA) (5% level of significance). After the preoperative 3D models of the root canals were obtained, the root canals were divided into three groups according to different root canal morphology as follows, round canals with curvature less than 15°, round canals with curvature of 20°-35°, and oval-shaped root canals [19,20]. Each group was further divided into two subgroups according to endodontic cavities of CECs and TECs. Then root canals in each subgroup were randomly divided into 24 groups treated with XPS, PTG, HPT and Hero Shaper (n = 10).

Root canal preparation

CECs or TECs were prepared in the teeth of each group respectively, and then the canals were accessed. The working length (WL) was determined [21] and the glide path was established with a #15 K-file (FKG Dentaire SA) to the WL. Subsequently, in the XPS group, the tip was inserted into the canal and the instrument was activated in the X-Smart motor (Dentsply Maillefer; 800 rpm and 1.0 Ncm), applying the 3 to 5 up-and-down movements. After reaching the WL, five more up-and-down movements were applied over the entire WL (tip size 30, 0.04 taper) [18]. In the PTG or HPT group, the SX and S1 file first shaped the coronal two-thirds of the canal, followed by S2, F1 and F2 (tip size 25, 0.08 taper) to WL. PTG enlarged the root canals up to the WL using a continuous rotary movement powered by the X-Smart motor (Dentsply Maillefer; 300 rpm and 2.0 Ncm) [15], while HPT was hand used for shaping the root canal to WL [22] for a comparison between motor use and hand use under the CECs. In the Hero Shaper group, the instruments progressed the root canals to the WL from size 15 to 30 (tip size 30, 0.04 taper) in sequence with an X-Smart motor (Dentsply Maillefer; 300 rpm and 2.0 Ncm) [23]. For all groups, canals were irrigated canals with 3 mL 17% EDTA (5 min), 3 mL 2.5% sodium hypochlorite (5 min) and 2 mL distilled water (1 min) and dried with paper points. After the root canal preparation, the specimens were scanned again under a micro-CT using the same parameter settings as above to obtain the post-preparation images.

Evaluation of apically extruded debris

Based on the Myers and Montgomery model [24], the weight of the Eppendorf tubes was measured seven times with an electronic scale of 10⁻⁵ g (Sartorius, Gottingen, Germany). After removing the maximum and the minimum values of tube weight, the mean weight value of the remaining tube was taken. Then the teeth were fixed by the cover of Eppendorf tubes, and a 27-G needle was inserted to balance the air pressure inside and outside the tube. The tube was fitted into a vial with a gingival barrier, and a rubber dam was used to isolate the crown and the apparatus.

After completion of the canal instrumentation, the Eppendorf tube was removed from the vial. The exterior surface of the root apex was washed using 1 mL distilled water to collect the debris in the tube. The tubes of samples were stored in an incubator at 68°C for 5 days and the dry debris was weighed. The weight of the apically extruded debris was calculated by subtracting the original weight of the empty Eppendorf tube from the gross weight.

Evaluation of root canal transportation and centering ability

Superimposed initial and post-preparation images were used to measure the distances between two central axes at 0 mm, 1 mm, 2 mm, and 4 mm from the WL. The Gambill method employed in this experiment is the most commonly used way to calculate root canal transportation and centering ability [25]. The amount of canal transportation was determined by the following equation described by Gambill [25]. Each value was measured three times, and a mean value was taken.

The following formula was used for the calculation of transportation [26,27]: (X1 − X2) − (Y1 − Y2), where X1 was the mesial dentin wall thickness uninstrumentation, Y1 was the distal dentin wall thickness uninstrumentation, and X2 was the mesial dentin wall thickness instrumentation, and Y2 was the distal dentin wall thickness instrumentation. A result equivalent to 0 indicates that no transportation occurred.

The centering ratio was calculated using the following ratio: (X1 − X2)/ (Y1 − Y2) or (Y1 − Y2)/ (X1 − X2). X1, X2, Y1, and Y2 represent the same meaning as the formula used for the calculation of transportation. If (X1 − X2) or (Y1 − Y2) is not equal, a lower figure is considered as the ratio’s numerator. According to this formula, a result of 1 indicates perfect centering.

Evaluation of root canal shaping ability

Pre- and postoperative models of the canals were rendered with CTAn v.1.14.4 software (Bruker microCT, Bruker) and coregistered with their respective preoperative data sets using the rigid registration module of the DataViewer v.1.5.1.9 software (Bruker microCT). A qualitative comparison among groups was performed using color-coded models of the matched root canals (red and green colors indicate pre- and postoperative canal surfaces) with CTVol v.2.2.1 software (Bruker microCT).

The increase of postoperative 3D parameters at 2 mm from WL (volume and surface area) was acquired with Mimics Research 21.0 software (Materialise, Leuven, Belgium).

Statistical analysis

Every group had normality and homoscedasticity after using the Shapiro-Wilk test and Levene test to identify them, respectively. The range of the P value of Levene test was from 0.051 to 0.976, and the number of samples in each group was the same. Then, the amount of apically extruded debris, the degree of canal transportation, and the parameters of shaping ability were analyzed statistically using the one-way analysis followed by the Tukey post hoc test for multiple comparisons. The level of significance was set at α = 0.05.

Results

CEC and TEC in mandibular molars

CECs (Fig. 1A, B) or TECs (Fig. 1C, D) were prepared in the teeth of each subgroup, respectively. CECs were accessed at the central fossa and extended only as necessary to access canal orifices while preserving the pericervical dentin and chamber roof. TECs were accessed to establish straight access to canal orifices without coronal interference.

Fig. 1

CECs and TECs in mandibular molars. The occlusal view from micro-CT cross sections perpendicular to the occlusal plane of the CECs (A) and the TECs (C). The sagittal view of the CECs (B) and the TECs (D) from 3D volumetric representations.

Apically extruded debris

All the instruments used in this study extruded debris out of the root apical.

Under CECs: for round canals with curvature less than 15°, the XPS group excluded less apical debris than the PTG, HPT and Hero Shaper group in the same situation (P < 0.05); moreover, for round canals with curvature between 20°-35° and oval-shaped root canals, the XPS and PTG group extruded less apical debris than the HPT and Hero Shaper group in the same situation (P < 0.05), respectively.

Under TECs: for round canals with curvature less than 15° and with a curvature of 20°-35°, there were no significant differences in apical debris prepared with the four Ni-Ti instruments (P > 0.05); however, for oval-shaped root canals, the XPS and PTG group excluded less apical debris than HPT and Hero Shaper group in the same situation (P < 0.05), respectively.

The comparison between CECs and TECs: for round canals with curvature of 20°-35°, the XPS group under CECs extruded less debris from the apical than that under TCEs (P < 0.05); however, there was no significant difference in apical debris for PTG, HPT and Hero Shaper groups in the same situation.

The mean values and standard deviations for all groups are listed in Table 1.

Table 1 Apically extruded debris under CECs and TECs after preparation with Ni-Ti instruments (mg, mean values ± standard deviations)

	CECs	TECs
Curvature <15°
XPS	0.45 ± 0.17aA	0.60 ± 0.31aA
PTG	0.73 ± 0.15bA	0.82 ± 0.23aA
HPT	0.85 ± 0.21bA	0.76 ± 0.40aA
Hero Shaper	0.72 ± 0.33bA	0.83 ± 0.56aA
Curvature of 20°-35°
XPS	0.61 ± 0.33aA	1.29 ± 0.21aB
PTG	0.92 ± 0.54aA	1.16 ± 0.54aA
HPT	1.91 ± 1.16bA	1.53 ± 0.89aA
Hero Shaper	1.25 ± 0.38bA	1.72 ± 0.75aA
Oval-shaped
XPS	0.65 ± 0.28aA	0.91 ± 0.23aA
PTG	1.01 ± 0.64aA	0.67 ± 0.60aA
HPT	2.13 ± 1.26bA	2.52 ± 0.98bA
Hero Shaper	1.98 ± 0.66bA	1.56 ± 0.72bA

Different superscript lowercase indicates a statistically significant difference for the different instrument groups under the same endodontic cavity at a 5% significant level (P < 0.05).

Different superscript uppercase indicates a statistically significant difference for the same instrument groups under the different endodontic cavities at a 5% significant level (P < 0.05).

Canal transportation

Under CECs: for round canals with curvature less than 15° and 20°-35°, the XPS and PTG groups formed less canal transportation and had higher centering ratios than the HPT and Hero Shaper groups at 0 mm, 1 mm, 2 mm, and 4 mm from the WL of root canals (P < 0.05). For oval-shaped canals, the XPS group formed less transportation and had a higher centering ratio at 0 mm from the WL of root canals than the PTG, HPT and Hero Shaper groups (P < 0.05), and XPS and PTG groups formed less canal transportation at 1 mm and 2 mm from the WL of root canals than the HPT and Hero Shaper groups (P < 0.05). Moreover, the XPS and HPT formed less canal transportation at 4 mm from WL than the PTG and Hero Shaper groups (P < 0.05).

Under TECs: for round canals with curvature less than 15° and 20°-35°, the Hero Shaper formed more canal transportation and a lower centering ratio than the other three groups at 0 mm, 1 mm, 2 mm, and 4 mm from WL of root canals (P < 0.05). Additionally, for the oval-shaped canal, the XPS and PTG formed less canal transportation and a higher centering ratio than the HPT and Hero Shaper groups at 0 mm and 2 mm from WL of root canals (P < 0.05); moreover, the Hero Shaper groups formed more canal transportation and a lower centering ratio than the other three groups at 1 mm from WL of root canals (P < 0.05); further, at 4 mm from WL, there was no significant difference in the canal transportation after preparation with four Ni-Ti instruments (P > 0.05); however, the XPS and PTG had higher centering ratios than the HPT and Hero Shaper groups in the same situation (P < 0.05).

The comparison between CECs and TECs: for round canals with curvature less than 15° at 0 mm, 1 mm, 2 mm, and 4 mm from WL, the centering ratios of four tested Ni-Ti instruments were all higher under TECs than those under CECs (P < 0.05). For the round canals with curvature 20°-35°, at 0 and 4 mm from WL, the centering ratios of XPS, PTG and HPT groups were all higher under TECs than those under CECs (P < 0.05), while the Hero Shaper did not show significant differences of that between TECs and CECs (P > 0.05); at 1 mm and 2 mm from WL, the centering ratios of four tested Ni-Ti instruments were all higher under TECs than those under CECs (P < 0.05). For the oval-shaped canal, at 0 mm, 1 mm, and 2 mm from WL, the centering ratios of four tested Ni-Ti instruments were all higher under TECs than those under CECs (P < 0.05); moreover, at 4 mm from WL, the centering ratios of XPS and PTG were higher under TECs than those under CECs (P < 0.05).

The matching of pre-operative (dark gray) and post-shaping (white) root canals are shown in Fig. 2. The mean values and standard deviations of transportation and centering ratio for each group are listed in Tables 2,3,4,5.

Fig. 2

(A) XPS group under CECs. (B) PTG group under CECs. (C) HPT group under CECs. (D) Hero Shaper group under CECs. (E) XPS group under TECs. (F) PTG group under TECs. (G) HPT under TECs. (H) Hero Shaper group under TECs. X1: the mesial dentin wall thickness uninstrumentation. Y1: the distal dentin wall thickness uninstrumentation. X2: the mesial dentin wall thickness instrumentation. Y2: the distal dentin wall thickness instrumentation.

Table 2 Transportation and centering ratio under CECs and TECs after preparation with Ni-Ti instruments at 0 mm from WL (mm, mean values ± standard deviations)

	Transportation		Centering ratio
	CECs	TECs	CECs	TECs
Curvature <15°
XPS	0.01 ± 0.01aA	0.01 ± 0.02aA	0.66 ± 0.37aA	0.98 ± 0.03aB
PTG	0.02 ± 0.01aA	0.03 ± 0.03aA	0.63 ± 0.24aA	0.97 ± 0.07aB
HPT	0.11 ± 0.02bA	0.02 ± 0.03aB	0.39 ± 0.08bA	0.74 ± 0.21aB
Hero Shaper	0.18 ± 0.09bA	0.07 ± 0.01bB	0.21 ± 0.15bA	0.37 ± 0.26bB
Curvature of 20°-35°
XPS	0.01 ± 0.01aA	0.01 ± 0.01aA	0.72 ± 0.41aA	0.94 ± 0.12aB
PTG	0.01 ± 0.01aA	0.01 ± 0.01aA	0.75 ± 0.46aA	0.90 ± 0.32aB
HPT	0.05 ± 0.04bA	0.03 ± 0.03aA	0.40 ± 0.39bA	0.85 ± 0.50aB
Hero Shaper	0.09 ± 0.07bA	0.08 ± 0.04bA	0.42 ± 0.15bA	0.48 ± 0.26bA
Oval-shaped
XPS	0.01 ± 0.02aA	0.01 ± 0.01aA	0.65 ± 0.44aA	0.77 ± 0.43aB
PTG	0.04 ± 0.03bA	0.01 ± 0.02aB	0.34 ± 0.33bA	0.70 ± 0.44aB
HPT	0.04 ± 0.04bA	0.04 ± 0.02bA	0.35 ± 0.43bA	0.50 ± 0.53bB
Hero Shaper	0.07 ± 0.05bA	0.05 ± 0.01bA	0.41 ± 0.27bA	0.66 ± 0.29aB

Table 3 Transportation and centering ratio under CECs and TECs after preparation with Ni-Ti instruments at 1 mm from WL (mm, mean values ± standard deviations)

	Transportation		Centering ratio
	CECs	TECs	CECs	TECs
Curvature <15°
XPS	0.01 ± 0.01aA	0.01 ± 0.01aA	0.86 ± 0.20aA	0.96 ± 0.08aB
PTG	0.02 ± 0.02aA	0.02 ± 0.01aA	0.81 ± 0.18aA	0.93 ± 0.05aB
HPT	0.08 ± 0.02bA	0.02 ± 0.02aB	0.54 ± 0.26bA	0.82 ± 0.30aB
Hero Shaper	0.13 ± 0.07bA	0.07 ± 0.05bB	0.37 ± 0.22bA	0.64 ± 0.29bB
Curvature of 20°-35°
XPS	0.01 ± 0.02aA	0.01 ± 0.02aA	0.75 ± 0.21aA	0.84 ± 0.10aB
PTG	0.03 ± 0.03aA	0.02 ± 0.01aA	0.64 ± 0.12aA	0.85 ± 0.22aB
HPT	0.07 ± 0.06bA	0.04 ± 0.03aB	0.42 ± 0.29bA	0.68 ± 0.41aB
Hero Shaper	0.08 ± 0.03bA	0.08 ±0 .01aA	0.39 ± 0.17bA	0.47 ± 0.25bB
Oval-shaped
XPS	0.02 ± 0.02aA	0.01 ± 0.01aA	0.70 ± 0.25aA	0.80 ± 0.14aB
PTG	0.04 ± 0.02aA	0.02 ± 0.01aB	0.72 ± 0.38aA	0.82 ± 0.23aB
HPT	0.07 ± 0.06bA	0.02 ± 0.02aB	0.53 ± 0.28bA	0.71 ± 0.32aB
Hero Shaper	0.08 ± 0.02bA	0.04 ± 0.03bB	0.47 ± 0.31bA	0.58 ± 0.21bB

Table 4 Transportation and centering ratio under CECs and TECs after preparation with Ni-Ti instruments at 2 mm from WL (mm, mean values ± standard deviations)

	Transportation		Centering ratio
	CECs	TECs	CECs	TECs
Curvature <15°
XPS	0.01 ± 0.01aA	0.01 ± 0.01aA	0.80 ± 0.29aA	0.99 ± 0.02aB
PTG	0.02 ± 0.01aA	0.02 ± 0.01aA	0.78 ± 0.23aA	0.97 ± 0.01aB
HPT	0.05 ± 0.01bA	0.02 ± 0.03aB	0.51 ± 0.17bA	0.88 ± 0.11aB
Hero Shaper	0.10 ± 0.04bA	0.06 ± 0.02bB	0.47 ± 0.26bA	0.64 ± 0.31bB
Curvature of 20°-35°
XPS	0.01 ± 0.01aA	0.01 ± 0.01aA	0.63 ± 0.28aA	0.74 ± 0.16aB
PTG	0.02 ± 0.03aA	0.01 ± 0.01aA	0.58 ± 0.37aA	0.69 ± 0.30aB
HPT	0.05 ± 0.04bA	0.04 ± 0.04aA	0.40 ± 0.41bA	0.68 ± 0.37aB
Hero Shaper	0.14 ± 0.06bA	0.10 ± 0.03bB	0.29 ± 0.11bA	0.45 ± 0.20bB
Oval-shaped
XPS	0.02 ± 0.02aA	0.01 ± 0.01aA	0.69 ± 0.47aA	0.83 ± 0.39aB
PTG	0.03 ± 0.03aA	0.02 ± 0.02aA	0.62 ± 0.45aA	0.87 ± 0.23aB
HPT	0.06 ± 0.04bA	0.04 ± 0.03bA	0.50 ± 0.36bA	0.64 ± 0.32bB
Hero Shaper	0.11 ± 0.05bA	0.05 ± 0.03bB	0.41 ± 0.27bA	0.53 ± 0.31bB

Table 5 Transportation and centering ratio under CECs and TECs after preparation with Ni-Ti instruments at 4 mm from WL (mm, mean values ± standard deviations)

	Transportation		Centering ratio
	CECs	TECs	CECs	TECs
Curvature <15°
XPS	0.01 ± 0.02aA	0.02 ± 0.02aA	0.85 ± 0.12aA	0.96 ± 0.02aB
PTG	0.02 ± 0.01aA	0.02 ± 0.01aA	0.84 ± 0.14aA	0.93 ± 0.01aB
HPT	0.06 ± 0.01bA	0.02 ± 0.01aB	0.68 ± 0.31bA	0.85 ± 0.24aB
Hero Shaper	0.08 ± 0.05bA	0.04 ± 0.02bB	0.64 ± 0.19bA	0.66 ± 0.21bA
Curvature of 20°-35°
XPS	0.01 ± 0.01aA	0.01 ± 0.02aA	0.67 ± 0.31aA	0.80 ± 0.16aB
PTG	0.02 ± 0.02aA	0.01 ± 0.01aA	0.59 ± 0.22aA	0.71 ± 0.30aB
HPT	0.04 ± 0.02bA	0.02 ± 0.02aA	0.44 ± 0.36bA	0.68 ± 0.37aB
Hero Shaper	0.10 ± 0.06bA	0.08 ± 0.05bA	0.40 ± 0.09bA	0.45 ± 0.20bA
Oval-shaped
XPS	0.01 ± 0.01aA	0.02 ± 0.02aA	0.75 ± 0.22aA	0.90 ± 0.06aB
PTG	0.04 ± 0.05bA	0.02 ± 0.01aB	0.77 ± 0.16aA	0.87 ± 0.18aB
HPT	0.02 ± 0.01aA	0.02 ± 0.02aA	0.69 ± 0.29aA	0.74 ± 0.28bA
Hero Shaper	0.04 ± 0.02bA	0.03 ± 0.01aA	0.62 ± 0.17bA	0.66 ± 0.23bA

Root canal shaping ability

Regarding the 3D parameters (volume and surface area), for all the root canals in the different groups, including round canals with curvature less than 15° and 20°-35° and oval-shaped canals, no significant difference was found among the groups with four tested Ni-Ti instruments under CECs and TECs (P > 0.05).

The mean values and standard deviations of changes in the root canal volume and surface areas for all groups are presented in Table 6. Representative images are presented in Fig. 3.

The instruments that can be used for the root canal under CECs or TECs are listed in Table 7, which indicates how to choose an available instrument for RCT with less debris, less transportation, and higher centering ratios.

Table 6 Increase of 3D parameters evaluated after preparation (mm, mean values ± standard deviations)

	Volume (mm3)		Surface area (mm2)
	CECs	TECs	CECs	TECs
Curvature <15°
XPS	2.84 ± 0.37aA	2.54 ± 1.23aA	1.06 ± 0.50aA	1.31 ± 1.05aA
PTG	2.63 ± 0.45aA	2.80 ± 0.84aA	1.38 ± 0.28aA	1.74 ± 0.73aA
HPT	2.91 ± 0.21aA	3.15 ± 0.23aA	1.50 ± 0.11aA	1.90 ± 0.96aA
Hero Shaper	2.71 ± 0.18aA	2.94 ± 0.31aA	1.04 ± 0.21aA	1.48 ± 0.57aA
Curvature of 20°-35°
XPS	2.73 ± 1.18aA	2.89 ± 0.39aA	1.14 ± 0.77aA	1.13 ± 0.43aA
PTG	2.71 ± 1.97aA	2.56 ± 0.59aA	1.80 ± 1.09aA	2.17 ± 1.50aA
HPT	2.92 ± 1.49aA	2.68 ± 1.45aA	1.90 ± 0.98aA	2.15 ± 1.18aA
Hero Shaper	2.89 ± 0.65aA	2.45 ± 0.79aA	1.61 ± 0.40aA	1.87 ± 1.32aA
Oval-shaped
XPS	3.21 ± 1.35aA	3.02 ± 1.34aA	2.59 ± 1.22aA	2.73 ± 0.43aA
PTG	3.57 ± 2.05aA	3.25 ± 0.82aA	2.37 ± 1.25aA	2.51 ± 0.24aA
HPT	3.24 ± 1.78aA	3.08 ± 1.41aA	2.47 ± 1.36aA	2.59 ± 1.16aA
Hero Shaper	3.55 ± 1.27aA	2.93 ± 0.93aA	2.32 ± 1.13aA	2.45 ± 1.76aA

Fig. 3

3D reconstruction of mandibular molars. (A) XPS group under CECs. (B) PTG group under CECs. (C) HPT group under CECs. (D) Hero Shaper group under CECs. (E) XPS group under TECs. (F) PTG group under TECs. (G) HPT under TECs. (H) Hero Shaper group under TECs. (1) before canal preparation (red). (2) after canal preparation (green). (3) the 3D reconstruction overlaps figures.

Table 7 XPS, PTG, HPT and Hero Shaper with less debris, less transportation and higher centering ratio under CECs and TECs

	Debris		Transportation		Centering ratio
	CECs	TECs	CECs	TECs	CECs	TECs
Curvature <15°	XPS	-
WL-0 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-1 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-2 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-4 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
Curvature of 20°-35°	XPS PTG	-
WL-0 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-1 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-2 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-4 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
Oval-shaped	XPS PTG	XPS PTG
WL-0 mm			XPS	XPS PTG	XPS	XPS PTG Hero Shaper
WL-1 mm			XPS PTG	XPS PTG HPT	XPS PTG	XPS PTG HPT
WL-2 mm			XPS PTG	XPS PTG	XPS PTG	XPS PTG
WL-4 mm			XPS HPT	-	XPS PTG HPT	XPS PTG

Discussion

In the present study, the apically extruded debris, canal transportation, and shaping ability were compared between CECs and TECs before and after instrumentation with XPS, PTG, HPT, and Hero Shaper systems in the curved and oval-shaped molar canals. The results showed that XPS and PTG systems may be more applicable to the CECs than Hero Shaper and HPT systems in the tested root canals. The present study provides operational guidance to endodontists for choosing appropriate Ni-Ti instruments under CECs (Table 7). Environmental setting of the cavities may affect the results from the viewpoint of instrument operability, and some strategies were carried out to reduce the effect from the environmental setting. First, the teeth were examined under an operating microscope, and teeth with preexisting environmental factors such as cracks, caries, and other decay were excluded from the samples. Second, each natural tooth is different, the environmental setting of the CECs includes anatomical morphology of the pulp cavity and root canal orifice in each isolated tooth, while this study focuses on the natural root canals rather than the cavities, and these root canals were divided into three groups according to different root canal natural morphology: round canals with the curvature less than 15°, round canals with curvature of 20°-35°, and oval-shaped root canals after the preoperative 3D models of the root canals were obtained [14,15], which reduced and limited the effect from the environment setting of the cavities.

In this experiment, four Ni-Ti file systems were used to prepare root canals, and their effects were compared between CECs and TECs. A single file Ni-Ti system of XPS with a tip size 30 and 0.04 taper is different from the five files Ni-Ti system of PTG or HPT with a tip size 25 and 0.08 taper, however, the comparison is among the Ni-Ti systems, not among the files or tip sizes; moreover, previous studies showed that apical preparation size did not affect the apically extruded debris [28]. In addition, the comparisons of root canal transportation and shaping ability were based on the data before and after the preparation with the same file for the same tooth, which agrees with the methods of researchers who used different tip sizes Ni-Ti instruments to study transportation and shaping ability [29].

Apical extrusion will cause postoperative pain, swelling, and persistent periapical lesions in the apical root canal [30]. However, up to now, the ideal file systems that prevent apical extrusion have not been available, especially for the situation under CECs. This study showed that the application of XPS, PTG, HPT and Hero Shaper caused apical extrusion of debris, regardless of CECs and TECs; however, XPS produced less apically extruded debris in round canals with curvature less than 15° under CECs than the other three tested files in the same situation. A possible explanation might be that XPS can shrink and expand according to the shape of the root canal, thus elevating the debris up to the coronal direction. Moreover, in this study, XPS preparation produced less apically extruded debris in the round root canal with a curvature of 20°-35° under CECs than that under the TECs, which is different from previous studies where the amount of apical debris extrusion in CECs groups was greater than that in TECs groups after preparation with Reciproc Blue and One Curve systems [31]. This deviation may be due to the different Ni-Ti instruments tested and XPS might have more super-elasticity and extreme flexibility than the Reciproc Blue. In addition, XPS can move through asymmetrical snake-shaped rotation, and the file presents a six-blade tip with flexible performance, forming a dynamic space in the root canal to remove dentin debris. Based on this, XPS can be a better choice for cases under CECs.

Due to the complexities of the root canal system and the physical properties of Ni-Ti instruments, root canal transportation is a common clinical complication, which increases the risk of prolonged periapical periodontitis and reduces the fracture resistance of the tooth. The CECs emphasizes tooth structure preservation for the concepts of minimally invasive dentistry, which may increase the risk of root canal transportation [6]. This study found that TECs groups prepared with four tested files preserved the original root canal anatomy with less transportation than that under CECs. This result was consistent with previous studies regarding ProGlider and WaveOne Gold where TECs groups formed less apical transportation than that under CECs after preparation [8]. Because CECs maintain part of the hard tissue of the crown, there will be an increase in the curvature of the root canal, which causes instruments to work mainly on the internal surface of the root canal, leading to root canal transportation. Moreover, the present study also showed that the XPS and PTG groups had less canal transportation and a higher centering ratio than the HPT and Hero Shaper groups for curved and oval-shaped canals, which suggests that XPS and PTG groups are better for cases under CECs than HTP and Hero Shaper. Instruments with higher flexibility decreased the transportation of the root canals [32]. XPS and PTG developed with proprietary advanced metallurgy for greater flexibility can preserve the original anatomy of a root canal and improve its fatigue resistance [15,33]. The Hero Shaper increased root canal transportation, and the original root canal anatomy of the apical segment became more variational than before the preparations, both under CECs and TECs, because the Hero Shaper fails to comply with the anatomy of the root canal because of its poor flexibility, leading to increased transportation after preparation. Fan et al. [34] have reported that apical transportation less than 0.3 mm is clinically acceptable and did not affect the effects of RCT. In the present study, apical transportation occurred in all samples, however, root canal transportation was under the clinically acceptable range.

Ni-Ti instrument preparation shapes the canals to facilitate cleaning and filling, preventing disease progression, promoting healing, and respecting the anatomy of the canal [35]. After root canal preparation with XPS, PTG, HPT and Hero Shaper, the final preparation 3D parameters (volume and surface area) of the four tested Ni-Ti instruments are similar when compared between CECs and TECs. Because the final width and taper of the four tested Ni-Ti instruments is 30#04 or 25#08, this may result in similar root canal morphology after preparation.

Mechanical preparation with PTG and hand preparation with HPT were chosen as a pair comparison between motor use and hand use with the same Ni-Ti system of RCT under the TECs and CECs. Both PTG and HPT consisted of SX, S1, S2, F1 and F2, and prepared the root canals to 25#08. There is no significant difference between PTG and HPT in the extruded debris, canal transportation and shaping ability for all the studied groups under TECs, which suggests that the mechanical preparation has a similar canal forming ability to the hand preparation. However, interestingly, under CECs, the PTG group formed less canal transportation and had a higher centering ratio than the HPT group. Although both files have the same geometry, PTG has an obvious advantage for its improved property of gold wire and mechanical preparation with a high rotary speed, which increases its flexibility and cyclic fatigue resistance under CECs, and shows a better centering ability than HPT.

In conclusion, recent advances in the available resources and technologies have made a significant impact on endodontic treatment procedures, allowing minimally invasive treatment procedures such as CECs to be used for endodontically treated teeth. This study provides theoretical and operational support for endodontists to choose an appropriate Ni-Ti instrument for the endodontically treated tooth under CECs.

Acknowledgments

This work was supported by the Foundation of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, 2018-87).

Conflict of Interest

The authors have no conflict of interest relevant to this article.

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