Introduction

Root canal preparation may be influenced by the design [1], kinematics [2,3,4], diameter and taper [5], and the number of instruments used [6, 7]. Reciprocating motion allows preparation with a lower number of instruments [8]. However, the importance of using more than one instrument to promote better root canal cleaning with less debris and uninstrumented root canal wall, including different diameters, has been demonstrated [6, 7, 9].

Oval-shaped and flattened root canals may make it difficult to root canal preparation, especially because they present areas that are difficult to access, favoring the accumulation of debris and microorganisms [10, 11]. The oval-shaped morphology has been studied [3, 4, 12, 13] and self-adjusting instruments [14] or different additional disinfection procedures [15, 16] have been proposed to improve cleaning. Nevertheless, some regions of the canal may remain unprepared [3, 17, 18].

The Unicone System (Medin, Nove Mesto Morave, Czech Republic) is composed of three instruments for reciprocating motion, with diameters #20, #25, and #40 and taper 6%, with conical core and smaller escape area [19]. It had less cyclic fatigue resistance when compared with the Reciproc R25 and WaveOne Primary instruments [20].

Reciproc is a single-file instrument made of a special NiTi alloy (M-Wire), used with reciprocating counterclockwise motion and available in three different sizes (R25, R40 and R50) to be used in different canal diameter. This instrument promotes preparation similar to Self-Adjusting File, WaveOne, and Protaper systems when used for oval-shaped root canal preparation [3] and greater increase in the root canal volume when compared with the BioRace rotary system size 40, .04 taper file in long oval root canals [21]. MTwo (VDW, Munich, Germany) is a system of NiTi files with S-shaped cross-sectional design, manufactured to operate in a rotary clockwise motion. In comparison to Reciproc, it demonstrated similar apical preparation in mandibular molars [22].

The aim of this study was to evaluate by means of micro-computed tomography (micro-CT) the shaping ability and cleaning effectiveness of different root canal preparation systems (Reciproc, Unicone and MTwo) in oval root canals.

Material and methods

Selection of teeth and preparation

Extracted mandibular incisors (CEP #31725014.7.0000.5416) were selected by using digital radiography (Kodak RVG 6100). Mesiodistal and buccolingual length measurements were used to define the diameter ratio (DR) at 9 mm from anatomic apex of each specimen. Single rooted teeth, with long oval canals (2 ≤ DR ≤ 4), completely formed apices and a root length of 20 ± 2 mm were initially selected. Sixty-six teeth were selected and stored in a glass bottle containing 0.1% thymol solution at 5 °C.

The selected specimens were scanned using a high-definition micro-CT scanner SkyScan 1176 (SkyScan 1176, Bruker-microCT, Kontich, Belgium) at 70 kV, 353uA, 360° rotations, a 0.5-mm-thick aluminum filter and 0.5° rotation step, resulting in an image with 17.42 μm voxel size. An initial reconstruction using NRecon software (NRecon v.1.6.3, Bruker-microCT) and analysis using CTAn software (CTAn v.1.14.4, Bruker-microCT) was performed according to morphological parameters of the root canals (length, volume, and surface area) to final selection of specimens. Fifty-four root canals (n = 54) were randomly assigned to one of the three different instrumentation groups.

After washing in water for 48 h, a conventional access to the root canals was created using high-speed diamond burs (n2, KG Sorensen, São Paulo, Brazil) and the patency of the root canals was confirmed when a #10 K-file (Dentsply Sirona, Ballaigues, Switzerland) was visible at the apical foramen. The working length was then set at 1.0 mm shorter and a single operator with clinical experience prepared all samples.

Root canal preparation with Reciproc 40 (n = 18): R40 (size 40, .06 taper) instruments were activated in reciprocating motion (VDW.SILVER, VDW GmbH), according to manufacturer’s instructions. Root canal preparations were performed using an open root canal model for all experimental groups. The instrument was gradually inserted into root canal in a slow in-and-out motion in the three levels (cervical, middle, and apical), with a brushing motion against the walls. After preparation of each third, the instrument was cleaned in a gauze and the root canal was irrigated. A new R40 instrument was used for each root canal preparation.

Root canal preparation with UniCone 20.06 and 40.06 (n = 18): a Unicone 20 (size 20, .06 taper) instrument was used before the Unicone 40 (size 40, .06 taper) instrument in reciprocating motion (VDW.SILVER, VDW GmbH). The recommended in-and-out motion was the same as described above for the R40 instrument.

Preparation with Mtwo files (n = 18): Mtwo files were used in sequence and in rotary motion (VDW.SILVER, VDW GmbH). For cervical, middle, and apical preparation, size 25, .07 taper, size 25, .06 taper and size 20, .06 taper instruments, respectively, were used. After this, a sequence of Mtwo files from size 25, .06 taper instrument up to size 40, .06 taper instrument were used. After the use of each instrument, root canal was irrigated.

Root canal irrigation during preparation was performed with 6 mL 2.5% NaOCl (2 mL for each third to Reciproc, 1 mL for each third after both instruments to Unicone and 1 mL for each instrument change to Mtwo rotary system). A 30-G NaviTip needle (NaviTip, Ultradent Products, South Jordan, UT, USA) placed to 1 mm short of working length in a 5-mL syringe (Ultradent Products, South Jordan, UT, USA) was used with a flow rate of 2 mL/min and simultaneous suction by using a 0.014-in tip (Capillary tips, Ultradent, USA). The continuous flow of the irrigant was associated with an in-and-out movement. A final irrigation with 5 mL 2,5% NaOCl followed by 2 mL 17% EDTA was performed in each sample. Root canals were aspirated with a capillary tip and dried with absorbent paper points (Dentsply Sirona), and the specimens submitted to postoperative scanning and reconstruction, applying the aforementioned parameters.

Micro-CT analysis

After reconstruction of the scans before and after root canal preparation by using NRecon software, the datasets were geometrically aligned by using the 3D registration function of the Data Viewer software (Data Viewer v.1.5.1, Bruker microCT). Images were quantitatively analyzed using CTan software (CTAn v.1.14.4, Bruker microCT) and defined parameters of increase in volume, percentage of debris, and percentage of uninstrumented surface were obtained as previously described [7, 23,24,25]. The analysis was performed in the total extension of root canal (from the coronal limit to the apex) and in thirds (cervical, middle, and apical).

Statistical evaluation

Data were compared using one-way ANOVA and Tukey to initial volume; to increase in volume (%) in the cervical, middle, and apical thirds; to debris (%) in the total and apical third; and to uninstrumented surface (%) in the total and middle and apical thirds (α = 5%). Kruskal-Wallis and Dunn tests were used to increase in volume (%) in the total; to debris (%) in the cervical and middle thirds; and to uninstrumented surface (%) in the cervical third (α = 5%).

Results

The results obtained showed that there was no difference among the samples evaluated with regard the initial volume, confirming the degree of homogeneity (baseline) of the groups (P > .05). Unicone group was associated with the highest increase in volume values (P < .05), with no difference among groups in the cervical and apical thirds (P > .05), as shown in Table 1. For this system, a higher percentage of debris was also observed in the entire root canal and in the middle third (P < .05). With regard to the percentage of uninstrumented surface, there was no significant difference among the systems in the middle and apical thirds (P > .05). However, a higher percentage of uninstrumented surface in the entire root canal and in the cervical third was observed for Unicone Group when compared with Reciproc group (P < .05) (Table 1). Figure 1 shows representative 3D reconstructions of mandibular incisors before and after canal preparation with the tested systems.

Table 1 Means and standard deviations (±)* or median percentage (maximum and minimum values)** of the parameters analyzed in the different experimental groups
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Fig. 1
figure 1

Tridimensional micro-CT scan reconstructions of the external and internal anatomy of oval canals of mandibular incisors, in the Reciproc, Unicone, and Mtwo groups. (A) The root canal before (green) and (B) after (red) preparation. (C) Superimposition of preoperative root canal (A) and post-instrumentation (B). (D) Superimposition of accumulated hard-tissue debris plus instrumented areas (black areas) on the postoperative anatomy (green). (E) Total accumulated hard-tissue debris plus instrumented areas (black areas). (F) Axial view of superimposed root canals before (green) and after (red) preparation at coronal (c), middle (m), and apical (a) thirds

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Discussion

The volumetric increase of the root canal, percentage of debris, and uninstrumented surface have been evaluated by means of micro-computed tomography. Oval canal preparation represents a clinical challenge [17, 18], and the use of different instruments [14] or even auxiliary means of cleaning [15, 16] did not promote complete preparation and cleaning of oval canals, corroborating with the present study. The teeth were first selected by means of radiographic analysis, according to other studies [3, 18, 26]. Selected teeth were then analyzed using micro-CT and they were randomly assigned to one of the experimental groups. The initial volume was similar among the experimental groups (Table 1).

In the present study, the higher increase in the root canal volume throughout the extension of the canal and in the middle third was observed for the Unicone reciprocating system, using two instruments. However, in spite of the increase in volumetric values, a higher percentage of untouched areas were also observed for this system. In previous studies, WaveOne, for example, promoted a higher increase in the volume value of the canal, with a similar percentage of uninstrumented surfaces of oval canals to Easy ProDesign Logic and OneShape systems [27]. It was also observed that although the SAF system touched the canal walls to a larger extent, it removed less dentin in the preparation of oval root canals [18]. Corroborating with our results, these data demonstrated that root canal preparation was influenced by diverse factors, such as instrument design, kinematics, and number of instruments. An increase of canal volume indicates greater wear of dentin walls, but does not mean that this occurred following the root canal morphology. The preparation with Unicone promoted greater volume increase than the other systems. However, this wear was probably not higher in flattening areas that presented greater accumulation of debris. Also, the orientation of blades associated with reciprocating motion may have favored less cleaning of the root canal walls.

In spite of standardizing the size (instruments with tip #40) and taper (taper .06), the taper may be influenced the preparation. The Unicone instruments present a constant taper, while Reciproc presents taper .06 in the apical 3 mm and a reduction to 0.04 mm up to the end of the active part. This reduction may have favored the greater action in the middle of the oval canal, which narrows significantly [10, 11], thereby promoting a larger area of instrumented surface and a reduction in the percentage of debris. It means that the reduction of taper for Reciproc probably improves its capacity to reach flattening areas. In the apical region, where the tip and taper were similar among the systems, no difference in the uninstrumented surface value or accumulation of debris was observed.

The apical enlargement of root canal preparation can be associated to apical crack formation [28]. However, larger instruments are significantly more effective in eliminating bacteria [29,30,31]. The root canal enlargement up three sizes larger than the first apical binding file is considered a proper preparation [32]. Mandibular incisors may present unprepared surfaces and debris in the root canal system [10, 11], due to the prevalence of oval canals [12, 33, 34], which make it difficult to obtain efficient cleaning [3, 17, 18]. With basis on these concepts, regarding the root canal morphology, the preparation in the present study was standardized up to file #40, .06 taper.

The highest percentage of uninstrumented surface in the preparation with Unicone and the highest percentage of debris may also be related to its smaller metal mass, as observed after evaluating its preparation capacity [19]. Furthermore, the Unicone instruments presented smaller areas and lengths of flutes than the Reciproc instruments [35]. The smaller metal mass of the Unicone instruments could promote less contact of the instrument with the dentin walls. Areas that remained untouched during preparation may be colonized by biofilm, capable of compromising the endodontic treatment [16, 36].

In relation to cleaning (considered by the percentage of debris), the preparation of oval canals were shown to be cleaner (lower percentage of debris) with the reciprocating R40, and rotary system with the Mtwo sequence of instruments. A lower percentage of debris in the middle third of oval canals prepared with MTwo or R40 instruments was observed when compared with the preparation performed with a single Mtwo 40.06 instrument [7]. Favorable results were also obtained with the use of the Mtwo sequence of instruments, in the analysis of preparation in mandibular molars with two separate mesial canals and severe curvature [37]. The accumulation of debris during root canal preparation usually involves areas as isthmus, irregularities, and ramifications [25]. Oval canals present straightening in the middle region of the root canal [11], making this region critical to cleaning.

This Micro-Ct study presents limitations inherent to an in vitro root canal preparation study [38], as different NiTi systems and evaluated parameters. Also, root canal preparations were performed using an open root canal model. It is difficult to compare the results of this study with other experimental design and methodological studies. While considering these limitations, important considerations could be observed for root canal preparation and cleaning of oval-shaped root canals.

Conclusion

The Unicone system using two instruments in reciprocating motion resulted in higher increase in volume values. However, less remaining debris was present when Reciproc single-file and Mtwo rotary system were used. Root canal preparation and cleaning depend on root canal morphology, kinematics, and instrument design.