**1. Introduction**

The presence of bacteria within the root canal system poses a risk factor for the appearance of pulp and periapical diseases in both primary and permanent dentition [1,2]. The biomechanical preparation of the root canal system is therefore considered to be a fundamental step in the root canal treatment process in order to adequately eliminate bacteria, necrotic tissue, and infected dentin [3]. In addition, the root canal system must be funnel shaped, becoming narrower in the apical direction in order to maintain the original anatomy and enable sufficient obturation [4,5]. A pulpectomy is widely recommended for primary teeth so as to preserve arch length, maintain primary teeth, including their

**Citation:** Faus-Llácer, V.; Pulido Ouardi, D.; Faus-Matoses, I.; Ruiz-Sánchez, C.; Zubizarreta-Macho, Á.; Ortiz, A.M.R.; Faus-Matoses, V. Comparative Analysis of Root Canal Dentin Removal Capacity of Two NiTi Endodontic Reciprocating Systems for the Root Canal Treatment of Primary Molar Teeth. An In Vitro Study. *J. Clin. Med.* **2022**, *11*, 338. https://doi.org/10.3390/jcm11020338

Academic Editors: Massimo Amato and Gianrico Spagnuolo

Received: 13 December 2021 Accepted: 7 January 2022 Published: 11 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

functional and aesthetic properties, and guide the proper eruption of permanent dentition [6]. Hand files are widely used to work with the root canal system in primary dentition; however, the root anatomy makes successful endodontic treatment difficult [7,8]. In primary teeth, the root canal system is characterized by high anatomical variability, including accessory and curved canals, as well as physiological root resorption that can alter the formation of the root canal system [9]. Nickel–titanium (NiTi) endodontic rotary instruments enable clinicians to maintain the original anatomy of curved canals, reducing the likelihood of potential mishaps during root canal system preparation [10]. Recently, novel NiTi endodontic rotary files have been specifically developed for the root canal treatment of primary teeth. Endogal Kids Rotary can be used either with a rotary or reciprocating motion; however, this latter movement is recommended for use in children, since it reduces the working time. This endodontic reciprocating system is manufactured in a NiTi alloy with heat treatment, and has a 17 mm length, 4% taper, 300μm apical diameter, and triangular cross-section design. Moreover, Reciproc Blue NiTi endodontic pediatric files also performs a reciprocating motion and is manufactured in a CM-Blue Wire NiTi alloy with heat treatment, and has a 17 mm length, 300μm apical diameter, and double-S cross-section design. The heat treatment improves the physical properties of NiTi endodontic rotary files, increasing their cyclic fatigue resistance and helping them adapt to different curvatures and angulations. Some studies have described the use of single files in a reciprocating motion for the root canal treatment of primary molars and reported significant advantages in pediatric dentistry, such as a decrease in working time, low risk of iatrogenic errors, or the prevention of cross-contamination [11–13]. That being said, root canal treatments can be affected by various factors, including anatomical design, diameter, kinematics, taper, and the number of files used during the procedure [14,15]. In addition, several techniques have been used to measure the amount of dentin removal, including plastic models, histologic sections, serial sectioning, scanning electron microscopic studies, radiographic comparison, and the silicone impression of un-instrumented root canal systems [15]. However, few studies have used the micro-CT with primary molars, which is a conservative, accurate, and nondestructive measurement procedure [11]. Micro-computed tomography (micro-CT) analysis has become a conservative measurement technique for obtaining an accurate 3D analysis, enabling both the quantitative and qualitative assessment of the root canal system anatomy after the shaping procedures [6,16].

The objective of the present study was to evaluate and compare the dentin removal capacity of Endogal Kids and Reciproc Blue NiTi endodontic reciprocating systems for the root canal treatment of primary second molar teeth via a micro-CT scan, with a null hypothesis (H0) that there are no differences in root dentin removal capacity between the Endogal Kids and Reciproc Blue NiTi endodontic reciprocating systems for root canal treatments in primary molar teeth.

#### **2. Materials and Methods**

#### *2.1. Study Design*

Sixty root canal systems were chosen from a total of fifteen primary second molar teeth (8 upper and 7 lower) that had been extracted for orthodontic or restorative reasons. Between January and March 2021, around three root canal systems were selected for study from cases at the Department of Stomatology at the University of Valencia in Valencia, Spain. All of the selected root canal systems presented no prior root canal filling materials or root resorption. A power of 80.00% was calculated using the bilateral Student's *t*-test for two independent samples. When used to calculate the variation from the null hypothesis H0: μ1 = μ2, the significance level of 5.00% and power of 80.00% meant that 60 root canal systems were necessary for the purposes of this study. The study was carried out as a randomized controlled experimental trial, in keeping with the norms outlined by the statement of the German Ethics Committee on the use of organic tissues as part of medical research (Zentrale Ethikkommission, 2003). Additionally, the study was reviewed and approved by

the Ethics Committee of the University of Valencia under Process No. H1512122849636. All study participants provided their prior informed consent for participation in this study.

#### *2.2. Experimental Procedure*

The sixty root canal systems in the fifteen selected primary second molar teeth were assigned randomly (Epidat 4.1, Galicia, Spain) to one of the following NiTi endodontic reciprocating systems: A. EK3 Endogal Kids (Endogal, Galician Endodontics Company, Lugo, Spain) (*n* = 30) (EDG) or B. R25 Reciproc Blue (VDW, Baillagues, Switzerland) (*n* = 30) (RB). Impressions of the teeth were taken using polyvinyl siloxane material (Ref.: 7000054992, Express ™ 2 Putty Soft, 3M ESPE ™, Saint Paul, MN, USA) to enable the access cavity to be prepared using the technique described by Rover et al. [17]. The root canal working length was determined with a stainless steel #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) and observed under magnification (OPMI pico, Zeiss Dental Microscopes, Oberkochen, Germany) until the far end of the file became visible through the epical foramen. Each root canal system was manually prepared with up to a #25 K-file (Dentsply Maillefer, Ballaigues, Switzerland) before being performed upon according to the NiTi endodontic reciprocating system to which it had been assigned. Root canal systems randomly assigned to the EDG study group were prepared with a reciprocating movement, and the root canal systems randomly assigned to the RB study group were also prepared with a reciprocating motion. In addition, the root canal systems were irrigated using a 5 mL sterile saline solution (Braun, Jaén, Spain) with 5 mL of 17% EDTA (SmearClear; SybronEndo, CA, USA) and 5 mL of 5.25% NaOCl (Clorox; Oakland, CA, USA), administered using a 0.3 mm endodontic needle (Miraject Endo Luer; Hager & Werken, Duisburg, Germany) inserted into the working length up to 1 mm. The teeth were kept in an incubator (mco-18aic, Sanyo, Moriguchi, Osaka, Japan) and stored at 37 ◦C with 100% relative humidity. A single clinician performed all the root canal procedures.

#### *2.3. Micro-CT Scanning*

Preoperative and postoperative micro-CT scans (Micro-CAT II, Siemens Preclinical Solutions, Knoxville, TN, USA) were performed to analyze and compare the amount of root canal dentin removed by the Endogal Kids and Reciproc Blue NiTi endodontic reciprocating systems subsequent to the root canal treatment of the primary second molar teeth. The scans were taken using the following exposure parameters: 88 μA, 90 kV, 360◦ rotation, and 50 μm isotropic resolution. Tomographic 3D images of the entire tooth showed a total of 512 slices, with an isotropic voxel size of 50 microns and a 512 × 512-pixel resolution for each slice (Figure 1A–F).

#### *2.4. Measurement Procedure*

The analysis of the change in the volume of dentin removed after the root canal procedures was carried out using image processing software (ImageJ, National Institutes of Health, Bethesda, MD, USA) after the root canal systems had been defined and segmented (ROI: 10 × 10 × 10 mm) using the preoperative and postoperative micro-CT scans (Micro-CAT II, Siemens Preclinical Solutions, Knoxville, TN, USA). In addition, transverse section images were also analyzed in the apical, middle, and coronal root thirds (Figure 2).

#### *2.5. Statistical Tests*

Statistical analysis was carried out using SAS 9.4 (SAS Institute Inc., Cary, NC, USA). The mean and standard deviation (SD) were used for the descriptive analysis of quantitative data. For each of the variables, the difference between the pre- and postoperative values was analyzed using a t-test for independent samples or a nonparametric Mann–Whitney– Wilcoxon test based on compliance with the application criteria. *p* < 0.05 was determined to be the level for statistical significance.

**Figure 1.** Reconstructed 3D micro-CT images of (**A**) preoperative (blue), (**B**) postoperative (red), and (**C**) superimposed pre- and postoperative images of the EDG study group (blue and red) and (**D**) preoperative (blue), (**E**) postoperative (red), and (**F**) superimposed pre- and postoperative images of the RB study group (blue and red).

**Figure 2.** Transverse section images after aligning preoperative (blue) and postoperative (red) micro-CT scans of the EDG study group at the (**A**) coronal, (**B**) middle, and (**C**) apical root third and (**D**) coronal, (**E**) middle, and (**F**) apical root third of the RB study group.
