Comparative Micro-CT Assessment of Manual and Mechanized NiTi Files versus Stainless Steel Files in Prototyped Primary Teeth

ABSTRACT

Objective:  To compare the adequate time, canal transportation, remaining resin quantity, and volume after instrumentation with different manual and mechanized files.

Material and Methods:  Eighty root canals from prototyped deciduous molars were divided into four groups according to instrumentation: WOG - reciprocating file; SMF - nickel-titanium manual file; K - manual stainless steel file; SBF - rotary file for deciduous teeth. Micro-CT scans were taken before and after instrumentation, and data were collected and statistically analyzed. Two-way ANOVA and Tukey's test were used for parametric results, and Kruskal-Wallis and Dunn's test for nonparametric results.

Results:  Mechanized groups showed shorter times than manual groups (p≤0.05). In canal transportation, the WOG group showed greater deviation than the other groups (p=0.002). The SBF group showed the highest remaining resin quantity (p=0.008). All groups showed statistically significant differences between apical and cervical thirds (p=0.008), although no differences in volume were observed among groups (p≥0.05).

Conclusion:  NiTi files (SMF) are a viable alternative for pulpectomy in deciduous teeth, preserving original canal anatomy without excessively altering root canal volume, providing efficient treatment in reduced time, whether used manually (SMF) or mechanized nickel-titanium files (WOG and SBF).

Keywords:
Pulpectomy; Pediatric Dentistry; Endodontics.

Introduction

Despite several advances in Pediatric Dentistry over the last decade, caries remains the most prevalent disease in childhood [¹]. In view of this situation, preserving the primary teeth until their physiological exfoliation is essential to promote a functional occlusal relationship, contributing to phonation, aesthetics, and chewing, and to maintaining natural space [²].

Pulpectomy is the primary clinical procedure used to disinfect root canals and maintain primary teeth. Removal of organic debris is performed biomechanically using manual or mechanized instruments, followed by irrigation and filling the root canals with a resorbable paste [³].

For decades, researchers have used extracted teeth to compare materials and instruments available on the market. However, this method has drawbacks, such as the lack of sample standardization due to the complex morphology of primary teeth and a limited number of specimens [⁴]. Extracted primary teeth are often unsuitable because they frequently exhibit pathological root resorption, which complicates their use in studies. To overcome these issues, researchers have begun using prototyped teeth in conjunction with micro-computed tomography (micro CT). This combination provides high-resolution images and standardized samples with identical morphological details, ensuring reliable reproducibility of the canal while using a material with a density similar to dentin [⁵], resulting in more conclusive and unbiased results [⁶]. This methodology serves as a valid model for research in pediatric endodontics and applies to various areas of endodontic investigation, including irrigation, instrumentation, and obturation [⁷].

Conventionally, to the present day, manual instrumentation with stainless steel files is the gold standard in Pediatric Dentistry. Still, it has limitations, such as the risk of instrument fracture, curvature deviation, and longer care time, which compromise child management. Nickel-titanium files offer greater instrument flexibility, allowing better modeling, preserving anatomy and root curvature, and avoiding canal transportation [⁸].

The use of mechanized systems (rotary or reciprocating) in primary teeth was first described by Barr [⁹], who demonstrated that rotary instruments promote a more conical canal configuration in a shorter time than manual instruments. The main advantage of reciprocating kinematics is the clockwise and counterclockwise cutting direction, providing less chance of instrument fracture and resistance to cyclic fatigue [¹⁰].

Many studies [¹¹-¹⁶] have compared various on-market instruments designed for permanent teeth that can result in iatrogenic effects such as perforations, excessive wear, and tears when used in primary teeth [¹⁷]. However, to date, few studies have focused on using prototyped primary teeth to compare manual and mechanized nickel-titanium instruments designed for primary teeth, mostly on modifications of the root canal space and changes in curvature during instrumentation [⁵,⁶,¹²,¹⁸].

Therefore, this study aimed to compare the adequate time, canal transportation, remaining resin quantity, and volume after instrumentation with different manual and mechanized files in prototyped primary molars. The null hypothesis was that there would be no statistically significant differences between the groups.

Material and Methods

Sample Size Calculation

A significance level of 5% and a power of 90% (G*Power - v3.1.9.2, University of São Paulo, SP, Brazil), with a standard deviation of 1.13mm, was adopted according to the study of Barasuol et al. [¹⁹]. After the calculation, a minimum of fifteen root canals per group was required. The mesial canals of the mesial root from a commercial, validated, prototyped deciduous mandibular molar (#75 - Denart, São Paulo, SP, Brazil) [⁵,¹⁸] were used as the unit of study, yielding a final sample of 80 root canals per group.

Although the sample size calculation indicated 15 canals per group, 18 were used to strengthen the statistical power. The mesiolingual and mesiobuccal canals were instrumented in each group using the type of file detailed in the following paragraphs. In each group, the mesiobuccal canal was instrumented first, followed by the mesiolingual canal, to standardize the instrumentation sequence.

Micro CT Scanning

Root canals were scanned with a micro-CT scanner (SkyScan 1174v2, Bruker Belgium SA, Kontich, Belgium) at a resolution of 19.7 μm, with the X-ray tube operated at 50 kV and 800 mA, and a 180° rotation with a 1.0 step around the vertical axis. A pre-instrumentation scan was executed to provide values and evaluate initial parameters, such as volume, resin quantity, and prototype centralization.

Canal Preparation

A single pre-trained operator and examiner had carried out all mechanical preparations. Pulp chamber opening had been performed with a round diamond bur (FG 1014; KG Sorensen, Serra, ES, Brazil) and the removal of the entire pulp chamber roof with an inactive conical diamond bur (FG 3082; KG Sorensen, Serra, ES, Brazil). Then, with a K#10 file, the working length (WL) was determined by advancing the file to the apical foramen and retraction of 2.0 mm. Then, the prototypes had been randomly divided into four groups:

• WOG (WaveOne Gold reciprocating file): The canals were prepared with WaveOne Gold reciprocating files (Dentsply Maillefer - Dental Products, Ballaigues, Switzerland) in reciprocating rotation. We use the ''K'' type file #10 for opening the cervical third and the files #20.07 (WOG Small) and #25.07 (WOG Primary) for instrumentation of the entire working length. The E-Connect S motor (MK-life Medical and Dental Products, Porto Alegre, RS, Brazil) was used with torque and speed pre-established by the manufacturer in the "Waveone - Dentsply Standard" system.

• SMF (Sequence Manual File nickel-titanium manual file): The canals had been instrumented with Sequence Manual File nickel-titanium files - ''SMF'' (MK-life Medical and Dental Products, Porto Alegre, RS, Brazil). We used file #17.08 to open the cervical third, and files #15.04, #20.04, #25.04, and #30.04 for the entire working length.

• K (stainless steel manual file): The root canals had been instrumented with Kerr, ''K'' files (Dentsply Maillefer - Dental Products, Ballaigues, Switzerland) using the conventional step-back technique, employing balanced force for the mechanical preparation of the root canals. We had used ''K'' type file #10 for canal negotiation and ''K'' type #15, #20, and #25 files for the manual instrumentation of the apical third and ''K'' #30 file for the manual instrumentation of the cervical third, with a 1 mm indentation for the cervical third. The step-back technique was chosen due to its long-standing use in pediatric endodontics and its consistent results in shaping canals in primary teeth [¹⁴].

• SBF (Sequence Baby File rotary file for deciduous teeth): The canals had been prepared with the rotary files for use in deciduous teeth - Sequence Baby File- ''SBF'' (MK-life Medical and Dental Products, Porto Alegre, RS, Brazil) at a continuous rotation. We used file #17.08 to open the cervical third, and files #20.04, #25.04, and #30.04 for instrumentation of the entire working length. The E-Connect S motor (MK-life Medical and Dental Products, Porto Alegre, RS, Brazil) with 1.5N/cm torque and speed of 350 rpm was used. The instrumentation protocol was performed according to the study by Souza et al. [¹⁸].

All groups had received irrigation with 2 ml of saline solution at each file exchange.

Comparative Analysis of Post-Instrumentation Images

After instrumentation, all samples were scanned again in micro-CT using the same parameters as the initial scan. A stopwatch has counted the adequate instrumentation time from the insertion of the first file into the root canal.

Initial and final images had been reconstructed three-dimensionally using NRecon^® software (v1.6.9, Bruker, Kontich, Belgium), with post-alignment corrections and optimized ring-artifact corrections as necessary. Smoothing and beam-hardening adjustments were set to 10% and 60%, respectively, and the 3D models of the preand postoperative images were co-registered with an accuracy greater than one voxel (DataViewer^®, v1.5.1.2, Bruker, Kontich, Belgium) (Figure 1).

Figure 1
Superimposition of the images before (red color) and after (green color) instrumentation of the canals, indicating canal transportation and remaining resin quantity.

Canal transportation was determined using the formula of Gambill et al. [²⁰], which assigned positive and negative values to deviations from the mesial and distal sides, respectively. Also, data were calculated as a percentage (%) of the resin thickness remaining in the third thirds of the root canals, according to the study by Souza et al. [¹⁴].

The percentage of volume increase had been determined by the difference between the volume of the post-instrumentation and pre-instrumentation canal, using the formula (Vf - Vi) divided by Vi and multiplied by 100%, where Vf is the final volume and Vi is the initial volume. The percentage increase in the total canal volume was obtained using CTAn^® software (v1.12, Bruker Belgium SA, Kontich, Belgium).

Statistical Analysis

Statistical analyses were performed using SigmaPlot^® 12.0 (Systat Software, 2011) and Statistica^® 10.0 (StatSoft Inc., Tulsa, OK, USA). The results were analyzed using a two-way ANOVA followed by Tukey's test, and a Kruskal-Wallis test followed by Dunn's test, with a significance level of 0.05 (p<0.05).

Results

The median adequate time (in seconds) across the groups indicates that the mechanized instruments (WOG and SBF) required less time than the manual instruments (SMF and K). Among the manual groups, SMF (40.960 seconds) had a shorter working time than K (111.020 seconds), as shown in Table 1.

Table 1
Median of adequate time in seconds (n=80).

Group	Median
WOG	16.065a
SMF	40.960b
K	111.020c
SBF	19.625a

No statistically significant differences were observed between the thirds or between the group and thirds interaction on root canal transportation. There was a statistically significant difference between groups WOG and K (p=0.002) and between WOG and SBF (p=0.006), indicating that WOG showed greater deviation (Table 2).

Table 2
Mean and standard deviation of transportation between thirds (mm²) (n=240).

Groups	Mean	Thirds	Transportation
			Mean ± Standard Deviation
WOG	0.0474	Apical	0.009 ± 0.082B, a
		Medium	0.024 ± 0.135B, a
		Cervical	0.109 ± 0.127B, a
SMF	0.00628	Apical	-0.032 ±0.079AB,a
		Medium	-0.007 ± 0.131AB,a
		Cervical	0.058 ± 0.092 AB, a
K	0.0297	Apical	-0.018 ± 0.132A,a
		Medium	-0.043 ± 0.096A,a
		Cervical	-0.028 ± 0.146A,a
SBF	0.0237	Apical	-0.017 ± 0.100A,a
		Medium	-0.018 ± 0.121A,a
		Cervical	-0.036 ± 0.164A,a

Regarding the resin remaining after instrumentation, Table 3 shows a statistically significant difference between the SMF and SBF groups (p=0.008) and between the K and SBF groups (p=0.034). SMF exhibited the lowest amount of resin remaining, and SBF exhibited the highest resin remnant among all groups. There were statistically significant differences between the thirds (apical vs cervical) of all groups, indicating, on average, a greater amount of resin remaining in the apical third than in the cervical third (p=0.005).

Table 3
Mean and standard deviation of remaining resin between thirds (mm²) (n=240).

Groups	Mean	Thirds	Remaining Resin Quantity
			Mean ± Standard Deviation
WOG	15.852	Apical	20.092 ± 15.359AB,b
		Medium	15.650 ± 9.111AB,ab
		Cervical	11.815 ± 9.452AB,a
SMF	14.057	Apical	18.333 ± 14.367A,b
		Medium	15.193 ± 10.388A,ab
		Cervical	8.645 ± 6.688A,a
K	15.147	Apical	14.442 ± 10.405A,b
		Medium	13.422 ± 8.499A,ab
		Cervical	17.579 ± 11.114A,a
SBF	21.283	Apical	25.894 ± 19.467B,b
		Medium	21.787 ± 14.860B,ab
		Cervical	16.169 ± 13.296B, a

No statistically significant differences in root canal volume (pré and post-instrumentation) were observed among the groups (Table 4). The increase in instrumented volume was higher for K-files than for the other groups. The SBF rotary system and the WOG reciprocating system resulted in similar instrumented root canal volumes, but K resulted in the highest increase in instrumented root canal volume.

Table 4
Median of percentage of volume increase (mm³) (n=80).

Groups	Median
WOG	19.102
SMF	28.544
K	40.765
SBF	26.824

Discussion

This study aimed to evaluate instrumentation time, transportation time, the amount of resin remaining, and the percentage volume increase of the canals after instrumentation with different files and kinematics using microcomputed tomography (micro-CT) analysis. Several methodologies, such as microscopic analysis, radiographs, and cone beam computed tomography (CBCT), can be used to evaluate these comparative parameters in research [²¹-²²]. However, micro CT is the most recommended conservative technique, as it allows the obtaining of an accurate three-dimensional image, thus enabling the quantitative and qualitative anatomical evaluation of root canals after endodontic procedures [²³].

The adequate instrumentation time for the mechanized groups was shorter than that for the manual groups, corroborating the studies by Boonchoo et al. [²⁴] and Kalita et al. [²⁵]. This result is related to the use of an endodontic motor, which reduces operator fatigue and speeds the procedure. However, in the comparison of manual files, it was possible to verify in SMF files (NiTi instruments) a shorter instrumentation time, even with the same number of files for the instrumentation sequence, indicating that this manual instrumentation option favors child management (shorter clinical sessions). This reduced instrumentation time is related to the used alloy, which provides malleability to the file, and to the direction of rotation (clockwise) in which the instrument is used [²⁶].

Canal transportation is characterized by marked wear on one side of the canal wall, indicating that the instrument does not have the flexibility to keep up with excessive curvature or taper for the diameter of the instrumented canal. WOG showed greater transportation than the other groups, which is justified by the taper of the instruments used; SBF (designed for primary teeth) showed lower transportation. The rationale behind these results is the lowest instrument taper, which has a proportional effect on transport [²⁷]. However, in the analysis of thirds, no statistically significant differences were observed among groups, indicating that the use of mechanized instruments in deciduous teeth is a safe option for instrumentation. To date, no other literature on this methodology presents comparable results.

The amount of resin remaining demonstrates wear of the canal walls after instrumentation, indicating the level of force employed, which is proportional to the level of resin remaining [⁶]. Although stainless steel instrumentation is the most commonly used in Pediatric Dentistry, it leaves less resin remaining than mechanized instruments [¹¹]. This corroborates the results of the present study, which showed that the manual file groups showed statistically significant differences compared to SBF [¹²], demonstrating the safety of mechanized instrumentation in deciduous teeth without excessive root wall wear, which can result in iatrogenic effects such as tears or perforations.

All groups showed statistically significant differences between the cervical and apical thirds, with less resin remaining in the cervical third and more in the apical third. This indicates conical root canal morphology, with the cervical third wider and a narrowing towards the apical third, maintaining the original shape of the root anatomy [¹⁹].

The volumetric analysis contributes to verifying the efficacy of the instruments. In the present study, the percentage increase in root canal volume between groups did not show statistically significant differences, indicating that the instruments maintained the root canal's original anatomy. Esentürk et al. [²⁷], Poornima et al. [¹³], and Hidalgo et al. [¹⁴] demonstrated that mechanized instruments can increase canal volume, which disagrees with the results of the present study, likely due to differences in methodology. However, the K files showed a greater increase in volume percentage than the other groups. This occurred due to the instrument material (stainless steel, which is less flexible than NiTi) and the operator's performance during instrumentation [¹⁵].

Although the prototyped models used in this study offer significant advantages, such as anatomical standardization, control of morphological variation, and compatibility with high-resolution three-dimensional analyses, it is crucial to acknowledge their inherent limitations. The primary concern is the homogeneous composition of the resin, which does not accurately replicate the structural heterogeneity of natural dentin, including dentinal tubules, accessory canals, collagen fibers, and pulp remnants, all of which influence the physical-mechanical interaction with endodontic instruments during canal preparation [⁶,¹⁶,¹⁸]. Furthermore, the absence of physiological resorption and the material's uniform rigidity may alter the kinematic behavior of the files, particularly during the learning curve of mechanized instruments and in cyclic fatigue tests [⁵,²⁶]. These factors may affect the direct clinical translatability of the findings. Future studies should consider incorporating biomimetic materials with hardness gradients or hybrid models that combine prototyped structures with natural elements. Additionally, the use of validated ex vivo models, such as freshly extracted primary teeth with partial histological preservation, may improve the external validity of the results [⁴,⁷,²⁷].

The findings of the present study carry relevant clinical implications for Pediatric Endodontics, particularly regarding the rational selection of root canal instrumentation systems for primary teeth. The significant reduction in clinical time observed in the mechanized groups (WOG and SBF) represents a noteworthy advancement in behavioral management, as shorter appointments tend to improve child cooperation, reduce professional fatigue, and minimize the risk of treatment interruption [²⁴-²⁶]. The SBF rotary file, specifically designed for the anatomy of primary teeth, demonstrated satisfactory performance in preserving the original canal volume and showed a low incidence of transportation, supporting its safe application in routine pediatric care [¹⁸,²⁷]. Conversely, while the reciprocating WOG system was effective in reducing working time, it exhibited greater canal transportation, indicating the need for cautious use in anatomically complex canals, particularly those with pronounced curvature or thin dentinal walls [¹²,¹³]. In this context, the NiTi manual file (SMF) may serve as a viable alternative in clinical settings lacking endodontic motors, offering enhanced flexibility compared to stainless steel files and intermediate performance in both instrumentation time and anatomical preservation [⁸,¹¹]. The widespread clinical adoption of mechanized systems for primary teeth still requires further validation through randomized clinical trials evaluating not only procedural parameters but also microbiological success rates and long-term treatment outcomes [¹⁶,²⁸,²⁹].

Conclusion

The results of the present study encourage the use of manual and mechanized nickel-titanium files in the instrumentation of deciduous teeth, since they are faster and, depending on the system, can demonstrate reduced root canal transportation compared to conventional stainless-steel files. However, as observed, the WOG system showed greater transportation, suggesting caution in its application in anatomically challenging canals. All systems provided a satisfactory amount of remaining resin and a percentage volume increase, providing a model that preserves the original anatomy of the root canals and enabling a more comfortable and effective treatment for both the professional and the child. Further studies on the creation of reciprocating systems for primary teeth should be considered.

• Financial Support
  Brazilian Council for Scientific and Technological Development (CNPQ: Process #130718/2022-0), Multi-User Laboratory of School of Dentistry (FOA-UNESP), FINEP (FINEP/CT-INFRA - Process FINEP: #01.12.0530.00 - PROINFRA 01/2011), and FAPESP (Process #2021/06342-7).

Data Availability

The data used to support the findings of this study can be made available upon request to the corresponding author.

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Edited by

• Academic Editor:
  Fabio Luiz Cunha D'Assunção

Publication Dates

• Publication in this collection
  23 Mar 2026
• Date of issue
  2026

History

• Received
  31 Mar 2025
• Reviewed
  02 June 2025
• Accepted
  14 July 2025