Laboratory Research
Impact of traditional, conservative and ultraconservative access cavities on root canal shaping
Luis F. Jiménez-Rojas, MSc 1
Kaline Romeiro, PhD 1
Sabrina C. Brasil, PhD 1
Renata Perez, PhD 2
José C. Provenzano, PhD 1
Fábio R. Pires, PhD 3
Flávio R. F. Alves, PhD 1
Isabela N. Rôças, PhD 1
José F. Siqueira Jr, PhD 1
https://doi.org/10.71347/kba25jr7
1 Postgraduate Program in Dentistry, University of Grande Rio (UNIGRANRIO), Rio de Janeiro, RJ, Brazil.
2 North of Minas United Faculty, Department of Endodontics, Ipatinga, MG, Brazil.
3 Postgraduate Program in Dentistry, Estácio de Sá University, Rio de Janeiro, RJ, Brazil.
Corresponding author:
Flávio R. F. Alves, PhD, Postgraduate Program in Dentistry, University of Grande Rio (UNIGRANRIO) Rua Professor José de Souza Herdy, 1160, Duque de Caxias, RJ, Brazil 25071-202
e-mail: flavioferreiraalves@gmail.com
Key words: Access cavity; micro-computed tomography; minimally invasive endodontics; root canal preparation.
Acknowledgements: Supported by grants from the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazilian Governmental Institutions.
The authors deny any conflicts of interest.
Abstract
Objective: This study evaluated the impact of traditional, conservative/contracted, and ultraconservative/”ninja” access cavities on root canal shaping using micro-computed tomography (micro-CT).
Methods: Mesial canals from freshly extracted mandibular molars with pulp vitality were scanned in micro- CT for sample selection. Specimens matched by anatomic similarities were distributed into 3 experimental groups according to the endodontic access cavity types (traditional, conservative, and ultraconservative). After canal preparation using R-Motion instruments size 25.06, the specimens were once again scanned to assess diverse shaping parameters.
Results: There were significant differences in the increment rate of root canal volume and area between traditional and ultraconservative access cavities (P < .05). The amount of unprepared canal areas was significantly higher with the ultraconservative access cavity in comparison with the other groups (P < .05). Intergroup analysis did not reveal significant differences in the structure model index, amount of accumulated hard tissue debris, dentin removed, and remaining thickness of pericervical dentin (P > .05).
Conclusions: Ultraconservative access cavities resulted in significantly less prepared canal areas than traditional and conservative endodontic cavities. The latter two, in turn, showed no significant differences in the parameters evaluated.
Introduction
Conservative access cavities have been developed for endodontics following the concept of minimally invasive dentistry (Gutmann 2013). The rationale behind this concept takes into account that preserving the maximum extent of the pulp chamber roof during the access preparation would improve fracture resistance and favor the long-term survival of root canal-treated teeth (Clark & Khademi 2010). This is crucial because the removal of substantial amounts of healthy dentin during endodontic and restorative procedures is a significant factor contributing to tooth susceptibility to fracture (Kishen 2015).
Remains of infected pulp tissue can be a source of persistent infection, leading to posttreatment apical periodontitis (Siqueira & Rôças 2008). Consequently, effective disinfection of the canal system becomes of paramount importance to a favorable outcome. It has been demonstrated that minimally invasive approaches may result in a greater number of unprepared areas (Krishan et al. 2014, Barbosa et al. 2020). These untreated areas can harbor unaffected residual bacterial biofilms, which represents a reservoir for persistent infections to cause or sustain periradicular inflammation (Siqueira et al. 2018b).
A new generation of reciprocating files has been designed to contribute to the philosophy of minimally invasive endodontics. The R-Motion instrument presents a triangular cross-section with sharp edges, rounded free faces, and an optimized tip with a patented heat-treated alloy. According to the manufacturer, it features reduced core sizes that are less invasive than equivalent competitive files, reducing the risk of excessive dentin removal. In this way, the shaping ability is similar to instruments with a higher taper, achieved with less dentin removal in the danger zones (Carvalho et al. 2022). Moreover, R-Motion instruments have demonstrated high resistance to cyclic fatigue and rotation angle (Basturk et al. 2022). However, very few studies have evaluated the shaping ability and the total volume of dentine removed associated with reciprocating MaxWire instruments (Basturk et al. 2022, Guedes et al. 2022).
To date, there is no consensus on the amount of prepared canal walls after conservative access in mandibular molars. Discrepant findings are evident across studies, with two reporting no difference (Krishan et al. 2014, Augusto et al. 2020), and two favoring the conventional approach (Barbosa et al. 2020, Lima et al. 2020). Therefore, the present study aimed to evaluate the impact of conservative and ultraconservative access cavities on root canal shaping using R-Motion instruments in extracted mandibular molars, as evaluated by micro-CT imaging. Traditional endodontic access cavities served as the reference technique for comparative analysis. The null hypothesis was that there are no differences among the access cavities designs.
Materials and methods
Sample selection
The study protocol was approved by the Bioethics Committee of the School of Dentistry (CB-172- 2024). The specimens used in this study were human mandibular molars with vital pulps freshly extracted for reasons not related to this study. Immediately after extraction, the teeth had their external surfaces decontaminated with 2.5% NaOCl, they were placed in sterile saline solution (0.9% NaCl) and stored until the time of use. The teeth were not stored dry or in formalin, in order to prevent pronounced changes to the dentin structure and features. Pulp vitality was confirmed by the presence of a bulk of soft tissue in the pulp chamber, after access preparation.
For the initial selection of molars having two mesial canals that merged into one in the apical root segment, periapical radiographs were taken in the buccolingual and mesiodistal directions, using digital radiography system RVG First (Trophy Imaging, Atlanta, GE, USA). Each tooth was also inspected under an operating microscope to verify that the apices were completely formed and that the root and coronal surfaces were free of internal and/or external fractures, fissures, or resorptions. Ultrasonic cleaning of the outer root surfaces was performed to remove any tissue remnants.
Later, micro-computed tomography (micro- CT) scans of the selected teeth were performed as described below. Teeth confirmed to have two mesial canals that merged to one in the apical segment were selected and matched in trios, based on micro-CT data regarding anatomical similarities, preoperative canal volume and surface area, and tooth length.
Specimen preparation
The final sample comprised mesial root canals from 39 mandibular molars. Because of the mesial root configuration, the two mesial canals were evaluated as a unit. The sample size was calculated using the G*Power 3.1 software (Heinrich Heine Universität, Düsseldorf, Germany) and was based on a previous study (Santosh et al. 2021). It was performed with the alpha-type error set at 0.05 and power at 80%, resulting in 10 specimens per group. Thirteen teeth were used in each group to compensate for possible losses.
Access cavity preparation
Micro-CT images were used to plan the preparation of conservative and ultraconservative access cavities, by designing its trajectory towards the entrance orifice of the root canal, in the sense that it required the least possible removal of dental structure. The pre-established micro-CT measurements were transferred to the occlusal face of the specimen with the help of a fine point indelible ink marker. This indicated the exact place to perform the access cavity. The access was made under magnification with an operating microscope in all groups. The shaft diameter of the RaCe R-Motion instrument (FKG Dentaire, La Chaux-de-Fonds, Switzerland) was measured using a digital caliper so that the access dimension was not less than this measurement.
Traditional access cavity (TradAC). The access cavity was prepared by completely removing the pulp chamber roof in order to achieve straight-line access to the root canal orifices, with slight divergence of the axial walls, so that all orifices could be visualized at the same time. For the access cavity procedure, #2 and #4 round carbide burs at high speed were used, until reaching the pulp chamber, and then the Endo-Z bur was used to prepare the final shape.
Conservative (or contracted) access cavity (ConsAC). Preparation started in the central fossa of the occlusal surface and extended with slight convergence of the axial walls towards the occlusal surface, only as far as it was necessary to detect the root canal orifices, preserving part of the pulp chamber roof. The procedure was performed using #2 round carbide and Endo-Z burs.
Ultraconservative access cavity (UltraAC). Also known as “ninja” access cavity. Preparation started as described in the conservative group, but without any extension, keeping as much as possible of the pulp chamber roof. Only a #2 round carbide bur was used in this group. The “ninja” access is derived from the oblique projection towards the central fossa of the root canal orifices in the occlusal plane. By doing this, localization of all root canal orifices from different visual angles was possible.
Root canal preparation
Size 10 and 15 manual K-files (Dentsply Maillefer, Tulsa, OK, USA) were used to explore each root canal until the tip of the instrument was visible at the apical foramen under an operating microscope. This measurement was recorded as the working length (WL). The apical foramen was sealed with TopDam sealer (FGM, Joinville, SC, Brazil) to create a closed system. The root was wrapped in moistened gauze during handling. A single dental operator performed all intracanal procedures.
Each mesial canal was initially irrigated with 2 mL 2.5% NaOCl. Canal preparation was performed with RaCe R-Motion instruments (FKG Dentaire) driven by a VDW Silver motor (VDW, Munich, Germany) in reciprocating motion. The R-Motion Glider instrument (15.03) was used to establish patency up to the WL, while the R-Motion instrument (25.06) completed preparation. The instruments were used with gentle back-and-forth movements of 2- to 3-mm amplitude and minimal apical pressure. After 3 back-and-forth motions, the instrument was removed, cleaned, and the canal was irrigated with 3 mL 2.5% NaOCl. A 30-gauge needle (NaviTip, Ultradent, UT) was used for irrigation, placed 3 mm short of the WL. Patency was always confirmed with an ISO #10 manual file.
The same instrumentation procedures, including irrigation time and volume, were applied for all three groups, with a total NaOCl irrigation volume of 14 mL. Each instrument prepared 4 canals (2 roots).
There was no instrument fracture during the mechanical root canal preparation, but one instrument exhibited severe plastic deformation after its first use. The distal roots served as the non- instrumented control group.
Micro-CT analysis
Micro-CT scans were used to assess the root canal anatomy for sample selection, standardize the distribution between groups by matching teeth by volume and anatomical similarities, plan access cavity preparation, and analyze the root canal shaping. The specimens were mounted on a custom platform and scanned in a micro-CT device (SkyScan 1174. v2; Bruker-microCT Kontich, Belgium), before and after chemomechanical preparation. The same acquisition parameters were used before and after instrumentation: 50 kV, 800 μA, pixel size of 18.99 μm, 0.5 mm aluminum filter, 5200 ms exposure, 1° rotation step, and 180° rotation around the vertical axis.
Images were reconstructed using NRecon v.1.6.9 (Brucker-microCT) with a 50% beam hardening correction and 14-ring artifact correction, resulting in the acquisition of 700–800 transverse cross- sections per tooth in a bitmap format. The volume of interest extended from the cementoenamel junction to the apex of the mesial roots.
Thereafter, image dataset pairs (same specimen, before and after instrumentation) were registered to ensure they were positioned in the same spatial coordinates. To perform the co-registration procedure, the datasets were exported in ‘’.nrrd’’ format to the 3DSlicer software (http://www.slicer. org). The sound canal volume (initial dataset) was considered as the reference volume, to which the other datasets corresponding to the different instrumentation steps were registered. An ‘’Affine’’ algorithm with 12 degrees of freedom was used.
After reconstruction and registration, all subsequent image analysis steps were performed using the FIJI software (open access). The first segmentation step was a subtraction operation to obtain the initial volume of the uninstrumented canal. The obtained image volume was duplicated, and the root canal was digitally filled from the duplicated stack. Then, the stack of the initial root canal was digitally filled from the duplicated stack. Then, the stack of the initial root canal after digital filling was subtracted from the initial segmented root canal volume to have the uninstrumented canal volume.
Similar procedures were applied to obtain the instrumented root canal volume and quantify removed dentin volume, accumulated debris, and unprepared areas, following the methodology outlined in a previous study (Moraes et al. 2021). Volumetric calculations were executed using ImageJ software (FIJI version) plug-in 3D Object Counter.
For calculation of removed dentin volume from the canal during instrumentation steps, the initial canal volume was subtracted from the instrumented root canal volume. The presence of material with a density similar to dentin in regions previously occupied by air in the root canal space was considered as hard tissue debris. Therefore, the image dataset was subtracted from the volume of the instrumented canal to obtain the volume of accumulated hard tissue debris along the root canal. After this procedure, for the calculation of unprepared areas, the instrumented canal volume was subtracted from the initial canal volume. The amount of unprepared areas was calculated in terms of percentage determined by the number of static voxels (voxels present in the same position on the canal wall before and after instrumentation as unprepared area) divided by the total number of voxels present on the initial root canal surface. The structure model index (SMI) is a measure of surface convexity and was used to determine the root canal roundness; values range from 0 to 4, with 0 corresponding to parallel flat planes and 4 to a perfect ball.
Three-dimensional models of the canal before and after preparation, non-instrumented areas, and total debris were rendered using CTVol v.2.2.1 software (Bruker-microCT) for visualization and qualitative evaluation of the 3D models.
Statistical analysis
The Shapiro-Wilk test was applied to verify the data normality. The Kruskal-Wallis test was used to compare groups regarding the canal area and the volume increase, amount of unprepared areas, and SMI before and after preparation.
Table 1. Data from micro-computed tomographic analyses before and after root canal preparation in teeth with different access cavities [mean (median; range)]
The same test was used to evaluate the amount of accumulated hard tissue debris, dentin removed, and remaining thickness of pericervical dentin after preparation. For intragroup analyses, Mann-Whitney U and t-tests were used to verify differences within canal volume, area, and SMI before and after preparation. The significance level was set at 5% for all statistical tests (P < .05).
Results
Intergroup analysis
Table 1 depicts the results for the evaluated shaping parameters and Figure 1 shows representative specimens from the experimental groups. There were no significant differences between groups regarding the initial volume, area, and SMI (P > .05). The increment rate of canal volume with root canal preparation was 47.63% (median, 51.03%) for the TradAC group, 45.62% (median, 36.16%) for the ConsAC group, and 22.40% (median, 26.61%) for the UltraAC group in the total canal length. There were significant differences between the UltraAC and the TradAC groups (P < .05). The increment rate in the canal area with root canal preparation was 28.40% (median, 21.37%) for the TradAC group, 27.05% (median, 11.08%) for the ConsAC group, and 3.63% (median, 3.40%) for the UltraAC group. Significant differences were observed between the UltraAC and TradAC groups (P < .05). The amount of unprepared surface area of the root canal was 16.03% (median, 16.02%) for the TradAC group, 17.32% (median, 16.60%) for the ConsAC, and 30.35% (median, 29.88%) for the UltraAC group, with significant differences between the UltraAC group and the two other groups (P < .05). No intergroup differences were observed for accumulated hard tissue debris, dentin removed, and remaining thickness of pericervical dentin after preparation (P > .05).
Figure 1. Representative 3D reconstructions of micro-computed tomography scans of teeth with traditional, conservative and ultraconservative access cavities. Images were obtained before (initial) and after root canal preparation with RaCe R-Motion instruments and show the amount of prepared and unprepared areas, as well as the accumulated hard tissue debris.
Intragroup analysis
In the TradAC group, canal volume, area, and SMI values significantly increased after preparation (P < .05). In the ConsAC group, the observed increase in canal volume and SMI reached statistical significance (P < .05), but not for the increase in area (P > .05). In the UltraAC group, no significant differences were noted in the canal volume, area, and SMI values before and after preparation (P > .05).
Discussion
This micro-CT study evaluated the impact of 3 different types of access cavities (TradAC, ConsAC, UltraAC) on the shaping of mandibular molar mesial canals. Freshly extracted teeth with vital pulps were used in order to avoid significant changes that may occur in dentin during drying and storage. The results indicated that the UltraAC design resulted in significantly less prepared canal surface areas, and a minimum increase in the total canal volume and surface area, in comparison with the other 2 groups. These findings reject the null hypothesis that the different cavity designs do not influence the root canal shaping. Other evaluated parameters, such as accumulated debris, dentin removed, and remaining dentin thickness at the pericervical area were not significantly affected by the access design.
The mean amount of unprepared canal surface area ranged from 16.03% (TradAC group) to 31.35% (UltraAC group). These values are within the range reported for reciprocating preparation of mesial canals of mandibular molars in previous studies: 18.1% (Siqueira et al. 2018b), 20.7% (Siqueira et al. 2013), 20.55%-35.31% (Gergi et al. 2015), and 25.8%-27.4% (Vertucci 1984). Previous studies demonstrated statistically significant differences in unprepared walls in root canals treated with TradAC versus ConsAC (Krishan et al. 2014, Barbosa et al. 2020, Xia et al. 2020) and TradAC versus truss access cavities (Barbosa et al. 2020). In contrast, some studies showed no significant differences in unprepared walls between TradAC versus ConsAC (Moore et al. 2016, Rover et al. 2017, Silva et al. 2020) and TradAC versus UltraAC (Silva et al. 2020). In posterior teeth, there was disagreement between studies, with 4 showing no differences (Moore et al. 2016, Rover et al. 2017, Augusto et al. 2020, Silva et al. 2020), and 3 favoring traditional access for affecting more canal walls (Krishan et al. 2014, Barbosa et al. 2020, Xia et al. 2020).
These findings in the UltraAC group are a reason of concern because significant limiting effects were observed in canal shaping. The intangible increase in both volume and area after preparation is in line with the minimally invasive concept, and this may have occurred by the fact that the UltraAC design restricted the lateral displacement of the instrument, resulting in less enlargement. However, the larger amount of unprepared walls pose a significant limitation, as residual biofilms and tissue remnants may persist on those areas, even with NaOCl irrigation (Siqueira et al. 2018a). Bacteria persisting on unprepared walls have been demonstrated to negatively influence the treatment outcome (Ricucci et al. 2016). Further studies are necessary to assess the long-term outcomes of endodontic treatment using minimally invasive access cavity designs.
The lack of difference in unprepared areas between TradAC and ConsAC aligns with some previous studies (Vieira et al. 2020, Freitas et al. 2021, Miranda et al. 2022). The possibility exists that the type of instrument may compensate for the restriction in lateral displacement posed by the contracted cavity walls. However, a study using an expanding instrument revealed a significantly poorer antibacterial results in the ConsAC group in comparison with TradAC, even though no significant differences were observed in shaping (Vieira et al. 2020). Further studies evaluating cleaning and disinfection, in addition to shaping, are required to evaluate the effects of minimally invasive cavities on the main objectives of chemomechanical preparation.
Three studies (Mustafa 2021, Carvalho et al. 2022,
Guedes et al. 2022) evaluated the behavior of this instrument in mesial canals of mandibular molars and one (Moriya et al. 2023) assessed its stress distribution patterns along simulated canal walls. The unprepared areas in the present study were similar to those reported by a previous study using TradAC (Guedes et al. 2022). Studies comparing the effects of access cavity designs on preparations using different instrument systems should be encouraged.
One limitation of ex vivo studies employing extracted teeth, like the present one, is the difficulty to replicate the dynamic and challenging clinical conditions that are faced by endodontists during in vivo procedures. The static nature of extracted teeth eliminates factors like patient movement and mouth opening, which could limit visualization and preparation procedures. Consequently, caution should be exercised when extrapolating the results of ex vivo studies to the clinical practice, and the findings should be considered within the context of their limitations.
Conclusions
In conclusion, ultraconservative access cavities did not offer advantages compared with traditional and conservative ones in some important shaping parameters. The latter two, in turn, showed no significant differences in the evaluated parameters. Future studies should adopt a correlative approach to assess the influence of access cavity designs on the cleaning, shaping, and disinfection effectiveness in infected root canals.
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