ORIGINAL RESEARCH

https://doi.org/10.5005/jp-journals-10015-2570
World Journal of Dentistry
Volume 16 | Issue 1 | Year 2025

Influence of Contracted Endodontic Cavity Design on the Debridement Efficacy of Three Different Irrigant Activation Systems in Human Permanent Mandibular Molars: A Scanning Electron Microscopy Analysis

Vignesh Srinivasan¹, Srilekha Jayakumar², Janani Karunakaran³, Jwaalaa Rajkumar⁴, Vashni Solomon⁵, Aarthi Thiagarajan⁶

^1–5Department of Conservative Dentistry and Endodontics, Chettinad Dental College and Research Institute, Chengalpattu, Tamil Nadu, India

⁶Department of Oral and Maxillofacial Surgery, Chettinad Dental College and Research Institute, Chengalpattu, Tamil Nadu, India

Corresponding Author: Janani Karunakaran, Department of Conservative Dentistry and Endodontics, Chettinad Dental College and Research Institute, Chengalpattu, Tamil Nadu, India, Phone: +91 9790819879, email: janani6694@gmail.com

Received: 10 January 2025; Accepted: 14 February 2025; Published on: 13 March 2025

ABSTRACT

Aim: To evaluate the influence of contracted endodontic cavity (CEC) design on debridement efficacy of laser-activated (LA), ultrasonic-activated (UA), and sonic-activated (SA) irrigant systems in the distal canals of human permanent mandibular molars using scanning electron microscopy (SEM) analysis.

Materials and methods: Sixty (N = 60) freshly extracted human intact permanent mandibular first molars were randomly assigned to one of two groups: traditional endodontic cavity (TEC) and CEC (n = 30 per group). A preoperative cone-beam computed tomography (CBCT) scan was taken. After access cavity preparation, the canals were instrumented with an Mtwo rotary system up to size #30, 0.06 taper. During instrumentation, each canal was irrigated with 3 mL of 5.25% NaOCl followed by 3 mL of 17% ethylenediaminetetraacetic acid (EDTA) for 1 minute. Following cleaning and shaping, samples were randomized for final irrigation into three treatment subgroups: laser, ultrasonic, and sonic (n = 10 per subgroup). Final irrigant activation was done with 5 mL of 5.25% NaOCl, 5 mL of 17% EDTA, and lastly, 5 mL of 5.25% NaOCl using either LA, UA, or SA. All specimens were decoronated, and the distal roots were separated from the mesial using a diamond disk. The distal roots were then split longitudinally and viewed under an SEM for debris removal. Statistical analysis was done using an unpaired t-test.

Results: Overall, debris scores were similar between TEC and CEC at all levels of the root canals. In the intergroup analysis, LA irrigants showed maximum debris removal at all levels of the root canals compared to UA and SA irrigants in both TEC and CEC.

Conclusion: The debridement efficacy of CEC in the distal root of the mandibular molar was comparable to TEC irrespective of the activation system. However, LA irrigants were more efficient in removing debris in both TEC and CEC at the coronal third, middle third, and apical third of the root canal.

Clinical significance: CEC design preserves more pericervical dentin compared to traditional access cavities, thereby increasing fracture resistance. However, the reduced cavity size may pose a challenge in cleaning and shaping of the canal system, which could compromise debridement efficacy. Hence, irrigation activation plays a pivotal role. The findings of this study influence clinical guidelines, helping practitioners decide when and how to use CEC designs and which irrigant activation system to employ for achieving optimal results.

Keywords: Contracted endodontic access cavity, Debridement efficacy, Endo activator, Passive ultrasonic irrigation, Pericervical dentin, Traditional endodontic access cavity

How to cite this article: Srinivasan V, Jayakumar S, Karunakaran J, et al. Influence of Contracted Endodontic Cavity Design on the Debridement Efficacy of Three Different Irrigant Activation Systems in Human Permanent Mandibular Molars: A Scanning Electron Microscopy Analysis. World J Dent 2025;16(1):62–68.

Source of support: Nil

Conflict of interest: None

INTRODUCTION

The success of root canal treatment depends on proper shaping, cleaning, and obturation of the root canal system.¹ However, the long-term survival of endodontically treated teeth (ETT) depends upon their ability to withstand masticatory forces. Numerous factors, such as carious destruction, access cavity preparation, irrigants,² intracanal medicaments,³ and excessive removal of radicular dentin, can cause irreparable damage to the tooth structure.⁴ Previous studies have reported that the risk of fracture in ETT is directly related to the amount of dentin tissue lost.⁵ Traditional endodontic cavity (TEC) preparation tends to focus on the operator’s needs more than the structural needs, thereby reducing its fracture resistance,⁶ and is the second-largest cause of loss of tooth structure.⁷

Recently, various conservative access cavity designs, such as contracted endodontic cavity (CEC), ninja access/ultraconservative (NEC), and truss/orifice-directed dentin conservation (DDC) access, have been developed as an alternative to TEC. These designs aim to minimize the removal of pericervical dentin (PCD), which directly influences the longevity of an endodontically treated tooth, thus maintaining the biomechanical integrity of the tooth and its prognosis. CEC cavity designs permit greater preservation of PCD and soffit. Directed dentin preservation is an approach that emphasizes the “preservation of PCD” during root canal therapy and restorative procedures. Conservative endodontic cavity design and biomimetic obturation allow for “targeted dentin conservation” without compromising tooth strength.⁸

However, there is a possibility of missing some root canal orifices, which may negatively impact instrumentation efficacy,⁹ thereby preventing complete disinfection and debridement of the root canal system and pulp chamber. However, this can be overcome by the use of cone-beam computed tomography and a dental operating microscope.¹⁰ Debridement of the pulp chamber, especially in CEC design, becomes potentially difficult due to the preservation of soffit, which requires the use of irrigant activation methods and ultraflexible instruments. However, clinicians should take the following benefits of CEC into consideration: CEC design has advantages such as PCD preservation, less disruption of tooth morphology, and increased fracture resistance of the tooth.¹¹

Irrigation activation is a critical aspect of root canal therapy (RCT) and plays a vital role in the cleaning and disinfection of the root canal system. The primary goal of irrigation is to remove debris, accumulated hard tissue debris (AHTD), and smear layer from the canal system, ensuring a successful treatment outcome. The use of an irrigant activation system improves the penetration of irrigants, effective removal of biofilm and smear layer, and clinical outcomes. The efficacy of irrigant activation systems in debriding complex root canal anatomy, especially in oval-shaped distal canals of mandibular molars using TEC, is well documented.¹² Some of the most commonly used activation systems include ultrasonic activation (UA), sonic activation (SA), and laser activation (LA).¹³ LA of irrigants is an advanced technique for endodontic disinfection based on the principle of “photoacoustic agitation.” SA and UA are based on using low-frequency (sonic) and high-frequency (ultrasonic) sound waves to activate the irrigant solution, respectively.¹⁴^,¹⁵

The use of irrigant activation systems could potentially improve debridement efficacy in teeth with contracted endodontic access cavities, which has not been investigated before. Hence, this study aimed to evaluate the influence of CEC on the debridement efficacy of three different irrigant activation systems—UA, SA, and LA—in the distal canal of human permanent mandibular first molars using scanning electron microscopy (SEM) analysis.

MATERIALS AND METHODS

Sample Selection and Preparation

The study was carried out between June 2024 and August 2024 after obtaining clearance from the Institutional Human Ethical Committee (IHEC-CDCRI/2024/FAC/0041). Sixty freshly extracted intact human permanent mandibular first molars were selected for the study. After cleaning the root surfaces with 2.5% sodium hypochlorite (NaOCl), the samples were stored in individual vials containing 5 mL of 2% thymol solution at room temperature until use. Teeth devoid of caries and cracks, teeth with three canals, and intact crowns with mature root apices were included in the study. Teeth with extra canals, aberrant root anatomy, and root curvatures greater than 25° (Schneider’s method) were excluded. All samples were scanned using CBCT (Planmeca, Finland) with Romexis software (voxel size: 75–150 μm). Teeth with a root length of 15 mm and an apical canal diameter of ISO size 15 were selected for the study.

Sample Size Calculation and Randomization

The sample size was calculated based on a pilot study and was estimated to be sixty teeth (N = 60). The power of the study was set at 80%, with an α error of 5%. The selected specimens were randomly allotted to one of two groups (TEC or CEC), which were further subdivided into one of three subgroups (UA, SA, or LA).

Access Cavity Preparation

All the specimens were randomly allotted to two groups (n = 30).

Traditional endodontic cavity (TEC): In this group, access cavities were prepared using an Endo-Access bur, 21 mm, size 2 (Dentsply Maillefer, Ballaigues, Switzerland) with a high-speed handpiece under DOM (Labomed Prima DNT Surgical Microscope). The pulp chamber was completely deroofed, and an unimpeded (straight-line) access to the primary curvature of the root canal was established.

Contracted endodontic cavity (CEC): Using CBCT dimensions as guidance, an access cavity was prepared using size 856 diamond points (SS White Dental, Lakewood, NJ) with a high-speed handpiece under DOM. The tooth was accessed at the central fossa and extended only as much as necessary to locate canal orifices, preserving the PCD and part of the chamber roof. No further attempt was made to remove the coronal tooth structure for improved visibility of the three orifices.

Root Canal Preparation

Root canals were negotiated with a size 10 K-file (M.access Dentsply Maillefer, Ballaigues, Switzerland) until the tip was visualized at the apical foramen, and the working length (WL) was established 1 mm short of the apex. All canals were instrumented to WL with two nickel-titanium rotary instruments (VDW, Munich, Germany), up to size #30, 0.06 taper, in a closed apical system by blocking the apical foramen with resin. Irrigation parameters, such as volume and duration, were standardized for both groups. During instrumentation, the canals were irrigated with 3 mL of 5.25% NaOCl (Parcan, Septodont, India) between each instrument. Each sample was then irrigated with 3 mL of 17% EDTA (Dent Wash, Prime Dental Products, India), followed by 3 mL of normal saline at the end of instrumentation.

Final Irrigation Protocol

An equal number of specimens in each group received one of the following final irrigation protocols: UA, SA, or LA.

Ultrasonic Activation of Irrigants

The final irrigation protocol consisted of three activation cycles: first with 5 mL of 5.25% NaOCl, followed by 5 mL of 17% EDTA, and finally with 5 mL of 5.25% NaOCl. Each irrigant was activated for 20 seconds using a size 25 IrriSafe file (Satelec, Acteon Group, Mérignac, France) positioned 2 mm from WL. The IrriSafe file was mounted on a Suprasson P5 Booster ultrasonic unit (Satelec, Acteon Group) with a frequency of 30 kHz and a power setting of 5.

Sonic Activation of Irrigants

The final irrigation was carried out with 5 mL of 5.25% NaOCl, followed by 5 mL of 17% EDTA, and finally 5 mL of 5.25% NaOCl. Each irrigant was activated for 20 seconds (three activation cycles) using a 25/.04 EA tip (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) positioned 2 mm from WL. The handpiece was activated at 10,000 cycles/minute, which translates to 0.166–0.3 kHz.

Laser Activation of Irrigants

A 2940-nm wavelength Er:YAG laser equipped with a handpiece H14 was used (LightWalker AT, Fotona, Ljubljana, Slovenia). The laser parameters were 0.3 W, 15 Hz, 20 mJ, SSP mode, with air and water turned off. The pulse length was 50 ms. A 9-mm-long conical 600-μm fiber tip (PIPS 600/9, Fotona) was placed into the pulp chamber, kept stationary, and activated.

The irrigant flow rate was 2.5 mL/minute for all groups. All specimens were then irrigated with 5 mL of normal saline and dried with sterile paper points.

Sample Preparation for Scanning Electron Microscopy Analysis

Following the root canal procedure, all specimens were removed from the resin block. The distal roots were separated from the mesial root using a diamond disc. The selected samples were then split longitudinally. Two shallow longitudinal grooves were cut on each side of the root in a buccolingual direction, ensuring that the grooves did not penetrate the canal. The roots were then split using a chisel and mallet, and both root halves were prepared for SEM investigation (JSM-7001F; JEOL, Tokyo, Japan).

All root canals were evaluated for canal wall debridement in the coronal, middle, and apical thirds of the root canal. The area with the greatest amount of debris was photographed for each third of the canal at ×1000 magnification. The dentin debris was scored based on the Hülsmann grading system.¹⁶

Score 1: Clean root canal wall, only a few small debris particles.

Score 2: Few small agglomerations of debris.

Score 3: Many agglomerations of debris covering <50% of the root canal wall.

Score 4: More than 50% of the root canal wall covered by debris.

Score 5: Complete or nearly complete root canal wall covered by debris.

The SEM evaluation was conducted by two senior endodontists who were blinded to the groups. Both endodontists were calibrated using SEM images before evaluation.

Statistical Analysis

The data were analyzed using Statistical Package for the Social Sciences (SPSS) for Windows (ver. 26.0, IBM Corp., Armonk, NY). Statistical analysis was performed using the unpaired t-test and one-way ANOVA, with the significance level set at p ≤ 0.05.

RESULTS

The kappa statistic for interobserver reliability was 0.711, indicating good agreement between the examiners. There was no statistically significant difference in debris scores at the coronal third, middle third, and apical third of the root between TEC and CEC for ultrasonic, sonic, and laser activation of irrigation (p > 0.05) (Table 1).

**Table 1:** Mean debris scores between TEC and CEC at coronal third, middle third, and apical third of root canal according to different activation of irrigants
		Coronal third Mean ± SD	Middle third Mean ± SD	Apical third Mean ± SD
Ultrasonic-based	TEC	2.6 ± 0.771	2.9 ± 0.5	2.5 ± 0.730
	CEC	3 ± 1.032	2.5 ± 0.655	2.51 ± 0.516
	p-value	0.760 (NS)	0.133 (NS)	0.69 (NS)
Sonic-based	TEC	3.687 ± 0.478	3.5 ± 0.447	3.01 ± 0.5
	CEC	3.93 ± 0.573	3.2 ± 0.655	2.7 ± 0.516
	p-value	0.21 (NS)	0.65 (NS)	0.483 (NS)
Laser-based	TEC	2.2 ± 0.23	2.3 ± 0.31	2.2 ± 0.54
	CEC	2.05 ± 0.34	2.201 ± 0.45	2.425 ± 0.65
	p-value	p = 0.78 (NS)	p = 0.89 (NS)	p = 0.85 (NS)

CEC, contracted endodontic cavity; NS, not significant using unpaired t-test; TEC, traditional endodontic cavity

Traditional Endodontic Cavity

It was found that LA irrigants showed better debridement efficacy in the coronal, middle, and apical thirds of the root canal, followed by UA, with the least efficacy observed for SA irrigants. SA-activated irrigants had higher debris scores than LA at the coronal, middle, and apical thirds of the root canals. This difference was statistically significant (p < 0.05) (Table 2).

**Table 2:** Comparison of mean debris scores between three different irrigation systems for TEC and CEC at coronal third, middle third, and apical third of root canal
		Ultrasonic-based activation Mean ± SD	Sonic-based activation Mean ± SD	Laser-based activation Mean ± SD	p-value
TEC	Coronal third	2.6 ± 0.771	3.687 ± 0.478	2.2 ± 0.23	0.002**
	Middle third	2.9 ± 0.5	3.5 ± 0.447	2.3 ± 0.31	0.037*
	Apical third	2.5 ± 0.730	3.01 ± 0.5	2.2 ± 0.54	0.001**
CEC	Coronal third	3.00 ± 1.032	3.93 ± 0.573	2.05 ± 0.34	0.005**
	Middle third	2.5 ± 0.619	3.2 ± 0.655	2.201 ± 0.45	0.001**
	Apical third	2.51 ± 0.516	2.7 ± 0.517	2.425 ± 0.65	0.000**

CEC, contracted endodontic cavity; TEC, traditional endodontic cavity; Statistically significant at *p < 0.05 and **p < 0.01 using one-way ANOVA test

Contracted Endodontic Cavity

It was found that laser-activated irrigants had the lowest debris scores and demonstrated the highest debridement efficacy in the coronal, middle, and apical thirds of the root canal. In contrast, sonic-activated irrigants had higher debris scores across all three thirds of the root canal. This difference was statistically significant (p < 0.05).

SEM evaluation revealed that specimens subjected to laser-activated irrigants had the least debris, whereas those subjected to sonic-activated irrigants had the highest debris scores (Fig. 1 for TEC; Fig. 2 for CEC).

Figs 1A to F: A representative SEM image of debris covering the canal wall in the apical third for laser-activated (A, D), ultrasonic-activated (B, E), and sonic-activated (C, F) irrigants at ×1000 magnification in TEC

Figs 2A to F: A representative SEM image of debris covering the canal wall in the apical third for laser-activated (A, D), ultrasonic-activated (B, E), and sonic-activated (C, F) irrigants at ×1000 magnification in CEC

DISCUSSION

The emerging concept of contracted endodontic cavity (CEC) design is an alternative to TEC, aiming to minimize the removal of tooth structure while maintaining the biomechanical integrity of the tooth. CEC preserves pericervical dentin (PCD) and soffit; however, the real challenge lies in shaping and cleaning the root canals with limited access.⁹ Hence, it becomes imperative to evaluate debridement efficacy in CEC, particularly with the use of suitable irrigant activation systems. Irrigants play a crucial role in disinfecting and cleaning the root canal complex.¹⁷ Their effectiveness is enhanced by agitation through advanced techniques such as ultrasonic and laser activation.¹⁸ The irrigating solution used in the present study was NaOCl due to its antimicrobial and tissue-dissolving properties, while EDTA was used for its ability to remove the inorganic component of the smear layer.

In this study, we found that none of the activation systems could completely remove debris from the root canal walls. This suggests that all activation systems had a similar effect, regardless of access cavity design (CEC or TEC). This finding aligns with studies by Rödig et al.¹⁹ and Varela et al.,²⁰ which reported incomplete debris removal from root canals in TEC. Similarly, Gündüz and Ozlek reported no significant differences in debris and smear removal between CEC and TEC when activated using laser, ultrasound, or conventional irrigation methods.²¹ A meta-analysis by Virdee et al. found improved cleanliness of root canals when irrigants were activated; however, they could not recommend the best activation technique due to data heterogeneity.²² Regardless of the activation method, irrigation enhances the flow of solutions into minute accessory canals, aiding in debris removal.²³^,²⁴ This likely explains the insignificant differences between TEC and CEC in our study.

We also compared the effectiveness of different activation systems by assessing debris scores at the coronal, middle, and apical thirds of the root canal. Our findings showed that, for both TEC and CEC, laser-activated (LA) irrigants resulted in significantly lower debris scores compared to ultrasonic-activated (UA) and sonic-activated (SA) irrigants across all levels of the root canal. These results are supported by Uslu et al. and Tong et al., who reported that laser-activated irrigants demonstrated superior smear removal, particularly at the apical third, when compared to ultrasonic and sonic activation.²⁵^,²⁶

Recently, laser-activated (LA) irrigation has gained popularity in dentistry, particularly with Er:YAG lasers, which have demonstrated superior cleaning efficiency in root canal treatment.²⁷ When activated by lasers, irrigants are heated beyond their boiling point, forming a vapor bubble after each pulse at the fiber tip. This bubble collapses upon reaching maximum volume, generating a cavitation effect that produces turbulent photoacoustic agitation. This process facilitates three-dimensional irrigant flushing within the root canal system, effectively removing debris and smear layers.²⁸ Despite the inherent challenges in cleaning the apical third due to root canal complexities, our study demonstrated that laser activation is the most effective method for debris removal, irrespective of access cavity design.

However, some authors, such as Passalidou et al., found no significant differences in smear scores between laser-activated irrigants and needle-syringe irrigation.²⁹ Additionally, in a literature review, Do and Gaudin et al. concluded that laser activation significantly improved debris removal in only four out of eleven studies when compared to other methods.³⁰ In our study, while LA had lower debris scores, no statistically significant difference was observed between laser-activated and ultrasound-activated irrigants at any level of the root canal. Kamaci et al. similarly reported that ultrasonically activated irrigation removed more debris than conventional methods but had comparable efficacy to laser activation.³¹

Ultrasonically activated irrigants (UA) are widely used in clinical settings and have demonstrated promising results due to acoustic microstreaming and cavitation effects.³² Additionally, UAI offers advantages such as vapor lock reduction, increased NaOCl reaction rate, and ultrasonic oscillation, which exerts circumferential shear stress on canal walls, aiding in debris removal.³³ UAI is often chosen for irrigant activation, either alone or in combination with other systems.³⁴

In contrast, our study found that sonic-activated (SA) irrigants had the highest debris scores compared to the other two systems. SA is a commonly used activation device that has been reported to yield significantly better results in debris removal than ultrasonic-based systems for final irrigation. Interestingly, our findings contradict studies by Chu et al.,¹⁸ Rödig et al.,²¹ Gadaalay et al.,³⁵ and Raza et al.,³⁶ which reported that sonically activated irrigants had superior smear removal compared to ultrasonic activation or comparable results between the two. However, a systematic review and network meta-analysis by Natanasabapathy et al. concluded that SA was less effective than UA, which aligns with our findings.³⁷ The high debris scores associated with SA may be attributed to its lower oscillation frequency (1–10 kHz) compared to UA (25–40 kHz), leading to reduced streaming velocity and shear stress, ultimately limiting its cleaning ability.²⁶ While our findings contrast with many previously published studies, we recommend further research to establish scientific consensus.

Clinical Significance

The clinical significance of this study lies in its potential to improve debridement efficacy during root canal therapy, particularly in mandibular molars with contracted access cavity designs. Understanding which activation system is most effective in CEC can help clinicians optimize irrigation protocols. For example, if a particular system, such as laser activation, is proven superior—as demonstrated in this study—it could become a standard recommendation for cases with restricted access or complex canal anatomies.

Limitations

This study had a few limitations:

The in vitro design excluded certain variables that may be unavoidable in clinical settings.
Further in vivo studies evaluating debridement efficacy in the pulp chamber and curved root canals are necessary to fully understand the influence of CEC when different irrigant activation systems are used.

CONCLUSION

It can be concluded that, within the limitations of this in vitro study, complete debridement of the root canal system remains a challenge for clinicians. The access cavity design (TEC or CEC) did not impact debris scores with any of the irrigant activation systems. Laser-activated irrigants significantly reduced debris scores at all levels of the root canal, followed by ultrasonic-activated and sonic-activated irrigants, respectively. Additionally, the outcome of endodontic treatment depends on various factors, including the preoperative condition of the tooth, patient-related factors, and the type of tooth involved, all of which clinicians should consider.

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