The primary objective of endodontic therapy is the thorough debridement and disinfection of the root canal system to facilitate healing and prevent reinfection [1]. This is achieved through a combination of mechanical instrumentation and chemical irrigation, a process known as chemomechanical preparation. However, mechanical instrumentation inevitably produces a smear layer, an amorphous layer of debris composed of dentin shavings, organic pulp remnants, and microorganisms, which covers the canal walls and occludes the dentinal tubules [2]. The presence of the smear layer is controversial, but it is generally considered detrimental to treatment outcomes. It can harbor bacteria, protect them from intracanal medicaments, and act as a physical barrier, preventing irrigants and sealers from penetrating the dentinal tubules [3]. Its removal is therefore advocated to enhance disinfection, improve the adaptation of obturation materials to the canal walls, and increase the bond strength of resin-based sealers, ultimately contributing to a superior apical seal [4]. Achieving complete smear layer removal is particularly challenging in root canals with complex anatomy, such as severe curvatures. In these cases, conventional needle irrigation (CNI) has been shown to be ineffective, especially in the apical third, due to the inability of the irrigant to penetrate and flush this critical region effectively, leading to the "vapor lock" effect [5]. To overcome these limitations, several irrigation activation systems have been developed to enhance the hydrodynamics and chemical action of irrigants within the canal space. These systems include sonic devices like the EndoActivator (Dentsply Sirona), which operates at a lower frequency to produce hydrodynamic agitation [6]; Passive Ultrasonic Irrigation (PUI), which utilizes higher-frequency oscillations to induce powerful acoustic streaming and cavitation effects [7]; and novel flexible instruments like the XP-Endo Finisher (FKG Dentaire), which employs a shape-memory NiTi alloy that adapts to the canal's morphology to mechanically scrub the walls [8]. More recently, Laser-Activated Irrigation (LAI) using Erbium family lasers (e.g., Er:YAG) has gained prominence. LAI generates powerful photoacoustic shockwaves within the irrigant, leading to explosive vapor bubble formation and collapse, which results in highly effective cleaning [9]. Several studies have compared the efficacy of these systems, often with conflicting results. Some have found PUI to be superior to sonic activation [10], while others report comparable efficacy between newer systems like the XP-Endo Finisher and PUI [11]. The performance of LAI has been consistently reported as excellent, yet direct comparisons against the full spectrum of modern activators in a standardized model of curved canals are limited. This research gap is significant because the geometric constraints of a curved canal can differentially affect the performance of each activation technology. Therefore, this study was designed to provide a comprehensive, direct comparison of these four distinct activation technologies. The aim of this in vitro study was to compare the effectiveness of the XP-Endo Finisher, EndoActivator, Passive Ultrasonic Irrigation, and Er:YAG Laser-Activated Irrigation on smear layer removal in the coronal, middle, and apical thirds of curved root canals. The null hypothesis was that there would be no significant difference in smear layer removal among the tested activation systems.

Study Design and Sample Selection Seventy-five recently extracted human mandibular molars, stored in 0.1% thymol solution, were selected. Inclusion criteria were: intact crowns, mature apices, no signs of cracks or fractures, no previous endodontic treatment, and a single, patent mesiobuccal canal with a curvature between 20° and 40°, as determined by Schneider's method on periapical radiographs. Teeth with calcifications, open apices, or root resorption were excluded. Sample Preparation The teeth were decoronated using a high-speed diamond bur under water coolant to create a flat occlusal surface and standardize the reference point. The working length (WL) was established by inserting a #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) into the mesiobuccal canal until its tip was visible at the apical foramen, and then subtracting 1 mm. Apical patency was confirmed by passing a #10 K-file 1 mm beyond the foramen. Instrumentation Protocol All root canals were instrumented using the ProTaper Gold rotary system (Dentsply Sirona, Ballaigues, Switzerland) with an X-Smart Plus electric motor (Dentsply Sirona) according to the manufacturer's instructions, up to an F2 file (size 25, .08 taper). During instrumentation, each canal was irrigated with 2 mL of 5.25% sodium hypochlorite (NaOCl) between each file change using a 30-gauge side-vented needle (Vista Dental Products, Racine, WI, USA) inserted to 2 mm short of the WL. After instrumentation, each canal was flushed with 5 mL of distilled water. Group Allocation and Final Irrigation Protocol The 75 samples were randomly divided into five groups (n=15) using a random number generator. The final irrigation protocol for all groups consisted of a 1-minute rinse with 1 mL of 17% ethylenediaminetetraacetic acid (EDTA) to target the smear layer, followed by the specific activation protocol for each group. • Group 1: XP-Endo Finisher (XPF) An XP-Endo Finisher file (size 25, .00 taper; FKG Dentaire, La Chaux-de-Fonds, Switzerland) was used with the canal full of EDTA. The file was activated using an endodontic motor at 800 rpm and 1 Ncm torque for 60 seconds, with gentle up-and-down movements of 7–8 mm. • Group 2: EndoActivator (EA) A size 25, .04 taper polymer tip was placed on the EndoActivator handpiece (Dentsply Sirona). With the canal full of EDTA, the tip was activated for 60 seconds (in three 20-second cycles) using short, vertical pumping strokes. • Group 3: Passive Ultrasonic Irrigation (PUI) An IrriSafe ultrasonic tip (size 25, .00 taper; Satelec, Acteon, Mérignac, France) attached to an ultrasonic unit (P5 Newtron, Satelec) was used. The tip was placed 2 mm short of the WL and activated passively (without touching the canal walls) at a power setting of 4 for 60 seconds (in three 20-second cycles). The irrigant was replenished between cycles. • Group 4: Laser-Activated Irrigation (LAI) An Er:YAG laser (LightWalker AT, Fotona, Ljubljana, Slovenia) with a 300 μm diameter conical fiber tip was used. The tip was positioned 2 mm short of the WL. The laser was activated at a setting of 50 mJ, 20 Hz, and very short pulse mode for 60 seconds (in three 20-second cycles) while the canal was filled with EDTA. The tip was moved in a gentle helicoidal motion. • Group 5: Conventional Needle Irrigation (CNI - Control) The canal was irrigated with 1 mL of 17% EDTA for 60 seconds using a 30-gauge side-vented needle placed 2 mm short of the WL, with no activation. After the activation/control protocol, all canals were flushed with 5 mL of distilled water to remove the EDTA and any remaining debris. The canals were then dried with sterile paper points. Scanning Electron Microscopy (SEM) Evaluation Longitudinal grooves were prepared on the buccal and lingual surfaces of each root using a diamond disc, without penetrating the canal. The roots were then split into two halves using a chisel. The half containing the most visible part of the canal was selected, dehydrated, mounted on aluminum stubs, and sputter-coated with gold-palladium. The coronal, middle, and apical thirds of each canal were examined at 1500× magnification using an SEM (JSM-6510, JEOL, Tokyo, Japan). Two calibrated and blinded examiners, with a pre-established inter-examiner agreement (Cohen’s Kappa κ = 0.88), scored the presence of the smear layer using the following 5-point scale: 1. No smear layer; dentinal tubules open. 2. Small amounts of smear layer; most tubules open. 3. Homogeneous smear layer covering the surface; few tubules open. 4. Homogeneous smear layer covering the entire surface; no open tubules. 5. Heavy, non-homogeneous smear layer covering the entire surface. Statistical Analysis The data were analyzed using SPSS software (Version 25.0, IBM Corp., Armonk, NY, USA). As the data were ordinal, non-parametric tests were used. The Kruskal-Wallis test was employed to compare smear layer scores among the five groups for each canal third. Pairwise comparisons were then performed using the Mann-Whitney U test with a Bonferroni correction for multiple comparisons. The level of statistical significance was set at p < 0.05.

All activation systems resulted in significantly better smear layer removal compared to the CNI control group across all canal thirds (p < 0.001). The mean smear layer scores, standard deviations, and statistical comparisons for the coronal, middle, and apical thirds are presented in Table 1. In general, smear layer removal was most effective in the coronal third and least effective in the apical third for all groups. The CNI group consistently showed the highest (worst) smear layer scores, with most surfaces covered by a heavy, homogeneous smear layer. In the critical apical third, the LAI and PUI groups demonstrated the lowest mean scores (1.67 ± 0.49 and 1.80 ± 0.56, respectively), indicating superior cleaning efficacy. There was no statistically significant difference between the LAI and PUI groups (p = 0.512). Both LAI and PUI were significantly more effective than XPF (2.27 ± 0.60; p < 0.05), EA (3.13 ± 0.74; p < 0.001), and CNI (4.60 ± 0.51; p < 0.001). The XPF group performed significantly better than the EA group (p = 0.008) in the apical region. Pairwise comparisons of the mean smear layer scores in the apical third are detailed in Table 2. The percentage distribution of smear layer scores provides a clearer picture of cleaning efficacy (Table 3). In the apical third, 87% of samples in the LAI group and 80% in the PUI group achieved scores of 1 or 2 (indicating clean or nearly clean surfaces). This contrasts sharply with the XPF group (47%), the EA group (13%), and the CNI group (0%). Conversely, 100% of samples in the CNI group and 87% in the EA group had scores of 3, 4, or 5 in the apical third. Table 1: Mean Smear Layer Scores (± Standard Deviation) for All Groups in Coronal, Middle, and Apical Thirds. Group Coronal Third Middle Third Apical Third XPF 1.47 ± 0.52 ᵃ 1.80 ± 0.56 ᵇ 2.27 ± 0.60 ᶜ EA 2.07 ± 0.60 ᵇ 2.60 ± 0.74 ᶜ 3.13 ± 0.74 ᵈ PUI 1.20 ± 0.41 ᵃ 1.47 ± 0.52 ᵃ 1.80 ± 0.56 ᵇ LAI 1.13 ± 0.35 ᵃ 1.33 ± 0.49 ᵃ 1.67 ± 0.49 ᵇ CNI (Control) 3.40 ± 0.63 ᶜ 4.07 ± 0.59 ᵈ 4.60 ± 0.51 ᵉ *Different superscript letters (a, b, c, d, e) within a column indicate statistically significant differences (p < 0.05). Table 2: Pairwise Comparisons (p-values) of Smear Layer Scores between Groups in the Apical Third. Comparison p-value Significance LAI vs. PUI 0.512 NS LAI vs. XPF 0.015 S LAI vs. EA < 0.001 S LAI vs. CNI < 0.001 S PUI vs. XPF 0.041 S PUI vs. EA < 0.001 S PUI vs. CNI < 0.001 S XPF vs. EA 0.008 S XPF vs. CNI < 0.001 S EA vs. CNI < 0.001 S *S = Significant (p < 0.05); NS = Not Significant. (Mann-Whitney U test with Bonferroni correction). Table 3: Percentage Distribution (%) of Smear Layer Scores (1–5) in the Apical Third for Each Group. Score XPF (%) EA (%) PUI (%) LAI (%) CNI (%) 1 7 0 27 33 0 2 40 13 53 54 0 3 40 47 20 13 20 4 13 33 0 0 40 5 0 7 0 0 40

The findings of this in vitro study demonstrated that all tested irrigation activation systems were significantly more effective at removing the smear layer from curved root canals than conventional needle irrigation. This rejects the null hypothesis and underscores the necessity of active irrigation techniques for achieving adequate canal debridement. The superiority of activation systems is attributed to their ability to improve the circulation, penetration, and chemical activity of irrigants, overcoming physical barriers like the vapor lock effect that limit the efficacy of passive irrigation [5, 12]. The most significant finding was the superior performance of Laser-Activated Irrigation (LAI) and Passive Ultrasonic Irrigation (PUI), which showed no statistical difference between them. Both systems achieved the highest level of cleanliness, particularly in the challenging apical third. The efficacy of PUI is well-documented and is primarily based on two physical phenomena: acoustic streaming and cavitation [7]. Acoustic streaming creates a powerful hydrodynamic shear force along the canal walls that dislodges debris, while cavitation—the formation and collapse of microbubbles—generates shockwaves that further disrupt the smear layer [13]. The Er:YAG laser used for LAI operates on a different principle. Its wavelength is highly absorbed by water, causing explosive vaporization of the irrigant. This process generates large, expanding, and collapsing vapor bubbles, which create powerful shockwaves and high-velocity fluid movement within the confined canal space [9]. This "photoacoustic" effect is extremely effective at disrupting and flushing debris, which explains its top-tier performance in our study. These results are consistent with previous reports that have highlighted the profound cleaning ability of LAI in complex anatomies [14]. The XP-Endo Finisher (XPF) also demonstrated commendable efficacy, significantly outperforming sonic activation and conventional irrigation. The unique mechanism of the XPF, which relies on its shape-memory MaxWire® alloy, allows the instrument to expand and conform to the canal's three-dimensional morphology upon reaching body temperature [8]. This creates a light mechanical scrubbing action combined with fluid agitation. Its high flexibility makes it particularly suitable for curved canals, as it can navigate curvatures without causing iatrogenic damage. While highly effective, it was statistically less effective than LAI and PUI in the apical third. This might be because its primary action is a combination of mechanical contact and moderate fluid dynamics, which may be less potent than the intense, non-contact forces generated by cavitation and photoacoustic shockwaves. Our findings align with studies by Keskin et al., who also found PUI to be superior to XPF in smear layer removal [15]. Sonic activation with the EndoActivator (EA) was the least effective among the active irrigation groups, although it was still a significant improvement over CNI. Sonic systems operate at lower frequencies (1-6 kHz) compared to ultrasonic systems (25-40 kHz), resulting in less powerful hydrodynamic activation [6]. While sonic energy can improve irrigant exchange, it appears insufficient to generate the high shear forces needed to completely remove the tenacious smear layer in the apical part of curved canals. This finding is in agreement with a large body of literature that consistently ranks PUI as more effective than sonic activation for canal debridement [10, 16]. This study specifically focused on curved canals, which introduces a critical variable. The rigidity of some ultrasonic tips can be a concern, as contact with the canal wall can dampen the oscillation and reduce efficacy or even lead to ledging [7]. The use of a flexible, non-cutting PUI tip and the non-contact nature of LAI are significant advantages in such anatomies, allowing energy to be transmitted effectively without iatrogenic risk. The XPF's inherent flexibility is its core design feature for these scenarios. The limitations of this study must be acknowledged. As an in vitro investigation, it cannot replicate the complex clinical environment, which includes the presence of vital or necrotic pulp tissue, systemic circulation, and temperature variations. The study used 17% EDTA as the final irrigant, and results might differ with other protocols, such as alternating irrigation with NaOCl. Furthermore, only one type of canal curvature and size was evaluated. Future studies should investigate these systems' efficacy against microbial biofilms, their performance in different anatomical variations (e.g., C-shaped canals, isthmuses), and ultimately, their impact on clinical treatment outcomes through well-designed randomized controlled trials.

Within the methodological limitations of this in vitro study, the following conclusions can be drawn: 1. All tested irrigation activation systems (LAI, PUI, XPF, and EA) were significantly more effective than conventional needle irrigation for smear layer removal in curved root canals. 2. Laser-Activated Irrigation (LAI) and Passive Ultrasonic Irrigation (PUI) demonstrated the highest efficacy, particularly in the apical third, with no significant difference between them. 3. The XP-Endo Finisher was also highly effective, performing significantly better than sonic activation. 4. Sonic activation with the EndoActivator was the least effective of the active systems but still represented a significant improvement over passive irrigation. These findings highlight that the selection of an advanced irrigation activation system is a critical determinant for achieving optimal canal cleanliness, especially in anatomically complex cases like curved canals.

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