Carrier-Based Obturation: Effect of Sonication Technique on Sealer Penetration in Dentinal Tubules: A Confocal Laser Scanning Microscope Study

Abstract

(1) Background: In endodontics, anatomical complexities and irregularities in the root canal system may interfere with sealer penetration, increasing the risk of treatment failure. This work aimed to investigate the effects of sonicated Thermafil (Dentsply, Tulsa Dental Specialties, Johnson City, TN, USA) on sealer penetration into the dentinal tubules. (2) Methods: Thirty teeth with single round-shaped root canals were used to compare Sonicated Thermafil with sonication (ST—10 teeth), System B (EIE Analytical Technology, Orange, CE, USA) (SB—10 teeth), and Thermafil without sonication (T—10 teeth). A confocal laser scanning microscope (CLSM) was used to determine the depth, area, and percentage of sealer penetration into the dentinal tubules. (3) Results: ST showed the deepest average penetration at 6 mm from the apex compared to SB (p < 0.01) and T (p < 0.01) and at 3 mm compared to SB (p < 0.01). In addition, it had the highest penetration values at both levels compared to SB (p < 0.01) and T (p < 0.05) and a higher penetration area at 6 mm compared to T (p < 0.05) and SB (p < 0.05). (4) Conclusion: Sonic activation can improve the carrier-based obturation technique thanks to deeper sealer penetration and thus better retention of materials.

1. Introduction

In endodontics, anatomical complexities and root canal system irregularities can reduce the capacity of a three-dimensional sealing in eradicating bacteria. Therefore, ultrasonic and sonic activations have been suggested during obturation procedures to improve the quality of the seal. They enhance the injection of filling materials into isthmuses [1], lateral canals [2,3], and dentinal tubules [1,4] and reduce gaps with denser root filling [5,6], although they do not affect calcium silicate-based cement tubular penetration [7]. Sealer penetration is crucial in increasing the interaction between filling material and dentin, enhancing the ability of the sealing and promoting entombment of residual bacteria within the tubules [8,9,10,11,12,13,14,15].

Obturation techniques have changed over the years, improving root canal filling to reduce the risk of coronal, lateral, and apical leakage and bacterial recolonization, factors that have a significant impact on the long-term outcomes of endodontic treatments [16]. Although several studies have been conducted to improve the quality of sealing and to perfect the different techniques, lateral canals and anatomical complexities such as isthmus or delta continue to be factors of uncertainty in endodontic treatments [17]. Among the different techniques, continuous wave compaction and Thermafil (Dentsply/Maillefer, Ballaigues, Switzerland) are the best known. The former, a variant of Schilder’s warm vertical obturation compaction, consists of the Elements Obturation Unit, which contains a System B (EIE Analytic Technology, Orange, CA, USA) device and a gutta-percha extruder in a motorized handpiece; disposable cartridges of gutta-percha are heated rapidly, and the unit shuts off automatically to prevent overheating of the material. The latter consists of a plastic core coated with alpha-phase gutta-percha and a heating device that controls the temperature. After heating the carrier in a special oven for a certain time, the dentist inserts it into the canal without twisting or turning it. Although continuous wave compaction is considered a time-consuming technique with a long learning curve and the impossibility of applying it correctly in curved and narrow canals, especially in correspondence with very conservative conicity and preparation diameters (20.05; 25.04), which is related to instrumentation performed in the apical third, Thermafil has been proposed as an efficient obturation system for vertical condensation of warm thermo-plasticized gutta-percha, characterized by rapid execution, predictable results, and a short learning curve [18], and is particularly effective in narrow and anatomically complex root canals [19,20]. Nevertheless, it has some shortcomings, such as difficulties in post space preparation and the risk of making ledges or lateral root perforations [21]. Another limitation results from the rapid temperature drop due to the heating process, which takes place outside the tooth and leads to a reduction in the thermoplastic effect; this may have a negative impact on the filling of the lateral canals and the quality of the seal [22]. To overcome these limitations, sonication applied during carrier-based obturation could create a thermohydrodynamic phenomenon that keeps Thermafil warm during insertion and pushes gutta-percha and sealer deeper into root canal irregularities, increasing the packability and compressive strength of the sealer [23].

This study aimed to determine the effects of sonication of the carrier-based obturation on the sealer penetration into dentinal tubules. In the current literature, there are several studies describing the crucial role of sonication in improving the sealer penetration [24,25], but the possibility of combining the filling system with the sealer has not been investigated. The simultaneous activation of the sealer and the obturation system, which is the novelty of our work, could be an optimal way to increase the sealing ability of gutta-percha by acting as a mediator for the activation, thus better filling the dentinal tubules to achieve a better intracanal micro-tug and, above all, a better filling of isthmuses or accessory canals. The main objective of our work was to compare sonicated Thermafil (ST) with two conventional root-fillings techniques, namely, System B (SB) and Thermafil, without sonic activation (T) to determine the capacity of ST in enhancing sealer penetration. The null hypothesis was that there was no difference among System B and sonicated and non-sonicated Thermafil in depth, percentage, and area of sealer penetration.

2. Materials and Methods

Thirty teeth with single round-shaped root canals extracted for periodontal or orthodontic reasons were selected. The crowns were removed with a 0.3 mm low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA), and the root canal length was standardized to 12 mm. Root canal preparation was performed using the Wave One Gold (Dentsply, Tulsa Dental Specialties, Johnson City, TN, USA) reciprocating system in the working length up to the Wave Gold Medium instrument (35.08). A K-File 40 was used to ensure and standardize the apical diameter after shaping. After instrumentation, 2.5 mL NaOCl 5.25% (Niclor, Ogna, Muggiò, Italy) was used as irrigant. The canals were subsequent irrigated with 17% EDTA (Ogna), which was allowed to act for 5 min, and subsequently by 2 mL of 5% NaOCl (Ogna) for one minute and sterile water for another minute. Canals were then dried with paper points, and the dentin walls were coated with Topseal sealer (Dentsply, De Trey, Constance, Germany). For this purpose, a paper point was moved 40.06 mm to 1 mm short of the WL in a pumping motion for 5 s. The sealer was previously labeled with a 0.1 wt% rhodamine B dye (Carlo Erba Reagenti, Arese, Italy) for confocal laser scanning microscopy (CLSM). Samples were divided into three groups characterized by the different obturation techniques.

• Group 1 (SB—10 teeth): System B (EIE Analytic Technology, Orange, CA, USA) and Calamus (Dentsply, Maillefer).
• Group 2 (T—10 teeth): Thermafil obturators 40 (Dentsply, Tulsa Dental Specialties, Johnson City, TN, USA).
• Group 3 (ST—10 teeth): Thermafil obturators 40 (Dentsply. Tulsa Dental Specialties, Johnson City, TN, USA) with sonication performed by SONICflex quick 2008/L Air Scaler (KaVo Dental Ltd., Corinium Industrial Estate, Amersham Bucks, UK), specifically adapted for Thermafil obturators (Figure 1). Vibration time: 10 s at a frequency of 6.5 Hz.

The teeth were stored at 100% humidity in a dark environment and at 4 °C. After seven days, teeth were cut transversely with a 0.3 mm Isomet saw (Isomet, Buehler, IL, USA) 3 and 6 mm from the apex to obtain sections approximately 1 mm thick. The surfaces were then polished with superfine Sof-Lex disks (3M) on a low-speed handpiece to remove any dentin debris that may have been generated during cutting. Specimens were examined at 10x magnification under CLSM 510 (Zeiss, Steven Schallhorn, MD, USA). The ImageJ program was used to measure the depth, area, and percentage of penetration of the sealer penetration into the dentinal tubules. The emission wavelengths were 540/590 nm. The sealer penetration depth was determined as the mean depth of eight different measurements. In addition, the maximum penetration was measured from the canal wall to the end of the maximum depth of sealer infiltration. Instead, the area was calculated by subtracting the space of the root canal from the areas infiltrated by the sealer [26].

Statistically significant differences in sealer penetration between groups at 3- and 6-mm depths were investigated through a non-parametric test (Kruskal–Wallis test with Dunn’s procedure). The normality was checked through the Shapiro–Wilk test. Statistical significance was set at 5% (p < 0.05). Power post hoc was calculated. Statistical analysis was performed using STATA16 (StataCorp, College Station, TX, USA).

3. Results

ST showed the deepest mean penetration at 3- and 6-mm levels (3 mm: 348.4 ± 120.1 μm; 6 mm: 640.2 ± 163.43 μm) in comparison with that achieved using System B (3 mm: 214.1 ± 152.4 μm, p < 0.05; 6 mm: 263.6 ± 175.4 μm; p < 0.01), while compared with T, ST reported the statistically significant deepest mean penetration at 6 mm (470.3 ± 291.5 μm; p < 0.05) (Figure 2).

ST registered the highest values of infiltration at both levels (3 mm: 773.2 ± 292.0 μm; 6 mm: 1156.8 ± 117.9 μm), reporting statistically significant differences at 6 mm in comparisons with T (3 mm: 594.2 ± 188.5; 6 mm: 708.2 ± 420.3, p = 0.015) and SB (3 mm: 545.91 ± 242.58, 6 mm: 564.5 ± 388.4, p < 0.001) respectively.

Although the percentages of sealer penetration were higher for T at 3 mm (0.6 ± 0.1) and ST at 6 mm (0.7 ± 0.1), no statistically significant differences emerged.

The ST area of penetration at 6 mm (15,997.1 ± 8566.5 μm²) resulted in significantly higher values than those observed with T (8092.9 ± 6115.0, p = 0.0282) and SB (7696.0 ± 7585.4, p = 0.0282). At 3 mm, no statistically significant differences emerged. All results are reported in

Table 1. Comparisons.

		SB	T	ST	p-Value
3 mm	Depth (µm)	214.07 ± 152.40	346.38 ± 137.47	348.39 ± 120.13	p < 0.05 a
	Maximum depth (µm)	545.91 ± 242.58	594.19 ± 188.50	773.17 ± 291.98	NS
	Percentage (%)	51 ± 18	59 ± 11	54 ± 14	NS
	Area (µm2)	5396.05 ± 4384.00	6962.35 ± 5045.51	9092.69 ± 3278.76	NS
6 mm	Depth (µm)	263.57 ± 175.41	470.30 ± 291.48	640.22 ± 163.34	p < 0.05 b
	Maximum depth (µm)	564.46 ± 388.44	708.24 ± 420.28	1156.79 ± 117.87	p < 0.05 c
	Percentage (%)	53 ± 19	59 ± 13	65 ± 13	NS
	Area (µm2)	7696.03 ± 7585.38	8092.89 ± 6114.97	15,997.06 ± 8566.48	p < 0.05 d

Data are reported as mean ± standard deviation. SB: System B, T: non-sonicated Thermafil, ST: Sonicated Thermafil. Significance: (a) ST vs. SB (p = 0.029, 1 − β = 0.52), (b) ST vs. CWT (p = 0.0019, 1 − β = 0.99), (c) ST vs. T (p = 0.015, 1 − β = 0.85), ST vs. SB (p < 0.001, 1 − β = 0.99), (d) ST vs. T (p = 0.028, 1 − β = 0.59), ST vs. SB (p = 0.028, 1 − β = 0.56).

.

4. Discussion

In the presence of root canal infections, bacteria can penetrate dentin permeability at a depth of approximately 300 µm, reducing the effectiveness of obturation techniques [27]. Therefore, this study evaluated the sealer penetration of three obturation methods evaluating the improvement achieved by sonicated techniques compared to conventional warm gutta-percha obturation techniques. The study was carried out using CLSM analysis, an investigation technique with several benefits compared to other imaging techniques [7,11].

From our findings, Thermafil with sonicated activation ensures a significant deeper sealer penetration than that observed using either Thermafil without sonic activation or System B, according to previous studies, which demonstrated how sonicated activation may increase the sealer penetration and enhance lateral canal obturation [2,11].

The deeper sealer penetration observed with Sonicated Thermafil could be due to a thermo-hydrodynamic effect able to push the sealer into dentinal tubules more intensely to obtain a more effective and hermetic seal thanks to the higher frequency, and the decreased time of the activation method could reduce the sealer noise and prevent voids and gap formation [2]. Our findings are similar to those of Wiesse et al., according to which the intratubular penetration was similar using sonic or non-sonic activation techniques [28]. Moreover, it seems that the sealer penetration does not depend on the smear layer removal, but it is due to the physicochemical properties of the sealer and the used obturation technique [26], although the current literature is not unanimous on the role of the obturation technique in the sealer penetration [29].

According to Moon et al. (2012), the sealer penetration percentage is a more noteworthy measurement than the maximum penetration; penetration of the sealer into dentinal tubules along the root canal circumference entails a mechanical improvement of the interface and contributes to preventing reinfection [10]. However, in this study, the percentage of sealer penetration was similar among the three groups, both at 3 and 6 mm; therefore, the sonic activation seemed unable to improve this parameter significantly. Nevertheless, higher percentages of penetration were registered in ST at both the apical third and the middle third of the root, so underlining as sonic energy could improve the rate of sealer penetration [3]. With attention to the apical third, our findings appear encouraging. According to Dash et al. (2017), the ultrasonic devices do not perform in the apical third because, touching the walls, the phenomenon of acoustic streaming and cavitation is reduced [30]. In our case, the sonic activation, delivered for the entire duration of the filling, did not seem affected by the dentinal walls, as the shutter received sonic energy from the soniflex handpiece outside the endodontic space.

Observing the sealer penetration area in some samples, a heterogeneous distribution of the sealer along the canal root perimeter emerged with a peculiar concentration along the vestibule–lingual area of each section. This phenomenon, known as the “butterfly effect” [4,13,31], is probably due to dentinal tubular sclerosis in the mesiodistal direction in some dental elements. Although the butterfly effect was not studied in this work, future studies should consider its possible influence as a confounding element [31] because it could be one of the causes of the wide penetration ranges observed in this study as in the previous ones [32,33].

According to some recent studies about the sealer penetration [4,11,12,13,26], in our work, all analyzed parameters increased from the apical (3 mm) toward the coronal third (6 mm), probably determined either by the lesser density and diameter of the dentinal tubules in the apical third [34,35] or by a better removal of the smear layer and more effective irrigation in the middle third rather than in the apical third [36].

The sealer penetration into dentinal tubules avoids bacterial colonization, entombs the remaining bacteria and improves the retention of canal obturation materials. Therefore, deeper tubular penetration of the sealer could represent an excellent outcome because it could allow lesser instrumentation of the apical third and the middle third of the root canal, removing those pathogens responsible for the endodontic disease, economizing on the quantity of dental materials, and reducing errors during the shaping. In addition, sonic activation of Thermafil, in addition to transferring energy waves from the carrier to the sealer, keeps gutta-percha warm and, therefore, its thermoplastic characteristics over an extended period, postponing its hardening.

A recent study demonstrated that sonic and ultrasonic activations could modify the physicochemical properties of some endodontic sealers [37] when the activated carrier comes into contact with the sealer directly. This phenomenon did not happen in this study because the sonic energy was directly delivered to Thermafil without direct contact with the sealer. From our results, it has emerged that sonic activation of carrier-based obturation achieves an optimal distribution of the sealer into dentinal tubules and, consequently, it allows deeper levels of infiltration to be reached than those obtained with conventional Thermafil or with System B. Furthermore, the used frequency (6 Hz) and the activation time (10 s) do not negatively influence the sealer behavior. Since colonization of dentinal tubules by bacteria can reach distances of at least 300 μm [38], the sonicated Thermafil presented here can be developed as a new device that combines the great adaptability of Thermafil to canal walls with the stronger sonic capability to increase penetration of endodontic materials.

The main limitation of this work is due to the small sample size. Second, there could be cofounder bias due to the butterfly effect observed in some CLSM images, which may have biased some measurements. Future studies need to be conducted with larger sample sizes and to consider the role of the butterfly effect as a confounding factor.

5. Conclusions

Sonic activation can improve the carrier-based obturation technique thanks to a deeper sealer penetration and, therefore, a better retention of materials, significantly reducing the risk of the new reinfection. Future studies should investigate the effect of sonicated Thermafil with bioceramics.