Introduction

An ideal root-end filling material should have some properties as dimensional stability, radiopacity, antimicrobial activity, biocompatibility, and ability to stimulate mineralized tissue [1]. The marginal adaptation of materials used for root canal filling or retrograde filling is important for clinical outcome after treatment. Among the physicochemical properties, the bond strength between root dentin and root-end filling should promote adaptation and decrease interface between material and dentin. Thus, the bond strength of endodontic materials has been evaluated by using the push-out tests [2].

New MTA-based root-end filling materials have been developed [3–7]. MTA Sealer is an experimental endodontic sealer composed by white Portland cement, zirconium oxide (radiopaque filler), calcium chloride (additive), and resin [3]. The addition of calcium chloride to MTA-based materials increases its capacity of releasing calcium [8]. MTA Sealer has demonstrated calcium ions release [9], proper setting time and flow for clinical use [10], and biocompatibility similar to MTA in rats [11].

Ultrasonic preparation is the most suitable technique for retrograde cavity preparation [12] promoting parallel walls with adequate retention for the filling material and reduced exposure of dentin tubules [13]. In addition, ultrasonic preparation requires less bone removal for access to the apex [14] and allows root-end preparation according to the long axis of the root canal [15]. Different ultrasonic tips are available differing in diameter, angle, and cutting surface. The different surface characteristics can result in qualitative differences in root-end preparation [16–22] and time required to preparation. The conventional tips with a diamond-coated surface are the most widely used for root-end preparations. The conventional diamond tips are manufactured by means of galvanic soldered diamond powder, with a roughened structure and spaces between the grains [23].

An improved type of diamond-coated tip manufactured by chemical vapor deposition (CVD) has recently been introduced [24]. Diamond surfaces are created using the CVD technique by depositing a uniform polycrystalline diamond film resulting in a single body diamond film. This surface property is the basis for the cutting capacity of CVD tips when compared to conventional diamond tips [24]. Bernardes et al. [24] observed that root-end preparations established with CVD diamond tips required less preparation time than conventional tips, with no difference in quality between the three preparation tips studied. Batista de Faria-Junior et al. [12] evaluated ErCr:YSGG laser to perform retrograde cavities and observed a longer preparation time and lower quality preparations when compared to ultrasonic tips.

The push-out mechanical test has been used to evaluate bond strength of filling materials and posts to the root dentin [23, 25–27]. Optimal adhesion of the root-end filling material minimizes the occurrence of displacement that may cause voids and cracks, resulting in failure of endodontic treatment.

The impact of the root-end preparation tip in bond strength has not been described in the literature. The aim of this investigation was to evaluate the influence of different ultrasonic tips (CVD T0F-2; Trinity diamond; Satelec S12̸90 L) used for root-end preparation on the bond strength and type of failure of different root-end filling materials. The null hypothesis is that different ultrasonic tips do not influence the bond strength of retrograde materials.

Material and methods

The composition and manufacturers of tips and root-end filling materials evaluated are described in Table 1.

Table 1 Root-end filling materials, compositions, ultrasonic tips, and manufacturers
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The study was approved by the Ethics Committee of the Araraquara School of Dentistry—UNESP. The MTA Angelus cement was prepared using a 3:1 ratio by weight (powder:liquid) according to the manufacturer’s instructions. The MTA Sealer was prepared using a 5:1 ratio by weight (powder:liquid) to obtain the puttylike consistency of a retrofilling material. The ZOE cement was prepared using a 5:1 ratio (powder:liquid) [28].

Ninety slices (2-mm thick) from single-rooted human teeth were stored in 0.5 % chloramine-T trihydrate solution (Formula & Action Magistral Pharmacy, São Paulo, SP) for no longer than 1 week and, thereafter, stored in distilled water in a refrigerator at 4 °C, according to ISO/TS 11405:200 standards. The slices were embedded in resin (Resina Poliéster Automotiva Natrielli–Natrielli Química Ltda., Santana do Parnaíba, São Paulo). The initial root canal preparation was performed with a 1.5-mm diameter cylindrical bur (Vortex, Produtos Odontológicos, São Paulo, SP) at a speed of 2000 rpm. Then, for the retrograde preparation, the specimens were placed in a delineator device in order to place the tip parallel to the walls of the preparation.

The tips used for retrograde preparation were CVD (CVD-Valley, São José dos Campos, São Paulo, Brazi), Trinity (Trinity, São Paulo, SP, Brazil), and Satelec (Satelec, Paris, France). All tips are diamond with 3 mm in length. The slices were randomly divided into nine groups of 10 samples each. Group CVD MTAA; Group CVD MTAS; Group CVD ZOE; Group Trinity MTAA; Group Trinity MTAS Group Trinity ZOE; Group Satelec MTAA; Group Satelec MTAS; Group Satelec ZOE.

The preparations were performed with CVD ultrasound (CVD-Vale, São José dos Campos, SP, Brazil) at 50 % power under copious irrigation with saline solution. The retropreparation tip was positioned against all the walls, providing an average increase of approximately 0.5 mm in diameter of the cavity. The preparation time was 17 s for the CVD tip [21] and 45 s for the Trinity and Satelec tips (Zuolo et al. [17]). After the preparation, the materials were placed in the cavities. The samples were kept in an incubator at 37 °C in the presence of humidity for 48 h. After this time, the excess cement was removed with scalpel blades and 220–600 grain sandpaper until the entire cement/dentin interface was observed.

For the mechanical test, each resin disc/dentin/retrofilling material was placed in the testing machine (EMIC DL 2000, São José dos Pinhais, PR, Brazil) with a 5-kN load cell. The progressive compression test was carried out at a speed of 1 mm/min from the contact until the displacement of the sealer from the root canal walls was observed. The apparatus had a cylindrical tip with a diameter of 1.3 mm and was positioned so that during charging, the contact only occurred with the cement. The values were obtained in N and transformed to MPa. The recorded value was divided by the surface adhesion filling area, calculated by the following formula: 2πrh, where r is the radius of the root canal and h is the thickness of the dentin.

The sample specimens were examined under a stereomicroscope to analyze the type of failure. After tabulating the data, they were analyzed for normality using the Shapiro–Wilk test, where a normal distribution of the data was observed. Data were submitted to the parametric ANOVA statistical test and to Tukey multiple comparison test, with significance level set at 5 %.

Results

The mean and standard deviation (push-out) mechanical testing are shown in Table 2 (values in MPa). In the analysis of the effect of ultrasonic tip on the bond strength, the CVD tip promoted a significantly greater bond strength than the Satelec and Trinity tips (P < 0.05). When the effect of the root filling material was analyzed, independent of the ultrasonic tip used in the retropreparation, ZOE demonstrated significantly less bond strength (P < 0.05) when compared to the other tested materials.

Table 2 Mean (MPa) and standard deviation of bond strength values (MPa)
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The most common mode of failure was adhesive failure, with the exception of the groups obturated with ZOE, in which mixed failure predominated.

Discussion

The push-out test is the most widely used to evaluate the bond strength between the dentin and root-end filling materials [25, 27, 29, 30]. The tip diameter may influence the results of the push-out test [31, 32]. In the present study, a device with approximately 1.3-mm tip diameter was used [33–35].

The most widely used method for performing root-end preparations is ultrasonic preparation [13], which has contributed to the increase in success rate of endodontic surgery [14]. Paz et al. [36] investigated the cutting efficiency of two ultrasonic units with two different tips, Satelec and Spartan, and observed the former to be more effective for dentin removal than the latter. The present study evaluated the influence of retropreparation tips with different manufacturing methods in the bond strength of retrofilling materials. The results showed that irrespective of the filling material, the CVD T0F-2 tip showed the highest bond strength values. The characteristics and arrangement of the surface of the CVD diamond tip can be correlated to the obtained results [21]. The Satelec and Trinity ultrasonic tips have small diamond crystals embedded as if they were incorporated into a solid material. Both tips showed loss of total diamond after use, and the loss was higher for the Satelec tip [21]. In the present study, the lost diamond particles could have remained in the dentin surface and interfered with the adhesion of the materials.

The CVD tips were manufactured using chemical vapor deposition, applying a thick layer of pure diamond, forming a single stone on the entire surface of the tip. Perhaps, because of the manufacturing technique, the tip retained its shape after use, thus maintaining the cutting power for a longer time (Bernardes et al. [21]), which may explain the better results of the CVD tip observed in the present study. Bernardes et al. [21] did not find any significant difference in the quality of the root-end preparation, but they observed more regular root-end preparations for the Satelec and Trinity tips than for the CVD tip. This increased irregular root-end preparation by the CVD tip may have generated greater attrition and retention areas between the material and root canal wall.

The results from the present study show that the MTA Angelus and MTA Sealer produced the highest bond strength values irrespective of the ultrasonic tip used in the root-end preparation. Hong et al. [37] reported similar values without using ultrasonic preparation. MTA-based materials undergo expansion after setting, which can explain the results [38]. The MTA Sealer, besides the resin, contains white Portland cement which could increase the mechanical strength through the expansion [38].

After retrograde cavity preparation, the use of EDTA has not been recommended to prevent exposure of a greater amount of dentinal in the apical region after apicoectomy. Also, according Celik et al. [39], irrigation regimes using EDTA have no effect on the push-out bond strength of the calcium silicate cements. The most probable reason for the MTA bond strength is related to the friction effect, since MTA has volumetric expansion, which can promote higher bond strength [40, 41]. Sluyk et al. [40] showed that the presence of some moisture after perforation sealing using MTA does not influence the MTA retention. Storm et al. [41] suggested that the possible reason for the sealing ability of mineral trioxide aggregate (MTA) is its slight expansion upon setting.

The ZOE cement had the lowest values independent of the ultrasonic tip used. In the present study, the consistency used was greater than that for the root canal filling (5:1, powder:liquid) and the results can be explained by the presence of zinc ions from the zinc oxide, which can affect the mineral component of the dentin [42].

Regarding the type of failure, adhesive failure predominated, agreeing with the results of Shokouhinejad et al. [42], with the exception of the groups where the material was the ZOE in which mixed failure predominated. The type of adhesive failure in this study may be related to the storage time before the push-out test, which was 3 days. Other authors [42, 43] used periods of 4 and 7 days. The longer time may have favored higher material adhesion to the dentinal walls.

Conclusion

The results of this study demonstrate that the root-end preparation with the CVD tip positively influences the bond strength of root-end filling materials. MTA Angelus and experimental MTAS presented bond strength to dentin prepared with ultrasonic tips.