Influence of adhesive system on quartz fiber post dislocation resistance in endodontically treated teeth

Aim: To evaluate the dislocation resistance of the quartz fiber post/cement/dentin interface after different adhesion strategies. Methods: Forty bovine lower central incisors were selected and prepared with K-files using the step-back technique, and irrigated with 3 mL of distilled water preceding the use of each instrument. Prepared teeth were stored at 37ºC and 100% humidity for 7 days. The roots were prepared and randomized into 4 groups. The quartz fiber post was cemented with an adhesion strategy according to the following groups: G BisCem - BISCEM; G OneStep±C&B - One Step ± C&B; G AllBond±C&B - AllBond3 ± C&B; G AllBondSE±C&B - AllBondSE ±C&B with a quartz fiber post. Cross-sectional root slices of 0.7 mm were produced and stored for 24 h at 37° C before being submitted to push-out bond strength. Results: The mean and standard deviation values of dislocation resistance were G BisCem : 1.12 (± 0.23) MPa, G OneStep±C&B : 0.81 (± 0.31) MPa, G AllBond±C&B : 0.98 (± 0.14) MPa, and G AllBondSE±C&B : 1.57 (± 0.04) MPa. G AllBondSE±C&B showed significantly higher values of dislocation resistance than the other groups. Conclusions: Based on this study design, it may be concluded that adhesion strategies showed different results of quartz post dislocation resistance. Simplified adhesive system with sodium benzene sulphinate incorporation provided superior dislocation resistance.


Introduction
The objectives of the restorative treatment of endodontically treated teeth with large crown destruction are giving the teeth more resistance to masticatory forces, reestablishing function, and esthetics 1 . Thus, sometimes the use of intraradicular post and cores is necessary 2,3 . Ideally, the intraradicular post and cores should not move into the root canal and forces are distributed in the teeth homogeneously, decreasing the risk of tooth fractures 4,5 .
Cast metal posts have been used for a long time in clinical practice and have presented high survival rates 6 . Metal posts have high elastic modulus, which increases fracture and catastrophic root failure. Therefore, glass fiber posts have been introduced as an alternative. Due to their low elastic modulus compared with cast metal posts, glass fiber posts have decreased the number of catastrophic failures 7 . In addition, they are more aesthetic and result in less clinical time 3,8,9 .
Carbon-fiber posts were introduced in the 1990s, and after that other types were produced such as glass fiber posts and zirconia and quartz posts. Fiber posts are cemented with a polymer luting cement to dentin due to chemistry affinity for its own epoxy resin matrix with the resin cement and quartz posts presenting a 98% survival rate after 9 years 9 . However, difficulty in adhesive cementation is observed and debonding is the main type of failure [10][11][12][13][14][15] . It is important to know which adhesive strategy should be used to decide the best choice for fiber post cementation 16 . Thus, the objective of this study was to evaluate the dislocation resistance of the quartz fiber post/ cement/dentin interface after different adhesion strategies. The null hypothesis was that the different adhesion strategies would not affect the dislocation resistance.

Teeth preparation
Forty bovine incisors were selected for this study. To be included, the following criteria had to be met: straight roots and a root length of at least 15 mm. External debris were removed with a periodontal curette and scalpel blade. The teeth were sectioned transversely 15 mm from the apex using a slow-speed diamond disc under water coolant, the pulp tissue was removed, and chemomechanical preparation was performed according described previously 17,18 . The root canals were prepared with K-files using the step-back technique and irrigated with 3 mL of distilled water preceding the use of each instrument. After that, they were stored at 37ºC and 100% humidity for 7 days. Subsequently, the roots were prepared ( Figure 1) with a specific Bisco burr (BISCO Int., Schaumburg, Illinois, USA), and randomized into 4 groups. Quartz fiber post was cemented according to the adhesive system and cements (Table 1)

Push-out test
After cementation of the quartz post, the teeth were sectioned transversely into 7 slices that were approximately 0.7 mm thick using a low-speed disc (Isomet, Buehler Ltd, Lake Bluff, IL, USA) with constant water cooling. The internal diameter of the canal of each slice was measured with a digital caliper (Digimess, 100.174BL, Digimess Instrumentos de Precisão Ltda, São Paulo, SP, Brazil) and the contact area between the filling and dentin of each slice was calculated. Each slice was placed with the apical side up on a mechanical testing machine (DL-2000, EMIC Equipamentos e Sistemas de Ensaio Ltda, São José dos Pinhais, PR, Brazil). A force was applied to the shutter toward the apical neck using a 500 N load cell and a crosshead speed of 1 mm/min with a 0.8-mm diameter cylindrical device. The dislocation resistance (MPa) was obtained by dividing the force (N) required to displace the filling material by the adhesive area (mm 2 ). The adhesive area was calculated using (1 and 2): (2) where, g is the root taper, R1 represents the apical radius, R2 represents the coronal radius, h is the thickness of the slice, and A is the adhesive area.
Apical and coronal radii were obtained from photographs and measured with Image Tool software 10 times from different sides to obtain the radii means ( Figure 2).

Statistical Analysis
Data normality was checked by the Kolmogorov-Smirnov test. Data were analyzed by ANOVA and the Tukey post-hoc test. A significance level of 5% was used for analysis.

Results
The results of the dislocation resistance are shown in Table 2 and Figure 3. The G AllBondSE±C&B group presented the highest dislocation resistance value (1.57 ± 0.04 MPa), with a significant difference from the other groups, p < 0.05. G BisCem (1.12 ± 0.23 MPa), G OneStep±C&B (0.81 ± 0.31 MPa), and G AllBondSE±C&B (0.98 ± 0.14 MPa) did not show statistically significant differences between each other, p > 0.05.

Discussion
In this study, the dislocation resistance of root dentin/cement/ post was evaluated after different adhesive strategies. The materials in this study showed easy clinical use, furthermore, the decreased catastrophic failure could have occurred as result of correct dissipation of forces in the middle third of teeth. The use of fiber post with resin cement leads to a block formation with dental tissue since the elastic modulus is similar to dentin (18 GPa), once the values for fiber post is between 16-40 GPa and resin cement between 6.8-10.8 GPa 19 . However, the adhesive system strategies used in this study showed different results for dislocation resistance, and the highest values were found in the G AllBondSE±C&B group.
Difficulty in hybrid layer formation results in ineffective adhesion and a decrease in dislocation resistance due to: high factor cavity configuration, high polymerization shrinkage, and difficulty in the homogeneous acid condition of the root walls 8 . In the G BisCem group, self-adhesive cement was used. This cement compounds the acid monomers, which demineralize the dentin substrate while infiltrating the monomers to form a hybrid layer. It is likely that the lower dislocation resistance occurred due to the fact that there was no formation of a homogeneous hybrid layer 14 .
The simplified adhesive systems (primer with adhesive) showed statistically significant lower dislocation resistances between groups (G OneStep±C&B and G AllBondSE±C&B ), p < 0.05%. One Step adhesive is a universal self-adhesive of one plot that presents hydrophilic monomers (hydroxyethyl methacrylate), acid monomers, and acetone as solvent. Hydroxyethyl methacrylate presents low hydrolytic stability when in the same environment with acid monomers and degrades in little time. Furthermore, acetone solvents have high volatility, which makes the operatory technique difficult and justifies the low value of dislocation resistance 20 . However, in this study, a significant difference between the G OneStep±C&B (universal adhesive), 0.81 (± 0.31), and G AllBond±C&B (etch-and-rinse adhesive), 0.98 (± 0.14), groups was not observed, p > 0.05%. Conventional adhesive systems, with 3 steps etch-and-rinse, present difficulties related to incomplete acid filling of the root and incorrect evaporation of the primer solvent, compromising the adhesion process, mainly in the medium and apical thirds 13,14,21,22 .
The highest values of dislocation resistance were found in the G AllBondSE±C&B group. It is likely that the compound present in part I of the adhesive, sodium benzene sulphinate, acts as a co-initiator in the acid-base reaction between benzoyl peroxide and tertiary amine of the chemically cured resin cements, increasing the results of dislocation resistance [23][24][25] . The sodium benzene sulphinate allows the acidity of the adhesive system to demineralize the dentin substrate without interfering in the resin cement degree of conversion. Thus, it is possible that the incorporation of the coinitiator avoids the chemical incompatibility between simplified adhesives and chemically cured resin cements.
Based on this study design, it is licit to conclude that the adhesion strategies showed different results of quartz post dislocation resistance. The simplified adhesive system with sodium benzene sulphinate incorporation presented superior dislocation resistance.