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ORIGINAL ARTICLE
Year : 2021  |  Volume : 11  |  Issue : 3  |  Page : 393-399

Effect of conventional and experimental silanes on the adhesion of fiberglass posts to root canals: In vitro study


1 Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, SP, Brazil
2 Department of Restorative Dentistry, Division of Biomaterials and Biomechanics, Oregon Health and Science University, Portland, Oregon, USA

Date of Submission25-Nov-2020
Date of Decision06-Jan-2021
Date of Acceptance25-Jan-2021
Date of Web Publication3-Sep-2021

Correspondence Address:
Prof. Rafael Leonardo Xediek Consani
Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, 901 Limeira Ave., 13414-903, Piracicaba, SP
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_297_20

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  Abstract 

Introduction: Thiourethane addition to methacrylate of resin cements improves properties of flexural strength, toughness modulus, and fracture toughness. The aim of this study was to evaluate the effect of experimental silanes based on thiourethane in the adhesion of fiberglass posts to the root canal submitted to thermal and mechanical cycling.
Materials and Methods: Two oligomers were synthesized with catalytic amounts of triethylamine. Multifunctional thiol–pentaerythritol tetra-3-mercaptopropionate was combined with difunctional isocyanates – 1,6-hexanedioldiissocyante (aliphatic) or 1,3-bis (1-isocyanato-1-methylethyl) benzene (aromatic). Human dental root canals were treated with conventional and experimental silanes, establishing six experimental groups (n = 10) endodontically rehabilitated with fiberglass posts. The specimens were submitted to 1.2 x 106 mechanical cycles and 500 thermal cycles (5°C and 55°C). One slice from each root region (cervical, middle, and apical) was submitted to the push-out bond test, and the fracture pattern evaluated. Scanning electron microscopy micrographs were obtained from the marginal gaps occurred in the adhesive interfaces. Data were analyzed with two-way ANOVA followed by Tukey's test at α = 0.05 confidence level.
Results: Conventional and experimental silanes improved bond strength (P < 0.001) with similar values among the root regions (P = 0.199), decreased the occurrence of adhesive failures in dentin-cement bond, and increased the cohesive failures in dentin and the mixed failures. The marginal gaps showed in scanning electron microscopy micrographs were similar, except for the Angelus conventional silane, showing greater slit.
Conclusions: Conventional and experimental silanes provided greater adhesive strength, reduced dentin-cement failures, and the marginal gaps were similar, except for the control group Angelus, showing greater slit.

Keywords: Bond strength, fiberglass post, marginal gap, root canal, silane


How to cite this article:
Pomini MC, Pfeifer CS, Piovezan Fugolin AP, Piccolli VM, de Paula Ramos RA, Xediek Consani RL. Effect of conventional and experimental silanes on the adhesion of fiberglass posts to root canals: In vitro study. Saudi Endod J 2021;11:393-9

How to cite this URL:
Pomini MC, Pfeifer CS, Piovezan Fugolin AP, Piccolli VM, de Paula Ramos RA, Xediek Consani RL. Effect of conventional and experimental silanes on the adhesion of fiberglass posts to root canals: In vitro study. Saudi Endod J [serial online] 2021 [cited 2021 Dec 4];11:393-9. Available from: https://www.saudiendodj.com/text.asp?2021/11/3/393/325407


  Introduction Top


Fiberglass posts have been widely used in endodontic treatments for teeth with limited coronal structure, having large acceptance among dentists.[1] The fiberglass posts are adhesively bonded to the root canal, and a strong adhesion is the best way to rehabilitate the root canal with this technique.[2] However, the fiberglass postdebonding is considered the most common failure occurred in the method.[3],[4] This failure type has been related to many factors such as insufficient remnant of the dental crown and anchoring features,[5] cementation techniques sensitivity,[4] and the fiberglass post surface treatments.[1]

Different fiberglass post treatments are approaches aiming to increase the adhesive strength in bonding procedures.[6] Evidences suggest that the surface treatment is a mandatory clinical step, since it positively affects the adhesive strength.[7] Among the surface treatments, the silanization is the most frequently method used by dentists for root canal treatments.[1] This technique is a fast clinical procedure[2] and able to increase chemical bond due to reaction among the adhesive bifunctional molecules, the inorganic glass fiber, and the organic resin cement.[1],[8]

The silanization is also recommended in the root canal treatment associated to the self-adhesive resin cement. The covalent linkage of the methacrylate monomer cement with the silane is stronger than the ionic bond with the glass fibers.[8] Therefore, better silanization procedures may enhance the fiberglass post biomechanical behavior in root canal treatments and decreasing adhesive failure rate.

Thiourethane oligomers added to the light-cured resin cements decreased the polymerization shrinkage stress and increased the degree of conversion.[9] This improvement for the methacrylate-based resin materials was obtained due to low glass transition temperature and actives behavior of the oligomer, as well as to the fact that the additive is prepolymerized.[10] The thiourethane result from a combination of two multifunctional thiols with three difunctional isocyanates (aliphatic, aromatic, or cyclic), and different combinations result in oligomers with different values of bond strength. Thus, the reaction of the thiol (pentaerythritol tetra-3-mercaptopropionate [PETMP]) was higher when associated to the cyclic isocyanate, followed by the aliphatic.[11]

Better mechanical results with these oligomers showed the efficacy for adhesive materials where higher impact resistance is needed,[12] such as fiberglass post for root canal treatments. Significant evidences for the efficacy and feasibility of the thiourethane oligomers additions on dual-cured[13] or light-cured[9] resin cements, as well as combined or not with the hexanediol dimethacrylate crosslink agent as additive for denture base acrylic resin[14] were observed.

Cement viscosity was increased when thiourethane was added,[15] result that may have a positive effect over the resin cement during the fiberglass post fixation in the root canal, region where exists confinement restriction. The cements are affected by rheological properties, and there was a significant difference between the flow of different cements and the root canal diameter affected the cement capillary flow.[16]

The aim of this in vitro study was to evaluate the effect of conventional silanes and experimental silanes based on thiourethane on the adhesive strength of fiberglass posts in root canals submitted to thermal and mechanical cycling.


  Materials and Methods Top


Experimental materials composition

Thiourethane oligomers used to make the experimental silanes were synthesized as previously described,[10] and based on the best adhesive results for the thiol–PETMP.[11] Two oligomers were synthesized in solution with catalytic amounts of triethylamine. Multi-functional thiol–PETMP was combined with di-functional isocyanates–1,6-hexanedioldiissocyante (HDDI-aliphatic) or 1,3-bis (1-isocyanato-1-methylethyl) benzene (BDI - aromatic). Precipitation in hexanes and rota evaporation were used to purify the oligomers, followed by characterization by 1-H Nuclear Magnetic Resonance (Bruker AMX-400 MHz, Santa Barbara, CA, USA) and mid-IR spectroscopy. Isocyanate complete reaction was confirmed by the disappearance of the peak in mid-IR (2270 cm−1) and thiourethane bond formation confirmed by the appearance of resonance signals at 3.70 ppm. After, a titration with Ellman's reagent was used to determine the concentration of each oligomer. Purified oligomers were added at 20 wt% to ethoxylated bisphenol.[10] The commercial silanes RelyX CP (3M ESPE, St Paul, MN, USA) or Angelus (Angelus, Londrina, PR, Brazil) were used with the PETMP-HDDI or PETMP-BDI experimental silanes, respectively.

Specimen selection and preparation

The University Ethics Committee approved the study (# 95564518.7.0000.5418/2019). Sixty extracted sound human premolars teeth with root of 14 mm length and single canal collected in the Human Dental Bank (State University of Ponta Grossa, PR, Brazil) were used. The crowns were separated from the roots by cutting 2 mm above the cement-enamel junction using double face diamond disk (KG Sorensen, Barueri, SP, Brazil) and water cooling. The root canals were prepared using the step-back technique using Kerr type files (Dentsply Maillefer, Ballaigues, Switzerland) up size #50 and 1 mm short of the root apex. The instrumentation was performed interspersed by 1% sodium hypochlorite (Asfer Chemical, Sao Paulo, SP, Brazil) irrigations and final rinse with 17% ethylenediamine tetracetic acid (Biodinamica, Ibipora, PR, Brazil), both followed by 0.9% saline solution (ADV, Nova Odessa, SP, Brazil). The canals were dried with sterile absorbent paper points (Dentsply Maillefer) and filled by the lateral condensation technique using gutta-percha natural polymer points (Dentsply Maillefer) and canal sealer (Sealer 26; Dentsply Maillefer).

Postluting procedures

After 7 day storage at relative humidity in microbiological greenhouse (ProLab; Sao Paulo, SP, Brazil) at 37°C, the roots received a post space preparation to a depth of 10 mm, initially with Gates Glidden drill (#1; Microdont, Sao Paulo, SP, Brazil), followed by Peeso reamers (Microdont), and completed with the corresponding post drill (# 0.5; Angelus). A final rinse with 20 mL distillated water was performed and the root canal dried with absorbent paper points (Dentsply Maillefer). The roots were randomly divided into six groups (n = 10) according to the conventional or experimental silanes: RX (RelyX CP-Control); RX-PETMP-HDDI; RX-PETMP-BDI; AN (Angelus-Control); AN-PETMP-HDDI; and AN-PETMP-BDI, respectively.

Before luting, Angelus fiberglass posts were checked for proper length (15 mm) and the excess removed with a water-cooled diamond bur (Acerdent; London, UK), cleaned with 70% alcohol (Proderma, Piracicaba, SP, Brazil) and air-dried. The 15 mm post length was standardized so that 12 mm restored the root canal and the remaining 3 mm were used to establish the distance between the light-curing unit tip and root top. The bonding procedure was according to the manufacturer's recommendations of the chemically activated adhesive (Fusion Duralink; Angelus). The root canal dentin was etched with 37% phosphoric acid (Attaque Gel, Biodynamic, Ibipora, PR, Brazil) for 15 s, rinsed with water, and dried with absorbent paper points (Dentsply Maillefer). A layer of Fusion Duralink Primer (Angelus) was applied with microbrush (Kg Brush, KG Sorensen, Cotia, SP, Brazil), rubbing it lightly on the canal walls for 30 s followed by air drying for 10 s. RX and AN control groups were silanized with RelyX CP or Angelus, respectively. Following, the experimental silanes RX-PETMP-HDDI; RX-PETMP-BDI; AN-PETMP-HDDI, and AN-PETMP-BDI were appliqued with microbrushes (KG Sorensen) and air dried for 5 s.

All posts were luted with a dual-polymerizing resin cement (Variolink N, Ivoclar Vivadent, Schaan, Liechtenstein) using a Centrix syringe (DFL, Rio de Janeiro, RJ, Brazil). The posts were placed into the root canal and the cement excess removed. After, the resin cement was photoactivated with 1200 mW/cm2 irradiance (Bluephase; Ivoclar Vivadent) for 20 s for each face.

After luting procedure, the cores were built up with a photo-activated nanohybrid composite resin (Tetric N-Ceram, Ivoclar Vivadent) with 5 mm-height above the enamel-cement limit. Impressions of the cores were made with Vinyl polysiloxane (Zetaplus; Zhermack, Rovigo, Italy) using stock tray, and the molds poured with type IV gypsum (Durone IV, Dentsply). All root cores received laboratory-made indirect composite resin crowns (SR Adoro; Ivoclar Vivadent) luted with Variolink N resin cement (Ivoclar Vivadent), following the manufacturer's guidelines.

Thermal and mechanical cycling

For aging treatment, the roots were placed up to 2 mm below the enamel-cement limit in polyvinyl chloride tubes (Tigre; Sao Paulo, SP, Brazil) filled with self-curing acrylic resin (VIPI, Pirassununga, SP, Brazil). The roots were removed and reinserted into the acrylic resin until to form a root mold. After 24 h, the specimens were submitted to 1.2 × 106 mechanical cycles in a fatigue machine (11000; Erios, Sao Paulo, SP, Brazil), positioned in a metallic base, and submitted to 50 N axial force on the crown occlusal face, with a frequency of 2 Hz, and immersed in water at 37°C ± 1°C.[17] Following, the specimens were removed from the acrylic resin molds and submitted to thermocycling (MSCT-3E; Sao Carlos, SP, Brazil) with 500 cycles between 5°C and 55°C with a dwell time of 30 s at each temperature, and a transfer time of 5 s between baths.[18]

Push-out bond strength evaluation

A low-speed diamond blade saw (Isomet 1000; Buehler, Lake Bluff, IL, USA) was used to make the slices of 1 mm (±0.1) of each root region. A digital caliper (Mitutoyo; Suzano, SP, Brazil) confirmed the slice thickness (h), as well as measured the coronal (R) and apical (r) inner diameters of the root canal in order to calculate the bonding surface. The following formula determined the bonding surface:

A = π (R + r) (h2+ [Rr] 2) 0.5

Where: A = Bonding surface, π=13.1416; R = Inner coronal diameter; r = Inner apical diameter, and h = slice thickness.

Each root slice was submitted to the push-out test using a universal loading machine (Instron model 441, High Wycombe, Buckinghamshire, UK) equipped with a metal device for slice stabilizing. The load was applied on root slice in the apex-crown direction using a 1-mm cylindrical plunger at crosshead speed of 1 mm/min until the post portion was dislodged from the root slice. The maximum failure load was recorded in N and converted into MPa dividing the load by the bonded area (N/mm2).

The failure modes were analyzed by means of a stereomicroscope (Meiji EMZ-TR, Meiji Techno, Tokyo, Japan) at ×40 magnification and classified into the following categories: adhesive failure at the cement-dentin interface; adhesive failure at the post-cement interface; cohesive failure in cement; cohesive failure in post; cohesive failure in dentine or mixed failures.

Representative slices of each failure type were covered with a gold-palladium layer in a metallizer device (Bal-Tec SCD050; Sputter Coater, Sao Paulo, SP, Brazil), and the marginal gap observed with scanning electron microscopy (JEOL, model JSM 5600 LV; Tokyo, Japan) at a ×400 magnification using 15 kV accelerating voltage.

Statistical analysis

The bond strength data distribution normality and variances homogeneity were analyzed using Shapiro–Wilk and Levene tests, respectively. The interaction between silane types and root regions on push-out bond strength was assessed by two-way ANOVA followed by Tukey's test post hoc comparisons among the groups. All analyses were performed using the SPSS 24.0 software (SPSS Inc., Chicago, IL, USA) at α = 0.05 significance level.


  Results Top


Push-out bond strength

[Table 1] shows that the experimental silanes promoted a significantly higher bond strength compared to the conventional silanes in each group (P < 0.001). No statistically significant difference was shown among the experimental silanes in each group. The root region-silane interaction (P = 0.398) and the root region factor (P = 0.199) did not showed statistically significant difference.
Table 1: Push-out bond strength (mean values±standard deviation) in MPa for conventional and experimental groups

Click here to view


No cohesive failure in cement or posts was observed [Table 2]. The most frequent failure was adhesive in the dentin-cement bond, followed by mixed failures. The experimental silanes applied on the conventional silanes reduced the number of adhesive failures in dentin-cement bond and increased the number of cohesive failures in dentin and mixed failures.
Table 2: Debonding modes (failure number/percentage) for conventional and experimental groups

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Scanning electron microscopy micrographs of gaps in the adhesive interfaces of root slices related to the groups RX (a); RX-PETMP-HDDI (b); RX-PETMP-BDI (c); AN (d); AN-PETMP-HDDI (e); and AN-PETMP-BDI (f) are shown in [Figure 1]. The marginal gap was similar for all groups, except for the group AN (d) that showed greater gap when compared to the others.
Figure 1: Scanning electron microscopy micrographs showing gap formation for the groups: RX (a); RX-pentaerythritol tetra-3-mercaptopropionate-HDDI (b); RX-pentaerythritol tetra-3-mercaptopropionate-BDI (c); AN (d); AN-pentaerythritol tetra-3-mercaptopropionate-HDDI (e); and AN-pentaerythritol tetra-3-mercaptopropionate-BDI (f)

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  Discussion Top


This study evaluated two silane types based on thiourethanes applied associated to conventional silanes in root canal treatments with fiberglass post. The experimental silanes provided a significant increase on bond strength when compared to the control groups in the three root canal regions (cervical, middle and apical).

Recent studies have shown that the thiourethane oligomers addition to methacrylate matrix of resin cements improves mechanical properties. Among the improved properties, flexural strength, toughness modulus, and fracture toughness are the most noticeable. This result is due to the pendant thiols from the thiourethane matrix that are free to react through chain-transfer reactions with the methacrylate matrix vinyl bonds of the resin cement, resulting in important advantages in physical properties.[11],[15] These properties were also related to the active behavior and lower glass transition of the thiourethanes, contributing for less stress in the methacrylate structure.[10]

The chain-transfer reactions delay the polymer gelation and vitrification increasing the degree of conversion, since the total reaction is prolonged.[10],[11],[15] In addition, the polymer structure resulted from the reaction is more homogenous compared to conventional networks, as proved by the reduction of the tan delta peaks during dynamic mechanical analysis.[11],[12]

Considering the current study purpose, it is expected that the application of the experimental silanes to conventional silane showed similar reactions of the pendant thiols from the experimental silanes with the resin cement matrix, as showed in previous study.[9]

The higher bond strength values found for the experimental silanes in the current study [Table 1] may be related to the higher toughness modulus and fracture toughness resulted from the reaction, promoting an adhesive interface more stable.[12],[13] Therefore, this result suggests the need of a resin adhesive material with an improved behavior for dental rehabilitations submitted to repetitive impacts such as occur in endodontically treated teeth with fiberglass posts.

The polymerization stress reduction found in materials modified by thiourethanes showed a relevant effect in the bond strength, especially in confined regions such as the cementation line.[11] It is possible to think that the experimental silanes associated to conventional silanes would also promote improvements on the mechanical properties of resin cements, contributing to increase the bond strength of fiberglass posts to root canal.

The debonding failure pattern was different when both experimental silanes were compared to the conventional silanes [Table 2]. Conventional silanes showed predominantly adhesive failures between fiberglass post and resin cement. However, the thiourethane addition increased bond strength, resulting in decreased adhesive failures and increased cohesive failures in dentin and mixed failures.

An earlier study showed that the silane impaired or generally had no effect on the bond strength of self-adhesive cements to the fiberglass posts. In addition, the bond strength of all self-adhesive cements was higher than the conventional cement when the posts were not silanated; therefore, it seems that silanization of fiberglass posts is not necessary when self-adhesive cements are used.[8] Considering these allegations, it would be necessary further evidence on the effect of silanes in the increase of the bond strength of fiberglass posts to root canal dentin.

On the other hand, the surface of silanized fiberglass posts increases the mechanical interlocking with the adhesive cement.[19] As the experimental silanes increased the adhesive bond strength in the current study, it is assumed that the greater the strength required for fiberglass posts debonding, the higher the chance to occur catastrophic fractures such as cohesive in dentin or mixed failures; therefore, confirming the findings.

Another fact related to decrease of adhesive failures with experimental silanes is the stress amount concentrated at interface fiberglass post and root canal. As the elastic modulus of the resin cement is smaller than those for fiberglass posts and root canal dentin, greater stress concentration occurs in this interface.[20] In this sense, the reaction between a low glass transition oligomer and the methacrylate network increases the elastic modulus, toughness, and fracture toughness of the resin cement when compared to methacrylate without additives.[10]

Toughness of the resin material is an important clinical performance prediction and should guide the dental restorative material selection,[20] as well as the elastic modulus in relation to restoration mechanical strength.[15] In addition, thiourethane reduced the polymerization shrinkage, resulting in lower residual stress around inorganic particles of dental resin materials.[10],[13],[15] The reduction of this stress also increased the fiberglass posts retention, resulting in lower amount of adhesive failures,[21] similar to the results of the current study.

In previous studies, thiourethane addition reduced the volumetric contraction due to low polymerization shrinkage for the experimental resin materials when compared to control,[9],[11],[15] result that could have also occurred in this study with the experimental silanes. However, it is important to clarify that these aforementioned studies[9],[11],[15] verified the mechanical properties of the resin cement after the specimens construction without no specific treatment, while in the current work, the root canals were endodontically treated in vitro and submitted to mechanical and thermal cycling. Thus, it is possible that the aging of the interface between the root canal and the fiberglass posts fixed with thiourethane-based materials results in a similar bond strength decrease due to similar gap formation among the groups. By a

nalogy, the difference between the marginal gap values of ceramic materials was negligible but with a significant increase after cementation and thermo mechanical aging.[22]

Nevertheless, it is worth noting that the thiourethane-based silanes showed higher bond strength and lower adhesive failure number in this study, fact that would justify its application in endodontic procedures. A previous study has shown that the direct composite resin restoration with and without fiberglass post groups presented significantly higher fracture resistance values, and no significant differences for the fiberglass post system.[23] Water absorption in the interface may also have a deleterious effect on the structure and mechanical properties of resin dental materials.[24]

Mechanical properties of adhesives are directly related to bonding long-term behavior, as well as the silane based on thiourethane would be for the endodontic treatment success with fiberglass posts. Although the result is promising, the findings must be interpreted with caution due to limitations that simulated the conditions in vivo in this in vitro study. The flexural strength values for quartz, carbon, and fiberglass posts are significantly different,[25] and differences in bond strength were demonstrated between the different areas, with the coronal and apical having higher values when the intermediate root treatment with calcium hydroxide was associated with fiberglass posts and RelyX Unicem self-adhesive resin.[26]

Therefore, further studies regarding mechanical properties of different post types and oral behavior of adhesive materials containing thiourethane are required to predict the benefit for clinical use.


  Conclusions Top


Conventional and experimental silanes provided greater adhesive strength; reduced dentin-cement failures and the marginal gaps were similar, except for the control group Angelus, showing greater slit.

Acknowledgments

The authors are grateful to the Dr. Stella Kossatz Pereira, Coordinator of the Human Teeth Bank of the State University of Ponta Grossa; Ivoclar Vivadent and Angelus manufacturers for the materials employed in the study, and to the Coordination for the Improvement of Higher Education Personnel (CAPES) for financial support.

Financial support and sponsorship

This study was financially supported by Coordination for the Improvement of Higher Education Personnel (CAPES) of the Brazilian Ministry of Education.

Conflicts of interest

There are no conflicts of interest.

 
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