Effect of cleaning protocols on bond strength of etch-and-rinse adhesive system to dentin.

AIM: This study aimed to evaluate the cleaning protocol of dentin impregnated with epoxy resin-based sealer (AH Plus) and the effects on the adhesive interface using 95% ethanol (E) or formamide solution (F) associated with microabrasion with glass microparticles (M).
METHODOLOGY: Forty dentin specimens were impregnated with sealer (AH Plus). Specimens were cleansed according to the following protocols (n = 10): G1 (E); G2 (F); G3 (E = M); and G4 (F = M). No treatment was performed in G5 (control). Specimens were subjected to standard error of the mean to evaluate the residues persistence (×500) and open dentinal tubules counting (×2000). Another fifty specimens were subjected to similar procedures and cleaning protocols. Seven days later, composite-resin-made specimens were performed on dentin using etch-and-rinse adhesive system, then submitted to microshear test.
RESULTS: G2 and G4 showed higher residues' persistence and lower incidence of open dentinal tubules than G1 and G3 (P < 0.05). No difference was found between G1 and G3 or G2 and G4 (P > 0.05). Adhesive system bond strength was similar in all groups (P > 0.05).
CONCLUSIONS: Microabrasion negatively affected the epoxy resin-based sealer residues' removal protocols' action; all groups presented residues on the dentin surface. However, 7 days later, the protocols did not affect the bond strength of adhesive system to sealer-impregnated dentin.

INTRODUCTION

Pulp chamber dentin tends to be covered by endodontic sealer after residues after the root canals' obturation; it may cause undesirable esthetic and functional effects.[12] Ethanol, xylol, amyl acetate, or eucalyptol have already been recommended for surface cleansing; however, their effectiveness is still questionable.[3456] It favors the sealer residues persistence, which may cause discoloration of dental crown and/or negatively affect the adhesive interface between dentin and adhesive system, compromising the cosmetic restorations longevity.[12345678]

Ethanol is the most commonly used substance in cleaning protocol of dentin impregnated with endodontic sealer.[68] It is used alone or associated with diamond bur debridement in the cleansing treatment of dentin surface impregnated with zinc oxide-eugenol-based sealer (Endomethasone), reverting the bond strength of self-etching adhesive system to dentin.[8] On the other hand, ethanol is ineffective in cleansing dentin with epoxy-based sealer residue.[5]

Formamide solution (Endosolv R) is another alternative since it presents a satisfactory action on epoxy resin-based sealers. In addition, it is also recommended in root canals obturated with AH Sealer retreatment.[79] This solution prevents the bond strength reduction of self-etching adhesive systems (Clearfil SE Bond e Clearfil Tri-S Bond) after cleaning protocol is carried out in AH Plus impregnated dentin.[7] However, the effects are still unclear when etch-and-rinse adhesive system is used instead of self-etching adhesive system.

Since the cleaning solutions do not provide the complete removal of endodontic sealer residues,[34567] their association with mechanical and/or abrasive methods can be an appropriate alternative. Recently, microabrasion with glass microparticles associated with 2% chlorhexidine gel (Concepsis Scrub) has been recommended on temporary cement residues' removal in dentin surface; moreover, it showed to reverse the bond strength of resin cement to dentin.[10] However, there have been no studies evaluating the cleaning protocol in dentin impregnated with endodontic sealer.

Thus, the aim of this study was to evaluate the efficiency of 95% ethanol (Rinse-N-Dry; Vista Dental, Racine, WI, USA) and formamide solution (Endosolv R; Saint-Maur-des-fossés Cedex, FR), with microabrasion using an antimicrobial gel containing glass microparticles (Concepsis Scrub; Ultradent, South Jordan, UT, USA) and their effects on the bond strength of etch-and-rinse adhesive system to dentin previously impregnated with an epoxy resin-based sealer (AH Plus; Dentsply De Trey, Konstanz, GE) and submitted to several residues removal protocols. The null hypothesis is that cleaning protocols do not completely remove the sealer residues and affect the bond strength of adhesive system to impregnated dentin.

METHODOLOGY

Groups evaluated

G1 (E): A cotton pellet was embedded in 95% ethanol (Rinse-N-Dry, Dental View, Racine, WI, USA) and rubbed until endodontic sealer residue could not be visually detected on the dentin surface

G2 (E + M): Similar to G1, followed by microabrasion with 2% chlorhexidine gel-containing glass microparticles (Concepsis Scrub; Ultradent, South Jordan, UT, USA) was carried out using a brush, for 15 s at low speed in all dentin surface

G3 (F): Similar to G1, however, formamide solution (Endosolv R; Saint-Maur-des-fossés Cedex, France) was used

G4 (F + M): Similar to G2, however, formamide solution was used

G5 (control): No cleaning treatment was performed.

Endodontic sealer residues persistence evaluation

Fifty dentin specimens (5 mm × 5 mm × 3 mm) were obtained from the middle third of bovine incisors crown, using a hard tissue-cutting machine (Isomet; Buehler, Lake Bluff, IL, USA) under water cooling. Afterward, they were individually immersed in 5 mL of 2.5% NaOCl (Asfer, São Caetano do Sul, SP, BR), for 15 min, and then in 5 mL of 17% EDTA (Biodinamica, Ibiporã, PR, Brazil) for 3 min. In sequence, 5 mL of 2.5% NaOCl was used as a final irrigation solution. Finally, all specimens were dried using absorbent paper, according to methodology reported in Jordão-Basso et al.[6]

Ten specimens were kept untouchable and used as control group (G %5). For the other forty specimens, an epoxy resin-based sealer (AH Plus; Dentsply De Trey, Konstanz, Germany) was handled, in a 1:1 proportion, according to the manufacturer's instructions. The endodontic sealer was then spread over pulp chamber dentinal surface using a microbrush (FGM, Joinville, SC, Brazil) and left untouched for 15 min. Then, these specimens were randomly divided into four experimental groups (n = 10), according to endodontic sealer residues cleaning protocol, as previously described in groups evaluated session.

In experimental groups, epoxy resin-based sealer was removed using a cotton pellet embedded in 95% ethanol (G1 and G2) or formamide (G3 and G4) and rubbed onto pulp chamber dentinal surface until the sealer had been visually removed. However, in G2 and G4, dentinal surface was also submitted to microabrasion using a prophy cup and 2% chlorhexidine gel-containing glass microparticles (Concepsis Scrub; Ultradent, South Jordan, UT, USA) for 15 s. In sequence, theses specimens were washed, for 1 min, with distilled water.

The specimens were dehydrated inside a sealed chamber containing silica gel for 24 h. All specimens were mounted onto metal stubs, gold-sputter coated (single cycle-120 s) under vacuum inside a metalizing chamber (MED 010, Balzers Union, Balzers, Liechtenstein), and examined using scanning electron microscopy (JEOL 6060; JEOL Ltda, Tokyo, Japan), operating at 15 kV.

Four different fields were evaluated in each specimen, and an image representative of the specimen was obtained at × 500 and × 1000 magnification. Two independent and proper calibrated examiners evaluated the residues persistence on dentin surface, using microimages at × 500, according to Kuga et al.[3] Parameters: Score 1, absence or discrete residues presence with visible open dentinal tubules; Score 2, discrete residues presence, with more than 50% residue-free dentin surface; Score 3, moderate residues presence, with < 50% residue-free dentin surface; and Score 4, intense residues presence, with dentinal tubules opening practically or totally obstructed. The open dentinal tubules counting without residues presence was obtained using the Photoshop CS5 program (Adobe Systems, San Jose, California, USA), as described by Jordão-Basso et al.[6]

Interference on adhesive interface

Fifty specimens (10 mm length × 5 mm width) were obtained from the bovine incisor crowns. The buccal surface fragment was flattened with a polishing machine (DP-10; Panambra, Struers, Ballerup, DEN) using #320 and #600 silicon sandpapers, for 20 s, to exposure and planning dentinal surface. Then, the fragments were placed into a polystyrene matrix mold (16.5 mm width × 25.0 mm length) and embedded with acrylic resin (Classic Jet, São Paulo, SP, BR) left undisturbed for 1 h.

Forty specimens were impregnated with endodontic sealer (AH Plus), subjected to several cleaning protocols, as described in endodontic sealer residues persistence evaluation study. Therefore, specimens were divided into four experimental groups (n = 10) and one control group (G5). The specimens were stored at 37°C, in a humidity-free environment, to final set epoxy resin based.

After 7 days, dentin surface was submitted to acid etching using 37% phosphoric acid, for 15 s, washed with distilled water for 30 s, and drying with absorbent paper, keeping dentin slightly moistened. In sequence, an etch-and-rinse adhesive system (Scothbond Multi-Purpose; 3M ESPE, St. Paul, MN, USA) was applied on dentin surface, and light cured using a LED-laser system (Bluephase; Ivoclar Vivadent, Schaan, Liechtenstein, AL), with 1.200 mW/cm2 light intensity for 20 s. In sequence, four cylinders (0.7 mm internal diameter × 1.0 mm height) made of composite resin (Filtek Z-250; 3M, St. Paul, MN, USA) specimens were bonded on the buccal surface, two at mesial and two at distal.

Afterward, the specimens were stored in a 100% relative humidity environment at 37°C, and the microshear bond test was performed after 24 h. All specimens were fixed inside a metal matrix, so the composite cylinders specimens were placed perpendicularly to a 500 N load cell. An orthodontic wire (0.2 mm diameter) held the composite cylinder base, and all specimens were subjected to compressive loading with 0.5 mm/min crosshead speed with EMIC DL2000 electromechanical testing machine (EMIC, São José dos Pinhais, PR, Brazil) until obtaining composite specimens displacement. Bond strength was obtained from maximum force (N) divided by union area (mm2) in MPa. Arithmetic average was calculated for the four composite specimens from each fragment and called mean specimen.

The failure mode was analyzed in stereomicroscope (×20) and classified as: (1) adhesive, between dentin and adhesive system; (2) cohesive, in the adhesive system; and (3) mixed, partial between dentin and adhesive system.

Statistical analysis

Residues' persistence data were subjected to Kruskal–Wallis and Dunn tests. Open dentinal tubules counting and bond strength of adhesive system data were submitted to ANOVA at one criterion and Tukey tests. All evaluations were performed at 5% significance level.

RESULTS

Regarding the residues persistence on dentin surface, G1 and G3 presented lower incidence than G2 and G4 (P < 0.05). No difference was found between G1 and G3 or G2 and G4 (P > 0.05). G5 presented no residues.

In relation to open dentinal tubules, G1 and G3 presented a higher number than G2 and G4 (P < 0.05). No difference was observed in the amount between G1 and G3 or G2 and G4 (P > 0.05). G5 showed the highest number of open dentinal tubules. Figure 1 shows residues' presence and open dentinal tubules incidence in all groups.

Figure 1

Representative image of residues persistence (×500) according to the groups: G1 (a), G2 (b), G3 (c), G4 (d), and G5 (e). Image representative of open dentinal tubules incidence (×1000) according to the groups: G1 (f), G2 (g), G3 (h), G4 (i), and G5 (j). G1, 95% ethanol; G2, 95% ethanol and microabrasion with glass microparticles; G3, formamide solution; G4, formamide solution and microabrasion with glass microparticles; and G5, control, untreated. Vmx: maximum value and Vmin: minimum value

Seven days later, cleaning protocols did not affect the bond strength of etch-and-rinse adhesive system to dentin impregnated with epoxy resin-based sealer (P > 0.05). Table 1 shows the median, maximum, and minimum score values attributed to endodontic sealer residues persistence in microscopic images and the arithmetic mean and standard deviation of open dentinal tubules (units) amount and bond strength of adhesive system to dentin (MPa), according to the evaluated groups.

Table 1

Median, maximum and minimum values score value attributed to endodontic residues and the arithmetic mean and standard deviation of open dentinal tubules (units) amount and bond strength of adhesive system to dentin (MPa)

Figure 2 displays the failure mode incidence according to the groups. Mixed failure was the most observed one among the groups.

Figure 2

Failure mode incidence according to the groups. G1, 95% Ethanol; G2, 95% ethanol and microabrasion with glass microparticles; G3, formamide solution; G4, formamide solution and microabrasion with glass microparticles; and G5, control, untreated. Red: adhesive failure, yellow: mixed failure, and blue: cohesive failure

DISCUSSION

No cleaning protocol completely removed the epoxy resin-based sealer (AH Plus) resides from dentin surface and immediate abrasion with glass microparticles in 2% chlorhexidine gel presented higher residues dispersion causing a higher dentinal tubule opening obstruction. However, after 7 days, these cleaning protocols no affected the bond strength of etch-and-rinse adhesive system in dentin. Thus, the null hypothesis was rejected.

“Like dissolves like” concept means a polar solute dissolves in polar solvents.[7] However, 95% ethanol cleaning protocol showed endodontic sealer residues persistence on the dentin surface in all situations.[5] Although epoxy resin is a nonpolar substance and ethanol is a polar solvent, but presenting both polar and nonpolar characteristics, the residues' persistence may have occurred due to water presence in 95% ethanol solution (Rinse-N-Dry) used in our study, since its pharmaceutical formula is extremely volatile.[11] As epoxy resin-based sealer is insoluble in water, possibly its residues remained scattered over the dentinal surface in G1 and G2, in agreement to Kuga et al.[5]

Although formamide is a nonpolar solvent,[7] in our study, its cleaning potential was similar to 95% ethanol. Jordão-Basso et al.[6] observed that, when used before final setting sealer, xylol has similar epoxy resin-based impregnated dentin surface cleaning potential to ethanol. On the other hand, we believe that the results obtained are due to the similar cleaning potential and chemical properties between formamide and xylol and consequently to 95% ethanol.

Although, Saraç et al.[10] showed that Concepsis Scrub and Sikko Tim are the most efficient cleaning agents to remove the temporary cement from dentin, this result was not observed in the present study. The greatest AH Plus residues persistence endodontic sealer on dentin was observed in G2 and G4. It can be related to its final setting time.[1213]

In our study, when the microabrasion with prophy cup and 2% chlorhexidine gel-containing glass microparticles gel was performed, the endodontic sealer residues presented in a fluid consistency. As epoxy resin is hydrophobic, this cleaning protocol caused sealer residues dispersion, and consequently lower open dentinal tubules incidence, as observed in Figure 1.

On the other hand, cleaning protocols did not affect the bond strength of etch-and-rinse adhesive system to dentin, possibly due to acid etching being performed after 7 days, once 37% phosphoric acid has a satisfactory removal effect from the chemical smear layer.[1415] Therefore, as there is a larger area of exposure intertubular dentin and incidence of open dentinal tubules, this possibly favored the adhesion between etched-and-rinse dentin adhesive system and dentin, as demonstrated by previous studies.[16171819202122]

Despite Roberts et al.[7] observed that only formamide (Endosolv R) reversed the bond strength of two-step self-etching (Clearfil SE Bond) or one-step self-etching (Clearfil Tri-S Bond) adhesives after cleansing dentin previously impregnated with sealer (AH Plus), in our study, we observed that 95% ethanol also was efficient to reverse the bond strength, but when used etch-and-rinse dentin adhesive system.

Adhesion between dentin and etch-and-rinse dentin adhesive system occurs mainly by mechanical action, through the hybrid layer formation.[23] This adhesive system is considered as reference and gold standard to compare with other systems.[1624] As self-etching adhesive system has a chemical bond with hydroxyapatite, any residues can interfere negatively on this chemical reaction and consequently on adhesive interface.[25] Therefore, in our study, the bond strength was not affected possibly due to dentin acid etching and adhesive strategy used.

Furthermore, these adhesives present higher mechanical bonding with dentinal substrate and lower adhesive interface degradation by hydrolysis,[1819] due to hydrophobic resin presence in the primer chemical composition,[202122] which minimized differences among dentin cleaning protocols.

Future studies should be carried out to find the best endodontic residues sealer removal protocols, favoring the longevity of esthetic restorations and endodontic treatment.

CONCLUSION

Immediate microabrasion, using 2% chlorhexidine with glass microparticles, had a negative effect on epoxy resin-based sealer residues' removal protocols' action, when used 95% ethanol or formamide. However, 7 days later, the treatments did not affect the bond strength of etch-and-rinse dentin adhesive system to dentin.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.