Influence of Drying Protocol with Isopropyl Alcohol on the Bond Strength of Resin-based Sealers to the Root Dentin

Abstract

Introduction: This study compared the bond strength, interfacial ultrastructure, and tag penetration of resin-based sealers applied to smear-free radicular dentin using 70% isopropyl alcohol as the active final rinse.

Methods: Eighty root canals were prepared and assigned to 2 groups (n = 40) according to the drying pro- tocol: paper points or 70% isopropyl alcohol. Then, roots were divided into 4 subgroups (n = 10) with respect to the sealer and obturation material: AH Plus (Dentsply De Trey GmbH, Konstanz, Germany) and gutta-percha (AH/GP), Hybrid Root SEAL (Sun Medical, Tokyo, Japan) and gutta-percha (HR/GP), Epiphany SE (Pentron Clinical Technologies, Wallingford, CT) and gutta-percha (EP/GP), and Epiphany SE and Resilon (EP/RS). Roots were sectioned, and the push-out test was performed. Failure modes were examined under stereomicroscopy and sealer penetration into the dentinal tubules under scanning electron microscopy. Data were statistically analyzed by 2-way analysis of variance post hoc Tukey tests with a significant level of 5%.

Results: Overall, canals dried with isopropyl alcohol showed significantly higher bond strength values (2.11  1.74 MPa) than with paper points (1.81 1.73 MPa) (P < .05). The HR/GP group showed lower bond strength than the AH/GP group (P < .05) but higher than the EP/GP and EP/RS groups (P < .05). The most frequent type of failure was cohesive in the AH/GP and HR/GP groups and adhesive in the EP/GP and EP/RS groups. Scanning electron microscopic evaluation revealed better adaptation of the adhesive interface in the AH/GP and HR/GP groups in comparison with the EP/GP and EP/RS groups.

Conclusions: A final rinse with EDTA and 70% isopropyl alcohol improved the bond strength and penetration of the sealers into dentinal tubules of the root. (J Endod 2014;■:1–5)

Primary infection or infection secondary to root filling procedures is the main cause of apical periodontitis and endodontic failure. It follows that the root filling functions, such as entombment and prevention of bacterial penetration, are paramount. Conventional root fillings consist of a core material, usually gutta-percha or Resilon, that should be closely adapted to the canal wall and a sealer that fills voids and gaps between the core and the dentin. In endodontic research, epoxy resin–based sealers, such as AH Plus (Dentsply De Trey GmbH, Konstanz, Germany), are frequently used as a control material because of their reduced solubility, long-term dimensional stability, and adequate microretention to dentin. However, its sealing ability remains controversial partly because AH Plus does not bond to gutta-percha.

Improvements in adhesive technology have fostered attempts to incorporate adhesive dentistry in endodontics by introducing methacrylate-based sealers focusing on forming a single cohesive unit between the core material, sealing agent, and root canal dentin. Recently, acidic resin monomers were incorporated into these sealers to render them self-adhesive to dentin substrates, aiming to reduce the application time and errors that might occur during bonding steps. However, sealer adhesion to dentin may be affected by the moisture condition of the root canals before filling procedures. Thus, making smear-free dentin more wettable may improve sealer penetration.

According to the manufacturers, keeping the root canals in a moist state after the removal of the smear layer with EDTA is recommended to improve the dentin hybridization of methacrylate-based sealers. Considering that no clear instructions have been provided for achieving such an ideal degree of residual moisture, various chemicals, including alcohol in different concentrations, have been tested to improve dentinal wettability. Recent studies have shown that excessive desiccation may remove the water residing in the dentinal tubules, which may in turn hamper effective penetration of hydrophilic sealers, compromising the quality of adhesion. Conversely, a final rinse with 70% isopropyl alcohol has shown promise to improve zinc oxide–based sealer penetration into the dentinal tubules, but its effect is still unclear when using resin-based sealers in the obturation procedure.

Therefore, this study aimed to compare the bond strength, interfacial ultrastructure, and tag penetration of AH Plus and 2 self-adhesive methacrylate resin–based sealers (Hybrid Root SEAL; Sun Medical, Tokyo, Japan; and Epiphany SE; Pentron Clinical Technologies, Wallingford, CT) applied to smear-free radicular dentin using 70% isopropyl alcohol as the active final rinse. The null hypothesis tested was that different drying protocols of root dentin would not affect the bond strength and the penetration of dentinal tubules of different resin-based endodontic sealers.

Materials and Methods

Sample Selection

This study was approved by the local ethics committee (protocol #0086.0.138.000-09). Eighty straight single-rooted maxillary canines with fully formed apices and similar root morphology were obtained from a pool of extracted teeth and stored in 0.1% thymol solution at 5◦C. The specimens were decoronated by transversally sectioning the roots at 17 mm from the apex with a double-faced diamond disc (#6911H; Brasseler Dental Products, Savannah, GA) at a low speed with air/water spray coolant. Preliminary periapical radiographs were exposed in both buccolingual and mesiodistal directions for each tooth. All teeth presenting more than 1 root canal, isthmus, resorption, calcifications, or apical curvature were excluded. Teeth not patent to the canal length with a size 10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) were also discarded. Within 3 months after extraction, teeth were washed under running water for 24 hours, blot dried, stored in normal saline, and transferred to a chamber maintained at 37◦C and 95% relative humidity

Root Canal Preparation

Conventional access cavities were made, and apical patency was confirmed by inserting a 10 K-file through the apical foramen before and after completion of root canal preparation. The working length (WL) was established at 1 mm from the canal length, and a single experienced operator performed all experimental procedures. The canals were prepared using a crown-down technique with hand K-files (Dentsply Maillefer) up to size 60, flushed with 2 mL 1% sodium hypochlorite between each file size, and delivered in a syringe with a 30-G needle placed 1 mm short of the WL. After preparation, the canals were irrigated with 5 mL 17% EDTA (pH = 7.7) for 5 minutes followed by a final 5-minute 5-mL rinse with bidistilled water.

Experimental Groups

The specimens were randomly assigned to 2 experimental groups (n = 40) according to the drying protocol. In group 1, the canals were blot dried with size 60 paper points (Dentsply Maillefer) until complete dryness of the last point was confirmed visually. In group 2, after the removal of excess normal saline with size 60 paper points, as in group 1, the canals were filled with 70% isopropyl alcohol (Pizzani Qúımica Industrial, São José dos Campos, SP, Brazil) using a syringe with a 30-G blunt-tip needle carried to the WL. The alcohol was left in the canal for 5 seconds and immediately aspirated with a size .014 capillary tip (Ultradent, South Jordan, UT) at a low vacuum with a gentle up-and- down motion for 5 seconds. For each drying protocol, the specimens were further assigned to 4 subgroups (n = 10) with respect to the sealer and obturation material: AH Plus and gutta-percha (AH/GP), Hybrid Root SEAL and gutta-percha (HR/GP), Epiphany SE and gutta-percha (EP/GP), and Epiphany SE and Resilon (EP/RS).

The sealers, shown in Supplemental Table S1 (available online at www.jendodon.com) were prepared according to the manufacturer’s recommendations and introduced at large amounts into the canal orifice with a Lentulo spiral (Dentsply Maillefer) rotated at 500 rpm in a clockwise direction with a slow-speed handpiece inserted up to 1 mm short of the WL. Thereafter, a prefitted size 60, 0.02 taper cone (Dentsply Maillefer) was inserted into the full WL, and nickel-titanium finger spreaders (Dentsply Maillefer) were used to conduct the lateral compaction using 3 fine-medium accessory cones (Dentsply Maillefer) per canal. A heated instrument was used to cut the coronal surplus, after which the filling was vertically compacted with a size 10 plugger (Dentsply Maillefer). The coronal root surfaces of the specimens obturated using Hybrid Root SEAL and Epiphany SE sealers were light cured (Curing Light 2500; 3M ESPE, St Paul, MN) for 20 and 40 seconds, respectively. The roots were radiographed from buccolingual and mesiodistal directions to check the length of the filling material and the presence of voids, and samples were stored (37˚C and 95% humidity) for 7 days to allow the complete setting of the sealers. If voids were observed in the obturation mass, the specimen was replaced.

After this period, each root third (coronal, middle, and apical) was sectioned perpendicularly to its axis into three 1-mm-thick serial slices using a low-speed saw (Isomet 1000; Buehler, Lake Forest, IL) rotating at 300 rpm with a 75-g load with water coolant. Thus, 9 slices were obtained from each specimen, with a total of 90 sections per group. Each slice was marked on its apical side with an indelible marker.

Push-out Bond Strength Test and Failure Analysis

The first slice obtained from each root canal third was submitted to the push-out test in a universal testing machine (Instron 4444; Instron, Canton, MA), operating at a crosshead speed of 1.0 mm/min. Four- millimeter-long shafts with tip diameters of 0.4 mm, 0.6 mm, and 1.0 mm were used for the apical, middle, and coronal sections, respectively, until bond failure. The apical surface displaying the ink dot was placed facing the punch tip, ensuring that loading forces were introduced from an apical to coronal direction, to push the filling material toward the larger part of the root slice, thus avoiding any limitation to the material movement. This method ensured the alignment of the specimen in an accurate and reproducible manner, maintained the shaft centralized, and avoided its contact with the dentin when the material was pushed and dislodged from the canal wall. Bond strength data were converted to MPa by dividing the load (in kN) by the adhesion area of the filling material in millimeters squared. The adhesion area was calculated as the lateral surface area of a truncated cone using the formula p(R + r)[h2+ (R — r)2]0.5, where p is the constant 3.14, R is the mean radius of the coronal canal, r is the mean radius of the apical canal, and h is the thickness of the slice. The widest and narrowest diameters of the filling material and the thickness of the slice were individually measured by a digital caliper with 0.001-mm accuracy (Mitutoyo Messgerate GmbH, Neuss, Germany).

The failure mode of each debonded specimen after the push-out test was assessed with a stereomicroscope (Stemi 2000-C; Zeiss, Jena, Germany) at a magnification of 25×. Failures were classified as follows:

1. Adhesive between dentin and sealer (no sealer visible on dentin walls)
2. Cohesive in sealer (dentin walls totally covered with sealer)
3. Mixed when both adhesive and cohesive failures could be observed

Scanning Electron Microscopic Analyses

The second slice from each canal third was selected and prepared for scanning electron microscopic (SEM) analysis (JSM 5410; JEOL Ltd, Tokyo, Japan), as previously described. The specimens were mounted to observe the dentin/filling interface regarding the presence of a hybrid layer and resin tag formation at magnifications of 50×, 500×, and 1,000×.

The third slice obtained from each root third was used for the SEM evaluation of the filling material surface. The sections were demineralized in hydrogen chloride for 48 hours and deproteinized in 2.5% sodium hypochlorite for 15 minutes. After the complete dissolution of dentin, the filling material was washed twice in distilled water for 4 minutes, slightly dried, sputter coated with a gold-palladium layer, and observed at a magnification of 500×. The qualitative surface analysis of the filling material allowed for the morphologic evaluation of the tags.

SEM analyses were performed double-blinded by 2 operators independently. In case of disagreement, a third expert made an independent review, and consensus was sought.

Statistical Analysis

The normal distribution of the push-out strength data was first verified using the Shapiro-Wilk test. Two-way analysis of variance post hoc Tukey test was used to determine whether a statistically significant 2-factor interaction existed between sealers and the drying protocols. Statistical comparisons within and between the experimental groups was performed by using SPSS v. 17.0 for Windows (SPSS Inc, Chicago, IL) with the significance level set at 5%.

Results

Push-out Bond Strength Test and Failure Analysis

Means and standard deviations of push-out bond strength of the experimental groups in each third are summarized in Table 1. Bond strength values were significantly affected by either the drying protocol or filling material (P < .05). Overall, root canals dried with isopropyl alcohol showed significantly higher bond strength values (2.11 1.74 MPa) than the conventional drying protocol (paper points) (1.81  1.73 MPa) (P < .05).

The AH/GP group displayed a significantly higher bond strength than the other groups (P < .05). A statistical ranking for bond strength values was AH/GP > HR/GP > EP/GP = EP/RS. The level of root had no significant effect on the bond strength values in the HR/GP and EP/GP groups considering both drying protocols (P > .05). Conversely, a statistically significant difference regarding the drying protocol was observed in the middle and apical thirds of the AH/GP group (P < .05) and in the coronal third of the EP/RS group (P < .05).

Analysis of the bond failure after the push-out test (Table 2) revealed that the most common type of failure mode was cohesive in the AH/GP and HR/GP groups and adhesive in the EP/GP and EP/RS groups. No adhesive failure was observed in the AH/GP group.

SEM Evaluation

Representative SEM images of the dentin/filling interface in each third of the experimental groups after drying protocol with either paper points or isopropyl alcohol are shown in the Supplemental Figures S1 and S2 (available online at www.jendodon.com), respectively. SEM evaluation revealed better adaptation of the adhesive interface in the AH/GP (Supplemental Figures S2A–C and S3A–C are available online at www. jendodon.com) and HR/GP groups (Supplemental Figures S2D–F and S3D–F are available online at www.jendodon.com) with juxtaposition of the sealer to the canal walls in almost the entire extension of the interface regardless of the drying protocol. In the EP/GP (Supplemental Figures S2G–I and S3G–I are available online at www.jendodon.com) and EP/RS groups (Supplemental Figures S2J–L and S3J–L are available online at www.jendodon.com), gaps between the filling material and the dentin walls as well as dentin surface areas completely devoid of sealer with no tag formation were observed in all thirds.

The analysis of the surface of filling materials after dentin demineralization showed dense areas of relatively long, continuous, and coherent tags with a parallel arrangement in the AH/GP (Supplemental Figure S3A and B is available online at www.jendodon.com) and HR/GP (Supplemental Figure S3C and D is available online at www.jendodon. com) groups. In the EP/GP (Supplemental Figure S3E and F is available online at www.jendodon.com) and EP/RS (Supplemental Figure S3G and H is available online at www.jendodon.com) groups, the resin tags were collapsed and interwoven.

Discussion

Interests in adhesive endodontics have led to the introduction of different resin-based root canal sealers. Despite the inadequate levels of bond strength of most current sealers to dentin, adhesion is necessary to maintain the integrity of the sealer-dentin interface during mechanical stresses caused by tooth flexure, operative procedures, or subsequent preparation of a post space. Although bond strength testing may not be a reliable predictor of the clinical behavior of sealers, push-out bond strength test has been considered suitable for ranking root filling materials regarding root canal dentin adhesion.

In the present study, the drying protocols differentially affected the push-out bond strength and dentinal tubule penetration of the resin-based sealers; therefore, the null hypothesis was rejected. Overall, canals dried with 70% isopropyl alcohol showed significantly higher bond strength values than those with paper points. Considering the hydrophilic propensity of the resin-based sealers, it may be speculated that isopropyl alcohol (C3H7OH), which has lower polarity than ethanol (C2H5OH), promoted less removal of the water from dentinal tubules, enhancing the dentin wettability, increasing the degree of conversion of the sealers, and consequently improving their adhesion. However, a standardized degree of residual moisture may be difficult to achieve in all regions of the root. It happens because of differences in the dentinal tubule density and the limited accessibility of solutions to the most apical portions of the canal, which may explain significant differences in the results observed among the canal thirds in some groups.

Among the filling materials, the AH/GP group had the highest bond strength mean values regardless of the dentin surface treatment. It was shown that because of the flowability, long polymerization time, and high cohesion among the molecules, epoxy resin–based sealers can penetrate deeper into the microirregularities of the dentin, enhancing mechanical interlocking and increasing their displacement resistance. These inherent physicochemical properties may explain the higher bond strength, the better adaptation of the adhesive interface, and the presence of denser areas of tags observed in the AH/GP group in comparison with the methacrylate-based sealers.

Hybrid Root SEAL has a 4-META monomer in its composition, a hydrophilic radical that bonds to dentin, and a hydrophobic radical that bonds to the solid filling material. The 2 carboxylic groups attached to the aromatic group produce acidification and demineralization of the dentin surface in order to promote its adhesion. The lower bond strength of the HR/GP group com-pared with the AH/GP group may be justified by the incomplete polymerization of the sealer inside the canal, whereas its slow self-curing mechanism, creating a stress relief via prolonged plastic flow during setting, may explain its better results compared with the EP/GP and EP/RS groups.

Epiphany SE self-adhesive sealer results from the replacement of the monomer urethane dimethacrylate (UDMA), which provides relative viscosity to the sealer, by the highly hydrophilic monomer hydroxyethylmethacrylate (HEMA) and the incorporation of an acidic primer. Although compounds containing HEMA produced adequate collagen wetting and interpenetration, increasing the penetration of the material into the dental substrate, this was not observed in this study. The lowest bond strength observed in the EP/GP and EP/RS groups may result from its incomplete polymerization promoted by the presence of oxygen inside the dentinal tubules and the incomplete photoactivation caused by the decrease in light exposure in the deepest regions of the root canal. Besides, its fast self-curing setting time associated with curing stress generated in the adverse geometric configuration of the root canal may be so intense that the resin may detach from the dentin walls, creating interfacial gaps, which is in accordance with previous reports.

Although recently launched sealers have been proposed as innovative filling materials, the ideal root canal sealer has yet to be found. To date, supported by the plethora of ex vivo studies, bondable methacrylate resin–based sealers do not seem to be better alternatives for root canal obturation than their nonbonding counterparts. The use of EDTA-conditioning combined with a final irrigation with 70% isopropyl alcohol seemed to improve adhesion of methacrylate resin–based sealers to the dentin more easily than the conventional paper point drying protocol. Further research is warranted to determine what would be the better strategy for improving the adhesiveness of the so-called self-adhesive sealers to the root canal dentin.

Conclusions

Overall, removal of the smear layer followed by a drying protocol using 70% isopropyl alcohol before canal obturation improved bond strength and penetration of the sealers into dentinal tubules of the root.

Authors: Kleber Campioni Dias, Carlos Jose Soares, Liviu Steier, Marco Aurelio Versiani, Fuad Jacob Abi Rached-Junior, Jesus Djalma Pecora, Yara Terezinha Correa Silva-Sousa and Manoel Damiao de Sousa-Neto

References:

1. Ørstavik D. Materials used for root canal obturation: technical, biological and clinical testing. Endod Topics 2005;12:25–38.
2. Nagas E, Uyanik MO, Eymirli A, et al. Dentin moisture conditions affect the adhesion of root canal sealers. J Endod 2012;38:240–4.
3. Carneiro SM, Sousa-Neto MD, Rached FA Jr, et al. Push-out strength of root fillings with or without thermomechanical compaction. Int Endod J 2013;45:821–8.
4. Vilanova WV, Carvalho-Júnior JR, Alfredo E, et al. Effect of intracanal irrigants on the bond strength of epoxy resin-based and methacrylate resin-based sealers to root canal walls. Int Endod J 2011;45:42–8.
5. Teixeira CS, Alfredo E, Thome LH, et al. Adhesion of an endodontic sealer to dentin and gutta-percha: shear and push-out bond strength measurements and SEM analysis. J Appl Oral Sci 2009;17:129–35.
6. Borges RP, Sousa-Neto MD, Versiani MA, et al. Changes in the surface of four calcium silicate-containing endodontic materials and an epoxy resin-based sealer after a solubility test. Int Endod J 2012;45:419–28.
7. Resende LM, Rached-Júnior FJ, Versiani MA, et al. A comparative study of physicochemical properties of AH Plus, Epiphany, and Epiphany SE root canal sealers. Int Endod J 2009;42:785–93.
8. Ørstavik D, Nordahl I, Tibballs JE. Dimensional change following setting of root canal sealer materials. Dent Mater 2001;17:512–9.
9. Kim YK, Grandini S, Ames JM, et al. Critical review on methacrylate resin-based root canal sealers. J Endod 2010;36:383–99.
10. Prado M, Simão RA, Gomes BP. Effect of different irrigation protocols on resin sealer bond strength to dentin. J Endod 2013;39:689–92.
11. Engel GT, Goodell GG, McClanahan SB. Sealer penetration and apical microleakage in smear-free dentin after a final rinse with either 70% isopropyl alcohol or Peridex. J Endod 2005;31:620–3.
12. Zmener O, Pameijer CH, Serrano SA, et al. Significance of moist root canal dentin with the use of methacrylate-based endodontic sealers: an in vitro coronal dye leakage study. J Endod 2008;34:76–9.
13. Stevens RW, Strother JM, McClanahan SB. Leakage and sealer penetration in smear-free dentin after a final rinse with 95% ethanol. J Endod 2006;32:785–8.
14. Wilcox LR, Wiemann AH. Effect of a final alcohol rinse on sealer coverage of obturated root canals. J Endod 1995;21:256–8.
15. Cecchin D, de Almeida JF, Gomes BP, et al. Influence of chlorhexidine and ethanol on the bond strength and durability of the adhesion of the fiber posts to root dentin using a total etching adhesive system. J Endod 2011;37:1310–5.
16. Pane ES, Palamara JE, Messer HH. Critical evaluation of the push-out test for root canal filling materials. J Endod 2013;39:669–73.
17. Goracci C, Grandini S, Bossu M, et al. Laboratory assessment of the retentive potential of adhesive posts: a review. J Dent 2007;35:827–35.
18. Tay FR, Loushine RJ, Lambrechts P, et al. Geometric factors affecting dentin bonding in root canals: a theoretical modeling approach. J Endod 2005;31:584–9.
19. Nunes VH, Silva RG, Alfredo E, et al. Adhesion of Epiphany and AH Plus sealers to human root dentin treated with different solutions. Braz Dent J 2008;19:46–50.
20. Haragushiku GA, Sousa-Neto MD, Silva-Sousa YT, et al. Adhesion of endodontic sealers to human root dentine submitted to different surface treatments. Photomed Laser Surg 2010;28:405–10.
21. Sousa-Neto MD, Silva Coelho FI, Marchesan MA, et al. Ex vivo study of the adhesion of an epoxy-based sealer to human dentine submitted to irradiation with Er : YAG and Nd : YAG lasers. Int Endod J 2005;38:866–70.
22. Chang JC, Hurst TL, Hart DA, Estey AW. 4-META use in dentistry: a literature review. J Prosthet Dent 2002;87:216–24.
23. Van Landuyt KL, Snauwaert J, De Munck J, et al. Systematic review of the chemical composition of contemporary dental adhesives. Biomaterials 2007;28:3757–85.
24. Lawson MS, Loushine B, Mai S, et al. Resistance of a 4-META-containing, methacrylate-based sealer to dislocation in root canals. J Endod 2008;34:833–7.
25. Finger WJ, Lee KS, Podszun W. Monomers with low oxygen inhibition as enamel/ dentin adhesives. Dent Mater 1996;12:256–61.
26. Rached-J´unior FJ, Souza-Gabriel AE, Alfredo E, et al. Bond strength of Epiphany sealer prepared with resinous solvent. J Endod 2009;35:251–5.
27. Costa JA, Rached-J´unior FA, Souza-Gabriel AE, et al. Push-out strength of methacrylate resin-based sealers to root canal walls. Int Endod J 2010;43:698–706.
28. Babb BR, Loushine RJ, Bryan TE, et al. Bonding of self-adhesive (self-etching) root canal sealers to radicular dentin. J Endod 2009;35:578–82.
29. Onay EO, Ungor M, Ari H, et al. Push-out bond strength and SEM evaluation of new polymeric root canal fillings. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;107:879–85.