Photomedicine and Laser Surgery Volume 32, Number 9, 2014 ª Mary Ann Liebert, Inc. Pp. 495–499 DOI: 10.1089/pho.2014.3721
Bond Strength of Restorative Material to Dentin Submitted to Bleaching and Er:YAG Laser Post-Treatment Fabiana Almeida Curylofo, DDS, Danielle Cristine Furtado Messias, DDS, PhD, Yara T. Correa Silva-Sousa, DDS, PHD, and Aline Evangelista Souza-Gabriel, DDS, PhD
Abstract
Objective: The purpose of this study was to assess the bond strength of a restorative material to bleached dentin, pretreated with Er:YAG laser. Background data: Laser irradiation for dental surface treatment may increase the bond strength of restorative material to tooth surface. There are no reports of using Er:YAG laser on dentin bleached with 35% hydrogen peroxide. Methods: Forty maxillary canines were sectioned, resulting in 80 fragments (5 · 5 mm) of intracoronary dentin that were divided into eight groups (n = 10) according to the dental bleaching (present or not), surface conditioning (with or without laser) and the post-bleaching time to perform restoration (immediately or 7 days). The bleached specimens received two applications of 38% hydrogen peroxide. Er:YAG laser was applied for 20 sec with 400 mJ/15 Hz. Restorative procedure was performed using phosphoric acid, an adhesive system, and resin using a split Teflon matrix. The specimens were submitted to shear bond strength test and the data (MPa) were analyzed by ANOVA and Tukey’s test (a = 0.05). Results: There were significant differences among the three factors ( p < 0.05). The highest values were obtained for unbleached specimens compared with bleached, for those treated with laser compared with those only conditioned with acid, and for those restored after 7 days compared with those having immediate restoration. In the interaction of factors, the bleached specimens treated with laser and restored immediately were statistically similar ( p > 0.05) to those bleached and restored after 7 days. Conclusions: Er:YAG laser can restore the bond strength of the dentin/restorative material interface even if the restoration is performed immediately after bleaching.
Introduction
D
ental bleaching is a common procedure in general dentistry, most especially in the field of cosmetic dentistry.1 Bleaching is achieved from an oxi-reduction reaction in which the reactive oxygen and some free radicals (released from the degradation of the bleaching agent) attack the longchained, dark-colored chromophore molecules present in the dental tissues, and thus split them into smaller, less colored and more diffusible molecules, producing the whitening effect.2 Several agents are proposed to perform bleaching in vital and non-vital teeth, such as carbamide peroxide, hydrogen peroxide (H2O2), and sodium perborate.3,4 Although (H2O2) is effective in lightening the tooth color, its use has been associated with undesirable complications in the dental hard tissues, including the reduction of adhesive capacity.5,6 This reduction is related to the exposure time to bleaching agent, product concentration, and postbleaching period to perform the bonding procedure.7
The residues of bleaching gel present inside the dentin tubules and collagen matrix inhibit the light activation of resin materials, with subsequent reduction in bond strength of composite restorations.8,9 This negative effect should be avoided, because after bleaching treatment, restorations needs to be changed in most cases, because their color does not match with the color attained by the natural teeth.4 In order to have the total liberation of byproducts of peroxides and, thus achieve a satisfactory final restoration, the post-bleaching time should range from 10 to 21 days.10,11 Clinically, this interval may be too long for patients who have aesthetic restorations in several teeth and seek immediate treatment.12 In light of this impasse, it is speculated that the use of laser irradiation can accelerate the release of free residual radicals and modify the dentin, creating a favorable substrate to receive the restorative procedures,13 and that different wavelengths can be analyzed for this purpose.
School of Dentistry, University of Ribeira˜o Preto, Ribeira˜o Preto, SP, Brazil.
495
496
The Er:YAG laser emits a length of 2.94 lm, which coincides with the peak absorption of water in the mineral tissues.14,15 In this system, the energy generated during irradiation is absorbed by the water of the superficial portion of the tissue, which is heated to its vaporization temperature, increasing the internal pressure of the tissues and causing microexplosions that lead to the ejection of the substrate in the form of microscopic particles.16 This laser modifies the permeability and acid resistance of dentin,17 and surface appearance becomes more irregular, with no smear layer and exposed tubules.16–18 Although the literature has several research studies of bleaching in dental substrate, there are few studies assessing the interaction among bleaching agents (particularly at high concentrations) with the restorative materials and Er:YAG laser.19 Some recent investigations verified that Er:YAG laser irradiation could improve adhesion to bleached teeth;19,20 however, all these studies used enamel surfaces. The purpose of this study was to evaluate in vitro the effect of Er:YAG laser on shear bond strength of the restorative system to dentin submitted to bleaching with 38% (H2O2). Materials and Methods
The study protocol was reviewed and approved by the local Ethics Committee (064/2010). The design of the present study is presented in Fig. 1. Superior human canines stored in steam of 0.1% thymol solution at 4C were washed in running water for 24 h to eliminate thymol residues. Teeth were examined under a 20 · magnifier (Leica Microsystems, Wetzlar, Germany), those with structural defects were discarded, and 40 teeth were selected. Roots were sectioned 1 mm below the cementoenamel junction. Crowns were fixed in wax bisected longitudinally in a mesiodistal direction using a double-faced diamond disk (#7015, KG Sorensen, Barueri, SP, Brazil) mounted in a low-speed handpiece (Dabi Atlante, Ribeira˜o Preto, SP,
CURYLOFO ET AL.
Brazil). Each crown hemisection was sectioned in the incisal, mesial, distal, and cervical faces to obtain square samples with 5 mm width and 5 mm high (25 mm2), providing 80 fragments (40 for the vestibular face and 40 for the lingual face). Fragments of crown were embedded in autopolymerized acrylic resin ( Jet Classic, Sa˜o Paulo, SP, Brazil) surrounded by a polyvinyl chloride (PVC) cylinder (1.5 cm diameter and 1.5 cm high) with the intracoronary dentin face up. After resin polymerization, the PVC cylinder was removed, and the dentin surface was grounded with 400 grit silicon carbide (SiC) paper (Norton, Lorena, SP, Brazil) under water cooling to flat the surface. Complementary grinding was accomplished with 1200 grit SiC paper for 1 min to produce a standardized smear layer. The specimens were washed with 10 mL of 1% sodium hypochlorite to simulate the irrigation during biomechanical preparation of the radicular canals. The specimens were randomly assigned to two groups of equal size (n = 40), according to the treatment received in the coronary surface: no bleaching procedure (control) and bleaching with high-concentrated agent. The bleaching agent used was 38% (H2O2) (Opalescence X-tra Boost; Ultradent Products, Inc., South Jordan, UT), available in gel form, which is mixed with red colorant at the moment of use. A session of bleaching was performed, and the protocol consisted of a bleaching gel application on the fragment surface (intracoronary dentin) for 20 min, according to the manufacturer’s instructions. This procedure was repeated twice during the session, waiting 5 min between each application. The teeth were then divided into two subgroups according to surface treatment after bleaching: I) without irradiation and II) irradiation with Er:YAG laser (400 mJ, 15 Hz, noncontact mode). The Er:YAG laser with a spot size of 1.0 mm (Opus 20; Opus Dent, Tel Aviv, Israel) was applied for 20 sec, and the irradiation distance was 10 mm, standardized by acrylic device suitable for this purpose. The safety standards for use
FIG 1. Distribution of the experimental groups (n = 10).
Er:YAG LASER IRRADIATION ON BLEACHED DENTIN
497
Table 1. Shear Bond Strength Means (MPa) and Standard Deviations of the Restorative Material to Dentin in the Different Experimental Groups Unbleached dentin
Without laser Er:YAG laser
Bleached dentin with 38% HP
Restored immediately
Restored after 7 days
Restored immediately
Restored after 7 days
(5.66 – 1.77)a (7.48 – 1.31)
(4.55 – 1.64) (6.57 – 2.52)
(1.90 – 0.80) (7.30 – 1.58)
(5.94 – 1.71) (6.54 – 1.88)
a Control group. HP, hydrogen peroxide.
of the laser equipment were strictly followed. Absorbent paper was placed over the dentin surface and the specimens were sealed with temporary cement (Coltosol; Vigodent, Rio de Janeiro, RJ, Brazil) until the moment of the restorative procedure, aiming to simulate clinical situation. After surface treatment, the specimens were subdivided again, according to the post-bleaching time for the restorative procedure (n = 10): restored immediately or after 7 days (specimens were kept in relative humidity at 37C until the moment of restorative procedure). The teeth were conditioned with 37% phosphoric acid (3M ESPE St. Paul, MN) for 15 sec, rinsed for 30 sec under running tap water, and dried with absorbent paper. The conventional adhesive system (Adper Single Bond; 3M ESPE, St. Paul, MN) was used according to the manufacturer’s instructions. Restorative procedure was performed using a split Teflon matrix (3 · 3 · 4 mm) stabilized with silicone plus (Profile Denso, Vigodent, Bonsucesso, RJ, Brazil) in order to obtain composite resin cylinders with the abovementioned measures. The specimens received an application of the Z250 resin (3M ESPE, St. Paul, MN), which consisted of three increments with the aid of a spatula n. 1 (Duflex, Rio de Janeiro, RJ, Brasil), and polymerized for 40 sec in each increment using a light-curing unit (Dabi Atlante, Ribeira˜o Preto, SP, Brazil). The light-curing device was kept at distance of 10 mm from the resin surface. Sequentially, the matrix was carefully opened and the specimens were kept in relative humidity at 37C. After 24 h, the specimens were submitted to shear bond test in a universal testing machine (Instron 4444; Instron Corporation, Canton, MA) at a 0.5 mm/min cross-head speed and a 2 kN load cell until the restoration displacement. Shear bond strength values were registered in kN and transformed in MPa. Fracture types at the resin/dentin interface were analyzed under a stereoscopic microscope at 40 · magnification (Leica Microsystems, Wetzlar, Germany), and displayed in percent. Failure was considered adhesive if it occurred at
the dentin/adhesive interface, cohesive if it occurred in the material or the substrate, and mixed if it involved both the interface and the material. Statistical analysis
Averages and standard deviations of the mechanical test (bond strength) were calculated, and data were analyzed using ANOVA. Multiple comparisons were performed using Tukey–Kramer test at a 0.05 significance level (Instat Program; GraphPad Software, San Diego, CA). Results
Table 1 shows the bond strength of the composite material to intracoronary dentin in the different experimental groups. There were significant differences among the three tested factors ( p < 0.05). The highest mean values were obtained for unbleached specimens compared with bleached ( p < 0.05). The groups treated with Er:YAG laser after bleaching had superior bond strength to those treated by only being conditioned with phosphoric acid (without laser) ( p < 0.05). Regarding post-bleaching time, it was verified that specimens restored with composite material after 7 days had higher shear bond strength to dentin that those that received the restorative material immediately after bleaching and surface treatment ( p < 0.05). In the interaction of factors, bleached specimens treated with Er:YAG laser and restored immediately were statistically similar ( p > 0.05) to those bleached without Er:YAG laser and restored after 7 days. The analysis of failure type after the shear bond strength test demonstrated the predominance of mixed failure in the control group (unbleached). The specimens that received the bleaching treatment, regardless of the post-bleaching time interval, exhibited a predominance of adhesive failures. Cohesive failures were verified only in unbleached specimens treated with or without Er:YAG laser (Table 2).
Table 2. Failure Types (%) After the Shear Bond Strength Test Unbleached dentin Restored immediately
Without laser Er:YAG laser
Bleached dentin with 38% HP
Restored after 7 days
Restored immediately
Restored after 7 days
A
C
M
A
C
M
A
C
M
A
C
M
30 20
20 20
50 60
40 40
10 0
50 60
60 70
0 0
40 30
70 60
0 0
30 40
HP, hydrogen peroxide; A, adhesive failure; C, cohesive failure; M, mixed failure.
498 Discussion
Darkening of anterior teeth presents an aesthetic problem that often requires corrective action, and can often be successfully treated through a whitening procedure.21 H2O2 is the most common agent used for whitening procedure at concentrations varying from 30 to 38%;22,23 however, its use has been associated with undesirable complications in hard tissues, including the reduction of adhesive capacity.5,6 The results of this study showed that there were higher bond strength values obtained in unbleached dentin, in other words, that dental bleaching negatively affected the bond strength of restorative material. This can be explained by the presence of bleaching agents (OH-radical) on dentin surface, which inhibits resin polymerization.2,24 Being a strong oxidant agent and having a high penetrating power, the OHradical acts in the peritubular and intertubular dentin through degradation of the organic portion, breaking the polypeptide chain as a result of destruction of amino acids of the matrix.24–26 Therefore, the bond strength may have been reduced because of the presence of bleaching agents on the dentin surface.2,27 To minimize the effect of residual oxygen in dentin tubules, there was an interval of 7 days before the resin restoration, as was recommended in the literature,24 and it was found that the highest bond strength values were obtained for the specimens restored after 7 days when compared with those restored immediately. After the bleaching procedure in our study, it was believed that oxygen was not fully released from the dentin, because the surface was sealed with temporary cement, just as occurs in clinical situations. The oxygen present inside the tubules inhibits the polymerization of the adhesive system and composite resin, and consequently reduces the strength of the dentin/restorative material interface.9,28–30 Recently, it was demonstrated that bleaching agents increased dentinal tubule diameter and promoted alterations in the mineral content of dentin.31,32 Regarding post-bleaching time intervals, Spyrides et al.33 suggest that the dentin treated with different bleaching agents is only restored 14 days after bleaching. This period would be sufficient for total removal of possible byproducts confined in the substrate that inhibit the adhesive process. A post-bleaching period of 1–3 weeks after tooth whitening is also recommended, to re-establish the normal bond strength.13,20 Seeking to reduce this waiting time, this study tested the Er:YAG laser as a conditioning dentin procedure after tooth bleaching. Overall, the present study supports that the bond strength values of composite resin to dentin bleached and treated with Er:YAG laser were similar to those observed for unconditioned bleached dentin. Therefore, the laser conditioning of the dentin surface after bleaching increased the adhesion of restorative material to dentin. It is speculated that the energy generated during irradiation is absorbed by water in the superficial portion of the tissue, which is heated to its vaporization temperature, increasing the internal pressure of the tissues and causing microexplosions that lead to the ejection of the substrate in the form of microscopic particles;16 moreover, the laser Er:YAG acts on the dentin surface by creating a smear layer free area with open tubules with certain density energy
CURYLOFO ET AL.
output. As dentin substrate is filled by residual bleaching agents after dental bleaching, and these agents are free radicals that are very unstable, the heat generated by Er:YAG laser may accelerate the release of free radicals and modify dentin into a substrate better able to receive the adhesion procedures.13 The surface area for adhesion on irradiated dentin surfaces is enlarged based on the scaly and flaky surface that appears following Er:YAG irradiation.34 Samad-Zadeh et al.35 showed that the adhesion between composite resin and irradiated dentin surfaces is greater than that for unirradiated surfaces. In this study, prevalence of the adhesive failure mode in all the bleached specimens, regardless of the protocol or post-bleaching times, was verified. This fact can be explained by the presence of residual oxygen on the dentin surface, which may impede the penetration of resin into dentin tubules, creating an adhesive interface more susceptible to fracture. In summary, the results of this study demonstrate that H2O2 in high concentrations can reduce the adhesion between dentin and restorative materials. However, the Er: YAG laser can remove the superficial layer of dentin that was changed by H2O2, and, therefore, the restoration with composite resin can be performed in the same clinic session. Direct comparisons of the findings of this study to the literature were not performed because of the lack of studies that evaluate the bleached dentin after the action of bleaching agents at high concentration and conditioning by means of laser, and subsequent restoration. Although laboratory studies do not reproduce intraoral conditions, they offer a controlled environment for preparing and testing the specimens, thus allowing a comparable evaluation of the variables under investigation. This study gives support to better understanding of the interaction of bleaching agents with dental surface. The results of this study encourage further clinical studies aiming to increase the bond strength of the dentin submitted to high concentration bleaching agents. Conclusions
The Er:YAG laser can restore the bond strength of the material to dentin, even if the restoration is performed immediately after bleaching. Author Disclosure Statement
No competing financial interests exist. References
1. Sulieman, M., Addy, M., MacDonald, E., and Rees, J.S. (2004). A safety study in vitro for the effects of an in-office bleaching system on the integrity of enamel. J. Dent. 32, 581–590. 2. Plotino, G., Buono, L., Grande, N.M., Pameijer, C.H., and Somma, F. (2008). Nonvital tooth bleaching a review of the literature and clinical procedures. J. Endod. 34, 394–407. 3. Dahl, J.E., and Pallesen, U. (2003). Tooth bleaching: a critical review of the biological aspects. Crit. Rev. Oral Biol. Med. 14, 292–304. 4. Joiner, A. (2006). The bleacing of teeth: a review of literature. J. Dent. 34, 412–419.
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5. Chng, H.K., Palamara, E.A., and Messer, H.H. (2002). Effect of hidrogen peroxide and sodium perborate on biomechanical properties of human dentin. J. Endod. 28, 62–67. 6. Tam, L.A., Kuo, V.Y., and Noroozi, A. (2007). Effect of prolonged direct and indirect peroxide bleaching on fracture toughness of human dentin. J. Esthet. Restor. Dent. 19, 100–110. 7. Spalding, M., Taveira, L.A., and de Assis, G.F. (2003). Scanning electron microscopy study of dental enamel surface exposed to 35% hydrogen peroxide: alone, with saliva, and with 10% carbamide peroxide. J. Esthet. Restor. Dent. 15, 154–164. 8. Nikaido, T., Takano, Y., Sasafuchi, Y., Burrow, M.F., and Tagami, J. (1999). Bond strengths to endodontically– treated teeth. Am. J. Dent. 12, 177–180. 9. Shinohara, M.S., Peris, A.R., Pimenta, L.A.F., and Ambrosano, G.M.B. (2005). Shear bond strength evaluation of composite resin on enamel and dentin after nonvital bleaching. J. Esthet. Restor. Dent. 17, 22–29. 10. Basting, R.T., Rodrigues, J.A., Serra, M.C., and Pimenta, L.A. (2004). Shear bond strength of enamel treated with seven carbamide peroxide bleaching agents. J. Esthet. Restor. Dent. 16, 250–260. 11. Cavalli, V., Reis, A.F., Gianinni, M., and Ambrosano, G. (2001). The effect of elapse time following bleaching on enamel bond strength on resin composite. Oper. Dent. 26, 597–602. 12. Lai, S.C.N., Tay, F.R., Cheung, G.S., et al. (2002). Reversal of compromised bonding in bleached enamel. J. Dent. Res. 81, 477–481. 13. Leonetti, E.D., Rodrigues, J.A., Reis, A.F., Navarro, R.S., Aranha, A.C., and Cassoni, A. (2012). Microtensile bond strength of resin composite to dentin treated with Er:YAG laser of bleached teeth. Lasers Med. Sci. 27, 31–38. 14. Martinez–Insua, A., Da Silva Dominguez, L., Rivera, F.G., and Santana–Penin, U.A. (2000). Differences in bonding to acidetched or Er:YAG-laser-treated enamel and dentin surfaces. J. Prosthet. Dent. 84, 280–288. 15. Visuri, S.R., Gilbert, J.L., Wright, D.D., Wigdor, H.A., and Walsh, J.T. (1996). Shear strength of composite bonded to Er:YAG laser-prepared dentin. J. Dent. Res. 75, 599–605. 16. Ramos, R.P., Palma Dibb, R.G., Chimello, D.T., Chinelatti, M.A., and Pe´cora, J.D. (2002). Effect of Er:YAG laser on bond strength to dentin of a self-etching primer and two single-bottle adhesive systems. Lasers Surg. Med. 31, 164–170. 17. Schein, M.T., Bocagel, J.S., Nogueira, G.E.C., and Schein, P.A.L. (2003). SEM evaluation of the interaction pattern between dentin and resin after cavity preparation using ER:YAG laser. J. Dent. 31, 127–135. 18. Ceballos, L., Osorio, R., Toledano, M., Garcia–Godoy, F., Flaitz, C., and Hichs, J. (2001). Er:YAG laser pretreatment effect on in vitro secondary caries formation around composite restorations. Am. J. Dent. 14, 46–49. 19. Rocha Gomes Torres, C., Caneppele, T.M., Del Moral de Lazari, R., Ribeiro, C.F., and Borges, A.B. (2012). Effect of dental surface treatment with Nd:YAG and Er:YAG lasers on bond strength of resin composite to recently bleached enamel. Lasers Med. Sci. 27, 755–760. 20. Leonetti Edos, S., Rodrigues, J.A., Reis, A.F., Navarro, R.S., Aranha, A.C., and Cassoni, A. (2011). Effects of Er:YAG laser irradiation on the microtensile bond strength to bleached enamel. Photomed. Laser. Surg. 29, 551–558.
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21. Elfallah, H.M., and Swain, M.V. (2013). A review of the effect of vital teeth bleaching on the mechanical properties of toothenamel. N. Z. Dent. J. 109, 87–96. 22. Attin, T., Manolakis, A., Buchalla, W., and Hannig, C. (2003). Influence of tea on intrinsic colour of previously bleached enamel. J. Oral Rehabil. 30, 488–494. 23. Ernst, C.P., Marroquı´n, B.B., and Willershausen–Zo¨nnchen, B. (1996). Effects of hydrogen peroxide-containing bleaching agents on the morphology of human enamel. Quintessence Int. 27, 53–56. 24. Kawamoto, K., and Tsujimoto, Y. (2004). Effects of the hydroxyl radical and hydrogen peroxide on tooth bleaching. J. Endod. 30, 45–50. 25. Barkhordar, R.A., Kempler, D., and Plesh, O. (1997). Effect of nonvital tooth bleaching on microleakage of resin composite restorations. Quintessence Int. 28, 341–344. 26. Rodrigues, L.M., Vansam, L.P., Pe´cora, J.D., and Marchesan, M.A. (2009). Permeability of different groups of maxillary teeth after 38% hydrogen peroxide internal bleaching. Braz. Dent. J. 20, 303–306. 27. Elkhatib, H., Nakajima, M., Hiraishi, N., Kitasako, Y., Tagami, J., and Nomura, S. (2003). Surface pH and bond strength of a self-etching primer/adhesive system to intracoronal dentin after application of hydrogen peroxide bleach with sodium perborate. Oper. Dent. 28, 591–597. 28. Shinohara, M.S., Peris, A.R., Rodrigues, J.A., Pimenta, L.A., and Ambrosano, G.M. (2004). The effect of nonvital bleaching on the shear bond strength of composite resin using three adhesive systems. J. Adhes. Dent. 6, 205–209. 29. Ferreira, E.A., Souza–Gabriel, A.E., Silva–Sousa, Y.T., Sousa– Neto, M.D., and Silva, R.G. (2011). Shear bond strength and ultrastructural interface analysis of different adhesive systems to bleached dentin. Microsc. Res. Tech. 74, 244–250. 30. Haywood, V.B. (1992). History, safety, and effectiveness of current bleaching techniques and applications of the nightguard vital bleaching technique. Quintessence Int. 23, 471–488. 31. Ubaldini, A.L., Baesso, M.L., Medina, Neto, et al. (2013). Hydrogen peroxide diffusion dynamics in dental tissues. J. Dent. Res. 92, 661–665. 32. Maleknejad, F., Ameri, H., and Kianfar, I. (2012). Effect of intracoronal bleaching agents on ultrastructure and mineral content of dentin. J. Conserv. Dent. 15, 174–177. 33. Spyrides, G.M., Perdiga˜o, J., Pagani, C., Araujo, M.A., and Spyrides, S.M. (2000). Effect of whitening agents on dentin bonding. J. Esthet. Dent. 12, 264–270. 34. Stiesch–Scholz, M., and Hanning, M. (2000). In vitro study of enamel and dentin marginal integrity of composite and compomer restorations placed in primary teeth after diamond or Er:YAG laser cavity preparation. J. Adhes. Dent. 2, 213–222. 35. Samad–Zadeh, A., Harsono, M., Belikov, A., et al. (2011). The influence of laser-textured dentinal surface on bond strength. Dent. Mater. 27, 1038–1044.
Address correspondence to: Aline E. Souza–Gabriel University of Sa˜o Paulo Ribeira˜o Preto School of Dentistry Av. do Cafe´ s/n Monte Alegre, Ribeira˜o Preto-SP CEP 14040-940 E-mail: [email protected]
Bond Strength of Restorative Material to Dentin Submitted to Bleaching and Er:YAG Laser Post-Treatment Fabiana Almeida Curylofo, DDS, Danielle Cristine Furtado Messias, DDS, PhD, Yara T. Correa Silva-Sousa, DDS, PHD, and Aline Evangelista Souza-Gabriel, DDS, PhD
Abstract
Objective: The purpose of this study was to assess the bond strength of a restorative material to bleached dentin, pretreated with Er:YAG laser. Background data: Laser irradiation for dental surface treatment may increase the bond strength of restorative material to tooth surface. There are no reports of using Er:YAG laser on dentin bleached with 35% hydrogen peroxide. Methods: Forty maxillary canines were sectioned, resulting in 80 fragments (5 · 5 mm) of intracoronary dentin that were divided into eight groups (n = 10) according to the dental bleaching (present or not), surface conditioning (with or without laser) and the post-bleaching time to perform restoration (immediately or 7 days). The bleached specimens received two applications of 38% hydrogen peroxide. Er:YAG laser was applied for 20 sec with 400 mJ/15 Hz. Restorative procedure was performed using phosphoric acid, an adhesive system, and resin using a split Teflon matrix. The specimens were submitted to shear bond strength test and the data (MPa) were analyzed by ANOVA and Tukey’s test (a = 0.05). Results: There were significant differences among the three factors ( p < 0.05). The highest values were obtained for unbleached specimens compared with bleached, for those treated with laser compared with those only conditioned with acid, and for those restored after 7 days compared with those having immediate restoration. In the interaction of factors, the bleached specimens treated with laser and restored immediately were statistically similar ( p > 0.05) to those bleached and restored after 7 days. Conclusions: Er:YAG laser can restore the bond strength of the dentin/restorative material interface even if the restoration is performed immediately after bleaching.
Introduction
D
ental bleaching is a common procedure in general dentistry, most especially in the field of cosmetic dentistry.1 Bleaching is achieved from an oxi-reduction reaction in which the reactive oxygen and some free radicals (released from the degradation of the bleaching agent) attack the longchained, dark-colored chromophore molecules present in the dental tissues, and thus split them into smaller, less colored and more diffusible molecules, producing the whitening effect.2 Several agents are proposed to perform bleaching in vital and non-vital teeth, such as carbamide peroxide, hydrogen peroxide (H2O2), and sodium perborate.3,4 Although (H2O2) is effective in lightening the tooth color, its use has been associated with undesirable complications in the dental hard tissues, including the reduction of adhesive capacity.5,6 This reduction is related to the exposure time to bleaching agent, product concentration, and postbleaching period to perform the bonding procedure.7
The residues of bleaching gel present inside the dentin tubules and collagen matrix inhibit the light activation of resin materials, with subsequent reduction in bond strength of composite restorations.8,9 This negative effect should be avoided, because after bleaching treatment, restorations needs to be changed in most cases, because their color does not match with the color attained by the natural teeth.4 In order to have the total liberation of byproducts of peroxides and, thus achieve a satisfactory final restoration, the post-bleaching time should range from 10 to 21 days.10,11 Clinically, this interval may be too long for patients who have aesthetic restorations in several teeth and seek immediate treatment.12 In light of this impasse, it is speculated that the use of laser irradiation can accelerate the release of free residual radicals and modify the dentin, creating a favorable substrate to receive the restorative procedures,13 and that different wavelengths can be analyzed for this purpose.
School of Dentistry, University of Ribeira˜o Preto, Ribeira˜o Preto, SP, Brazil.
495
496
The Er:YAG laser emits a length of 2.94 lm, which coincides with the peak absorption of water in the mineral tissues.14,15 In this system, the energy generated during irradiation is absorbed by the water of the superficial portion of the tissue, which is heated to its vaporization temperature, increasing the internal pressure of the tissues and causing microexplosions that lead to the ejection of the substrate in the form of microscopic particles.16 This laser modifies the permeability and acid resistance of dentin,17 and surface appearance becomes more irregular, with no smear layer and exposed tubules.16–18 Although the literature has several research studies of bleaching in dental substrate, there are few studies assessing the interaction among bleaching agents (particularly at high concentrations) with the restorative materials and Er:YAG laser.19 Some recent investigations verified that Er:YAG laser irradiation could improve adhesion to bleached teeth;19,20 however, all these studies used enamel surfaces. The purpose of this study was to evaluate in vitro the effect of Er:YAG laser on shear bond strength of the restorative system to dentin submitted to bleaching with 38% (H2O2). Materials and Methods
The study protocol was reviewed and approved by the local Ethics Committee (064/2010). The design of the present study is presented in Fig. 1. Superior human canines stored in steam of 0.1% thymol solution at 4C were washed in running water for 24 h to eliminate thymol residues. Teeth were examined under a 20 · magnifier (Leica Microsystems, Wetzlar, Germany), those with structural defects were discarded, and 40 teeth were selected. Roots were sectioned 1 mm below the cementoenamel junction. Crowns were fixed in wax bisected longitudinally in a mesiodistal direction using a double-faced diamond disk (#7015, KG Sorensen, Barueri, SP, Brazil) mounted in a low-speed handpiece (Dabi Atlante, Ribeira˜o Preto, SP,
CURYLOFO ET AL.
Brazil). Each crown hemisection was sectioned in the incisal, mesial, distal, and cervical faces to obtain square samples with 5 mm width and 5 mm high (25 mm2), providing 80 fragments (40 for the vestibular face and 40 for the lingual face). Fragments of crown were embedded in autopolymerized acrylic resin ( Jet Classic, Sa˜o Paulo, SP, Brazil) surrounded by a polyvinyl chloride (PVC) cylinder (1.5 cm diameter and 1.5 cm high) with the intracoronary dentin face up. After resin polymerization, the PVC cylinder was removed, and the dentin surface was grounded with 400 grit silicon carbide (SiC) paper (Norton, Lorena, SP, Brazil) under water cooling to flat the surface. Complementary grinding was accomplished with 1200 grit SiC paper for 1 min to produce a standardized smear layer. The specimens were washed with 10 mL of 1% sodium hypochlorite to simulate the irrigation during biomechanical preparation of the radicular canals. The specimens were randomly assigned to two groups of equal size (n = 40), according to the treatment received in the coronary surface: no bleaching procedure (control) and bleaching with high-concentrated agent. The bleaching agent used was 38% (H2O2) (Opalescence X-tra Boost; Ultradent Products, Inc., South Jordan, UT), available in gel form, which is mixed with red colorant at the moment of use. A session of bleaching was performed, and the protocol consisted of a bleaching gel application on the fragment surface (intracoronary dentin) for 20 min, according to the manufacturer’s instructions. This procedure was repeated twice during the session, waiting 5 min between each application. The teeth were then divided into two subgroups according to surface treatment after bleaching: I) without irradiation and II) irradiation with Er:YAG laser (400 mJ, 15 Hz, noncontact mode). The Er:YAG laser with a spot size of 1.0 mm (Opus 20; Opus Dent, Tel Aviv, Israel) was applied for 20 sec, and the irradiation distance was 10 mm, standardized by acrylic device suitable for this purpose. The safety standards for use
FIG 1. Distribution of the experimental groups (n = 10).
Er:YAG LASER IRRADIATION ON BLEACHED DENTIN
497
Table 1. Shear Bond Strength Means (MPa) and Standard Deviations of the Restorative Material to Dentin in the Different Experimental Groups Unbleached dentin
Without laser Er:YAG laser
Bleached dentin with 38% HP
Restored immediately
Restored after 7 days
Restored immediately
Restored after 7 days
(5.66 – 1.77)a (7.48 – 1.31)
(4.55 – 1.64) (6.57 – 2.52)
(1.90 – 0.80) (7.30 – 1.58)
(5.94 – 1.71) (6.54 – 1.88)
a Control group. HP, hydrogen peroxide.
of the laser equipment were strictly followed. Absorbent paper was placed over the dentin surface and the specimens were sealed with temporary cement (Coltosol; Vigodent, Rio de Janeiro, RJ, Brazil) until the moment of the restorative procedure, aiming to simulate clinical situation. After surface treatment, the specimens were subdivided again, according to the post-bleaching time for the restorative procedure (n = 10): restored immediately or after 7 days (specimens were kept in relative humidity at 37C until the moment of restorative procedure). The teeth were conditioned with 37% phosphoric acid (3M ESPE St. Paul, MN) for 15 sec, rinsed for 30 sec under running tap water, and dried with absorbent paper. The conventional adhesive system (Adper Single Bond; 3M ESPE, St. Paul, MN) was used according to the manufacturer’s instructions. Restorative procedure was performed using a split Teflon matrix (3 · 3 · 4 mm) stabilized with silicone plus (Profile Denso, Vigodent, Bonsucesso, RJ, Brazil) in order to obtain composite resin cylinders with the abovementioned measures. The specimens received an application of the Z250 resin (3M ESPE, St. Paul, MN), which consisted of three increments with the aid of a spatula n. 1 (Duflex, Rio de Janeiro, RJ, Brasil), and polymerized for 40 sec in each increment using a light-curing unit (Dabi Atlante, Ribeira˜o Preto, SP, Brazil). The light-curing device was kept at distance of 10 mm from the resin surface. Sequentially, the matrix was carefully opened and the specimens were kept in relative humidity at 37C. After 24 h, the specimens were submitted to shear bond test in a universal testing machine (Instron 4444; Instron Corporation, Canton, MA) at a 0.5 mm/min cross-head speed and a 2 kN load cell until the restoration displacement. Shear bond strength values were registered in kN and transformed in MPa. Fracture types at the resin/dentin interface were analyzed under a stereoscopic microscope at 40 · magnification (Leica Microsystems, Wetzlar, Germany), and displayed in percent. Failure was considered adhesive if it occurred at
the dentin/adhesive interface, cohesive if it occurred in the material or the substrate, and mixed if it involved both the interface and the material. Statistical analysis
Averages and standard deviations of the mechanical test (bond strength) were calculated, and data were analyzed using ANOVA. Multiple comparisons were performed using Tukey–Kramer test at a 0.05 significance level (Instat Program; GraphPad Software, San Diego, CA). Results
Table 1 shows the bond strength of the composite material to intracoronary dentin in the different experimental groups. There were significant differences among the three tested factors ( p < 0.05). The highest mean values were obtained for unbleached specimens compared with bleached ( p < 0.05). The groups treated with Er:YAG laser after bleaching had superior bond strength to those treated by only being conditioned with phosphoric acid (without laser) ( p < 0.05). Regarding post-bleaching time, it was verified that specimens restored with composite material after 7 days had higher shear bond strength to dentin that those that received the restorative material immediately after bleaching and surface treatment ( p < 0.05). In the interaction of factors, bleached specimens treated with Er:YAG laser and restored immediately were statistically similar ( p > 0.05) to those bleached without Er:YAG laser and restored after 7 days. The analysis of failure type after the shear bond strength test demonstrated the predominance of mixed failure in the control group (unbleached). The specimens that received the bleaching treatment, regardless of the post-bleaching time interval, exhibited a predominance of adhesive failures. Cohesive failures were verified only in unbleached specimens treated with or without Er:YAG laser (Table 2).
Table 2. Failure Types (%) After the Shear Bond Strength Test Unbleached dentin Restored immediately
Without laser Er:YAG laser
Bleached dentin with 38% HP
Restored after 7 days
Restored immediately
Restored after 7 days
A
C
M
A
C
M
A
C
M
A
C
M
30 20
20 20
50 60
40 40
10 0
50 60
60 70
0 0
40 30
70 60
0 0
30 40
HP, hydrogen peroxide; A, adhesive failure; C, cohesive failure; M, mixed failure.
498 Discussion
Darkening of anterior teeth presents an aesthetic problem that often requires corrective action, and can often be successfully treated through a whitening procedure.21 H2O2 is the most common agent used for whitening procedure at concentrations varying from 30 to 38%;22,23 however, its use has been associated with undesirable complications in hard tissues, including the reduction of adhesive capacity.5,6 The results of this study showed that there were higher bond strength values obtained in unbleached dentin, in other words, that dental bleaching negatively affected the bond strength of restorative material. This can be explained by the presence of bleaching agents (OH-radical) on dentin surface, which inhibits resin polymerization.2,24 Being a strong oxidant agent and having a high penetrating power, the OHradical acts in the peritubular and intertubular dentin through degradation of the organic portion, breaking the polypeptide chain as a result of destruction of amino acids of the matrix.24–26 Therefore, the bond strength may have been reduced because of the presence of bleaching agents on the dentin surface.2,27 To minimize the effect of residual oxygen in dentin tubules, there was an interval of 7 days before the resin restoration, as was recommended in the literature,24 and it was found that the highest bond strength values were obtained for the specimens restored after 7 days when compared with those restored immediately. After the bleaching procedure in our study, it was believed that oxygen was not fully released from the dentin, because the surface was sealed with temporary cement, just as occurs in clinical situations. The oxygen present inside the tubules inhibits the polymerization of the adhesive system and composite resin, and consequently reduces the strength of the dentin/restorative material interface.9,28–30 Recently, it was demonstrated that bleaching agents increased dentinal tubule diameter and promoted alterations in the mineral content of dentin.31,32 Regarding post-bleaching time intervals, Spyrides et al.33 suggest that the dentin treated with different bleaching agents is only restored 14 days after bleaching. This period would be sufficient for total removal of possible byproducts confined in the substrate that inhibit the adhesive process. A post-bleaching period of 1–3 weeks after tooth whitening is also recommended, to re-establish the normal bond strength.13,20 Seeking to reduce this waiting time, this study tested the Er:YAG laser as a conditioning dentin procedure after tooth bleaching. Overall, the present study supports that the bond strength values of composite resin to dentin bleached and treated with Er:YAG laser were similar to those observed for unconditioned bleached dentin. Therefore, the laser conditioning of the dentin surface after bleaching increased the adhesion of restorative material to dentin. It is speculated that the energy generated during irradiation is absorbed by water in the superficial portion of the tissue, which is heated to its vaporization temperature, increasing the internal pressure of the tissues and causing microexplosions that lead to the ejection of the substrate in the form of microscopic particles;16 moreover, the laser Er:YAG acts on the dentin surface by creating a smear layer free area with open tubules with certain density energy
CURYLOFO ET AL.
output. As dentin substrate is filled by residual bleaching agents after dental bleaching, and these agents are free radicals that are very unstable, the heat generated by Er:YAG laser may accelerate the release of free radicals and modify dentin into a substrate better able to receive the adhesion procedures.13 The surface area for adhesion on irradiated dentin surfaces is enlarged based on the scaly and flaky surface that appears following Er:YAG irradiation.34 Samad-Zadeh et al.35 showed that the adhesion between composite resin and irradiated dentin surfaces is greater than that for unirradiated surfaces. In this study, prevalence of the adhesive failure mode in all the bleached specimens, regardless of the protocol or post-bleaching times, was verified. This fact can be explained by the presence of residual oxygen on the dentin surface, which may impede the penetration of resin into dentin tubules, creating an adhesive interface more susceptible to fracture. In summary, the results of this study demonstrate that H2O2 in high concentrations can reduce the adhesion between dentin and restorative materials. However, the Er: YAG laser can remove the superficial layer of dentin that was changed by H2O2, and, therefore, the restoration with composite resin can be performed in the same clinic session. Direct comparisons of the findings of this study to the literature were not performed because of the lack of studies that evaluate the bleached dentin after the action of bleaching agents at high concentration and conditioning by means of laser, and subsequent restoration. Although laboratory studies do not reproduce intraoral conditions, they offer a controlled environment for preparing and testing the specimens, thus allowing a comparable evaluation of the variables under investigation. This study gives support to better understanding of the interaction of bleaching agents with dental surface. The results of this study encourage further clinical studies aiming to increase the bond strength of the dentin submitted to high concentration bleaching agents. Conclusions
The Er:YAG laser can restore the bond strength of the material to dentin, even if the restoration is performed immediately after bleaching. Author Disclosure Statement
No competing financial interests exist. References
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Address correspondence to: Aline E. Souza–Gabriel University of Sa˜o Paulo Ribeira˜o Preto School of Dentistry Av. do Cafe´ s/n Monte Alegre, Ribeira˜o Preto-SP CEP 14040-940 E-mail: [email protected]
