Bond strength of resin composite to enamel submitted to at-home desensitizer and bleaching agents

Aim: This study evaluated the effect of a desensitizer agent (DES) during bleaching with 10% carbamide peroxide (CP) on enamel microshear bond strength (μSBS). Methods: Sixty bovine incisors were obtained and randomly distributed into groups (n=15): (C) Control: no desensitizing or bleaching, (DES) desensitizing gel application, (CP) bleaching with 10% CP and (CP/DES) bleaching with 10% CP combined with DES. Bleaching was performed for 6 h/day for 14 consecutive days. DES was applied for 8 h only on the 7th and 14th days of therapy. Specimens were stored in artificial saliva among the CP or DES applications and submitted to μSBS testing at three postrestoration times (n=5): 24 h, 7 days, and 14 days after bleaching using a universal testing machine. Failure modes were observed under a stereomicroscope. Data were analyzed by two-way ANOVA and Tukey’s test (α=5%). Results: Immediately after bleaching (24 h), CP promoted lower μSBS than the C and DES groups (p<0.05) but with no differences from the CP/DES. μSBS increased in the DES, CP, and CP/DES groups (p<0.05) when bonding was performed for 7 or 14 days elapsed from bleaching. CP/DES exhibited the highest μSBS among the groups 14 days after bleaching (p<0.05). Cohesive failure in enamel was predominant in the CP groups, while adhesive failure was mostly observed for the other groups. Conclusion: The use of a desensitizer during at-home bleaching maintained the enamel immediate bond strength, and its application favored bonding when the restoration was delayed for 14 days.


Introduction
Tooth bleaching is a widespread technique commonly performed and desired by patients due to its conservative approach and efficacy in providing color alterations 1,2 . Carbamide (CP) and hydrogen (HP) peroxides are active compounds of bleaching agents, and their concentrations vary according to the bleaching technique 3 . In-office bleaching uses high concentrations of CP or HP (higher than 25%), while the home-applied technique uses lower CP (up to 22%) or HP (up to 10%) concentrations. Although a highly concentrated agent could accelerate the bleaching effect, both methods achieve acceptable outcomes 1,3 . Studies have reported that at-home bleaching resulted in mild tooth sensitivity 4 , but a recent systematic study revealed that the high variation among bleaching protocols could not confirm that at-home bleaching yielded a lower risk or intensity of sensitivity 5 . Therefore, the management of this clinical symptom still represents a challenge during tooth bleaching, even when performed at lower concentrations.
In vitro studies have reported an increase in enamel surface roughness 6 and a decrease in microhardness 7 and immediate enamel bond strength 8 following at-home bleaching. These alterations to the enamel surface could result in pore formation and consequent entrapment of oxygen residuals released from the peroxide agents, which could, in turn, negatively interfere with the light curing of the adhesive agent. Poor light curing could lead to a decreased number, dimension, and quality of the resin tags in the etched enamel 9 . However, despite the cause, enamel bond strength may recover by surface remineralization promoted by saliva or remineralizing solutions 10,11 .
Although several in vitro studies proposed applying antioxidant agents to reverse the immediate bond strength of bleached enamel [12][13][14] , no randomized clinical trials attest to the long-term performance of restorations placed immediately after bleaching with the antioxidant application. Consequently, delaying the bonding procedures to up to 3 weeks still seems to be an effective method to guarantee satisfactory restorative results after tooth bleaching 10 .
In this scenario, studies investigating the effect on the enamel bond strength of potassium nitrate-and sodium fluoride-based desensitizer use before, during, or after tooth bleaching are scarce 15 . Although desensitizer application could be a feasible solution to overcome tooth sensitivity 16 and maintain the resin composite bond strength to enamel due to its composition, a study showed that the application of a desensitizer containing potassium nitrate and sodium fluoride reduced the enamel bond strength when the bonding procedure was performed immediately after bleaching 17 . The authors credited the bonding reduction to the fluoride deposition on the surface, thereby compromising the adhesion between resin and teeth 17 . However, it is necessary to determine if a desensitizer agent influences the enamel bond strength even in delayed bonding procedures.
Based on these facts, this study evaluated the enamel bond strength at different postrestorative times after home-applied bleaching combined or not with a desensitizer agent. The null hypothesis was that the use of home-applied bleaching combined with a desensitizer agent would not influence enamel bond strength immediately or after 7 and 14 days of bleaching treatment.
The variable responses evaluated were microshear bond strength (μSBS, in MPa) and fracture mode analysis, observed at 40 χ magnification. Table 1 displays the composition of the materials used. Specimen preparation. Sixty extracted bovine incisors, stored in 0.1% thymol solution, were cleaned with periodontal scalers, and the remaining debris was removed with a bicarbonate jet. The roots were cut at the amelocement junction, and enamel/ dentin blocks (5 x 5 mm and 3 mm of thickness) were obtained from the central area of the crown using a double face diamond disc (KG Sorensen, Barueri, SP, Brazil).
Bleaching and desensitizing treatments. Specimens were randomly distributed into four groups according to bleaching and desensitizing treatments as described previously. A 1-mm layer of the bleaching agent (10% CP) was applied on the exposed enamel surface of groups CP and CP/DES for 6 h/day for 14 days. During bleaching, specimens were stored underneath moisture gauze at 37 0 C and relative humidity. After bleaching, specimens were thoroughly rinsed with deionized water and stored in artificial saliva [1.5 mM calcium (CaCl 2 ), 0.9 mM phosphate (NaH 2 PO 4 ), 0.15 mM potassium chloride (KCl)], adjusted to pH 7.0 and 3.125 mL of solution for each mm 2 of exposed enamel until the next gel application 18,19 .
The desensitizer agent in the CP/DES group was applied over the enamel, similar to the bleaching gel, for 8 h. However, desensitizer application was only performed on the 7 th and 14 th days of treatment. Specifically, in the CP/DES group, DES application was carried out after the bleaching procedure. After treatments, the specimens were thoroughly rinsed with deionized water and stored in artificial saliva. Desensitizer gel was also applied on the 7 th and 14 th days in the DES group, but no bleaching agent was applied to the specimens; instead, they were kept in artificial saliva in the remaining hours. In the C group, neither bleaching nor desensitizing treatment was performed; specimens were kept in artificial saliva throughout the 14 days of treatment. The artificial saliva was replaced daily.
Restorative procedure. The specimens were submitted to restorative procedures according to the posttreatment times (   Table 2 displays the results of the μSBS test. Immediately after bleaching (24 h), CP promoted lower μSBS than the C and DES groups (p<0.05) but with no differences from CP/DES. Seven days after bleaching, the C group exhibited the lowest μSBS values (p<0.05), and DES, CP, and CP/DES displayed no significant differences (p>0.05). In addition, at this time point (7 d), the μSBS of the DES, CP, and CP/DES groups increased in comparison to that of the immediate (24 h) corresponding groups (p<0.05).

RESULTS
Fourteen days elapsed from bleaching, CP/DES exhibited the highest μSBS among groups, with no significant differences between the 7 th and 14 th days elapsed from treatment (p>0.05). In addition, no differences were detected among C, DES, and CP at this time point (14 d, p>0.05). C exhibited no bond strength differences, regardless of the evaluation time (p>0.05). The DES and CP groups produced higher μSBS at the 7 th and 14 th days elapsed from bleaching than at the immediate time point.

DISCUSSION
A potassium-and nitrate-based desensitizer combined with CP maintained the enamel μSBS and displayed values higher than those of the other groups when bonding was performed 14 days after bleaching. In addition, the desensitizer alone promoted higher μSBS than the control on the 7 th day of bonding postponement. Therefore, the results rejected the null hypothesis because this protocol influenced the enamel μSBS after bleaching. The fluoride effect in the remineralization process could explain this outcome 20 . The addition of Ca and F to low-concentrated CP has been demonstrated to control enamel mineral loss during bleaching 21,22 . These findings support the theory that the fluoride in the desensitizer maintained enamel integrity and hence the bond strength.
Contrary to these findings, a report from Amuk et al. 17 (2018) showed that the application of a desensitizer (UltraEZ) during 22% CP bleaching decreased the bond strength.
The authors stated that desensitizer deposition weakened the adhesive interface. The differences in both studies could rely on the different CP concentrations, the frequency of the desensitizer applications, and mainly by the time elapsed from the bleaching and bonding procedures. In that study, only immediate bond strength was carried out, and the desensitizer was applied daily for 14 days 17 . Therefore, divergences in the protocols might impact the effects of desensitizers on enamel bonding.
Even though bleaching with low-concentrated CP decreased immediate enamel μSBS 20 , the postponement (7 or 14 days) of bonding reversed that result. The oxygen release and entrapment in the structures during bleaching may decrease enamel/dentin bond strength 23 . In addition, HP could interfere with enamel morphology due to the inorganic content changes produced during bleaching 24 . These have been the rationale for replacing restorations on bleached teeth only after a few weeks 3 . Although the desensitizing gel did not increase the immediate bond strength, the results suggested that the combination of CP with a fluoride-and potassium nitrate-containing desensitizer favored bonding in the case of the restoration's postponement. Thus, the 10% CP protocol combined with DES would benefit the enamel μSBS 14 days elapsed from bleaching.
Fluoride interacts in the tooth demineralization process when chemically soluble, increasing remineralization and reducing mineral loss 20 . The presence of fluoride ions during an acidic challenge, promoted by a solution with pH 6.5, leads to the exchange of Ca of hydroxyapatite (HA) for fluoride, thereby forming fluorapatite (FA). As FA has a lower solubility and dissociation constant than HA, the pH will have to drop more dramatically (approximately at 4.5) to demineralize enamel 25 . Therefore, it is likely that the fluoride in the desensitizer gel controlled the μSBS of bleached enamel similarly to fluoride-containing bleaching agents controlling enamel mineral content 22 . It is worth mentioning that bleaching gels dispensed in syringes often display neutral or even acidic pH to extend the product's shelf life 1 . Moreover, pH may drop over the gel's application 26 . Therefore, contact of the enamel surface with a fluoride-containing desensitizer after acidic challenges may have reversed this condition.
Several studies have investigated enamel bond strength after at-home and in-office bleaching treatment [10][11][12][13] . Cavalli et al. 10 (2001) evaluated the enamel bond strength of an etch-and-rinse adhesive after bleaching with CP gels (10 to 20%) for ten days. The authors concluded that bond strength was recovered three weeks elapsed from bleaching and artificial saliva storage 14 . In contrast, others 27 reported that enamel treated with 10% CP and restored with self-etching adhesive exhibited similar bond strength to the control group 24 h after bleaching. Furthermore, bond strength after 35% HP bleaching presented similar values to untreated enamel after one week 27 . Such discrepancies between studies 14,27 could rely on differences in bonding and bleaching procedures. Nevertheless, no consensus exists regarding shear bond strength for bleached teeth bonded with self-etching adhesive 9 .
The predominance of cohesive failures in enamel (COE) in the CP groups at all times corroborates the action of HP byproducts on enamel. The decrease in enamel strength was previously reported even after CP treatment 24 . According to that study, CP produced morphological alterations, with loss of enamel prism core, decreasing enamel cohesive strength 24 . In addition, the predominance of adhesive failures of the CP/DES group at 7 and 14 days after bleaching indicates an increase in enamel resistance to fracture, which was probably promoted by remineralization. Conversely, Faria de Lacerda et al. 28 (2016) observed that 0.05% sodium fluoride remineralized enamel exhibited mixed failure modes for etch-and-rinsed and self-etched restorations. That study observed that the μSBS of the F-treated group bonded with the self-etching system was lower than that of the control group 28 . This contrast may be explained by the 8-week fluoride daily application, which may have hypermineralized the enamel.
In summary, the μSBS behavior of bleached enamel and its recovery or even increase within the following weeks reaffirms previous data [10][11][12][13][14] . Due to fluoride incorporation in the tooth structure, the desensitizer may have enhanced enamel bond strengths.
Since esthetic dental treatments frequently begin by tooth bleaching, these findings suggest that desensitizer application during bleaching could favor enamel bond strength of restorative procedures when postponed for two weeks.
The manufacturer recommends the desensitizer's application from 15 min to 1 h, but the frequency depends on the tooth condition, patient sensitivity, and professional decision (Table 1) 29 . Because of that recommendation, several application times can be found in the literature (from 15 min 30 to overnight use 31 ). In this study, the desensitizer's gel application occurred on the 7 th and 14 th days of bleaching for 8 h without intervals, which corresponds to 1 h 15 min of daily use. However, the prolonged application adopted is a limitation of this in vitro design and cannot be extrapolated to a clinical situation because of the risks of swallowing the desensitizer. Furthermore, authors have attested 32 that the shear bond test also holds limitations because shear stress concentrates where the load is applied, causing an uneven tension distribution and a higher % of cohesive failures within the resin composite (COR), underestimating the μSBS values. However, all groups exhibited a lower % of COR (10-30%), contradicting this idea and supporting the μSBS test performed.
Clinically, a desensitizer agent containing fluoride and potassium nitrate attenuates the risk and intensity of tooth sensitivity 19 during bleaching by obstructing dentin tubules and preventing depolarization of nerve fibers 33 . Even though this study presents the inherent limitation of an in vitro design, it demonstrated that the desensitizer might positively impact the bond strength of postponed procedures. Further investigations could attempt different protocols to determine an acceptable effect on immediate enamel bonding.
In conclusion, the combination of a desensitizer agent with bleaching gel (10% CP) upheld the enamel bond strength immediately after bleaching. The protocol combining low-concentrated CP with a desensitizer agent presented the highest bonding performance under a 14-day postponement.