Basic Research—Technology

Effect of 35% Sodium Ascorbate Treatment on Microtensile Bond Strength after Nonvital Bleaching Jason R. Hansen, DDS,* Kenneth J. Frick, DDS, MS,* and Mary P. Walker, DDS, PhD† Abstract According to previous reports, adverse effects of tooth bleaching on bond strength can be reversed by delaying bonding for 1–3 weeks or by applying 10% sodium ascorbate (SA) for 3 hours or more. This study evaluated the effectiveness of the short-term application of 35% SA to counteract the effects of a 7-day 35% hydrogen peroxide (H2O2) bleaching regimen on bond strength. Methods: Forty extracted third molars were mounted and sectioned to obtain a flat dentin surface and then randomly assigned into 4 groups: group 1: restored, no bleach; group 2: bleached, bonded immediately; group 3: bleached, treated with two 1- minute 35% SA applications before bonding; and group 4: bleached, treated with two 5-minute 35% SA applications before bonding. For bleach treatment in groups 2–4, flattened dentin surfaces were exposed to H2O2 for 7 days at 37 C. Subsequent to respective treatments, dentin surfaces were built up with composite (TPH3 and Prime &Bond NT, Dentsply Caulk, Milford, DE). After 24 hours of storage (100% humidity, 37 C), the specimens were sectioned into 1-mm2 dentin-composite beams. Four beams from each tooth (n = 40/group) were subjected to microtensile bond strength testing. Results: Results were as follows: group 1: 18.1  8.1MPa, group 2: 11.3  5.7MPa, group 3: 11.2  5.2MPa, and group 4: 12.6  6.1MPa. A 1-factor analysis of variance and the Tukey post hoc test (a = 0.05) indicated that bleaching had a detrimental effect on bond strength and that short-term SA treatments after bleaching did not significantly improve bond strength. Conclusions: The application of 35% SA in a clinically relevant timeframe was not effective at reversing bleaching effects on bond strength. Bonding procedures should be delayed following tooth bleaching. (J Endod 2014;40:1668–1670)

Key Words Microtensile bond strength, non-vital bleaching, sodium ascorbate treatment

From the *Advanced Specialty Education Program in Endodontics and †Research and Graduate Programs, University of Missouri-Kansas City, School of Dentistry, Kansas City, Missouri. Address requests for reprints to Dr Mary P. Walker, 650 East 25th Street, Kansas City, MO 64108. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists.


Hansen et al.


leaching procedures are commonly performed in dentistry to treat discolored teeth. Often, endodontically treated teeth have nonvital (internal) bleaching procedures performed, with subsequent restoration of the endodontic access with composite resin materials. It has been reported that the bond strength and sealing ability of composite resin are adversely affected when placed immediately after bleaching procedures (1–3). Some researchers have proposed that the decrease in bond strength is caused by residual oxygen in the dentinal tubules, which can interfere with resin infiltration or inhibit polymerization (4, 5). It has also been proposed that the decrease in bond strength is caused by changes in the proteins and mineral contents of the tooth after bleaching (6). Although bleaching affects tooth structure in a way that decreases bond strength, the effect is not permanent. Depending on the study, it has been reported that if bonding is delayed 1–2 weeks after the termination of nonvital bleaching, then bond strengths return to normal values (7–10). Studies have also shown that use of sodium ascorbate (SA), a potent antioxidant, can reverse the negative effects of bleaching on bond strengths and/or the sealing ability of composite restorations after nonvital bleaching (9–11). Most of the antioxidant studies have used 10% SA, with the bleaching effect on bond strengths to dentin reversed after as little as 3 hours or as much as 3 days of SA application. In a recent bleaching study using colorimetry to assess the amount of residual peroxides present after the use of 35% SA, it was found that after 2 separate 1-minute applications of 35% SA, there were no peroxides that could be detected (12). No bonding was tested in the colorimetry study, but it was suggested that if the residual peroxides are responsible for the typical reduction in bond strength after bleaching, then the two 1-minute applications of 35% SA may be sufficient to reverse the negative effects on bonding. To date, no studies have evaluated bond strength subsequent to the counteractive effects of 35% SA used for short-term application times after nonvital bleaching. The purpose of this study was to evaluate the effect of 35% SA used for short-term periods on microtensile bond strength of composite resin to dentin immediately after a simulated nonvital bleaching procedure.

Materials and Methods Forty previously extracted third molars were collected in accordance with an institutional review board–approved protocol with no patient identifiers associated with the teeth. The extracted teeth were stored in 0.9% phosphate buffered saline with 0.002% NaN3 included to inhibit microbial growth. The teeth were then mounted in acrylic resin (Great Lakes Ortho, Tonawanda, NY) and randomly assigned into 4 groups of 10 teeth each: Group 1: Restored without bleaching Group 2: Bleached with 35% hydrogen peroxide for 7 days and then bonded immediately Group 3: Bleached with 35% hydrogen peroxide for 7 days and then treated with two 1-minute 35% SA applications and then bonded Group 4: Bleached with 35% hydrogen peroxide for 7 days and then treated with two 5-minute 35% SA applications and then bonded The occlusal one third was removed from each tooth using a water-cooled diamond blade saw (Isomet 1000; Buehler, Lake Bluff, IL) to obtain a flat dentin surface. The dentin surface was then rubbed against wet 600-grit sand paper for 30 seconds to simulate the formation of a smear layer followed by 15 seconds of air and water spray. The prepared teeth were inverted and suspended on a plastic grate over a 5-mm layer of

JOE — Volume 40, Number 10, October 2014

Basic Research—Technology

Figure 1. (A) Teeth suspended in 35% H2O2. (B) Tooth to be serially sectioned into dentin-composite beams with 1-mm2 cross-sectional area. (C) Dentincomposite beam attached to a microtensile tester.

35% hydrogen peroxide solution, allowing for only the occlusal surface to be in contact with the bleaching agent (Fig. 1A). The teeth suspended in the bleaching agent were stored in a covered container at 37 C for 7 days. After completion of the 7-day bleaching period, the teeth were washed with air and water spray for 20 seconds and then treated in accordance with the protocol of their assigned group classification. For groups 3 and 4, 35% SA (Sigma Aldrich, St Louis, MO) was applied with a microbrush and scrubbed along the dentin surface. Group 3 was exposed for 1 minute, and group 4 was exposed for 5 minutes. The SA was removed with air and water spray, and the SA application was then repeated a second time for each group. An approximate 4-mm layer of composite was then added to all tooth specimens using the following protocol. After a 10-second total etch using 34% phosphoric acid, adhesive (Prime and Bond NT; Dentsply Caulk, Milford, DE) was applied for 20 seconds and then thinned with air and light cured for 20 seconds. Composite resin (TPH3, Dentsply Caulk) was then applied to the prepared surface in 3 or more increments of less than 2 mm with each layer cured for 40 seconds. The teeth with bonded composite were stored for 24 hours in a closed container with 100% humidity at 37 C and then serially sectioned into 1-mm-thick vertical dentin-composite slabs with the water-cooled diamond blade (Fig. 1B). The specimens were then rotated 90 and sectioned again with the diamond blade to obtain rectangular dentin-composite beams with a cross-sectional area of approximately 1 mm2. The surface area of the bonded interface was calculated for each specimen by measuring the narrowest portion with a digital caliper (Marathon, Richmond Hill, ON, Canada). Four dentincomposite beams were obtained from each tooth specimen for microtensile testing. The beams were attached to a microtensile tester fixture (Microtensile Tester; Bisco, Schaumburg, IL) by applying cyanoacrylate (Zapit; Dental Ventures of America, Corona, CA) to the composite and dentin ends of the beams, ensuring that the dentin-composite interface was centered in the fixture (Fig. 1C). Adhered beam specimens were subjected to tensile forces at a crosshead speed of 1 mm/min. The microtensile bond strengths were expressed in MPa based on the following calculation: force at debond (N) divided by the dentin–composite beam interface area (mm2).

Results The microtensile bond strength results from the various groups are presented in Table 1. Statistical analysis was performed with SPSS Statistics v21.0 (IBM, Armonk, NY). A 1-factor analysis of variance and the Tukey post hoc test revealed that group 1 (no bleaching) showed significantly higher bond strength (P # .05) than groups 2–4, which had no significant differences (P > .05) among them. Bleaching had a detrimental effect on bond strength, and short-term SA treatments after bleaching did not significantly improve bond strength. JOE — Volume 40, Number 10, October 2014

Discussion This study assessed the effects of short-term applications of 35% SA on dentin bond strength after a simulated nonvital bleaching procedure. It has been reported that the application of 10% SA for as little as 3 hours or as long as 3 days can reverse the negative effects of bonding to teeth receiving nonvital bleaching procedures (9–11). A major weakness with those findings is that the reported application times are not clinically applicable. Outside of the context of nonvital bleaching, there are studies that have shown that a 10% SA solution can reverse the effects of bleaching after 10 minutes (13–16). Major differences between those studies and the current study include some or all of the following: 1. 2. 3. 4.

They use weaker bleaching agents. They perform external bleaching procedures. The bleach is applied for less time. They are testing bond strengths to enamel rather than dentin.

Although clinicians may be inclined to cite findings from such studies to justify the short-term use of 10% SA after nonvital bleaching procedures, the external validity supporting such a citation is very weak. A study was performed using a colorimetric assay to assess the effectiveness of 35% SA in removing residual peroxides in a clinically relevant manner after 40 minutes of bleaching with 35% H2O2 (12). It was found that two 1-minute applications of 35% SA facilitated complete elimination of residual peroxides. Because the presence of residual peroxides is thought to be a major factor in the decreased bond strength seen after tooth bleaching (4, 5), it would be expected that elimination of the peroxides would lead to increased bond strengths. Although 35% H2O2 was used as the bleaching agent in that study and the current study, bleaching occurred for 7 full days in the current study versus only 40 minutes in the other. The current study found that short-term application of 35% SA did not counteract the decreased bond strength after nonvital bleaching with 35% H2O2. The standard deviations obtained with the results of each group were higher than expected. Although perhaps it would not be a complete explanation of the higher deviations, it has been reported that inter- and intradentin bond strength variability exists (17, 18). Each tooth in the TABLE 1. Microtensile Bond Strength Mean and Standard Deviation Values Groups (N = 40 dentin-composite beams/group)

Mean ± SD (MPa)

Group 1: No bleaching Group 2: 35% H2O2 only Group 3: 35% H2O2 + 2  1 min 35% SA Group 4: 35% H2O2 + 2  5 min 35% SA

18.1a  8.1 11.3b  5.7 11.2b  5.2 12.6b  6.1

SA, sodium ascorbate; SD, standard deviation. There was a significant difference (P < .05) between groups; superscript letters indicate subsets.

SA Treatment on Microtensile Bond Strength


Basic Research—Technology study was mounted and then sectioned to reveal a flat dentin surface. The intention was to have each tooth sectioned at the same level, but differences in the amount and distribution of enamel invariably resulted in each specimen being sectioned to a level dictated by its individual morphology. It is possible that the variation in the level and orientation of the flat dentin surface of the specimens may have, at least minimally, contributed to the higher than expected standard deviations observed. Of the numerous bonding adhesive options available, Prime and Bond NT was the bonding agent used in this study. The mean bond strength for the control group was somewhat low but was similar to the bond strength reported in other microtensile studies using Prime and Bond NT (19). Although one would expect the results of this study to apply to other adhesive systems, it is plausible that alternative bonding agents may yield different results. It has been reported that waiting 1–2 weeks will reverse the adverse effects of bleaching on bond strength (7–10). Because the short-term 35% SA in this study did not effectively restore the bond strengths and because other nonvital bleaching studies using SA do not involve a clinically relevant application time, the clinician is left with the decision to either delay bonding or accept the compromised bond strength that accompanies immediate bonding after bleaching. An endodontic access has a high ratio of bonded surfaces and thus is associated with higher stress levels within the resin materials as polymerization shrinkage occurs (20). Accordingly, the effects of polymerization shrinkage can be detrimental to the long-term success of composite restorations (21). Considering the negative effects of bleaching on bond strength and the unfavorable stress levels placed on composite restorations in an endodontic access, the best clinical recommendation is to wait 1–2 weeks before restoring teeth that have had nonvital bleaching performed.

Conclusion The application of 35% SA in a clinically relevant time frame was not effective at reversing the negative effects of 7 days of bleaching with 35% H2O2on bond strength. Bonding procedures should be delayed after tooth bleaching in order to ensure adequate bond strength and sealing ability.

Acknowledgments The authors thank Dr Changqi Xu and Ms. Rachel Reed for assistance and training for the microtensile bond strength testing. Supported in part by gifts from the American Association of Endodontists Foundation and the UMKC Rinehart Foundation. The authors deny any conflicts of interest related to this study.


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JOE — Volume 40, Number 10, October 2014

Effect of 35% sodium ascorbate treatment on microtensile bond strength after nonvital bleaching.

According to previous reports, adverse effects of tooth bleaching on bond strength can be reversed by delaying bonding for 1-3 weeks or by applying 10...
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