The use of teeth whitening has become a prominent esthetic dental procedure in recent years, even for orthodontic patients and those in orthodontic treatment.[1] Modern bleaching agents are mainly made of hydrogen peroxide (HP) or carbamide peroxide (CP), which generates reactive oxygen species capable of oxidizing the chromogenic molecules in dental tissues.[2] Bleaching can be an adequate esthetic enhancement; however, the release of residual oxygen and free radicals that are related to peroxide can change the surface of the enamel and affect the subsequent polymerization of the adhesives.[3] These effects are significant in orthodontics, as strong bonding between the enamel and the bracket is crucial for ensuring bracket stability during the process of tooth movement.[4]

The problem of bracket debonding still exists in clinical situations, and bracket debonding rates reported in the literature vary from around 0.5% to 17.6%.[5] Failure of the brackets more than once can make treatment take longer, result in more chairside appointments, and may cause further enamel damage during bracket rebonding.[6] The resistance of the orthodontic bond to shear force is known as shear bond strength (SBS), and is commonly used to measure bond strength.[7] The literature has set about 6-8 MPa as a clinically acceptable level for orthodontic bonding.[8] Therefore, any technique that significantly decreases the bond between the enamel and the adhesive has an effect on the efficiency and predictability of orthodontic treatment.

In the recent experimental and review literature, it has been shown that the adhesive strength has been reduced as a result of bleaching. In a systematic review and meta-analysis of the in-vitro studies, Savian et al., (2021) found that the bond strength of various adhesive systems to enamel and dentin was significantly affected by vital bleaching.[9] Likewise, orthodontic brackets were specifically studied by Perciano et al. (2021), and they showed that the shear bond strength of esthetic brackets might be affected by previous bleaching.[10] More recently, Boccuzzi et al. (2023) evaluated 11 studies with a total of 1,000 teeth and found that bracket bonding after bleaching may be negatively affected by the treatment, while postponing the bonding of brackets until after bleaching generally results in a better SBS.[11]  In addition, Zaki et al. (2023) concluded that antioxidant interventions might help to repair orthodontic bracket SBS that has been affected by bleaching, reinforcing the contribution of the remaining oxidative products in the decreased adhesive performance.[12]

Nevertheless, there are still major questions to be answered about the relative efficiencies of various bleaching materials, and how quickly the adhesive strength is restored in the initial post-bleaching phase. The few experimental studies that do compare 10% carbamide peroxide, sodium perborate, and 10% hydrogen peroxide to assess shear or microtensile bond strength at specified times following bleaching. The clinical challenge, however, is not just to make teeth whiter, but to not lose enough bonding ability to be able to treat them orthodontically with predictable success. Clinicians might be able to better decide when orthodontic bonding can be done after bleaching without bracket failure by determining the changes in the adhesive strength between 24 hours and 7 days. Therefore, the aim of the present study was to assess and compare the shear bond strength and microtensile bond strength after 10% hydrogen peroxide, 10% carbamide peroxide, and 2 g sodium perborate bleach treatments at 24 h and seven days, compared with non-bleached enamel.

This is an in-vitro experimental study in the Department of Orthodontics, University College of Dentistry (UCD), University of Lahore, in cooperation with the Department of Civil Engineering, University of Lahore (UOL). The aim of the study was to assess the effect of the various bleaching agents and post-bleaching time periods on orthodontic shear bond strength and microtensile bond strength. The study was carried out for six months from October, 2025 to March 2026. A total of 240 observations were made in the study. The sample size was calculated as n = Z² × p × q / E² (where n = the sample size, Z = 1.96, the level of confidence at 95%, p = 50% (0.50), q = 1 − p (0.50), and E = the margin of error (0.0635)). Therefore: n = (1.96)² × 0.50 × 0.50 / (0.0635)² = 240 The teeth were randomly selected from the available teeth in the archive, based on the predetermined criteria. The teeth were included if they were stored in 0.1% thymol solution for about one week at room temperature and then kept in distilled water. Teeth with intact buccal enamel surfaces that could be used for orthodontic bracket bonding were included. The bonding site was placed in the middle of the buccal enamel surface, and a mounting jig was employed to keep the buccal surface perpendicular to the bottom of the mounting tube. If the enamel surface had irregularities, hypoplastic areas, dental fluorosis, if the dental brackets were accidentally debonded, or if the enamel was previously treated with chemical substances like formalin, alcohol, hydrogen peroxide, or other bleaching agents, the teeth were not included. Endodontically treated teeth were not included. The specimens were randomly divided into four groups. Group 1 was the control group which did not receive any bleach treatment. Group 2 included teeth that were bleached with 10% hydrogen peroxide, Group 3 included teeth that were bleached with 10% carbamide peroxide and Group 4 included teeth that were treated with 2 g sodium perborate. The bleaching agents used were 10% hydrogen peroxide (Pola Office, SDI), 10% carbamide peroxide (Whiteness Perfect, FGM, Joinville, Brazil), and sodium perborate (Proderma, Piracicaba, Brazil). The buccal enamel surfaces were first cleaned with a rubber cup and water for 5 seconds at a slow-speed handpiece prior to bleaching and bonding. The surfaces were then washed with water for 10 seconds and air dried. The etching time for the control group was 30 seconds of phosphoric acid (37%) followed by 10 seconds of washing, and 10 seconds of air drying. The bleaching agents were prepared and applied according to the manufacturers' instructions. The experimental specimens were uniformly covered with about 1 mm of the corresponding bleaching material. After the bleaching, the specimens were washed and rehydrated in distilled water at room temperature. The specimens were assessed after 24 hours and after 7 days post bleaching. The teeth were stored in distilled water for the period of time between orthodontic bracket bonding and mechanical testing. The central buccal enamel surface was covered with metallic orthodontic brackets and a light-cured orthodontic adhesive was used to bond them. O-rings were soldered on brackets for proper placement in brackets. Excessive adhesive resin was removed from around the brackets and the brackets were well compressed against the enamel surface during bonding. The bonded teeth were then stored in distilled water at room temperature until testing. Shear bond strength was determined by a Universal Testing Machine (NLC 500 N, Lloyd Instrument Plc). The machine was run at a crosshead speed of 0.5 mm/minute, and the load cell was 50 kg. The debonding forces at failure were recorded for each orthodontic bracket. The failure load recorded was converted to bond strength based on bonding area of the bracket and presented in megapascal (MPa) unit. The shear bond strength was measured at 24 hours and seven days and compared with the non-bleached control group for each bleaching group. The microtensile bond strength was also evaluated by applying standard mechanical loading on Universal Testing Machine. Mounting was done so that the specimens could be subjected to tensile force on the bonded area with minimum deformation of the metallic bracket and its wings. Failure loads and microtensile bond strength in MPa were noted at failure. Microtensile strength was evaluated for each bleaching group at 24 hours and 7 days after bleaching and compared with the non-bleached group. Shear bond strength and microtensile bond strength (in MPa) were the main outcome measures. The three bleaching agents were evaluated at 24 hours and seven days after bleaching, with the non-bleached teeth as the control. For each specimen, the treatment group, the treatment bleaching agent, the bleaching interval, the type of mechanical test, and the measured failure load were noted. The bond strength values were computed and added to the study database. Data were then sorted by the four experimental groups and subsequently compared statistically. The data gathered were fed into and analyzed with the Statistical Package for the Social Sciences (SPSS) (Version 20). The shear bond strength and microtensile bond strength were measured and analyzed in each study group using descriptive statistics such as mean, standard deviation, minimum, and maximum values. Before inferential analysis, the normalities of continuous variables was investigated. One-way analysis of variance (ANOVA) was employed to compare the mean bond strength between the study groups, when the data were normally distributed. The least significant difference (LSD) post hoc test was applied for pairwise comparisons when the ANOVA test showed a statistically significant overall difference. If the assumptions for parametric analysis were not met, an appropriate non-parametric test was considered. P value ≤ 0.05 was statistically significant.

Shear bond strength (SBS) of the bleached specimens was dependent on the bleaching agent and post-bleaching interval. For 10% carbamide peroxide, the mean SBS increased from 5.32 ± 1.09 MPa at 24 hours to 6.08 ± 1.62 MPa after 7 days. The same trend was shown with specimens treated with 2 g sodium perborate, which increased from 6.54 ± 1.38 to 10.79 ± 2.90 MPa after 7 days. The bond strength of all 3 bleaching agents improved with increasing post-bleaching interval from 5.56 ± 1.27 MPa at 24 hours to 8.53 ± 2.20 MPa after 7 days for the mean SBS following 10% hydrogen peroxide. (Table 1)

The SBS for the non-bleached control specimens was significantly higher than the bleached specimens. The individual SBS values were in the range of 9.49 to 16.33 MPa, and the mean SBS was 13.37 ± 1.76 MPa. (Table 2)

The different bleaching agents and post-bleaching time also had an effect on the microtensile bond strength. The mean of the microtensile strength of 10% carbamide peroxidase rose from 0.76 ± 0.31 MPa at 24 h to 1.54 ± 0.97 MPa after 7 days. The same values were 0.88 ± 0.14 MPa and 1.37 ± 0.37 MPa, respectively, in the sodium perborate group. The mean microtensile strength at 24 hours for the 10% hydrogen peroxide group was the lowest at 0.40 ± 0.06 MPa, and after 7 days it increased significantly to 1.82 ± 0.94 MPa. (Table 3)

Microtensile bond strength values of the non-bleached control specimens range from 3.05 to 4.92 MPa, with a mean of 3.89 ± 0.52 MPa. These values were higher than all three of the bleached groups at both post-bleaching time periods. (Table 4)

Statistically significant differences in mean bond strength were shown in comparison among the four study groups (p < 0.001). The non-bleached control group had the highest mean SBS (13.37 ± 1.76 MPa), while the hydrogen peroxide group had a mean of 8.67 ± 2.14 MPa, the sodium perborate group had a mean of 7.04 ± 1.73 MPa, and the carbamide peroxide group had a mean of 5.70 ± 1.35 MPa. The overall mean of the SBS was 7.30 ± 1.75 MPa, which showed a significant difference in the strength of SBS among the study groups. (Table 5)

Likewise, a significant difference was found between study groups in terms of microtensile bond strength (p < 0.001). The mean microtensile strength was the highest in the non-bleached control group (3.89 ± 0.52 MPa), while the mean values were significantly lower in the carbamide peroxide (1.11 ± 0.98 MPa), sodium perborate (1.16 ± 0.82 MPa), and hydrogen peroxide (1.13 ± 0.38 MPa) groups. The overall mean microtensile strength was 1.82 ± 0.68 MPa. (Table 6)

Table 1. Shear Bond Strength (SBS) of Bleached Teeth According to Bleaching Agent and Post-Bleaching Interval

Table 2. Shear Bond Strength of Non-Bleached Teeth (Control Group)

Table 3. Microtensile Bond Strength of Bleached Teeth According to Bleaching Agent and Post-Bleaching Interval

Table 4. Microtensile Bond Strength of Non-Bleached Teeth (Control Group)

Table 5. Comparison of Shear Bond Strength Among the Study Groups

Table 6. Comparison of Microtensile Bond Strength Among the Study Groups

In the present in-vitro study, it was found that bleaching had an adverse effect, with significant shear bond strength (SBS) and microtensile bond strength (MTBS) differences between the non-bleached control and three bleaching groups. The highest SBS (13.37 ± 1.76 MPa) was obtained in the non-bleached control, while the other values of SBS were lower after 10% carbamide peroxide, 2 g sodium perborate and 10% hydrogen peroxide. The same trend was seen with microtensile bond strength, with a mean value of 3.89 ± 0.52 MPa in the control group, 1.11 ± 0.98 MPa in the carbamide peroxide group, 1.16 ± 0.82 MPa in the sodium perborate group, and 1.13 ± 0.38 MPa in the hydrogen peroxide group. The findings suggest that the changes that occur in enamel during bleaching may affect the interface between the orthodontic adhesive and enamel. The results are similar to those of Savian et al. (2021), who performed a systematic review and meta-analysis of 52 in-vitro studies, which concluded that vital bleaching adversely affected the adhesive bond strength to enamel and dentin. The mean analysis revealed a significant decrease in bond strength after bleaching, regardless of the bleaching agent or substrate used. They also noted that the negative impact was no longer statistically significant after two and three weeks, indicating that a bleaching-induced impairment, if it is occurring, at least partially relies on time. The current study also showed that there was a significant increase in the SBS for all three bleaching agents from 24 hours to 7 days, but 7 days SBS was still lower than the non-bleaching control.[13] The present results are especially similar to those obtained by the orthodontic study of Perciano et al. (2021), which analyzed the impact of dental whitening on the SBS of esthetic orthodontic brackets. In their experimental research, they used bovine incisors and evaluated the bonding strength of brackets on unbleached teeth and teeth that had undergone 1 or 3 bleaching periods prior to bonding. Brackets were ready to bond 7 days after the bleaching. The authors determined that the bleaching procedure and the type of brackets had an effect on SBS. The results of both studies showed that it is important to consider the history of bleaching when assessing orthodontic bracket adhesion, although the experimental conditions varied from that of this study. Differences in absolute SBS values may also be attributable to the different materials used in the brackets and the seven-day bonding period in the Perciano et al. study.[10] More recent research by Wang, Li, Shang, and Xiao (2026) also finds that there is a decrease in SBS immediately following bleaching. They studied how tooth bleaching and desensitization affect the SBS of metal brackets for orthodontics. They showed an SBS of 12.24 ± 3.71 MPa for their control, while bonding 24 hours after bleaching showed a significantly lower SBS of 7.50 ± 1.77 MPa. Importantly, for bonding to be delayed after bleaching and desensitizing, it returned to control levels after 1 week. The same general trend was found in the present study, with an increase in SBS from 5.32 ± 1.09 to 6.08 ± 1.62 MPa with carbamide peroxide, from 6.54 ± 1.38 to 10.79 ± 2.90 MPa with sodium perborate, and from 5.56 ± 1.27 to 8.53 ± 2.20 MPa with hydrogen peroxide between 24 hours and 7 days. Therefore, the results of Wang et al. offer recent experimental confirmation of the significance of the post-bleaching interval.[14] Miranda-Castro et al. (2026) further tested 22% carbamide peroxide and 35% hydrogen peroxide brackets that were bonded after varying post-bleaching waiting times. They found statistically significant differences between the bleaching agents and the post-bleaching time in terms of SBS. After 7 days, the 35% hydrogen peroxide group obtained a highest value of SBS, 12.63 ± 4.26 MPa, and the 22% carbamide peroxide group obtained 9.73 ± 3.76 MPa. Their results are very applicable to the present study, as they also compared the use of bleaching agents and a 24-hour versus seven-day interval. The current study also showed that after 7 days, there was an improvement in the SBS when compared to the control group for both groups of hydrogen peroxide and carbamide peroxide. Comparisons of the numerical values should be done with caution, however, as the concentrations used in the bleaching and the adhesive system/bracket protocol were quite different between the two studies.[15] The findings of Sharma et al. (2025) also corroborate the present finding that bleaching causes orthodontic bracket adhesion to decrease. The evaluation of 10% carbamide peroxide and 35% hydrogen peroxide was done by Sharma, Soni, Sahu, Sarita, Shah, Ganavadiya, Ghadage, and Patil on extracted human premolars. No significant difference was found in their mean SBS values among the three groups, which were 18.5 ± 2.1 MPa for the unbleached control, 14.3 ± 2.4 MPa after 10% carbamide peroxide, and 11.7 ± 1.9 MPa after 35% hydrogen peroxide. The current study presented similarly, in that the control had the highest SBS, and all bleaching groups had lower values. Consequently, the current study revealed the overall mean of SBS of 7.04 ± 1.73 MPa and 8.67 ± 2.14 MPa for carbamide peroxide and hydrogen peroxide, respectively. Lower values could be attributed to variations in bleaching protocol/conditions, bleaching concentration, bleaching exposure time, storage conditions, bracket adhesive, and post-bleaching interval for bonding.[16] Roy, Manjusha, Nishad, Aravindan, Krishnajith, and Jameema (2024/2025) carried out another recent study on the performance of orthodontic metal brackets bonded with 35% hydrogen peroxide solution and tested two different primer systems under dry and moist conditions. The study also reaffirms the significance of the adhesive system and environmental factors in assessing bracket SBS following bleaching. This is important for the present investigation since all specimens were bonded with the same light-cured orthodontic adhesive and stored in a controlled environment. Thus, any differences found between the studies cannot be ascribed solely to the bleaching agent, as other factors, such as the type of primer and adhesive used, moisture conditions, bracket design, enamel preparation, and testing method may affect the strength of the bond measured.[17,19] The study by Ambersari et al. (2024) also showed that orthodontic adhesion can be altered by the work environment after bleaching. Their in-vitro study analysed metal brackets following bleaching with 37% hydrogen peroxide, and compared desensitizing treatments using fluoride with those using non-fluoride. The authors found that significantly greater SBS occurred for the fluoride desensitizing treatment compared to the other treatments. It is important to note that the findings of these studies help to interpret the present study as it did not use an antioxidant or desensitizing intervention. This may partly explain the lower bond strengths reported soon after bleaching, as the bond strengths could still be influenced by the presence of bleaching-induced oxidative effects at the enamel-adhesive interface.[18, 20] The results are also in line with the scoping review by Zaki, Ghorab, and Shamaa (2023) that assessed 23 in vitro studies of antioxidants to restore orthodontic bracket SBS following bleaching. The review found that bleaching can negatively affect the bracket SBS and that antioxidants like sodium ascorbate and ascorbic acid can enhance bond strength in many experimental protocols. The authors also stated that there is variability depending on the type of antioxidant, its concentration, application time, bracket material, and bonding protocol. In the present study, antioxidants were not used; however, the improvement in SBS between 24 hours and seven days would be consistent with the concept that the adverse bleaching effect reduces with time and the dissipation of any residual oxidative products.[12] The present study also used microtensile testing to confirm that bleaching had more of an impact on the adhesive interface than just changing the response to shear loading. The mean microtensile bond strength for the control specimens was 3.89 ± 0.52 MPa, whereas the carbamide peroxide specimens had a mean value of 1.11 ± 0.98 MPa, the sodium perborate specimens had a mean value of 1.16 ± 0.82 MPa, and the hydrogen peroxide specimens had a mean value of 1.13 ± 0.38 MPa. The hydrogen peroxide group was especially noteworthy, with microtensile bond strength going from just 0.40 ± 0.06 MPa after 24 hours to 1.82 ± 0.94 MPa after seven days. The results of this time-dependent improvement are comparable to the SBS results and indicate that the deteriorated enamel-adhesive interface partially recovers as bleaching time increases. The relative differences of the three bleaching agents should be interpreted with caution, however. The greatest improvement of SBS from 24 hrs to 7 days was achieved with sodium perborate (6.54 ± 1.38 to 10.79 ± 2.90 MPa). Hydrogen peroxide increased from 5.56 ± 1.27 to 8.53 ± 2.20 MPa, whereas carbamide peroxide showed a more modest increase from 5.32 ± 1.09 to 6.08 ± 1.62 MPa. These differences do not necessarily mean that one bleaching material is more harmful than another since the concentration of the peroxide, length of exposure, formulation, release of oxygen, condition of the enamel, storage medium, adhesive composition, and bonding interval are highly variable across the various experimental studies. The effects of bleaching on adhesion were also reported by Savian et al. who also cited methodological variation as a source of differences in the results. Therefore, the present results provide an important additional time dependent component to the existing evidence. All three groups of bleaching materials showed relatively low SBS and microtensile strength at 24 hours, with a general increase in these properties after seven days. However, all the bleached groups failed to get back to the same levels as the controls. The clinical significance of this discovery is that it suggests that immediately or shortly after bleaching, orthodontic brackets could be subjected to a weaker enamel-adhesive interface. The recent study by Miranda-Castro et al. (2026) also showed a significant difference in SBS depending on the gap between bleaching and bracket bonding, and Wang et al., (2026) found that SBS recovered to the level of the control group after 1 week of their unique bleaching and desensitization protocol. The present results are in accordance with the recent literature that suggests that tooth bleaching and orthodontic bracket bonding can have a negative effect on orthodontic bracket adhesion, especially if bonding is attempted shortly after bleaching. The observed recovery over 7 days suggests that there is some degree of recovery of adhesive performance with time. But there is also a wide disparity in the literature regarding use of bleaching agents, concentrations, adhesive systems, bracket materials, and waiting periods. This means that the present results are not conclusive proof of the superiority or inferiority of one bleaching agent over another; rather, they are evidence of a time-and protocol-dependent decrease in bond strength. The results of the present study combining the two methods suggest that bleaching may adversely affect the enamel-adhesive interface and that this influence decreases with increasing post-bleaching time, confirming the findings of both methods. Limitations This study was done in in-vitro conditions, which does not necessarily reflect the actual complexity of the oral cavity, such as the saliva, mastication, temperature changes, and oral hygiene factors. These results were limited to only three bleaching protocols and two post-bleaching intervals, and thus cannot be generalized to other concentrations, formulations, and post-bleach waiting periods. The study also employed extracted teeth and only one orthodontic adhesive system/bracket type. In addition, the microtensile test is a sensitive technique and can be affected by specimen preparation and loading configuration. There was no long term clinical follow-up to see if the observed differences equate to greater bracket failure clinically.

The shear and microtensile bond strengths of orthodontic brackets were significantly lower when they were bleached than when they were not bleached. The strength of the bond was steadily increasing as the bonding was postponed from 24 hours to 7 days, but this value was still not equivalent to the non-bleached control. Sodium perborate yielded the highest recovery with regard to SBS and hydrogen peroxide showed a significant improvement in microtensile strength after 7 days. These results indicated that immediate orthodontic bonding following bleaching could be detrimental to the bond, and a long post-bleaching interval could be beneficial to bracket bonding.