Tooth Whitening in Association with Clear Aligner Treatment

Along with dental alignment, tooth shade is a major factor in the perception of smile esthetics.⁹ Whitening is being requested more frequently, especially by orthodontic patients¹⁰; in one study, 88% of orthodontic patients asked for whitening during or after their treatment.¹¹ Krug and Green found a significantly higher 90% satisfaction rate among patients when whitening was included with orthodontic treatment, compared with orthodontic treatment alone.¹²

This trend has prompted several investigators to assess the efficacy of whitening agents applied during multibracket appliance treatment.^13-15 Results have been mixed, however, and no combination of orthodontic treatment and tooth-whitening strategy has proved entirely satisfactory.

Although conventional whitening trays incorporate reservoirs for the whitening agent, the close fit of a clear aligner—while essential for control and predictability during treatment—leaves little room for a whitening gel. We hypothesized that a uniform distribution could be achieved by spraying a liquid whitening agent directly onto the interior surfaces of the aligner trays. The optimal composition and concentration of an agent that could whiten the teeth without causing deterioration in the microstructure or esthetic qualities of the aligner material had yet to be determined. Therefore, the present study was designed to evaluate different at-home whitening protocols during orthodontic clear aligner treatment.

Materials and Methods

The efficacy of the whitening spray formulation was tested on a sample of 38 volunteers who were undergoing orthodontic treatment with F22 aligners at the University of Ferrara Orthodontic Clinic. An adult over age 18 was included in the study if he or she had six vital maxillary anterior teeth, no need for attachments or divots on the labial surfaces of the upper canines or incisors, no direct or indirect reconstruction of the upper canines or incisors, no history of dentin hypersensitivity, and no history of professional bleaching during the previous three years. Before beginning the treatment, each participant signed an informed-consent document describing the assigned whitening treatment and its potential adverse effects.

The participants were randomized as follows:

• Group 1 (eight patients): Application of 3% hydrogen peroxide for nine hours per day on days 7-14 of aligner wear.
• Group 2 (eight patients): Application of 10% carbamide peroxide for nine hours per day on days 7-14 of aligner wear.
• Group 3 (eight patients): Application of 16% carbamide peroxide for nine hours per day on days 7-14 of aligner wear.
• Group 4 (14 patients): Application of 16% carbamide peroxide for nine hours per day on days 1-14 of aligner wear.

According to previous cone-beam computed tomography analyses, the fit of the F22 aligner provides less than 40µm of space between the tray and the anterior tooth surface.¹⁶ Realistically, the thickness of the bleaching solution will never exceed this value. To qualitatively analyze the distribution pattern of a liquid within the aligner, we randomly selected four patients. A blue-dyed whitening solution was applied using the same standardized protocol that would be followed during the efficacy portion of the study. The blue liquid was sprayed onto the internal labial surface of the aligner, with one spray at the central incisors and one spray in each of the canine regions. The upper aligner was fitted in the patient’s upper arch as it would normally be worn, and a frontal photo­graph was taken to enable visual evaluation of the whitening agent’s distribution pattern between the aligner and the upper anterior teeth.

On the day the patient was provided with the appropriate whitening spray (T1), a spectrophotometric analysis was performed using a VITA Easyshade V^*** compact, which was chosen for its proven accuracy and reliability.^17,18 Three values were read for each of the six maxillary anterior teeth—one each at the incisal, middle, and cervical thirds of the labial surfaces (Table 1).

The 16 shades of the VITA classical A1-D4^*** color scale were reorganized from highest (B1) to lowest (C4), as recommended by the ADA,¹⁷ and each value was assigned a numerical equivalent (Table 2).

Comparison of the median shades for each site in the upper anterior segment before whitening, using the Kruskal-Wallis H test, found no statistically significant differences among the four patient samples, making it possible to state that all four groups belonged to the same population.

At the same appointment, the patient was instructed to apply the whitening spray as described above after dinner and toothbrushing, at about 10-11 p.m., and to leave the aligner in place for nine consecutive hours, until about 7-8 a.m.

Each patient returned three days later to be checked for side effects. Because no gingival inflammation or changes in dental sensitivity were detected in any patients, all participants continued following the initial instructions. No changes in the bleaching protocol or prescription of desensitizing agents were required during the study. The color analysis was repeated at the two-week follow-up (T2), following the same protocol as before.

A scanning electron microscope^† (SEM) was used to conduct a qualitative analysis of the F22 aligners before and after application of the whitening spray. We tested one unused aligner sample, one aligner that had been worn for 14 days before whitening (T1), and one from each of the four whitening groups at T2. A section measuring 5mm × 5mm was taken from the median third of the aligner’s labial surface at the level of the upper right central incisor. Each sample was gilded, and SEM images were taken at magnifications 250×, 1,000×, and 5,000×.

For each group, we used the mean as the measure of central tendency and the relative standard deviation as the measure of variability. The analysis of variance F test (p < .05) and Bonferroni post hoc test (p < .05) were used to identify any statistically significant differences among the mean percentage variations in each group. Nonparametric evaluations were chosen because the data did not fit a normal distribution.

A binomial z-test was performed, using the mean values for each test group, to identify the at-home whitening protocols that had resulted in statistically significant improvements in whiteness (p < .05). The percentage variation from T1 was then calculated for each site, with B1 as the upper limit of 100%, to better delineate the degree of whitening observed in each group. The Kolmogorov-Smirnov goodness-of-fit test was used to determine which variations did not display significantly different distributions from normal (p < .05).

Whiteness improved in a majority of sites in all four treatment groups, ranging from 51% in Group 1 (Fig. 3) to 73% in Group 4 (Fig. 4).

Only Group 1 included any sites that worsened in color after whitening treatment (2.7%). On the other hand, only Groups 3 (p = .013) and 4 (p < .0001) showed significantly greater numbers of whiter sites relative to unchanged sites. Group 4 patients achieved a significantly greater shade improvement (Fig. 5) than patients in Groups 1 (p = .02), 2 (p = .01), or 3 (p = .01).

In the SEM analysis of F22 aligners after whitening agent application, the six samples showed no appreciable microstructural differences at magnifications 250×, 1,000×, or 5,000×. In every case, the filamentous appearance typical of the aligner material (Fig. 6) was hidden by a continuous layer of organic material, deposited during normal wear. Crystalline deposits of mineral salts were evident at higher magnifications (Fig. 7).

In the spectrophotometric analysis of nine aligners, the aligners maintained their transparency after 14 days of using either a hydrogen peroxide or carbamide peroxide whitening agent (Table 4). The amount of light passing through the aligner was greater than 80% at T1 and remained high at T2, as demonstrated by the mean transmittance curves for the visible wavelength range of 400-700nm (Fig. 8).

Discussion

Simultaneous orthodontic and whitening treatment is likely to be well received by patients, since it combines two major factors involved in smile esthetics.^10,19 This is particularly important because patient compliance plays a decisive role in the outcome of aligner treatment.

At-home whitening procedures can provide excellent results, often better than with in-office treatment.²⁰ Moreover, whitening agents that can be applied at home are associated with fewer adverse effects, as they generally contain lower concentrations of the active peroxide ingredient.²¹

After confirming that liquid whitening agents could be evenly distributed within the small space between F22 aligner trays and the teeth, we tested the efficacy of various concentrations of sprayed whitening agents. Our results confirmed other reports that the concentration of whitening agent and the length of time in contact with the teeth are the key factors.^22,23 We found that the greatest percentage improvement in whiteness was achieved with the application of 16% carbamide peroxide for nine hours per day over 14 days (Group 4).

SEM analysis indicated that the whitening agents had no effect on the F22 aligner material, because the microfilamentous structure was covered by a layer of organic material. Although some surface irregularities were visible at high magnifications, these appear to have been caused by the heat applied during the gilding process, rather than by chemical degradation. Striations that probably resulted from normal cleaning were also visible.