INTRODUCTION

To minimize potential damage to teeth during the removal of orthodontic brackets, the ideal location for mechanical failure is the adhesive–bracket interface, resulting in the bulk of the resin remaining on the tooth.^1–4 Subsequently, complete removal of adhesive is important as remnants can result in increased plaque accumulation, white spot lesions, periodontal inflammation, and discoloration.^5–8 To achieve complete and efficient removal of adhesive as well as produce minimal damage to enamel, dentists need to differentiate adhesive from enamel accurately and rapidly. This task has challenges as a result of the similarity in shade between resin and enamel, as documented in both orthodontic and restorative settings.^7,9–15

Fluorescence allows a substance to emit more visible light than it receives. Teeth naturally fluoresce when exposed to light sources containing ultraviolet (UV) wavelengths, projecting a quality of vitality and health.¹⁰ Fluorescent additives have been incorporated into resins to help mimic a natural tooth and also aid in detection of the resin. Illumination with the correct wavelength helps distinguish resin from enamel.^9–12,14,16

Multiple studies evaluated UV light as an adjunct in the detection of composite restorations in standard dental examinations and in forensic identification in postmortem odontograms.^9,11,12,14 A recent study by Ribeiro and associates¹⁷ evaluated the application of UV light in an orthodontic setting. This and other studies concluded UV illumination was a valuable adjunct when detection or complete removal of resin is warranted.^{9,11,12,14,17}

The aim of this study was to evaluate the effectiveness and efficiency of UV light versus white (W) light in the detection of florescent adhesive during orthodontic debonding. The null hypothesis was there were no significant differences in effectiveness and efficiency when using UV versus W light. Results unique to this study included the amount of time spent on adhesive removal, comparison of two adhesives, and use of scanning electron microscopy (SEM) for detailed assessment of adhesive removal.

MATERIALS AND METHODS

This in vitro study was approved by the University at Buffalo Institutional Review Board (Buffalo, N.Y.; no. 00001372). All experimental procedures were performed by one investigator (Dr Lai). A pilot investigation involved seven commonly used orthodontic adhesives to evaluate for inclusion (Table 1). Inclusion criteria were (1) manufacturer's indication for use as bracket adhesive for direct bonding and (2) fluorescence visible under UV illumination.

Table 1 Adhesives Evaluated for Study Inclusion

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To examine for fluorescence, the adhesives were illuminated with a 5 light emitting diode (LED) ultraviolet flashlight at 395 nm wavelength (INOVA, Nite Ize, Boulder, Colo). Only Pad Lock (P) and Opal Bond MV (O) met the inclusion criteria.

Teeth Selection

Extracted human premolars (n = 135) were obtained from a dental research facility and stored in 0.5% chloramine-T.^18,19 Inclusion criteria were premolars with sound buccal enamel and no caries or previous restorations when inspected visually under W and UV illumination. A total of 108 teeth met the inclusion criteria from which 80 were randomly selected for the four experimental groups (Figure 1).

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Sample Preparation

The teeth were mounted into a ModuPRO Endo module (Acadental, Oakland Park, Kans) using the maxillary left first molar socket. Each tooth's buccal surface was cleaned and polished with pumice using a rubber cup in a low-speed handpiece for 10 seconds, then rinsed and dried using an air-water syringe. The enamel was then etched with 35% phosphoric acid (Ultra-Etch, Ultradent, South Jordan, Utah) for 30 seconds, rinsed with water for 10 seconds, and air dried for 5 seconds and primer (Assure PLUS, Reliance Orthodontics, Itasca, Ill) was applied using a microbrush according to the manufacturer's instructions.

Premolar orthodontic brackets (Dentsply GAC, Islandia, N.Y.) were bonded to the teeth, half using P and the remainder using O. The bracket was pressed firmly onto the enamel surface, excess adhesive was removed with an explorer, and the adhesive light was cured for 3 seconds from occlusal, gingival, mesial, and distal aspects using the “extra power” setting of a VALO LED curing light (Ultradent). The teeth were then stored in a humid chamber at 37°C for 24 hours.

Brackets were debonded by compressing and distorting the bracket using a bracket removing plier (#098; Orthopli, Philadelphia, Pa). The amount of adhesive remaining was assessed with UV light under a stereomicroscope (Figure 2) and scored using a modified Adhesive Remnant Index (Table 2).²⁰ Only teeth with a score of 4 and 5 were included.

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Table 2 Modified Adhesive Remnant Index

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RESULTS

Intrarater Reliability

Intrarater reliability was excellent in three groups (ICC: P–W = 0.997, O–W = 1.000, O–UV = 0.904) and good in one (ICC: P–UV = 0.795; Table 3). Excellent overall reproducibility (ICC = 0.999) was also found when all 40 repeat measurements were combined.

Table 3 Intraclass Correlation Coefficient for Adhesive Remnant Measurements^a

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Surface Area of Adhesive Remnants

Median adhesive surface area ranged from 0.72 to 1.38 mm² in groups with W light and from 0.02 to 0.10 mm² in groups with UV light (Table 4, Figure 3). Data for surface area of adhesive remnants in three groups did not fulfill the assumption of normality and therefore the Kruskal-Wallis test was used (Table 5). Statistically significant differences in median surface area of adhesive remaining were found among the groups (P ≤ .001). Post hoc pairwise Mann-Whitney tests found significant differences between all pairings that compared a group with W light versus UV light, despite the adhesive type (Table 6). No significant differences were found between pairings of groups that used the same type of light but different adhesive.

Table 4 Descriptive Statistics for Surface Area of Adhesive and Time Per Group^a

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Table 5 Shapiro-Wilk Test for Surface Area of Adhesive and Time^a

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Table 6 Post Hoc Pairwise Mann-Whitney Test for Surface Area of Adhesive^a

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Time Needed for Adhesive Removal

Mean times for removal, in ascending order, were the following: O–UV = 33.4 seconds, P–UV = 36.1, P–W = 38.3, and O–W = 43.7 (Table 4). Data related to time was assessed with analysis of variance as the assumption of normality was upheld, and Levene's test confirmed homogeneity of variance (P = .074; Table 5). Statistically significant differences in mean time for adhesive removal were found among groups (P ≤ .001; Table 7). Post hoc Tukey HSD showed that O with W light was statistically significantly higher in mean time for removal than O and P both with UV light (Table 7). There was no significant difference in mean time between the two types of adhesives with either light source.

Table 7 Analysis of Variance for Time^a

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Correlation Between Time and Surface Area of Adhesive

A statistically significant negative correlation was found between time and surface area of adhesive remaining when P was used with W light (r = −0.707, P ≤ .001; Table 8). No other significant correlations existed in the remaining groups.

Table 8 Correlation Between Time and Surface Area of Adhesive^a

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DISCUSSION

The importance of complete adhesive removal following orthodontic treatment and the negative side effects that may occur if adhesive remains has been well documented.^5–8,15 Although studies have investigated various adhesive removal techniques that may yield the least damage to the tooth structure, few studies have suggested a technique that may aid in the detection of the adhesive.^24,25

Current findings showed that, with use of UV illumination, smaller amounts of adhesive remained relative to use of W light. These results agreed with the findings of Ribeiro and associates who also found adhesive to be more effectively removed when using UV light.¹⁷ Their study, also investigating Opal Bond MV as the fluorescent adhesive, found median adhesive remnants of 0.80 mm² with W light and 0.25 mm² with UV light.¹⁷ Their results using Opal Bond MV were similar to the current study when using W light, with a median of 0.72 mm², but current results were much lower under UV light, with a median of 0.02 mm². The specific wavelength and handling of the UV light during adhesive removal were not specified in the previous study; those factors and differences in the operators likely accounted for the variation in results.

Potential limitations in the current study included the wavelength of UV light used, which was 395 nm. Although this was determined to be the optimal wavelength for several composite resins, the ideal wavelength for each orthodontic adhesive is potentially different.^11,14

Under both light conditions, there was a trend for Opal Bond MV to have lower median adhesive remnant scores than Pad Lock. This result under UV light may have been a result of the stronger fluorescence intensity observed for Opal Bond MV. An explanation for the finding with W light could be that the reflective behavior of a thinner layer of Pad Lock camouflaged it with the reflective behavior of the enamel surface. In contrast, Opal Bond MV was observed to have a matte, chalky appearance during removal, allowing for easier distinction from enamel.

Regarding the efficiency of adhesive removal with the aid of UV light when compared with W light, the differences were statistically significant between Opal Bond MV with W light and Opal Bond MV with UV light, and Opal Bond MV with W light and Pad Lock with UV light. Although Pad Lock with W light had relatively low mean removal times, it had the highest mean surface area of adhesive remaining. As discussed previously, the reflective nature of Pad Lock may have resulted in camouflage, causing the operator to prematurely assume complete removal. Although the removal time of Pad Lock with W light was similar to Pad Lock with UV and Opal Bond MV with UV light, the objective to remove all adhesive was not accomplished as effectively without the UV light.

Other investigators have examined differences in removal efficiency among various methods of adhesive removal.^24,25 Of four studies that evaluated a tungsten carbide bur using high speed, mean times ranged from 5.26 to 10.18 seconds per tooth.^2,22,25,28 Mean times in this study were much higher, ranging from 33.4 to 43.7 seconds across groups. This difference may be attributable to variations in the adhesives and burs used and the Adhesive Remnant Index prior to adhesive removal. Another factor was the difference in operator technique. Oliver and Griffiths²⁹ found large differences in time for removal (65.9 seconds vs 191 seconds) when two operators with different experience levels were used. The current study used a single operator to eliminate operator variability.

Findings using SEM/energy-dispersive X-ray spectroscopy, where penetration of the electron beam has been shown to be 2 μm,²⁷ demonstrated that resin detectable under the stereomicroscope with UV light was >2 μm in thickness. The assessment also showed that enamel that appeared free of resin under UV light can have thin (<2 μm) adhesive present. No previous study has determined the minimum thickness of adhesive detectable by UV light that was found to be >2 μm in the current study. This study, however, did not investigate enamel damage after removal. If significantly more enamel is removed when using UV light, then such negative effects may outweigh the benefits.³⁰ Future research should be done to evaluate enamel damage.

Based on the findings of this study, the null hypothesis was rejected, and it is recommended that fluorescent adhesives be used in conjunction with UV LED illumination to ensure more complete adhesive removal. Additional advantages of UV LED illumination include portability, affordability, and ease of use intraorally.

CONCLUSIONS

• The use of UV light resulted in less fluorescent adhesive resin remaining on tooth surfaces when compared with W light.

• When Opal Bond MV was used with UV light, there was a statistically significant decrease in mean time spent on adhesive removal when compared with W light.

• There was no significant difference between the two fluorescent adhesives regarding effectiveness and efficiency under UV illumination.

• Fluorescent adhesives with a thickness <2 μm may not be detectable by UV illumination. For thicker remnants, UV light at 395 nm is a reliable method for adhesive detection.