INTRODUCTION

The adhesion between orthodontic resins and enamel is unique in dentistry in that it is intended to be temporary, yet durable enough to withstand orthodontic forces. Following completion of orthodontic therapy, the brackets and bonding resins must be removed with minimum trauma to the tooth and, ideally, without any resin remnants. Complete elimination of the residual adhesive resin attached to the enamel surface is mandatory to avoid prolonged accumulation of bacterial plaque that may further lead to decalcification and periodontal problems.1,2

Removal of the residual adhesive can cause physical changes on enamel, ranging from surface roughening to microscopic fractures.1,3–5 Eliades et al.4 reported that the color of the enamel is also affected by debonding and subsequent cleaning procedures. In addition to the effects of iatrogenic surface roughness, changes in the color of enamel may also result from discoloration of the residual resin that has irreversibly penetrated the surface despite the cleaning procedures.4,6,7 Resin residuals may change tooth color because of both the internal changes via physicochemical reaction of the adhesive resin and the external changes caused by superficial absorption of food pigments.6,7 However, even under laboratory conditions, it is virtually impossible to eliminate the entire adhesive residue on the enamel surface without the aid of strong magnification. Provided that this theoretical goal is accomplished, such complete reduction of resin-penetrated enamel may lead to considerable loss of sound tooth structure. Thus, availability of an orthodontic bonding agent with both the least discoloration potential and the ability to remove its residue by a simple protocol would be most desirable.

Despite a plethora of studies evaluating the bond strength of different orthodontic adhesives and surface alterations after bracket debonding,8,9 only a few studies have focused on possible changes in the color of enamel after treatment. In a clinical study, the color values were measured in patients before and after orthodontic treatment, and it was reported that the color of natural teeth changed after orthodontic treatment.10 Other laboratory studies have evaluated the discoloration of orthodontic adhesives11–15 and the color alterations on enamel after finishing and polishing.4 Results obtained in those studies are still inconclusive regarding the contributory effects of different protocols and resin removal techniques on the color changes of the enamel during treatment. Based on these considerations, the purpose of this study was to investigate the color alterations on enamel following the use of different orthodontic bonding adhesives and adhesive residue–removal burs.

MATERIAL AND METHODS

The material of the study consisted of freshly extracted premolars obtained from patients for whom extraction treatment had been indicated. The research protocol, the laboratory study protocol including the use of extracted human teeth, and the consent form were evaluated and approved by the Baskent University Institutional Review Board and Ethics Committee.

According to power analysis, 22 samples for each group were needed to obtain a statistical significance of at least a 0.2-unit difference in terms of ΔE (ΔE  =  total color difference) at 80% power and 5% error. Assuming possible specimen loss during the procedures, 25 teeth were assigned to each group. The teeth were obtained from patients aged 15 to 20 years, with the strict inclusion criteria of being sound, noncarious, and free of restorations, fractures, white spot lesions, and iatrogenic damage during extraction. All extracted teeth were immediately cleansed of tissues and debris and were stored in distilled water at room temperature until the experiments.

Adhesive Procedures

Of the 175 teeth, 25 served as control specimens. The remaining were randomly assigned to three experimental groups (n  =  50 each) with respect to the adhesive tested (Figure 1). The following adhesive protocols were employed:

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Group 1 (control)

The enamel surfaces were left untreated and were only subjected to color assessment before and after photoaging.

Group 2

Enamel was etched with 37% orthophosphoric acid for 15 seconds, rinsed with air-water spray for 20 seconds, and air dried for 10 seconds. Transbond XT Adhesive Primer (3M Unitek, Monrovia, Calif) was used in conjunction with Transbond XT Adhesive Resin (3M Unitek) according to the manufacturer's instructions for bonding metal brackets with 0.018-inch slots (Ormco; Sybron Dental Specialties, Glendora, Calif). Following removal of excess adhesive from the margins, light curing was performed with an LED source (3M Elipar S10; 3M Unitek) for 10 seconds.

Group 3

A self-etch adhesive system (Transbond Self-Etching Primer [SEP]; 3M Unitek) was used in conjunction with Transbond XT Adhesive Resin as with group 2.

Group 4

Following pretreatment of enamel surfaces with 20% polyacrylic acid, the brackets were bonded with light-cured resin-modified glass ionomer cement (RMGIC; Fuji Ortho LC; GC Corp, Tokyo, Japan). The cement was light cured for 20 seconds from both the mesial and distal aspects.

Before photoaging, the bonded specimens were kept in distilled water at room temperature for 24 hours (Figure 2).

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Color Assessment

Color assessment was performed three times in the experimental groups and two times in the control group. Measurements were made from the rectangular area in the middle of the adhesive tags using a handheld spectrophotometer (SpectroShade Micro; MHT, Verona, Italy; Figure 3). Before each measurement, the spectrophotometer was calibrated. For each specimen, three measurements were made in order to minimize the margin of error and calculate the average of three measurements. Color evaluation was made in accordance with the CIE (Commission Internationale de l'Eclairage) L*a*b* color system (lightness, red/green, and blue/yellow). The following formula was used for color comparisons:

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The Δ values calculated in this study were as follows:

ΔE 1: The difference between the values obtained at the beginning of treatment (before adhesive procedures) and after cleaning the surfaces (baseline-debonding). Clinically, this value indicates the color change throughout the orthodontic treatment.

ΔE 2: The difference between the values at the beginning of the treatment and the final values (baseline-ageing). Clinically, this value indicates the color change that occurred both during and after the orthodontic treatment.

ΔE 3: The difference between the values obtained after cleaning the surfaces and the final values (debonding-ageing), and clinically indicates the coloring that occurs after the treatment.

In the control group, color measurement was conducted at the beginning and after photoaging. Thus, only the ΔE 1 value was calculated. In the experiment groups, tooth colors were measured before adhesive procedures, after cleaning of the enamel surfaces, and after the second photoaging. Thus, three Δ values were calculated.

RESULTS

Eight teeth had enamel cracks after debonding and were excluded from the study. For each measurement, the ICC values indicating the reliability of the L, a, and b measurements repeated at T0, T1, and T2 were above 99% (Table 1). The median, interquartile range, and minimum-maximum ΔE values as well as the number and percentage of ΔE values that are over the clinical threshold (ΔE  =  3.7) are presented in Table 2.

Table 1. Intraclass Correlation Coefficients and 95% Confidence Intervals for Repeatability of the L, a, and b Measurements at T0, T1, and T2^a

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Table 2. The Median, Interquartile Range, and the Minimum and Maximum ΔE Values of the Groups^a

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The color change was similar for the ΔE 1 and ΔE 2 values in groups 2A, 3A, 3B, and 4A, and both were significantly higher than that of ΔE 3 (Table 3). There was no significant difference between the ΔE 1 and ΔE 3 values in group 2B, whereas the ΔE 2 value was significantly higher than that of ΔE 3. No significant difference was observed between the Δ values in group 4B (Table 3).

Table 3. Comparisons Between the ΔE 1, ΔE 2, and ΔE 3 Levels Within the Adhesive Systems and Cleaning Methods^a

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In both the etch-and-rinse and the SEP groups, the color change observed with the tungsten carbide bur was significantly higher than that obtained with the Stainbuster bur (Table 4), with the former being significantly higher than that of the control. Within the RMGIC subgroups, the amount of change for the ΔE 1 value was similar for both the cleaning methods and the control group (Table 4).

Table 4. Comparisons Between the Adhesive System and the Cleaning Method With Respect to the ΔE 1 (Baseline-Debonding) Value^a

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Cross-comparisons of the ΔE 1, ΔE 2, and ΔE 3 levels with respect to the adhesive systems and cleaning methods are presented in Table 5. Within the etch-and-rinse subgroups, the color change observed at the ΔE 1, ΔE 2, and ΔE 3 levels in subgroup 2A were significantly higher than those of 2B. Within the SEP subgroups, the color change ΔE 1 and ΔE 2 levels in subgroup 3A were significantly higher than those of subgroup 3B. However, there was no significant difference between the subgroups at level ΔE 3. Within the RMGIC subgroups, the color change at level ΔE 3 in group 4B was higher than that of subgroup 4A. However, there was no significant difference between the subgroups at levels ΔE 1 and ΔE 2 (Table 5).

Table 5. Cross-Comparisons of the ΔE 1, ΔE 2, and ΔE 3 Levels With Respect to the Adhesive Systems and Cleaning Methods^a

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DISCUSSION

The CIE L*a*b* system is considered to be the standard color space, and the mathematical magnitude of color changes is indicated as ΔE.17,18 The human eye has restricted capability to see such differences and cannot perceive ΔE values below 1.19 The ΔE values between 2 and 3.7 represent the clinically perceivable but acceptable range of differences. It has been reported that ΔE values of 3.7 and higher cannot be accepted under clinical conditions.20 Therefore, as with previous studies, the ΔE threshold value was accepted as 3.7 units herein.4,10–15

Enamel discoloration after orthodontic treatment is often overlooked in daily practice. According to Karamouzos et al.,10 the optical characteristics of enamel are changed during orthodontic treatment, with the color change being affected by several factors. External coloring occurs as a result of superficial absorption of food pigments, while internal coloring occurs during aging. In the present study, photoaging was performed to stimulate internal coloring. A color change above the threshold (ie, ΔE  =  3.7) observed in 56% of the control specimens confirms the efficacy of this method. As explained previously,14,15,21 the lack of saliva, the food coloring, and the inability to simulate the mechanic abrasion caused by brushing are the limitations of this methodology. In the present study, the magnitude of color change was greater during orthodontic treatment than after treatment, with the latter being below the threshold. These findings corroborate those of Eliades et al.4 and Jahanbin et al.,15 who observed the highest color change after debonding.

Tungsten carbide burs are known to produce minimal damage to the enamel during removal of adhesive residue.22–24 Fiber-reinforced composite burs require a longer time to remove adhesive remnants after debonding but may offer the advantage of providing a smoother surface compared to tungsten carbide.14,25 In the present study, the total color changes observed in the etch-and-rinse and SEP groups were significantly lower in the Stainbuster subgroups than in the tungsten carbide group. This finding can be explained by the fact that Stainbuster burs provide a smoother enamel surface and increase light reflection.14,25

In the present study, the finding that the etch-and-rinse/tungsten carbide bur group exhibited the highest total color change can be explained by the occurrence of thicker and longer resin extensions formed after phosphoric acid etching.26 Accordingly, this combination may not be suitable for clinical use. Compared to the etch-and-rinse adhesive, the lesser extent of total color change observed in the self-etch/tungsten carbide bur combination strongly suggests shorter resin extensions are produced by the SEP,27–29 which in turn causes less discoloration.30 As for the RMGIC, the changes observed after adhesive removal by both types of burs were low, confirming that the RMGIC did not produce resin extensions and that the bond between the enamel and the cement was predominantly chemical rather than micromechanical.26 However, with regard to the total change (ie, discoloration after the orthodontic treatment), it was interesting to observe that the RMGIC/Stainbuster group exhibited the only value among the test groups that was above the clinic threshold (ΔE  =  3.7). This unexpected result suggests that RMGIC residuals that are invisible to the eye at clinical magnification may still exist on the enamel surface after cleaning. Coupled with the unfavorable color stability of RMGIC,31 these residuals may lead to clinically unacceptable discoloration values as observed herein. Further studies using elemental analysis should be carried out to confirm this assumption. However, based on the present results, the use of Stainbuster burs cannot be recommended for removal of RMGIC.

CONCLUSIONS

• Discoloration occurs on enamel during and after orthodontic treatment. Both the orthodontic adhesive systems and the burs used to remove their residuals on tooth surfaces are responsible for this effect.

• The highest color change was observed in the etch-and-rinse/tungsten carbide bur group. This combination is not recommended for clinical use in terms of enamel discoloration.

• When the brackets are bonded with the etch-and-rinse or SEP systems, cleaning the adhesive residuals with Stainbuster burs is recommended for minimal color change. For the RMGIC, tungsten carbide burs may provide less enamel discoloration in the long run.