INTRODUCTION

Careful examination of teeth with an intense light source can often reveal microcracks in the enamel. These enamel microcracks (EMCs) have generally been attributed to the abnormalities in the maturation process, occlusal forces, temperature variations, and restorative processes.^1,2 As previous studies have shown, EMCs can be noticed after debonding at the end of orthodontic treatment.^3–5 While small EMCs may not result in tooth fracture, over prolonged periods their growth can be detrimental as they may serve as sites for demineralization and increase susceptibility to carious lesions.^3,6,7

Nowadays, patients have high esthetic demands and pay more attention to the possible enamel damage that takes the form of EMCs after debonding.^8,9 Enamel irregularities and visible EMCs are often noticed by patients at the beginning of orthodontic treatment and thus arise questions regarding the wisdom of bonding brackets on such teeth. Using ceramic brackets (CB) causes more concern, because the physical properties of ceramics such as hardness, high bond strength, and low fracture toughness or brittleness have led to many reports of irreversible enamel surface damage during the removal procedure.^10–12 Thus, as patient awareness is growing and documentation of EMCs is difficult, it is important to develop an understanding of the effect of debonding metal brackets (MB) and CB on visible EMCs and those EMCs that can be visualized only under scanning electron microscopy (SEM).^3,12

Many attempts have been made to assess enamel damage after bracket removal.^8,12–16 Several investigations have taken a deeper view to evaluating various EMC characteristics.^5,9,17–19 However, there are no reports on a consistent examination of the effect of debonding teeth having visible EMCs.

Therefore, the purposes of this study were to (1) compare the characteristics (location, length, and width) of EMCs having varying degrees of severity before and after removal of MB and CB, (2) ascertain whether there is a correlation between the original dimensions of EMCs and the likelihood of their increasing during debonding followed by RAR, (3) determine whether EMCs visibility is of any prognostic value, and (4) determine whether invisible EMCs might progress to visible ones.

MATERIALS AND METHODS

Tooth Selection and Enamel Surface Evaluation

The teeth were prepared in accordance with the guidelines of the International Organization for Standardization.²⁰

Ninety freshly extracted maxillary premolars were included in the final study. They had been extracted for orthodontic reasons and were used with the patients’ informed consent. Selection criteria for the teeth are listed in Table 1. The extracted teeth were decontaminated in 0.5% chloramine-T solution and then stored in distilled water that was changed weekly before preparation and testing.

Table 1. Teeth Selection Criteria

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The sample size was estimated using the sample size calculator,²¹ by which a sample size of 90 was required to detect differences with a 5% confidence interval (CI) and 90% confidence level (population size, 133).

The study was performed according to the protocol presented in Figure 1.

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The buccal enamel surfaces of all the teeth included in the study were evaluated using SEM (TM-1000, Hitachi, Tokyo, Japan).¹⁹ The SEM was operated at 15 kV, at ≤5 × 10⁻² Pa (electron gun vacuum) and at ≈ 30–50 Pa (specimen chamber vacuum). The teeth were not coated with a conductive layer prior to SEM examination. The initial evaluation of EMCs was performed at ×50–×100.

Examination of the buccal enamel surface is shown in Figure 2. Montages (stitching together of multiple images) of the SEM micrographs were made to reconstruct images of some larger crowns. From these micrographs the height (h) of each crown was measured. For detailed mapping of the EMCs, the buccal surface was divided into three zones of equal height: first zone—cervical third, second zone—middle third, third zone—occlusal third.^9,12,19 After direct inspection by the naked eye under normal room illumination and initial examination with the SEM, the teeth were divided into three groups of 30: group 1, teeth having pronounced EMCs; group 2, teeth showing weak EMCs; and group 3, a control group (comprised of an equal number of teeth with pronounced and weak EMCs) to study the effect of dehydration on existing EMCs. EMCs were classified into pronounced (visible with a naked eye under normal room illumination) and weak (not apparent to the naked eye under normal room illumination but visible under SEM based on their visibility).^3,12

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Using a digitally sketched ruler, each zone was divided into 10 measurement areas (MAs); a total of 30 MAs of every tooth was obtained (Figure 2). Using our derived formula, a measurement step (x, the distance between two MAs) was quantified. Despite the existing number of EMCs, one, the longest, was chosen and analyzed in detail. The width of the longest EMC in each zone was measured (10 MAs of the width could be registered in each zone). The length of the longest EMC was calculated (Figure 2).

The location (cervical, middle, and occlusal third), length, and width of the longest EMC in groups 1 and 2 were determined before and after debonding. The width of the longest EMC was examined in the same segment before and after bracket removal, regardless of changes in its length.

In group 3 (control group), the teeth were subjected to the same analysis but without bonding. All the teeth from group 3 were examined twice by SEM, as were the other specimens after the same time and means of storage. All evaluations were performed by the same examiner.

Bonding Procedure

The teeth from groups 1 and 2 were randomly assigned to one of two subgroups, using the lottery method. Each tooth was assigned a unique number. The numbers were placed in a bowl and thoroughly mixed. Without looking, the researcher selected 15 numbers for subgroup 1, and the rest were assigned to subgroup 2. The teeth that were assigned those numbers were then included in the sample. The procedure was performed twice for groups 1 and 2 separately.

In subgroup 1, 15 teeth from groups 1 and 2 were bonded with maxillary premolar MB (Discovery; Dentaurum, Ispringen, Germany). In subgroup 2, 15 teeth from groups 1 and 2 were bonded with maxillary premolar CB (Clarity; 3M Unitek, Monrovia, Calif). Bracket characteristics and bonding procedures are presented in Table 2. After bonding, the specimens were placed in distilled water at 37°C and stored for 24 hours prior to testing.²⁰

Table 2. Bracket Characteristics, Bonding and Debonding Procedures

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RESULTS

Assessing the visibility of the EMCs was performed repeatedly by the same investigator three times every second day. For standardization, the same location, time of day, and tooth position were chosen. Following repeated EMC visibility assessments, no significant discrepancies between results were observed.

Measurement errors were analyzed using a method suggested by Bland and Altman.²² The enamel surface was reimaged and the measurements repeated for 10 teeth. There was 100% agreement between two length measurements of EMCs. The mean of the differences between two width measurements was 0.073 µm, while the limits of agreement were −0.459 and 0.606, indicating that 95% of the differences between these two measures, excepting the six measures, were within this range (Figure 3).

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The mean widths and lengths of pronounced and weak EMCs before and after MB removal are presented in Table 3. Pronounced EMCs showed higher L_overall before and after debonding (P > .05) compared with weak ones. W_overall was greater for pronounced EMCs only after removal (P > .05; Figure 4). Width values increased for both types of EMCs following debonding, although a significant result was found only in the pronounced EMCs group (0.57 µm; P < .05). The greatest width increase in the pronounced EMCs was observed in the first zone (cervical third; 0.72 µm; P > .05), followed by the third zone (occlusal third; 0.58 µm; P < .05). In the weak EMCs group, the extent of width increase did not differ between zones. Following MB debonding, none of the weak EMCs progressed to visible ones.

Table 3. Width and Length of Pronounced and Weak Enamel Microcracks (EMCs) Before Bonding and After Removal of Metal Brackets (MB)^a

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Descriptive statistics of the width and length of pronounced and weak EMCs before and after debonding CB are shown in Table 4. Pronounced EMCs possessed significantly greater W_overall and L_overall (P < .05; Figure 5). However, we recorded the same amount of increase in W_overall after removal for both types of EMCs (0.30 µm; P > .05). After debonding, the difference in width was greatest in the third zone (occlusal third) for pronounced EMCs (0.57 µm; P < .05) and the first zone (cervical third) for weak EMCs (0.59 µm; P < .05). Following removal of the CB, four (26.67%) weak EMCs progressed to pronounced ones.

Table 4. Width and Length of Pronounced and Weak Enamel Microcracks (EMCs) Before Bonding and After Removal of Ceramic Brackets (CB)^a

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Both types of EMCs showed higher W_overall (P < .05 for pronounced and P > .05 for weak EMCs) and L_overall (P > .05) before and after debonding CB compared with MB (Figures 6 and 7).

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A less-than-moderate negative correlation between EMCs width and their increase during debonding followed by RAR was found (Spearman rho  =  −.310; P < .05). No correlation between the length of EMCs and their progress with bracket removal was observed (Spearman rho  = −.053; P > .05).

Changes in width and length measurements of EMCs for group 3 are given in Table 5. Differences in W_overall and L_overall of pronounced and weak EMCs were quite small and nonsignificant.

Table 5. Width and Length of Pronounced and Weak Enamel Microcracks (EMCs) for Group 3^a

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DISCUSSION

Since the purpose of this investigation was to evaluate in detail qualitative EMCs characteristics, the model of an in vitro study was chosen. It allowed us to collect the proper sample size, and analyze and make direct precise measurements of the location, length, and width characteristics using SEM.

The findings of this study showed that visible EMCs possessed higher W_overall and L_overall compared with weak EMCs (except for the width measurement before bonding MB). Although we found no investigation addressed to this specific subject (pronounced vs weak EMCs), we calculated that the W_overall and L_overall of both types of EMCs were lower than those found in other studies (evaluating the dimensions of EMCs without considering their visibility).^17–19

A different amount of EMCs increase was noticed not only comparing pronounced and weak EMCs, but also in analyzing the effect of debonding MB and CB. Both easily visible EMCs and weak ones showed higher W_overall and L_overall before and after CB removal. The greatest difference was observed in the pronounced EMCs group. However, investigations evaluating the impact on the enamel of debonding MB and CB have not yet come to one common conclusion regarding which bracket type could lead to the greatest tooth structure defects.^5,12

Following bracket removal, the W_overall of all EMCs increased. Changes for pronounced EMCs were 0.57 µm with MB and 0.30 µm with CB; for weak EMCs, − 0.32 µm with MB and 0.30 µm with CB. The apparent tendency of the increase in the width parameter after debonding is consistent with the findings of the previously published study.¹⁹ However, it is important to emphasize that bond strength in vitro is higher than that in vivo (because of the oral humidity, etc), thus the increase in EMCs may be greater than in a clinical situation.²³

The lack of statistically significant differences in width and length between weak and pronounced EMCs before bonding poses the question, what makes EMCs visible? Experimental evaluation of the depth parameter of these two groups of EMCs using a confocal optical profilometer (COP; Sensofar PLµ 2300, Barcelona, Spain) did not show significant differences, either. Thus, such findings suggest that not the EMC morphology (length, width, or depth) determines its visibility, but the instrument used for evaluation (SEM, COP, or human eye). The latter possesses all the features of the above-listed devices—although having a much lower spatial resolution—plus cognitive averaging. Therefore, solid and unbroken EMCs were visible in contrast to fragmented or branched.

Further examination of the results revealed that the highest increase in the width of pronounced EMCs was observed in the cervical third (0.72 µm) for MB and occlusal third (0.57 µm) for CB. The greatest changes in the width for weak EMCs were found in the cervical third (0.59 µm) with CB but did not differ between the zones using MB (0.31 µm). These results show that forces during debonding are more concentrated on the cervical portion of the buccal tooth surface, followed by the occlusal and middle thirds.^13,15,19 Different enamel quality in these zones might be the reason why the location on the enamel surface has an effect on EMCs. Thus, a greater EMC increase in the cervical area after debonding followed by RAR may be due to a thinner enamel layer.²⁴

Analyzing the changes in the length parameter, we noted that the increase in L_overall was found only after removal of the CB from the teeth having pronounced EMCs. There are conflicting results in the literature regarding the effect of debonding on the length of EMCs. Whereas some studies reported an increase of this measurement after MB and CB removal,^17,18 others found no significant difference following debonding,²⁵ or even a decrease in L_overall on MB removal.¹⁹

CONCLUSIONS

• Teeth having pronounced EMCs showed higher W_overall and L_overall before and after debonding followed by RAR compared with weak EMCs.

• Greater width and length values were noticed using CB for both types of EMCs compared with MB.

• During debonding CB, 26.67% of invisible EMCs progressed to visible ones.

• Following bracket removal, W_overall in all cases increased, but the teeth having pronounced EMCs were not predisposed to a greater EMCs increase after debonding followed by RAR.

• Visibility of EMCs before bonding is of low prognostic value for predicting EMCs increase after bracket removal.