The Influence of Thermocycling Testing on Enamel Microcracks following the Metal Orthodontic Brackets Debonding

Abstract

Enamel microcracks (EMCs) arising during the removal of metal orthodontic brackets represent a considerable challenge in dentistry. This in vitro study aims to explore the impacts of thermocycling, the types of orthodontic bonding agent, and curing techniques on the enamel surface of the tooth structure following the debonding of orthodontic metal brackets. It also examines the incidence, number, length, and direction of EMCs on the buccal surface of the tooth. Additionally, the study compares adhesive remnant index (ARI) scores and bracket failure post-debonding. Forty extracted human maxillary canines were divided into ten groups, including intact enamel negative controls (groups 1, 2) and groups (3–8) with metal brackets bonded using two different bonding agents and curing techniques. Following bonding, half of the groups underwent thermocycling testing. EMCs, ARI scores, and bracket failure modes were evaluated. The formation, length, and direction of cracks did not significantly differ among groups, regardless of experimental conditions. Thermocycling had no significant effect on ARI scores or bracket failure modes. However, significant variations were observed among curing technique groups, with seventh-generation bonding agents demonstrating potential effectiveness in achieving complete adhesive removal. The study underscores the importance of considering bonding agent systems and curing protocols to optimize bond outcomes and minimize the risk of metal bracket failure in orthodontic treatment.

1. Introduction

Orthodontic brackets, pivotal in levelling and aligning teeth, have evolved significantly since their inception. Initially, the gold standard involved bonding brackets onto stainless-steel bands, transitioning in the late 1970s to direct bonding onto enamel, which became the predominant method. Subsequent modifications aimed to enhance bond strength through base design alterations and adhesive use, facilitating the effective application of orthodontic forces. The rising demand for aesthetic solutions fueled advancements in bracket design and bonding techniques [1].

The process of bonding orthodontic brackets has undergone significant advancements, largely driven by innovations such as enamel acid-etching, pioneered by Buonocore in 1955, which marked a pivotal moment in adhesive dentistry [1,2]. Since the advent of orthodontic bracket bonding by Newman, various bonding agents have been developed to enhance adhesion strength and precision [3]. One notable advancement is the introduction of light-cured materials, initially described by Tavas and Watts, which enable precise bracket placement and ensure a robust bond at the enamel-composite interface [4].

Debonding is the procedure of removing the brackets, along with adhesive resin from the tooth surface. Various methods exist, each with its potential impact on enamel integrity. Factors such as bracket type, bonding agent type, and debonding instruments influence the extent of enamel damage. Metal or stainless-steel brackets have shown less risk of enamel fracture than ceramic brackets [5].

Evaluating the adhesion quality between the composite, tooth, and bracket base is crucial. This assessment is commonly conducted using the Adhesive Remnant Index (ARI) method, which provides insights into the distribution and strength of adhesive remnants following bracket removal [6,7]. The integrity of the bonds between metal–composite and composite–enamel are crucial for treatment efficacy and patient oral health. These bonds must withstand orthodontic forces and intraoral stresses throughout treatment [8,9]. However, conducting in vitro studies to assess these adhesive systems presents challenges. Thermocycling involves subjecting materials to alternating temperature variations to simulate oral conditions and assess their durability [10,11,12]. It is a laboratory method that exposes dental materials and teeth to ranges of temperatures comparable to those occurring in the oral cavity, which could result in adverse consequences such as damage or weakening of the materials due to various coefficients of thermal expansion between the tooth structure and the filling material. During these cycles, thermal stresses may impact the bond strength between the repairing or filling material and the tooth structure [13,14,15,16]. Thermocycling is considered one of the most severe thermal environments [17]. Adhesive failure can occur at any interface between the bracket, adhesive, and enamel or at a number of different sites. Factors influencing detachment site include the tooth preparation technique (curing technique), bonding material type, and debonding method [1,8].

Previous research [8,18,19] evaluated the effect of the removal of different types of brackets on tooth enamel. Enamel microcracks (EMCs), often overlooked but crucial, can result from debonding procedures, potentially leading to long-term complications, including tooth fracture, demineralization, and esthetic concerns [20].

Given the critical role of metal bracket bonding and subsequent debonding in orthodontic procedures, this study aimed to investigate the impact of thermocycling, orthodontic bonding agents, and curing techniques on the enamel layer following metal bracket debonding. While previous research has explored the influence of different bonding agents and curing methods on EMCs, limited attention has been given to thermocycling tests as an evaluation parameter. Therefore, this study seeks to fill this gap by incorporating thermocycling tests to assess the incidence, number, length, and direction of EMCs. Understanding these effects is crucial for optimizing orthodontic procedures, minimizing enamel damage, and improving treatment outcomes.

2. Materials and Methods

2.1. Study Setting and Sample

The study was performed at the Advanced Technology of Dental Research Laboratory (ATDRL), Faculty of Dentistry, King Abdulaziz University, Jeddah City, Kingdom of Saudi Arabia. Forty healthy extracted human upper canine teeth were procured under the specifications certified by the Jazan University ethical panel and in congruence with the guidelines of the Helsinki Declaration. These teeth, confirmed to be clinically free from caries, cracks, or fractures, were included in this study. While in the case of the presence of carious lesions, hypoplastic lesions, restoration, or fracture, teeth were excluded.

2.2. Sample Allocation

Each canine was cleaned with tap water and then stored in a 10% formalin solution at room temperature. Specimens were then assigned a number and randomly allocated into one of the ten groups using Random Allocation Software, Version 1.0 (Saghaei & Kazemi, Esfahan, Iran) computer software.

2.3. Samples Preparation and Intervention

The teeth were randomly divided into ten equal groups (n = 4) as follows:

Group 1 (G1): Intact enamel (no orthodontic brackets) as the negative control group without thermocycling test.

Group 2 (G2): Intact enamel (no orthodontic brackets) as the negative control group with thermocycling test.

Group 3 (G3): Buccal enamel surface bonded with metal orthodontic bracket by 5th generation bonding agent using pre-curing (curing technique 1) and adhesive composite cement without thermocycling test.

Group 4 (G4): Buccal enamel surface bonded with metal orthodontic bracket by 5th generation bonding agent using co-curing (curing technique 2) and adhesive composite cement without thermocycling test.

Group 5 (G5): Buccal enamel surface bonded with metal orthodontic bracket by 5th generation bonding agent using pre-curing (curing technique 1) and adhesive composite cement with thermocycling test.

Group 6 (G6): Buccal enamel surface bonded with metal orthodontic bracket by 5th generation bonding agent using co-curing (curing technique 2) and adhesive composite cement with thermocycling test.

Group 7 (G7): Buccal enamel surface bonded with metal orthodontic bracket by 7th generation bonding agent using pre-curing (curing technique 1) and adhesive composite cement without thermocycling test.

Group 8 (G8): Buccal enamel surface bonded with metal orthodontic bracket by 7th generation bonding agent using co-curing (curing technique 2) and adhesive composite cement without thermocycling test.

Group 9 (G9): Buccal enamel surface bonded with metal orthodontic bracket by 7th generation bonding agent using pre-curing (curing technique 1) and adhesive composite cement with thermocycling test.

Group 10 (G10): Buccal enamel surface bonded with metal orthodontic bracket by 7th generation bonding agent using co-curing (curing technique 2) and adhesive composite cement with thermocycling test.

2.4. Bonding Procedures

The metal orthodontic brackets (0.022-inch slot, Roth, standard, with 3-4-5 hooks, Eterfant, CA, USA) were bonded to the mid-buccal surface of the teeth by using a bracket holding tweezer.

For this purpose, two different types of bonding agents were used as the following:

• Bonding agent (5th generation): Coltene One Coat Bond Sl. All the teeth used this bonding agent by applying 37% phosphoric acid gel (3M/Unitek, Monrovia, CA, USA) for 30 s as acid etching,
• Bonding agent (7th generation): Kulzer Heraeus Gluma Bond Universal Bottle (Kulzer GmbH, Hanau, Germany), and one adhesive light curing orthodontic cement (orthocem, FGM DENTAL Group, Fort Lauderdale, FL, USA).

After exerting gentle pressure to fit the brackets to the teeth properly and removing excess adhesive resins around the brackets, curing was performed with the same device for 3 s.

2.4.1. Primer Pre-Curing (Curing Technique No. 1)

In the groups which required pre-curing (Groups III, V, VII, and IX), the recommended primer was cured for 3 s on the buccal surface of the tooth individually, and then the adhesive light-curing orthodontic cement was applied to the base of metal brackets (0.022-inch slot, Roth, standard, with 3-4-5 hooks, Eterfant, USA) and positioned on the buccal surface of the tooth. Excess bonding material was carefully removed, and the mesial and distal sides of buccal surface were light cured for 3 s each using an H LED curing unit (CORDLESS LED, Woodpecker, Foshan, China).

2.4.2. Primer Co-Curing (Curing Technique No. 2)

In the groups in which co-curing was planned (Groups IV, VI, VIII, and X), the primer was applied on the tooth surface and air-thinned, followed by adhesive light curing orthodontic cement, metal bracket (0.022-inch slot, Roth, standard, with 3-4-5 hooks, Eterfant, USA) placement, and removal of excess material. The primer and adhesive resin cement were cured together simultaneously at the same time on both the mesial and distal aspects of the buccal tooth surface using an H LED curing unit (CORDLESS LED, Woodpecker, China).

Additionally, all specimens were stored in distilled water at 37 C for 24 h. Then, 5 groups were subjected to the thermocycled (SD Mechatronic, Feldkirchen-Westerham, Germany) (Figure 1) for 5000 cycles within the temperature range of 5–55 °C with the dwell time of 30 s at each temperature, while the other 5 groups were investigated directly under the stereomicroscope (MEIJI, EMZ-13TRD, MEIJI Techno, Saitama, Japan) (Figure 2) at a magnification of 70× without thermocycling testing. All teeth were prepared and bonded by the same operator.

2.5. Debonding Procedures

All the groups with brackets were debonded by using a stainless steel angled bracket remover orthodontic dental plier (Surgicalonline manufacturer, New York, NY, USA). The residual adhesive on the enamel surface of each tooth was carefully removed with a porcelain resin polishing stone using a slow-speed handpiece for the evaluation of EMCs after the orthodontic brackets debonding.

2.6. Outcome Assessment

After debonding, the enamel surface of each tooth and the brackets were examined using a stereomicroscope to examine the type of failure (Figure 3). In addition, the type of bracket failure was measured using the Adhesive Remnant Index (ARI) Artün and Bergland–1984 [21]. This index was divided into four grades as follows:

0 = when no adhesive remains on the tooth surface.

1 = when less than half of the adhesive remains on the tooth surface.

2 = when more than half of the adhesive remains on the tooth surface.

3 = when the entire adhesive remains on the tooth surface.

The bracket/adhesive interface can be considered the most favorable failure site for safe debonding, leaving most of the adhesive on the enamel surface, as seen in scores 2 and 3. This interface can be considered safe, since there is less chance of enamel fracture.

The mode of failure and any bracket fracture were also examined visually. Then, the buccal surfaces were examined under the stereomicroscope at a magnification of 70×. The incidence, number, direction, and length of enamel cracks were recorded and compared between the groups after the debonding. The directions of the enamel cracks were classified as vertical, oblique, horizontal, and mixed.

2.7. Data Management and Analysis:

The data analysis procedure involved several steps to examine the collected data systematically. Initially, descriptive statistics were calculated to summarize the characteristics of the variables, including frequencies, percentages, means, and standard deviations. Normality testing, employing the Shapiro–Wilk test, was conducted to assess the distribution of the variables. Following this, group comparisons were performed using the Mann–Whitney U test for the comparison between thermocycling and non-thermocycling groups, as well as the Kruskal–Wallis test for comparing different curing technique groups. The chi-square test was employed to assess the association between two categorical variables: the presence or absence of certain characteristics (e.g., vertical, horizontal, or oblique position) and the groups (thermocycling vs. non-thermocycling) for each curing technique. Post hoc tests, Tukey, were carried out to determine specific group differences when significant differences were detected. p-value (<0.05) was adopted as the level of significance. Finally, the statistical package for the social sciences (SPSS) software (SPSS, V.23, IBM Corp, Armonk, NY, USA) was utilized for data analysis.

3. Results

The Shapiro–Wilk test was employed to assess the normality of various variables. Results presented in

Table 1. Normality testing results for various variables.

Items	Shapiro–Wilk
	Statistic	df	p-Value
Incidence of cracks	0.634	40	0.000 **
Adhesive Remnant Index (ARI) scores (0)	0.621	16	0.000 **
Adhesive Remnant Index (ARI) scores (1)	0.591	16	0.000 **
Adhesive Remnant Index (ARI) scores (2)	0.273	16	0.000 **
Adhesive Remnant Index (ARI) scores (3)	0.172	32	0.000 **
Mode of bond failure for metal bracket: Mixed	0.632	32	0.000 **
Mode of bond failure for metal bracket: Bracket/Adhesive	0.172	32	0.000 **
No failure–Success (100%)	0.637	16	0.000 **

** Significant at a significance level of 0.01.

indicate that all variables, including the incidence of cracks, ARI scores (0, 1, 2, and 3), mixed mode of bond failure for metal bracket, bracket/adhesive mode of bond failure for metal bracket and no failure–success (100%), demonstrated statistically significant deviations from normality (p < 0.01). These findings suggest that the assumption of normality is violated for these variables, necessitating caution when employing parametric statistical tests that assume normality. Non-parametric alternatives may be more appropriate for analyzing these variables to ensure accurate and reliable results.

The analysis of crack incidence within the thermocycling and non-thermocycling groups yielded interesting findings regarding the presence or absence of cracks among different experimental conditions (

Table 2. Comparison of incidence of cracks in thermocycling and non-thermocycling groups with various curing techniques.

Group		Incidence of Cracks		p-Value
		Absent	Present
		n(%)	n(%)
Thermocycling Groups	Group 2: Control	4(100)	0(0.0)	0.077 a
	Group 5: G5_Curing technique1	0(0.0)	4(100)
	Group 6: G5_Curing technique2	2(50)	2(50)
	Group 9: G7_Curing technique1	1(25)	3(75)
	Group 10: G7_Curing technique2	2(50)	2(50)
Non-Thermocycling Groups	Group 1: Control	4(100)	0(0.0)	0.111 a
	Group 3: G5_Curing technique1	2(50)	2(50)
	Group 4: G5_Curing technique2	1(25)	3(75)
	Group 7: G7_Curing technique1	2(50)	2(50)
	Group 8: G7_Curing technique2	4(100)	0(0.0)
p-value	0.289 b

^a Kruskal–Wallis test was conducted; ^b Mann–Whitney U test.

). In the thermocycling groups, while group 2: control group demonstrated no incidence of cracks, the experimental groups exhibited varying degrees of crack formation. However, the differences were not statistically significant (p = 0.077). Notably, group 5: g5_curing technique1 showed 100% cracks. Conversely, group 6: g5_curing technique2 and group 10: g7_curing technique2 displayed a 50% incidence of cracks, indicating a moderate susceptibility to crack formation with these curing techniques. In contrast, group 9: g7_curing technique2 showed a 75% incidence of cracks, suggesting a higher susceptibility to crack formation than the other experimental groups.

On the other hand, in the non-thermocycling groups, crack formation was observed in some group 1: control and experimental samples, with no statistically significant differences observed across the groups (p = 0.111). In group 2: the control group, no cracks were observed, indicating a lack of crack formation under standard conditions. However, varying degrees of crack formation were evident among the experimental groups. For instance, group 3: g5_curing technique1 and group 7: g7_curing technique1 exhibited a 50% incidence of cracks, suggesting a moderate susceptibility to crack formation with these curing techniques. In contrast, group 4: g5_curing technique2 displayed a higher incidence of cracks at 75%, indicating a greater susceptibility to crack formation than the other experimental groups. Interestingly, group 8: g7_curing technique2 mirrored group 2: control group, with no observed cracks, suggesting a potential protective effect or minimal impact on crack formation with this curing technique.

In comparing crack formation between thermocycling and non-thermocycling conditions, the p-value of 0.289 indicates no significant difference in crack incidence between the two groups.

Figure 4 showcases the occurrence of crack formations (crack1, crack2, and crack3) in various groups under thermocycling and non-thermocycling conditions, alongside the corresponding p-values for each comparison. For instance, in the thermocycling groups, the counts for crack formations were consistent across different curing technique groups, with p-values of 0.373, 0.501, and 0.626 for crack1, crack2, and crack3, respectively, indicating no significant differences between the counts within each group. Similarly, in the non-thermocycling groups, comparable results were observed with p-values, suggesting no significant variations in the occurrence of crack formations across different groups.

In comparing crack numbers between thermocycling and non-thermocycling conditions, the obtained p-values of 0.375, 0.536, and 0.791 for crack1, crack2, and crack3, respectively, indicate no significant difference in crack number between the two groups.

Moreover, the assessment of crack length in dental materials across thermocycling and non-thermocycling conditions revealed notable findings (Figure 4). In the thermocycling groups, diverse crack lengths were observed for the first crack, with group 5: g5_curing technique1 displaying a mean length of 31.04 ± 4.56, and group 6: g5_curing technique2, group 9: g7_curing technique2, and group 10: g7_curing technique2 exhibiting mean lengths of 28.95 ± 0.20, 29.57 ± 1.05, and 25.71 ± 5.08, respectively. However, statistical analysis did not reveal significant differences among these groups (p = 0.655). Similarly, no significant differences were observed in the second crack lengths between thermocycling groups (p = 0.221), with group 5: g5_curing technique1 recording a mean length of 29.50 ± 0.23 and group 10: g7_curing technique2 at 31.04 ± 0.0. However, for the third crack, only group 5: g5_curing technique1 showed a mean length of 21.44 ± 0.0, while data for the remaining groups were unavailable due to no record of any cracks formed after the orthodontic debonding with thermocycling.

In the non-thermocycling groups, group 3: g5_curing technique1, group 4: g5_curing technique2, and group 7: g7_curing technique1 exhibited mean crack lengths of 27.14 ± 2.38, 31.28 ± 2.35, and 24.52 ± 11.85, respectively, with no significant differences observed between these groups (p = 0.555). However, data for the second and third cracks length was unavailable for group 3: g5_curing technique1, group 4: g5_curing technique2, and group 7: g7_curing technique1, due to no record of any cracks formed after the orthodontic removal without thermocycling, precluding statistical analysis for these groups.

In comparing crack length between thermocycling and non-thermocycling conditions, the p-value of 0.791 for crack1 indicates no significant difference in crack length between the two groups.

In

Table 3. Directional characteristics of cracks in cracked teeth among different curing technique groups under thermocycling and non-thermocycling conditions.

Group		Vertical			Horizontal			Oblique
		No	Yes	p-Value	No	Yes	p-Value	No	Yes	p-Value
		n(%)	n(%)		n(%)	n(%)		n(%)	n(%)
Thermocycling Groups	Group 5: G5_Curing technique1	1(25)	3(75)	0.809 a	4(100)	0(0.0)	1.00 a	3(75)	1(25)	0.809 a
	Group 6: G5_Curing technique2	0(0.0)	2(100)		2(100)	0(0.0)		2(100)	0(0.0)
	Group 9: G7_Curing technique1	1(33.3)	2(66.7)		3(100)	0(0.0)		2(66.7)	1(33.3)
	Group 10: G7_Curing technique2	0(0.0)	2(100)		2(100)	0(0.0)		2(100)	0(0.0)
Non-Thermocycling Groups	Group 3: G5_Curing technique1	0(0.0)	2(100)	0.513 a	2(100)	0(0.0)	1.00 a	2(100)	0(0.0)	0.513 a
	Group 4: G5_Curing technique2	1(33.3)	2(66.7)		3(100)	0(0.0)		2(66.7)	1(33.3)
	Group 7: G7_Curing technique1	0(0.0)	2(100)		2(100)	0(0.0)		2(100)	0(0.0)
p-value		0.829 a			ND +			0.829 a

^a Chi-Square Test was conducted. ⁺ Not determined.

, the incidence of vertical crack formation within different groups under thermocycling and non-thermocycling conditions is presented. In the thermocycling groups, group 5: g5_curing technique1 showed vertical cracks in 75% of cases, while group 6: g5_curing technique2 exhibited vertical cracks in 100% of cases. For group 9: g7_curing technique2, vertical cracks were observed in 66.7% of cases, and in group 10: g7_curing technique2, vertical cracks were also observed in 100% of cases. In the non-thermocycling groups, group 3: g5_curing technique1 and group 4: g5_curing technique2 both showed vertical cracks in 100% of cases, while group 7: g7_curing technique1 also exhibited vertical cracks in 100% of cases. These percentages illustrate the variation in vertical crack occurrence across different groups under different conditions. However, statistical analysis (p-values of 0.809 for thermocycling groups and 0.513 for non-thermocycling groups) suggests no significant difference in the incidence of vertical cracks among the respective groups.

Across all thermocycling groups, including group 5: g5_curing technique1, group 6: g5_curing technique2, group 9: g7_curing technique2, and group 10: g7_curing technique2, horizontal cracks were absent in all cases, resulting in 100% absence within each group. Similarly, in the non-thermocycling groups, namely group 3: g5_curing technique1, group 4: g5_curing technique2, and group 7: g7_curing technique1, horizontal cracks were also absent in all cases, resulting in 100% absence within these groups as well. The p-values of 1.00 for both thermocycling and non-thermocycling groups suggest no significant difference in the incidence of horizontal cracks among the respective groups (Table 3).

The provided data in Table 3 outlines the occurrence of oblique crack formations within various groups under thermocycling and non-thermocycling conditions. In the thermocycling groups, for instance, group 5: g5_curing technique1 exhibited a presence of oblique cracks in 25% of cases, while the absence was noted in 75%. In contrast, group 6: g5_curing technique2 and group 10: g7_curing technique2 showed 100% absence of oblique cracks. For non-thermocycling groups, such as group 3: g5_curing technique1 and group 7: g7_curing technique1, oblique cracks were absent in all instances, resulting in 100% absence within these groups. The calculated p-values of 0.809 and 0.513 for the thermocycling and non-thermocycling groups, respectively, suggest no significant difference in the incidence of oblique cracks among the respective groups. In comparing crack length between thermocycling and non-thermocycling conditions, the p-values of 0.829 for both the occurrence of vertical and oblique crack formations indicates no significant difference in crack length between the two groups.

The analysis of ARI (0) indicates noteworthy differences between thermocycling and non-thermocycling groups (

Table 4. Comparison of adhesive remnant index (ARI) scores between thermocycling and non-thermocycling conditions in different curing technique groups.

Group		ARI (0)		ARI (1)		ARI (2)		ARI (3)
		n(%)	p-Value	n(%)	p-Value	n(%)	p-value	n(%)	p-Value
Thermocycling Groups	Group 5: G5_Curing technique1	1(25)	0.029 a,*	2(50)	0.061 a	1(25)	0.392 a	0(0.0)	1.00 a
	Group 6: G5_Curing technique2	1(25)		3(75)		0(0.0)		0(0.0)
	Group 9: G7_Curing technique1	4(100)		0(0.0)		0(0.0)		0(0.0)
	Group 10: G7_Curing technique2	4(100)		0(0.0)		0(0.0)		0(0.0)
Non-Thermocycling Groups	Group 3: G5_Curing technique1	0(0.0)	0.007 a,*	4(100)	0.025 a,*	0(0.0)	1.00 a	0(0.0)	0.392 a
	Group 4: G5_Curing technique2	0(0.0)		3(75)		0(0.0)		1(25)
	Group 7: G7_Curing technique1	4(100)		0(0.0)		0(0.0)		0(0.0)
	Group 8: G7_Curing technique2	3(75)		1(25)		0(0.0)		0(0.0)
p-value		0.381 b		0.381 b		0.780 b		0.780 b

^a Kruskal–Wallis test was conducted; ^b Mann–Whitney U test; * significant at a significance level of 0.05.

). In particular, thermocycling groups 5 (g5_curing technique1) and 6 (g5_curing technique2) exhibited statistically significant results (p = 0.029), with 25% of cases in each group showing ARI (0) presence. This suggests that adhesive remnants were present on the tooth surface after bracket debonding in these groups despite thermocycling exposure. Conversely, all cases in thermocycling groups 9 (g7_curing technique2) and 10 (g7_curing technique2), as well as non-thermocycling group 7 (g7_curing technique1), showed no adhesive remnants (0%). Notably, non-thermocycling group 3 (g5_curing technique1) also demonstrated a significant difference (p = 0.007), indicating the absence of ARI (0). In contrast, non-thermocycling group 8 (g7_curing technique2) displayed 75% of cases with ARI (0).

The analysis of ARI (1) reveals interesting patterns between thermocycling and non-thermocycling groups (Table 4). Thermocycling groups 5 (g5_curing technique1) and 6 (g5_curing technique2) showed 50% and 75% of cases with ARI (1) presence, respectively, although the p-value for group 5 (g5_curing technique1) was not significant (p = 0.061). In contrast, all cases in thermocycling groups 9 (g7_curing technique2) and 10 (g7_curing technique2) had no ARI (1). Among the non-thermocycling groups, group 3 (g5_curing technique1) displayed a significant difference (p = 0.025), with all cases exhibiting ARI (1). Group 4 (g5_curing technique2) showed a similar pattern to thermocycling group 6 (g5_curing technique2), with 75% of cases showing ARI (1). Notably, non-thermocycling group 8 (g7_curing technique2) exhibited a lower percentage (25%) of cases with ARI (1).

The analysis of ARI (2) reveals no significant differences between thermocycling and non-thermocycling groups (Table 4). In thermocycling group 5 (g5_curing technique1), 25% of cases exhibited ARI (2), although the p-value (0.392) indicates no statistically significant difference compared to other groups. Similarly, all other thermocycling groups (groups 6, 9, and 10) showed no instances of ARI (2), reflecting consistency across these groups. Among non-thermocycling groups, all cases in groups 3, 4, 7, and 8 demonstrated no presence of ARI (2), with a p-value of 1 for group 3 (g5_curing technique1), signifying no significant difference compared to other groups.

The analysis of ARI (3) reveals no significant differences between thermocycling and non-thermocycling groups. In thermocycling groups 5, 6, 9, and 10, there were no instances of ARI (3), indicating consistency across these groups with p-values of 1. Among non-thermocycling groups, group 3 (g5_curing technique1) showed no presence of ARI (3) with a p-value of 0.392, suggesting no significant difference compared to other groups. However, in group 4 (g5_curing technique2), 25% of cases exhibited ARI (3), but the overall p-value suggests no significant difference. Similarly, groups 7 and 8 (g7_curing technique1 and g7_curing technique2) demonstrated no presence of ARI (3). In comparing adhesive remnant index (ARI) scores between thermocycling and non-thermocycling conditions, the obtained p-values of 0.381, 0.381, 0.780, and 0.780 for ARI (0), ARI (1), ARI (2), and ARI (3), respectively, indicate no significant difference in adhesive remnant index (ARI) Scores between the two groups.

Figure 5 illustrates the distribution of adhesive remnant index (ARI) scores across various groups subjected to thermocycling and non-thermocycling conditions. In the thermocycling groups, for instance, group 5: g5_curing technique1 and group 6: g5_curing technique2 predominantly exhibit ARI scores of 0 and 1 in 25% and 50% of cases, respectively, suggesting efficient removal of adhesive material. Conversely, group 9: g7_curing technique2 and group 10: g7_curing technique2 display ARI scores of 0 in 100% of cases, indicating complete adhesive retention. Similarly, in non-thermocycling groups, group 3: g5_curing technique1 and group 7: g7_curing technique1 show ARI scores of 1 and 0 in 100% of cases, respectively. This implies full adhesive retention in both of the groups. Meanwhile, group 4: g5_curing technique2 and group 8: g7_curing technique2 present a mix of ARI scores, indicating varying degrees of adhesive retention, with 25% and 75% of cases showing ARI scores of 1.

The comparison of mixed outcomes among different groups yielded statistically significant results (

Table 5. Comparison of mixed, bracket/adhesive and no failure–success (100%) outcome incidence between thermocycling and non-thermocycling groups.

Group		Mixed		Bracket/Adhesive		No Failure–Success (100%)
		n(%)	p-Value	n(%)	p-Value	n(%)	p-Value
Thermocycling Groups	Group 5: G5_Curing technique1	3(75)	0.029 a,*	0(0.00)	1.00 a	1(25)	0.029 a,*
	Group 6: G5_Curing technique2	3(75)		0(0.00)		1(25)
	Group 9: G7_Curing technique1	0(0.00)		0(0.00)		4(100)
	Group 10: G7_Curing technique2	0(0.00)		0(0.00)		4(100)
Non-Thermocycling Groups	Group 3: G5_Curing technique1	4(100)	0.025 a,*	0(0.00)	0.392 a	0(0.0)	0.007 a,*
	Group 4: G5_Curing technique2	3(75)		1(25)		0(0.0)
	Group 7: G7_Curing technique1	0(0.00)		0(0.00)		4(100)
	Group 8: G7_Curing technique2	1(25)		0(0.00)		3(75)
p-value		0.564 b		0.78 b		0.381 b

^a Kruskal–Wallis test was conducted; ^b Mann–Whitney U test; * significant at a significance level of 0.05.

). In the thermocycling groups, group 5 (g5_curing technique1) and group 6 (g5_curing technique2) both exhibited mixed outcomes at a rate of 75%, with p-values of 0.029. However, in the case of group 9 (g7_curing technique2) and group 10 (g7_curing technique2), there were no instances of mixed outcomes, resulting in p-values of 0.00. Similarly, among the non-thermocycling groups, group 3 (g5_curing technique1) had a mixed outcome frequency of 100%, with a p-value of 0.025, while group 4 (g5_curing technique2) exhibited a 75% occurrence. Groups 7 (g7_curing technique1) and 8 (g7_curing technique2) did not show any mixed outcomes.

In Table 5, the comparison of bracket/adhesive outcomes among different groups showed no statistically significant differences in the thermocycling groups, with all groups exhibiting no instances of bracket/adhesive issues (p-value = 1.0). Similarly, most non-thermocycling groups did not report any bracket/adhesive problems. However, in group 4 (g5_curing technique2), 25% of cases exhibited such issues, although this was not statistically significant (p-value = 0.392).

The analysis comparing the outcomes of “no failure–success (100%)” among different groups revealed some noteworthy findings (Table 5). In the thermocycling groups, groups 5 (g5_curing technique1) and 6 (g5_curing technique2) had similar results, with 25% of cases showing no failure or 100% success, although this difference was not statistically significant (p-value = 0.029). Conversely, groups 9 (g7_curing technique2) and 10 (g7_curing technique2) in the thermocycling groups exhibited 100% success rates, which were significantly higher compared to the other groups (p-value < 0.05). In the non-thermocycling groups, significant differences were observed between groups 3 (g5_curing technique1) and 8 (g7_curing technique2), with group 3 having no instances of failure or 100% success compared to group 8, which had a 75% success rate. This difference was statistically significant (p-value = 0.007).

In comparing the mode of failure for brackets across thermocycling and non-thermocycling groups, the obtained p-values of 0.564, 0.78 and 0.381 for mixed, bracket/adhesive and no failure–success (100%), respectively, indicate no significant difference in crack length between the two groups.

In evaluating the mode of bond failure categorized as “mixed”, the results demonstrate notable trends across both thermocycling and non-thermocycling groups (Figure 6). Among the thermocycling groups, groups 5 (g5_curing technique1) and 6 (g5_curing technique2) exhibit a similar distribution, with 25% showing an absence of mixed failure and 75% indicating its presence. Conversely, groups 9 (g7_curing technique1) and 10 (g7_curing technique2) in the thermocycling category displayed a complete absence of mixed failures, with all cases showing successful bonding. In the non-thermocycling groups, group 3 (g5_curing technique1) and group 7 (g7_curing technique1) achieved a 100% success rate, with no instances of mixed failures observed. However, groups 4 (g5_curing technique2) and 8 (g7_curing technique2) showed a distribution, with 75% and 25% of cases experiencing mixed failure, respectively.

In analyzing the presence or absence of brackets after orthodontic treatment, the results indicate consistent outcomes across both thermocycling and non-thermocycling groups (Figure 6). All thermocycling groups (groups 5, 6, 9, and 10) showed a complete absence of brackets, indicating successful bonding and retention of brackets following treatment. Similarly, non-thermocycling groups (groups 3, 4, 7, and 8) exhibited predominantly successful outcomes, with only group 4 (g5_curing technique2) displaying a single instance of bracket presence out of four cases.

The “no failure–success (100%)” category indicates the successful outcome of orthodontic treatments without any failure in bracket bonding (Figure 6). In both thermocycling and non-thermocycling groups, the majority of cases displayed successful results, with only a few exceptions. In the thermocycling groups, g5_curing technique1 and g5_curing technique2 each had one case where the success was achieved, while all cases in g7_curing technique2 (groups 9 and 10) resulted in successful outcomes, indicating robust bonding under thermocycling conditions. Conversely, in non-thermocycling groups, all cases in g5_curing technique1 and g5_curing technique2 (groups 3 and 4) exhibited successful results, whereas g7_curing technique1 (group 7) had all successful cases, and g7_curing technique2 (group 8) displayed a slight deviation with three successes out of four cases.

4. Discussion

Enamel damage often manifests as cracks, which have the potential to extend further during the debonding process. Such cracks pose a risk to the structural integrity of the enamel and may result in aesthetic concerns for patients [22]. The current study examined the influence of thermocycling, bonding agent, and curing techniques on enamel surfaces after debonding metal brackets. Our investigation revealed that crack incidence, length, and direction did not differ significantly across different groups. Although thermocycling did not significantly impact ARI and bracket failure modes, significant variations in them among different curing technique groups were observed, highlighting the importance of adhesive and protocol selection in orthodontic treatment.

The current study did not find significant differences in crack incidence, length, and direction after the debonding of metal brackets across various experimental groups, regardless of thermocycling conditions, curing techniques (pre-curing or co-curing), and bonding agents (fifth generation or seventh generation). These results suggest that these factors do not significantly impact crack formation during bracket debonding. Similarly, Yuasa et al. [11] revealed an insignificant difference in bond strength after thermocycling. However, contrary to our findings, a study by Jurubeba et al. [23] reported a significant increase in enamel microcracks after bracket debonding following thermocycling. They suggested that thermal stresses may weaken the enamel structure, predisposing it to crack formation during bracket debonding. Another research by Vukelja et al. [24] examined the influence of different curing techniques on bond reliability. They noted that the co-curing method led to diminished bond reliability compared to the conventional pre-curing method. However, in the same line as our results, Ghaffari et al. [25] found that surface treatment with either laser-etching or acid-etching exhibited insignificant differences in the frequency or length of enamel cracks after bracket debonding. Studies comparing the performance of different bonding agents have yielded mixed results regarding their impact on enamel integrity. When microleakage was examined after using fifth generation bonding agents compared with the seventh generation bonding agents, seventh-generation agents showed significantly less leakage than the other group [26]. It is important to consider the differences in experimental design, sample size, and methodology.

The comparison of ARI scores between thermocycling and non-thermocycling conditions across different curing technique groups revealed interesting findings. Notably, all cases in group 9, group 10, and group 7, as well as 75% of cases in group 8 (all bonded with a seventh generation bonding agent), exhibited an ARI score of zero, regardless of thermocycling. While variations were observed in the presence of ARI (1), particularly in group 3, where all cases showed ARI (1) in non-thermocycling conditions, overall comparisons did not show significant differences between the two conditions. Additionally, no significant differences were noted in the presence of ARI (2) and ARI (3) across different curing techniques or between thermocycling and non-thermocycling conditions. These findings suggest that thermocycling did not significantly influence the adhesive remnants remaining on the tooth surface after bracket removal across the various curing technique groups. These findings provide valuable insights into the effects of thermocycling testing with different orthodontic bonding agents and curing techniques on adhesive remnants after metal orthodontic brackets debonding. The observation that all cases in certain groups, particularly those bonded with a seventh generation adhesive (groups 7, 9, and 10), exhibited an ARI score of zero regardless of thermocycling indicates a consistent pattern of adhesive remnant removal. This finding aligns with previous studies demonstrating the effectiveness of seventh generation bonding agents in achieving complete adhesive removal upon bracket debonding [26,27]. The variations observed in the presence of ARI (1), particularly in group 3, where all cases showed ARI (1) in non-thermocycling conditions, suggest potential differences in adhesive bonding and debonding behavior under different environmental conditions. Previous research has highlighted the impact of environmental factors, such as temperature fluctuations, on adhesive performance and durability [8,28]. These findings emphasize the importance of considering environmental conditions, such as thermocycling, when evaluating adhesive remnants and bonding efficacy.

The comparison of bond failure and success rate after metal bracket debonding revealed significant differences among different curing technique groups. Thermocycling groups 5 and 6 exhibited mixed outcomes in 75% of cases, while groups 9 and 10 showed no instances of mixed outcomes. Non-thermocycling groups 3 (100%) and 4 (75%) had mixed outcomes, with group 4 showing bracket/adhesive issues in 25% of cases. Groups 5 and 6 showed similar success rates (25%), while groups 9 and 10 had significantly higher success rates (100%). Non-thermocycling groups 3 and 4 exhibited showed mixed outcomes in 10% and 75% of cases, respectively. On the other hand, group 7 and 8 had significantly higher success rates. Overall, thermocycling did not significantly influence bracket failure modes. The observed differences in bond failure and success rates among curing technique groups underscore the impact of various factors on orthodontic treatment outcomes. The significant variation in outcomes suggests that the choice of adhesive and curing technique plays a crucial role in bond integrity. The influence of adhesive properties and application methods on bond strength and durability was evident [29]. Additionally, research has highlighted the importance of considering environmental factors such as thermocycling, which can simulate oral conditions and affect bond performance over time [10,11]. These findings emphasize the need for careful consideration of adhesive systems and curing protocols to ensure optimal bond outcomes and minimize the risk of bracket failure during orthodontic treatment.

In this study, following debonding, we examined the enamel surface of each tooth and the brackets using a stereomicroscope. Petrescu et al. demonstrated the potential of optical coherence tomography (OCT) for non-invasive visualization of near-surface alterations in complex tissues, suggesting its utility for enhancing orthodontic procedures to restore the tooth surface to its pre-treatment condition by analyzing the enamel structure [30].

It has been reported that using Er,Cr:YSGG laser (Waterlase MD, Biolase technology, Inc., Irvine, CA, USA) (4 W/20 Hz) during debonding of ceramic brackets helps protect enamel topography compared to conventional manual methods. Proper adjustment of laser settings is essential to prevent structural alteration beneath the bracket [31].

Another study investigated changes in Enamel Microcracks (EMCs) following metal bracket removal, noting new EMC formation post-debonding. These findings underscored the need for further examination of EMCs to understand tooth structure changes during orthodontic treatment [32].

In this study, metal brackets were used, which are known for lower enamel fracture risk compared to ceramic brackets [5]. We employed stainless steel angled bracket remover orthodontic dental pliers for debonding, a method that Fan et al. concluded reduces the risk of enamel cracks compared to chiseling [33]. The method employed for investigation, along with the various parameters assessed, including in-depth examination of orthodontic bracket failure post-debonding and the analysis of ARI scores, all contribute to this study’s strengths. This research provides valuable insights into integrating thermocycling tests to evaluate the occurrence, quantity, length, and orientation of EMCs, thereby enhancing the field given its restricted accessibility and significance.

On the other hand, the limitations inherent in this in vitro study stem from its experimental design. Moreover, the sample size is relatively small, potentially limiting the generalizability of the findings. Furthermore, the short-term evaluation and absence of clinical correlation restrict the extrapolation of results to real-world orthodontic practice. Additionally, the study did not account for potential confounding factors that could influence the outcomes. Therefore, continued investigation is warranted, encompassing larger sample sizes, long-term assessments, and the incorporation of potential confounding variables to yield more definitive conclusions. The comparison of various methods for examining enamel surface is also recommended for future research.

5. Conclusions

This study found that after debonding metal brackets, there were insignificant differences in crack formation, length, and direction across various thermocycling groups, curing techniques, or bonding agents used. Additionally, thermocycling did not significantly impact ARI scores or bracket failure modes. However, significant variations were observed among curing technique groups. Our findings highlight the potential effectiveness of seventh-generation bonding agents in achieving complete adhesive removal, and emphasize the importance of careful bonding agent selection and curing protocols to ensure optimal outcomes and minimize bracket failure risk during orthodontic treatment. Meanwhile, the findings suggested that thermocycling did not significantly affect the adhesive remnants remaining on the tooth surface after bracket removal across the various curing technique groups. This adds valuable input into the effects of thermocycling testing with different orthodontic bonding agents and curing techniques on adhesive remnants after metal orthodontic brackets debonding. Generally, these results underscore the need for careful consideration of bonding agent systems and curing protocols to ensure optimal bond outcomes and minimize the risk of metal bracket failure during orthodontic treatment.