Effect of the CO₂ laser on enamel surface roughness after debonding ceramic brackets and the application of nanohydroxyapatite: in-vitro study

Abstract

Objective

Evaluate the effect of the CO₂ laser on enamel surface roughness after debonding ceramic brackets and applying nanohydroxyapatite.

Methodology

Sixty extracted premolars were randomly assigned to four groups (n = 15): G1, G2, G3, G4. Ceramic brackets were bonded to the buccal surface for G1 and G2 groups. Before debonding, the CO₂ laser group (G2) was irradiated with a power of 6 W for 5 s; shear bond strength was calculated using a universal testing machine and the adhesive remnant index with a stereomicroscope. Enamel roughness for the four groups was assessed using a profilometer; then, each group was subdivided for the application of nanohydroxyapatite paste and stored in saliva or deionized water. The surfaces were re-evaluated after 15, 30, and 60 days; the Friedman and Nemenyi post-hoc tests were used for statistical analysis, while Kruskal–Wallis and Dunn's multiple comparison post-hoc tests with Holm's correction adjustment to compare between groups.

Results

The group irradiated with CO₂ laser significantly reduced bond strength and roughness parameters upon debonding compared to the bracket group (p < 0.001). No significant differences were observed in roughness reduction between the group treated with the CO₂ laser and the application of nanohydroxyapatite paste; however, the surface roughness decreased in the bracket group G1b.

Conclusions

CO₂ laser irradiation reduced the force required for debonding ceramic brackets and minimized enamel surface roughness compared with conventional debonding. This contributes to the development of enamel-preserving orthodontic treatments; however, this property does not improve when applying nanohydroxyapatite paste.

1. Introduction

Fixed orthodontic appliances aim to achieve an optimal occlusion that allows a balanced structural and functional adjustment, periodontal health, and aesthetic harmony of the orofacial region (1, 2). Notably, aesthetic aspects play a very important role in orthodontic treatment. Although ceramic brackets were introduced in 1980 (3), they continue to play a key role today, especially in the anterior region, where less visibility is desired (4). In addition, these brackets offer advantages such as biocompatibility, optimal esthetics, resistance to chemical and thermal changes, and adequate bond strength, while maintaining the biomechanical performance required by the clinician (4, 5). Brackets are composed of aluminum oxide ([Al₂] [O₃]) and were classified by Michel L. Swartz according to their structure: polycrystalline and monocrystalline ceramic brackets, with mechanical or chemical retention (6). The difference between both types lies in the optical clarity since polycrystalline ceramic brackets reflect light, giving rise to a certain degree of opacity (1, 7, 8).

Once orthodontic treatment has been completed, the appliance must be removed without damaging the tooth structure; however, enamel and bracket fractures are common during conventional debonding because the shear bond strength (SBS) exceeds 10 MPa (9). It is estimated that between 5 and 20 µm of enamel is lost, leaving rough surfaces that make proper cleaning difficult, promoting the accumulation of plaque and bacteria that will favor demineralization (10, 11). Recent methods, such as nanohydroxyapatite (nHAp) application, have been studied to achieve remineralization of the enamel surface (12). The nHAp is a form of synthetic apatite with chemical and structural characteristics similar to dental hard tissue, its 20 nm particles promote the rearrangement of calcium phosphate ions in the form of hydroxyapatite, leading to the formation of a thin but firmly adherent layer on the tooth surface, significantly increasing surface hardness and remineralization (12–14).

Several methods have been suggested to facilitate the debonding of ceramic brackets by promoting a reduction in shear bond strength (15). Four approaches stand out: mechanical debonding, ultrasonic debonding, electrothermal debonding, and laser debonding (16, 17). Laser-assisted bracket removal has been reported to achieve painless debonding without damaging pulp and with a reduction in removal time between 1 and 5 s. Various laser devices have been studied for debonding ceramic brackets including carbon dioxide (CO₂) diode, erbium-doped yttrium aluminum garnet (Er:YAG), and neodymium-doped yttrium aluminum garnet (Nd:YAG) (15, 18).

Studies have concluded that the mode of action for debonding is based on the thermal softening of the resin adhesive caused by laser-induced heat, which is transmitted from the bracket to the resin. The latter can occur by thermal ablation, and photoablation (18–21). The CO₂ laser is available primarily at a wavelength of 10.6 μm, which is highly absorbed by water, minerals such as hydroxyapatite, and ceramic materials, making it a suitable instrument for the treatment of ceramic surfaces and for the debonding of ceramic brackets (18, 22, 23).

An effective way to assess the different degrees of enamel damage after debonding ceramic brackets and to analyze surface morphology is the profilometer, which analyzes characteristics such as profile height and roughness at the nanometer level (21, 24).

Various debonding methods, enamel cleaning and polishing techniques, as well as remineralizing agents, have been studied. However, to date, no method has demonstrated the ability to preserve the integrity of enamel without causing any degree of damage, nor to completely restore its physicochemical properties to its original pre-treatment condition. Therefore, we seek to put alternative methods forward that enable the enamel surface repair after debonding, by focusing on less invasive and more comfortable techniques for the patient.

Therefore, the aim of this study is to evaluate the use of a CO₂ laser for debonding ceramic brackets together with the effect of the subsequent application of nanohydroxyapatite on the roughness of the enamel surface. The analysis focuses on SBS and enamel surface characterization, followed by the evaluation of nHAp effects on enamel roughness under different conditions and time points.

2. Materials and methods

This study was carried out under the Declaration of Helsinki and approved by the Research Ethics Committee of the Center for Research and Advanced Studies in Dentistry of the School of Dentistry at the Autonomous University of the State of Mexico with registration number: (CEICIEAO-2023-009).

2.1. Sample preparation

Sound human premolars extracted for orthodontic reasons were included, free of caries, cracks, restorations, or developmental defects. The teeth were cleaned under running water to remove blood and tissue debris, polished with fluoride-free prophylactic paste using a rubber cup at low speed, and stored in distilled water in a 37 °C incubator for a maximum of three months until testing. Specimens were then distributed randomly allocated into four groups (n = 15) using a computer-generated sequence with the RAND() function in Microsoft Excel to minimize selection bias (25): G1 (bracket), G2 (bracket laser), G3 (polished enamel), and G4 (healthy enamel).

The buccal surface was conditioned following the same protocol as employed in the G1 and G2 groups. Initially, 35% phosphoric acid (Ultra-etch, Ultradent Products Inc., South Jordan, UT, USA) was applied for 15 s. Subsequently, the surface was rinsed with water for 20 s to ensure thorough removal of the etchant, followed by air-drying for 5 s.

A thin layer of adhesive (3M Unitek, Monrovia, CA, USA) was applied to the enamel surface, dispersed using oil-free air, and subsequently light-cured for 10 s.

In group G3, the buccal surface was polished with aluminum oxide tips (Onegloss, Shofu Inc., Kyoto, Japan) by applying gentle pressure for 10 s in conjunction with the use of diamond polish (Diamond polish, Ultradent Products Inc., South Jordan, UT, USA).

2.2. Bracket bonding

Thirty polycrystalline ceramic brackets (Clarity Advanced, 3M Unitek) were bonded to the center of the buccal surface of the teeth in groups G1 and G2 using Transbond Plus Color Change Adhesive (3M Unitek). Excess resin was removed with the tip of a dental explorer. The adhesive was light-cured for 6 s on the mesial and distal sides of the bracket using a light-emitting diode (LED) curing unit with an output intensity of 1600 mW/cm² (Ortholux, 3M Unitek).

2.3. CO₂ laser irradiation and shear bond strength test

Prior to the debonding procedure, samples from the G2 group were irradiated using a CO₂ laser (The Yoshida Dental MFG, CO., LTD, Tokyo, Japan) operating in continuous mode at a wavelength of 10.6 μm and a power of 6 W.

The light was applied perpendicularly at 5 mm from the bracket surface, performing horizontal sweeping movements for 5 s. Immediately following irradiation, a chisel-shaped blade was positioned at the enamel/bracket base interface to perform the debonding procedure.

SBS testing was performed using a universal testing machine (EZ Graph, Shimadzu Corporation, Kyoto, Japan), applying a force in the occluso-gingival direction to produce detachment at a crosshead speed of 1 mm/min, the resulting values were recorded in megapascals (MPa).

2.4. Evaluation of the adhesive remnant index (ARI)

A stereomicroscope at 10× magnification (VE-S5C, Velab, Guadalajara, Jalisco, México) was used to quantify the amount of residual resin adhered to the enamel surface.

Residual resin was removed using a 12-blade tungsten carbide bur (C375R, Jota AG, Rüthi, Switzerland) and polished with a OneGloss tip (Shofu Inc., Kyoto, Japan) and diamond paste for 10 s. ARI scoring was performed by a single experienced observer, without knowing the teeth group allocation, using standardized criteria. Consistency was ensured through repeated evaluation of the samples.

2.5. Enamel surface assessment

The buccal surface of each tooth was sectioned at a thickness of 4 mm using a micromotor with 0.20 mm diamond cutting discs.

Samples from the four groups were evaluated with a profilometer (Mitutoyo Surftest SJ-301, Tokyo, Japan), using surface roughness parameters Ra, Rq and Rz recorded in micrometers. Three distinct locations at the center of each specimen were measured, and the values were averaged to produce a single representative value per tooth. In total, 15 profilometric measurements were acquired per experimental condition. A diamond tip of 5 μm radius was employed, operating with a cutoff value (λc) of 0.08 mm, a cross-sectional length of 0.5 mm, a scanning speed of 0.25 mm/sec, and a Gaussian filter algorithm. These parameters were selected according to internationally accepted standards and recommendations for contact surface profilometry (ISO 21920-2:2021; ISO 21920-3:2021), which replaced the earlier ISO 4287/4288 widely cited in biomaterials research (26, 27).

2.6. Application of the nanohydroxyapatite paste

Groups G1, G2, and G3 were further subdivided into three experimental subgroups n = 5, (Figure 1). One group was stored untreated in artificial saliva (a), while the remaining groups received nanohydroxyapatite paste (Apagard, Sangi Co., Ltd., Japan) for a period of two months and were stored in artificial saliva (b) and deionized water (c), respectively. The artificial saliva (Viarden S.A. de C.V., México) was composed of mineral salts (Na⁺, K⁺, F⁻, Cl²⁻, Mg²⁺, Ca²⁺, PO₄²⁻), xylitol, enzymes, and flavoring agents, according to the manufacturer's formulation. The enamel surface was brushed twice daily for 3 min using an electric toothbrush, with a bean-sized amount of nanohydroxyapatite paste applied during each session and rinsed with distilled water to ensure complete removal of the paste. The use of an electric toothbrush ensured a standardized brushing motion and minimized variability in force application, and the operator was previously trained to keep handling consistently across all specimens. All specimens were stored in an incubator at 37 °C, with artificial saliva and deionized water refreshed every 48 h. Surface evaluations were conducted at 15, 30, and 60 days using a profilometer.

Figure 1.

Sample distribution into groups according to the treatment performed.

2.7. Scanning electron microscopy (SEM) evaluation

The enamel surfaces were examined with a scanning electron microscope (JEOL JSM-6510LV, JEOL Ltd., Akishima, Tokyo, Japan) in three representative sections located in the center of the tooth crown. Fields were selected within the area previously covered by the bonded bracket, ensuring that the evaluation focused specifically on the region subjected to bonding and debonding procedures. SEM observations were performed to qualitatively describe representative morphological features of the enamel surface. Micrographs were obtained at magnifications of ×200, ×1000, and ×1500, with a field of view of approximately 10 µm × 10 µm. Images were acquired under low-vacuum conditions and a working distance of approximately 10 mm.

2.8. Statistical analysis

The collected numerical data were analyzed by the Statistical Software SPSS version 25.0 (IBM, Armonk, New York, USA) and Python 3.13 (Python Software Foundation PSF).

Data normality was assessed using the Shapiro–Wilk test, while the comparison of SBS values between the G1 and G2 groups was conducted using Student's t-test. The Mann–Whitney U test was employed to analyze ARI scores. Surface roughness parameters across 0, 15, 30, and 60 days were evaluated using the Friedman tests and Nemenyi post-hoc tests (Nemenyi-Damico-Wolfe); for changes between groups, Kruskal–Wallis and Dunn's multiple comparison post-hoc tests with Holm's correction adjustment method were used. To further confirm the adequacy of the sample size, a post hoc power analysis was conducted using G*Power software (version 3.1.9.7, Heinrich-Heine University, Düsseldorf, Germany).

3. Results

3.1. SBS

The group irradiated with the CO₂ laser (G2) exhibited a statistically significant reduction in bond strength during debonding when compared to the bracket group (G1) (p < 0.001), as shown in Table 1.

Table 1.

Shear bond strength (MPa).

Group	Mean (SD) MPa
G1	22.36 (6.11)a
G2	10.96 (5.51)b

Different letters: statistically significant difference (p < 0.05).

3.2. ARI

The Mann–Whitney U test showed a statistically significant difference in the location of bond failure between the two groups (p < 0.044), as presented in Table 2.

Table 2.

Frequency distribution of the ARI Index.

Group	% ARI Index
	0	1	2	3	Total
G1	13	27	20	40	100a
G2	40	40	0	20	100b

Different letters: statistically significant difference (p < 0.05).

3.3. Roughness

Statistically significant differences in roughness values were observed prior to paste application of groups G2, G3 and G4 compared to group G1 (p < 0.001). In contrast, no significant differences were observed among Groups G2, G3, and G4 (p ≥ 0.605) (Table 3).

Table 3.

Results of enamel roughness prior to nHAp paste treatment.

Group	Ra	Rq	Rz
G1	0.210 ± 0.022a	0.278 ± 0.038a	1.210 ± 0.213a
G2	0.130 ± 0.011b	0.165 ± 0.017b	0.732 ± 0.110b
G3	0.131 ± 0.015b	0.166 ± 0.017b	0.727 ± 0.112b
G4	0.138 ± 0.026b	0.178 ± 0.032b	0.728 ± 0.124b

Significance level ≤0.05, Different letters: statistically significant difference.

Group G2b showed a significant within-group reduction in Ra and Rq after 30 days of toothbrushing with nHAp paste, in contrast, a decrease in Rz was observed only when compared with the group G2a. Group G1b showed a significant between-group reduction in surface roughness with Ra decreasing after 15 days, and Rq, Rz showing further reduction after 60 days of toothbrushing (Table 4).

Table 4.

Descriptive statistics and statistical analysis results of enamel roughness at multiple time points among the groups.

Roughness parameter	Group	T0	T15	T30	T60
Ra	G1a	0.212 ± 0.010 A,a	0.185 ± 0.008 A,B,a	0.171 ± 0.016 B,a	0.172 ± 0.012 B,a,b
	G1b	0.206 ± 0.034 A,a	0.175 ± 0.024 B,C,a	0.175 ± 0.007 B,a,b	0.158 ± 0.009 C,a
	G1c	0.213 ± 0.014 A,a	0.192 ± 0.007 B,a	0.246 ± 0.028 A,b	0.224 ± 0.013 A,b
	G2a	0.129 ± 0.004 A,a	0.132 ± 0.006 A,a	0.177 ± 0.017 B,a	0.143 ± 0.011 A,a
	G2b	0.126 ± 0.009 A,a	0.129 ± 0.014 A,B,a	0.139 ± 0.005 B,C,b	0.148 ± 0.009 C,a
	G2c	0.136 ± 0.015 A,a	0.141 ± 0.015 A,B,a	0.169 ± 0.011 B,C,a	0.221 ± 0.007 C,b
	G3a	0.124 ± 0.004 A,a	0.116 ± 0.008 A,a	0.138 ± 0.006 B,a	0.128 ± 0.001 A,B,a
	G3b	0.133 ± 0.008 A,B,a	0.127 ± 0.002 A,a	0.150 ± 0.009 C,a	0.140 ± 0.004 B,C,a,b
	G3c	0.136 ± 0.014 A,a	0.159 ± 0.007 B,b	0.155 ± 0.009 A,B,a	0.158 ± 0.005 A,B,b
Rq	G1a	0.280 ± 0.018 A,a	0.240 ± 0.019 A,B,a	0.217 ± 0.023 B,a	0.212 ± 0.022 B,a
	G1b	0.277 ± 0.058 A,a	0.220 ± 0.030 B,C,a	0.219 ± 0.019 A,B,a	0.199 ± 0.011 C,a
	G1c	0.277 ± 0.025 A,a	0.234 ± 0.009 B,a	0.308 ± 0.033 A,a	0.273 ± 0.014 A,b
	G2a	0.163 ± 0.009 A,a	0.171 ± 0.009 A,a	0.231 ± 0.025 B,a,	0.181 ± 0.012 A,a
	G2b	0.158 ± 0.014 A,a	0.160 ± 0.016 A,a	0.170 ± 0.007 A,b	0.191 ± 0.012 B,a
	G2c	0.175 ± 0.019 A,B,a	0.174 ± 0.019 A, a	0.206 ± 0.014 B,C,a	0.271 ± 0.007 C,b
	G3a	0.158 ± 0.006 A,a	0.147 ± 0.010 A,a	0.176 ± 0.008 B,a	0.160 ± 0.002 A,B,a
	G3b	0.167 ± 0.009 A,B,a	0.163 ± 0.005 A,a	0.192 ± 0.02° C,a	0.183 ± 0.008 B,C,a
	G3c	0.171 ± 0.016 A,a	0.205 ± 0.013 B,C,b	0.192 ± 0.010 A,B,a	0.211 ± 0.006 C,b
Rz	G1a	1.206 ± 0.077 A,a	1.117 ± 0.151 A,B,a	1.020 ± 0.129 A,B,a,b	0.927 ± 0.114 B,a
	G1b	1.226 ± 0.246 A,a	0.901 ± 0.162 B,a	0.889 ± 0.171 B,a	0.816 ± 0.072 B,a
	G1c	1.199 ± 0.152 A,a	0.962 ± 0.130 B,a	1.269 ± 0.100 A,b	1.132 ± 0.054 A,b
	G2a	0.713 ± 0.052 A,a	0.836 ± 0.075 A,B,a	1.029 ± 0.113 C,a	0.861 ± 0.058 B,C,a
	G2b	0.698 ± 0.098 A,a	0.690 ± 0.098 A,a	0.718 ± 0.022 A,B,b	0.829 ± 0.053 B,a
	G2c	0.784 ± 0.088 A,a	0.723 ± 0.087 A,a	0.810 ± 0.108 A,a,b	1.109 ± 0.048 B,b
	G3a	0.699 ± 0.026 A,B,a	0.676 ± 0.051 A a	0.782 ± 0.027 B,a	0.699 ± 0.033 A,B,a
	G3b	0.758 ± 0.050 A,a	0.735 ± 0.061 B,a,b	0.761 ± 0.063 C,a	0.795 ± 0.061 D,a,b
	G3c	0.723 ± 0.031 A,a	0.864 ± 0.073 B,b,	0.824 ± 0.104 A,B,a	0.890 ± 0.091 B,b

T0 no treatment, T15 after 15 days of nHAp application, T30 after 30 days of nHAp application, T60 after 60 days of nHAp application.

Different uppercase letters within the same row indicate statistically significant differences between groups at different time points (p < 0.05).

Different lowercase letters within the same column indicate significant statistically differences within groups at the same time point (p < 0.05).

Friedman tests revealed significant differences in surface roughness parameters within the experimental groups. Most analyses demonstrated moderate to large effect sizes (Kendall's W), with post hoc statistical power exceeding 98%. For intergroup comparisons, Kruskal–Wallis analyses consistently yielded large effect sizes (ε² = 0.64–0.85), while pairwise comparisons conducted using Dunn's test with Holm-adjusted p-values confirmed robust differences among groups, further supporting the adequacy of the selected sample size.

3.4. SEM

The SEM micrographs in group G1 revealed a surface with irregular elevations and depressions, exposing enamel prisms. In group 1b, a smoother surface was observed, characterized by tubule obliteration (Figure 2).

Figure 2.

SEM micrographs depicting samples bonded to brackets (G1), (A) T0 without treatment, (B) T60 stored in artificial saliva (G1a), (C) T60 saliva + nHAp (G1b), (D) T60 deionized water + nHAp (G1c).

In group G2, the surface initially revealed an irregular pattern with micro-reliefs and depressions. After immersion in saliva, the surface appeared smoother, and this morphology remained even after the application of nHAp paste (Figure 3).

Figure 3.

SEM micrographs depicting samples of the bracket laser group (G2), (A) T0 without treatment, (B) T60 stored in artificial saliva (G2a), (C) T60 saliva + nHAp (G2b), (D) T60 deionized water + nHAp (G2c).

An irregular pattern with marked lines as well as the classic hexagon pattern with the presence of porosities similar to healthy enamel was shown in group G3 (Figure 4). After immersion in saliva, a softened surface was observed; however, it still maintained the presence of scratches, these being to fade in the group where the nHAp paste was applied, showing a much more homogeneous and smoother surface.

Figure 4.

SEM micrographs depicting polished enamel (G3) samples at different storage conditions, (A) T0 without treatment, (B) T60 stored in artificial saliva (G3a), (C) T60 saliva + nHAp (G3b), (D) T60 deionized water + nHAp (G3c).

The images obtained in group G4 show microtextured surfaces with the appearance of dotting; overall, a smooth surface was observed (Figure 5).

Figure 5.

SEM micrographs of healthy enamel (G4). (A) enamel surface at x200, (B) x1500.

4. Discussion

The primary goal of any orthodontic treatment is to establish structural, functional, and aesthetic harmony between the teeth and the occlusion. Once the treatment with fixed appliances is completed, the attached devices are removed. However, the enamel surface gets damaged, so it must be rehabilitated to restore it as fully as possible to its previous state. Nevertheless, the process of bracket removal and subsequent enamel polishing has raised clinical concerns, since it can compromise the integrity of the enamel structure (21). This issue has prompted an intense search for safe and effective methods to prevent this, leading to the development and implementation of various specialized tools and techniques (1, 28, 29).

In response to these limitations, laser technology has emerged as an effective and minimally invasive alternative for debonding aesthetic brackets. CO₂ laser has gained clinical relevance due to its thermal softening mechanism, which facilitates controlled bracket debonding without excessive mechanical stress on the enamel (21, 30, 31). In our study, group G2 demonstrated the lowest debonding forces using CO₂ laser at 6 W for 5 s. Furthermore, Macri et al. (32) achieved greater SBS reduction with 10 W. They also confirmed pulpal safety with temperature increases remaining below the critical threshold of 5.5 °C. Finally, an ARI of 2 was observed, so they concluded that CO₂ laser was a safe alternative for debonding. In our case, adhesive fracture occurred at the enamel-resin interface (ARI 1), which is clinically favorable, as it reduces enamel damage and facilitates safer adhesive removal and polishing, in line with findings by Saito et al. (33), who reported no significant changes in ARI values.

The efficacy of CO₂ laser is attributed to the alignment of its wavelengths (9.3–10.6 μm) with the absorption bands of phosphate, carbonate, and hydroxyl groups in enamel and dentin. This interaction enables structural modification without inducing thermal damage (23, 34).

The removal of orthodontic brackets represents a critical phase in treatment, as it may compromise enamel surface integrity if inappropriate techniques are employed. The extent of enamel damage is influenced by the type of instrument used for bracket debonding and residual adhesive removal (35, 36). In the present study, tungsten carbide burs and aluminum oxide tips were selected, as previous research has identified these tools as being the least aggressive alternatives in conventional debonding procedures. Thawaba et al. (35) reported that tungsten carbide burs produced varied surface markings, including deep scratches; however, based on the Enamel Surface Index (ESI), the resulting surfaces were deemed acceptable. These findings align with our SEM observations in group G1, where pronounced enamel irregularities were evident, likely due to the hardness of the bur relative to the adhesive and enamel.

In contrast, prior studies have confirmed that CO₂ laser reduces adhesive bond strength while preserving enamel morphology, yielding roughness values comparable to conventional polishing (10, 30). Our results corroborate these findings, as groups G3a and G2a exhibited similar mean roughness values, whereas group G1a showed significantly increased roughness.

Complementary evidence from Vidor et al. (29) indicated smooth enamel surfaces following bracket debonding and resin removal with aluminum oxide tips, although their study lacked quantitative roughness data. In contrast, our profilometric analysis revealed that group G1 exhibited the highest roughness values, with surface depressions and a characteristic honeycomb enamel pattern. This group also showed a predominance of ARI scores 2 and 3, consistent with our findings.

Despite these advancements, residual roughness following debonding remains a clinical concern, not only due to bracket removal but also because of the subsequent enamel treatment required to eliminate adhesive remnants, particularly when using carbide burs (36, 37).

In this context, nHAp has been investigated as a remineralizing agent capable of repairing microscopic enamel defects (38, 39). In our study, samples stored in artificial saliva without nHAp application showed stable or improved roughness, whereas those stored in deionized water exhibited progressive roughness increases, even with nHAp treatment. This may be attributed to the absence of minerals and buffering agents that mitigate the acidifying effects of the medium (40). SEM analysis of group G2c revealed a partially smooth surface with localized enamel irregularities.

According to the results obtained in this study, the combination of CO₂ laser and nHAp did not yield significant reductions in surface roughness. The only group showing sustained roughness reduction was the G1 treated with nHAp, particularly in samples stored in saliva. Ra values decreased at 15 days, while Rq and Rz parameters improved at 60 days, suggesting a potential interaction between nHAp and salivary components. This may be due to the ability of nHAp nanoparticles (50–1000 nm) to penetrate enamel pores and act as nucleation sites for calcium and phosphate ions, promoting remineralization (41) in synergy with the salivary buffering system.

These findings are consistent with Verma et al. (14), who reported significant roughness reduction following 15 days of brushing with nHAp post-debonding. However, Ajami et al. (42) found no statistically significant changes in roughness after 10 and 60 days of nHAp serum or toothpaste application and observed irreversible enamel surface alterations in both study groups. Vicente et al. (37) emphasized the need to protect enamel post-debonding, citing SEM evidence of enamel loss and increased susceptibility to demineralization compared to intact enamel.

Ultimately, while CO₂ laser and nHAp demonstrated individual benefits under the conditions of this in-vitro study, their combined application did not reveal a synergistic effect. This limits their consideration as a complementary strategy at the current stage. Nonetheless, their selective use in specific contexts may hold potential for enhancing outcomes by minimizing iatrogenic damage and supporting enamel recovery, although further clinical validation is required. In addition, intrapulpal temperature changes were not measured, which represents a limitation regarding the assessment of thermal safety during CO₂ laser irradiation in this study.

5. Conclusions

The CO₂ laser showed potential for debonding ceramic brackets by reducing forces and preserving enamel morphology. At the same time, nHAp also demonstrated promise for repairing post-treatment defects in conventional bracket debonding.

Funding Statement

The author(s) declared that financial support was not received for this work and/or its publication.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Ethics statement

The studies involving humans were approved by Research Ethics Committee of the Center for Research and Advanced Studies in Dentistry of the School of Dentistry at the Autonomous University of the State of Mexico with registration number: CEICIEAO-2023-009. The studies were conducted in accordance with the local legislation and institutional requirements. The human samples used in this study were acquired from gifted from another research group. Written informed consent for participation was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and institutional requirements.

Author contributions

AM-R: Methodology, Investigation, Data curation, Conceptualization, Funding acquisition, Writing – original draft, Visualization. LR: Formal analysis, Supervision, Project administration, Methodology, Data curation, Conceptualization, Writing – review & editing. RS-V: Writing – review & editing, Supervision, Investigation, Funding acquisition. MN: Validation, Writing – review & editing, Supervision. RC: Formal analysis, Methodology, Investigation, Data curation, Writing – review & editing.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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