Evaluation of the effects of Er,Cr:YSGG laser irradiation on enamel demineralization and orthodontic shear bond strength: an in vitro comparative study with topical fluoride

Abstract

Background

Orthodontic appliances increase the risk of enamel demineralization. Dental lasers have been reported to enhance enamel resistance to demineralization when applied prior to orthodontic procedures. This study aimed to evaluate the effects of Er,Cr:YSGG laser application, alone or in combination with fluoride, on enamel demineralization, surface hardness, and orthodontic shear bond strength.

Methods

A total of 135 extracted human premolars were used. For Vickers hardness and ICP-OES analyses, 75 samples were divided into five groups: (1) negative control, (2) topical fluoride, (3) Er,Cr:YSGG laser, (4) laser combined with fluoride, and (5) positive control. Samples from Groups 2, 3, 4, and 5 were subjected to a demineralization procedure, and enamel hardness was subsequently measured. The demineralization solutions were analyzed using ICP-OES to determine calcium and phosphorus levels. For shear bond strength evaluation, four groups were prepared from the remaining 60 specimens. The same surface treatment protocols as those applied in Groups 1–4 of the microhardness test were used. Data were analyzed using SPSS software.

Results

The surface hardness of Groups 2, 3, and 4 was significantly higher than that of Group 5 (p < 0.05). The amounts of both calcium and phosphorus released in Group 5 were significantly higher than in the other groups. There were no statistically significant differences in calcium and phosphorus levels among Groups 2, 3, and 4. No significant differences in the shear bond strength of orthodontic brackets were observed among the groups.

Conclusion

Within the limitations of this in vitro study, the application of Er,Cr:YSGG laser, either alone or in combination with fluoride, improved enamel resistance to demineralization compared to the demineralized control group, without adversely affecting the bond strength of orthodontic brackets. The combined application did not demonstrate a statistically significant additional benefit compared to single-modality treatments.

Introduction

Fixed orthodontic treatment may compromise oral hygiene, facilitating bacterial plaque accumulation and adhesion to tooth surfaces. This increases caries formation and susceptibility to periodontal diseases [1, 2].

Enamel demineralization in the form of white spot lesions (WSLs) is one of the most common iatrogenic effects associated with fixed orthodontic treatment [2, 3]. The white appearance of incipient caries lesions such as WSL is the result of mineral loss from the surface and subsurface hydroxyapatite crystals of the enamel [4, 5]. Mineralised hydroxyapatite crystals can regain their original form if exposed to an oral environment that can remineralise them [6, 7].

Topical application of fluoride before or during fixed orthodontic treatment is the most commonly used method for prevention [8, 9] and has been shown to reduce the amount of demineralisation [10, 11].

Dental lasers have also been investigated as an alternative approach to enhance enamel resistance. Erbium-doped solid-state laser systems have been studied for their effects on enamel surface properties during orthodontic treatment. Treatment of enamel with Er: YAG laser has been reported to improve the crystalline structures on the enamel surface [12]. Erbium laser energy causes the water in the hard tissue to turn into vapour. This causes micro-explosive effects on the enamel surface, leading to ablation (removal) of the surface tissue.

The Er,Cr:YSGG laser, another erbium-doped system, has become widely utilized in dentistry due to its versatility in hard and soft tissue applications. Although both Er: YAG and Er,Cr:YSGG are erbium-doped lasers, they differ in wavelength and in the technology used to generate energy. Er,Cr:YSGG lasers have been reported to enhance enamel resistance to demineralization [13]. However, the effects of Er,Cr:YSGG lasers on orthodontic bond strength have been investigated in only a limited number of studies.

Although previous studies have investigated the effects of erbium lasers on enamel demineralization, findings regarding the additional benefit of combining laser irradiation with topical fluoride remain variable [14–16]. Furthermore, studies evaluating the influence of Er,Cr:YSGG laser parameters on both enamel demineralization resistance and orthodontic shear bond strength within the same experimental model are relatively limited. Considering that preventive surface treatments applied prior to bonding may influence adhesion performance, assessing both mineral loss and bond strength together may provide a more comprehensive understanding. Therefore, a comparative evaluation of laser and fluoride applications under controlled in vitro conditions appears appropriate.

The main purpose of this study was to evaluate the effects of Er,Cr:YSGG laser application on enamel demineralization resistance and to investigate its potential influence on the shear bond strength of orthodontic brackets. Topical fluoride, as a well-established preventive approach, was incorporated as a comparative treatment group.

Materials and methods

The study protocol was conducted in accordance with the Declaration of Helsinki and received approval from the Non-Interventional Clinical Research Ethics Committee of Ordu University (Decision No: 2026/69). The investigation was carried out using extracted human teeth. All patients provided written informed consent allowing their extracted teeth to be used for research purposes.

Sample size calculation was performed using G*Power software (version 3.1.9.4; Heinrich Heine University, Düsseldorf, Germany). The study of Damar et al. was taken as a reference for sample size calculation. Based on the reported data, an effect size of f = 0.648 was determined. To achieve a 95% confidence level (1 − α) and 95% statistical power (1 − β), separate power analyses were performed according to the experimental designs [17]. For the Vickers microhardness and ICP-OES analyses (five groups), a minimum total sample size of 50 specimens (10 specimens per group) was required. For the shear bond strength analysis (four groups), a minimum total sample size of 48 specimens (12 specimens per group) was required. To increase statistical power and ensure balanced group allocation, 15 specimens were included in each group. Accordingly, 75 specimens were allocated to the Vickers microhardness and ICP-OES analyses and 60 specimens to the shear bond strength analysis, resulting in a total of 135 human premolars included in the study.

Extracted human premolars obtained for orthodontic and periodontal indications were used in the study. Teeth with intact enamel surfaces were selected and stored in a 0.01% thymol solution prepared with distilled water at 4 °C until use. Prior to the experimental procedures, the buccal enamel surfaces were cleaned using pumice prophylaxis. The roots were then separated at the cemento-enamel junction with a low-speed precision cutting device (Minitom, Struers Inc.; Westlake, OH, USA). Following specimen preparation, the teeth were randomly assigned to the experimental groups using a computer-generated randomization list.

For Vickers microhardness (VHN) and elemental analysis (ICP-OES) evaluations, samples were divided into five groups (n = 15) as follows: Group 1: Negative control group which was not subjected to demineralization, Group 2: Topical fluoride was applied on the bonding surface of the enamel, Group 3: Laser was applied on enamel surface, Group 4: Laser and topical fluoride were applied together on the bonding surface of the enamel, Group 5: Positive control group which was subjected only to demineralization.

For shear bond strength evaluations, samples were divided into four groups (n = 15) as follows: Group 1: Control group which was not subjected to treatment, Group 2: Topical fluoride was applied on the bonding surface of the enamel, Group 3: Laser was applied on the bonding surface of the enamel, Group 4: Laser and topical fluoride were applied together on the bonding surface of the enamel. Unlike the specimens used for hardness and elemental analyses, the samples used for shear bond strength testing were not subjected to the demineralization protocol.

For the microhardness and ICP-OES groups (Groups 2–5), treatment applications were performed prior to demineralization, and surface hardness was measured after the demineralization procedure. The demineralization solutions obtained after the microhardness test were used for ICP-OES analysis. Teeth were placed in a flat plane with their whole facial surfaces completely horizontally touching modeling wax. Self-curing acrylic resin was used to form blocks to stabilize the samples. Given that shear bond strength testing on completely demineralized enamel would lack clinical relevance, no fifth positive control group was incorporated into the bond strength analysis (Fig. 1).

Fig. 1.

Flow diagram representing the experimental design

Enamel surface treatments

Fluoride treatment

Before fluoride application, all specimens were removed from distilled water and gently air-dried. A 5% sodium fluoride varnish (Flor-Opal, Ultradent Products Inc., South Jordan, UT, USA) was applied to the enamel surfaces for 30 min in accordance with the manufacturer’s instructions.

Laser treatment

Samples were irradiated immediately after removal from distilled water. Pre-defined 4 × 4 mm enamel areas were irradiated using an Er,Cr:YSGG laser (Waterlase Millennium, Biolase Technologies Inc., San Clemente, CA, USA). The device emitted photons at a wavelength of 2.78 μm with an output power of 0.25 W. The repetition rate was fixed at 20 Hz and the pulse duration at 140 µs. The energy density per pulse was calculated as 4.46 J/cm² (power: 0.25 W; frequency: 20 Hz; pulse energy: 12.5 mJ; focal spot area: 0.28 × 10⁻² cm²). These parameters were selected to provide sub-ablative energy levels, as previous studies have shown that low-energy Er,Cr:YSGG irradiation within this range can enhance enamel acid resistance without causing surface ablation [13–15].

Before each experimental session, the laser parameters were checked, and the output power was verified using a power meter to ensure consistent laser performance throughout the study. Irradiation was performed in focused mode with the handpiece positioned 1 mm from the enamel surface. The working distance was standardized throughout the procedure using an endodontic file attached to the handpiece as a reference. To minimize laser beam reflection, the handpiece was maintained at an angle of approximately 3–5° relative to the enamel surface. All specimens were treated manually using standardized horizontal and vertical scanning movements across the entire treatment area, with equal irradiation time allocated to each direction.

Fluoride + laser treatment

Fluoride and laser were applied consecutively on the enamel surfaces according to the aforementioned protocols for Groups 2 and 3. The samples were kept in distilled water after surface treatments.

Demineralization procedure

Demineralization protocol was applied according to the study of White et al. [18]. Enamel specimens were immersed in a demineralizing solution containing 0.1 M lactic acid and 0.2% Carbopol 907, which was 50% saturated with hydroxyapatite and adjusted to pH 5.0. Samples were kept in the solution for 96 h at 37 °C to produce subsurface enamel lesions.

Demineralization treatment was applied to groups 2, 3, 4 and 5 prior to Vickers microhardness measurements. Group 1 remained untreated and served as the negative control group. The demineralization solutions obtained after the procedure were used for ICP-OES elemental analysis.

Vickers microhardness (VHN) test

A Vickers Diamond Microhardness Tester (Buehler Micromet 5101, obj. Lens: 50x) was used for measurement of hardness in Vickers Hardness Units/Numbers. Three indentations were made with a 300 g load, applied perpendicularly to the surface for 15 s, and the mean values were recorded.

Before starting the measurements, repeated measurements were taken at different times on 5 randomly selected samples to ensure the calibration of the researcher performing the measurement. Intra-measurement reliability was evaluated using the intraclass correlation coefficient, and a high level of measurement consistency was found (ICC = 0.94). Following the calibration process, all experimental measurements were performed by the same researcher to eliminate inter-observer variability and ensure a standard evaluation process.

ICP-OES analysis

The amounts of calcium and phosphorus released into the demineralization solutions were measured (in ppm) with an inductively coupled plasma optical emission spectrometer (ICP-OES; Spectro Flame Blue EOP TI, Spectro Analytical Instruments GmbH and Co. KG, Kleve, Germany) under the following conditions: Plasma power: 1430 W, Pump speed: 30 rpm, Coolant Flow: 13 L/min, Auxiliary Flow: 0.80, Nebulizer Flow: 0.75. Before elemental determination, calibration was performed using calcium and phosphorus standard solutions (Merck KGaA, Darmstadt, Germany).

Bonding procedure

A conventional etch-and-rinse system was used for bonding. The enamel surface was dried with compressed air after removal from the water. The enamel was etched with 37% phosphoric acid for 15 s, rinsed with atomized water and dried thoroughly afterward. A light-cure primer (Transbond XT primer, 3 M Unitek, CA, USA) was applied to the etched surface, gently air-dried for 2–3 s, and then light-cured for 15 s using an LED unit (Elipar S10, 3 M Espe, St. Paul, CA, USA).

Mesh-based stainless steel premolar brackets with a surface area of 9.08 mm2 (Gemini, 3 M-Unitek, CA, USA) were directly bonded on the enamel with a light-cure adhesive resin (Transbond XT, 3 M-Unitek, CA, USA). The excess resin was removed with a scaler and the adhesive was light-cured for 20 s. The samples were stored in 37 °C distilled water for 24 h for maximum bond strength before the shear bond strength tests.

Shear bond strength test

A universal testing machine (MCE 2000ST, Quicktest Prifpartner GmbH, Langenfeld, Germany) was used for the measurement of shear bond strength. Specimens were mounted on the jig of the testing machine. A crosshead speed of 0.5 mm/minute was used until the fracture was observed. Measurements were recorded in Newtons and converted to megapascals using the formula: Shear bond strength (MPa) = Debonding force (N) / Bracket base area (mm²), where 1 MPa = 1 N/mm².

All specimens were coded prior to analysis, and all measurements were performed by a blinded operator who was unaware of the group allocation.

Statistical Analysis

The data were transferred to IBM SPSS Statistics V.21 (IBM Corp. NY, USA) software for analysis. A Shapiro-Wilk test showed that the data were not normally distributed (p < 0.05). Kruskal-Wallis test was used to determine the differences between groups and Mann-Whitney U test was used to analyze the inter-group differences. Descriptive statistics and 95% confidence intervals (CI) were calculated. A value of p < 0.05 was considered statistically significant. For Kruskal–Wallis analyses, effect sizes were calculated using epsilon squared (ε²) to estimate the magnitude of group differences.

Results

The results of the microhardness, elemental analysis, and shear bond strength tests are presented in Tables 1, 2, 3 and 4. The Kruskal-Wallis test showed significant differences in the Vickers microhardness numbers and in the ICP-OES elemental analysis results for both calcium and phosphorus among the groups (p < 0.001).

Table 1.

Mean vickers microhardness (VHN) values of the enamel surfaces

Group	VHN (Mean ± SD) [95% CI]	Median (IQR)	Min	Max
Group 1. Sound enamel	389.53 ± 39.61a [367.60–411.47]	391 (60)	321	464
Group 2. Fluoride	127.87 ± 12.38b [121.01–134.72]	129 (18)	103	151
Group 3. Laser	129.40 ± 22.03b [117.20–141.60]	121 (29)	101	169
Group 4. Laser+Fluoride	131.13 ± 20.41b [119.83–142.44]	123 (21)	110	176
Group 5. Demineralized group	26.13 ± 7.10c [22.20–30.07]	27 (12)	16	40

Groups sharing the same letter are not significantly different from each other

p < 0.001; ε² = 0.757 Different superscript letters (a, b, c) indicate statistically significant differences among the groups according to the Mann-Whitney U test with Bonferroni correction (p < 0.05)

Table 2.

Mean calcium (Ca) release values of enamel surfaces

Group	Ca Release (ppm ± SD) [95% CI]	Median (IQR)	Min.	Max.
Group 1. Sound enamel	Not detected	-	-	-
Group 2. Fluoride	10.72 ± 1.44a [9.92–11.52]	10.77 (2.02)	8.00	13.02
Group 3. Laser	10.96 ± 1.65a [10.04–11.87]	10.96 (2.50)	8.82	13.85
Group 4. Laser+Fluoride	11.96 ± 1.92a [10.90–13.02]	12.16 (2.20)	8.50	15.60
Group 5. Demineralized group	44.61 ± 3.77b [42.52–46.70]	45.65 (4.58)	36.72	50.59

p < 0.001; ε² = 0.582 Different superscript letters (a, b) indicate statistically significant differences among the groups according to the Mann-Whitney U test with Bonferroni correction (p < 0.05)

Table 3.

Mean phosphorus (P) release values of enamel surfaces

Group	P Release (ppm ± SD) [95% CI]	Median (IQR)	Min.	Max.
Group 1. Sound enamel	Not detected	-	-	-
Group 2. Fluoride	3.39 ± 0.74a [2.97–3.86]	3.23 (1.00)	2.22	4.87
Group 3. Laser	3.40 ± 0.8a [2.98–3.80]	3.65 (1.09)	2.39	5.10
Group 4. Laser+Fluoride	4.23 ± 0.69a [3.85–4.61]	4.27 (0.99)	3.13	5.36
Group 5. Demineralized group	17.72 ± 1.67b [16.80–18.65]	18.02 (2.10)	14.70	20.18

p < 0.001; ε² = 0.646 Different superscript letters (a, b) indicate statistically significant differences among the groups according to the Mann-Whitney U test with Bonferroni correction (p < 0.05)

Table 4.

Mean shear bond strength (MPa) values of the study groups

Group	Shear Bond Strength (MPa ± SD) [95% CI]	Median (IQR)	Min.	Max.
Group 1. Sound enamel	9.41 ± 1.41 [8.63–10.20]	9.12 (2.12)	7.12	11.92
Group 2. Fluoride	9.92 ± 3.56 [7.95–11.89]	9.85 (1.68)	8.46	19.45
Group 3. Laser	9.59 ± 1.16 [8.95–10.23]	9.88 (1.04)	6.64	11.50
Group 4. Laser+Fluoride	9.75 ± 1.01 [9.19–10.31]	10.09 (1.96)	7.91	11.20

No statistically significant differences were detected among the groups according to the Kruskal-Wallis test (p > 0.05); ε² = 0.000

Vickers microhardness (VHN)

The untreated control group exhibited significantly higher Vickers microhardness values than all experimental groups, whereas the demineralized enamel group presented the lowest values (Table 1).

For enamel surface microhardness, the fluoride, laser, and laser+fluoride groups exhibited significantly higher microhardness values than the demineralized group, but significantly lower values than the sound enamel group (all p < 0.001). No statistically significant differences were observed among the fluoride, laser, and laser+fluoride groups (fluoride vs. laser, p = 0.713; fluoride vs. laser+fluoride, p = 0.870; laser vs. laser+fluoride, p = 0.624).

Elemental analysis (ICP-OES)

The Kruskal–Wallis test revealed significant differences in both calcium and phosphorus ion release among the groups (p < 0.001). The demineralized group exhibited the highest mean calcium (44.61 ± 3.77 ppm) and phosphorus (17.72 ± 1.67 ppm) release values, indicating the greatest mineral loss (Tables 2 and 3).

For calcium ion release, pairwise Mann–Whitney U comparisons showed that the demineralized group differed significantly from the fluoride, laser, and laser+fluoride groups (all p < 0.001). No statistically significant differences were observed among the fluoride, laser, and laser+fluoride groups (fluoride vs. laser: p = 0.713; fluoride vs. laser+fluoride: p = 0.056; laser vs. laser+fluoride: p = 0.116).

For phosphorus ion release, pairwise Mann–Whitney U comparisons showed that the demineralized group differed significantly from the fluoride, laser, and laser+fluoride groups (all p < 0.001). No statistically significant differences were observed among the fluoride, laser, and laser+fluoride groups (fluoride vs. laser: p = 0.902; fluoride vs. laser+fluoride: p = 0.057; laser vs. laser+fluoride: p = 0.054).

Shear bond strength

The mean shear bond strength values ranged between 9.41 ± 1.41 MPa and 9.92 ± 3.56 MPa across the groups, with no statistically significant differences (p = 0.766) (Table 4).

Discussion

Demineralisation and caries formation around orthodontic brackets is considered one of the most important complications of long orthodontic treatments [19]. Acid roughening prior to orthodontic bonding may reduce the hardness of the enamel surface, leading to easier accumulation of bacterial plaque around the bracket, increased Streptococcus mutans colonisation and a decrease in ambient pH below the remineralisation threshold. This may increase the risk of caries formation [20].

Fluoride is one of the most common agents promoting remineralisation of dental enamel tissue preventing its demineralisation [21, 22]. Topical fluoride forms a fluorapatite-like structure and increases resistance to the acidic environment [23].

The present study investigated the effects of Er,Cr:YSGG laser irradiation, topical fluoride application, and their combined use on enamel surface microhardness, calcium (Ca) and phosphorus (P) release, and the bond strength of orthodontic brackets. The findings suggest that fluoride and laser applications, both separately and in combination, may contribute to increased enamel resistance to demineralisation under the experimental conditions of this study without adversely affecting the bond strength of orthodontic brackets.

In our study, the microhardness of the enamel surface was analysed to evaluate the degree of mineral loss and recovery. This method is a simple, reliable, and rapid approach frequently preferred in the literature for assessing the effects of preventive treatments on enamel demineralisation [24–26].

When examining the results of our study, the microhardness of the demineralised enamel surface was found to be lower compared to the other groups. These results are consistent with the study by Shihabi et al. [27].

There are many studies in the literature showing that fluoride application significantly increases the surface hardness and provides mineral recovery in demineralised areas. In their in vitro study, Molaasadolah et al. observed an increase in the microhardness of demineralised enamel surface after application of two different fluoride varnishes [25]. Salehzadeh Esfahani et al. reported that fluoride-containing agents not only prevented demineralisation but also significantly increased surface microhardness [28]. In this study, compared to the control group without surface treatment after demineralisation, the demineralised study groups showed lower surface hardness, while the topical fluoride treated groups showed an increase in microhardness and a decrease in mineral loss.

Er,Cr:YSGG laser irradiation has also been reported to reduce enamel mineral loss in simulated caries conditions [29]. It has been shown that the significant increase in surface temperature during laser application creates acid-resistant molten areas, which makes acid diffusion to the surface difficult. Er: YAG lasers, similar to Erbium-doped Er,Cr:YSGG, have been shown to reduce the potential for demineralisation while maintaining a high level of shear bond strength in brackets [30].

Bezerra et al. reported that the combination of Er,Cr:YSGG laser and fluoride was more effective in reducing mineral loss compared to single treatments [31]. Gonzalez et al. suggested that laser irradiation may enhance the effects of topical fluoride on enamel surface properties [32]. Zhao et al. described a remineralisation effect following the combined application of Er,Cr:YSGG laser and a fluoride-containing varnish (Duraphat^®), which they associated with improved enamel protection and surface morphology compared to single applications [33]. However, although these previous studies suggested a potential advantage of combining laser and fluoride, the present study did not find statistically significant differences among the treated groups. In agreement with our findings, Tagliaferro et al. reported that there was no significant difference between the resistance to demineralisation achieved by laser treatment alone and by the combination of laser and fluoride in deciduous teeth [34]. Similarly, Molaasadollah et al. reported that the combination of Er,Cr:YSGG laser and 1.23% APF gel did not result in significantly greater remineralisation compared with fluoride application alone [16]. This discrepancy among studies may be attributed to differences in laser parameters, the type of fluoride agent used, application protocols, or experimental conditions.

In this study, the ICP-OES method was used to determine the amount of minerals (Ca and P) released into the demineralization solution after the experimental procedures. ICP-OES is a trace-level elemental analysis technique that has been increasingly used in dentistry for the quantitative evaluation of mineral content [35, 36].

Elemental analysis revealed no statistically significant difference between the group treated with the combination of laser and fluoride and the groups treated with either fluoride or laser alone, suggesting that, under the present experimental conditions, both the combined and individual applications may be effective in reducing mineral loss from enamel.

These findings also coincide with some studies in the literature. For example, Ana et al. reported that fluoride application following Er,Cr:YSGG laser irradiation reduced mineral loss by promoting the formation of CaF₂-like complexes; nevertheless, it did not show significant superiority over fluoride used alone [14].

Similarly, Eymirli et al. reported that the combined application of Er,Cr:YSGG laser and a fluoride varnish containing CPP-ACP enhanced mineral uptake in both deciduous and permanent teeth. The authors observed that this laser-supported combination resulted in a significant increase in phosphorus and calcium percentages, particularly in deciduous enamel. In contrast, for permanent teeth, the effect of laser irradiation alone was limited, while its combination with fluoride-based agents significantly improved the mineral content [37].

One of the primary objectives of the present study was to determine whether the combined application of Er,Cr:YSGG laser irradiation and topical fluoride would provide an additional protective effect against enamel demineralization. The absence of a statistically significant superiority of the laser+fluoride group compared with the fluoride-only or laser-only groups suggests that the combined application did not provide an additional benefit over the individual treatment modalities. From a clinical perspective, this finding may indicate that the choice of preventive approach can be guided more flexibly according to individual patient needs and treatment conditions.

It should be taken into consideration that the structural changes discussed in the present study were not directly evaluated using advanced characterization methods such as SEM, micro-computed tomography (micro-CT), or atomic force microscopy (AFM), which can provide more detailed and quantitative information regarding enamel surface and structural alterations. Instead, the interpretations were based on surface microhardness measurements and elemental solubility findings.

All SBS values were within or above the range commonly reported as clinically acceptable for orthodontic bracket bonding (approximately 6–8 MPa) [38].

In a study by Domantaitė and Trakinienė, the bond strength of brackets was evaluated after fluoride varnish application. In this study, a significant decrease in bond strength was observed in bracket bonds made one day after fluoride varnish application, but this difference was reported to disappear in bonds made after 30 days [39]. They observed that no statistically significant difference was found in the bond strength of the topical fluoride group compared with the control group. This suggests that the effect of fluoride on bond strength may vary depending on the time after application and material interactions.

Some previous studies have reported that fluoride application to enamel surfaces has little to no impact on shear bond strength, which is consistent with the findings of the present study [40, 41].

In an in vitro study by Nabawy et al., Er,Cr:YSGG laser was applied before bonding orthodontic brackets to the enamel surface, followed by remineralisation treatment with an agent containing casein phosphopeptide-amorphous calcium phosphate (CPP-ACP). The results demonstrated no statistically significant difference in bond strength between the laser-only and the laser + CPP-ACP groups. This suggests that the combined use of laser and remineralisation agents does not adversely affect the orthodontic bonding protocol while protecting the enamel against demineralisation [42]. The findings are in line with the bond strength results obtained in our study. Similarly, Gonçalves et al. reported no significant difference in shear bond strength when Er: YAG laser was applied to acid-etched enamel surfaces [43].

It should also be noted that the fluoride group demonstrated greater variability in shear bond strength values than the other groups. This finding may be related to the presence of several comparatively high bond strength values in the fluoride group, resulting in a wider distribution of measurements. In addition, the inherent heterogeneity of extracted human teeth may have contributed to this variability. Differences in enamel mineralization and surface characteristics associated with factors such as donor age, duration of intraoral exposure, and previous fluoride exposure may have influenced the fluoride–enamel interaction and, consequently, the bonding response among specimens, resulting in a broader range of values.

In this study, we applied a conventional acid etching method before orthodontic bonding. The additional acid etching of the laser-treated enamel surface may have masked the laser’s influence, which could explain the lack of a statistically significant difference in bond strength. Phosphoric acid etching is a standard clinical procedure prior to orthodontic bracket bonding. Therefore, conventional acid etching was applied to all specimens in order to simulate routine clinical conditions. Although inclusion of a non-etched laser subgroup could have clarified the isolated effect of laser irradiation, such a design would not have reflected current clinical practice. It is also possible that the sub-ablative Er,Cr:YSGG laser parameters used in the present study did not produce surface modifications sufficient to significantly influence orthodontic bond strength. Future studies may further investigate this interaction.

This study has certain limitations. It was conducted under in vitro conditions, which do not fully replicate dynamic intraoral factors such as salivary flow, pH fluctuations, and microbial activity. Therefore, the findings should be interpreted with caution and should not be directly extrapolated to clinical conditions. Furthermore, only a single Er,Cr:YSGG laser parameter was evaluated, and different irradiation settings may produce different outcomes. The lack of a dynamic pH-cycling model, together with the absence of thermocycling or artificial aging procedures, limits the assessment of the long-term stability of the study outcomes and orthodontic bond strength. Future studies incorporating more comprehensive experimental designs, including different laser parameters, aging protocols, pH-cycling models, as well as in vivo and long-term clinical investigations, are warranted to further validate the present findings and better establish their clinical applicability.

Conclusion

Within the limitations of this in vitro study, Er,Cr:YSGG laser irradiation may be considered a potential preventive approach for enhancing enamel resistance to demineralization without adversely affecting orthodontic bracket bond strength. However, the combined application of laser irradiation and fluoride did not demonstrate a statistically significant additional benefit compared with either laser irradiation or fluoride application alone. Therefore, preventive strategies during orthodontic treatment may be selected according to clinical requirements, patient-related factors, and clinical applicability rather than assuming the superiority of combined protocols. Further studies incorporating pH-cycling models, artificial aging procedures, surface characterization analyses, and long-term clinical investigations are needed to more comprehensively evaluate the clinical applicability of these approaches.

Authors' contributions

Conceptualization: S.B.; Methodology: S.B. and A.Ş.; Formal analysis and investigation: A.Ş. and H.Ö.; Material preparation and data collection: A.Ş. and H.Ö.; Writing - original draft preparation: A.Ş. and H.Ö.; Writing - review and editing: S.B.; Supervision: S.B. All authors read and approved the final manuscript.

Clinical trial number

Not applicable.

This study is an in vitro experimental design and does not involve any human participants or clinical interventions; therefore, clinical trial registration is not required.

Gels and blots

Not applicable (no gels or blots were used in this study).

Funding

No funding was received for this study.

Data availability

The datasets generated and analyzed during the current study are available from the corresponding author upon request.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the Declaration of Helsinki and was approved by the Non-Interventional Clinical Research Ethics Committee of Ordu University (Decision No: 2026/69). The study utilized extracted human teeth obtained for orthodontic and periodontal purposes. Written informed consent was obtained from all patients for the use of their extracted teeth for research.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.