Introduction

Orthodontic treatments have become increasingly common across a broader patient demographic, driven by technological advancements and the growing emphasis on aesthetics in modern society. Among adult patients, aesthetic concerns are particularly pronounced, often accompanied by the use of composite and prosthetic restorations. This shift highlights the importance of aesthetic orthodontic options in clinical practice1,2,3.

Orthodontic brackets, a cornerstone of these treatments, are classified based on their material, design, connection type, size, and shape4. These brackets are generally grouped into three categories: metal, ceramic, and plastic5. Ceramic brackets are preferred over plastic ones for their superior durability and aesthetic advantages compared to metal brackets, although allergenic properties remain a critical factor in bracket selection6.

Nowadays, different types of brackets are being manufactured, such as self-ligating brackets, which offer several advantages, such as shorter procedure times, fewer appointments, reduced overall treatment time, and lower friction resistance7. However, like all orthodontic brackets, they are subject to harsh oral conditions, such as humidity, pH, and temperature fluctuations, mechanical forces, and plaque accumulation. These factors contribute to bracket failure through mechanisms such as corrosion and friction8,9,10.

Corrosion is a significant issue in the oral environment, influenced by factors such as chewing forces, occlusion patterns, oral hygiene, dietary habits, and medication use11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27. While primarily associated with metals, it also affects ceramics, polymers, and cements, compromising durability and altering material properties. Intraoral corrosion can release harmful ions, posing local and systemic risks16,17,18.

Gastroesophageal reflux disease (GERD) is a condition in which stomach acid flows back into the esophagus, exposing orthodontic materials to highly acidic conditions (pH as low as 1.1). This acidity accelerates corrosion, leading to the release of metal ions such as chromium (Cr) and nickel (Ni), which can negatively affect oral tissues and pose systemic risks, including hypersensitivity reactions and potential carcinogenic or mutagenic effects19,20,21.

Previous studies have examined parameters such as enamel surface discoloration and enamel erosion in patients with gastroesophageal reflux disease by simulating gastric acid. However, research on the corrosion behavior of different orthodontic brackets under specific acidic conditions remains limited27,28.

This study aims to investigate the impact of gastric acid on the corrosion of metal, ceramic, and self-ligating (SL) brackets by analyzing ions such as nickel and chromium and determining their surface roughness at different time intervals.

The null hypothesis stated that there would be no significant differences in the corrosion and surface characteristics of various orthodontic bracket types when immersed in solutions with varying pH levels.

Material and methods

Materials and grouping

This study used three groups: metal brackets (M)(Orthos Metal Twin Brackets, Ormco,California, USA), Self-Ligating brackets (SL)(Damon Ultima, Ormco,California, USA), and ceramic brackets (C) (Aesthetic Twin, Ormco,California, USA) (N = 198, n = 66). The percentages of chromium (chrome), manganese (Mn), iron (Fe), nickel (Ni), Aluminum oxit (Alo), and Oxite composite (Oc) elements are given in Table 1 and Fig. 1 describes the workflow of the present study.

Table 1 Elemental composition of orthodontic brackets (%), presented with bracket types as rows and elements as columns.
Full size table
Fig. 1
figure 1

Schematic of sample prepration and experimental flow.

Full size image

Prepratıon of solutions

The samples in each group were divided into three subgroups, with two different gastric solutions (pH 1.5 and 3) and artificial saliva (pH 7) serving as the control group (n = 22). All solutions were prepared in the Biruni University Faculty of Pharmacy Laboratory.

Artificial saliva was using the following ingredients: 7.69 g/L dipotassium phosphate, 2.46 g/L dipotassium hydrogen phosphate, 5.3 g/L sodium chloride, and 9.3 g/L potassium chloride. The pH was adjusted to 7.0 by adding an appropriate amount of lactic acid.

The gastric solutions mimicking human gastroesophageal reflux were prepared with the following ingredients, 2.0 g of sodium chloride (NaCl), 3.2 g of pepsin, 7.0 mL of hydrochloric acid (HCl), and water. They were adjusted to a pH of 3.0 to represent a moderate gastric insufficiency condition and to a pH of 1.5 to represent a severe gastric insufficiency condition27,28.

All bracket specimens were placed in airtight glass tubes containing 1.5 mL of the respective solution and incubated at 37 °C for three different time intervals: 30 min, 24 h, and 1 month9. The incubations were carried out using an Elektro-mag M5040 P oven (Istanbul, Turkey) with a sample size of n = 22 for each time interval.

Ions release

The release of metal ions from the brackets in the study was quantitatively analyzed using an inductively coupled plasma mass spectrometry (ICP-MS) instrument (Thermo Scientific,Germany)21, specifically the Thermo X Series model. This analysis measured the concentrations of nickel (Ni) and chromium (Cr) ions released into the solution over time.(Fig. 2).

Fig. 2
figure 2

Scanning electron microscopy (SEM) observations were performed at 200× magnification for group M (metal) under the following conditions: pH 1.5 and 3 (gastric acid), and pH 7 (artificial saliva), with evaluation time intervals of 3 min, 24 h, and 1 month. The imaging parameters were standardized across all groups: acceleration voltage (HV) of 15.00 kV, spot size of 3.0, magnification of 1000× , detector type (ETD), mode (SE), working distance (WD) of 8.3–9.8 mm, and scale bar of 200 µm. The analyses were conducted using a Quanta FEG SEM system.

Full size image

Surface profilometer and SEM (scanning electron microscope)

A surface profilometer equipped with Zeiss Axio CSM700 (Carl Zeiss AG,Göttingen, Germany) Controller software was used to measure the surface roughness of the brackets. The instrument analyzed a total area of 1,087,313.578 µm2. It is capable of non-contact optical measurements and provides high-resolution 3D imaging of surfaces, allowing for detailed analysis of surface roughness, topography, and layer thickness. Additionally, an electron microscope (SEM, Model JEOL JSM-6060, Japan) was used to examine the surface morphology of the specimens. The imaging was conducted at 1000× magnification with an accelerating voltage of 15 keV for M and SL brackets, while 5 keV was used for ceramic brackets to prevent reflections and ensure high-resolution imaging. The measurement results were digitally recorded to form the dataset for this study. (Figs. 2, 3, 4).

Fig. 3
figure 3

Scanning electron microscopy (SEM) observations were performed at 200 × magnification for group SL (self-ligating) under the following conditions: pH 1.5 and 3 (gastric acid), and pH 7 (artificial saliva), with evaluation time intervals of 3 min, 24 h, and 1 month. The imaging parameters were standardized across all groups: acceleration voltage (HV) of 15.00 kV, spot size of 3.0, magnification of 1000× , detector type (ETD), mode (SE), working distance (WD) of 8.3–9.8 mm, and scale bar of 200 µm. The analyses were conducted using a Quanta FEG SEM system.

Full size image
Fig. 4
figure 4

Scanning electron microscopy (SEM) observations were performed at 200 × magnification for group C (ceramic) under the following conditions: pH 1.5 and 3 (gastric acid), and pH 7 (artificial saliva), with evaluation time intervals of 3 min, 24 h, and 1 month. The imaging parameters were standardized across all groups: acceleration voltage (HV) of 15.00 kV, spot size of 3.0, magnification of 1000 × , detector type (ETD), mode (SE), working distance (WD) of 8.3–9.8 mm, and scale bar of 200 µm. The analyses were conducted using a Quanta FEG SEM system.

Full size image

Statistical analyses

The statistical power analysis was conducted using the G*Power software program (v3.0.10). With a significance level (α) set at 0.05, an effect size (f) of 0.4, and a power of 80%, the sample size was determined accordingly to ensure sufficient statistical robustness11.

Statistical analyses were performed using SPSS 22.0 (Statistical Package for the Social Sciences). Descriptive statistics were applied, followed by the Kruskal–Wallis test and Mann–Whitney U test to compare the release of nickel (Ni) and chromium (Cr) ions between metal and self-ligating (SL) brackets exposed to gastric fluid for 30 min, 24 h, and 7 days, as the data did not follow a normal distribution.

To determine statistically significant group differences, a Tukey post-hoc test was performed following ANOVA for multiple comparisons of surface roughness (Ra values) across different bracket types and exposure durations. The Tukey test was chosen due to its ability to control the Type I error rate while conducting multiple comparisons.

Results

The evaluated data, presenting the average values of Nickel (Ni) and Chromium (Cr) ion release and surface roughness for M, SL, and C brackets, are summarized in Tables 2, 3, 4 and Figs. 5, 6, 7, 8.

Table 2 Median and standard deviation (± SD) of nickel (ni) ion release findings for m and sl brackets (mg/l).
Full size table
Table 3 Median and standard deviation (± SD) of chromium (cr) ıon release for m and sl brackets (mg/l).
Full size table
Table 4 Median and standard deviation (± SD) surface roughness (Ra and Rz) for M, SL and C brackets (µm).
Full size table
Fig. 5
figure 5

Median values Nickel (ni) ion release for M, SL, and C brackets under different conditions: pH 1.5 and pH 3 (gastric acid) and pH 7 (artificial saliva), at different time 30 min, 24 h, and 1 month.

Full size image
Fig. 6
figure 6

Median values of chromium (Cr) ion release for M, SL, and C brackets under different conditions: pH 1.5 and pH 3 (gastric acid) and pH 7 (artificial saliva), at different time 30 min, 24 h, and 1 month.

Full size image
Fig. 7
figure 7

Median of surface roughness (Ra) for M, SL, and C brackets under different conditions: ph 1.5 and ph 3 (gastric acid) and ph 7 (artificial saliva), at different time 30 min, 24 h, and 1 month.

Full size image

Ni ion release, a significant difference was observed between all brackets at all pH levels after 30 min: M brackets released less Ni at pH 1.5 (t =  − 2.842, p < 0.05), more at pH 3.0 (t = 4.689, p < 0.05), and less at pH 7.0 (t =  − 8.317, p < 0.05). Cr ion release was consistently higher in SL brackets than in other brackets across all pH levels (pH 1.5: t =  − 13.830; pH 3.0: t =  − 8.317; pH 7.0: t =  − 11.684, all p < 0.05). (Tables 2, 3, Figs. 5, 6).

Fig. 8
figure 8

Median of surface roughness (Rz) for M, SL, and C brackets under different conditions: pH 1.5 and pH 3 (gastric acid) and pH 7 (artificial saliva), at different time 30 min, 24 h, and 1 month.

Full size image

For 24 h exposure, Ni release was higher in M brackets at pH 1.5 (t = 24.186, p < 0.05) and higher in SL brackets at pH 3.0 and 7.0 (p < 0.05). Over 1 month, M brackets showed higher Ni release at pH 1.5 (t = 5.314, p < 0.05) and 7.0 (t = 18.434, p < 0.05), while no significant difference was observed at pH 3.0 (t = 0.233, p > 0.05). (Tables 2, 3, Figs. 5, 6).

Surface roughness analysis indicated that M brackets had the highest Ra values, while C brackets had the lowest across all pH levels for both 30 min and 1 month exposures (p < 0.017) Similarly, after 1 month of exposure to gastric fluid, M brackets again exhibited the highest surface roughness, with C brackets showing the lowest at pH 1.5 and 3.0 (p < 0.017). At pH 7.0, a significant difference was found between M and SL brackets, and between M and C brackets, with M brackets maintaining the highest surface roughness compared to the other types. (Table 4, Figs. 7, 8).

Discussion

This study demonstrated significant differences in the corrosion behavior and surface properties of orthodontic brackets under varying pH levels and time intervals, leading to the rejection of the null hypothesis. The results highlight that gastric acid environments, as seen in gastroesophageal reflux disease (GERD), exacerbate corrosion and surface changes, particularly in metal and self-ligating brackets9. These findings align with previous studies showing that acidic conditions promote electro-galvanic corrosion, ion release, and biocompatibility concerns3,9,25.

ICP-MS, used in this study, provided precise measurements of ion release, reinforcing its role as a gold-standard technique for detecting trace elements. Consistent with prior research, nickel (Ni) release was significantly higher than chromium (Cr) release, with Ni posing greater biocompatibility risks, including hypersensitivity reactions10.

Consistent with previous studies, this research confirmed that nickel ion release was significantly higher than chromium ion release. House et al16 and Kerosuo et al22 identified nickel as a major concern due to biocompatibility risks like hypersensitivity. Bhaskar et al23 reported elevated nickel release within the first seven days, highlighting the early susceptibility of orthodontic materials to ion leaching. Similarly, Mikulewicz et al12 found Ni release to exceed that of Cr and other ions.

Self-ligating brackets released more nickel ions than conventional brackets from the same manufacturer, highlighting the impact of design and material properties on corrosion and ion release, with potential implications for biocompatibility and patient safety in early orthodontic treatment24.

In this study, self-ligating brackets exhibited the highest nickel ion release during the first 24 h, with metal brackets surpassing them after one month. The consistently higher chromium ion release from self-ligating brackets across all time intervals suggests that material composition and design significantly influence corrosion behavior. Acidic environments (pH 1.5 and 3.0) further intensified nickel and chromium ion release, with the most pronounced effects observed at pH 1.5. These findings reinforce the need for careful consideration of both material properties and bracket design in clinical practice to minimize adverse biological effects and improve long-term stability.

Interestingly, ceramic brackets exhibited superior resistance to corrosion, attributed to the incorporation of Al₂O₃ particles25. Al₂O₃ enhances corrosion resistance by forming a dense, protective oxide layer that increases compactness and reduces porosity in ceramic coatings. This barrier minimizes electrolyte penetration, limiting ion exchange and preventing material degradation. As a result, ceramic brackets remain more stable and less reactive in acidic conditions, consistent with previous findings25,26.

Mann et al27 and Cengiz et al28 investigated the impact of pH levels simulating GERD on corrosion and surface characteristics, confirming the deleterious effects of highly acidic environments. This study effectively simulated real-world conditions by using pH values of 1.5 and 3.0 to represent severe and moderate reflux, providing clinically relevant insights27,28.

The analysis of surface roughness revealed significant pH-dependent variations. Metal and self-ligating brackets displayed the highest surface roughness at pH 1.5, while ceramic brackets maintained the lowest roughness values throughout the study period. Surface roughness peaked within the first 24 h and subsequently decreased over one month. These variations likely reflect differences in material composition and electrochemical interactions with the acidic medium29.

Conclusion

This study demonstrated that acidic conditions significantly increase corrosion rates and ion release from orthodontic brackets, with self-ligating and metal brackets being most affected. In contrast, ceramic brackets enhanced with Al₂O₃ particles showed superior corrosion resistance and minimal surface roughness changes. Nickel ion release was highest in self-ligating brackets within the first 24 h, while metal brackets exhibited increased release over time. Surface roughness changes were most pronounced in metal brackets, particularly in acidic environments, underscoring the impact of material composition.

A key limitation of this study was the use of a single corrosion assessment method. While the weight-loss technique provides valuable data, it may not fully capture corrosion dynamics. Future studies should incorporate additional methods, such as electrochemical impedance spectroscopy and SEM, and investigate the effects of temperature and prolonged exposure to enhance clinical relevance and improve the durability of orthodontic materials. These findings emphasize the importance of material selection and design, especially for patients with acidic oral conditions like GERD.