Salivary Nickel and Chromium Levels in Orthodontic Patients with and Without Fixed Metallic Appliances

Highlights

What are the main findings?

• Orthodontic fixed metallic appliances increase nickel and chromium in saliva.
• Nickel and chromium cause toxic effects like allergic reactions.
• Food pH affects metal release.

What are the implications of the main findings?

• Evaluate the risk of intoxication for the patient.
• Choose ceramic appliances.

Abstract

Background/Objectives: Dental malocclusions are often treated with appliances made of metal alloys. These alloys biodegrade in oral cavity and release toxic metals such as nickel and chromium. This study aimed to assess nickel and chromium content in the saliva of patients with and without fixed metallic orthodontic appliances. Methods: This was a descriptive cross-sectional study aiming to assess nickel and chromium content in saliva. A survey was conducted to record socio-demographic characteristics and clinical signs due to the wearing of fixed metallic orthodontic appliances. A 10 mL saliva sample was used to measure salivary pH and assess nickel and chromium concentrations using atomic emission spectrophotometry. A Student’s t-test compared saliva metal levels in non-wearers and wearers of metal orthodontic appliances. A Chi-square test was used to assess the influence of pH on metal release in patients. Results: A total of 92 participants, divided in two groups; 46 without appliance and 46 wearing appliance were received during the study period. Their mean age was 17.05 ± 6.46 years. Patients’ mean saliva pH was 6.97 ± 0.44. The mean nickel concentration was 4.39 ± 4.01 µg/L in the saliva of non-appliance wearers and 20.41 ± 18.56 µg/L in the saliva of appliance wearers, respectively. The chromium mean concentration was 1.3 ± 1.33 µg/L for non-appliance wearers and 9.38 ± 19.49 µg/L and for appliance wearers. Metal release is influenced by the pH of foods. Conclusions: Metal orthodontic appliances increase the release of nickel and chromium in saliva. It is necessary to monitor the risk of intolerance and optimize treatment duration.

1. Introduction

Correcting dental misalignment enables the dentition to fulfill its occlusal wedging, centering and guiding functions [1,2]. At the end of orthodontic treatment, oral functions such as chewing, phonation, swallowing and breathing are easily performed. Facial aesthetics and smile improve. Also, growth potential is optimal for the young person. To achieve these therapeutic goals, the multi-attachment metal alloy orthodontic appliance is usually used.

Unfortunately, the insertion in mouth of a fixed metallic orthodontic appliance is often accompanied by a metallic taste, irritative oral symptoms, the appearance of allergic reactions or hyperplastic gingivitis refractory to standard treatments [3,4]. This appliance includes molar bands or tubes, brackets, arch wires. These components are made of metals with varying rates of iron, nickel, chromium, molybdenum, cobalt, copper, cadmium or titanium [4,5].

In the oral environment, these metals are subject to variations due to saliva pH, oral temperature and the actions of enzymes or bacteria in dental plaque, food and then oral hygiene products [6,7,8]. The consumption of food or drinks with acidic or alkaline pH varies the pH of the oral cavity [9], increasing the formation of an oxide layer on the surface of dental alloys [10,11].

A series of oxidation and reduction reactions occur on the metals surface. Also, various mastication forces cause mechanical abrasion of orthodontic appliances and potentially release metal ions [12]. This biodegradation of fixed orthodontic appliances leads to the release of metal ions (nickel, chromium, iron, copper) in the oral cavity [13,14,15].

Food habits and oral hygiene methods vary from one individual to another. In rice, corn, peanuts, dried fish and tomatoes consumed in Burkina Faso, Bazié et al. found varying levels of nickel, chromium, lead, zinc, cadmium, manganese, mercury and arsenic [16]. The local context is also marked by the use of homemade cooking utensils made of metal alloys (aluminum, silicon, copper, zinc, chromium, etc.) which contaminate food during cooking [17,18]. These metals add to those potentially released by orthodontic appliances in mouth. Metals released in saliva go through the digestive tract and reach the bloodstream of exposed subjects and accumulate mainly in some organs such as the liver or kidneys.

In their studies on the release of toxic components by orthodontic appliances, authors found variable amounts of nickel, chromium or bisphenol A in the saliva of patients wearing orthodontic appliances [8,19,20,21,22,23]. According to Ousehal et al. [13], fixed orthodontic appliances release fairly high concentrations of nickel during the first days of their bonding in the oral cavity. As for Al-Rawe et al. [24], the increase in the release of metal ions is mainly due to the consumption of food containing acidic pH.

Furthermore, nickel and chromium cause more toxicological effects than other metals in dental alloys [25,26]. The prevalence of nickel allergy is estimated at 28.5% of the general population by Zigante et al. [27]. Cytotoxic, mutagenic and carcinogenic effects are reported due to the bioaccumulation of metals in the body, and the length of orthodontic treatments [5,26,28,29].

The high prevalence of orthodontic anomalies combined with the fact that alternative appliances such as clear aligners or resin braces also release toxic components such as bisphenol A or methyl methacrylate monomer, fixed metal braces are still widely used [30]. They are fairly less expensive and highly effective for correcting dental malpositions. It is the most commonly used appliance in our context.

Despite this interest for metallic orthodontic appliances, the toxicity remains a health concern. These reasons have raised the interest in investigations to characterize the risk of patient exposure to metals during orthodontic treatment.

We report the results of a study which general objective was to assess nickel and chromium content in the saliva of Burkinabe patients with and without fixed metallic orthodontic appliances.

2. Materials and Methods

2.1. Context, Type and Duration of the Study

This prospective and descriptive cross-sectional study was conducted from 21 July to 10 October 2025. Data collection involved subjects received between 9 am and 6 pm during consultation at Yalgado Ouédraogo University Hospital, at Eras dental clinic and at clinic Frany. Sample analysis was performed in the Laboratory of Environment, Toxicology and Health (LATES), University Joseph Ki-Zerbo, Ouagadougou.

2.2. Population Studied and Sampling

The study covered two groups of patients. Group 1 included subjects who did not wear orthodontic appliances. Group 2 consisted of subjects who wore multi-attachment metal orthodontic appliances. The sample size was decided for convenience, each group met the same inclusion criteria. After obtaining their informed consent, they were included when they came for consultation.

2.3. Inclusion and Non-Inclusion Criteria

Subjects included in this study were non-smokers and were not under medical treatment. Also, they were carefully examined for any oral disease, systemic disease, oral piercing, metallic dental restorations, dentures or welding on orthodontic appliances (diabetes, renal or hepatic insufficiency, Parkinson, antidepressant, antihistamine, antihypertensive). Patients with a known allergy to jewelry, watches or any other source of nickel were excluded.

The control group of subjects who did not wear orthodontic appliances was formed based on the same inclusion criteria. They were selected among patients who came for initial consultation.

2.4. Variables Studied

Using a clinical examination form, we reported the socio-demographic profile of the subjects selected for the study and their initial malocclusions according to the dental health component table of the Index of Orthodontic Treatment Need (IOTN). Then, we identified the clinical signs of appliance toxicity in patients, listed their food habits and assess their plaque index using the Silness and Löe method. Lastly, we measured the pH in each saliva sample as well as nickel and chromium quantities.

2.5. Appliances

2.5.1. Orthodontic Appliance

The fixed metallic orthodontic appliance for each patient comprised twenty brackets (BK Unix Roth 022 B704 22021HK) bonded with 3M transbond XT composite on the incisors, canines and premolars and four bands (B704 22092HK BWP Washplus) cemented with Fuji One glass-ionomer cement on the first molars, two preformed maxillary and mandibular orthodontic arch wires ligated with a power chain or elastomeric ligatures. This metallic orthodontic device was made by Orthoplus (Paris, France).

2.5.2. Laboratory Appliances

A Voltcraft PH-100 ATC (made by Volcraft in Taiwan and purchased in Germany) series pH meter was used to measure pH in saliva. It was calibrated every Monday morning to ensure the reliability of the measurements. A 190-L LG freezer was used to store saliva samples at −20 °C before transferring them to the laboratory for analysis.

PerkinElmer^® OptimaTM 8000 (made by PerkinElmer in Hainesport, NJ, USA) inductive coupling plasma atomic emission spectrometer was used for metal quantification. It uses a dual-spectrum optical system with a range of 160 to 900 nm, and a resolution of 0.009 to 200 nm. It enables simultaneous multi-element analysis and equipped with a dual-view detector.

2.5.3. Small Materials, Substances and Reagents

A set of small materials or reagents were used for the study. Sterile 50 milliliter polypropylene Falcon tubes were used to facilitate the access of the pH meter probe to saliva. Artificial saliva with a pH of 6.8 was used as a reference to calibrate the pH meter weekly.

2.5.4. Saliva Collection and Sample Incubation

Saliva sampling was made two hours after any oral hygiene protocol and at least thirty minutes after any food intake. The mouth was previously rinsed with 15 mL of distilled water for 30 s. Saliva was collected without any stimulation.

The patient was at rest with his mouth closed for 5 min and then directly spit up to 10 mL of saliva in a sterilized 50 mL polypropylene tube. The tube was pre-washed with nitric acid to dissolve any traces of metals and rinsed with demineralized water. Then, samples were anonymously labeled and placed in the freezer at −20 °C to ensure sample stability after saliva pH measurement.

2.5.5. Saliva pH Measurement Technique

The pH was measured immediately after saliva collection (maximum 30 min). The measurement was made using a Voltcraft portable electronic pH meter, pH-100 ATC series. After each measurement, the pH meter probe was rinsed with distilled water contained in a rinse bottle equipped with a soft pouring plastic spout and then dried with a soft, lint-free cloth.

2.6. Ethical Considerations

Participants were informed of the study objectives and how it will be conducted. They gave their informed consent in writing. Throughout the study, patient anonymity was respected and the confidentiality of the data collected was ensured. This study received authorization from the Health Research Ethics Committee (CERS) through its deliberation no. 2024-06-181 from 3 October 2024.

2.7. Data Processing and Analysis

Data collected were processed with SPSS 21 and Word 2016 software. The significance threshold p was set at 0.05. To highlight the interaction of orthodontic appliances in saliva, Student’s t-test was used to compare the averages of nickel and chromium in the saliva of subjects wearing and not wearing orthodontic appliances. A Chi-square test was used to assess the influence of pH on metal release in patients. Our variables are described by their mean and standard deviation.

3. Results

A total of 92 patients participated in this study, divided into two groups: 46 patients equipped with a multiband appliance and 46 people without any appliance. Participants were mainly pupils and students living in the Ouagadougou city as shown in

Table 1. Participants’ socio-demographic characteristics.

Socio-Demographic Variable	Without Appliance	With Appliance	Total
Participant number (n)	46	46	92
Age (average and standard deviation)	17.84 ± 7.07	16.26 ± 5.85	17.05 ± 6.46
Gender (n/%)
Male	18	27	45 (51.09)
Female	28	19	47 (48.91)
Residence (n/%)
Ouagadougou	40	43	83 (90.22%)
Others	6	3	9 (9.78%)
Occupation
Pupil	26	30	56 (60.87)
Student	17	16	33 (35.87)
Computer scientist	1	0	1 (1.09)
Engineer	1	0	1 (1.09)
Pharmacist	1	0	1 (1.09)

. The average age was 16.26 ± 5.85 years for those without appliance, and 17.84 ± 7.07 years for those equipped with appliance. Their food habits were dominated by sweet foods (see

Table 2. Distribution of participants per types of food eaten just before saliva sampling.

Diet Habit	Frequency	Rate
Favorite drink
Sugary soft drink	37	40.22%
Water	6	6.52%
Natural juice	49	53.26%
Food taken before saliva sampling
Cereals and cereal products (rice, pasta, couscous, porridge and bread)	73	79.35
Protein foods (meat, milk and eggs)	6	6.52
Leguminous foods (bean)	6	6.52
Roots and tubers (cassava, yam, potato)	4	4.35
Sugar-sweetened beverages (tea, coffee)	3	3.26
Total	92	100%

).

These patients consulted mainly for aesthetic reasons due to dental malposition summarized in

Table 3. Patient distribution according to clinical aspects.

Clinical Aspect	Frequency	Rate
Diagnosed orthodontic anomalies (n = 46)
Deep bite	7	15.21%
Open bite	5	10.87%
Missing tooth	1	2.17%
Retained tooth	2	4.35%
Diastema	4	8.70%
Dental crowding	15	32.61%
Crossbite	4	8.70%
Overjet	8	17.39%
Dental restorations (n = 46)
Composite	5	10.87%
Glass ionomer cement	2	4.35%
None	39	84.78%
Clinical signs due to the orthodontic appliance placement (n = 46)
Pruritus	3	6.52%
Irritation	22	47.82%
Metallic taste	19	41.30%
Burning	2	4.39%
Clinical procedure performed before saliva sampling (n = 46)
Arch wire replacement	16	34.78%
Bracket bonding	3	6.52%
Ring sealing	1	2.17%
Chain replacement	12	26.08%
Ligature replacement	38	81.61%

which also lists the clinical signs observed after the appliance was placed in the mouth and the procedure performed just before saliva sampling. Oral hygiene (

Table 4. Plaque index scores of patients accoding to Loe and Silness (n = 92).

Plaque Index	Without Appliances (n/%)	With Appliances (n/%)
Excellent oral hygiene	0	0
Good oral hygiene	10 (21.74%)	10 (21.74%)
Moderate oral hygiene	33 (71.74%)	22 (47.83%)
Poor oral hygiene	3 (6.52%)	14 (30.43%)

) was generally moderate, particularly among those wearing orthodontic appliances.

The results showed that the pH (see

Table 5. Repartition of participants according to saliva pH (n = 92).

Saliva pH	Without Appliances (n/%)	With Appliances (n/%)
[6–6.5]	2 (4.35%)	8 (17.39%)
[6.5–7]	16 (34.78%)	14 (30.43%)
[7–7.5]	27 (58.70%)	17 (36.96%)
[7.5–8]	1 (2.1%)	7 (15.22%)
Total	46 (100%)	46 (100%)
Saliva pH average	7.03 ± 0.32	6.97 ± 0.44

) was on average 6.97 ± 0.44 in participants wearing appliances and slightly higher in those who did not wear appliances. As shown in

Table 6. Salivary nickel and chromium levels (n = 92).

Metal (μg/L)	Without Appliance (n = 46)		With Appliance (n = 46)		p-Value
	Frequency	Rate	Frequency	Rate
Nickel					0.0002
<2	13	28.26%	2	4.35%
2 to 10	30	65.22%	18	39.13%
11 to 20	3	6.52%	11	23.91%
>20	0	0.00%	15	32.61%
Total	46	100%	100.00%	100.00%
Nickel average	4.39 ± 4.11		20.41 ± 18.56
Chromium					0.0031
<2	41	89.13%	17	36.96%
2 to 10	5	10.87%	22	47.83%
11 to 20	0	0.00%	4	8.70%
>20	0	0.00%	3	6.52%
Total	46	100%	46	100.00%
Chromium average	1.30 ± 1.33		9.38 ± 19.49

, on average nickel and chromium in patients were respectively 20.41 ± 18.56 and 9.38 ± 19.49. There is a statistically significant association between salivary pH and diet (p < 0.001), foods cause a variation in salivary pH (

Table 7. Relationship between salivary pH and dietary factors (n = 92).

Food	Salivary pH (%)				p-Value
	[6–6.5]	[6.5–7]	[7–7.5]	[7.5–8]	0.001
Cereals and cereal products	8 (11.59)	26 (37.68)	32 (46.38)	3 (4.35)
Protein foods	0	2 (33.33)	3 (50)	1 (16.67)
Leguminous foods	0	2 (16.67)	8 (66.67)	2 (16.67)
Roots and tubers	2 (66.67)	0	1 (33.33)	0
Sugar-sweetened beverages	0	0	0	2 (100)

). No significant association was found between nickel release and salivary pH (

Table 8. Relationship between nickel release and salivary pH (n = 46).

	Salivary pH (%)				p-Value
Nickel Release (µg/L)	[6–6.5]	[6.5–7]	[7–7.5]	[7.5–8]	0.08
<2	1 (50)	1 (50)	0	0
2 to 10	3 (20)	7 (46.67)	3 (20)	2 (13.33)
11 to 20	2 (13.33)	2 (13.33)	8 (53.33)	3 (20)
>20	2 (14.29)	4 (28.57)	6 (42.86)	2 (14.29)

). In

Table 9. Relationship between chromium release and salivary pH (n = 46).

	Salivary pH (%)				p-Value
Chromium Release (µg/L)	[6–6.5]	[6.5–7]	[7–7.5]	[7.5–8]	0.02
<2	2 (11.76)	6 (35.29)	8 (47.06)	1 (5.88)
2 to 10	6 (27.27)	4 (18.18)	7 (31.82)	5 (22.73)
11 to 20	0	3 (75)	0	1 (25)
>20	0	1 (33.33)	2 (66.67)	0 (0)

, a statistically significant association was observed between released chromium levels and salivary pH (p = 0.02).

4. Discussion

The results of this study show nickel and chromium release increase in patients’ saliva. Saliva seems to be a biological sample easier to study than blood, urine, hair, nails or skin. Sampling this biological fluid is less invasive than serum, easier to analyze than urine or hair samples.

The use of the freezer ensured the stability of the saliva samples before their analysis. Nickel and chromium concentrations in saliva samples remain stable for 6 months if stored at −20°. The pH is measured immediately, within thirty minutes after collection to avoid any change due to loss of CO₂ or temperature fall. Since the sample is taken at least thirty minutes after having eaten, saliva buffering capacity brings the pH back to an equilibrium value close to neutral. Nevertheless, we noticed that the appliance reduces salivary pH.

The spectrometer used had a quantification limit of 2 µg per liter of saliva. Below this limit, the exact value of nickel or chromium was unknown. This limited the accurate reading of low concentration of trace elements, particularly in the saliva of participants who do not wear appliances. We applied the calculation method that divides the limit of quantification by two [31]. Atomic absorption spectrometry is a reliable and sensitive method for quantifying trace metals in liquid samples [32].

Most of participants in this study were pupils and students living in the Ouagadougou city. Indeed, they seek orthodontic treatment for aesthetic reasons, as they are more concerned with their physical appearance. Discrepancy was the main reason for consultation. Orthodontic treatment with a multi-bracket metal device is indicated in these cases after the eruption of permanent teeth, generally above twelve years. Fixed metallic orthodontic appliances provide precise three-dimensional control of tooth movements. These patients had no metallic dental restorations to avoid any other source of metal release. This exclusion criterion led to the exclusion of some adult participants with dental amalgam restorations from the present study. Therefore, the study results cannot be generalized to the entire population.

This type of treatment is most often for adolescents whose diet is rich in fermentable sugars. Indeed 93.48% of the subjects surveyed consumed soft drinks and sweetened natural juices. Soft drinks and sugar affect saliva pH, which becomes acidic and increases the corrosion of dental alloys [7].

Furthermore, the presence of metals in subjects without orthodontic appliances shows that there are other sources of exposure to metals. Hence the need for an individual approach by considering other potential sources of metal exposure for each participant to orthodontic treatment with metal appliances. Metal levels in in-vivo studies could be altered by factors such as food, drinks, air, or tooth brushing [22]. The main mechanisms of ion release are related to galvanic corrosion and mechanical friction.

There are many sources of food exposure: meat, chocolate, cocoa, dried fruits (hazelnuts, almonds), cereals, legumes, tea, coffee, spinach. Canned goods can also be contaminated by the migration of metals from the container. Similarly, some stainless steel or homemade aluminum kitchen utensils widely used in our context can contribute to this contamination during cooking. Drinking water can also be contaminated by metals. The WHO guideline values for drinking water currently tolerate nickel concentration up to 70 μg/mL and chromium concentration up to 50 μg/mL [33].

Occupational sources of exposure to nickel and chromium include metalworking jobs (welder, factory), industries (automotive, jewelry, watchmaking, electrical, chrome plating) [34]. Preventive measures must be taken by wearing personal protective equipment, ventilating work areas, and regularly monitoring ambient air.

All these sources contribute to oral exposure which increases the doses of metals in patients wearing metal orthodontic appliances. Practitioners may, for some patients, recommend the use of alloys whose composition does not contain nickel [35]. For these potential risks, alternatives to metal components during orthodontic treatment can be ceramic brackets or clear aligners.

Ceramic brackets are made of zirconium oxide or aluminum oxide. They are more stable in saliva [36], but their cost is high and their implementation more delicate; easily breakable during treatment and their removal is very difficult. Indeed, the polished surface of the ceramic is less smooth compared to that of the metal, which facilitates the retention of dental plaque. The adhesive capacity of the composite is important in the ceramic interface, the breakage during removal refers to the tooth–composite interface. Enamel fractures regularly occur during the removal of ceramic brackets. All these drawbacks tend to limit the use of ceramic brackets.

The use of aligners is currently flourishing, notably among adult patients, due to their aesthetic appeal. They are designed using artificial intelligence. The cost of orthodontic treatment with aligners is higher compared to that of treatment using metal brackets. In addition, orthodontic aligners release bisphenol A, an endocrine disruptor, into saliva. It affects hormones and fertility by affecting spermatozoids and ovaries health [23].

The presence of the appliance significantly increases nickel and chromium in saliva. This corroborates the results found by some authors who have highlighted the increase of these metals in saliva after wearing orthodontic appliances. This increase can be attributed to the initial release of ions from newly exposed metallic surface before relative stabilization by a protective oxide layer. Values obtained in this study remain lower than those found by Ousehal et al. [13]. Bonding brackets, changing orthodontic arch wires or sealing molar bands increase metal release. Ours results appear to contradict those of Kocadereli et al. [37], who reported that orthodontic appliances did not lead to a significant increase in nickel and chromium levels in saliva.

Due to its strong buffering capacity, saliva restricts extreme pH fluctuations. In this study, the observed salivary pH range (6–8) affected chromium passivation, while exerting minimal impact on nickel release kinetics. Acidity appears to be the main factor driving substantial nickel release [7]. The composition of the alloys may further account for the observed significant chromium release.

The observation of the food consumed by our patients shows a predominance of starch which sticks to dental structures in areas difficult to access by brushing due to the presence of brackets and orthodontic arch wires. They arrived for consultation after eating a meal without brushing their teeth. Food debris accumulates into bacterial plaque [38]. Bacteria residing in dental plaque metabolize dietary residues to produce lactic acid [39,40,41]. The salivary pH decreases, promoting increased corrosion of metal alloys and the release of metals into the oral cavity. This rise in metal exposure, combined with other sources, can lead to the bioaccumulation of metals in organs such as the liver and kidneys, or trigger allergic reactions. Experimental studies have demonstrated cytotoxic, mutagenic, and carcinogenic effects associated with chronic nickel and chromium exposure [42,43,44,45,46].

By considering a patient who swallows one liter of saliva per day, according to the results of this study, he could swallow nearly twenty micrograms of nickel and ten micrograms of chromium per day during the treatment period, which can last up to two years. Since orthodontic appliances are made of several metals, the combination of both becomes a serious concern for the human body. As Sawadogo et al. [18] point out, other metals such as aluminum intervenes and could become a set of toxic metals for patients. Paradoxically, nickel and chromium were added to the alloy for its shine and to increase its resistance to corrosion.

However, the doses released are minimal, below the daily safety limit of 600 µg per day which corresponds to the toxicity threshold of nickel [47]. Many authors reiterate that the doses of metals released are significant at the beginning of appliance use, during the first week, and gradually decrease [48,49,50]. Clinically, nearly half of the orthodontic patients experienced irritative symptoms or a metallic taste. Nickel, in particular, is a well-known allergen. Nickel and chromium exposure entail toxicological risks, including oxidative stress, mutagenicity and carcinogenicity.

Finally, this was a cross-sectional study with minimal assessment in saliva with pH nearly null. A longitudinal study on larger cohorts before, during, and after orthodontic treatment would enable to better monitor the kinetics of metal release in saliva and long-term biological effects.

5. Conclusions

This study highlights the release of metals in saliva environment by the metallic orthodontic appliance consisting of bands, brackets, arche wires. The main results show that nickel and chromium significantly increase in saliva with a metallic orthodontic appliance. The presence of an orthodontic appliance in the oral cavity resulted in a fourfold increase in salivary nickel levels and a sevenfold increase in salivary chromium levels. Quantities released remain tolerable under sound clinical practice that considers the risks of sensitization and intolerance to metals, particularly nickel. This release is related to salivary pH.

Clinical signs report a sensation of metallic taste upon the fitting of the appliance.

While wearing fixed orthodontic appliances, it would be advisable to avoid foods with extreme alkaline and particularly acidic pH levels. As a preventative measure, it also seems necessary to reduce the duration of wearing metallic orthodontic appliances to the bare minimum.