Determination of metal ions release from orthodontic archwires in artificial saliva using inductively coupled plasma-optical emission spectrometer (ICP-OES)

Abstract

Over the last decades many concerns have been raised regarding the migration of potentially toxic metals from the orthodontic appliances to the oral environment due to the dynamic dominant conditions. The current study aimed to investigate the effect of the oral environment acidity and aging time on the ion release from orthodontic archwires. For this purpose, dental archwires consisted of three different alloys were immersed in artificial saliva of varied pH values for 7 and 30 days at 37±1°C. The liquid extracts were then analyzed with inductively coupled plasma-optical emission spectrometer (ICP-OES). It was found that the released ion species and the measured concentrations were not in accordance with manufacturers’ data. Furthermore, the leachates were mainly enriched with Cr and Ni ions by decreasing the saliva pH, while most of the archwires released the highest amounts of Ni, Mn and Cr ions after 30 days aging at pH = 3.5. Independent of the material type or the aging conditions, the total release of Ni and Cr ions was within the considered average dietary intake levels.

Keywords

Ion release orthodontic archwires inductively coupled plasma-optical emission spectrometry (ICP-OES)artificial saliva

1 Introduction

Orthodontic bands, brackets and archwires, consist of stainless steel containing nickel (Ni), chromium (Cr), manganese (Mn), copper (Cu), titanium (Ti) and iron (Fe) [1]. Clinical requirements for shape memory and superelasticity properties are usually met by Ni-Ti wires also capable of initial alignment. Particularly, strength and springiness attitude can be achieved by using beta-titanium concurrently imparting good formability to the orthodontic alloy [2].

However, the complicated dynamic conditions dominating in the oral environment such as mechanical stresses, chemical, thermal, biological and enzymatic changes [3 –5] possibly comprise threats against the long-life activity of archwires. A chemically aggressive environment established by human saliva favors the well-defined corrosion, namely the gradual degradation of wire by electrochemical attack, leading to a metal ion release even if a protective oxide film exists on the surface of the alloy [6, 7]. The aforementioned process not only deteriorates the surface performance thus increasing the friction at the interface between the wire and bracket [8], but can also be associated with adverse biological effects provoked by interactions between the released metal ions and the human tissues. The latter are of great concern regarding the biological response of archwires in terms of the chemical risk assessment during the period of time that the orthodontic alloys are used. Although Ti ions are not considered as possible factors impairing cytotoxic effects [9 –11], Ni and Cr can prompt allergic reactions [12 –18]. On the other hand, metal ions leached from orthodontic appliances could have carcinogenic, mutagenic, and cytotoxic effects [19 –23]. Moreover, an increase of Ti leakage may weaken the protective surface film of NiTi alloys [6], facilitating the Ni ion release [6, 7] which, in turn, may involve NiTi alloys in cytotoxic reactions as reported by both in vitro [24] and vivo studies [25].

Various analytical techniques such as atomic absorption spectrometry (AAS) [26, 27], inductively coupled plasma-atomic emission spectrometry or optical emission spectrometry (ICP-AES or ICP-OES) [20 , 28–30], ICP-mass spectrometry (ICP-MS) [23 , 31–33], have been utilized in order to estimate the metal ion release from several orthodontic appliances. Among them, the ICP-OES technique constitutes a widespread, powerful analytical tool for the determination of trace elements, well-known about its simplicity, low thresholds and simultaneous measurements for up to 70 elements [34]. In this context, metal constituents were investigated in a variety of simple or complex matrices. For instance, ICP-OES wasused to determine the Ni ion release amount from Ti₅₀Ni_47.2Co_2.8 orthodontic alloy in artificial saliva [35]. Furthermore, both saliva and hair samples were subjected to multi-elemental analysis for toxic metals during orthodontic treatment in cleft lip and palate patients.Velasco-Ibáñez et al. [36] evaluated the release of mainly Ni and Ti in urine and plasma by means of ICP-OES, under orthodontic treatment with Ni–Ti heat activated wires and stainless steel wires. Wołowiec et al. [37] conducted hair mineral analysis regarding Cr and Ni levels using ICP-OES, on the basis of dietary habits of the studied population.

The present study aims at the detection and quantitation of metal ions which may be leached from six different types of commercially available orthodontic archwires to the saliva using the ICP-OES analytical technique. The effect of the artificial saliva pH values and the time period of contact between these specific dental archwires and oral environment on the extent of ion release were studied for the first time, thus adding new data toward the estimation of the safe exploitation of modern appliances in orthodontic treatment.

2 Experimental section

2.1 Chemicals and reagents

Detailed information of the studied archwires is listed in Table 1. Sodium chloride (NaCl) was provided by Centralchem, potassium chloride (KCl) from Chem-Lab NV, urea (CH₄N₂O) from Honeywell-Fluka, while calcium chloride dehydrate (CaCl₂·2H₂O), sodium phosphate monobasic (NaH₂PO₄) and sodium sulfide nonahydrate (Na₂S·9H₂O) were all purchased from Sigma-Aldrich. DL-lactic acid and potassium hydroxide (KOH) were obtained from Sigma-Aldrich and Honeywell-Fluka respectively. The multielement standard solution 6 for ICP (TraceCERT^®) containing 100 mg/L of Ni, Cr, Mn, Ti and Mo in 5%w/w HNO₃ was used as stock solution for the plotting of each element calibration curve and was supplied from Sigma-Aldrich. HNO₃, 65%w/w, (ultrapure) provided by Chem-Lab was diluted to 5%w/w with double distilled water and used for standard dilutions.

Table 1
Orthodondic materials used in the study

Material (archwire) Alloy Trade name Size (inches) Manufacturer

1 SS Wire SS Upper 016 Form III 0.016×0.016 American Orthodontics (AO, Sheboygan, Wisconsin)

2 NiTi Wire NiTi Form I Upper 016 0.016×0.016 American Orthodontics (AO, Sheboygan, Wisconsin)

3 CuNiTi Tanzo^® Copper Nickel Titanium (Tanzo Low Wire Upper 016) 0.016×0.016 American Orthodontics (AO, Sheboygan, Wisconsin)

4 SS Tru-Arch^® UM 0.016×0.016 Ormco (Ormco, Glendora, CA)

5 NiTi Tru-Arch^® UM 0.016×0.016 Ormco (Ormco, Glendora, CA)

6 CuNiTi Tru-Arch^® CuNiTi 35°C UL 0.016×0.022 Ormco (Ormco, Glendora, CA)

Material (archwire)	Alloy	Trade name	Size (inches)	Manufacturer
1	SS	Wire SS Upper 016 Form III	0.016×0.016	American Orthodontics (AO, Sheboygan, Wisconsin)
2	NiTi	Wire NiTi Form I Upper 016	0.016×0.016	American Orthodontics (AO, Sheboygan, Wisconsin)
3	CuNiTi	Tanzo^® Copper Nickel Titanium (Tanzo Low Wire Upper 016)	0.016×0.016	American Orthodontics (AO, Sheboygan, Wisconsin)
4	SS	Tru-Arch^® UM	0.016×0.016	Ormco (Ormco, Glendora, CA)
5	NiTi	Tru-Arch^® UM	0.016×0.016	Ormco (Ormco, Glendora, CA)
6	CuNiTi	Tru-Arch^® CuNiTi 35°C UL	0.016×0.022	Ormco (Ormco, Glendora, CA)

2.2 Methods

2.2.1 Inductively coupled plasma-optical emission spectrometer (ICP-OES) analysis

Measurements were conducted by means of an axial viewing ICP-OES system (Perkin Elmer, Optima 2100 DV). The optimum instrumental conditions were set as: 0.8 L min^-1 nebulizer argon flow rate, 1300 W incident power and 1.5 mL min^-1 sample flow rate. The following characteristic spectral lines were selected to determine the amount of each element: Ni: 231.604 nm, Mn: 260.568 nm, Cr: 205.560 nm, Mo: 203.845 nm and Ti: 334.940 nm. Three readings per element were conducted during sample running. Each sample was prepared and analyzed three times and an average value per element was taken to obtain accurate results. Artificial saliva was also measured as blank solution for the baseline correction. Calibration curves were plotted as regression lines (y = bx+a) for each element separately by diluting the stock multielement standard solution in the appropriate concentration ranges. The slope values (b), intercept values (a) and the correlation coefficients (R) of the regression lines, as well as the detection (LOD) and quantitation (LOQ) limits calculated for all elements are given in Table 2.

Table 2
Regression data and ICP detection (LOD) and quantitation (LOQ) limits for the selected wavelengths used for elemental analysis of archwires’ leachates

Element, wavelength (nm) b (cps ppb^-1) a (cps) R LOD (ppb)^† LOQ (ppb)^‡

Ni, 231.604 86.99 93.86 0.9957 1.53 4.63

Mn, 260.568 1336.55 2341.45 1.0000 0.20 0.61

Cr, 205.560 85.88 58.13 0.9940 0.28 0.86

Mo, 203.845 30.96 –3.13 0.9958 0.45 1.36

Ti, 334.940 2035.04 1143.02 0.9962 0.44 1.35

Element, wavelength (nm)	b (cps ppb^-1)	a (cps)	R	LOD (ppb)^†	LOQ (ppb)^‡
Ni, 231.604	86.99	93.86	0.9957	1.53	4.63
Mn, 260.568	1336.55	2341.45	1.0000	0.20	0.61
Cr, 205.560	85.88	58.13	0.9940	0.28	0.86
Mo, 203.845	30.96	–3.13	0.9958	0.45	1.36
Ti, 334.940	2035.04	1143.02	0.9962	0.44	1.35

^†Concentration corresponding to five standard deviations (SD) of the signals of blank samples (HNO₃ 5%w/w) divided by slope (LOD = 3.3 SD/b). ^‡Concentration corresponding to five standard deviations (SD) of the signals of blank samples (HNO₃ 5%w/w) divided by slope (LOQ = 10 SD/b).

2.3 Sample preparation

Each group of the Table 1 was studied for three pH values (3.50, 5.00, 7.00) at two different time periods (7 and 30 days). To simulate the oral environment, artificial saliva was prepared by dissolving 0.40 g NaCl, 0.40 g KCl, 0.80 g CaCl₂·2H₂O, 1.00 g CH₄N₂O, 0.78 g NaH₂PO₄·2H₂O, and 0.005 g Na₂S·9H₂O in 1 L of water for injection [38]. The final pH value was adjusted to 3.5, 5 and 7 by either DL-Lactic acid or 1M KOH and measured by means of a benchtop pH meter (Thermo Scientific™ Orion™ 3-Star) at 25°C.

All tested wires were cut to 8 cm length using a stainless steel wire cutter (World Precision Instruments, Sarasota, FL), and rinsed with acetone and distilled water. Afterwards they were immersed in 22 mL of each different pH artificial saliva and stored at 37±1°C for 7 and 30 days respectively. The liquid medium was then separated from wires, immediately filtered and subjected to elemental analysis.

2.4 Statistical analysis

Classic Student’s t-test followed by Bonferroni corrections was applied for the measured ion release concentrations to determine significant differences where applicable (p < 0.01), taking into account both the parameters of pH and aging time. The IBM SPSS statistics (version 25) software was used to perform the statistical analysis. The values of the quantified concentrations represent mean values±standard deviation of replicates (n = 3).

3 Results and discussion

The present investigation focuses on the importance of particular factors that can influence the release of metal ions from orthodontic archwires to artificial saliva. Total accurate mean values followed by standard deviations (SD) of the determined metal ion release concentrations from the six different orthodontic archwires, examined after immersion in 7 and 30 days at medium pH values of 3.50and 5.00, are presented in Table 3. Non-detected ions (ND) or elemental responses found under the calculated LOQ’s of the selected ICP analysis technique are also included. It can be seen that for the overall selected aging conditions the number of samples containing quantifiable concentrations (n = 22) is slightly higher in relation to those under the estimated LOQs (n = 19). Especially, a pH decrease and a shorter immersion time period in artificial saliva medium somehow favor the direct quantitation procedure for some ion species.

Table 3
Average metal ion release (ppb) by time period from different orthodontic archwires, for artificial saliva pH = 3.50 and 5.00 respectively

Ni Mn Cr Mo Ti

pH pH pH pH pH

3.50 5.00 3.50 5.00 3.50 5.00 3.50 5.00 3.50 5.00

Archwire Time period (days) Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD

Ormco SS 7 4.16^a 0.92 ND^† – ND^† – ND – 14.79 2.20 ND – ND – ND – ND – ND –

30 7.96^a 0.52 ND – 12.59 1.73 ND – 17.96 5.24 ND – <LOQ – ND – ND – ND –

Ormco NiTi 7 <LOQ^‡ – ND – ND – ND – ND – ND – ND – ND – ND – ND –

30 <LOQ – <LOQ^‡ – ND – 2.20 1.20 ND – ND – ND – <LOQ – ND – ND –

Ormco CuNiTi 7 <LOQ – 5.61^b 0.19 ND – ND – ND – 24.98 1.33 ND – ND – ND – ND –

30 10.60^b 0.60 ND – ND – 4.37 1.62 ND – ND – ND – <LOQ – ND – ND –

AO SS 7 <LOQ – ND – <LOQ – ND – <LOQ – ND – ND – ND – ND – ND –

30 <LOQ – ND – ND – ND – 5.61 0.81 ND – <LOQ – ND – ND – ND –

AO NiTi 7 8.39^c 3.75 7.40^d 1.15 ND – ND – ND – ND – ND – ND – ND – ND –

30 17.95^c,d,e 0.74 4.88^e 0.69 2.40 0.65 ND – ND – ND – ND – ND – ND – 1.39 0.81

AO CuNiTi 7 14.65 4.40 <LOQ – 12.02 2.72 ND – <LOQ – ND – <LOQ – ND – 23.39 4.77 ND –

30 8.52 0.42 <LOQ – 4.82 1.88 <LOQ – ND – ND – ND – ND – ND – ND –

		Ni	Mn	Cr	Mo	Ti
Ormco SS	7	4.16^a	0.92	ND^†	–	ND^†	–	ND	–	14.79	2.20	ND	–	ND	–	ND	–	ND	–	ND	–
	30	7.96^a	0.52	ND	–	12.59	1.73	ND	–	17.96	5.24	ND	–	<LOQ	–	ND	–	ND	–	ND	–
Ormco NiTi	7	<LOQ^‡	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	<LOQ	–	<LOQ^‡	–	ND	–	2.20	1.20	ND	–	ND	–	ND	–	<LOQ	–	ND	–	ND	–
Ormco CuNiTi	7	<LOQ	–	5.61^b	0.19	ND	–	ND	–	ND	–	24.98	1.33	ND	–	ND	–	ND	–	ND	–
	30	10.60^b	0.60	ND	–	ND	–	4.37	1.62	ND	–	ND	–	ND	–	<LOQ	–	ND	–	ND	–
AO SS	7	<LOQ	–	ND	–	<LOQ	–	ND	–	<LOQ	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	<LOQ	–	ND	–	ND	–	ND	–	5.61	0.81	ND	–	<LOQ	–	ND	–	ND	–	ND	–
AO NiTi	7	8.39^c	3.75	7.40^d	1.15	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	17.95^c,d,e	0.74	4.88^e	0.69	2.40	0.65	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	1.39	0.81
AO CuNiTi	7	14.65	4.40	<LOQ	–	12.02	2.72	ND	–	<LOQ	–	ND	–	<LOQ	–	ND	–	23.39	4.77	ND	–
	30	8.52	0.42	<LOQ	–	4.82	1.88	<LOQ	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–

^†ND denotes not detected ion responses. ^‡<LOQ denotes ion responses under the quantitation limit. ^†‡ The groups with the same superscript letters exhibit statistically significant differences (p < 0.01) under a common labelling process.

Regardless either pH conditions or aging time, it is obvious that no Molybdenum (Mo) traces were quantified for all archwires, while Ni ions were occurred in each case (Fig. 1). Particularly, AO NiTi wires (Fig. 1c) exhibited the highest prevalence of metal ion release, among others, while having the maximum Ni contents followed by AO CuNiTi wires (Fig. 1d). AO CuNiTi (Fig. 1d) and Ormco SS (Fig. 1a) extracts were found to contain the highest loadings of Mn ions. Moreover, AO CuNiTi seems to be the most prone archwire to Ti ion release, followed by AO NiTi. All other archwires showed a relative resistance to Ti ion diffusion from the metal to aqueous medium. Remarkably increased Cr concentrations deal with OrmcoCuNiTi (Fig. 1b) and Ormco SS. Concerning the different type of the examined archwires, the observed diversity of ion release behavior implies a differential corrosion resistance at constant aging conditions. This behavior could be probably attributed to their particular chemical composition according to manufacturers’ declaration, as well as to different processing techniques selected for archwires’ production, leading to specific metallurgic structures which could affect their surface topography and generating special manufacturing defects capable of accelerating the corrosion process [6, 8]. The results showed that the highest levels of leaching were detected for Cr ions in both archwires, even if it is not stated in the provided safety datasheets (SDSs). Moreover, Mo ions were also detected for Ormco SS, OrmcoCuNiTi and AO CuNiTi, while Mn was measurable for AO NiTi and AO CuNiTi without being presented in the relative SDSs. Similar contradiction between the experimental release of metal ions in aqueous media and the content of metal in the alloy was also reported previously [23]. The notable Ti release found in AO CuNiTi extract accompanied by the concomitant determined amounts of Ni and Mn verified the corruption of the TiO₂ protective surface layer against chemical corrosion [39].

Fig. 1

Representative graphs of total ion release concentrations for a) Ormco SS, b) Ormco CuNiTi, c) AO NiTi, and d) AO CuNiTi archwires after 7 and 30 days immersion at different pH values of artificial saliva. †Single-asterisk marking corresponds to not detected ion species. †‡Double-asterisk marking corresponds to ion responses < LOQ.

At pH = 3.5, an augmentation of Ni ion content is observed over time (Fig. 2a) reaching the maximum levels for Ormco SS, Ormco CuNiTi and AO NiTi after 30 days immersion in artificial saliva. According to the Table 3 data, statistically significant differences are observed for Ni concentrations between 7 and 30 days regarding not only the Ormco SS but also the AO NiTi archwire (p < 0.01). Similar increasing trend is also recorded for Mn ion concentration (Fig. 2b) in Ormco SS and AO NiTi, as well as regarding the Cr ion concentration (Fig. 2c) in Ormoco SS and AO SS. On the contrary, the AO CuNiTi archwire seems to remain the most resistant in terms of Ni, Mn and Cr extraction against aging period time exhibiting lower ion concentrations.

Fig. 2

Comparative plots with ion release concentrations of a) Ni, b) Mn, and c) Cr ions from different archwires after 7 and 30 days immersion time in artificial saliva of pH = 3.50. †Single-asterisk marking corresponds to not detected ion species. †‡Double-asterisk marking corresponds to ion responses < LOQ.

By keeping the acidity of the oral environment at pH = 5, Ni ions were just detected for Ormco NiTi (Fig. 3a), whereas Mn ion levels (Fig. 3b) were finally found to be increased for Ormco NiTi and Ormco CuNiTi. In the case of AO NiTi archwires both Ni and Ti ion release was kept at lower levels even after 30 days immersion in saliva. The acceleration of ion release rates revealed for Ni, Mn and Cr even after 1 month immersion in highly acidic artificial saliva in the majority of dental archwires seem to be in contrast to previous results [6 , 40]. The observed differences have been explained by diverse experimental designs [3 , 41]. Additionally, AO CuNiTi wires seem to be more prone to chemical corrosion within 1 week in lowest pH medium and further withstand ion release over time, while Ni ions were released in pH = 5.00 in the same way. This behavior could be attributed to either a possible saturation of the solution with the released metal ions or to the occurrence of insoluble precipitates in the form of stable oxide layers which could act as barriers against ions release. Under these surface conditions limited amounts of ions are available in solutes for elemental analysis and thus remain undetectable [8, 23].

Fig. 3

Comparative plots with ion release concentrations of a) Ni, and b) Mn ions from different archwires after 7 and 30 days immersion time in artificial saliva of pH = 5.00. †Single-asterisk marking corresponds to not detected ion species. †‡Double-asterisk marking corresponds to ion responses < LOQ.

It is worth mentioning that a pH regulation to 7.00 rather inhibits the ion release for the majority of the tested archwires regardless the aging time. Some traces of Mn and Mo ions were just detected only for AO SS eluates after a 7 days immersion in neutralised saliva.

It can be seen that a gradual decrease of pH value to 3.5 results in the progressive detection of Ni ions in the series of Ormco NiTi, Ormco CuNiTi and AO SS leachates, as well as in the highest Ni contents for Ormco SS, AO NiTi and AO CuNiTi, for the shorter selected aging time of 1 week (Fig. 4a). In a similar manner, Cr ions started to become detectable in the AO SS extracts, while Cr levels were also enhanced for Ormco SS at pH = 3.50 and for Ormco CuNiTi at pH = 5.00, even if a remarkable resistance against Cr ion release is confirmed for the latter at pH = 3.50 (Fig. 4b).

Fig. 4

Comparative plots with ion release concentrations of a) Ni, and b) Cr ions from different archwires after 7 days immersion time in artificial saliva of pH values 3.50, 5.00 and 7.00. †Single-asterisk marking corresponds to not detected ion species. †‡Double-asterisk marking corresponds to ion responses < LOQ.

Regarding the selected immersion time of 1 month, the Ni ion concentration rises until it reaches the highest values at pH = 3.5 for Ormco SS, Ormco CuNiTi, AO NiTi and AO CuNiTi, while Ni becomes detectable for AO SS archwire (Fig. 5a). Statitistically significant differences were found for Ni ion content regarding the AO NiTi archwire over 30 days aging, when pH was altered from 3.50 to 5.00 (p < 0.01, Table 3). Furthermore, Mn ion levels increase in the extreme pH value of 3.5 in the cases of Ormco SS, AO NiTi and AO CuNiTi, while Mn ions are not yet detectable for Ormco NiTi and Ormco CuNiTi denoting an achieved resistance against chemical corrosion (Fig. 5b). Likewise, Cr concentration ascended for both Ormco SS and AO SS under the same condition tendencies (Fig. 5c). In the oral environment, pH fluctuations of saliva are expected to occur in high prevalence. The consumption of acidic meals and drinks in combination with the microbiological activity frequently taking place on the retainer wires may establish periods of low pH contributing to high ion release rates [7]. Indeed, the largest raise for Cr ions levels sparked by a gradual decrease of pH in artificial saliva as detected in the present work was previously confirmed by Kuhta et al. [23] when Cr ion leaching was compared to that of Ni and Ti in acidic conditions. The observed increment of Ni ion levels for the majority of the studied orthodontic archwires, mostly reflected by a typical 3.7–fold increase recorded for AO NiTi wire, denotes an intensive weakness of TiO₂ surface protection. The aforementioned process was also described previously [6, 7]. In fact, statistically significant differences were found for Ni content in AO NiTi under diverse acidic conditions between 7 days and 1 month aging. Significant differences were also detected in Ormco CuNiTi archwire (p < 0.01, Table 3). Milheiro et al. [42] showed that the acidity of the oral environment might have more impact on Ni release than mechanical loading. Staffolani et al. [27] claimed that orthodontic appliances are more susceptible to both Ni and Cr ion release when pH was decreased from pH = 6.5 to 3.5.

Fig. 5

Comparative plots with ion release concentrations of a) Ni, b) Mn, and c) Cr ions from different archwires after 30 days immersion time in artificial saliva of pH values 3.50, 5.00 and 7.00. †Single-asterisk marking corresponds to not detected ion species. †‡Double-asterisk marking corresponds to ion responses < LOQ.

Herein, the largest amount release measured for Ni corresponds to 0.013μg/day, for Cr to 0.078μg/day, for Mn to 0.009μg/day and for Ti to 0.073μg/day (Fig. 6). Food chain is supposed to be the main source of Ni and Cr for human being due to the impact of a modern life-style. The presence of Ni, Cr, as well as of their respective compounds has been associated with possible cancer risk by well-documented literature data [43, 44]. However, the total release of Ni and Cr values was well below the normal dietary intake of Ni (200–300μg/day) and Cr (50–200μg/day) [3, 38]. Even if no systemic toxic reactions have been mentioned at such low Ni levels, however allergic reactions in a previously sensitized patient may be induced [45 –47] specifically when taking into account the relatively extended presence of fixed orthodontic appliances in the oral cavity [31, 48].

Fig. 6

Maximum values of total Ni, Cr, Mn and Ti ion release per day from different orthodontic archwires.

4 Conclusions

In the present study, the ICP-OES technique was used to estimate the ion release from orthodontic appliances in artificial saliva under different aging conditions. It was found that the type and amount of the released metal ion is independent of the metal content in the alloy as stated by SDSs. A gradual decrease of saliva pH favors mainly the raise for Cr and Ni ion levels implying possible intensive weakness of TiO₂ surface protection. In highly acidic saliva, ion release rates for Ni, Mn and Cr metals were found to accelerate after 30 days aging for the majority of the examined dental archwires. The total release of Ni and Cr ions from the tested orthodontic appliances was finally quantified much less the considered average dietary intake levels, regardless either the alloy type or the aging conditions. These findings could provide critical information targeting the improvement of safe orthodontic treatments in the framework of the contemporary clinical practice.

Conflicts of interest

There no competing interest to declare.

Footnotes

Acknowledgments

The experimental procedures were performed at the Department of Basic Dental Sciences, Division of Dental Tissues Pathology and Therapeutics, School of Dentistry, Aristotle University of Thessaloniki.

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		Ni				Mn				Cr				Mo				Ti
		pH				pH				pH				pH				pH
		3.50		5.00		3.50		5.00		3.50		5.00		3.50		5.00		3.50		5.00
Archwire	Time period (days)	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Ormco SS	7	4.16^a	0.92	ND^†	–	ND^†	–	ND	–	14.79	2.20	ND	–	ND	–	ND	–	ND	–	ND	–
	30	7.96^a	0.52	ND	–	12.59	1.73	ND	–	17.96	5.24	ND	–	<LOQ	–	ND	–	ND	–	ND	–
Ormco NiTi	7	<LOQ^‡	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	<LOQ	–	<LOQ^‡	–	ND	–	2.20	1.20	ND	–	ND	–	ND	–	<LOQ	–	ND	–	ND	–
Ormco CuNiTi	7	<LOQ	–	5.61^b	0.19	ND	–	ND	–	ND	–	24.98	1.33	ND	–	ND	–	ND	–	ND	–
	30	10.60^b	0.60	ND	–	ND	–	4.37	1.62	ND	–	ND	–	ND	–	<LOQ	–	ND	–	ND	–
AO SS	7	<LOQ	–	ND	–	<LOQ	–	ND	–	<LOQ	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	<LOQ	–	ND	–	ND	–	ND	–	5.61	0.81	ND	–	<LOQ	–	ND	–	ND	–	ND	–
AO NiTi	7	8.39^c	3.75	7.40^d	1.15	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–
	30	17.95^c,d,e	0.74	4.88^e	0.69	2.40	0.65	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–	1.39	0.81
AO CuNiTi	7	14.65	4.40	<LOQ	–	12.02	2.72	ND	–	<LOQ	–	ND	–	<LOQ	–	ND	–	23.39	4.77	ND	–
	30	8.52	0.42	<LOQ	–	4.82	1.88	<LOQ	–	ND	–	ND	–	ND	–	ND	–	ND	–	ND	–