Nickel release from orthodontic retention wires—The action of mechanical loading and pH


Nickel (Ni) is a potent sensitizer and may induce innate and adaptive immune responses. Ni is an important component of orthodontic appliances (8–50 wt%). Due to chemical and mechanical factors in the oral environment, Ni is released from these appliances. Retention wires are in situ for a long period of time.


To quantitatively evaluate the influence of mechanical loading and pH on the nickel release from orthodontic retention wires.


Five different types of multi-stranded wires (Original Wildcat, Noninium, Lingual retainer, Dentaflex 3-s, Dentaflex 6-s), were submersed for 24 h in either 10 ml of distilled water or lactic acid, both submitted to cyclic loading in a 3-point bending test (0×, 1000×, 10,000×). The solutions were analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and the data was statistically analyzed (ANOVA, p < 0.05).


Mechanical loading has a strong effect on the Ni release from orthodontic retention wires, especially in distilled water. Acidity has more impact on Ni release when compared to mechanical loading. Manganese-steel “ Ni-free ” wires released quantifiable amounts of Ni due to trace elements of Ni within the wire.


All investigated wires release considerable amounts of Ni to which exposure may have biological implications.


Nickel (Ni) is the most common cause of allergic contact dermatitis, with an estimation of incidence up to 17% in women and 3% in men . The difference in Ni allergy incidence between male and female is related to daily contact to jewelry and especially (ear) piercings in the female population, and is considered the most important cause of Ni sensitization . Nevertheless, other important sources of exposure are cosmetics, detergents, coins, the professional environment, and dentistry .

In dentistry, nickel is used in a broad spectrum of applications, but mainly in orthodontics for arch wires, bands, headgear, and brackets. During active orthodontic treatment, those appliances are used for a relatively short period of time with an average of approximately 2 years. Many studies report on the release of nickel from these fixed orthodontic appliances in both static and dynamic conditions . To achieve teeth position stability and prevent immediate relapse after the active orthodontic treatment it is common practice to apply a retainer wire. These wires remain in the oral cavity for many years or even decades . The most commonly used wires are made of stainless steel containing approximately 8 wt% nickel. Nowadays, these wires are multi-stranded instead of plain round or rectangular wires. In this way, they can achieve increased mechanical retention and improved physiologic movement of the teeth. The cross section of a strand can be round or rectangular and a wire is formed from three to six fine strands . Little attention is paid to adverse effects on dental or periodontal health after placement and if it is mentioned it is mostly related to the oral hygiene . To our knowledge reports on metal release, or specific the nickel release, from orthodontic retainer wires are scarce. One case report shows the clinical relevance of these retainer wires in a Ni allergic patient . The most important factor influencing Ni release from orthodontic appliances is corrosion, i.e., deterioration of a (dental) material with metallic ion release. One case study reports clinical signs of corrosion from a solder joint of a removable retainer containing Ni. However, the causative agent of the reaction is not reported .

The oral environment is dynamic, where factors like pH variation, temperature, salivary conditions, mechanical load, microbiological and enzymatic activity/bacterial acidic production have effect on the corrosion rates of metals . As mentioned above, retainer wires are twisted multi-strands and the Ni release rate of these wires might in principle be different compared to straight, plain wires, especially when subjected to movement induced by teeth function and para-functional habits. Furthermore, plaque is frequently accumulated on the retainer wires . This can locally lower the pH and subsequently enhance Ni release.

The aim of this study was to report on the nickel release of orthodontic retention wires which are subjected to mechanical loading at different pH. The Ni release of the wires was measured under standardized in vitro conditions. Furthermore, different types of wires were investigated, i.e., triple or six-stranded, with different diameters and different chemical composition.

Materials and methods

The type and brand of 4 stainless steel and 1 “nickel-free” retention wires investigated in this study are shown in Table 1 . The multi-stranded wires were sectioned in specimen of 22.0 mm in length. Three hundred and sixty specimens of the 5 different types of wires were divided into two main groups: (1) wires submersed in distilled water and (2) wires submersed in lactic acid 90%. Each main group was further divided into three subgroups with (A) wires not exposed to cyclic loading; (B) wires exposed to 1000 cyclic loading cycles; (C) wires exposed to 10,000 cyclic loading cycles. Each subgroup consisted of 12 wires.

Table 1
Brand, composition (wt%) of the wires by EDAX analysis, dimensions and surface area ( A w ) of the wires used in the study. Other relevant data are: r wire radius of the wire, r int radius of the circular space in the center of the strand; n number of turns of the strand; N number of strands of each wire, and the calculated length of strand L s . L wire is the length of the wire and was in all experiments 22.0 mm.
Wire Ni Fe Cr Mn d A w r wire r int n N L s
Noninium a , d 0.0 57.4 23.1 19.5 0.45 46.0 0.105 0.015 9.5 3 23.1
Original Wildcat b , e 8.9 69.7 19.5 1.2 0.45 46.6 0.105 0.015 10.6 3 23.4
Lingual retainer c , e 8.0 70.4 19.8 1.2 0.81 84.8 0.188 0.027 6.6 3 23.7
Dentaflex 3-s a , e 8.6 72.0 17.9 1.0 0.45 48.0 0.105 0.015 13.2 3 24.1
Dentaflex 6-s a , e 8.8 69.7 19.6 1.3 0.45 69.8 0.065 0.095 16.7 5 + 1 27.7

a Dentaurum, Ispringen, Germany.

b Dentsply International, USA.

c Unitek 3M, Monrovia, USA.

d Ni trace elements of <0.2% can be included according the manufacturer.

e Contain also traces of Cu (∼0.3 wt%) and Co (∼0.3 wt%).

For the cyclic loading, a metal-free setup was used where the wires were exposed to a three-point bending test. The test setup consisted of a modified injection syringe (Terumo ® Syringe, Luer Lock Tip, 10 ml) in which two parallel holes were made in each side of the syringe. Two wires per syringe were placed in the holes and were loaded by an air driven piston with a frequency of 1 Hz and a loading cycle of 50%. Movement of the piston of the syringe led to a bending force in the wires, with a deflection of 2–3 mm. Before testing, the specimens were ultrasonically cleansed with ethanol during 1 min, rinsed with distilled water and dried. The syringe with the 2 wires was placed in a plastic vial filled with 10 ml of distilled water or the lactic acid solution (ISO standard 10271: pH = 2.3, [NaCl] = 5.83 g, [Lactic acid] = 9 g/l) and mechanically loaded. After 24 h the wires were removed from the solution and 1 ml of nitric acid 6.5% was added to each sample, for measurement purposes.

The solutions were analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) (ELAN 6100, SCIEX – Toronto, Canada) to quantify the amount of ions released. The ICP-MS was calibrated against standard solutions consisting of 1% nitric acid and two solutions of Ni, Cr, Co, Fe, Mn and Cu ions in a concentration of 100 ppb and 10 ppb. Control samples were also analyzed and the values for each ion were subtracted from each analyzed sample. These controls were the experimental media (i.e., distilled water or lactic acid) without contact with the specimens. The composition of the wires was evaluated by Scanning Electron Microscopy-Energy-Dispersive X-ray spectrometry (SEM-EDAX) (model XL20, FEI Company, Netherlands), which has a detection limit of 0.2 wt%.

For the calculation of the surface area of each specimen a few formulas were used. The length of each strand of a helix shaped wire can be calculated according to Eq. (1) , where L s is the length of the strand, L wire the length of the wire, and n is the number of turns of the strand. The radius of the circular space in the center of the strand is given by the r int and the radius of the wire by r wire .

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Ls=((2πn(rint+rwire))2+Lwire2′>Ls=((2πn(rint+rwire))2+L2wireLs=((2πn(rint+rwire))2+Lwire2
L s = ( ( 2 π n ( r int + r w i r e ) ) 2 + L w i r e 2

The area of the wire can be calculated according to Eq. (2) and for the Dentaflex 6-s wire, with 5 strands turned around one central strand, Eq. (3) is used.

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Aw=2Nπrwire2+2NπrwireLs’>Aw=2Nπr2wire+2NπrwireLsAw=2Nπrwire2+2NπrwireLs
A w = 2 N π r w i r e 2 + 2 N π r w i r e L s
<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='Aw=2Nπrwire2+2NπrwireLs+2πrintLwire’>Aw=2Nπr2wire+2NπrwireLs+2πrintLwireAw=2Nπrwire2+2NπrwireLs+2πrintLwire
A w = 2 N π r w i r e 2 + 2 N π r w i r e L s + 2 π r int L w i r e
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Nickel release from orthodontic retention wires—The action of mechanical loading and pH
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