# Analysis of Coating Loss from Coated Stainless Steel Orthodontic Wire
## 1 Introduction  
The incidence of metal allergies has been increasing in recent years [1]. Although non-metal orthodontic appliances, such as plastic and ceramic brackets and elastomeric materials, have been used clinically and shown to be effective for the treatment of patients with metal allergies, only metal orthodontic wire is available, greatly limiting the types of orthodontic treatments that can be applied in such patients. Four types of alloy wire, i.e., stainless steel (SS), cobalt-chromium (Co-Cr), nickel-titanium (Ni-Ti), and \(\beta\)-titanium (\(\beta\)-Ti) wire, are used clinically as orthodontic arch wires for multi-bracket treatment [2].  
Selective application of these alloy wires can provide a broad range of orthodontic forces at various stages of treatment. Most orthodontic wires contain metals that cause metal allergies, including nickel (Ni), cobalt (Co), and chromium (Cr), and therefore the potential for release of metal ions into the oral cavity represents a problem precluding their use in patients with metal allergies [3, 4]. [PERSON] et al. [5] showed that elution of metal ions can occur from orthodontic appliances containing metal materials and reported significantly higher Ni ion concentrations in the dental plaque of patients with orthodontic appliances compared to non-orthodontic patients. Therefore, only biocompatible \(\beta\)-Ti wire can be applied in patients with metal allergies, but their poor rigidity restricts their application and the establishment of proper orthodontic force systems [3]. To solve these problems and enable the use of appropriate types of wire for patients with metal allergies, it is necessary to modify the surface of orthodontic wires to prevent the leaching of metal ions into the oral cavity. Wire can be plated to increase corrosion and wear resistance, increase surfacehardness, reduce friction, and improve aesthetic appearance [6; 7; 8]. Orthodontic wires plated with gold (Au) and rhodium (Rh) or coated with resin are already commercially available. Our previous studies [9; 10] showed that plating of SS orthodontic arch-wire with titanium nitride (TiN) improved corrosion resistance and reduced friction without compromising the original mechanical properties of the substrate metal. However, coated wires may be susceptible to coating loss due to delamination or wear in the oral cavity [6]. The resulting exposure of the substrate metals in the oral cavity is associated with increased risk of developing metal allergy [11] due to metal ion leaching [12; 13; 14; 15]. However, the details of coating loss from coated wires have yet to be elucidated.  
The present study focused on the effects of low pH, mechanical deformation, and brushing as possible causes of metal exposure in the oral cavity. Low pH conditions can lead to degradation of coated orthodontic wires [12; 15; 16]. Mechanical deformation of the coated wire surface has been reported to result in coating loss, suggesting that coating loss may occur with bending of the wire [14; 17; 18]. Previous studies examined only the effects of bending at right angles [19], while orthodontic treatment often involves the bending of wires at various other angles, and it is therefore necessary to examine the effects of such bending to simulate clinical use. Brushing has also been reported to cause coating loss in epoxy resin-coated and chromium nitride (CrN)-coated wires [15; 20], and the mechanical effects of brushing may also cause coating loss in other types of coated wire.  
To our knowledge, there have been no previous investigations of corrosion resistance of orthodontic wires with multiple coating materials. The null hypothesis to be researched in this study was that exposure to low pH conditions (H01), bending (H02), and brushing (H03) of several types of coated orthodontic SS wire would not cause coating loss.