An Investigation into the Mechanical and Aesthetic Properties of New Generation Coated Nickel-titanium Wires in the As-received State and after Clinical Use

The orthodontic profession is constantly seeking to improve and optimize the aesthetics of orthodontic wires since the introduction of aesthetic brackets. Nickel-titanium (NiTi) wires since their introduction to orthodontics (Andreasen and Hilleman, 1971) have been extensively researched in vitro and are used as an initial levelling and aligning archwire because of its properties of springback and superelasticity (Burstone et al., 1985; Miura et al., 1986; Leu et al., 1990; Bishara et al., 1995; Bradley et al., 1996; Biermann et al., 2007; Berzins and Roberts 2010). Aesthetic wires are usually either coated NiTi wires or composite wires of reinforced polymers. Shape memory polymers have wide application in space technology and are being used currently in medicine and industrial applications (Jung and Cho, 2010; Hu et al., 2012). These wires have enormous potential for clinical application in orthodontics, and polyphenylene, a self-reinforced polymer composite, is close to being introduced to orthodontic practice (Burstone et al., 2011; Goldberg et al., 2011). However, these wires are still at the experimental stage. A fibre reinforced polymer is in clinical use (BioMers Products, LLC, Jacksonville, FL, USA) that is manufactured using a pultrusion process with a photo-cured resin (Gopal et al., 2005); however, these wires may be more likely to crack during bending and have been shown to deliver less consistent forces compared with alloy wires (Chang, 2012). The coated wires, which are currently available, either have an epoxy resin, polytetraflouroethylene (Teflon; Neumann et al., 2002), or a low reflectivity rhodium coating (Iijima et al., 2012) applied to the surface. Atomized Teflon particles are used to coat the wire using clean compressed air as a transport medium, which is then further heat treated in a chamber furnace (Husmann et al., 2002). The rhodium coating is applied by using a plasma-immersion ion implantation technique (Sridharan et al., 2004). The coated wires are found to be routinely damaged from mastication and activation of enzymes (Kusy, 1997). These wires have been shown to deliver lower forces in loading and unloading (Elayyan et al., 2010; Alavi and Hosseini, 2012; Iijima et al., 2012; Kaphoor and Sundareswaran, 2012). Poor colour stability has also been reported (Silva et al., 2013) and up to 25 per cent of the coating lost after 33 days in vivo (Elayyan et al., 2008). The coating itself, the process of its application and the fact that the NiTi component of the wire may be smaller to accommodate the thickness of the coating (Kaphoor and Sundareswaran, 2012), may account for these altered properties. The studies mentioned above concentrated on in vitro testing and comparison of these wires with the uncoated version. Only one previous study has investigated coated wires after clinical use (Elayyan et al., 2008). Therefore, the purpose of this study was to investigate key characteristics of four different types of Food and Drug Administration approved and commercially available wires in the as-received condition and after clinical use in the mouth