Ion Release and Surface Changes of Nickel–Titanium Archwires Induced by Changes in the pH Value of the Saliva—Significance for Human Health Risk Assessment

Abstract

The aim of this study was to explore whether changes in the salivary pH influence mechanical properties, surface roughness, and ion release from NiTi archwires with various surface coatings, and discuss the clinical significance of the findings. The uncoated, rhodium-coated, and nitrified NiTi wires were immersed into artificial saliva of different pH values (4.8, 5.1, 5.5, and 6.6). Released nickel and titanium ions were measured with inductively coupled plasma-optical emission spectroscopy at the end of 28 days. Atomic force microscopy was used to measure the arithmetic average surface roughness Ra, the root-mean-square roughness Rq, and the maximum height of the asperities RZ. The nanoindentation hardness (HIT) and Young’s modulus (EIT) measurements were performed. The change in the pH of artificial saliva is inversely proportional to the release of titanium from both coated and uncoated wires, and the release of nickel from uncoated wires. The surface roughness parameters of both coated and uncoated wires are unaffected by the change in the pH of artificial saliva. The change in the pH of saliva has minor influence on the hardness and Young’s modulus of elasticity of both coated and uncoated wires. The concentration of released metal ions measured was below the recommended upper limit for daily intake; nevertheless, hypersensitivity effects cannot be excluded, even at lower concentrations and at low pH.

Corrosion of Fixed Orthodontic Appliances: Causes, Concerns, and Mitigation Strategies

“…Rhodium-coated archwires-usually as bimetallic Rh-Au coatings [332][333][334]-are available on the market due to their pleasant color and superior corrosion resistance, without compromising the mechanical properties of the bulk alloys [253,310,[335][336][337][338].…”

“…Mlinaric et al [279] demonstrated that the alteration of the properties of Rh-coated NiTi archwires depends on the type of oral antiseptics used. In turn, Osmani and colleagues [338] advised that such archwires should be avoided if oral hygiene is compromised due to the higher release of metallic ions.…”

“…The titanium nitride system is commercially accessible as TiN-coated brackets and archwires [64,115,279,332,338,340,341,[344][345][346]. The typically gold-colored TiN coatings may be produced onto orthodontic-compatible metals by several methods, including pulsed highenergy density plasma treatment with Ti electrodes [457], nitrogen gas (N 2 ) diffusion [421] and ammonia gas (NH 3 ) treatments [417], plasma immersion ion implantation [422,458], ion beam assisted deposition [459], ion plating [420,423,424,426,428,460], and other PVD methods [396,429,431,461].…”