The aim of this study was to determine the nickel and titanium release from two different shapes of NiTi wires by immersion of the samples in artificial saliva. The wires had different cross sections but the same surface area. The cross sections were round and rectangular. In order for the rectangular and round NiTi wires to have the same surface area, 0.020-in. round and 0.016 × 0.016 in. rectangular wires were used in this study. The current study showed that with the same surface area, the concentration of Ni and Ti ions transferred from rectangular wires to the saliva is significantly higher than that of round wires. The difference might be due to the fact that cylindrical and rectangular bar shapes with the same surface areas do not have the same volume. The difference might also be due to the different edges of the cross sections. Further research is required to provide evidence for the effect of sharp and round edges on the release of ions into the saliva. The current study also showed that the amount of Ni and Ti ions significantly increased in both groups with the passage of time.
Ni is a widespread component of the Earth’s surface. Its presence in food and drinking water is determined by both natural and anthropogenic factors, the latter generically identifiable with industrial and technological sources. Ni and Ni compounds have been classified as carcinogenic to humans causing cancers of the lung and nasal cavity and paranasal sinuses after inhalation.
Numerous in vitro and in vivo studies have evaluated the release of metals from orthodontic appliances in biologic fluids such as saliva, blood, and urine.
Kuhta et al.  analyzed the effects of pH, composition of arch wire, and length of immersion on the release of metal ions from orthodontic appliances and found that release of metal ions was influenced by composition irrespective to the content of metal in the wire. Wataha et al.  also reported that although NiTi wires have a high percentage of nickel, the quantity of released nickel ions is smaller than that released from SS wire.
Most studies reported that the measurable amount of metals, released from orthodontic appliances in saliva or blood samples, was significantly below the toxic concentrations [3, 16]. Senkutvan et al.  analyzed the rate of Ni ion release from different types of arch wires used in orthodontics. They found that the quantities of metal ions released in their experimental conditions were not a concern in utilizing the appliance as the amount released was below the critical value necessary to induce allergy and below the daily dietary intake levels of 200–300 μg/day. Nonetheless, it cannot be excluded that even nontoxic concentrations might be sufficient to induce important biologic effects in the cells of the oral mucosa .
The release of metal ions from orthodontic appliances cannot be fully avoided; however, it would be advisable to use materials with lower amounts of ions released in the mouth. If the clinician has the option of using either round or rectangular NiTi wires, we suggest that round wires be given priority. However, more in vivo experiments are required to firmly determine the levels of dissolved nickel and titanium in different shapes of wires. Future studies with a lower pH to simulate intraoral conditions present with plaque buildup are also advisable. Comparing the surface roughness between round and rectangular wires from different manufacturers can also be enlightening for clinicians.