Release of metal ions from orthodontic appliances: an in vitro study

Abstract

In this paper, we report the results of an in vitro experiment on the release of metal ions from orthodontic appliances composed of alloys containing iron, chromium, nickel, silicon, and molybdenum into artificial saliva. The concentrations of magnesium, aluminum, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, manganese, iron, cobalt, copper, zinc, nickel, and chromium were significantly higher in artificial saliva in which metal brackets, bands, and wires used in orthodontics were incubated. In relation to the maximum acceptable concentrations of metal ions in drinking water and to recommended daily doses, two elements of concern were nickel (573 vs. 15 μg/l in the controls) and chromium (101 vs. 8 μg/l in the controls). Three ion release coefficients were defined: α, a dimensionless multiplication factor; β, the difference in concentrations (in micrograms per liter); and γ, the ion release coefficient (in percent). The elevated levels of metals in saliva are thought to occur by corrosion of the chemical elements in the alloys or welding materials. The concentrations of some groups of dissolved elements appear to be interrelated.

Fig. 1

a–c Mean concentrations of elements in artificial saliva with and without appliances

Fig. 2

Box plot graph for concentration of Ni ions (a) in artificial saliva with (Ni) and without (Ni0) orthodontic appliance and for Cr (Cr, Cr0) (b). c Scatter plot showing the correlation between Ni and Cr ions in artificial saliva incubated with orthodontic appliance

Fig. 3

Coefficient α for element release into artificial saliva

Fig. 4

Coefficient β for the difference between the concentration (in micrograms per liter) of various metal ions in artificial saliva before and after incubation

Fig. 5

Dependence between the concentrations of elements introduced with the appliance and the increase of their concentrations in artificial saliva