Release of metal ions from fixed orthodontic appliance: an in vitro study in continuous flow system

Abstract

Objective: To evaluate the release of metal ions from fixed orthodontic appliances.

Materials and methods: A new system for in vitro testing of dental materials was constructed and consisted of a thermostatic glass reactor that enabled immersion of the studied material. Experimental conditions reflected the human oral cavity, with a temperature of 37°C and a saliva flow rate of 0.5mL/min. The simulated fixed orthodontic appliance made of stainless steel was evaluated. Sampling was performed at several time points during the 28-day study, and the metal ion concentration was determined by inductively coupled plasma optical emission spectrometry.

Results: The total mass of released metal ions from the appliance during 4 weeks of the experiment was as follows nickel 18.7 μg, chromium 5.47 μg, copper 31.3 μg.

Conclusions: The estimated doses of nickel, chromium, and copper determined by extrapolation of experimental data released during the treatment period were far below the toxic dose to humans. This shows that orthodontic treatment might not be a significant source of exposure to these metal ions.

Figure 1.

Schematic diagram of the system for the evaluation of ion release from dental materials. The installation consisted of: (1) a glass reactor with a water jacket, (2) a water bath with an external circuit to enable operation of the reactor under temperature-controlled conditions, (3) a multi-channel peristaltic pump to feed the solution into the reactor, (4) a glass tank placed on a thermostatic magnetic stirrer, and (5) the collecting vessel. The receiving solution flowed down gravitationally. The various streams of solutions were distributed through the system by flexible, chemically resistant hoses made of Norprene tubes (Masterflex, Vernon Hills, IL, USA) for use with the peristaltic pumps. Adequate flow rate was achieved by the diameter of the tubing. The material was placed in a glass tank, which was immersed in the solution, which flowed into the system.

Figure 2.

(a) Kinetics of release of Ni ions from the orthodontic appliance into the artificial saliva. (b) Kinetics of release of Cr ions from the orthodontic appliance into the artificial saliva. (c) Kinetics of release of Cd ions from the orthodontic appliance into the artificial saliva. (d) Kinetics of release of Cu ions from the orthodontic appliance into the artificial saliva.

Figure 2.

Continued. (e) Kinetics of release of Fe ions from the orthodontic appliance into the artificial saliva. (f) Kinetics of release of Mn ions from the orthodontic appliance into the artificial saliva. (g) Kinetics of release of Mo ions from the orthodontic appliance into the artificial saliva. (h) Kinetics of release of Si ions from the orthodontic appliance into the artificial saliva.