Effects of water immersion on mechanical properties of new esthetic orthodontic wire.
New fiber-reinforced plastic orthodontic wire (FRP wire) was fabricated with polymethyl methacrylate (PMMA) for the matrix and biocompatible CaO-P(2)O(5)-SiO(2)-Al(2)O(3) (CPSA) glass fibers for fibers that have not only high esthetics but also mechanical properties similar to those of metal wires. The purpose of this study was to evaluate the effects of water immersion on the mechanical properties of this new wire. The fiber-reinforced plastic orthodontic wire specimens were 0.5 mm in diameter with 29.1% to 60. 4% volume fraction of fibers that were 20 microm in diameter. A three-point flexural test was performed to obtain the elastic modulus and flexural load at the deflection of 1 mm under dry and wet conditions. Stress relaxation of the wires was tested under dry and wet conditions, and the wire diameters were measured before and at 20 days after immersion. The results showed that there were changes in the elastic modulus up until 10 days and in flexural load up until 20 days after immersion. The values of these two at 30 days after immersion were 93% and 87%, respectively, of those before immersion. Stress relaxation occurred rapidly from the start of immersion until about 60 minutes under dry conditions and about 120 minutes under wet conditions and then approached saturation. The swelling of hydrated fiber-reinforced plastic orthodontic wires affected the wire diameter, although this effect was not significant. The results of this study therefore suggest that the mechanical properties of fiber-reinforced plastic orthodontic wires are reduced by water immersion in the initial stage.