Mechanical properties and aesthetics of FRP orthodontic wire fabricated by hot drawing

The FRP wires 0.5 mm in diameter with a multiple fiber structure were fabricated by drawing the fiber–polymer complex at 250°C for an esthetic, transparent orthodontic wire. Biocompatible CaO–P2O5–SiO2–Al2O3 (CPSA) glass fibers of 8–20 μm in diameter were oriented unidirectionally in the longitudinal direction in PMMA matrix. The mechanical properties were investigated by 3-point flexural test. The FRP wire showed sufficient strength and a very good elastic recovery after deformation. Young’s modulus and the flexural load at deflection 1 mm were nearly independent of the fiber diameter and linearly increased with the fiber fraction. The dependence on fiber fraction obeys well the rule of mixture. This FRP wire could cover the range of strength corresponding to the conventional metal orthodontic wires from Ni–Ti used in the initial stage of orthodontic treatments to Co–Cr used in the final stage by changing the volume ratio of glass fibers with the same external diameter. The estheticity in external appearance was excellent. Thus the new FRP wire can satisfy both mechanical properties necessary for an orthodontic wire and enough estheticity, which was not possible for the conventional metal wire.