Corrosion effects related to bending fracture resistance of orthodontic mini-implants

Aim: this study is to evaluate the effect of corrosion on flexural fracture resistance in orthodontic mini-implants composed of two materials and submerged in salivary substitutes with or without fluoride. Methods: twenty mini-implants were used, 10 from SIN Company (Ti6AL4V alloys) and 10 from Morelli (steel alloys), (G1: Ti6AL4V in fluoride-free saliva solution; G2: Ti6AL4V in saliva solution with 1500 ppm of fluoride; G3: Steel in saliva without fluoride; and G4: Steel in saliva with 1500 ppm of fluoride). The samples were taken to a potentiostat to evaluate the corrosion, and then were evaluated under scanning electron microscopy (SEM). Then, the mini-implants underwent flexural fracture resistance tests. Kruskal-Wallis test with the Student-Newman-Keuls comparison evaluated the corrosion and pitting potentials of each group. ANOVA and Tukey’s comparison test at a 1% significance level. Results: All groups suffered corrosion potential and pitting potential, but those that were in solutions with the presence of fluoride showed less resistance to the formation of corrosion pits (G1 and G3). In the SEM analysis after flexural resistance, small cavities suggestive of pitting corrosion were noted. The G4 was the only one that formed the passivation potential. In the fracture resistance test, mini-implants manufactured by Ti6AL4V fractured with less force applied (G1 and G2). Most steel mini-implants (G3 and G4) only deformed with a higher force application. Conclusion: Fluoride acts to corrode mini-implants, regardless of their manufacturing material. Regarding flexural resistence, the corrosion rate of the mini implants did`nt influence the fracture resistance values.