Effects of corrosion on orthodontic mini-implants related to removal torque fracture resistance.

Background: This study evaluated the effect of metallic corrosion on the torsional fracture resistance of mini-implants of different alloys in two solutions: artificial saliva and artificial saliva+fluoride.

Material and methods: The research included 60 mini-implants: 30 of Ti6Al4V and 30 of stainless steel from the brand Morelli. The groups were divided into G1: stainless steel control, G2: Ti6Al4V control, G3: stainless steel in saliva, G4: stainless steel in saliva+fluoride, G5: Ti6Al4V in saliva, and G6: Ti6Al4V in saliva+fluoride, all with n=10. A potentiostat conducted electrochemical corrosion tests. Subsequently, one mini-implant from each group underwent SEM analysis for corrosion examination (80 and 5.000x). Then, the mini-implants were removed from the rods and subjected to a mechanical torsion fracture test (500N) using a mandrel coupled to a universal mechanical testing machine. After fracture or deformation, one mini-implant from each group underwent SEM analysis again (80 and 5.000x).

Results: The statistical analysis showed no significant differences between the groups (stainless steel: 0.076 and Ti6Al4V: 0.199; pp>0.05). The Shapiro-Wilk test indicated that the data did not follow a normal distribution (p<0.05). The pitting potential analysis revealed no significant differences between G3 and G4, G5 and G6, or G4 and G6. Fracture resistance tests showed that most stainless steel mini-implants deformed rather than fractured completely (G1: 33.95N; G3: 40.60N; G4: 28.26N), requiring higher force for fracture. All Ti6Al4V mini-implants fractured at lower forces due to the material's brittleness (G2: 26.35N; G5: 27.50N; G6: 24.01N).

Conclusions: All analyzed groups experienced corrosion and pitting potentials, but none exerted sufficient influence to fracture or deform the devices under torsion. Key words:Mini-implants, corrosion, artificial saliva, fluoride, fracture resistance.