Surface characteristics and ion leaching of 17-4 precipitation hardening stainless steel orthodontic brackets exposed to salt water and fluoride-based oral rinses

The aim of this laboratory study is to analyze, compare, and contrast qualitatively and quantitatively the surface roughness and metal ion leaching of two brands of orthodontic precipitation hardening 17-4 stainless steel (SS) brackets exposed to salt water and commercially available NaF-based oral rinses. A total of 60 SS maxillary premolar orthodontic brackets from two brands (Victory Series™, and Mini Diamond Twin™) were assigned to three groups (n = 10). The brackets were immersed in NaF-containing oral rinses (Listerine™ or Orex™), or salt water rinse for 28 days. Microhardness (Vickers hardness number, VHN) and surface roughness (R _a) before and after immersion in oral rinses were evaluated using a non-contact optical profilometer. Inductively coupled plasma mass spectrometry (ICP-MS) was used to analyze the ion leaching into oral rinses. The local surface changes and elemental analysis of corrosion-free and corrosion spots after immersion were assessed and compared using scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS). Data analysis was performed using Statistical Package for the Social Sciences software using one-way analysis of variance, the Tukey’s post hoc test, and paired t-test (α = 0.05). The VHN of the brackets was significantly different at baseline and after immersion in the oral rinses (P < 0.05). The mean R _a before immersion in oral rinses for the two groups of the orthodontic brackets was significantly different (P < 0.01). The brackets immersed in salt water for Victory™ (0.65 ± 0.07 µm) and Mini Diamond™ (0.76 ± 0.08 µm) demonstrated significantly lower roughness values than NaF-based oral rinses (Listerine™ or Orex™) (P < 0.01). However, the difference in R _a between the bracket systems was significant only for Listerine™ and Orex™ rinses (P > 0.01). The ICP-MS analysis showed significant differences for Cu and Zn ions in Listerine™ and all ions except Ni for salt water oral rinses. On the contrary, the comparison between the bracket systems for the metal ions showed no significant difference in the Orex™ group (P > 0.05). Salt water oral rinses demonstrated significantly fewer surface changes than NaF-containing oral rinses. The leaching of the metal ions was significant for Fe²⁺ and Cr³⁺ but did not vary considerably for Ni²⁺ between the evaluated oral rinses.