Nickel Bioaccumulation in Oral Biofilm, Gingival Tissue, and Saliva During Fixed Orthodontic Treatment: A 12-Month Prospective Cohort Study on Predictors and Salivary pH Correlation.

Background: Nickel-based alloys in fixed orthodontic appliances are susceptible to electrochemical corrosion in the oral environment, releasing ions with cytotoxic and allergenic potential. While previous studies have documented initial nickel release patterns, the longitudinal dynamics of its accumulation across oral biospaces (saliva, biofilm, gingival tissue) and interactions with salivary pH remain poorly characterized. This study investigates nickel accumulation in saliva, oral biofilm, and gingival tissue over 12 months of orthodontic treatment and its association with salivary pH.

Material and methods: This 12-month prospective cohort study enrolled 120 participants undergoing fixed orthodontic treatment. Nickel concentrations were quantified via graphite furnace atomic absorption spectrophotometry in three biospaces at baseline (T0), 6 months (T1), and 9-12 months (T2). Salivary pH was measured concurrently. Statistical analyses included non-parametric longitudinal comparisons, inter-biospace contrasts, Spearman correlations, and multivariate regression modeling to identify predictors of nickel accumulation.

Results: Nickel exhibited distinct spatiotemporal patterns: progressive accumulation in biofilm (0.0008→21.5833 µg/L, p<0.001) versus biphasic kinetics in gingiva and saliva (peaking at T1 then declining). By T2, biofilm concentrations were 1000× higher than other biospaces (p<0.001). Treatment duration was the strongest predictor across all compartments (β=0.56-0.67, p<0.001), with biofilm accumulation additionally modulated by salivary pH (β=0.22, p=0.027) and age (β=-0.19, p=0.041). A time-dependent correlation emerged between salivary pH and biofilm nickel (T0: r=0.17, p=0.112; T2: r=0.41, p=0.008), suggesting pH-microbiome interactions.

Conclusions: Oral biofilm serves as the dominant long-term nickel reservoir during orthodontic treatment, with accumulation dynamics influenced by treatment duration, pH, and age. The pH-dependent biofilm-metal interaction highlights its potential as a biomarker for exposure monitoring and a target for preventive strategies. These findings support the development of pH-modulating oral care protocols and corrosion-resistant orthodontic materials to mitigate nickel exposure risks. Key words:Nickel, saliva, Spectrophotometry, Atomic, Orthodontic Appliances, Hydrogen-Ion Concentration.