Effect of topical fluoride application on titanium alloys: a review of effects and clinical implications

The purposes of this review were to: summarize the currently available evidence on the effect of fluoride on titanium alloys; discuss the mechanisms involved; and assess the clinical relevance and validity of statements deriving from in vitro approaches. The spectrum of effects noted include: morphological variations, such as increased roughness with adverse effects on sliding mechanics; mechanical properties of the wires, which may entail effects on the superelastic plateau of nickel-titanium wires, or reduction in the strength of wires, which can result in frequent intraoral failures; and release of ions during service. Reduced nickel release rates have been documented, however, from retrieved nickel-titanium wires presumably due to the passive layer formed. In relevant research, forming oxide on titanium alloys has been proposed to provide immunity to further degradation and ionic release, since nickel ions must diffuse through this layer to be released. The described evidence of fluoride on titanium alloys derives mostly from in vitro research, which includes oversimplifications in simulating the oral environment. The reactivity in laboratory experiments is dramatically increased relative to the actual clinical conditions, which exaggerates the effects noted. The effects shown have not been validated in vivo, since the only available evidence on intraorally fractured nickel-titanium archwires did not support the implication of hydrogen embrittlement as a failure mechanism. Rather, fractures were found to be related to: (1) mechanical factors associated with loading of the wire in specific arch sites; and (2) the masticatory forces. Clinically, the use of fluoride varnishes at specific, caries-risk sites may provide protection while minimizing the potential risk of adverse effects.