Intraoral aging of orthodontic materials: the picture we miss and its clinical relevance

Abstract

The purpose of this editorial is to systematically analyze the variety and potency of various aging variables affecting the morphology, structure, and mechanical properties of polymeric and metallic orthodontic materials. The effects of aging on the longevity of the bond strength and mechanotherapy were analyzed: aging-induced plasticization of resin adhesives might lead to bond failure at forces of magnitude lower than those sustained at the initiation of treatment. Standard in vitro methodologies cannot show this effect, and thus laboratory bond strength protocols require modification to become clinically meaningful. Also, the force transferred from an activated archwire to a preadjusted bracket slot, as well as friction during free sliding, seems to be affected by the intraorally induced alteration of materials. Although the effect of intraoral environmental conditions on the superelastic properties of nickel-titanium (Ni-Ti) archwires and coil springs requires further research to establish the true spectrum of effects, it has been suggested that intraoral temperature variations might transiently affect their properties and that the fracture resistance of used Ni-Ti wires is reduced. Clinical implications are discussed for (1) in vivo-aged elastomeric ligatures and chains, which can be postulated to express much higher creep than their in vitro-aged counterparts; (2) the largely unknown effect of aging on the spring component of self-ligated brackets and the associated effect on ligation force; and (3) the intraorally induced alterations in the structural conformation of Invisalign appliances (Align Technology, Santa Clara, Calif). The objective of future research efforts in the field of orthodontic materials should include the development of clinically relevant methodologies. A clear definition of limitations of laboratory experimental configurations might be instrumental in confining the clinical impact of research findings to their actual extent.