Orthodontic wire: a continuing evolution.

Wires have substantial structural presence in active and retentive orthodontic therapy. Wires, and auxiliaries fabricated from wire, may deliver force to produce dental displacements, they may attempt to prevent unwanted displacements, or they may simply carry force from one location to another within the dentofacial complex. Wires may have relatively simple or rather convoluted geometries. The most prominent wire in clinical orthodontic treatment is the "arch wire." Marketed in just two, symmetrical, cross-sectional shapes, its sizes are numerous, and arch wires are manufactured of a variety of metallic alloys and compositions. The primary objective of this article is to trace the history of arch wires over the past century, considering the evolution from the stiff arch bow to the wire exhibiting Hookean material behavior, from the precious metal to the stainless steel to the titanium alloys, and into the era of the "superelastic" wire and a thermomechanical behavior decidedly more complex than its predecessors. Timely, relevant questions that are addressed herein pertain to the current "state-of-the-art" of orthodontic wires, the extent to which the mechanics of the "old" and new wires are understood, and the anticipated arch wire advancements in the near future as a new millennium approaches.