Characterizing constraining forces in the alignment phase of orthodontic treatment.

OBJECTIVES: To describe the frictional forces (FF) that constrain wire sliding in the initial alignment phase of treatment using a new term, the "constraining force" (CF), and to hypothesize that CF is dependent on two factors: the hyperelastic behavior of archwires and the specific type of tooth geometric malalignment present.

MATERIALS AND METHODS: A laboratory device that simulates the four distinct malalignment types (in-out, rotation, tipping, and vertical step) was used to couple with an Instron testing apparatus. Incremental CF data for the four types of malalignment were recorded. Each type had five trials per increment of severity, from which the CF was averaged using 0.016-inch copper-nickel-titanium (CuNiTi) archwires.

RESULTS: Two types of friction curves were obtained: a traditional step function response and a power regression response. For all malalignment types, increasing degrees of irregularity increased power regression responses and CF. A severity turning point, displayed as a sudden increase in CF, occurred for each malalignment. The rotation type of malalignment yielded the lowest CF, while the vertical step type resulted in the highest CF.

CONCLUSIONS: The data infer a hypothesis that malrotation type having weak CF might act as a limiting factor in the alignment phase to unravel the neighboring teeth. Future investigations to compare clinical and bench data can help explain more fully the constraints impeding alignment resolution and the factors governing the ability to bring malaligned teeth into alignment.