Ideal arch force systems: A center-of-resistance perspective

Abstract

The analysis of force systems from an ideal arch has shown that the ratio of the moments produced by a straight wire connecting two malaligned brackets depends on the ratio of the angulations of the brackets to the interbracket axis. Although this result permits assessment of the relative forces and moments, prediction of future tooth movement requires knowledge of the position of center of resistance as well. In this study, the forces and moments produced by a straight portion of an arch wire were transferred from the brackets to the center of resistance. The purpose was to compare the force system at the brackets to the force system at the center of resistance and to assess whether bracket geometry can be applied to predict initial tooth movement. A computer model was used to simulate two teeth connected by a straight portion of wire. Forces and moments were calculated with the use of equations derived from elementary beam theory. The results show that the force system at the center of resistance may be of an entirely different “geometry” type than that at the bracket. Factors that influence the force system include the interbracket distance, the angulation of the teeth, the length of the tooth root, and the width of the bracket. (Am J Orthod Dentofacial Orthop 1998;114:256-64.)