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As a rectangular wire is inserted into edgewise brackets the wire exerts a force system three-dimensionally. The force system may include bending force in first and second orders and a torsional force in third order. Analytical and experimental studies on bending force have been introduced, but information about torsion is still lack. The purpose of this study was to estimate the torsional´moment in the force system of rectangular arch wires through theoretical and experimental studies. Wires most frequently used for third order control were selected as study materials. Cross sections of 0.016X17.022, 0.0174.025, 0.0194.025 inch rectangular wires in four different materials such as stainless steel (O mco), TMA (Ormco), NiTi (Ormco), and braided stainless steel (DentaFlex, Dentaurum) were used. The torque/twist rate of each test material was calculated using the torsion fotiuula Torque/twist rate, yield torsional moment, and ultimate torsional moment were measured with a torque gauge. The torsion formula assesses that the torque,/twist rate (T/0) is proportional to the characteristics of material (G) and cross section (J), and is inversely proportional to the length of wire (L). Most experimental results conesponded with the formula. The relative stiffness was calculated for reference to a logical sequence of wire changes.
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