The rotational tendency by the moment of this couple manifests irrespective of the location of the bracket on the tooth ( Isaacson et al., 1993). A rectangular archwire creates additionally a couple of forces in the bracket slot, which has a rotating effect that can be expressed as a moment. A force from a round archwire acting on a bracket introduces a rotational moment of a magnitude which depends upon the magnitude of the force and its distance from the centre of resistance. Orthodontic torque refers to the buccolingual root tipping in which movement of the crown is minimized and movement of the root apex is maximized ( Thurow, 1982). Torque causes the twist of a beam or a wire and torsion is the actual twisting that takes place in the material, generating shear strains and shear stresses, as the result of the torque. The Incognito and STb lingual brackets generated the highest moments. The lowest torque expression was observed at the self-ligating lingual brackets, followed by the conventional brackets. Additionally, the torque angles were evaluated at which the crown torque fell well below the minimum levels of 5.0 Nmm, as well as the moment/torque ratio at the last part of the activation/deactivation curve, between 10 and 15 degrees. The recorded differences of maximum moments were statistically significant, except between the Incognito and STb brackets. The magnitude of maximum moment at +15 degrees ranged 8.8, 8.2, 7.1, and 5.8 Nmm for the Incognito, STb, conventional Gemini, and the In-Ovation L brackets, respectively similar values were recorded at −15 degrees: 8.6, 8.1, 7.0, and 5.7 Nmm, respectively. One-way analysis of variance with post hoc multiple comparisons (Tukey test at 0.05 error rate) was conducted to assess the effect on bracket type on the generated moments.

After each activation, the bracket returned to its initial position and the moments in the sagittal plane were recorded during these rotations of the bracket.

A 15 degrees buccal root torque (+15 degrees) and then a 15 degrees palatal root torque (−15 degrees) were gradually applied to the right central incisor bracket. The wire was ligated with elastomerics into the Incognito, STb, and conventional brackets and each measurement was repeated once after religation. Each model was mounted on the orthodontic measurement and simulation system and 10 0.0175 × 0.0175 TMA wires were used for each bracket type. Incognito™ lingual brackets (3M Unitek), STb™ lingual brackets (Light Lingual System ORMCO), In-Ovation L lingual brackets (DENTSPLY GAC), and conventional 0.018 inch slot brackets (Gemini 3M Unitek) were bonded on identical maxillary acrylic resin models with levelled and aligned teeth. The aim of this study was to assess the effect of bracket type on the labiopalatal moments generated by lingual and conventional brackets.

Brackets lingual