OBJECTIVEThis study was undertaken to evaluate the influence of labial surface contours and collum angles of the maxillary anterior teeth on torque.

METHODS206 extracted maxillary teeth were selected, including 77 central incisors, 68 lateral incisors and 61 canines. All specimens were scanned by cone beam dental computed tomography (CT). Three-dimensional reconstructed images were made by using the CT software. The median sagittal planes of all teeth were selected and then analyzed by the Auto CAD software. For each tooth, the angles between tangent lines to the labial surface at four different heights along the surface and the longitudinal axis of the crown were measured. The collum angle was also measured.

RESULTSBetween 3.5 mm and 5.0 mm level of bracket heights, for the variation of 0.5 mm, the torque differed by 1.5 degrees for the maxillary central incisors and 2 degrees for the maxillary lateral incisors and canines. The mean collum angle values for the maxillary central incisors, lateral incisors and canines were 0.88 degree, 3.87 degrees and -3.30 degrees.

CONCLUSIONThe biological variation in tooth morphology would influence the torque after orthodontic treatment in different ways.