| Objective This study is undertaken to evaluate the influence of labial surface contours and collum angles of the maxillary anterior teeth and premolars on torque. The purpose of this study is to present a reference for torque adjusting to clinicians.methods 329 extracted maxillary teeth were selected,including 77 incisors,68 lateral incisors,61 canines,66 first premolars and 57 second premolars.All specimens were scanned by CBCT.Three-dimensional images were reconstructed by the CT software.The median sagittal planes of all teeth were selected and then analyzed by the AutoCAD 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 and coordinates of 14 continuous points on the labial surface were also measured.Results①The angles formed by the tangent lines to the labial surface at different heights along the surface and the longitudinal axis of the crown range from 2.73°to 5.70°in standard deviations for the maxillary anterior teeth and premolars.The maxillary second premolars have the greatest angle variation among different heights.Between 3.0 and 4.5 mm level the mean angle difference is 9.17°. The maxillary incisors have the smallest angle variation among different heights. Between 3.5 and 5.0 mm level the mean angle difference is 4.25°.②There are no significant differences among the angles formed by tangents of the adjacent heights on the labial surface for the maxillary anterior teeth(P＞0.05).There are significant differences among the same angles for the maxillary premolars(P=0.000).③For the variation between maximum collum angles and the minimum ones of the maxillary anterior teeth and the premolars,the greatest variation exists in the maxillary first premolars with the range of 19.34°.The smallest variation exists in the maxillary second premolars with the range of 14.78°.The long axes of the roots appear labial to the long axes of the crowns for the maxillary canines with the mean value of -3.30°. For the other maxillary anterior teeth and premolars,the long axes of the roots appear lingual to the long axes of the crowns,among which,the maxillary first premolars have the greatest mean collum angle value of 4.84°.The maxillary incisors have the smallest mean collum angle value of 0.88°.④There are higher correlations between the X-coordinates and the Y-coordinates of the points on the labial surfaces for the maxillary anterior teeth and premolars.The coefficients of determination are ranging from 0.574 to 0.952.The polynomial regression analyses show that the.best correlations are assumed square regression functions and all the regression equations are of statistical significance(P=0.000).Conclusion①Individual variation exists in the morphology of the maxillary anterior teeth and premolars,which increases gradually from the anterior teeth to premolars.The variation of torque varies between 1°and 4°according to the 0.5mm deviation of bracket heights in occlusogingival level of the maxillary anterior teeth and premolars.②The influence of the same variation of bracket heights on torque appears almost the same among different occlusogingival levels for the maxillary anterior teeth.The influence of the same variation of bracket heights on torque changes among different occlusogingival levels for the maxillary premolars.Among which,the variation of braket heights exerts a greater influence on torque at more gingival levels.③The collum angles,existing in the maxillary anterior teeth and premolars,have great individual variation.These should be all considered as the important factors affecting the root torque during orthodontic treatment.④Equations could be used to estimate the variation of torque under the influence of different bracket heights on the labial crown surfaces.Furthermore,the equations of labial surface contours could be used as guidance for bracket bases design.
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