The Relation between Force Magnitude and the Center of Rotation in the Maxillary Incisor

Abstract

When a tooth is tipped, it moves about a center of rotation, but the exact location of this center has been the subject of much discussion and confusion. This study was designed to locate the centers of rotation of upper central incisors by application of known forces to the teeth of fourteen patients. Records were taken to ascertain the rate of movement and pain threshold, and the axis of rotation was located by superposing accurate periapical films. The results show that there is no significant difference in the centers of rotation when forces of 51 and 121 gms. were used. The teeth tipped about points located apically at 1.2 and 1.2 mm. from the centroid of the root for groups studied. The teeth moved most rapidly immediately following the first application of force, and then a lag period followed with no tooth movement. This was followed by a steady small incremental rate until the space closed. The pain threshold dropped greatly during the first day of tooth movement. The direction and type of tooth movement were able to be observed on the films. Both the rate of movement and the pain response recorded concurred with observations made daily in the clinic. This study was designed to test the validity of the Moment/ Force Ratio used to locate the centers of rotation. The results obtained were very close to those found using this ratio, and therefore gave clinical support to this theory. The conclusion reached by this study indicates that the procedure was able to accurately locate the center of rotation, found to be just apical by 1.2 to 1.3 mm. from the centroid of the root.