Additional Palatal Force in Maxillary Canine Distalization

In orthodontic treatment involving extraction of the maxillary first premolars, the canine is distalized into the extraction space using sliding mechanics. Because the retraction force is applied at bracket level and therefore at a distance from the centre of resistance of the tooth, the canine tends to rotate distopalatally and to tip in the sagittal plane instead of translating bodily. These side effects lengthen treatment and increase the load on the posterior anchorage teeth. Power arms are used to bring the line of action of the force closer to the centre of resistance. In addition, because the centre of resistance of the maxillary canine lies palatal to the mid-buccolingual axis of the crown, a force applied from the palatal surface generates a rotational moment opposite in direction to that produced by a buccal force. This trial tested whether dividing a constant total distalizing force between the buccal and the palatal surfaces of the canine, so that the two rotational moments oppose one another, produces a more bodily canine movement than applying the whole force from the buccal side alone. The trial used a split-mouth design: in each participant one maxillary quadrant received the whole force from the buccal side and the contralateral quadrant received half the force buccally and half palatally, with the total force identical on both sides. Canine displacement, rotation, tipping and first molar anchorage loss were measured on superimposed digital models over 12 weeks.