The Effect of Increasing the Gaps between the Front Teeth on Torque and Intrusion Control of the Incisors for Anterior Retraction with Clear Aligners: A Prospective Study

Abstract Objective: To investigate the effect of sequential distalization on increasing gaps in the maxillary anterior teeth on the control of torque and three-dimensional tooth movement for anterior retraction with clear aligners in extraction cases. Methods: Twenty-four patients undergoing bilateral maxillary first premolar extraction with clear aligners were selected. According to a preset increase in maxillary anterior gaps, patients were divided into three groups: an integral retraction group (9 cases), a group in which a gap of 0.5mm was added (6 cases), and a group in which a gap of 1.0mm was added (9 cases). Plaster casts of the upper full dentures before and after 2mm retraction were acquired. Each pair of models was overlapped according to the palatal fold. The three-dimensional movement of the teeth, and the change of torque for the anterior teeth, were analyzed by Geomagic Studio 2014 software. Results: Mean torque changes for the central and lateral incisors were 4.03±0.91° mm and 3.86±0.94° mm in the integral retraction group; 1.35±0.97° mm and 1.21±0.89° mm in the 0.5 mm gap added group, and 1.74±0.86° mm and 1.75±0.95° mm in the 1.0 mm gap added group. Torque change in the gap added groups was significantly smaller than that in the integral retraction group (P < 0.05); there was no significant difference in this respect when compared between the 0.5mm gap added group and the 1.0mm gap added group (P > 0.05). Mean labial and lingual displacement of the central and lateral incisors were 2.34±0.40 mm and 1.98±0.30 mm in the integral retraction group, 1.61±0.27 mm and 1.49±0.43 mm in the 0.5 mm gap added group, and 1.69±0.44 mm and 1.58±0.28 mm in the 1.0 mm gap added group. Mean vertical displacement of the central and lateral incisors was 0.78±0.51 mm and 0.90±0.56 mm in the integral retraction group, 0.72±0.29 mm and 0.43±0.36 mm in the 0.5 mm gap added group, and 0.25±0.47 mm and 0.19±0.64 mm in the 1.0 mm gap added group. In the lip-lingual and vertical directions, displacements of the central and lateral incisors in the groups with added gaps were smaller than those of the integral retraction group (P < 0.05); however, there was no significant difference in this respect when compared between the 0.5mm gap added group and the 1.0mm gap added group (P > 0.05). The displacement of the second premolar to the second molar in the mesiodistal direction was compared between all groups; anchorage molars in the gap added groups was significantly smaller than that in the integral retraction group (P < 0.05) there was no significant difference between the 0.5mm and 1.0mm gap added groups (P > 0.05) and there was no significant difference between the groups in terms of the displacement of the anchorage molars in the buccal-lingual and vertical directions. Conclusion: Increasing the maxillary anterior gaps by sequential distalization of the anterior teeth was advantageous in terms of controlling the torque of the anterior teeth to achieve a desired pattern that was closer to normal bodily movement. Increasing the maxillary anterior gaps also led to better control in the vertical direction of the anterior teeth. However, this retraction strategy has higher requirements with regards to protecting the molar anchorage.