Effects of maximum retraction of anterior teeth on third order torque of incisors and soft and hard tissue changes by conventional and self-ligating brackets

Abstract

In recent times, the clinical application of mini-screws for maximum anchorage has allowed an increased amount of retraction in the anterior teeth, resulting in optimal changes in the facial profile. Maximum retractions of anterior teeth are performed frequently in Asians, given the large number of patients with dentoalveolar protrusion. However, how much torque is expressed by anterior brackets during retraction of anterior teeth, according to the bracket system used, under the same conditions remains not well understood. In addition, although there are experimental results that indicate that active-type self­ligating brackets are more advantageous for torque control than passive-type self­ligating brackets, there are no clinical studies on the subject, indicating the need for testing whether actual improvements can be obtained in clinical settings. The present study examined cases involving retraction of the anterior teeth after extraction of the maxillary first premolars or the maxillary first premolars and mandibular premolars for orthodontic treatment in Korean patients with lip protrusion. The objective was to compare (1) the patterns of anterior tooth movement when using conventional brackets versus self­ligating brackets, and (2) the characteristics of changes in hard and soft tissues based on the use of such brackets. The participants in the present study consisted of 64 patients who visited the Department of Orthodontics at Yonsei University Dental Hospital with the chief complaint of lip protrusion, and who subsequently underwent a retraction procedure after extraction of the maxillary first premolars or the maxillary first premolars and mandibular premolars. The participants were divided into active clip metal (ACM), active clip ceramic (ACC), passive clip metal (PCM), and conventional metal (CM) groups, based on the brackets used. Pre- and post-treatment lateral cephalograms acquired from these participants were analyzed and the skeletal, dental, and soft tissue changes from pre- to post-treatment were compared among the groups, and the following results were found: 1. Incisal tip of the maxillary anterior teeth at post-treatment showed retraction of 7.32 mm, 7.59 mm, 7.18 mm, and 7.29 mm in ACM, ACC, PCM, and CM groups, respectively, with no significant differences among the four groups (p > 0.05). 2. During retraction of the maxillary anterior teeth, changes in U1 to SN (°) was 10.2°, 11.6°, 13.5°, and 12.1° in ACM, ACC, PCM, and CM groups, respectively, with no significant differences among the four groups. Changes in IMPA (˚) also showed no significant differences among the four groups (p > 0.05). 3. No significant differences in treatment duration were found among the 4 groups (p > 0.05). 4. Both the upper lip to E-­line (mm) and the lower lip to E-­line (mm) were decreased post-treatment in all four groups, but there were no statistically significant differences (p > 0.05). Horizontal changes in soft tissues, such as Sn, A', Ls, Stms, Stmi, B', and Pog', were not significantly different among the four groups (p > 0.05). 5. In multiple linear regression analyses, Δ U1 to SN (˚) showed significantly positive linearity with only Δ VRP to U1t (mm) (p < 0.01).

The results above showed that the type of ligation used with the brackets did not affect the inclination of the anterior teeth after maximum retraction of the maxillary anterior teeth. The final tooth inclination was affected by the amount of retraction in the anterior teeth, according to the treatment goals and horizontal and vertical skeletal characteristics, more so than by the type of ligation used with the brackets.