Abstract: Objective To investigate the factors that influence the clinical efficacy of non-extraction treatment to moderate crowding with Damon self-ligating system. Materials and methods Forty cases with moderate crowding were selected and all of them were treated with Damon Q self-ligating appliances. Fourteen hard-tissue and nine soft-tissue measurements from cephalometric radiophotographs were analyzed before and after crowding correction. According to chin lip Z angle, all cases were divided into two groups (successful group and failure group). The characteristics of soft and hard tissues were analyzed with paired-samples t test and independent-samples t test between two groups using SPSS 19.0 software. Results of 40 cases, 22 cases were included in successful group, 18 cases were allotted in failure group. There was no significant difference in gender and age distributions between two groups. L1-AP angle significantly increased after treatment in two groups, while FMIA significantly decreased. Lower lip to E line distance significantly increased and Z angle was significantly decreased in failure group after treatment. L1-NB distance, mandibular plane angle (FMA), lower/upper lip to E line distance, as well as soft/hard tissue facial angle in failure group were more than that in successful group in both T1 and T2. The distribution of individuals with low angle, average angle and high angle was significantly different between two groups and the low angle cases achieved better therapeutic effects. Conclusion The upright lower incisor, ideal lip protrusions and lower mandibular plane angle are favorable prerequisites for the successful non-extraction treatment to moderate crowding with Damon self-ligating system.

Key words: Damon self-ligating system, Non-extraction, Moderate crowding, Cephalometries