The Effect of Size and Location of Composite Attachments on the Extrusive Force Delivered to a Maxillary Central Incisor by a Clear Aligner

Abstract

Introduction: One of the most challenging and least predictable tooth movements to achieve with clear aligner therapy is extrusion of maxillary central incisors. Composite attachments have been implemented to improve the biomechanical properties and make extrusion a more predictable tooth movement. Various attachment shapes have been investigated, but the effect of attachment size and location on the force delivered to a maxillary central incisor by a clear aligner has not been reported. Aim: To evaluate the effect of composite attachment size and location on the extrusive force delivered to a maxillary central incisor with a Zendura FLX clear aligner. Methods: Clear aligners designed to extrude a maxillary central incisor 0.25 mm using uLab software were fabricated using Zendura FLX .030 mil (0.75 mm) thickness clear aligner material. To evaluate the effect of attachment size, aligners were designed with three sizes of rectangular anterior extrusion attachments as well as no attachment. To evaluate the effect of attachment location, aligners were designed with a standard rectangular anterior extrusion attachment placed in the center of the facial surface of the crown, 2 mm apical, 2 mm incisal, in the center of the lingual surface of the crown and with no attachment. For each of the 9 groups, 5 aligners were fabricated with standardized margins trimmed 0.75 mm apical to the gingival margin. Extrusive force along the long axis of the crown was measured using a force gauge. Means and standard deviations were calculated and presented by group. The one-way analysis of variance (ANOVA) was conducted to compare the means between the groups and Tukey’s method was used as a post-hoc procedure for pairwise comparisons. P-values less than 0.05 were considered statistically significant. Results: Pairwise comparisons for all groups compared to No attachment were statistically significant (p <.0001). Pairwise comparisons for all attachment sizes were statistically significant (p < .0001). Although pairwise comparisons were statistically significant for the Incisal attachment group vs. Gingival attachment group (p = 0.033) and the Incisal attachment group vs. Mid-facial attachment group (p = 0.002), the observed differences may have been due to variation in aligner fabrication. No statistically significant differences were observed for pairwise comparisons of other attachment location groups. Conclusions: This is the first study in which the effect of attachment size and location on the extrusion of a maxillary central incisor with a clear aligner has been reported. In a benchtop model, all attachment configurations generated greater initial extrusive compared to the No attachment control. Attachment size has a positive relationship with initial extrusive force with larger attachments producing greater force. Attachment location did not appear to be as critical as attachment size in producing extrusive force. Biomechanically, larger attachments can be recommended to help generate the force required to predictably extrude maxillary incisors.