Effect of Gingival Margin Design on Retention of Thermoformed Orthodontic Aligners

Abstract

Purpose: The aim of this study was evaluate the effect of gingival margin design (scalloped vs. straight cut at gingival zenith vs. straight cut 2mm above gingival zenith) on the retention of thermoformed aligners. Retention of aligners is a critical requirement for efficient tooth movement. Methods: Two thermoform aligner materials were used, Invisacryl A and Invisacryl C, in 0.040 mil (1mm) thickness. Six aligner designs were fabricated for each of the two aligner materials (12 total aligner designs). Aligner designs are scalloped, straight cut at gingival zenith (0mm), and straight cut 2mm above gingival zenith on a model with attachments. These designs were tested with and without attachments. Three aligners were made for each of the 12 aligner designs for a total of 36 aligners. A Universal Testing Machine was used to pull each aligner off of a Kilgore dentoform in a direction perpendicular to the occlusal plane. The force needed to pull each aligner off of the dentoform was recorded as the retentive force of the aligner. A one way ANOVA with a Post Hoc Bonferroni test was completed on the average pull off force for each of the 12 aligner groups. Results: Of the 66 comparisons made 57 had significant differences when comparing each aligner group's average retentive pull off force. The highest retentive force was Invisacryl A, 2mm straight margin, with attachments while the lowest retentive force was Invisacryl C, scalloped with attachments. Conclusions: Invisacryl A material showed increased retention when compared to Invisacryl C material of the same aligner margin and attachment design. Straight line gingival margins (0 and 2mm) showed and increased retention when compared to scalloped margins for Invisacryl A and Invisacryl C with attachments. Aligners with attachments and scalloped margins had significantly less retention than aligners of the same material type with scalloped margins and no attachments. The 2mm straight gingival margin design had the highest retentive forces when compared to aligners of the same material and attachment type.