Retentive force of thermoformed and direct 3D-printed clear aligners with different margin designs and attachments:...

Clear aligner retention is essential for maintaining appliance stability, intimate aligner fit, and effective orthodontic force delivery. With the increasing interest in direct 3D-printed aligners (DPA), comparative evidence on their retention behavior relative to conventional thermoformed aligners (TFA) under different design conditions remains limited.

This in vitro study aimed to compare the retentive force of TFA and DPA under different margin designs and attachment conditions.

A total of 72 aligners were fabricated using two Methods: thermoforming with Zendura VIVA, 0.89 mm, and direct 3D printing with Tera Harz TC-85DAC, designed thickness 0.50 mm. Specimens were assigned to 12 groups according to fabrication method, attachment condition (NA or YA), and margin design: Straight 2 mm, Straight 0 mm, or Scalloped 0 mm. Retentive force was measured using a universal testing machine at a crosshead speed of 5 mm/min. Each aligner was tested five times. Group comparisons were performed using the Mann-Whitney U-test and Kruskal-Wallis test, with significance set at p < 0.05.

TFA generally demonstrated higher retentive force than DPA, but lower retentive force in the Scalloped 0 mm design without attachments. The Straight 2 mm design produced the highest retentive force across groups, whereas the Scalloped 0 mm design showed the lowest values. The highest mean retentive force was observed in the TFA-YA Straight 2 mm group (26.18 ± 1.67 N), and the lowest in the TFA-NA Scalloped 0 mm group (3.25 ± 0.94 N). Attachments increased retentive force in most conditions, with a pronounced effect in the Scalloped 0 mm design.

Clear aligner retention varied according to fabrication method, margin design, attachment condition, and material-related characteristics. Individualized aligner design may help optimize retention and improve biomechanical performance in clear aligner therapy.