Thickness-dependent biomechanical accuracy of additively manufactured and thermoformed orthodontic aligners: an in vitro comparative study

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Abstract
Additive manufacturing has introduced directly printed orthodontic aligners as an alternative to thermoforming. However, comparative data regarding thickness-dependent biomechanical accuracy remain limited. Therefore, this study aimed to evaluate the influence of fabrication method (thermoforming vs. direct 3D printing) and material thickness (0.76 mm vs. 1.0 mm) on the accuracy of rotational, tipping, and torque movements of different teeth under standardized in vitro conditions. Thirty-six standardized typodonts underwent digital staging (30° rotation, 10° mesial tipping, 10° positive torque). Aligners were fabricated via thermoforming (Zendura A) or direct 3D printing (Tera Harz TC-85). Movement was simulated using thermal activation (56 °C, 90 s). Pre- and post-movement scans were superimposed (Geomagic Control X), and accuracy was calculated as the percentage of achieved to programmed movement. Analyses were performed at both typodont and tooth levels. Statistical analysis was conducted using Jamovi (version 2.6.44), applying two-way ANOVA and Mann-Whitney U tests (p < 0.05). Aligner thickness significantly improved accuracy across all movement types (p < 0.001). In contrast, fabrication method showed inconsistent effects, with only isolated significant differences. Mesial tipping demonstrated the highest predictability (up to 92.67%), whereas torque showed the lowest (maximum 71.67%). Accuracy was generally higher in anterior teeth, with greater variability in posterior segments. No significant arch differences were observed. Aligner thickness is the primary determinant of biomechanical accuracy, with thicker aligners providing more predictable tooth movement. Fabrication method showed a secondary and variable influence, indicating that material thickness plays a more critical role in optimizing aligner biomechanics.

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