Thickness distribution and site-specific variability of thermoformed multilayer clear aligners: an in vitro study

Introduction: Thermoforming is a critical step in the fabrication of clear aligners and is associated with non-uniform deformation of thermoplastic materials, resulting in non-uniform thickness distribution. Because aligner thickness directly influences stiffness and force delivery, such variability may affect the accuracy and predictability of orthodontic treatment. Aim: This study aimed to evaluate the thickness distribution of thermoformed clear aligners across different tooth regions and anatomical sites, with a focus on identifying patterns of non-uniform material thinning. Methods: . Thickness measurements were obtained using a calibrated digital caliper at standardized anatomical reference points across anterior, canine, premolar, and molar regions. Generalized estimating equations (GEE) with robust standard errors were used to account for repeated measurements within the same aligner, and pairwise comparisons were adjusted using the Bonferroni correction. Results: < 0.001), with the lowest values in anterior teeth (404.0 ± 5.9 µm) and the highest in molars (481.6 ± 6.0 µm). Thickness reduction ranged from approximately 36% to 47% relative to the original material. Site-specific analysis demonstrated that the facial regions generally exhibited lower thickness values than corresponding palatal regions, indicating non-uniform thermoforming-induced deformation patterns. These findings demonstrated that anatomical location was a major determinant of thickness variability. Conclusion: Thermoformed clear aligners exhibit pronounced and systematic non-uniform thickness distribution strongly influenced by tooth morphology and local geometry characteristics. This non-uniform thickness distribution may contribute to variations in force delivery and should be considered in aligner design and clinical application.