BACKGROUND: Esthetic orthodontic aligners are removable polymer-based devices fabricated via thermoforming (TM) or direct three-dimensional printing (DTP). However, a gap remains in the literature regarding how these two manufacturing methods differ in their mechanical and physical properties. OBJECTIVES: This systematic review aimed to assess and compare changes in the physical and mechanical properties of orthodontic aligners fabricated by TM or DTP methods. However, it should be noted that the included evidence does not allow a clear separation between manufacturing method and material system, as TM and DTP aligners were based on fundamentally different polymer. SEARCH METHODS: A systematic search was conducted in six electronic databases in May 2025. Selection criteria: In vitro studies comparing these properties between TM and DTP aligners were included. Data collection and analysis: Two reviewers independently selected studies, extracted data, and performed risk-of-bias assessments and meta-analyses. Twenty-three studies were selected, and four were included in the meta-analysis. RESULTS: TM and DTP aligners exhibited distinct mechanical patterns. TM aligners generally showed higher flexural strength, storage modulus stability, and surface microhardness, reflecting greater initial rigidity. In contrast, DTP aligners demonstrated higher shape recovery potential, greater traction forces in vertical tests, and improved geometric accuracy under laboratory conditions. Regarding physical characteristics, TM aligners exhibited an average thickness reduction of -39.76% after fabrication, color change, and lower ultraviolet stability; while geometric accuracy tended to favor DTP aligners with a thickness increase of +11.02%, and better marginal adaptation. DTP aligners also exhibited a lower glass transition temperature (Tg = 42.3°C), suggesting differences in thermomechanical response. The meta-analysis showed that surface roughness assessments revealed higher Sa (mean surface amplitude) values in DTP aligners, whereas TM aligners exhibited greater Sv (valley depth). Following clinical use, DTP aligners maintained higher Sz (total amplitude) values, while TM aligners demonstrated reduced Sa values. There was no statistically significant difference in thickness variation between the two fabrication methods (standardized mean difference = 1.48; 95% confidence interval: -7.26-4.30; P = .62), with high heterogeneity (I2 = 97%). CONCLUSION: TM and DTP aligners show distinct physical and mechanical behaviors influenced by manufacturing and material characteristics. As current evidence is limited to in vitro studies, further clinical research is needed to confirm these findings and their relevance in patient care. Therefore, any clinical extrapolation should be taken with caution and interpreted within the limitations of the current evidence based on in vitro research. REGISTRATION: https://doi.org/10.17605/OSF.IO/BT98F.
Jorge et al. (Fri,) studied this question.