Direct-printed aligners in orthodontics: Integrating digital design, materials science, and clinical biomechanics

The transition from thermoformed aligners (TFAs) to direct-printed aligners (DPAs) represents an important development in contemporary clear aligner therapy. The inherent limitations of TFAs, including material thinning, geometric distortion, inconsistent force delivery, and limited predictability for complex tooth movements, have driven the development of DPAs, which use shape memory polymers (SMPs) and eliminate the thermoforming process. This narrative review synthesizes current evidence on the transition from TFAs to DPAs, focusing on advances in materials science, digital design, biomechanical performance, and clinical application. In vitro studies have shown that TFAs undergo substantial thickness reduction and degradation of mechanical properties after thermoforming, resulting in excessive initial forces and pronounced force decay that may compromise physiological tooth movement. In contrast, DPAs are fabricated using SMPs, which exhibit favorable viscoelastic behavior and can deliver light, continuous forces within physiological ranges with minimal force decay over clinically relevant wear periods. Additive manufacturing also enables advanced biomechanical design features, including localized thickness variation, reinforcement zones, pressure and relief areas, extended trimlines, and direct integration of auxiliaries into the aligner shell. Emerging clinical evidence and illustrative case applications suggest that these capabilities may improve torque expression, rotational control, extrusion efficiency, posterior expansion, and anchorage management while reducing reliance on bonded attachments and the number of aligners required. Although current findings suggest that DPAs may offer improved biomechanical control, predictability, and sustainability compared with TFAs, further research is needed to refine design protocols and evaluate their long-term cytotoxicity, intraoral aging behavior, and environmental impact.