Direct 3D printing versus thermoforming for clear aligners and retainers: a scoping review and evidence map

Direct fabrication of clear orthodontic aligners and retainers by three-dimensional (3D) printing is increasingly proposed as an alternative to thermoforming. Available evidence includes clinical outcome studies, in vivo ageing investigations, laboratory material-performance studies and safety data, which should not be interpreted as equivalent levels of evidence. This scoping review and evidence map aimed to identify, verify, classify and interpret the evidence comparing thermoformed and directly 3D-printed clear orthodontic appliances and materials. The review followed Joanna Briggs Institute guidance for scoping reviews and was reported using the PRISMA extension for Scoping Reviews (PRISMA-ScR). PubMed/MEDLINE, Embase, CENTRAL, Web of Science and Scopus were searched from inception to 31 March 2026. Evidence was charted by appliance type, evidence tier, material comparator, outcome domain and translational relevance. Descriptive proportions with Wilson score 95% confidence intervals were calculated; no hypothesis testing or pooled treatment-effect estimation was performed. Twenty-two verified sources were retained in the evidence map. Patient-level clinical studies represented 13.6% of mapped sources (3/22; 95% CI 4.7–33.3), and in vivo ageing studies represented 9.1% (2/22; 95% CI 2.5–27.8). Laboratory dimensional, mechanical or optical studies represented 59.1% (13/22; 95% CI 38.7–76.7), and safety or biological-response studies represented 18.2% (4/22; 95% CI 7.3–38.5). Dimensional/manufacturing and mechanical/material domains together accounted for 59.1% of the map, whereas patient-level clinical outcomes accounted for 13.6%. Comparative clinical evidence did not demonstrate statistically significant superiority of directly printed appliances over thermoformed comparators. Safety evidence indicated that resin chemistry, washing and post-curing require prospective monitoring. Direct 3D printing is a viable manufacturing route for clear orthodontic aligners and retainers, but current evidence supports technical feasibility and short-term comparability more strongly than clinical superiority. Future studies should prioritise adequately powered clinical comparisons, long-term retention outcomes, standardised mechanical and dimensional endpoints, transparent manufacturing protocols, safety monitoring, cost-effectiveness, environmental sustainability, patient-reported outcome measures and AI-assisted workflow evaluation.