MECHANICAL PROPERTIES OF DIRECTLY 3D-PRINTED ORTHODONTIC ALIGNERS: A NARRATIVE REVIEW

Abstract

Direct three-dimensional (3D) printing is an emergping approach to manufacturing orthodontic clear aligners. Unlike conventional thermoforming, which shapes a thermoplastic sheet over a printed dental model, direct printing fabricates the appliance from digital dental data without a physical model. The clinical value of a printed aligner depends not only on its geometric accuracy but also on its ability to deliver appropriate forces, resist deformation, and retain its properties in the mouth. This narrative review summarises the mechanical properties reported for directly printed aligner materials, including elastic modulus, tensile strength, hardness, viscoelastic behaviour, stress relaxation, and shape recovery. It also considers factors that may influence these properties, including resin composition, printing technology, post-curing, temperature, moisture, and intraoral ageing. Available laboratory studies demonstrate substantial variation between resin systems and testing conditions. Some printed materials exhibit shape-memory behaviour and promising recovery characteristics, whereas other studies report pronounced changes in modulus or residual stress under wet or thermally controlled conditions. These findings cannot be generalised to all directly printed aligners because material formulations and manufacturing protocols differ. Furthermore, results obtained from standardised specimens do not necessarily predict the forces delivered by a complete aligner during clinical tooth movement, and clinical data remain limited. Current evidence supports the potential of direct printing but highlights the need for standardised testing, transparent reporting of manufacturing parameters, and longer-term clinical validation.