Finite element analysis of 3D direct-printed shape memory polymer aligners for stress distribution and initial tooth movement in anterior open bite correction

Anterior open bite (AOB) correction with clear aligners remains biomechanically unpredictable due to inadequate control of incisor extrusion and a high tendency for uncontrolled lingual tipping. Conventional thermoformed polyethylene terephthalate (PET) aligners exhibit rapid force decay and localised stress concentration, limiting sustained vertical activation and increasing the risk of inefficient movement and periodontal overload. 3D direct-printed shape memory polymer (SMP) aligners provide programmable recovery and sustained force expression, representing a potential biomechanical advancement; however, their extrusion mechanics have not been quantitatively compared under controlled conditions. This study aimed to evaluate and compare initial tooth displacement, stress distribution, and extrusion-to-tipping ratio during maxillary incisor extrusion produced by thermoformed PET aligners with attachments and 3D direct-printed SMP aligners with and without attachments using finite element analysis (FEA). A validated patient-specific finite element model of the maxillary incisors was subjected to a standardised 0.25-mm programmed extrusive activation. Three configurations were analysed: (1) SMP aligner with attachments, (2) SMP aligner without attachments, and (3) PET aligner with attachments. Primary outcomes included von Mises stress within the periodontal ligament (PDL), cervical attachment stress, vertical displacement magnitude, and vertical-to-lingual tipping displacement ratio as a surrogate for moment-to-force control. PET aligners with attachments generated the highest mean PDL stress (3.44 MPa) and cervical attachment stress (5.26 MPa) compared with SMP aligners with attachments (2.43 MPa). Despite greater vertical displacement (0.48 mm), PET aligners demonstrated nearly equivalent lingual tipping (0.49 mm), yielding a vertical-to-tipping ratio ≈ 0.98, indicative of biomechanical inefficiency. The SMP aligners without attachments produced minimal extrusion (0.0031 mm) and the lowest PDL stress (1.36 MPa). In contrast, SMP aligners with attachments achieved controlled extrusion (0.32 mm) with reduced tipping (0.27 mm), resulting in the most favourable vertical control ratio (≈ 1.19) and the lowest cervical stress concentration (1.30 MPa), reflecting more balanced load distribution. In comparison to thermoformed PET aligners, 3D direct-printed SMP aligners with attachments exhibited a favourable biomechanical control of maxillary incisor extrusion under standardised activation. PET aligners with attachments exhibited increased PDL and cervical stress concentrations and uncontrolled tilting, while SMP aligners without attachments were unable to provide effective activation. The results of this investigation should be interpreted in the context of computational assumptions and should be subject to prospective clinical validation, as they assessed initial displacement using FEA.