Biomechanical evaluation of a customised 3D-printed metal appliance for intrusion of a supra-erupted maxillary second molar: comparative finite element analysis and an illustrative clinical case

Background Intrusion of a free-end maxillary second molar is biomechanically challenging because the absence of a distal adjacent tooth limits anchorage and force-delivery options. This study characterised the initial biomechanical response patterns of four intrusion approaches and investigated the effects of varying palatal traction hook heights in a customised three-dimensional (3D)-printed metal intrusion appliance. Methods Six finite element models were constructed from one patient’s cone-beam computed tomography data: a segmented arch (A), temporary anchorage device (TAD)-assisted mechanics (B), a clear aligner (C), and a 3D-printed metal intrusion appliance with palatal hook heights of 3, 5, and 7 mm (D1-D3). Prescribed forces were used in A, B, and D1-D3, and programmed displacement in C. Initial tooth displacement, the intrusion directionality index (η), periodontal ligament (PDL) and root stresses, and adjacent-tooth responses were evaluated. Clinical application was illustrated in the same patient. Results All models showed positive vertical crown-centroid displacement. Groups A and C showed larger mean nodal displacements, whereas B and D1-D3 had higher η values. Under the respective activation protocols, mean PDL and root stresses were lower in B and D1-D3 than in A and C. Adjacent-tooth displacement was smallest in B, intermediate in D1-D3 and A, with the largest displacement observed in C. Increasing palatal hook height modestly increased vertical displacement and η values but also changed the direction of buccopalatal displacement. Stress distributions and adjacent-tooth responses varied little across the tested hook heights. The clinical case showed approximately 1.5 mm of intrusion after 3 months. Conclusions In this patient-specific finite element model, the 3D-printed metal intrusion appliance produced predominantly vertical crown-centroid displacement. Palatal hook height influenced initial displacement direction, with little change in stress distributions or adjacent-tooth responses.