Combined use of clear aligners and mini-screws for intrusion of the maxillary first and second molars before prosthetic restoration: a finite element analysis

The long-term absence of molars might lead to the overeruption of the opposing molars, resulting in insufficient space for prosthetic restoration. Orthodontic intrusion of the opposing molars can assist in the reconstruction of the restoration space. This study aimed to explore the efficacy of the combined use of clear aligners and mini-screws in the multiple maxillary molar intrusion before restoration. Cone beam CT data from an adult male requiring intrusion of the maxillary left first and second molars were used. The maxilla and upper dentition were reconstructed using MIMICS software. Two mini-screws were placed between the upper first and second molars, and the buttons were placed close to the cervical region of the molar crowns. Six groups were established based on the intrusion protocols: Group A, mini-screw intrusion alone; Group B, clear aligner intrusion alone; Group C, both mini-screw and clear aligner intrusion; Group D, mini-screw first molar traction and clear aligner intrusion; Group E, clear aligner intrusion assisted by mini-screws; Group F, mini-screw first molar traction and clear aligner intrusion assisted by mini-screws. The amount of three-dimensional displacement, and the stress distributions on the surfaces of the periodontal ligaments of the upper left second premolar and molars were analyzed using ANSYS software. The second premolars were extruded in all groups except Group A. In Group A, the first molar moved distally while the second molar moved mesially. In Group B, both the first and second molars moved mesially. For the first molar, Group F showed the greatest amount of intrusion, while Groups A and B showed the least. A similar amount of intrusion of the buccal and palatal cusps was observed in Groups D and F. The amount of intrusion of the second molar was obviously less than that of the first molar in all groups, while the second molar in Group B exhibited extrusion. The greatest amount of intrusion was showed in Groups C and F. Obvious buccal tipping of the second molar was observed in Groups B, D, and E, while the intrusion was accompanied by distal displacement in Groups D and F. Similar amount of intrusion of the buccal and palatal cusps was observed in Group F. Stress on the periodontal ligaments was concentrated mainly on the apices of the palatal roots and cervical areas. In Group F, the stress was evenly distributed on the surfaces of the periodontal ligaments, with no obvious zones of stress concentration. The use of either mini-screws or clear aligners alone for the intrusion of the maxillary first and second molars was accompanied by unwanted bucco-palatal and mesio-distal displacement with low amount of intrusion. The combined application of clear aligners and mini-screws could increase the amount of intrusion and reduce the tipping displacement of the maxillary molars.