Effects of torque overcorrection on mandibular incisor intrusion using clear aligners in the first premolar non-extraction and extraction conditions: a finite element analysis study

BackgroundDuring intrusion and retraction, the mandibular incisors tend to tip labio-lingually, which might damage the surrounding alveolar bone. This study aimed to investigate the effect of torque overcorrection on the tooth movement and the stress distribution of the periodontal tissues.MethodsMandible and mandibular dentition were reconstructed in the digital software. Three groups were divided: first premolar non-extraction with incisor intrusion (NEI) group, first premolar extraction with incisor intrusion (EI) group, and first premolar extraction with incisor intrusion and retraction (EIR) group. The intrusion amount of 0.25 mm with different torque overcorrection degrees (0 degrees, 1 degrees, 2 degrees, 3 degrees) were sequentially added on the incisors. The biomechanical responses of the teeth and periodontal tissues were calculated in the ANSYS software.ResultsWith no overcorrection, the incisors were intruded with lingual tipping, while the canines and premolars were extruded with mesial tipping. The compressive stress was concentrated on the lingual cervical and labial apical areas of the periodontal ligaments (PDLs) and alveolar bone. The incisors in the EIR group showed the most lingual displacement and the highest stress level. When 1 degrees, 1 degrees and 3 degrees overcorrection degrees were added in the NEI, EI and EIR groups, respectively, the lingual displacement of the incisors decreased to zero and the stress was uniformly distributed on the PDLs and alveolar bone. With the increase of the overcorrection degrees, the incisors tended to tip labially, while the canines and premolars also gradually showed distal tipping tendencies. The compressive stresses were concentrated on the labial cervical and lingual apical areas of the PDLs and alveolar bone.ConclusionsDuring mandibular incisor intrusion, the torque overcorrection design may help reduce the lingual tipping tendencies of the incisors, relieve the compressive stress on the periodontal tissues, and protect the anchorage from the canines and premolars. When the first premolar was extracted, similar overcorrection degrees are required if the incisor retraction was not designed. However, more overcorrection degrees are required if the incisor intrusion and retraction were designed synchronously.