Effects of residual extraction space and torque overtreatment on mandibular incisor intrusion with clear aligners: a finite element analysis

Clear aligner therapy has expanded from simple to complex cases, including premolar extraction cases. However, the predictability of anterior tooth intrusion in extraction cases is limited and the biomechanics remains unclear. The aim of this study was to investigate the biomechanics of mandibular incisor intrusion with clear aligner therapy in premolar extraction cases, focusing on the influence of residual extraction space and torque overtreatment on tooth movement. A three-dimensional finite element model of mandibular extraction was used to simulate incisor intrusion under varying residual spaces (0–7.2 mm) and labial torque overtreatment levels (0–2°). Without torque overtreatment, incisor intrusion consistently produced lingual crown inclination. The required torque overtreatment (Y) for bodily movement correlated positively with residual extraction space (X) under the simulated 0.25 mm intrusion: Y = 0.05X + 0.72 for central incisors and Y = 0.025X + 0.60 for lateral incisors. Optimal overtreatment also promoted uniform periodontal ligament stress distribution. In extraction cases with clear aligner therapy, incisor intrusion causes lingual crown inclination, which is influenced by residual extraction space. Torque overtreatment effectively counteracts this inclination, with central incisors requiring more overtreatment than lateral incisors. The required optimal overtreatment amount is positively correlated with the residual extraction space.