The effect of an indirect anchorage reinforcement device on maxillary anterior intrusion with clear aligners: a three-dimensional finite element analysis and exploratory clinical validation.

BACKGROUND: Anterior intrusion is commonly incorporated into clear aligner protocols to correct deep overbite and maintain overbite control during anterior retraction. However, anterior intrusion remains difficult to achieve predictably, potentially due to insufficient vertical anchorage of the posterior segment. This study proposed an indirect anchorage reinforcement (IAR) device to guide occlusal contact and indirectly reinforce posterior vertical anchorage, and evaluated its biomechanical effects and clinical performance.

METHODS: A two part study was conducted. First, three-dimensional finite element analysis simulated initial responses in anterior intrusion (Model A) and combined anterior intrusion and retraction with first premolar extraction (Model B). Each model included a Control and an IAR condition (A-Control, A-IAR, B-Control, B-IAR). A 0.2 mm activation was applied for intrusion, and Model B additionally included a 0.2 mm activation for retraction. A 200 g vertical load was applied to the IAR device to simulate occlusal contact. Outcomes included initial tooth displacement, sagittal axial inclination, the sagittal force-moment system, and aligner displacement and von Mises stress. Second, a matched exploratory clinical study with a historical control evaluated a maxillary anterior intrusion protocol. The planned total intrusion was 2 mm, delivered in 10 aligners at 0.2 mm per aligner, with no other programmed tooth movements during these steps. Aligners alone were compared with aligners incorporating the IAR device. Intrusion accuracy was quantified by three-dimensional model superimposition and calculated as achieved divided by planned intrusion. Propensity score overlap weighting and overlap-weighted regression were used to compare the groups.

RESULTS: In finite element simulations, the IAR design increased anterior intrusion and suppressed posterior extrusion, with smaller axial inclination changes and a reduced moment arm d, consistent with a resultant force acting closer to the center of resistance. Clinically, the IAR group achieved higher intrusion accuracy. In overlap-weighted analyses, the weighted mean difference (IAR minus control) was 18.96% points for incisors (95% confidence interval 9.01-28.91; P < 0.001) and 33.56% points for canines (95% confidence interval 24.65-42.46; P < 0.001). Intra-examiner reliability was good (intraclass correlation coefficient = 0.85).

CONCLUSION: Across both non-extraction and extraction scenarios, the IAR concept improved the achievement and predictability of anterior intrusion while suppressing extrusion of the posterior anchorage teeth, indicating improved vertical anchorage control. The exploratory clinical findings supported its clinical feasibility, with a more pronounced improvement in canine intrusion.