Effectiveness of an anterior mini-screw in achieving incisor intrusion and palatal root torque for anterior retraction with clear aligners: A finite element study

Objectives: To analyze the biomechanical system of anterior retraction with clear aligner therapy (CAT) with and without an anterior mini-screw and elastics. Materials and Methods: Models including a maxillary dentition (without first premolars), maxilla, periodontal ligaments (PDLs), attachments, and aligners were constructed and imported to finite element software. Three model groups were created: (1) control (CAT alone), (2) labial elastics (CAT with elastics between the anterior mini-screw and buttons on central incisors), and (3) linguoincisal elastics (CAT with elastics between the anterior mini-screw and precision cuts on the lingual sides of the aligner). Elastic forces (0-300 g, in 50 g increments) were applied. Results: CAT alone caused lingual tipping and extrusion of the incisors. Labial elastics caused palatal root torquing and intrusion and mesial tipping of the central incisors, while linguoincisal elastics produced palatal root torquing and intrusion of both central and lateral incisors. Second premolars were intruded in all three groups, with less intrusion in the linguoincisal elastics group. For the control group, stress was concentrated on both labial and lingual root surfaces, alveolar ridge, and cervical and apical PDLs. Stress was more concentrated in the labial elastics group and less concentrated in the linguoincisal elastics group. Conclusions: CAT produced lingual tipping and extrusion of incisors during anterior retraction. Anterior mini-screws and elastics can achieve incisor intrusion and palatal root torquing. Linguoincisal elastics are superior to labial elastics with a lower likelihood of buccal open bite. Root resorption and alveolar defects may occur in CAT, more likely for labial elastics and less likely for linguoincisal elastics. (Angle Orthod. 2021;91:794-803.)

Automated summary

The study shows that clear aligner therapy with anterior mini-screws and elastics can achieve incisor intrusion and palatal root torquing. Linguoincisal elastics are superior to labial elastics with a reduced risk of buccal open bite.