Effects of Mandibular Extractions with Clear Aligners: A Finite Element Analysis

This study was aimed at analysing the mechanical characteristics of different mandibular extraction modes using a clear aligner. Three experimental schemes of different extraction patterns were designed to treat mandibular crowding, including extraction of one mandibular central incisor, bilateral first premolars, and bilateral second premolars. The stress distribution during the space closing was analysed using the finite element method. When a central incisor was extracted, a significant retraction force was found in the anterior region, in line with the design expectation. The posterior teeth, which were designed to move mesially, acted as anchorage for anterior retraction, and were subjected to a mesial force. The anterior teeth were retracted when the bilateral first premolars were extracted. The lateral incisors and canines were subjected to a significant distal force and moment, while the central incisors and canines were subjected to lingual forces and moments. Additionally, the canines were subjected to a non-designated intruding force. The molars were designed to move mesially when the bilateral second premolars were extracted. All molars were subjected to a significant mesial force, while the lingual force on the front teeth was slight. The bilateral second molars were subjected to non-design mesial moment and extrusive force. The bilateral first molars were subjected to a non-designated mesial moment, and the bilateral first premolars on both sides were subjected to non-designated intrusive force and distal moment. When one incisor was extracted, attachments on the anterior teeth had a controlling effect on the tooth axis, but the anterior teeth still tended to tilt. When the bilateral first premolars were extracted, the anterior teeth showed a tendency for lingual inclination. The risk of distal inclination of the canines and lingual inclination of the central incisor increased. When the bilateral second premolars were extracted and the posterior teeth were designed to move mesially, the teeth on both sides of the extraction sites showed an obvious bowing effect.