Abstract: Objective: This case reported the clear aligner therapy (CAT) of a patient with severe dental crowding, introverted deep overbite of the anterior teeth, and premolar impaction. Diagnosis and Treatment: A 28-year-old female patient, after being fully informed the treatment protocol and obtaining the patient’s informed consent, three first premolars and one lower first molar were extracted and the extraction space was used to relieve the crowding of the dentition, control the lingual movement of the root of the upper incisor, depress the lower anterior tooth to open the anterior tooth occlusion, and move the left molar mesially. Later stage of the first treatment, the segmented arch technology was used flexibly to right the $\left. {\overline {\, 5 \,}}\! \right| $ and align the left lower posterior teeth at the appropriate time. Finally, additional clear aligners were used to fine-tune the occlusion. Results: The orthodontic treatment course lasted for 29 months (54+34, a total of 88 sets of aligners). After treatment, the upper and lower dentitions were aligned, the overbite and overjet of anterior teeth were normal, with ClassⅠ relationship for right molars and ClassⅡrelationship for left molars. Conclusions: The precise design of the anterior tooth root control movement in the CAT plan and the flexible use of the advantages of the segmental arch, and the timely adjustment of the orthodontic technology, therefore improved the effectiveness of CAT.

Key words: clear aligner therapy, introverted deep overbite, molar mesialization, root torquing, raction of impacted teeth