Abstract: Abstract OBJECTIVE: This study was to investigate the effect of pre-formed muscle function appliance (T4K) on upper airway before and after orthodontic treatment of Class II mandibular retraction in the dentition period.METHODS:Twenty-three patients with Class II mandibular retraction who underwent treatment in the orthodontic department of Jiangsu Provincial Stomatological Hospital from February 2018 to August 2018, with an average age of 7.0 ± 1.04 years were used. The muscle function appliance (T4K) was used to correct the patient for 6 months. The cone beam CT was taken before and after treatment. The Dolphin 3D 11.9 software was used for 3D reconstruction. The statistical analysis was performed by SPSS 25.0 software. And after the measurement related indicators change. RESULTS: After 6 months of treatment, the upper airway total volume、Nasopharyngeal volume、Glossopharyngeal volume and pre-treatment results were excluded except for the pharyngeal segment.Statistically significant,the volume of the nasopharyngeal segment was the most significant; the minimum cross-sectional area of the upper airway nasopharyngeal segment and the pharyngeal segment was increased after treatment,The difference wasstatistically significant Conclusion:The maxillofacail function correction by pre-forming muscle function appliance (T4K) is beneficial to patients to get rid of bad oral habits, training the position of the tongue and using nasal breathing to restore the balance between the muscles of the mouth and the muscles. Proper growth with the airway

Key words: Pre-formed muscle function appliance, T4K appliance, Angle class II, early correction, upper airway