Abstract: Objective　 To evaluate the changes of the upper airway and hyoid position before and after the MRC muscle functional appliance in the treatment of children with malocclusion in the mixed dentition stage with mouth breathing by cone beam computed tomography (CBCT) and Dolphin software, in order to provide an important reference for clinical selection of MRC muscle functional appliances. Methods　20 children with malocclusion and accompanied by mouth breathing in the mixed dentition were included. There were 11 males and 9 females with an average age of 8.70±0.92 years. All the children were treated with MRC muscle appliance for 12 months. CBCT and Dolphin software were used to analyze and evaluate the effect of MRC appliance on the oropharyngeal airway and hyoid position. Results　 After 12 months of orthodontic treatment with MRC muscle appliance, the surface area of oropharyngeal airway was significantly increased (increased by 63.28±16.99 mm2, P<0.001), and the minimum axial area of oropharyngeal airway increased significantly (increased by 69.67±26.70 mm2, P<0.001).The total volume of oropharyngeal airway increased significantly (increased by 2896.44±688.86 mm3, P<0.001). The hyoid position moved significantly forward and downward (5.62±0.88 mm to the third anterior superior cervical margin, P<0.001; 3.43±0.92 mm to the third anterior lower cervical margin). Conclusions　 MRC muscle functional appliance has good effect on the anatomical structure of oropharyngeal airway and hyoid position in the mixed dentition children with malocclusion and accompanied by mouth breathing, which is conducive to break the oral breathing habit and relieve the adverse factors affecting the normal growth of cranial-maxillofacial and upper airway.

Key words: children in mixed dentition, mouth breathing, MRC appliance, CBCT