Abstract: Objective To evaluate MSM of the skeletal Class Ⅱ patients aged 10-20 in Jiangsu Province by CBCT before orthodontic treatment, combined with CVM, to provide a reference for the clinical choice of a reasonable RME method. Methods The study subjects included 89 male patients and 120 female patients. MSM were evaluated according to the method of Angelieri, and CVM were evaluated according to the CVM staging method updated and simplified in 2005. Results Age and gender were influencing factors of MSM in 209 patients, MSM of patients aged 16-20 were higher than those aged 10-15 (P<0.01)， MSM in female patients were higher than male (P<0.01)， MSM of CS4-CS6 were higher than that of CS1-CS3 (P<0.01). Gender and CVM were not influencing factors of MSM in patients at 10-15 years old; gender was the influencing factor of MSM in patients at 16-20 years old. Conclusion The results show that orthodontists should evaluate the MSM of skeletal Class Ⅱ patients aged 10-20 years before RME, so as to provide basis for the selection of RME method for patients at 10-15 years old. The influence of gender on 16-20yearold patients should be considered, so as to avoid RME failure caused by excessive tooth effects.

Key words: midpalatal suture, rapid maxillary expansion, skeletal Class Ⅱ, CVM, CBCT