Abstract: Objective: To detect the clinical effect of Twin-Block appliance in the treatment of the patients with Class Ⅱ division 1 malocclusion,and to provide the basis for clinical treatment. Methods: A total of 30 patients with class Ⅱ division 1 malocclusion were treated with Twin-Block appliance.The variables of the patients before and after treatment were measured with Ricketts cephalometric radiographys. Results: Compared with before treatment, the length of corpora mandibulae(Xi-Pm) of the patients after treatment was increased by 2.50 mm (P<0.05),the SNB angle was increased by 2.58°(P<0.05),and the ANB was decreased by 2.71°(P<0.01);the molar relationship distance and the Overjet were decreased by 2.81 mm and 3.68 mm,respectively (P<0.01);the L1-OP was decreased by 1.89 mm,the distance of L1-APog was increased by 1.84 mm, and the angle U1-APog was decreased by 7.35°(P<0.01);the Ls-TVL was increased by 1.85 mm,the Sm-TVL was increased by 2.12 mm (P<0.05),and the Pos-TVL was increased by 2.89 mm (P<0.05). Conclusion: The Twin-Block appliance could obviously promote the development of mandibular bone,upright the anterior teeth and improve the lip angle of upper anterior teeth and the proflie in the treatment of the patients with class Ⅱ division 1 malocclusion in the growth development period.

Key words: Twin-Block appliance, Class Ⅱ division 1 malocclusion, Ricketts analysis method, X-ray cephalometrics