Abstract: Objective: To compare clinical curative effect on Class Ⅱ with mandibular retrognathia by the two Twin-block appliance. Methods: 13 mandibular retrusion case with mixed dentition or early permanent dentition were selected, they received modified Twin-block treatment. According to the case control principle 13 case with Twin-block treatment were selected as the control group .The Cephalometrics analysis was performed before and after treatment for oberserving 18 hard tissue variables. Results: In the experiment group, the SNB increased, the Y axis decreased, MP-SN, FH-MP increased, the jaw and chin position relative to the skull base of change was obvious .Early treatment with two Twin-block appliance were effective in overjet and correction. Conclusion: It was almost the same effect for Class II Division 1 mandibular retrusion patients using Twin block or modified Twin-block in the early treatment. But the modified Twin-block appliance was more suitable for curing Angle’s class II division 1 mandibular retrusion cases.

Key words: Stepwise- mandible- forward, Twin-block appliance, Mandibular retrognathiaX, Cephalometrics