The aim of this study is to evaluate the treatment effects of conventional twin block appliance versus modified twin block appliance in the treatment of patients in growing stage having skeletal class II division 1 with mandibular deficiency.

in this randomized controlled trail there are 2 groups , the first group will receive modified twin block appliance ,in the second group will receive conventional twin block appliance . the follow up period will be 9 months till mandibular growth occur . the assessment will be through a questionnaire on the number of hours the appliance is worn each day , and another questionnaire assessing the difficulty of speech and aesthetics . Also by, photographs, x-rays and study models preoperative and post operative.

Inclusion Criteria:

1. Skeletal Class II relationship (ANB > 4°).
2. Mandibular retrognathy (SNB < 78°).
3. Overjet ≥ 5 mm.
4. Minimal crowding in dental arches (≤4 mm).
5. Class II molar relation.
6. Growing female patients.
7. Patients with CMV3 maturation stage of the cervical verbrae.

Exclusion Criteria:

1. No history of orthodontic treatment either prior to or during functional Appliance therapy.
2. Posterior cross bites or severe maxillary transverse deficiency.
3. Severe facial asymmetry determined by clinical or radiographical examination.
4. Poor oral hygiene.
5. Systemic diseases that may affect the orthodontic treatment results.
6. Patients past their peak growth spurt.

double blinded( participant and outcome assessor)