This clinical trial compares the effectiveness of a myofunctional appliance that uses a clear and modified twin block appliance with an expander made from 2mm biocompatible polyethylene terephthalate modified with Glycol thermoplastic material with a conventional twin block appliance with an expander. The trial will be conducted on adolescent patients with class II division 1.

In a multicenter single-blinded randomized clinical trial study with two-arm parallel groups, a computer random generator will develop a simple randomization with an equal allocation ratio (1:1) and without stratification. The investigator will initially assess the patients for eligibility to be included in the study. Those who will meet the inclusion criteria, the parents will be informed about the nature of the study verbally to get the initial approval for participation. Then they will be provided with the patient information sheet and consent form.

According to the sample size calculation, 48 patients will be recruited (24 for each group), and impressions and lateral cephalometric radiography will be taken before treatment and bite registration then the appliance inserted, and the patient will be checked every two weeks for expansion and every four weeks for overjet recording. The treatment will be continuing for 6 months to take final records. The collected data will include cephalometric evaluation, clinical evaluation, scanned upper model evaluation, and finally patient satisfaction from the appliance.

Data will be analyzed using Statistical Package for Social Sciences for Windows, version 26.0 including descriptive statistic (number, mean, and standard deviation), Reliability statistic (interclass correlation coefficient), and Inferential statistics (independent samples t-test).

Inclusion Criteria:

1. Patients' age at the start of treatment: at growth spurt [in stage 2 (Acceleration) - 3 (Transition) of cervical vertebral maturation].
2. Skeletal and dental relationships: skeletal class II with normal maxilla and retrognathic mandible. Moderate to severe dental class II division 1 malocclusion with overjet ≥ 8 mm.
3. Horizontal/normal growth pattern.
4. Convex profile with decreased or normal lower facial height (clinically and radiographically).
5. Well-aligned/mild crowding of maxillary and mandibular dental arches.
6. Good oral health free from caries and periodontal problems at the start of treatment.

Exclusion Criteria:

1. Patients with vertical growth pattern or backward mandibular rotation tendency.
2. Patients with open bite
3. Patients with obvious facial asymmetry.
4. Patients with orofacial clefting.
5. Patients with bad oral habits.
6. Active periodontal disease.

In the study, data collection and measurement will be conducted with the investigator. However, it won't be possible to blind the operator (investigator) due to the appliance design/shape difference, making the study single-blinded. Each patient will not know which type of twin block appliance will be used for them.