Patients who have severe crowding that require four premolars extraction will be treated in this study. The efficacy of the clear aligners and vestibular fixed appliances will be assessed. The treatment result of these two different methods will be explored using the Peer Assessment Rating (PAR) in two different times (T0: Before treatment, T1: After treatment) and the American Board of Orthodontics Objective Grading System (ABO-OGS) after treatment (T1).

There are two groups:

First group (Experimental): the patients in this group will be treated using clear aligners.

Second group (Control): the patients in this group will be treated using fixed appliances.

For years, orthodontists and dentists have used removable appliances for orthodontic treatment. Today, with the CAD/CAM technology, clear aligners treat a broader range of cases with greater precision. They consist of a series of plastic aligners that are intended to replace conventional wire and bracket technology for many orthodontic cases. Each custom manufactured aligner exerts gentle, continuous forces to move teeth incrementally from their original state to a final, treated state. Each aligner is worn for about two weeks, then replaced by the next in the series until the final position is achieved.

Inclusion Criteria:

• Class I malocclusion with severe crowding (more than 5 mm of tooth-size-arch-length-discrepancy).
• Good oral hygiene and periodontal health.
• No severe skeletal discrepancy.
• Normal proclination for the upper and lower incisors.
• No congenitally missing or extracted teeth (except for the third molars).
• No history of previous trauma to the maxillofacial region or surgical interventions.

Exclusion Criteria:

• Bimaxillary dentoalveolar severe protrusion.
• Previous orthodontic treatment.
• Subject with psychological abnormalities.
• Subject with systemic diseases.
• Subject has known allergy to latex and plastic