Patients who have severe crowding that requires four premolars extraction will be treated in this study. The efficacy and effectiveness of in-house clear aligners therapy compared with vestibular fixed appliances in the treatment of severe crowding malocclusion cases requiring the extraction of first premolars will be evaluated using Little's irregularity index (LII), Peer assessment rating index (PAR), and treatment duration.

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. 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 skeletal and dental malocclusion
• severe crowding (more than 6 mm of tooth size-arch length discrepancy)
• Good oral hygiene and periodontal health.
• 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:

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