Patients with severe crowding requiring the extraction of four premolars will be enrolled in this study. The degree of apical root resorption associated with using the clear aligner technique compared with vestibular fixed appliances in the treatment of severe crowding malocclusion cases requiring the extraction of first premolars will be evaluated using cone beam computed tomography (CBCT) volumes.

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.

With the increasing number of young adults requesting esthetic orthodontic treatment, clear aligners have continued to gain widespread use, driven by greater consideration of esthetic aspects and oral health-related quality of life compared to traditional fixed orthodontic appliances. Recently, clear aligners have been used in more complex malocclusion cases that require premolar extraction. Teeth root resorption occurs due to physiological or pathological causes that lead to loss of tooth structure. Apical root resorption (ARR) is a common side effect of orthodontic treatment that requires careful investigation of its location and frequency.

Inclusion Criteria:

• Adult patients aged 18-25 years.
• Class I malocclusion with severe dental crowding.
• Tooth size-arch discrepancy more than 6 mm.
• No congenitally missing or extracted teeth (except for the wisdom teeth).
• No history of previous trauma to the maxillofacial region or surgical interventions.

Exclusion Criteria:

• Previous orthodontic treatment.
• Presence of previously occurred ARR.
• Patients with psychological abnormalities or systemic diseases.
• Patients with known allergies to latex or plastic materials.