Design: Two-arm parallel randomized clinical trial in mixed dentition (6-12 years).

Arms: (1) 9 mm, 900 g leaf expander; (2) 9 mm Hyrax expander. Follow-up: From appliance insertion → completion of intended expansion → 6-month retention.

Primary outcome: Pain perception during the first 7 days post-insertion (daily patient-reported scale).

Secondary outcomes: Skeletal and dental changes on CBCT.

Participants receive a banded maxillary expansion appliance cemented to maxillary first permanent molars, assigned to:

Leaf expander arm: Nickel-titanium leaf-spring expander, 9 mm screw, delivering force of ~900 g.

Hyrax expander arm: Conventional tooth-borne jackscrew expander, 9 mm screw. Standard activation: two turns per week.

Retention: Both appliances kept passively in situ for 6 months of retention following achievement of planned expansion.

Inclusion Criteria:

• Male and female patients
• Mixed dentition stage
• Unilateral or bilateral posterior crossbite.
• Patients in prepubertal stage with cervical vertebral maturation stage 1-3.
• Fully erupted maxillary and mandibular permanent first molars.
• Skeletal class I or II.

Exclusion Criteria:

• Previous orthodontic treatment.
• Missing teeth.
• Inadequate oral hygiene.
• Patients with bad oral habits.
• Patients with medical conditions.
• Patients with cleft lip and palate.

statistician