The aim of this study is to evaluate and compare the effect of rapid maxillary expansion with and without microosteoperforation in patients with skeletal maxillary constriction.Theoretically, the force generated by the activation of the Hyrax expander leads initially to the compression of the periodontal ligament, bending of the alveolar bone, and tipping of the anchor teeth. This in turn contributes to the unfavorable changes in the supporting tissues; such as root resorption, buccal crown tipping, reduction of buccal bone thickness, marginal bone loss and alveolar bone fenestration and dehiscence. In this trial, it is assumed that by reducing the cortical bone resistance, using micro-osteoperforation (MOP), the alveolar bone bending and buccal tipping that usually occur during expansion will be reduced and thus facilitating the bone expansion.

The study consists of 2 groups that are divided divided equally and randomly.The first group will receive a tooth borne Hyrax expander supported on first premolar and first permanent molar , whereas the second group will receive the same appliance associated with microosteoperforation(MOP). The MOP will be performed on the buccal segment bilaterally along the maxilla starting from the distal of the canine till the distal of the first permanent molar. The assessment will be done using CBCT scans immediately after expansion and 6 months postretention.

Inclusion Criteria:

• Female adolescent patients with an age range 11- 14 years.
• Having apical constricted maxillary arch reflected by posterior cross-bite and verified through Howe's analysis on the dental study models.
• Normal vertical growth pattern.
• Fully erupted maxillary first premolars and first permanent molars.

Exclusion Criteria:

• Previous orthodontic treatment.
• Any congenital anomalies, systemic disease, or asymmetries that might have an influence on tooth movement.
• Active periodontal affection or severe gingival inflammation.