Compare the impact of using rapid maxillary expanders on masticatory muscle activities (Masseter and Anterior Temporalis muscles) among different posterior cross-bite patient (unilateral, bilateral) as well as compare the cross-bite side and non-cross-bite sides of the same preadolescent unilateral crossbite patient before the expansion and after one month of the completion treatment period using surface electromyography device.

Compare the impact of using rapid maxillary expanders on masticatory muscle activities (Masseter and Anterior Temporalis muscles) between the cross-bite side and non-cross-bite sides of the same preadolescent orthodontic patient before the expansion and after one month of the completion treatment period.

Two channels surface Electromyography sEMG (BioRadio), will be position to record the activity of anterior temporal (AT) and masseter (MM) muscles in three different statuses:

• Rest → patients will instruct to not bite on his/her teeth.

• Clench → patients will instruct to made a maximum voluntary clench on cotton rolls positioned around the first molar, for 5 sec "the cotton clench", and a maximum voluntary clench without the interposed rolls for 5 sec "the "clench".

  * This procedure followed the Sforza et al. protocol.

• During function "mastication" → The patients will instruct to chew on a chewing gum for 20 sec (20 continuous masticatory cycles).

Examination frequency:

The patients will be examined two times as follow:

Test 1: Immediately before rapid maxillary expanders cementation, Test 2: 5 months later (1 month after the retention phase).

the same procedure will be done on 35 patients (control group)

Inclusion Criteria:

1. Uni-lateral/ or bilateral posterior cross-bite.
2. Patients with erupted permanent maxillary and mandibular first molars.
3. Will be treated by rapid maxillary expanders, (the appliance planning to be worn for approximately 5 months).
4. Age ranged from 9 to 12.
5. Males and females.
6. Not under any medications.
7. Skeletal Class I or Class II (based on ANB angle, convexity, and Wits appraisal).
8. No active caries.
9. Healthy periodontal tissues.
10. Subjects with no signs of TMD or condylar problems.

Exclusion Criteria:

1. History of any functional appliance treatment.
2. Oral, systemic disease or any syndromes.
3. Patients under any medications.

6. Patients with symptoms of Temporomandibular disorders. 4. Neuropathic conditions. 5. Open and deep-bite.

data will be used for a research purpose only , will not be shared with any other institution