Thus study consists of evaluating the effect of low level therapy on pain induced by insertion of an orthodontic device (the "quadhelix"). Each day during the first seven days post procedure, the level of pain will be reported on an analogic visual scale.

As secondary objectives, intake of antalgics and type of swallowed food (soft or tough) will also be daily registered.

Patient will be randomized into one of the 3 following arms :

• arm 1 (experimental) : administration of Low Level Laser Therapy just after insertion of the quadhelix.
• arm 2 (sham procedure) : material necessary to perform Low Level Laser Therapy will be installed but the laser will not be activated.
• arm 3 (no intervention): material necessary to perform Low Level Laser Therapy will NOT be installed

Inclusion Criteria:

• Children aged between 8 and 15 years old who need to receive an orthodontic device ("quadhelix)
• informed consent form signed by parents who should be affiliated to French health insurance (french Social security)

Exclusion Criteria:

• malignant facial tumor
• not able to read or write correctly and thus not able to complete the questionnaire
• allergic or intolerant to paracetamol
• Informed consent not obtained