 The orthodontic thermoplastic devices became more affordable and with simple techniques, due to the technological advancement. The main advantage of this appliance is that it is nearly invisible, seen that patients do not want to show braces. Others reasons for searching this kind of treatment is because it is removable. While eating, brushing the teeth or even in a important social event, there is the facility to remove the appliance. In the other hand this appliance, according to the american board association, does not finish the treatment as well as the conventional braces. Another problem is that every device placed in the mouth is more plaque retentive, increasing the risk of enamel demineralization. Also, this device disrupts the normal flow of the saliva, preventing the buffering. This study will analyse the amount of streptococcus mutans and lactobacillus in the saliva and in the biofilm of the teeth and of the appliance, seeded in culture medium. Another point, is check the prognosis of the gingival health owing to the contact with the device during the hole treatment. 