## 1 Introduction  
The advent of the Internet and new digital technologies has completely changed the way we communicate. In recent years, a widespread phenomenon is that of social media, a group of Internet-based applications that allow the creation and exchange of user-generated contents [1]. Due to the diffusion of smartphones into people's daily routines, these technologies offer the innovative opportunity to reach a large audience and represent an inexpensive, rapid, and powerful way to communicate and disseminate information [2]. An exponentially increasing number of publications in medicine and dentistry have focused on the importance of using social media for health promotion interventions [3,4] and evidence suggests that new technologies can effectively contribute to improve knowledge and behavioral changes [5,6]. Social media have demonstrated to be a promising resource to facilitate chronic diseases self-management, medication adherence, tobacco cessation and support for mental health conditions [5,7,8]. Other recent studies used digital platforms to disseminate information related to skin cancer prevention [6], human papillomavirus vaccination [9], nutrition, and physical activity [10].  
The Lancet Commission on Adolescent Health and Wellbeing suggested that Digital Media offer \"outstanding new possibilities for engagement\" with youth populations [11]. In dentistry, several studies investigated the contents of social media on oral health and treatment procedures [12, 13, 14, 15, 16], but until now there has been little research on the effectiveness of social in relation to orthodontics. A good communication is an integral part of orthodontic treatments to obtain the patient's compliance for the oral hygiene, considering that the insertion of fixed orthodontic appliances makes hygiene procedures more difficult, restricts salivary and mucosal self-cleaning capacity, altering the microbial environment and increasing plaque build-up [17, 18, 19, 20]. As a result, gingivitis and enamel demineralization (including white spot lesions and dental caries) have been regarded as the most prevalent undesired effects that can lead to unsatisfactory results or premature termination of orthodontic therapy [21]. [PERSON] et al. [22] showed that 5% to 10% of orthodontic patients failed to complete their treatment just because of oral hygiene issues.  
It is well known that patient's compliance can be challenging during a long orthodontic therapy, especially in adolescents; therefore, both oral hygiene instruction (OHI) and patient motivation play a crucial role in maintaining an effective plaque control. To get better feedback, it would be appropriate to use communication tools that are familiar to youth population. As an innovative method, social media have the advantage of being constantly used, besides having interactive and engaging features such as animations [23,24]. The aim of this study is to investigate the effect of Instagram, the most used social media app among young adults [25], in improving knowledge and compliance for oral hygiene among young patients with fixed orthodontic appliances.  
The first null hypothesis of this study is that there is no significant difference in clinical parameters between young patients receiving conventional oral hygiene instructions with or without the additional use of a social media. Subsequently, the second null hypothesis is that no difference neither occurs when considering the increase of knowledge on oral health and oral practices as well as on dietary habits.