This study is part of a doctoral research project in Oral Health Sciences and Technologies, with a specialization in Dental Hygiene, at the Faculty of Dental Medicine, University of Lisbon.

With the advancement of technology, smartphones and mobile applications (apps) have become an integral part of daily life, especially among adolescents and young adults, who are the main users of these devices. In 2023, global app downloads reached an estimated 257 billion, including a growing number of health-related apps, known as Mobile Health or mHealth, and, more specifically, apps targeting oral health.

Oral diseases, such as dental caries and periodontal disease, remain a major public health issue due to their high prevalence and impact on quality of life. Although these conditions are preventable, the adoption and maintenance of healthy behaviors continues to be challenging. Mobile apps may serve as an effective complement to oral health consultations, promoting motivation, adherence to recommendations, and communication with healthcare professionals.

Despite the increasing availability of oral health apps, evidence regarding their effectiveness is still limited. Many of these tools lack scientific rigor and fail to adequately meet users' needs.

In this context, an original Portuguese-language mobile application for promoting oral health was developed, based on current scientific evidence and preliminary exploratory studies involving adolescents and oral health professionals. These studies aimed to identify the most relevant features for this type of tool. The app was designed using the Behaviour Change Wheel (BCW) model, widely recognized for structuring effective behavior change interventions in health.

This mobile app was developed by a multidisciplinary team from the University of Lisbon, involving experts from various faculties, including Psychology, Sciences, Dental Medicine, and Fine Arts. As an original tool, it is not yet available on app stores.

The main aim of this study is to test the effectiveness of this app as a complement to oral health consultations in a population of adolescents living in Portugal, contributing to the advancement of knowledge in this field and to the promotion of more innovative and user-centered clinical practices.

These are the specific objectives of the study:

1. To assess the effectiveness of the app when used as a complement to dental hygiene consultations in relation to several oral health indicators, namely risk of dental caries, gingival inflammation, dental plaque presence, and oral self-care behaviors.
2. To analyze the level of satisfaction and usability of the app's features.

The main hypotheses are:

1. The app will significantly improve oral health indicators, including plaque levels, gingival inflammation, and caries risk, compared to a control group.
2. The app will increase adherence to recommended oral hygiene behaviors (e.g., brushing frequency, interdental cleaning).
3. Adolescents using the app will report higher motivation and self-efficacy for oral self-care.
4. The app will demonstrate high usability and user satisfaction, supporting its potential for broader implementation.

The study is structured into two main phases: the development of the application, followed by a randomized controlled field trial to test its efficacy as a complementary tool to oral hygiene consultations.

The application was developed by a multidisciplinary team including professionals in dentistry, psychology, computer science, and design. It was built based on the Behaviour Change Wheel (BCW) model and integrates evidence-based behavioural change techniques from the Behaviour Change Technique Taxonomy v1 (BCTTv1). The app is designed to be personalized and user-friendly, providing features such as interactive toothbrushing guidance (with audio, video, and 3D animations), educational oral health content, gamified elements (badges and achievements), motivational messages, reminders, and a dedicated section where oral health professionals can upload patient-specific advice. The app adapts to user preferences, such as type of toothbrush (manual or electric), orthodontic appliance use, and daily routines.

The second phase of the study will consist of a parallel-group randomized controlled trial conducted in secondary schools in the Lisbon metropolitan area. The sampling strategy will be multi-stage. First, one or more schools will be selected by convenience, ensuring diversity in student profiles across different academic tracks (e.g., sciences, humanities, arts, and vocational education). Within the selected schools, entire classes will be randomly chosen for inclusion in the study. The principal investigator will visit the classrooms, at the beginning or end of a lesson, coordinated with teachers, to present the study objectives and procedures. Students who meet the inclusion criteria and provide informed assent/consent will be enrolled. For participants aged 18 or older, informed consent will be signed by the individual. For minors, both the adolescent and their legal guardian must sign the assent and consent forms, respectively.

After consent is obtained, participants will complete a baseline questionnaire developed by the investigator. The questionnaire is divided into two sections: the first collects demographic and health information, smartphone operating system details, and app usage frequency; the second assesses oral health knowledge and behaviours. Questionnaire content is based on a literature review and standardised oral health recommendations, and it has been reviewed by a panel of five experts (two dentists, two dental hygienists, and one psychologist).

Class-level randomisation (cluster randomisation) will be used to allocate participants to either the experimental group (which will use the app) or the control group (which will not), in a 1:1 ratio. This approach helps minimise the risk of cross-contamination between groups, as each class is expected to have different schedules and limited interaction with other classes. A total sample of 150 participants (75 per group) will be recruited, allowing for potential attrition, although the calculated minimum sample size was 128 based on an expected effect size of 0.5, a power of 0.80, and a significance level of 0.05.

The study will involve three time points across a 3-month follow-up period. At baseline, participants will undergo a clinical oral health assessment including the Simplified Oral Hygiene Index (OHI-S), Bleeding on Probing Index (BOP), Decayed, Missing and Filled Teeth Index (DMFT), and Caries Risk Assessment using the Cariogram model. All students will then receive an oral hygiene consultation, which includes scaling, polishing, and personalised oral health education. In the experimental group, the app will be used as a support tool during the consultation; in the control group, traditional educational materials (e.g., oral cavity models) will be used. A hygiene kit (manual toothbrush, interdental brush or dental floss and fluoride toothpaste) will be provided to all participants.

Thirty days later, a second clinical assessment (intermediate time point) will be conducted, re-evaluating the OHI-S and BOP. No educational reinforcement will be provided during this visit. A third assessment will take place 60 days after the second, repeating all baseline measurements, including a final questionnaire.

All clinical procedures will take place at the school facilities using portable equipment. The research team will include at least two members: the principal investigator, responsible for group allocation, the oral hygiene consultation, and initial data collection; and a second trained and calibrated examiner who will perform follow-up assessments. The second examiner will be blinded to group allocation to reduce bias. Additional field staff may be present at baseline to support data collection and reduce the time required for clinical observations.

The study will adhere to the principles of Good Clinical Practice and the Declaration of Helsinki. All necessary ethical approvals were obtained, including from the Ethics Committee of the Faculty of Dental Medicine of the University of Lisbon and from the school administrations. Participant anonymity will be strictly preserved: each student will be assigned a unique numerical ID, and only assent/consent forms will contain identifying information. These data will be used solely for contacting participants during follow-up and will be destroyed after data collection is complete.

App usage data will be stored locally on the participants' smartphones and will not be transmitted or shared automatically. Instead, participants will be asked to voluntarily share relevant app data with the research team during the follow-up assessments. This data will be identified only by participant ID numbers.

Data collected throughout the study will be manually entered into SPSS by the principal investigator. Descriptive statistics will be calculated for all variables. Inferential statistics will be applied using appropriate tests with a significance level set at 0.05. To ensure inter-examiner reliability, the Kappa coefficient will be used to assess calibration of the researchers. The Kolmogorov-Smirnov test will be used to assess the normality of the distribution of continuous variables, and the Levene's test will be used to assess homogeneity of variances between groups. Based on the results of these preliminary tests, the decision will be made whether to use parametric or non-parametric tests for further analyses.

If normal distribution is confirmed, comparisons of intraoral indices (e.g., plaque index, bleeding index, caries risk) across the three time points will be performed using the General Linear Model (GLM) Repeated Measures or MANOVA. If normality assumptions are not met, the Friedman test will be applied as a non-parametric alternative. For the analysis of associations between two continuous variables from independent samples, Pearson's correlation coefficient will be used, or Spearman's rank correlation coefficient in the case of non-parametric data.

For bivariate analyses involving independent samples, the t-test for independent means or one-way ANOVA will be applied if assumptions are met. If not, the Mann-Whitney U test or the Kruskal-Wallis test will be used, respectively.

Data management procedures include secure storage of all paper and electronic records. Clinical indices will be recorded using standardised forms and entered into a password-protected database. A 10% random sample will be double-checked against original forms to ensure accuracy. Data validation and cleaning will include range and logic checks. A full data dictionary will be maintained, detailing the origin, coding, and normal range of each variable. Missing data will be assessed for randomness and handled using appropriate imputation techniques or sensitivity analysis.

This project represents a pioneering effort in Portugal to develop and evaluate a theoretically grounded, evidence-based mHealth intervention for adolescent oral health promotion.

Inclusion Criteria:

• Be aged between 15 and 18 years at baseline.
• Be fluent in Portuguese.
• Have access to a mobile phone or computer/tablet with internet.
• Be willing to participate in the study by providing signed assent/consent. For participants under 18 years old, consent must be signed by a legal guardian.
• Use of the app at least twice during the first month of the study.

Exclusion Criteria:

• Current or previous use of any oral health-related mobile application.
• Not being responsible for their own oral hygiene.

The outcomes assessor is blinded to group allocation to minimize detection bias. This assessor will perform clinical evaluations (e.g., simplified oral hygiene index, bleeding on probing, DMFT and caries risk assessment) at intermediate and final follow-up points without knowledge of whether participants belong to the intervention or control group. Masking is maintained by using separate investigators for intervention delivery and outcome assessment. Participants, care providers, and the principal investigator are not masked due to the nature of the intervention, which requires active engagement with the mobile app or the standard consultation approach.