A parallel-group randomized clinical trial was conducted with 45 patients of the Orthodontic Clinic of Universidad Iberoamericana's (UNIBE) Postgraduate Unit. The study hypothesis was that motivational interview plus oral hygiene instructions would be more effective in maintaining long-term oral hygiene in comparison with conventional oral hygiene instructions alone.

A computer-generated list of numbers created with Statistical Package for the Social Sciences (SPSS) V21.0 was used to random allocate participants into the experimental or the control group. Monthly oral hygiene instructions and a G.U.M. kit were given to the sample. Additionally, the experimental group received motivational interviewing sessions by a trained periodontist. Simplified Oral Hygiene Index, Gingival Index, Periodontal Probing Depth and Bleeding on Probing were recorded at baseline, three and six months after the beginning of the study. Repeated-measures analysis of variance and chi-squared test were conducted.

Sample calculation A power analysis was carried out to determine the number of subjects needed to achieve a Simplified Oral Hygiene Index (SOHI) significant interaction with the study groups when applying repeated measures ANOVA. The total number of completed subjects needed to achieve power of 0.90 with an alpha (α) of 0.05, an effect size (f) of at least .25 was determined to be 36; taking into consideration a possible 10% attrition rate, 45 subjects were recruited, consented and randomized to either the experimental or control group.

Due to the matter that repeated measures ANOVA treats each measurement as a separate variable, and it's preferable to use listwise deletion, if one measurement is missing, the whole case gets dropped. For this reason and therefore the incontrovertible fact that the trial suffers attrition, the sample for the three measurements was reduced to 32 subjects.

Randomization A simple randomization procedure was executed by the principal investigator to allocate the participants into one of the study groups. A computer-generated list of numbers was created using the software SPSS V21.0, with a 1:1.25 allocation ratio.

Two dentists enrolled and allocated the participants according to the order of arrival and following the list of random numbers. A coding system was used as a concealment mechanism, which consisted of colored labels attached to the participants' file. Thus, the participant and the evaluator (the periodontist who recorded the data) were masked.

Study intervention An interviewer was in charge of providing oral hygiene instructions for both groups. All the participants received a G.U.M. kit with special orthodontic hygiene tools. Then, the patients of the experimental group received a motivational interviewing session.

Afterward, the clinical parameters were measured by another periodontist. Monthly follow-up appointments were scheduled for the orthodontic check-ups to deliver a new G.U.M. kit, to reinforce the OHI and MI, and to register the periodontal parameters. The important data for this research was the one recorded at baseline, three and six months after the intervention.

Statistical analysis Mean Simplify oral hygiene index (SOHI), gingival index (GI), periodontal probing depth (PPD), and bleeding on probing (BoP) scores were compared between groups across three-time points using repeated-measures mixed-model analysis of variance. Tests for equality of variances and sphericity to check for homoscedasticity were performed due to the problem of sample attrition. Nevertheless, it was not found any violation to these assumptions in any of the variables included in the analysis. SPSS software was used for all calculations (IBM SPSS 25th version). Due to the nominal level of measurement of the variable BoP, it was performed a chi-squared test for each three time points to check if there were any differences between the two experimental conditions.

Inclusion Criteria:

• Patients with 18-40 years old, systemically healthy, without active dental caries lesions or periodontal diseases, who were candidates for fixed orthodontic appliances.

Exclusion Criteria:

• Smokers and pregnant patients.
• Participants were discarded if they decided voluntarily to abandon the study of the orthodontic treatment or if they didn't attend to the periodical check-ups.

Two dentists were in charge of the study logistic. They enrolled and allocated the participants according to the order of arrival and following the list of random numbers. A coding system was used as a concealment mechanism, which consisted of colored labels attached to the participants' file. This system was only known by the logistics managers and the interviewer (the periodontist who provided the OHI and the MI). Thus, the participant and the evaluator (the periodontist who recorded the data) were masked.