The IAP CURAPROX calibrating probe is a graduated conical instrument with a rounded tip (Figure 2A and B). The working part comprises colored bands from the point to the base corresponding to IDBs by increasing diameter. The largest section of each colored band corresponds to the cleaning efficiency diameter of the relevant brush. The nonworking part has a click-fastening joint for the attachment of a handle for easier use and access to the interproximal spaces at the back of the mouth.

Interdental diameters were coded as 1=blue, 2=red, 3=pink, 4=yellow, and 5=green. The procedure consists of introducing the IAP CURAPROX probe into the vestibular interdental space, inserting it fully, then noting the color emerging from the interdental space on the vestibular side. This corresponds to the color code of the IDB most suitable for the interdental space in question. The pressure used to place the probe tip at the base of the interdental sites was approximately 50 N/cm² (0.20 gram force).

The IDBs used in this study are from the CPS range of CURAPROX (Curaden; Figure 2C and 2D). These IDBs consist of cylindrical nylon fiber brushes mounted on a CURAL wire core coated with synthetic material to prevent galvanic or traumatic effects. The brushes are supplied with ergonomic plastic handles to facilitate grip and insertion in posterior sites. Each CPS pack comprises five IDBs with the following characteristics: (1) a specific color code corresponding to the brush size (Table 1); (2) an access diameter, defined by the gauge of the CURAL wire core, which determines the minimal interdental space into which the brush can be inserted; and (3) an effective cleaning diameter, defined by the length and elasticity of the synthetic bristles covering the working part of the device.

The oral examination in accordance with clinical practices will be carried out by a dentist who will report the following clinical parameters:

PPD and CAL will be assessed following the criteria defined by the consensus report of the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions [13], using the US Williams PDT sensor probe (Zila-Pro-Dentec Inc) at a pressure of 20 g. Based on severity-defined stages of interdental CAL and tooth loss, complexity, extent, and distribution, they included a full oral examination of 6 sites on each permanent tooth. All measures will be performed on all teeth except for third molars. One calibrated dentist trained will perform the periodontal evaluations of all participants. Intraclass correlation coefficients for PPD and CAL will be determined at the level of the site. The intraexaminer coefficients for CAL will be, respectively, from 0.80 and 0.85, and from 0.75 and 0.85 for probing depth.

Interdental plaque will be collected from 4 sites (15-16, 25-26, 35-36, and 45-46). Each IDB used for sampling will be stored in RNase- and DNase-free tubes at –20°C within 4 hours and transferred later to the GenEPI platform (Hospices Civils of Lyon, Lyon, France) for long-term storage at −80°C. Analysis will be performed by 16S ribosomal RNA gene sequencing to determine microbiota composition in a single batch. Briefly, a 16S ribosomal RNA gene fragment comprising V3 and V4 hypervariable regions will be amplified using an optimized and standardized 16S amplicon library preparation. Sequencing will then be performed using a 250-base pair paired-end sequencing protocol on an Illumina MiSeq platform (Illumina). Bioinformatic analysis will then be performed using the QIIME2 software (Caporaso Lab, Northern Arizona University) in its latest version.

Participants will complete 2 questionnaires: (1) First, the baseline oral hygiene questionnaire at T0 (Multimedia Appendix 1) that analyzes habits, frequency, and duration of daily hygiene; and (2) second, the Theoretical Framework of Acceptability (TFA) administered at T1, T2, and T3 (Multimedia Appendix 2). The TFA includes dimensions of perceived effort, intervention coherence, affective response, perceived effectiveness, and self-efficacy, allowing assessment of feasibility and long-term integration. The trial participation schedule is detailed in Table 2.

Reduction in interdental inflammation, expressed as the proportion of sites with BoP at 3 months (T3), comparing calibrated versus noncalibrated IDBs.

Secondary outcome measures will be (1) biological outcomes: composition of interdental microbiota (16S ribosomal RNA sequencing, T0 and T3); (2) clinical outcomes: evolution of plaque index, GI, BoP, PPD, and CAL at T1, T2, and T3; and (3) patient-reported outcomes: acceptability and integration of daily interdental cleaning, assessed by the TFA questionnaire at T1, T2, and T3.

Participants will be randomly assigned to one of 2 arms (arm 1: calibrated IDBs and arm 2: noncalibrated IDBs). Randomization will be performed using an interactive web response system integrated into the electronic case report form (v8.2.30 build 3, EnnovClinical Group). Allocation will be 1:1 without stratification, as the sample size is limited and the population is considered homogeneous. Participants will remain in the same arm during the study period. A total of 50 participants will be enrolled, with 25 assigned to each group.

The study will be conducted under single-blind conditions. Participants will not know whether they are assigned to the calibrated or noncalibrated IDB group. To maintain blinding, both types of IDBs will be identical in packaging, color, and appearance. The investigator responsible for the allocation will not be blinded. Intervention implementation will be conducted by staff independent from data collection. To ensure impartiality, during the assessment and analysis phase, statisticians, clinical research associates, and clinicians will not know the group to which the participant belongs. The identification codes will be securely held by the study monitor and will remain sealed until the conclusion of the study to maintain confidentiality and prevent bias.

A case report form, specific to the HIJA study, will be created in Lyon, France. Only data required for the protocol and scientific publication will be recorded in the case report form. Each participant will be assigned a unique identification code to label all documents, forms, and data files. Thus, data recorded by investigators for each participant will be anonymized according to the Data Protection Act. As data are collected, they must be completed by authorized persons (investigators) with their own identifiers, according to the Data Protection Act. During the entry, the data are immediately checked thanks to consistency checks. The person in charge of filing must validate and justify any value change in the electronic case report form. Entries and modifications are subject to an audit trail. The quality, exactitude, and relevance of all input data will be the responsibility of the investigator. Accordingly, each page of the patient’s must be electronically dated and signed by the investigator to confirm agreement and responsibility for the collected data.

Experimental and personal data will be analyzed using a unique identification code with no directly identifiable personal data. Participants can be reidentified through a subject identification file, which will be securely maintained solely at the clinical study site. The signed informed consent documents will also be kept exclusively at the clinical study site for confidentiality and privacy purposes.

Statistical analyses will be conducted on both the intention-to-treat and per-protocol populations. All randomized participants will be included in the intention-to-treat analysis, while the per-protocol analysis will exclude those with major protocol deviations, such as nonuse of IDBs. Missing data will be addressed using multiple imputation by chained equations with Monte Carlo Markov Chain methods, assuming data are missing at random. Descriptive statistics, including means, SDs, medians, interquartile ranges, and frequencies, will be calculated. Between-group comparisons will be performed using Student 2-tailed t test or the Wilcoxon test for continuous variables and the Chi-square test for categorical variables. Changes over time will be assessed using mixed linear models, with log-transformed bleeding values when appropriate. Fixed effects will include independent variables such as age, socioeconomic status, and experimental group, while subjects will be included as a random effect to account for repeated clinical bleeding measurements. Estimated coefficients for each independent variable will be tested against the null hypothesis of zero, with significance set at P<.05. Analyses will be performed using SPSS (IBM Corp), SUDAAN (RTI International), and R software (R Development Core Team). Exploratory analyses will also be performed to assess potential correlations between CAL and participants’ adaptation and compliance with IDB use, as evaluated through the TFA questionnaire and the compliance logbook.