The objective is to determine whether the efficacy of a 38% silver diamine fluoride (SDF) gel is non-inferior to a 38% SDF solution for arresting carious lesions in preschool children when applied at half-yearly intervals over a 30-month period.

Aim: Silver diamine fluoride (SDF) is typically available as a 38% aqueous solution, which is runny and difficult to apply. A 38% SDF gel has been introduced but it has not been tested clinically. The objective is to determine whether the efficacy of a 38% SDF gel is non-inferior to a 38% SDF solution for arresting carious lesions in preschool children.

Methods / Design: This non-inferiority randomised clinical trial will recruit 630 3- to 4-year-old children to receive either SDF gel or SDF solution on their carious lesions every 6 months. The primary outcome is the proportion of active carious tooth surfaces that arrest at the 30-month follow-up. The same examiner will conduct all examinations at the kindergartens over 30 months. The examiner, the children, and the children's parents will be blinded to treatments. The parents will be surveyed on their child's oral health-related behaviours and socioeconomic background to allow adjustment for effect modification.

Significance: If the result is as anticipated, SDF gel will prolong contact of SDF with the carious lesion to promote remineralisation. As SDF gel is cost-effective, simple, non-invasive, and non-aerosol-generating, it can be widely recommended for caries control.

Inclusion Criteria:

• generally healthy, have at least one tooth with cavitated dentine carious lesion

Exclusion Criteria:

• uncooperative and difficult to manage, with severe forms of hypoplasia of fluorosis or other oral diseases, wearing orthodontic devices or under dental treatment, have major systemic diseases, or are on long-term medication