Searches were made for data using Medline, the Cochrane Central Register of Controlled Trials, Web of Science and the controlled-trials database of clinical trials (www.controlled_trials.com). Reference lists of potentially relevant reports and review articles were also searched. Attempts to obtain missing information and ‘grey’ literature were made through contact with selected investigators.
The treatment comparisons of interest included flossing versus no flossing, or a comparison of different frequencies of flossing use. Studies where the effect of flossing could not be separated from the effects of other treatments were excluded. The primary study outcome was a measure of caries incidence. There were no restrictions with respect to the study population. Study designs were limited to controlled clinical trials.
Data extraction and synthesis
Quality assessment was performed by evaluating the following items: random allocation, treatment allocation concealment, blinding of outcomes assessors, presentation of point estimates with a measure of variability for the primary outcome measure, ‘intention to treat’ analysis, report of baseline characteristics by treatment group, eligibility criteria, loss to follow-up, and missing values. For each trial, the number of surfaces at risk and the number of new interproximal caries lesions were derived from or estimated based on published data. Both relative risk (RR) and risk difference and their respective standard deviations were calculated. Heterogeneity was assessed with the I2 statistic (describes the percentage of the variability in effect estimates that is due to heterogeneity rather than sampling error. A value greater than 50% may be considered substantial heterogeneity). The effect of study characteristics such as fluoride, oral hygiene or caries risk on flossing effectiveness was estimated by meta-regression. Subgroup analyses and sensitivity analyses were also conducted.
The search identified 144 articles, of which six met the inclusion criteria. A total of 808 subjects, aged 4 to 13 years, were included in the meta-analysis. There were significant study-to-study differences and a moderate to large potential for bias. Professional flossing performed on school days for 1.7 years on predominantly primary teeth in children was associated with a 40% caries risk reduction [RR, 0.60; 95% confidence interval (CI), 0.48–0.76; P<0.001]. Both 3-monthly professional flossing for 3 years (RR, 0.93; 95% CI, 0.73–1.19; P 0.32) and self-performed flossing in young adolescents for 2 years (RR, 1.01; 95% CI, 0.85–1.20; P 0.93) did not reduce caries risk. No flossing trials in adults or under unsupervised conditions could be identified.
Professional flossing in children with low fluoride exposures is highly effective in reducing interproximal caries risk. These findings should be extrapolated to more typical floss-users with care, since self-flossing has failed to show an effect.
Regular flossing of children's teeth by a trained adult can dramatically reduce interproximal caries in those at high risk of caries.