Six controlled trials were included (the number of participants in the analysis was 732, ranging from n=35 to n=174). The duration of follow-up ranged from 1.7 to 3 years.
All the included studies showed evidence of a moderate to high risk of bias and were poorly reported. The methods of random allocation and allocation concealment were either unclear or inadequate in all studies. In the 4 studies reporting baseline caries rates there were differences between groups, with those assigned to flossing having fewer cavities. Two split-mouth studies did not take into account within-mouth clustering effects in their analysis, therefore the meta-analysis was based on imputed values for the clustering effect for these studies.
Using the fixed-effect model there was a lower risk of caries on flossed surfaces compared with control: RR 0.86 (95% confidence interval, CI: 0.76, 0.97, p<0.01) and RD -0.03 (95% CI: -0.05, -0.02, p=0.001). There was moderate to high heterogeneity for the RR analysis (70%, p<0.001) and moderate heterogeneity for the RD analysis (47%, p=0.10).
When a random-effects model was used, there was a lower risk of caries on flossed surfaces but for RR the difference was no longer statistically significant: RR 0.79 (95% CI: 0.61, 1.01, p=0.06), RD -0.03 (95% CI: -0.06, -0.01, p=0.02).
Subgroup analyses.
Professional flossing on school days (2 studies).
There was a statistically significant reduction in caries risk with flossing compared with control: RR 0.60 (95% CI: 0.48, 0.76, p<0.001) and RD -0.05 (95% CI: -0.07, -0.03, p<0.001). However, there was moderate to high statistical heterogeneity for both analyses.
Professional flossing every 3 months (2 studies).
There was no statistically significant reduction in caries risk: RR 0.93 (95% CI: 0.73, 1.19, p=0.56) and RD -0.02 (95% CI: -0.04, 0.01, p=0.32).
Self-performed flossing (2 studies).
There was no statistically significant reduction in caries risk: RR 1.01 (95% CI: 0.85, 1.20, p=0.93) and RD 0.00 (95% CI: -0.04, 0.04, p=0.96).