The Effects of Changing Caries Prevalence and Diagnostic Criteria on Clinical Caries Trials

In an area of water fluoridation (1 ppm), a clinical trial has been carried out with two examiners using two commercially available dentifrices containing 0.76% sodium monofluorophosphate (NaMFP). Regular and supervised use of these dentifrices resulted in significantly (p < 0.01) lower caries increments. Of the 853 subjects completing the study period of 30 months, 41 % were caries free at the outset. This subset experienced new caries (mean annual rate of 1 new decayed and filled surface per 100 surfaces at risk), but no significant treatment effect was observed in these subjects with zero initial caries prevalence (IDFS). In those subjects with non-zero IDFS, the treatment effect was statistically significant and approached the magnitude (22–34% reduction) observed in nonfluoride areas. Future trials may require larger numbers of subjects if caries prevalence continues to decrease. Preselection of subsets with specified caries prevalence may be necessary in order to reduce the sample size and increase discrimination of treatment effects. Comparisons of findings from two independent examiners demonstrate the importance of discriminatory diagnostic standards.