In Vitro Study of Root Caries

Abstract

The aim of this study was to investigate the fluoride distribution in root caries lesions, to establish a defined-species consortium model to study root caries in vitro, and to use this model to examine fluoride interactions with caries development. The distribution of fluoride in normal tooth roots, and in early and advanced root caries lesions (seven teeth) was mapped using proton microprobe analysis. Three-dimensional surface maps of F distributions showed higher fluoride peaks present in the surface of small lesions compared with normal root surfaces. Fluoride concentrations in the outer surface of large root cavities were greatly increased compared with sound inner dentine. High fluoride-containing minerals are considered a marker of remineralization. Thus, these results suggest that during the progression of root caries, ongoing remineralization occurs at the lesion surface. Streptococcus mutans, Streptococcus sobrinus, Actinomyces naeslundii, and Lactobacillus rhamnosus were selected for construction of a consortium plaque model, after screening several strains for cariogenic properties and suitability for cultural analysis. The bacteria were grown separately and as a consortium in a multi-plaque artificial mouth, supplied with a constant flow of a simulated oral fluid (basal medium mucin), and periodic sucrose to simulate meals. All the bacteria (except L. rhamnosus) individually formed large biofilms that demineralized bovine enamel and dentine blocks similarly to natural caries as assessed by microhardness measurement and microradiography. Four-species consortium plaques (resting pH 4.9-5.3) induced greater softening of enamel than did monocultures. A. naeslundii characteristically formed only a small percentage (up to 4%) of the final consortium. Substitution of sucrose by glucose had no effect on the cariogenicity to human roots or on the plaque composition. The consortium extended by including Streptococcus mitis and Veillonella dispar (resting pH 5.5-6.3) was cariogenic to bovine enamel and human roots but less so than that of the four-species consortium. A. naeslundii and S. mitis levels, however, were low. Enamel caries lesions showed an intact surface and pronounced subsurface lesion, whereas root lesions in the absence of fluoride had extensive demineralisation without a surface zone. These features are similar to natural caries, demonstrating that this artificial mouth defined-species biofilm model system is a realistic and powerful tool to study caries in both tooth enamel and root. Fluoride continuously supplied at 5 to 20 ppm, or intermittently supplied in sucrose at200 ppm, caused a substantial caries reduction in enamel, but less so in dentine. Increasing fluoride up to 20 ppm decreased S mutans and increased L. rhamnosus in the consortium plaques. The effect of fluoride as an anti-caries agent supplied in the environment compared to fluoride present in the tooth tissue was evaluated by testing the effect of 5 ppm F supplied continuously on caries progression in sheep tooth tissues developed in vivo under fluoridated and non-fluoridated regimes. The results indicated that fluoride in teeth had very little effect on caries in both enamel and tooth roots compared with environmentally supplied fluoride, the latter causing a major reduction in caries development.