The velocity of acid production (Vap) of Streptococcus mutans C180-2 and of 2 fluoride-resistant mutant strains S. mutans C180-2FR and S. mutans C180-2MFR was examined in vitro at pH values between 7.0 and 4.5. The Vap of the fluoride-resistant mutants was lower than the Vap of the parent strain at pH greater than or equal to 6.0. At pH 5.5 and 5.0, the Vap of the mutant strains was higher than the Vap of the parent strain, whereas at pH 4.5 no significant differences were observed between the Vap of the 3 strains. The fluoride sensitivity of all 3 strains was amplified by a low pH environment. The fluoride concentration necessary to inhibit the acid production of the parent strain completely was 27 mM at pH 7 and 0.1 mM at pH 4.5. For the 2 mutants, the corresponding concentrations were 65 mM at pH 7 and 0.5-0.7 mM at pH 4.5. The results suggest that, if S. mutans acquires fluoride resistance in vivo, the rate of acid production in dental plaque may be decreased at pH greater than or equal to 6, but increased at lower pH levels. Low concentrations of fluoride inhibit acid production less effectively.
Demineralization of dentin specimens proceeds at a faster rate than that of enamel. Although this is generally accepted, a quantification of the rate of formation of root lesions is hampered by the shrinkage of the lesions when these are dried prior to microradiographic analysis. This leads to a significant underestimation of the lesion depth and total mineral loss. The aim of this paper was to quantitate the rate of mineral loss during root lesion formation in vitro and to determine the shrinkage of root specimens as a result of drying. Unerupted roots of human teeth were subjected to a demineralizing system of 0.1 mol/L lactate buffer (pH = 4.8) with 0.2 mmol/L methanehydroxydiphosphonate during four, 11, 22, and 44 days. The root lesions were assessed by quantitative microradiography. The demineralizing solutions were analyzed to determine the amounts of root tissue dissolved. A comparison of these two sets of data showed that, with the demineralizing system used, root lesions may shrink up to 62%. Fixation of the specimens in fixative did not affect this shrinkage. Chemical analysis showed that mineral loss proceeded linearly with time. From the data-sets of this study, a model was developed to compensate for the shrinkage in the dentin specimens. In this way, it was possible to calculate the lesion depth at four demineralization times as being 130, 220, 320, and 530 microns, respectively. These values were in agreement with a microscopic determination of the lesion depth.
Since the introduction of the Intra-oral Cariogenicity Test by Koulourides et al. (1974), many groups around the world have been developing and using intra-oral models to test new caries-preventive products, as well as to study physiological processes in the oral cavity. In spite of the large numbers of papers reporting these methods, very little research has been done to determine the importance of the many variable parameters which influence the performance of these models. Among these, the following can be identified: (a) panelist criteria, (b) the use of sound vs. pre-demineralized enamel of human or bovine origin, (c) the use of gauze or a recess to accumulate plaque, (d) the method to create incipient lesions, (e) the duration of the experiment, (f) the number of panelists required for statistical significance to be obtained, (g) the assessment techniques for mineral and/or fluoride uptake/loss, and (h) the choice of contralateral, ‘cross-over’, or ‘monadic’ experimental designs. Our results, supported by data from the literature, indicate that the choices made with respect to these parameters are of paramount importance in determination of the outcome of the respective study.
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