The influence of varying fluoride concentrations on in vitro remineralisation of artificial dentinal lesions with differing lesion morphologies.

OBJECTIVE: The influence of low-level fluoride (F) concentrations and lesion characteristics on the remineralisation of sub-surface root caries was investigated in vitro. DESIGN: Experimentally produced dentinal carious lesions were exposed to artificial saliva in the presence of 0.00ppm, 0.48ppm, 2.49ppm or 4.91ppm F (as NaF) for 5 days. Calcium, phosphate and fluoride ion uptake was quantified by chemical assay. Baseline changes in the mineral content and distribution of the lesions were assessed by transverse microradiography (TMR). RESULTS: The uptake of calcium and phosphate was significantly increased (p<0.05) by fluoride, even at low concentrations. The action of fluoride was influenced by the mineral content and distribution of the lesions at baseline, however, with the location and degree of mineral deposition being strongly related to the mineral content of the lesions surface layer. CONCLUSIONS: The process of remineralisation of root dentine is multifactorial. The morphology of lesions at baseline significantly affected the location and quantity of mineral deposition.

The efficacy of techniques for the disinfection of artificial sub-surface dentinal caries lesions and their effect on demineralization and remineralization in vitro.

OBJECTIVES: The efficacy of three techniques for the disinfection of artificial sub-surface root caries lesions and their response to subsequent episodes of de- and remineralization was investigated quantitatively in vitro. METHODS: Sub-surface dentinal lesions (n=20), cut into four experimental blocks and deliberately contaminated with Streptococcus mutans, were subject to either steam autoclaving (121 degrees C, 5min), gamma irradiation (4100Gy), immersion in 0.1% (w/v) thymol-distilled water solution (24h) or reserved as a control. Next, the lesions were incubated aerobically in sterile nutrient broth for 24h at 37 degrees C and resultant cultures plated onto blood agar and neutralisation agar. Ten blocks from each experimental group were then immersed in an acidic buffer solution or exposed to artificial saliva for 5 days. Baseline changes in the mineral content and distribution of the lesions were assessed by transverse microradiography (TMR). RESULTS: Micro-organisms were recovered from each control block and one block treated by gamma irradiation. Steam autoclaving and immersion in a thymol solution significantly decreased (p<0.05) the amount of mineral lost from the body of lesions subject to a further acid challenge. Mineral ion uptake by lesions exposed to artificial saliva was significantly increased (p<0.05) through disinfection by steam autoclaving. CONCLUSIONS: Gamma irradiation proved the most acceptable method for the disinfection of sub-surface root dentine lesions having the least adverse effect on demineralization and remineralization.

Development of an in situ root caries model. A. In vitro investigations.

OBJECTIVES: The paper describes preliminary in vitro investigations, the objectives of which were to examine the influence of certain experimental parameters on artificial carious lesion formation in root hard tissues, and their remineralisation. These experiments formed part of a wider study that aimed to develop an in situ model of root caries, based on the existing coronal caries model used in Liverpool. The present studies examined the effects (a) of the anatomical origin of the dentine, the presence or absence of cementum, the exposure time and the type of demineralising system, on lesion development, and (b) of baseline lesion size on the extent and location of mineral re-precipitation. METHODS: Mineral content parameters in plano-parallel sections taken from dentine lesions were determined by computer-controlled transverse microradiography. RESULTS: The importance of the anatomical origin of the dentine on lesion formation was investigated by comparing in vitro lesion formation in premolar and molar dentine, and in dentine from apical, middle and coronal thirds of the root: no difference was observed between these sites. Lesions formed more rapidly in acid buffer solutions than in acid gel systems, and were more reliably produced when cementum was removed. The effect of baseline lesion size on subsequent in vitro remineralisation demonstrated that a small baseline mineral content was associated with a larger percentage mineral gain. The location of mineral deposition throughout the lesion was also influenced by baseline mineral content parameters. CONCLUSIONS: The results form a basis for the further development of an in situ dentinal caries model, providing data to suggest that manipulation of parameters involved in the preparation of artificial carious lesions has a significant effect on the behaviour of the lesion, particularly the phenomenon of remineralisation. Further work is needed to investigate the behaviour of the model in situ.

Development of an in situ root caries model B. In situ investigations.

OBJECTIVES: The aims of this pilot study were to assess the suitability of an in situ model for root caries studies, and to investigate the effect of sub-surface lesion mineral content and ultrastructural organisation on remineralisation. METHODS: Subjects were selected by criteria generally used for coronal caries models. Plaque pH responses and salivary flow rates were determined by standard methods. The chosen substrate was human dentine with artificially induced caries-like demineralisation and sterilised by gamma-irradiation (4100 Gy). Two levels of baseline demineralisation were induced by exposure to an acid buffer for 2 or 5 days. Mineral content was measured by computer-controlled quantitative microradiography. RESULTS: Remineralisation in situ was greater in smaller lesions with lower surface mineral content than in larger lesions in which the surface mineral content was high. CONCLUSIONS: The model is suitable for further development for investigating dentinal caries. More work is needed to validate the model by means of a fluoride dose-response study.

A study investigating the formation of artificial sub-surface enamel caries-like lesions in deciduous and permanent teeth in the presence and absence of fluoride.

OBJECTIVES: Clinical and in situ studies have shown that caries formation and progression is faster in primary than in permanent teeth. Ambient levels of fluoride are also known to influence the processes of carious lesion formation. The aims were, firstly to investigate the incidence of artificial sub-surface caries lesion formation in the enamel of deciduous and permanent teeth in vitro. Secondly, to compare quantitatively the mineral content and distribution of these lesions and assess the possible influence of fluoride upon the lesion parameters. METHODS: Twenty primary molars and 20 permanent premolars were randomly assigned to one of four experimental groups. Samples in each experimental group were immersed in a 0.05M lactic acid gel (6% w/v hydroxyethyl cellulose, pH 4.5) either in the absence or presence of fluoride for 7 days. Subsequently, lesion parameters of mineral loss (vol x % microm) and lesion depth (microm) were assessed by transverse microradiography and image analysis. RESULTS: No significant differences existed between the lesion parameters of permanent and primary teeth (P=0.20). Irrespective of tooth type, lesion parameters were significantly reduced in samples demineralised in the presence of fluoride (P< or =0.002). CONCLUSIONS: Fluoride significantly reduces the severity of sub-surface caries lesions formed in vitro and in contrary to previous studies, there were no significant differences in lesions parameters between permanent and primary teeth.

Effects of sulphur dioxide pollution on a Californian coastal sage scrub community.

Phytotoxic effects of sulphur dioxide emissions from an oil refinery (annual average of 0.09-0.17 ppm for a 25 year period) were studied near Santa Maria, on the rural central coast of California. Stands of coastal sage scrub along an SO(2) gradient downwind of the refinery were compared with stands in relatively pollution-free upwind sites. Physiological and growth changes in the dominant shrub, Salvia mellifera (black sage), were emphasised. Stomatal resistance of Salvia was found to be significantly lower on the polluted sites than on the pollution-free sites. Internodal stem length and heights of Salvia shrubs were also reduced. Significant reductions in photosynthetically active tissue of Salvia resulted from increased defoliation and decreased leaf size associated with SO(2) stress. Of particular ecological interest was the relative importance of species at different sites; abundance of perennial shrub species was significantly lower and abundance of annuals greater on the most polluted sites relative to the sites farthest from the SO(2) source. The influx of annuals and decrease in perennial cover resulted in greater species richness and reduced concentration of dominance similar to that observed in early post-fire sage scrub stands.