The influence of varying fluoride concentrations on in vitro remineralisation of artificial dentinal lesions with differing lesion morphologies
Introduction
Remineralisation is the process of a net mineral deposition within carious tissue.1, 2 Clinically, the results may be the reversal, diminishment or arrest of a caries lesion.3, 4 Compared with enamel the mechanisms of dentine remineralisation are more complex.5 This is because dentine contains not only mineral but substantial amounts of collagen and non-collagenous proteins.5 Many studies support the theory that remineralisation occurs from the re-growth of remaining mineral crystals and not by the spontaneous precipitation or nucleation of mineral on the organic matrix.6, 7, 8 Other investigators, however, have suggested that the removal of soluble phosphoproteins by acid dissolution allows de novo mineral nucleation, instigated by firmly bound non-collagenous matrix proteins.9, 10, 11, 12 It is likely, therefore, that the remineralisation of dentine is influenced not only by the reactive surface area of residual mineral crystals but also by the remaining organic matrix and structural organisation of the lesion.6, 13, 14, 15, 16
Apart from the influence of the lesion itself, ambient levels of fluoride have been demonstrated to enhance the rate and efficacy of dentine remineralisation.4, 14, 17, 18, 19 The cariostatic effect of fluoride is accomplished in several ways.17 Fluoride ions may be incorporated in the crystalline lattice of biological apatite, resulting in an enhanced stability and tendency for crystal growth.20, 21 The resultant fluorapatite also possesses a much lower solubility than the original mineral or indeed pure hydroxyapatite.18, 19, 22 Fluoride can also form calcium fluoride-like deposits on the surface of dental hard tissues, which act as pH-controlled releasing reservoirs.22 These deposits are formed more on and within cementum and dentine than enamel due to the relatively larger surface area, increased porosity and higher organic content of the carious root hard tissue substrates.17
With regard to factors that control the rate of the remineralising process, conflicting data exist. Both Arends et al.14 and Featherstone19 have demonstrated a linear relationship between the total amount of mineral redeposited within lesions compared with the square root of the fluoride concentration within the remineralising solution. These results would indicate that mineral deposition was controlled by the diffusion of fluoride into the dentine. Conversely, Kawasaki et al.5 found no such relationship whilst Klont and ten Cate6 suggested that the rate-determining step in the remineralisation of dentine was the rate of ion transport through surface layer pores.
This study aimed to investigate the process of sub-surface dentine lesion remineralisation. The remineralisation of artificial root caries lesions was assessed in vitro with respect to the influences of two experimental variables; fluoride concentration over the range 0.0–5.0 ppm and the mineral content and morphology of lesions at baseline. Fluoride concentration was not examined in excess of 5.0 ppm in order to avoid a precipitation of calcium fluoride from solution. Each experimental variable was assessed quantitatively using both chemical analysis techniques and transverse microradiography. The use of chemical assays allowed the uptake of calcium, phosphate and fluoride to be quantified on a time resolved basis, whilst the use of transverse microradiography allowed changes in the mineral content and mineral distribution of the lesions to be quantified.
Section snippets
Acid washing procedure
All glassware and pipette tips used in this study were acid washed for 24 h by immersion in a 10% (v/v) HCl solution followed by thorough rinsing in distilled deionised water. The procedure of acid washing was used to remove possible trace element contaminants, which if present, could have affected the sensitivities of the chemical assays.
Sample preparation and lesion formation
Twenty human molars were debrided of residual soft tissue and had cementum removed by wet abrasion using p1000-grit abrasive paper. The teeth were coated with
Chemical analysis
Fluoride significantly increased calcium ion uptake by lesions in both experimental groups (p < 0.05). This was found to be independent of time (data not shown) and concentration (Table 2). Whilst the presence of fluoride had no significant influence upon the uptake of phosphate by lesions in group B, a significantly greater amount of phosphate was taken up by group A lesions in the presence of 0.48 ppm F during the initial 72 h (data not shown) rising to 2.49 ppm latterly (Table 2). Again
Chemical analysis
This study showed that fluoride in low concentrations promoted a significant increase in the deposition of mineral during the remineralisation of sub-surface dentinal lesions. ten Cate et al.27 hypothesised that the amount of fluoride adsorbed by carious dentine rather than the concentration in solution is a primary factor influencing mineral deposition. The observation that calcium and phosphate uptake by lesions in both experimental groups was predominantly independent of fluoride
Conclusions
The process of remineralisation is multifactorial. Fluoride significantly increased the mineral uptake by sub-surface lesions with differing lesion morphologies. Remineralisation was, however, largely independent of ambient fluoride concentration and was not controlled by the rate of fluoride diffusion into the lesion. The mineral content of the lesions surface layer was found to be most influential upon the process of remineralisation, affecting the location and quantity of mineral deposition.
Acknowledgement
This work was funded by a BBSRC Case Award in conjunction with Glaxo-SmithKline.
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