CARIOSTATIC INFLUENCE OF Sr²⁺ ON HYDROXYAPATITE-DISC TOOTH ANALOGUE DEMINERALISATION

Objectives: Strontium (Sr²⁺) has been demonstrated to be cariostatic. The evidence is controversial and the exact mechanism by which strontium decreases dental caries is unclear. Our aim is to study the effect of the divalent metal cation Sr²⁺ on the kinetics of porous hydroxyapatite (HAP) disc dissolution using scanning microradiography (SMR) under artificial caries and erosion conditions. Methods: Compressed 1mm thick sintered HAP discs (Plasma-Biotal, UK. 20wt% nominal porosity) used as tooth analogues, were preconditioned, coated with acid-resistant varnish on all surfaces leaving one surface exposed, and located centrally in SMR cell. 1L 0.1% acetic acid pH4 (caries conditions) and 0.3% citric acid pH2.8 (erosion conditions) demineralising solutions were prepared with each of 0, 5, 10, 20 and 30ppm Sr²⁺ respectively. Demineralising solution was circulated at 0.80cm3.min-1, and the HAP demineralisation rate (RDHAP) was measured at a single centrally located point on each disc for 24h at 22±1°C using SMR. Each experiment was repeated twice for both increasing, and decreasing sequences of Sr²⁺ concentrations. Results: Caries conditions: mean RDHAP decreased significantly from 3.31E-4 (0ppm Sr²⁺) to 2.75E-4 (5ppm), 1.95E-4 (10ppm), 1.73E-4 (20ppm), and 1.44E-4 (30ppm) g.cm-2.h-1 for increasing concentration sequence, and from 2.08E-4 (30ppm Sr²⁺) to 1.83E-4 (20ppm), 1.23E-4 (10ppm), 9.71E-5 (5ppm) and 2.62E-4 (0ppm) g.cm-2.h-1 for decreasing concentration sequence. Erosion conditions: mean RDHAP decreased significantly from 4.22E-3 (0ppm Sr²⁺) to 3.66E-3 (5ppm), 3.23E-3 (10ppm), 2.99E-3 (20ppm) and 2.83E-3 (30ppm) g.cm-2.h-1 for increasing concentration sequence, and from 4.02E-3 (30ppm Sr²⁺)to 3.46E-3(20ppm), 3.05E-3 (10ppm), 2.58E-3(5ppm) and 3.33E-3 (0ppm) g.cm-2.h-1 for decreasing concentration sequence. Conclusion: Sr²⁺ decreased RDHAP under strictly controlled thermodynamic conditions relevant to dental caries and erosion. The non-reversibility in RDHAP throughout the increasing and decreasing Sr²⁺ sequences may be due to lasting effects of phase changes in HAP. This study demonstrates the potential usefulness of Sr²⁺ in caries prevention.