Mechanical recovery of dentin following remineralization in vitro--an indentation study

Abstract

This study sought to gain insights into the steps leading to remineralization and mechanical recovery of hydrated dentin. Mechanical recovery in water was hypothesized to result from effective mineral matrix binding and to occur from the innermost regions outwards due to an increase in the number of nucleation sites. Partially demineralized (0.05 M acetate, pH=5.0, 8h) dentin was remineralized using calcium and phosphate solutions of 10.1 or 9.8 degree of saturation (DS) for hydroxyapatite (pH=7.4) for 4, 8 or 24h. Remineralization used a constant solution composition approach, which allowed for a continuous mineral growth with relatively constant thermodynamic driving forces. Crystal growth rates (R) were calculated using concentrations of calcium and phosphate. Before and after de- and re-mineralization, specimens had their surface and cross-section elastic moduli measured using AFM-nanoindentation in water. DS=10.1 provided higher R and higher mechanical recovery at the surface (p<0.0001). Cross-sectional measurements showed that subsurface mechanical recovery occurred from the innermost demineralized areas gradually outwards for both groups with no statistical differences at different DS, thus suggesting that remineralization is driven by mineral growth within nucleation sites with preserved collagen fibrils. Further, mechanical recovery appeared to initially obey a heterogeneous pattern, which vanished with time. This study provides evidence of mechanical recovery of hydrated dentin after remineralization and novel insights into the steps leading to mechanical recovery of carious dentin.

Figure 1

Illustration of the constant composition system.

Figure 2

Rates of crystal growth obtained from experiments with DS=10.1 and 9.8. Growth rates (R) were higher for DS=10.1 and increased with time for both groups. The increase became less significant at each time point.

Figure 3

Values of reduced elastic modulus (E_r) obtained from surface (a) and cross-section measurements after 4 (b), 8 (c) and 24 hours (d) from remineralization treatments for groups DS=10.1 and DS=9.8.

Figure 4

Representative data obtained from one line of cross-section measurements from one single specimen at different time points. At 4 hours a heterogeneous pattern of mechanical recovery is noticed (a). The differences in properties decrease after 8 hours (b) and become more homogeneous at 24 hours (c). The schematic represents hypothetical steps for remineralization at a micro-scale: (d) after demineralization, (e) during the formation of “growth units”, and finally (f) having a relatively homogeneous remineralization.

Figure 5

Steps involved on de- and remineralization of collagen fibrils of dentin at a nano-scale. Dark ‘crystals’ are mineral present before acid attack and blue ‘crystals’ are re-grown mineral.