Although enamel demineralization is important for our understanding of caries formation, no consensus has been reached regarding the possible differences in susceptibility of primary and permanent enamel. We used the constant composition (CC) technique to investigate the acid-induced demineralization of these tissues at a relative undersaturation with respect to hydroxyapatite (HAP) of 0.902, pH = 4.5, and ionic strength = 0.15 mol L −1 . The demineralization rates showed significant differences, primary enamel having the greater susceptibility to dissolution during an initial linear stage: 1.5 ± 0.5 × 10 −10 mol mm −2 min −1 compared with 2.6 ± 0.5 × 10 −11 mol mm −2 min −1 for permanent enamel. During the reactions, we observed nanosized crystallites which attached to the enamel surfaces or escaped into the bulk solution. These nanosized crystallites were kinetically protected against further dissolution, even though the solutions remained undersaturated. It is hypothesized that they may contribute to the remarkable mechanical and dynamic characteristics of enamel.

The dissolution kinetics of human tooth enamel surfaces was investigated using nanomolar-sensitive constant composition (CC) and in situ atomic force microscopy (AFM) under simulated caries formation conditions (relative undersaturation with respect to hydroxyapatite = 0.902, pH = 4.5). Scanning electron microscopic (SEM) examination of the resulting etched enamel surfaces showed that deminerzalization, initiated at core/wall interfaces of rods, developed anisotropically along the c-axes. After an initial rapid removal of surface polishing artifacts, the dissolution rate decreased as the reaction proceeded in accordance with our recently proposed crystal dissolution model, resulting in hollow enamel cores and nanosized remaining crystallites, resistant to further dissolution. Generally, dissolution of minerals is regarded as a spontaneous reaction in which all the solid phase can be dissolved in undersaturated solutions. However, the dissolution of some biominerals may be suppressed when the crystallites approach nanometer size. This study shows that CC demineralization of enamel in acidic medium follows this new model that can be used to mimic carious lesion formation. In dissolution studies, nanosized enamel crystallites exhibit a remarkable degree of self-preservation in the fluctuating physiological milieu.

Kleiner geht's nicht! Nanometergroße Kristallite lösen sich nicht unbedingt weiter auf. Sind sie ähnlich groß wie die beim Auflösen entstehenden Vertiefungen (siehe Bild), können sie kinetisch stabilisiert sein, auch wenn die umgebende Lösung nicht gesättigt ist. In Biosystemen sorgt dieses Verhalten für die bemerkenswerte Selbsterhaltung von Biomineralien in wechselnder Umgebung.

Dissolution of nanosized crystallites may be inhibited and even suppressed due to their small size. Nanoparticles with size distribution similar to that of critical dissolution pits (see picture) can be kinetically stabilized against further dissolution even in undersaturated solutions. In biological systems, this behavior confers remarkable self‐preservation on biominerals in the fluctuating milieu.