Magnesium incorporation into primary dental enamel and its effect on mechanical properties.

Cross-sectional study of sound primary dental enamel revealed hardness zonation and, in parallel, significant change in the Mg content below the prismless layer. Mg content is known to play an important role in enamel apatite biomineralization, therefore, Mg ion exchange experiments were carried out on the outer surface of sound primary molars and on reference abiogenic Ca-phosphates using MgCl solution. Effects of Mg incorporation on crystal/particle size, ionic ratio and morphology were compared and the observed changes were explained by parallel diffusion and dissolution/reprecipitation processes. Based on depth profile analysis and high resolution electron microscopy of the Mg-exchanged dental enamel, a poorly ordered surface layer of approximately 10-15 nanometer thickness was identified. This thin layer is strongly enriched in Mg and has non-apatitic structure. Below the surface layer, the Mg content increased only moderately (up to ~3 at%) and the apatite crystal structure of enamel was preserved. As a common effect of the Mg exchanged volume, primary dental enamel exhibited about 20% increase of nanohardness, which is intrepreted by strengthening of both the thin surface layer and the region below due to the decreased crystallite size and the effect of incorporated Mg, respectively. STATEMENT OF SIGNIFICANCE: Dental enamel is the most durable mineralized tissue in the human body, which, in spite to be exposed to extreme conditions like mastication and acidic dissolution, is able to fulfill its biological function during lifetime. In this study we show that minor component magnesium can affect hardness properties of human primary dental enamel. Then, through Mg incorporation experiments we provide an additional proof for the poorly ordered Mg-containing intergranular phase which has been recently observed. Also, we report that the hardness of dental enamel can be increased by ca. 20% by Mg incorporation. These results contribute to a deeper understanding of sound primary dental enamel structure and may inspire new pathways for assisted remineralization of enamel and regenerative dentistry.