On Jul. 1, 2020, National Institutes of Health (NIH) funded scientists announced using a combination of advanced microscopy and chemical detection techniques to uncover the structural makeup of human tooth enamel at unprecedented atomic resolution, revealing lattice patterns and unexpected irregularities. The findings could lead to a better understanding of how tooth decay develops and might be prevented. The findings were published in Nature.

Your teeth are remarkably resilient, despite enduring the stress and strain of biting, chewing, and eating for a lifetime. Enamel — the hardest substance in the human body — is largely responsible for this endurance. Its high mineral content gives it strength. Enamel forms the outer covering of teeth and helps prevent tooth decay, or caries.

Tooth decay is one of the most common chronic diseases, affecting up to 90% of children and the vast majority of adults worldwide, according to the World Health Organization. Left untreated, tooth decay can lead to painful abscesses, bone infection, and bone loss. Tooth decay starts when excess acid in the mouth erodes the enamel covering. Scientists have long sought a more complete picture of enamel’s chemical and mechanical properties at the atomic level to better understand—and potentially prevent or reverse—enamel loss.

STEM studies have shown that at the nanoscale, enamel comprises tightly bunched oblong crystals that are about 1,000 times smaller in width than a human hair. These tiny crystallites are made mostly of a calcium- and phosphate-based mineral called hydroxylapatite. STEM studies coupled with chemical detection techniques had hinted at the presence of much smaller amounts of other chemical elements, but enamel’s vulnerability to damage from high-energy electron beams prevented a more thorough analysis at the necessary level of resolution.

To define these minor elements, a team of scientists at Northwestern University, Evanston, Illinois, used an imaging tool called atom probe tomography. By successively removing layers of atoms from a sample, the technique provides a more refined, atom-by-atom view of a substance. The Northwestern group was among the first to use atom probe tomography to probe biological materials, including components of teeth.

The scientists used atom probe tomography and advanced STEM techniques in a complementary fashion to overcome prior technical limitations. The Northwestern researchers worked with imaging experts led by Lena Kourkoutis, Ph.D., an associate professor of applied and engineering physics and director of electron microscopy at Cornell University’s national materials science user facility, PARADIM, in Ithaca, New York. At Cornell, the scientists coupled an ultra-fast chemical detector with STEM at very low temperatures to minimize enamel damage and gather more detailed chemical data. The complementary approaches enabled the team to piece together information at multiple levels of resolution to get a more complete view of the chemical and structural features of enamel crystallites.

The results showed that the crystallites were made of a continuous uniform lattice of hydroxylapatite atoms. However, the lattice structure appeared to be sprinkled with dark distortions, especially at the innermost core of the crystallites. A closer look at the core revealed that these defects were caused by the presence of minor elements that previous studies had hinted at. One such element was magnesium, which was highly concentrated in two distinct layers in the core. The central region was also rich in sodium, fluorine, and carbonate. Flanking the core was a “shell” with much lower concentrations of these elements.