Unprecedented Detail

Researchers Capture Human Dental Enamel in Unprecedented Detail

A research team led by Northwestern University scientists has uncovered the structural makeup of human dental enamel at unprecedented atomic resolution, revealing lattice patterns and unexpected irregularities.

Enamel is made up of tightly bunched, oblong crystals that are about 1,000 times smaller in width than a human hair. Image credit: Karen DeRocher, Northwestern University.

Dental enamel is a principal component of teeth, and has evolved to bear large chewing forces, resist mechanical fatigue and withstand wear over decades.

Its functional impairment and loss, caused by developmental defects or tooth decay (caries), affect health and quality of life, with associated costs to society.

Dental enamel covers the entire crown of human teeth, reaching thicknesses of several millimeters.

At the nanoscale, it 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.

“Earlier studies revealed the bulk composition of enamel, which is like knowing the overall makeup of a city in terms of its population,” said study senior author Professor Derk Joester, a researcher in the Department of Materials Science and Engineering at Northwestern University.

“But it doesn’t tell you how things operate at the local scale in a city block or a single house. Atom probe tomography gave us that more detailed view.”

“This work provides much more detailed information about the atomic makeup of enamel than we previously knew,” added Dr. Jason Wan, program officer at the National Institute of Dental and Craniofacial Research who was not involved in the study.

“The findings can broaden our thinking and approach to strengthening teeth against mechanical forces, as well as repairing damage due to erosion and decay.”

Impurities such as magnesium show up as dark distortions (indicated by white arrows) in the atomic lattice of human enamel crystallites. Image credit: Paul Smeets, Northwestern University Berit Goodge, Cornell University.

Professor Joester and colleagues used atom probe tomography and advanced scanning transmission electron microscopy (STEM) in a complementary fashion to overcome prior technical limitations.

The researchers were able to piece together information at multiple levels of resolution to get a complete view of the chemical and structural features of enamel crystallites.

Their 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.

“We assumed that human crystallites would be similar in composition to rodent enamel, which is widely used by researchers to understand human enamel,” said co-first author Dr. Paul Smeets, a researcher in the Department of Materials Science and Engineering at Northwestern University.

“But that was not the case — human enamel is much more chemically complex than we thought.”

In three views of the same sample of enamel crystallites, atom probe tomography reveals the distributions of three minor elements, with each colored dot representing a single atom. These maps show that magnesium is present in two distinct layers in the core, and fluorine and sodium are heavily concentrated in the areas between crystallites, known as the intergranular phase. Image credit: Karen DeRocher, Northwestern University.

The scientists suspected that the irregularities introduced by magnesium layers give rise to areas of strain in the crystallite.

Computer modeling supported their hunch, predicting higher stresses in the core than in the shell.

“Stress may sound bad, but in material science it can be useful, and we think it may make enamel stronger overall,” said co-first author Karen DeRocher, a graduate student in the Department of Materials Science and Engineering at Northwestern University.

“On the other hand, those stresses are predicted to make the core more soluble, which might lead to erosion of enamel.”

Indeed, when the researchers exposed crystallites to acid — similar to what happens in the mouth — the core showed more erosion than the shell.

“This new information will enable model-based simulation of enamel degradation that wasn’t possible before, helping us better understand how caries develops,” DeRocher said.

The study was published in the journal Nature.

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K.A. DeRocher et al. 2020. Chemical gradients in human enamel crystallites. Nature 583, 66-71; doi: 10.1038/s41586-020-2433-3