Advanced imaging reveals structure of tooth enamel

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

Teeth are covered with a thick, durable coating called enamel, which is the hardest substance in the human body. Enamel helps protect teeth from decades of chewing and provides a line of defense against tooth decay.

But enamel isn’t invincible. Bacteria that naturally live in the mouth produce acid, which can dissolve away enamel over time. This contributes to dental caries, or cavities. Cavities can cause pain, and may lead to infection and tooth loss if left untreated.

Enamel doesn’t readily repair itself after damage. While researchers have learned much about the minerals that make up tooth enamel, they would like to know more about its structure. Enamel is made up of rods, which are in turn composed of thousands of microscopic crystals called crystallites. A better understanding of these structures could lead to new strategies for protecting and repairing enamel.

An imaging technique called scanning transmission electron microscopy (STEM) has provided scientists with some rough pictures of enamel crystallites. However, the intensity of traditional STEM beams required for a clearer view would damage the enamel before a picture could be generated.

In new research, a team led by Karen DeRocher, Dr. Paul Smeets, and Dr. Derk Joester from Northwestern University used a combination of imaging techniques to produce a picture of enamel down to the atomic level. These techniques include a version of STEM performed at very cold temperatures and atomic probe tomography, which takes pictures of substances one layer of atoms at a time.

An enamel crystallite, looking down the long axis of the crystal. The dark areas show magnesium ions forming two layers on either side of the core. Northwestern University

The work was funded in part by NIH’s National Institute of Dental and Craniofacial Research (NIDCR). Results were published on July 1, 2020, in Nature.

The methods revealed a high-definition view of the crystallites, which are made mostly of a mineral known as hydroxylapatite. They also revealed the patterns in which other minerals were deposited within the crystals, including magnesium, sodium, and fluoride.

Atomic-level 3D image reconstructions showed that these other minerals tended to be found inside the core of the crystallites rather than in the outside layer, or shell. Computer modeling suggested that the minerals in the core contribute to their profound strength.

The mineral deposits may also explain some of the risk of dental caries. When the researchers exposed teeth to acid and then examined the resulting damage, the enamel crystal cores showed more erosion than the shells_. The team is planning further studies to understand more about this process.

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

The findings may lead to new approaches to strengthening teeth and repairing damage due to erosion and decay.