Dental caries, or tooth decay, is the most widespread noncommunicable disease worldwide.

According to the Centers for Disease Control and Prevention, more than $45 billion is lost in productivity in the U.S. each year, with untreated dental diseases as the number one cause.

Scientists have studied the composition and structure of tooth enamel on a scale larger than 50 micrometers and smaller than 100 nanometers. Enamel is the hardest substance in the human body and is the outermost layer of the human tooth.

A group of Northwestern researchers recently became the first scientists to successfully study crystalline orientation in single enamel rods, as well as in interrod enamel, as published in a December study. 

The novel research could one day help predict an individual’s future dental health by illuminating structural differences which impact enamel lifespan.

McCormick Prof. Derk Joester, primary investigator and co-author of the study, said enamel is a “poster child” for being a hierarchical material, in which structural features largely depend on the length scale, or on the size magnitude, of the material studied. 

“In order to understand how enamel dissolves when it’s exposed to acids that biofilms and bacteria in your mouth produce, you really need to understand it at all of these length scales,”  Joester said.

The study, a result of more than four years of research, was also co-authored by Karen DeRocher (McCormick ’20), a postdoctoral research associate at the National Institute of Standards and Technology. She worked in Joester’s lab in 2020 when the study method was published.

DeRocher said her main role with the lab was to run experiments at the beamline, an advanced photon source tool located at Argonne National Laboratory in Lemont, Illinois.

“We were looking at how things like composition or crystal packing affect the orientation of crystallites,” she said. “We were also looking to see if there were any chemical gradients across a single crystal.” 

The team found elements like magnesium, sodium and fluorine segregated to the crystal boundaries. DeRocher said element segregation in crystal cores and boundaries make them more susceptible to acid dissolution. Additionally, elemental composition and crystal packing can lead to varying dissolution behavior, impacting enamel lifespan. 

Feinberg Prof. and study co-author Stuart Stock ran the majority of the X-ray diffraction data, which was used to detect enamel rod signals — ultimately characterizing their orientations. 

Stock said thousands of long, thin crystallites make up each enamel rod and that enamel damage by dental caries causes chemical etching, dissolving some of the minerals in the outer layer of a tooth.

“It appears that the region between rods is a place which might be less organized and have more propensity to chemically be attacked,” Stock said “It’s more vulnerable.” 

The 0.5- to 10-micron scale research required a focused ion beam connected with a scanning electron microscope. DeRocher said a stream of gallium ions used to cut up the enamel acted like a very tiny blade or scalpel. 

But, she added that getting the right orientation of enamel on a micron-scale posed a challenge.

Joester said the team faced the challenge of being the first in the field to conduct lattice analysis on the mesoscale, which lies between the atomic scale and what the human eye can detect.

“We essentially had to invent the analysis and write the code,” said Joester. “We had to be very, very careful because ultimately, we used this technique to measure distances with a precision in the picometer range — so smaller than an atom.” 

Dr. Rares Raibulet and Dr. Marcos Montoya, co-owners and doctors at Stephens Dentistry in Evanston, said the most common day-to-day problems in their practice is either initial tooth decay, when outer enamel has been broken, or enamel decay around previous fillings. 

Raibulet said once a tooth is compromised and enamel loss occurs, it becomes more difficult to keep the mouth healthy. 

“We hope (the research) will be one of the puzzle pieces that really allow us to predict how enamel dissolves during caries,” Joester said. “Once we understand that, we have a foothold to design interventions that prevent that from happening.”

Correction: A previous version of this story misstated scientists’ experiences in studying the composition and structure of tooth enamel. The Daily regrets the error.

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