MADISON – Break any bone in the human body, and the body can repair the tissue and fix the damage. Yet tooth enamel – the strongest tissue in the human body – cannot repair itself. Still, our teeth last a lifetime.

“We apply huge pressure on tooth enamel every time we chew, hundreds of times a day,” says Pupa Gilbert, professor of physics at the University of Wisconsin-Madison. “Tooth enamel is unique in that it has to last our entire lifetime. How does it prevent catastrophic failure?”

In new research published Sept. 26 in the journal Nature Communications, Gilbert and her collaborators, including MIT engineering Professor Markus Buehler and University of Pittsburgh oral biology Professor Elia Beniash, used advanced imaging techniques to see a clearer picture of the organization of individual enamel crystals in human teeth. They found that these crystals are not perfectly aligned, as had been previously thought, and that this misorientation likely deflects cracks, leading to enamel’s lifelong strength.

“Prior to this study, we just didn’t have the methods to look at the structure of enamel,” Gilbert says. “But with a technique that I previously invented, called polarization-dependent imaging contrast (PIC) mapping, you can measure and visualize in color the orientation of individual nanocrystals and see many millions of them at once. The architecture of complex biominerals, such as enamel, becomes immediately visible to the naked eye in a PIC map.”

Tooth enamel is organized in micron-length rods made up of long, skinny crystals of hydroxyapatite. Gilbert and her group at UW-Madison applied PIC mapping to several human tooth samples and measured the orientation of each crystal in tooth cross sections.

“By and large, we saw that there was not a single orientation in each rod, but a gradual change in crystal orientations between adjacent nanocrystals,” Gilbert says. “And then the question was, ‘Is this a useful observation?'”


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