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Biomaterials

What is dental enamel, and how does it protect your teeth?

More complex than you’d think, dental enamel keeps our teeth intact through a combination of physics and chemistry

by Leigh Krietsch Boerner
October 20, 2020 | APPEARED IN VOLUME 98, ISSUE 41

 

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Credit: Shutterstock

Eating candy is a fun indulgence, but having a dentist drill holes in your teeth is a recipe for pain. Unfortunately, eating a lot of sweets may be an express ticket to the dentist’s chair, because sugar is bad for your dental enamel.

If you end up in the dentist’s office—whether for a checkup or because you’ve gotten a cavity—you might encounter a diagram of a tooth. You’ll see that the tooth is made of different layers, with the hard enamel on the outside protecting the dentin and pulp within. But our precious teeth are both extremely durable and vulnerable to a high-sugar diet. So what makes the enamel so tough and protective for our teeth but susceptible to sweet treats and sugar?

“Dental enamel is a really complex material,” says Derk Joester, a materials chemist at Northwestern University. “Enamel has to be really hard in order to protect the dentin, which is soft. And it has to be wear resistant so that we can chew for a lifetime without losing too much of our enamel,” he says.

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Credit: C&EN/Shutterstock
Human teeth have multiple sections. Shown here are the enamel, the dentin, and the pulp.

In simple terms, tooth enamel is kind of like a rock. It’s made of about 96% hydroxyapatite, says dentist Edmond Hewlett, from the University of California, Los Angeles, School of Dentistry. Apatites are a group of calcium phosphate compounds, with the generic formula Ca5(PO4)3X. The identity of the X ion determines the specific type of apatite. For example, when X = F, you get fluorapatite, Ca5(PO4)3F, a pretty mineral that comes in all kinds of colors. Hydroxyapatite, usually colorless or white, is Ca5(PO4)3OH.

But while all enamel is made of hydroxyapatite, it’s not the same inside and out. Enamel’s strength comes from how the hydroxyapatite is arranged, which scientists think reduces the risk of having a catastrophic crack propagate through enamel, Joester says. The outside of the enamel, which dentists refer to as the skin of the teeth, is also called aprismatic enamel. This layer is only a few micrometers thick, Hewlett says, and it has an amorphous form, “like a solidified liquid crystal.” Underneath that layer is where things start to get funky.

Below the aprismatic enamel are the outer and inner enamel. Here, enamel takes the form of rods about 5 µm wide and several micrometers long. And these rods are woven together in different 3-D structures. Outer enamel rods are mostly aligned parallel to one another, Joester says. In the inner enamel, the rods are organized in decussated layers, meaning the rods in each layer are at a different angle relative to the ones above and below, somewhat like the arrangement of wood fibers in plywood layers—but at alternating specific angles. This decussation makes the enamel stronger, Joester says, just as it does in plywood.

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Credit: Nat. Commun.
A polarization-dependent imaging contrast map of human enamel showing the different orientation of the crystals inside the rods

Inside the rods, there is another level of organization that adds to the enamel’s strength. A single rod consists of bundles of thousands of whisker-shaped hydroxyapatite crystals. These crystals are several micrometers long but only about 50 nm wide, says Pupa Gilbert, who works on the properties of biominerals at the University of Wisconsin–Madison. She and her research group have performed molecular dynamics simulations on hydroxyapatite crystals and found that the crystals inside the rods are not all lined up. Instead, they are misoriented, meaning the crystals gradually change their orientation with respect to one another, similar to a lever pivoting on a fulcrum. This structure makes the enamel resistant to fracture, through a mechanism called crack deflection, Gilbert says. The misoriented crystals give no straight path for cracks to propagate along, since energy gets dispersed each time the crack reaches the boundary between individual crystals. Her group found that the amount of misorientation strongly correlates with the hardness and stiffness of the enamel. However, having no misorientation or a large misorientation means cracks can propagate through the interfaces between the crystals, Gilbert says. In enamel whose crystals have only a small degree of misorientation, cracks are deflected.

While arrangements of both the rods and the crystals inside them add to the strength of tooth enamel, this biomaterial is even more complex. Joester studies what’s between the crystals, and it turns out it’s not just hydroxyapatite. His group found very small amounts of magnesium fluoride and magnesium carbonate between the crystals. Though tooth enamel contains very little of these compounds, “what there is really has a dramatic impact on the properties of enamel, both in terms of mechanical function and in terms of dissolution,” Joester says. This magnesium-containing material is not crystalline but amorphous, like the aprismatic enamel. It is soluble in acid, which is often in your mouth. Joester found that a low pH can dissolve the Mg between crystals, as well as hollow out the crystals themselves. He points out that this dissolving happens on a very small scale, and scientists don’t know yet how or if this causes cavities.Although

eating sugar does lead to cavities, it’s not the sugar itself that’s responsible for the rot—the bacteria that gobble up the sugar in your mouth are the problem.

When we get a dental cleaning, dentists scrape gunk called plaque off our teeth. Around 30–40% of plaque is made up of dead bacterial cells and proteins from our saliva. The remaining 60–70% is made up of bacterial hitchhikers, mostly Streptococcus sanguinis and Streptococcus mutans. These bacteria convert sugary sweets into harmful acids. “They metabolize simple carbohydrates, like sucrose, very quickly,” Hewlett says, and one of the by-products the bacteria make is lactic acid. The acid is what damages your teeth.

When the pH is lower than 5.5, acids will cause ions, such as Ca2+ and PO43–, to leach out of the enamel. As cavities form, acids start to eat away at the enamel, “not just at the very surface but below the surface as well,” Hewlett says. The enamel gets weaker as its structure breaks down. If this continues, the enamel at the surface starts to crumble, and that’s when you get a cavity, Hewlett says. “Imagine you have termites eating away at a hardwood floor. One day you step on it and it breaks, but they’ve actually been working away at it for some time,” he says.

Brushing with fluoride toothpaste can strengthen your teeth after ion loss and protect your teeth against further decay. Fluoride ions replace the Ca2+ and PO43– lost from acid reactions, and this replacement can shore up structural damage. In addition, extra F floating around in your mouth can turn the hydroxyapatite to fluorapatite, which is more resistant to acids, Hewlett says, so brushing your teeth is really important. You can have sugar in moderation, he says, but when you keep your teeth clean, “you’re going to reduce your risk of getting decay. It’s literally preventable.”

There are no cells that can regenerate enamel once the tooth erupts. “But if you take good care of your teeth, they can last you for more than 100 years,” Joester says.

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