Something to chew on: hard facts about tooth enamel

Science News, May 14, 2005 by Alexandra Goho

Although the DEJ appears under a light microscope as a thin, straight line, higher magnifications reveal a scalloped structure that increases the contact surface between dentin and enamel, explains Grayson. When a crack reaches the DEJ from the enamel side of the tooth, the scallops can deflect the trajectory of the propagating crack. This decreases the likelihood that the crack's tip will hit the junction head-on and with full force.

Even at the dentin, "the crack doesn't have smooth sailing from then on," says Grayson. "Another mechanism kicks in."

As the propagating crack begins to pry open the dentin structure, bridges made of so-called untracked ligaments form. By the time the crack has traveled less than a cell's thickness into the dentin, these bridges bring the crack to a stop.

The bridges form as a consequence of small cracks that develop ahead of that main crack's tip. Because the daughter cracks don't always line up perfectly with the mother crack, they create regions of unbroken tissue, or bridges, between themselves and the primary crack tip. The researchers observed several of these smaller cracks, which dissipate some of the energy driving the main crack.

Ritchie argues that empty tubules, left behind by dentin-forming cells after their job in tooth development is done, serve as seeds for the formation of these daughter cracks. "The presence of these tubules is critical," says Ritchie.

And since there are no such tubules in enamel, he adds, this crack-thwarting mechanism occurs only in dentin.

Ritchie and his colleagues have found that as teeth age, the tubules fill up with hydroxyapatite. "When they're plugged up, they can't [initiate] these microcracks quite the same way as they did before," he says. As a result, fewer uncracked ligament bridges form, and the aging teeth become brittler.

NIBBLING The revelations about how cracks move through teeth should spur the development of next-generation materials for dental restoration, such as crowns, caps, and cavity fillings, Grayson says. A clearer picture of how the dental-enamel junction connects two dissimilar materials could also help researchers design longer-lasting orthopedic implants, such as hip replacements, that stick better to patients' bones. "The DEJ embodies a whole bunch of secrets that we can mimic," says Grayson.

Scientists in Japan are already showing what might be possible. Knowing that enamel is almost entirely mineral, the researchers aimed to create a similar dental material. Past attempts at all-mineral materials have fallen short of the real thing.

Kazue Yamagishi of the FAP Dental Institute in Tokyo and her colleagues used a super-saturated solution of calcium and phosphate ions to make synthetic enamel that's 100 percent hydroxyapatite. The material starts off as a paste and hardens within 15 minutes of application into a tooth cavity.

Like current fillings do, the synthetic enamel seals the cavity and prevents acid-secreting bacteria from further eroding the tooth. However, unlike conventional treatments, the material doesn't require drilling of the damaged area of the enamel. Dentists often have to remove some of the healthy tooth because conventional fillings such as polymer resins and metal alloys, don't stick properly to small cavities or damaged surfaces.