Rebuilding Teeth

AT A CLINIC at the University of California, San Francisco, dentists in training hone their "drill-and-fill" skills for treating patients with cavities. But in a lab across the hall from the clinic, a group of biomaterials researchers has been examining alternative, longer-lasting restorations that go beyond simply patching holes in our pearly whites. Instead, they are working to perfect the chemistry necessary to mimic nature and help decayed teeth rebuild themselves.

Although dental filling materials have advanced over time, treating cavities with fillings hasn't fundamentally changed since the 1800s. Fillings are quick fixes that work well, but they wear down faster than regular teeth. And because they don't always seal tightly, fillings provide a hospitable environment for decay-causing bacteria.

Drilling teeth to prepare them for fillings can also cause problems: It often removes healthy tooth tissue along with the decay and can cause patients significant discomfort and anxiety.

On the horizon is a less stressful strategy called remineralization, which keeps healthy tissue and restores natural material lost to decay. Clinical dentists already use this approach to repair the top layer of teeth with fluoride and calcium. Now a research group at UCSF sees promise in using remineralization to fix deeper cavities. If that promise is realized, dentists may be treating these cavities with a restorative formulation.

Human teeth aren't as simple as they look. These dynamic structures develop via complex biological interactions that place inorganic crystals inside organic scaffolds, which is why even Mother Nature, when working at her best, still needs about five years to grow a tooth in the body. Dental researchers say growing a whole functional tooth outside the body is still many years away.

What researchers are much closer to is helping the body restore strength and function to a decayed tooth by adding back mineral crystals lost from the enamel and dentin. But "we're not just trying to duplicate what is done in nature. We are trying to make it go faster," says Sally J. Marshall, a professor of preventive and restorative dental sciences at UCSF who is working on remineralizing dentin.

She is married to Grayson W. Marshall Jr., a dentist-scientist, chair of the Division of Biomaterials & Bioengineering UCSF, and the principal investigator on the project to remineralize dentin. He speculates that a chemical process to restore decayed dentin will be ready for clinical testing in a few years.

Enamel, the outer coating on the natural crown of a tooth, is the strongest material in the human body. Its strength comes from its high concentration of tiny mineral crystals called apatite, which contain calcium, phosphate, fluoride, and carbonate. The human body grows apatite crystals inside enamel's organized lattice of proteins, which are known as amelogenins.

Under the enamel lies dentin, which consists of collagen fibrils reinforced with apatite. Dentin is roughly half apatite, one-third collagen fibrils, and 20% water. It is similar to bone in composition but different in microstructure.

Because enamel and dentin are made of minerals, they are susceptible to erosion by acid. Human teeth encounter acid not only in food and drink but also from bacteria that produce acid from sugar.

THE BODY constantly tries to add apatite back to the enamel to repair bits of damage. But when acid is eroding the minerals faster than the body can replenish them, decay descends through the enamel and into the dentin. And that spells even greater trouble.

Acid erodes dentin faster than enamel because dentin's apatite crystals are smaller, have more surface area, and contain more quick-dissolving carbonate than enamel's crystals. A hole—or cavity—forms after the dentin's collagen structure loses its mechanical properties and collapses into a goo that cannot be restructured.

Remineralization would restore to teeth the apatite crystals in enamel or dentin that dissolve during decay. If decay of dentin is caught early, remineralization can prevent the irreversible collapse of collagen supports and the resulting painful damage. Decay that continues past the dentin tissues will hit the tooth's soft pulp, where the dental nerve lives. People may wince when decay breaks through the enamel or the top layers of dentin; they howl when it gets to the pulp.

Enamel is the tooth's first line of defense against cavities. Because it is physically easier to access in the mouth than dentin, more investigators have been studying the structure and chemistry of enamel than those trying to figure out how to add minerals back to dentin. Enamel cavities also affect people of all ages, so dentists and researchers seem to place more emphasis on finding ways to remineralize enamel than dentin, says Brian H. Clarkson, a pediatric dentist who studies tooth decay at the University of Michigan School of Dentistry, in Ann Arbor.

Chemically, dentin is also more difficult to remineralize than enamel because dentin has a more complex, hydrated structure that must be built slowly from deeper in the tooth, Sally Marshall explains.

Figuring out how to remineralize dentin is important because an X-ray, the primary tool for diagnosing cavities, may not reveal a cavity until it attacks the dentin, says Van P. Thompson, a dentist and biomaterials professor at New York University's College of Dentistry. He adds that rebuilding dentin would ultimately be slower than filling a tooth but less prone to future problems.

The structural and chemical complexities within the different layers and junctions are what make enamel and dentin so difficult to rebuild. "The chemistry varies throughout the thickness of both the enamel and dentin," says Timothy P. Weihs, a professor of materials science studying the chemistry and structure of teeth at Johns Hopkins University. This variation leads to different properties at specific locations on the tooth, such as where the dentin meets the enamel, and among tooth types, such as a bicuspid or a molar.

Researchers still don't know exactly how the human body grows apatite crystals for enamel or dentin, or how to re-create those crystals in the lab. Microscopy shows that the crystals in enamel are actually rods that run the length of the tooth's crown and are roughly 2 mm long. Size varies by tooth type and location on the tooth's enamel. Engineering a crystal of that size is very difficult to do in the lab, says Stefan Habelitz, an assistant professor of preventive and restorative dental sciences at UCSF who works with the Marshalls and has a background in ceramics engineering. Silicon Valley crystal engineers can grow lots of big crystals, but they can't replicate the size of the crystals that naturally compose enamel, he notes. Habelitz has shown that amelogenin proteins help guide apatite crystal formation, but the longest apatite rods he has been able to engineer inside a glass matrix in the lab are 1 ??m in length.

Getting a crystal of the proper size bonded to the appropriate place in the scaffold is critical to tooth strength, Grayson Marshall says. A wrongly sized or misplaced crystal will not support the tooth, he adds.

An intact scaffold system is critical for remineralizing both enamel and dentin because apatite crystals have to have somewhere to roost. Therefore, only partially decayed teeth can be rebuilt—not completely rotten, cracked, or broken teeth.

Part of the Marshall group's research involves understanding where the apatite crystals position themselves in the dentin and how decay affects those positions. Raman spectroscopy and small-angle X-ray-scattering techniques tell them how much of the apatite is inside dentin's collagen fibrils (approximately 25%) and how much surrounds each fibril (approximately 75%). In addition, they have etched donated healthy teeth in the lab with acid and observed that the exterior apatite dissolves faster than interior apatite (J. Struct. Bio. 2008, 162, 404).

Cavities do not have universal patterns of decay either, say dental researchers. Atomic force microscopy and scanning electron microscopy demonstrate that each cavity has subtle structural differences from the top to bottom of the tooth.

DESPITE THE variations in dental decay, the trend in treatment is toward helping the natural material rebuild itself. Several products dispensed by dentists contain high concentrations of calcium phosphate or fluoride ions that may help the body remineralize enamel. These products, which are used by the patient at home, flood empty spaces in the enamel's lattice structure so the rate of remineralization can overtake the rate of decay.

Currently, however, no clinical products exist to remineralize dentin, but the Marshall group reports progress in laboratory studies. For those studies, the researchers used donated teeth that have decayed naturally, and they etched other donated teeth to mimic decay. They continually soaked the teeth in supersaturated mineralizing solutions with near-neutral pH that contained low-millimolar concentrations of calcium and phosphate ions, along with various amounts of carbonate or fluoride.

Via a technique called atomic force microscopy-based nanoindentation, they monitored the extent of remineralization with the various solutions. It took up to 28 days to partially rebuild dentin's apatite content in both naturally and artificially decayed teeth.

Because dentin must be able to withstand force to be functional in the mouth, the researchers also measured the wet elastic modulus, or the stiffness, of hydrated, remineralized dentin.

The UCSF researchers found that the best modulus measurements resulted from teeth soaked in a calcium and fluoride solution, but the reconstructed dentin was still not quite as strong as natural dentin. That's a problem they're still working to solve.

How laboratory results on remineralizing dentin would translate to a procedure for patient care in a dentist's office is still under study. The Marshalls point out that swish-and-spit rinses would not be practical because dentin remineralization would require several weeks of constant exposure. More than likely, they say, dentists would pack a patient's decayed tooth with a solid formulation containing the starting materials for forming apatite. Then, the materials would release into the dentin's collagen structure, forming apatite crystals over several weeks.

Grayson Marshall explains that the process might be similar to a technique currently used to treat patients with decayed teeth in developing countries. Lacking access to the anesthesia and other materials needed for drill-and-fill, dentists scoop decayed collagen from a tooth and fill the resulting hole with a plug of commercially available, malleable glass ionomer cement, which hardens in place and slowly releases fluoride. Patients report that this treatment makes their pain subside, and X-rays show that the decayed areas do not expand, he says. However, the plug eventually dissolves, so such a fix lasts only for a limited time.

Even in that limited time, some remineralization of dentin apparently takes place. As Grayson Marshall explains, research by other groups has shown that in primary teeth treated by the dissolving plug, what's left of the dentin is reinforced by fluoride.

Nevertheless, the clinical ability to remineralize dentin wouldn't necessarily eliminate drilling and filling in the developed world, Sally Marshall notes. This is because dentin can be remineralized only if the collagen structure remains intact. If part of the dentin structure has collapsed, a filling would be necessary to pack that space because collagen scaffolding can't be rebuilt in such teeth, she says.

So drilling and filling will continue for years to come. But the availability of remineralization techniques could begin to reduce the number of cavities that need to be filled and perhaps ease some patients' trepidation about a trip to the dentist.