Scientists have found that zapping teeth with very short pulses of low-energy laser light boosts resistance to cavities dramatically. Though promising, the technique has been tested to date only on extracted teeth in the laboratory; the scientists say more studies are needed before the technique is ready for the dentist's office.
The technique works by instantaneously melting and then fusing a tooth's enamel coating, making the enamel more chemically resistant to the acids that cause cavities. Scientists liken the technique to the annealing process used to strengthen steel and glass. When the laser technique is used in conjunction with a fluoride treatment, one experiment showed, cavities were completely stopped.
Scientists from the University of Rochester and Eastman Dental Center will present their most recent results at the annual meeting of the International Society for Optical Engineering (SPIE) in San Jose, Calif., February 5-10.
While there are a few dentists using lasers experimentally to remove decay caused by cavities, the Rochester program is one of just a couple of programs looking to use lasers to prevent cavities.
The work is led by John Featherstone, chair of the Department of Oral Sciences at Eastman Dental Center and associate professor at the University. Featherstone, a chemist and an expert on dental decay, says that the major mineral in enamel is calcium phosphate, also known as apatite, which dissolves in acid very slowly. But when our teeth are formed, some of the phosphate molecules in our teeth are pushed out and replaced by carbonate (which is abundant in the human body); carbonate dissolves easily in acid and leaves our teeth prone to cavities.
So Featherstone teamed up with Wolf Seka, a laser expert at the University's Laboratory for Laser Energetics, to study the effects of various laser wavelengths on teeth. The team collected hundreds of extracted teeth from area dentists, cleaned them and cut them into small pieces, then hit them with laser light.
The team eventually settled on a pulsed carbon dioxide laser tuned to just the right infrared frequency (either 9.3 or 9.6 microns, not the conventional 10.6 microns). This light is absorbed almost completely by the enamel, preventing the light from traveling into the delicate and sensitive pulp of the tooth, where it would cause damage.
The team uses about 25 100-microsecond pulses to heat the surface of the enamel (the outermost 5 microns, about one-tenth the width of a human hair) up to 1,000 degrees Celsius in a fraction of a second. The heat momentarily melts the crystalline structure, knocking out some of the decay-prone carbonate molecules. When the enamel fuses, it is 70 to 85 percent more resistant to acids.
"It's a delicate balance," says Featherstone, who will co- chair the SPIE session on lasers in dentistry. "You want enough heat to anneal the tooth, but obviously you don't want to damage the pulp." Because the light is absorbed by the enamel, and because the scientists use a pulsed laser to give the tooth time to cool, the pulp's temperature rises less than two degrees Celsius.
"To heat the tooth several hundred degrees without endangering the interior, you need to use a laser with a short pulse duration," adds Seka. "We use very little energy to heat the tooth; each pulse is about 100 millijoules." Seka ran extensive computer models to determine the right type of laser to use to prevent damage while still sealing the enamel.
To test the effects of laser treatment, Featherstone duplicates for a two-week period the daily cycle that teeth go through in the mouth. He puts them in an acid bath for seven hours (to mimic the acids created as we eat during the day) and puts them in a saliva-like solution for 17 hours to bathe the teeth in nutrients and minerals, remineralizing them. Then the team compares the amount of mineral loss from the laser-treated to non-treated teeth, which indicates resistance to cavities.
Featherstone and Seka emphasize that there is more work to do. For instance, they believe the effect will last many years, but they won't really know until they run further tests. Featherstone says the method could be used to treat the decay-prone surfaces of the crown or the tooth root. Since the laser only needs to heat the enamel for a few seconds, treatments wouldn't take long.
If dentists of the future use lasers to prevent decay, they probably would use them to supplement fluoride, which shows similar results in the prevention of cavities, says Featherstone.
Working with Featherstone and Seka on the NIH-funded project is a team that includes Daniel Fried, assistant professor at Eastman Dental Center, and senior technicians Richard Glena and Sandra McCormack.
Their results have appeared recently or will appear soon in a number of journals, including the SPIE Journal, Applied Optics and the Journal of Dental Research. A review of this and related work will appear in the March issue of Lasers in Surgery and Medicine. tr
Note to editors: Color slides of laser experiments in the laboratory are available.