Cold-Plasma Jets for Root Canals

Cold-plasma probes promise to kill bacteria deep inside the tooth, possibly preventing the recurrence of infection

PHOTO: Left, Huazhong University of Science and Technology; Right, Applied Plasma Technology Institute/Old Dominion University

Few things send a chill down the spines of grown men and women as the prospect of getting a root canal. Eighty percent of Americans are afraid of going to the dentist, with one out of every two polled dreading the infamous root canal the most, according to a recent survey by the American Association of Endodontists. Luckily, scientists have been working hard in the past few years to make future dentist visits a little less painful.

Researchers in China and Southern California are racing to develop easy-to-use devices that kill infectious bacteria by delivering a stream of ”cold” plasma, thereby cutting down on the number of patients who have to repeat root canal procedures.

Cold plasma, or atmospheric-pressure plasma, is ionized gas generated by applying brief pulses of high voltage to a controlled flow of air or mixtures of oxygen and inert gases, such as argon or helium. The resulting plasma is cool enough to touch. Cold plasma has caught the attention of biomedical researchers in the past decade because it has proved to be an excellent blood coagulator and sterilizer, making it ideal for surgical and dental procedures.

In a root canal procedure, the endodontist begins by drilling a hole in the infected tooth down to the root canal to reach the core of the tooth containing the blood supply and nerves. She then removes the infected pulp and sterilizes the root canal using a variety of techniques, including liquid antibiotics, which can sometimes be ineffective, and expensive lasers, which can burn the patient’s gums. Finally, the root canal is filled in, and a temporary filling caps the hole. If the infection was destroyed, a permanent crown can later replace the temporary filling.

Unfortunately, the disinfection step doesn’t always work. As many as 10 out of 100 patients treated for root canal infections return to the dentist’s chair for another round of treatment because the disinfectants failed to kill all the bacteria living deep inside the sick tooth.

Cold plasma might do a more reliable job, according to researchers. The latest plasma devices are capable of generating thin plasma plumes that can fit into a tooth’s root canal, which typically has a diameter of 3 millimeters.

At the University of Southern California, a team of engineers and dentists led by Chunqi Jiang have been working on a cold-plasma jet capable of forming a plasma plume that’s 3 centimeters long and less than 1 mm in diameter and that can reach all the way into the root canal. Jiang has used her plasma jet to kill saliva-derived biofilm, a slimy layer of bacteria that has shown some resistance to antibiotics.

”We are specifically targeting biofilm because those are the most resistant causes of infection,” says Jiang, whose device destroyed biofilms 99.99 percent of the time in experiments conducted in petri dishes. Jiang says that a dental company has shown interest in her cold-plasma jet.

At Huazhong University, in China, Xinpei Lu is working on a prototype device that actually generates the plasma plume inside the root canal. He is working with a Chinese company to bring the technology to market.

”When a patient needs to have root canal treatment, he or she needs to visit the dentist office at least two times,” Lu says. ”With our device, a patient only needs to visit the dentist’s office one time. Besides, there is no painful sensation. The patient only feels gas blowing.”

Although both devices show promise, Lu and Jiang say that more work lies ahead. Jiang says that the U.S. Food and Drug Administration will need to test the safety of the gases generated by her plasma jet. Lu, whose device was tested against one of the main types of bacteria that necessitate repeat root canals, wants to run experiments with other bacteria.

”It’s not a magic bullet that kills everything,” says Mounir Laroussi, one of the pioneers of the study of cold plasma in the United States and director of the Laser and Plasma Engineering Institute, at Old Dominion University, in Norfolk, Va.

Experiments conducted at Laroussi’s laboratory have found that cold plasma cannot kill bacteria that has developed endospores—a tough layer of protection that forms inside the cells of some types of bacteria. Yet Laroussi says his own device, called a plasma pencil, has shown good results against various types of oral bacteria in lab tests conducted at ODU’s dental school.

Lu and Jiang say that if clinical trials are successful, their devices could hit dentists’ offices by 2012.

To Probe Further

Chunqi Jiang reported on cold plasma for root canals at the 2008 IEEE International Power Modulators and High Voltage Conference. Her latest research is scheduled to appear in the August 2009 issue of IEE Transactions on Plasma Science .

Xinpei Lu’s cold-plasma device is described in the May 2009 issue of IEEE Transactions on Plasma Science.

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