Plasma Probes Pulp Painlessly

For some, nothing is more intimidating than a visit to the dentist’s office, especially when the appointment is for a root canal. The process is painful and can be a drawn-out experience.

Cold plasma emitted from a dental probe disinfects saliva-derived biofilm on an extracted human tooth. When the tooth is placed 5 mm below the tubular device nozzle for 5 min, 99.99 percent of the biofilm within the root canal is removed. (Images courtesy of Dr. Chunqi Jiang)

A routine root canal procedure, or endodontic treatment, involves drilling a hole inside the canal of a tooth to reach infected pulp tissue – where nerve fibers, veins and connective tissue are located. The diseased pulp is removed from the canal, leaving an empty cavity, which, after extensive cleaning with liquid antibiotics, is filled with a flexible plastic material called gutta-percha and then crowned.

However, completely disinfecting a root canal can require two or three return visits to the dentist’s office. To reduce the process to one visit, Dr. Chunqi Jiang of the electrical engineering and electrophysics department at the University of Southern California, and fellow researchers developed a cold-plasma jet that may prevent deposits of bacteria from being left behind, while also relieving pain. Her study was published in the July 2009 issue of IEEE Transactions on Plasma Science.

Replacing the drill

Cold plasma is becoming a staple in surgical procedures because its high-voltage ionized pulses of air or mixtures of oxygen and gases such as argon or helium are noninvasive and cool to the touch. Plasma has benefited those who use coagulation, sterilization and tissue engineering. Applying cold plasma in dental procedures will “help [clinicians] to identify novel therapeutic technologies based on cold plasma for root canal disinfection,” Jiang said.

Jiang designed a plasma dental probe that in one session can rid a root canal of saliva-derived biofilm, a layer of bacteria known to be resistant to antibiotics. The apparatus emits a plasma plume up to 2.5 cm long with a diameter of 3 mm, which fits inside the 2- to 3-cm-long canal. The short, intense electric pulses, which generate 6 kV and 1 kHz at about 100 ns, are low-powered at approximately 1 W and provide a noise- and pain-free method that is safe, with minimum heat.

Scanning electron microscope images reveal the inside of an extracted human tooth. The plasma plume reached and disinfected 1 mm of biofilm from the root canal, while below, biofilm is still present (a). A plasma-treated root canal surface shows open dentinal tubes within the root canal (b).

To test the plasma jet’s capability, Jiang used the equipment to disinfect the root canal of an extracted human tooth with a substantial layer of biofilm in the canal as observed by a scanning electron microscope. The tooth was placed a few millimeters beneath the tubular device nozzle for 5 min. The dental probe, with a 2-mm-diameter plasma plume, ranged from 15 to 28 mm in length with a gas mixture of helium and 1% dioxygen set at room temperature. Additionally, it applied a voltage of 4 to 8 kV with a flow rate of 1 to 5 liters/min-1.

During plasma treatment, electrons collide with atoms and molecules to initiate high dissociation, excitation and ionization in the plume. The researchers can widen and lengthen the plume to attain greater sterilization depth and surface coverage.

As a result, the plasma dental probe removed 99.99 percent of the saliva-derived biofilm in the tooth. A visible contrast line between the plasma-treated and nonplasma-treated areas was evident.

Don't sweat it

Compared to laser procedures, cold plasma is less expensive and pain-free, and it eliminates the risk of burning a patient’s gums. During testing, to make certain that the plasma did not produce too much heat, temperatures were monitored throughout exposure. The findings revealed that the plasma began at room temperature (22 °C) and increased to 31 °C within the first 30 seconds. Afterward, the temperature increased only gradually, by 0.8 °C per minute, confirming that heat-induced plasma is tolerable for patients undergoing bactericidal treatments.

To further determine what stops biofilm from recurring, Jiang used plasma emission spectroscopy with a monochromator and a photomultiplier tube to investigate the plasma plume and its primary reactive species. What the researchers found was that, compared with excited nitrogen molecules and with helium and nitrogen ions, atomic oxygen is more reactive during emission and is therefore the more likely agent in inactivating bacteria within the canal. Currently, Jiang is measuring concentrations of atomic oxygen while further investigating other plasma bactericidal mechanisms.

Amanda Francoeur
amanda.francoeur@laurin.com