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Diode lasers are used in dentistry mainly for oral surgery and disinfection of root canals in endodontic treatment. The
purpose of this study was to investigate and to improve the laser induced bacteria inactivation in endodontic treatment.
An essential prerequisite of the optimization of the irradiation process and device is the knowledge about the
determinative factors of bacteria killing: light intensity? light dosis? temperature? In order to find out whether high
power NIR laser bacterial killing is caused by a photochemical or a photothermal process we heated bacteria suspensions
of E. coli K12 by a water bath and by a diode laser (940 nm) with the same temporal temperature course. Furthermore,
bacteria suspensions were irradiated while the temperature was fixed by ice water. Killing of bacteria was measured via
fluorescence labeling.
In order to optimize the irradiation of the root canal, we designed special fiber tips with radial light emission
characteristic by optical ray tracing simulations. Also, we calculated the resulting light distribution in dentin by voxelbased
Monte Carlo simulations. Furthermore, we irradiated root canals of extracted human teeth using different fiber tip
geometries and measured the resulting light and heat distribution by CCD-camera and thermography.
Comparison of killing rates between laser and water based heating shows no significant differences, and irradiation of ice
cooled suspensions has no substantial killing effect. Thus, the most important parameter for bacterial killing is the
maximum temperature.
Irradiation of root canals using fiber tips with radial light emission results in a more defined irradiated area with minor
irradiation of the apex and higher intensity and therefore higher temperature increase on root canal surface.
In conclusion, our experiments show that at least for E. coli bacteria inactivation by NIR laser irradiation is solely based
on a thermal process and that heat distribution in root canal can be significantly improved by specially designed fiber
tips.
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