Bactericidal effects of diode laser on Streptococcus mutans after irradiation through different thickness of dentin.
OBJECTIVE
A reliable method to eradicate the bacteria of residual carious dentin has not yet been developed. The aim of this study was to evaluate the antibacterial effect of a diode laser on Streptococcus mutans through different thickness (500, 1,000, and 2,000 microm) of human dentin. The thermal effect of laser irradiation was also investigated.


METHODS
Dentin specimens were inoculated with 2 microl of S. mutans on one side and irradiated by a diode laser on the other side with a power output ranging from 0.5 to 7 W. The laser tip was swept with the whole irradiation area of 7 mm x 3 mm at a speed of about 10 mm/second with a total irradiation time of 30 seconds. Cooling with distilled water (30 ml/minute) was applied simultaneously during laser irradiation. After laser irradiation, the bacteria was removed from the dentin surfaces and cultured for 48 hours at 37 degrees C anaerobically to assess the colony forming units (CFU) per ml. The morphology of the lased bacteria and the temperature rise during laser irradiation were observed by scanning electron microscope (SEM) and measured by thermocouple, respectively.


RESULTS
The results revealed that 7 W of laser power could kill 97.7% of CFU through 500 microm thickness of dentin. However, the bactericidal efficiency was significantly reduced as the dentin thickness was increased. The morphological changes of lased bacteria ranged from less affected such as loss of their wall bands and existence of minicells to more severely degenerated, such as disintegration and fusion of cells with pores on the cell wall. Only the dentin specimens with a thickness of 500 microm exhibited a temperature rise greater than 5.5 degrees C after receiving 5 or 7 W of laser irradiation.


CONCLUSIONS
A diode laser can eliminate the Streptococcus mutans of the residual carious dentin without inducing high pulpal temperature rise when the remaining dentin thickness is greater than 1 mm.
Lee
Bor-Shiunn
BS
Graduate Institute of Clinical Dentistry, College of Medicine, National Taiwan University and National Taiwan University Hospital, Taipei 10016, Taiwan.
Lin
Yueh-Wen
YW
Chia
Jean-San
JS
Hsieh
Tseng-Ting
TT
Chen
Min-Huey
MH
Lin
Chun-Pin
CP
Lan
Wan-Hong
WH
D003731:Dental Caries / Q000382:microbiology*; D003804:Dentin / Q000382:microbiology* / Q000528:radiation effects / Q000648:ultrastructure; D006801:Humans; D066298:In Vitro Techniques; D028022:Low-Level Light Therapy; D008855:Microscopy, Electron, Scanning; D018709:Statistics, Nonparametric; D013295:Streptococcus mutans / Q000528:radiation effects*