DETERMINATION OF ENERGY DENSITY THRESHOLD FOR LASER-ABLATION OF BACTERIA - AN IN-VITRO STUDY

The Nd:YAG and CO2 lasers have been shown to be bactericidal at relati ve low energy densities. However, at energy densities exceeding 120 J/ cm(2) (CO2) and 200 J/cm(2) (Nd:YAG), laser irradiation also causes ir reparable root surface damage. The purpose of this study was to determ ine, in vitro, the energy density threshold at which microbial ablatio n could be achieved while inflicting the least amount of damage to the root surfaces of human teeth. Pairs of Escherichia coli colonies cult ured on broth agar were treated with a CO2 laser using a pulsed wavefo rm at approximate energy densities ranging from 3 to 110 J/cm(2). One of each colony-pair was then examined by scanning electron microscopy (SEM) and the other subcultured for viable microbes. Roots of extracte d teeth were lightly scaled and treated by CO2 laser, again with pulse d beam using approximate energy densities of 3 to 110 J/cm(2): and exa mined by SEM. Regardless of the level of energy density, residual bact eria could be subcultured from al laser treated microbial colonies. Th e inability of the laser to completely obliterate microbial colonies w as likely due to: depth of energy penetration, difficulty in precisely overlapping beam focal spots, irregular beam profile, and presence of microbes at the periphery of the beam focal spot. The threshold energ y density for bacterial obliteration was determined to be 11 J/cm(2) a nd that for root damage was 41 J/cm(2). Root damage was evident by cha rring, crater formation, meltdown and resolidification surface mineral , and increasing surface porosity. The results of this in vitro study indicate that when used at an energy density between II and 41 J/cm(2) the CO2 laser may destroy microbial colonies without inflicting undue damage to the tooth root surface.