Ideal process for dental hard tissues ablation: - no crack formation in the remaining tissue. - no heat transport in the residual tissues. - no adverse effect of heat in pulp.
Mechanism by : photo-thermal photo-chemical plasma induced ablation
1-photo-Thermal ablation : depend on the presence of water on the hard tissues, Maximum absorption at 3µm Er:YAG ……….2.94 µm Er:Ch:YSGG………..2.79µm
After certain temperature reached,melting,vaporization leading its removal by expansion of tooth material Micro explosion ….. Mechanical decomposition and blowing out of the generated small particles
In soft tissues heat is desired so long pulse duration and CW….. Sealing of vessels by coagulation. Also via regulation of pulse duration and repetition rate …you determine depth of coagulation zone.
Distribution of water in the tissues - enamel,dentin and caries … - enamel need more pulse E. - dehydration - water spray for cooling and increase absorption
2- Photochemical ablation : by Excimer at short wavelength UV Precise but not suitable because Low ablation rate per pulse.low pulse repetition rate. High cost Complicated Handling and maintenance Carcinogenicity of UV wavelength Can not transmitted by optical fibers Gases have limited lifetime
3- Ablation by ultra short laser pulses: From thermal ablation to plasma mediated ablation. Very promising approach. Bears high potential for fast,precise and damage free
USLP are pulses with duration in time regime in several tenth of picosecond down to some femtosecond or even below. Present record of shortest pulse is 4.5 fs 550 as.
The decrease of ablation threshold at short pulse length is caused by high power density The shorter duration of pulse with certain pulse energy can be made the higher peak of power of the pulse. High peak power with sufficient focusing of the beam gives very high power density (up to several TW/cm2) High P Density generate layer of plasma in the irradiated zone Plasma absorb all E contained in the laser pulse
When matter in the plasma state,the single atom have overcome the material bond and can move freely. Owing to the law of thermodynamic,the plasma expand rapidly blow out. Since pulses are short no time for heat to flow to surrounding lattice Also all energy deposited in the matter is removed again during ablation process –thus no temp. increase.