Momentum transfer in laser-induced ablation of hard tissue measured by laser Doppler vibrometry

ABSTRACT A laser Doppler vibrometer was used in a pendulum experiment to measure the recoil momentum induced in hard tissue by pulsed infrared laser exposure. A Holmium:YAG laserwas irradiated at bone and a superpulsed CO2 laser irradiated at dentin. Since the masses of thesamples were known and the ablated masses were measured, this method allowed an indirectdetermination of the velocity of the ablated particles. In a second experiment performed withthe CO2 laser the velocities ofthe ablated particles were measured directly by the time of flight detected by the laser beam of the vibrometer. The results of both methods are in goodagreement; at a mean power of 0.5 Watt of the CO2 laser the velocity was 50-60 m/s increasing at higher mean powers.Keywords: Holmium laser, CO2 laser, recoil momentum, laser Doppler vibrometer, ablation 1 . INTRODUCTION: To avoid thermal damage at the removal of tissue high power densities up to 106 W/cm2 andmore are used. Thus vaporisation, spalation or ablation is induced. The removed materialescapes from the surface with high velocities. Depending on the interaction time the irradiatedlaser energy is used to vaporise the removed material; additional energy is transferred intokinetic energy ofthe disappearing material and not into heat ofthe remaining tissue. Assumingthis idealistic version is reached, there is another risk of damage of the remaining tissue. Thedisappearing material contains momentum due to their mass and velocity. Regarding to theconservation of momentum, the same amount is transferred to the remaining tissue and maycause enormous acceleration. Unwanted effects could be the appearance of mechanical crackswhen working at hard tissue like enamel and dentin or that the laser induced momentum istransferred to delicate structures. The last one may happen when ablating middle ear bones inan uninterrupted ossicular chain. The acceleration of the middle ear bone is transmitted into thecochlea and may destroy hair cells in the inner ear.