Measuring by laser Doppler vibrometry the recoil momentum of biological tissue under infrared laser ablation

ABSTRACT The recoil momentum ofhard tissue induced by pulsed infrared laser exposure was measured in a pendulumexperiment using laser Doppler vibrometry. For the experiments bone was irradiated by holmium:YAG laserradiation and dentin by a superpulsed CO2 laser. Since the initial masses of the samples were known and theablated masses were measured, this method allowed also an indirect determination of the velocity of the ablatedparticles. In a second experiment performed with the CO2 laser the velocities ofthe ablated particles weremeasured directly by the time of flight detected by the laser beam of the vibrometer. The technical realisation aswell as the limitations of the method is discussed; Furthermore laser parameters are pointed out which inducecritical acceleration risking serious damage to sensitive organs.Keywords: Holmium laser, CO2 laser, recoil momentum, laser Doppler vibrometer, ablation 1 . INTRODUCTION Compared to mechanical drilling the adavantage of laser induced ablation is to remove tissue efficiently andavoiding thermal damage. The removed material contains of dust and small particles which escape from thesurface with high velocities. To achieve ablation power densities up to 106 W/cm2 and more are used. In theoptimal version of ablation the irradiated laser energy is used to vaporise the removed material and is nottransfered into heat of the remaining tissue: However depending on the interaction time, additional energy istransferred into kinetic energy of the disappearing material which may involve another risk of damage to theremaining tissue. The disappearing material contains momentum due to their mass and velocity. Regarding to theconservation of momentum, the same momentum is transferred to the remaining tissue inducing an acceleration.Unwanted effects could be the appearance of mechanical cracks when working at hard tissue like enamel anddentin or that the laser induced momentum is transferred to delicate structures. The last one may happen whenablating parts of middle ear bones in an unintemipted ossicular chain. The acceleration of the middle ear bone istransmitted into the cochlea and may destroy hair cells in the inner ear.The goal of the experiments described here was to determine the momentum transfer onto the remaining tissue.The measurements were carried out with samples of hard tissue like dentin or bone. The momentum transfer wasmeasured directly by monitoring the velocity of the sample or indirectly by measuring the velocity of thevaporised particles disappearing from the surface and determine in an additional measurement the mass of thevaporised material.