Photomechanical basis of laser ablation of biological tissue

Abstract

The photomechanical picture of laser ablation of biological tissue asserts that ablation is initiated when the laser-induced tensile stress exceeds the ultimate tensile strength of the target. We have developed a three-dimensional theoretical model of the thermoelastic response of tissue to short-pulsed laser irradiation which allows the time-dependent stress distribution to be calculated given the optical, thermal and mechanical properties of the target. In addition, we have developed a complimentary interferometric technique which can measure the laser- induced thermoelastic expansion of a material with nanometer spatial resolution on a nanosecond time scale. The complex features of this expansion allow the needed optical, thermal, and mechanical properties of the target to be determined, which then allows the stress distribution to be calculated. This work has led to several significant results which support the photomechanical model of ablation of biological tissue. First, unlike the one-dimensional model predicts the development of significant tensile stressed on the surface of the target, precisely where ablation is observed to occur. Experimental results from bone are consistent with mechanical fracture caused by laser-induced stresses. Experimental results from human meniscus, a representative soft tissue, show additional behavior consistent with the growth and collapse of cavitation bubbles within the tissue caused by laser-induced stresses.