Strong evidence that short-pulse laser ablation of biological tissues is a photomechanical process is presented. A full three-dimensional, time-dependent solution to the thermoelastic wave equation is compared to the results of experiments using an interferometric surface monitor to measure thermoelastic expansion. Agreement is excellent for calibrations performed on glass and on acrylic at low laser fluences. For cortical bone, the measurements agree well with the theoretical predictions once optical scattering is included. The theory predicts the presence of the tensile stresses necessary to rupture the tissue during photomechanical ablation. The technique is also used to monitor the ablation event both before and after material is ejected. TheoryExperimental results reported in the literature reveal that the energy density required to initiate ablation of biological tissue with nanosecond laser pulses is 10-fold less than that required for vaporization. This holds for a wide range of laser wavelengths (1, 2). When the laser pulse duration is shorter than a characteristic time, the material is "inertially confined"-i.e., it does not have time to expand, and heating takes place at constant volume.For ablation using short-pulsed lasers, there is evidence that photomechanical effects play the most significant role (1, 3). Here it is presumed that, since most materials are weaker in tension than in compression, the material will fail wherever the induced tensile stresses exceed the tensile strength. A onedimensional photomechanical model of laser-induced spallation correctly predicts the reduced energy density observed for nanosecond pulses. However, it also predicts that damage should first occur approximately one absorption depth beneath the surface (4, 5). In fact, ablation occurs at or near the surface. For the lasers and wavelengths used in ablating biological tissue, the optical absorption depth is usually comparable to the transverse laser dimension, and a onedimensional approximation is not appropriate. Onedimensional estimates made by our group did not correctly predict the observed surface movement, although they did account for the order of magnitude decrease in the energy required to reach threshold (1, 6). The various discrepancies can be reconciled by including three-dimensional effects. We have solved the full time-dependent three-dimensional equations, which predict that significant tensile stresses are created on the surface, precisely where ablation is observed to occur. All the time scales considered are much shorter than the time associated with thermal relaxation.When a material absorbs and is heated by laser energy, the resulting nonuniform temperature distribution causes internal forces, which lead to thermoelastic deformation. This deformation in a solid body is determined by the thermoelastic wave equation (7):3(l -2cr) [1] subject to the appropriate boundary and initial conditions, where u is the displacement vector, p is the density, E is Young's modulus, o is Poisson's ratio, X3...

Under certain conditions, laser light incident on a target material can induce an explosive removal of some material, a process called laser ablation. The photomechanical model of laser ablation asserts that this process is initiated when the laser-induced stresses exceed the strength of the material in question. Although one-dimensional calculations have shown that short pulsed lasers can create significant transient tensile stresses in target materials, the stresses last for only a few nanoseconds and the spatial location of the peak stresses is not consistent with experimental observations of material failure in biological tissues. Using the theory of elasticity, analytical expressions have been derived for the thermoelastic stresses and deformations in an axially symmetric three-dimensional solid body caused by the absorption of laser light. The full three-dimensional solution includes three stresses, radial, circumferential and shear, which are necessarily absent in the simple one-dimensional solution. These stresses have long-lived components that exist for eight orders of magnitude longer in time than the acoustic transients, an important point when the details of dynamic fracture are considered. Many important qualitative features are revealed including the spatial location of the peak stresses, which is more consistent with experimental observations of failure.

The photomechanical model 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 show that, unlike the one-dimensional thermoelastic model of laser-induced stress generation that has appeared in the literature, the full three-dimensional solution predicts the development of significant tensile stresses on the surface of the target, precisely where ablation is observed to occur. An interferometric technique has been developed to measure the time-dependent thermoelastic expansion, and the results for subthreshold laser fluences are in precise agreement with the predictions of the three-dimensional model.

Knee meniscus is a hydrated tissue; it is a fibrocartilage of the knee joint composed primarily of water. We present results of interferometric surface monitoring by which we measure physical properties of human knee meniscal cartilage. The physical response of biological tissue to a short laser pulse is primarily thermomechanical. When the pulse is shorter than characteristic times (thermal diffusion time and acoustic relaxation time) stresses build and propagate as acoustic waves in the tissue. The tissue responds to the laser-induced stress by thermoelastic expansion. Solving the thermoelastic wave equation numerically predicts the correct laser-induced expansion. By comparing theory with experimental data, we can obtain the longitudinal speed of sound, the effective optical penetration depth and the Grüneisen coefficient. This study yields information about the laser tissue interaction and determines properties of the meniscus samples that could be used as diagnostic parameters.

We present results from the application of laser interferometry to the study of short-pulsed laser ablation of biological tissue. The mechanical response of tissue to laser-induced stress is examined under subthreshold conditions to determine its role in initiating the ablation process. A theoretical model is developed to relate this surface displacement to the pressure within the tissue and the mechanical properties of the tissue. In the experiment, a 7.5 ns pulse of 355 nm light was used to irradiate bovine shank bone, human meniscus, and an aqueous dye solution. Interferometric monitoring of the tissue surface was used to determine its motion after laser irradiation. The surface movement of bone was qualitatively consistent with the theoretical predictions of the model. The movement of meniscus and an aqueous dye solution showed additional features that are consistent with the growth and collapse of cavitation bubbles.