| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. === Includes bibliographical references (leaves 128-132). === An experimental study was performed to determine the physical properties of knee meniscus using a low energy laser technique. Following irradiation by a 10 ns laser pulse, tissue undergoes thermoelastic expansion in response to laser-induced stresses. The stresses evolve, propagating through the tissue. If they exceed the material's strength, ablation occurs-the material ruptures. Below ablation threshold, the material remains in an expanded state until thermal relaxation occurs. We use numerical methods to solve the 3-D thermoelastic wave equation for a hydrated sample. In addition to thermoelastic expansion, expansion due to the formation of cavitation bubbles within the tissue was modeled. Cavitation occurs when tensile stresses rupture fluid. The laser-induced response of a gelatin phantom was measured with a Michelson interferometer and compared with predictions. Using gelatin as a tissue model provided a consistent experimental model of meniscus. Meniscus, like all biological tissue, is highly heterogeneous. By adapting the time dependent numerical solution of the wave equation, the measurement of physical properties of a hydrated sample became possible. The thermoelastic model depends on sound speed, Poisson's ratio, thermal expansion coefficient, and optical penetration depth. Once the behavior of gelatin was understood, human knee meniscus was studied. The thermoelastic model and experiment, allows measurement of physical properties of meniscus. Also, a numerical model of cavitation based on Rayleigh's equations was developed. By comparing experiment and theory in meniscus and water, we determined properties important to cavitation: threshold pressure, bubble density, surface tension and nucleation size. Finally, histology was compared with experiment. The presence and amount of cavitation displacement was correlated with the condition of meniscus. Physical properties can be used to diagnose degenerative cartilage. This research has increased understanding of the interaction of short laser pulses with cartilage tissue, and measured significant physical properties of knee meniscus with a minimally invasive laser technique. === by Marta Lyselle Dark. === Ph.D.
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