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In this study, we examined the effect of variation of pH environment on the thermal, mechanical and optical response of cartilage to laser-mediated heating. Our previous studies have demonstrated that cartilage undergoes accelerated stress relaxation during laser irradiation at fluences below the ablation threshold. Characteristic changes in integrated backscattered light intensity, radiometric surface temperature and internal stress are consistently observed during irradiation at physiologic pH. A peak in integrated backscattered light intensity occurs when surface temperature reaches approximately 65 degrees Celsius. Internal stress increases, plateaus, and subsequently decreases in a similar manner. In this study, porcine nasal septal cartilage specimens were immersed for 24 hours in physiologic buffer solution titrated to pH 1.1., 7, and 11. Cartilage samples were then irradiated (equals 1.3 micrometer, 4 W, spot size 4 - 5 mm) and integrated backscattered light intensity, radiometric surface temperature and internal stress were recorded. While specimens at pH 7 and 11 demonstrated qualitatively similar behavior, notable differences at pH 1.1 were observed, including: (1) temporal decoupling of the internal stress and light scattering responses; (2) rapid increases in integrated backscattered light intensity measurements during the early phases of laser irradiation; and (3) prolonged elevation in integrated backscattered light intensity and internal stress following laser irradiation. In addition, during successive laser irradiation to the same specimen, we observed a temporal decoupling between integrated backscattered light intensity and internal stress at pH 7 and 11. This was not observed at pH 1.1. These observations are discussed in the context of cartilage structure and chemical interactions within the extracellular matrix. Mechanisms for the observed differences are proposed, including interactions by the cartilage proteoglycan moieties, which are sensitive to changes in the pH environment and provide cartilage tissue with its characteristic viscoelastic properties.
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