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In this study, we examined (1) the effect of partial denaturation and (2) repetitive irradiation on porcine septal cartilage during Nd:YAG laser ((lambda) equals 1.32 micrometer, 25 W/cm2, 5 mm spot size) exposure. Diffuse reflectance from a HeNe probe laser and internal stress were measured in mechanically deformed cartilage specimens (2 X 10 X 25 mm) during Nd:YAG laser irradiation. Specimens were first partially denaturation in heated saline water baths at selected temperatures (50 degrees Celsius, 70 degrees Celsius, and 100 degrees Celsius all for 30 minutes). Native and water bath heated specimens were sequentially irradiated three times (pulse duration varying from 5 - 12 seconds, determined by noting the onset of accelerated stress relaxation) with a 5 minute cooling interval between pulses. After the first laser pulse, diffuse reflectance and internal stress changes occurred synchronously (coupled); the peak in internal stress occurred within less than 1 second following observation of the peak in diffuse reflectance. With repeated laser irradiation, this time interval lengthens with eventual decoupling of the diffuse reflectance and internal stress. With decoupling, internal stress increases during laser heating, and abruptly decreases when irradiation is terminated. Decoupling occurs with greater frequency in specimens pre-heated in the water bath. With the first laser exposure, only 15% of control, 8% of 50 degree Celsius heated, 0% of 70 degree Celsius heated, and 8% of 100 degree Celsius specimens exhibited decoupling. However, after two laser exposures, decoupling was observed in 83% and 60% of specimens heated in water baths at 70 degrees Celsius (N equals 12) and 100 degrees Celsius (N equals 12), respectively; in contrast, decoupling was observed in less than 20% of the native (N equals 24) and 50 degrees Celsius (N equals 12) water bath pre-heated specimens with two laser irradiations. The effect of partial denaturation using water bath immersion mimics findings observed with sequential laser irradiation. Hence cartilage likely undergoes partial denaturation during laser reshaping, and that sustained laser irradiation may result in irreversible changes in the tissue matrix.
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