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Mechanically deformed morphologic cartilage grafts undergo a temperature dependent phase transformation during sustained laser irradiation that results in reshaping of the specimen. While thermal, optical, and mechanical properties of cartilage undergoing laser heating have been previously investigated, the viability of these irradiated grafts has yet to be examined closely until now. In this study, chondrocyte viability following laser irradiation was determined by measuring the incorporation of radiolabelled sulfate (Na 35SO4-2) into proteoglycan (PTG) macromolecules. Proteoglycans are highly sulfated and are the principal molecular constituents of cartilage matrix. Their synthesis directly reflects chondrocyte viability. By measuring the scintillation counts of 35SO4-2 uptake and normalizing the value by the total protein content of each specimen we can determine the level of PTG synthesis rates following laser reshaping. Regional baseline PTG synthesis rates as a function of location was determined by dividing each specimen into six regions. All regions except the most cephalic are demonstrated similar PTG synthesis rates. The most cephalic region exhibited a significantly greater PTG synthesis rates. In order to establish a positive control for this study, specimens were immersed in boiling saline water for approximately 40 minutes. The boiled specimens demonstrated a fivefold increase in normalized radioisotope uptake and suggest that the non-specific uptake of radioactive Na35SO4-2 is caused by structural alterations in the collagen matrix caused by extensive thermal exposure. To avoid this thermal artifact, another positive control was established using nitric oxide was to induce apoptosis of the chondrocytes, resulting in significantly lower PTG synthesis compared to untreated tissue. Cartilage specimens (25 X 10 X 2 mm) were irradiated with light emitted from an Nd:YAG laser (25 W/cm2, (lambda) equals 1.32 micrometer) while radiometric surface temperature, internal stress, and backscattered light were simultaneously recorded. Individual specimens underwent either one, two, or three sequential laser exposures with the duration of each exposure determined in real-time from observation of characteristic changes in integrated backscattered light intensity that correlate with thermal mediated stress relaxation. A five-minute time interval between each irradiation was given to allow the cartilage to return to thermal equilibrium. Average laser exposure for each irradiation sequence was recorded (5, 8.3, 12.2 sec). PTG synthesis decreased with increasing laser exposure, but was noted to remain above baseline levels for NO treated tissue. To further refine these results and minimize the effect of regional tissue variations, 7 mm diameter discs excised from the most cephalic portions and a middle region of the pig nasal septal cartilages were irradiated. A reduction of PTG synthesis rates was noted with each successive irradiation, suggesting that laser mediated cartilage reshaping acutely does not eliminate the population of viable chondrocytes. The degree of reduction in PTG synthesis is dependent upon the time-temperature dependent heating profile created during laser irradiation, and carefully monitored dosimetry is necessary to ensure chondrocyte viability.
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