Er:YAG laser skin resurfacing using repetitive long‐pulse exposure and cryogen spray cooling: II. Theoretical analysis

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Abstract
Background and Objective To analyze the effects of laser pulse duration and cryogen spray cooling (CSC) on epidermal damage and depth of collagen coagulation in skin resurfacing with repetitive Er:YAG laser irradiation. Study Design/Materials and Methods Evolution of temperature field in skin is calculated using a simple one‐dimensional model of sub‐ablative pulsed laser exposure and CSC. The model is solved numerically for laser pulse durations of 150 and 600 μsec, and 6 msec cryogen spurts delivered just prior to (“pre‐cooling”), or during and after (“post‐cooling”) the 600 μsec laser pulse. Results The model indicates a minimal influence of pulse duration on the extent of thermal effect in dermis, but less epidermal damage with 600 μsec pulses as compared to 150 μsec at the same pulse fluence. Application of pre‐ or post‐cooling reduces the peak surface temperature after laser exposure and accelerates its relaxation toward the base temperature to a different degree. However, the temperature profile in skin after 50 msec is in either example very similar to that after a lower‐energy laser pulse without CSC. Conclusions When applied in combination with repetitive Er:YAG laser exposure, CSC strongly affects the amount of heat available for dermal coagulation. As a result, CSC may not provide spatially selective epidermal protection in Er:YAG laser skin resurfacing. Lasers Surg. Med. 28:131–137, 2001.