Extent of lateral epidermal protection afforded by a cryogen spray against laser irradiation.

BACKGROUND AND OBJECTIVES Cryogen spray cooling (CSC) has become an integral part of dermatologic laser surgery because of its ability to remove selectively large amounts of heat from human skin in short periods of time, thereby protecting the epidermis from unintended thermal injury. The objective of the present study is to investigate the extent of lateral epidermal protection afforded by a cryogen spray during laser irradiation. MATERIALS AND METHODS CSC experiments on skin phantoms are conducted using a commercial nozzle (GentleLase, Candela) to characterize epidermal cooling in time and space; namely, surface temperatures and heat fluxes during a 60 milliseconds spurt and 30 milliseconds delay. Numerical methods are used to model the light distribution (755 nm), heat diffusion and thermal injury within the epidermis and dermis. A 755 nm laser (GentleLase, Candela) was used to assess in vivo the extent of lateral epidermal protection against irradiation in human skin. RESULTS The commercial nozzle produced an uneven deposition and spread of liquid cryogen, thereby creating zones of high and low heat extraction on the surface. Numerical and in vivo studies show that 18 mm diameter laser beams may induce skin injury at the periphery of the irradiated areas. However, a 10 mm diameter beam provides the safest therapy because only the zone of highest heat extraction is exposed to laser irradiation. Beyond 10 mm, heat extraction is no more than a third of the maximum heat extraction within this diameter. CONCLUSIONS Accumulation of heat within the epidermis is always greater at the laser beam periphery, away from the CSC nozzle tip, where heat extraction is lowest. Therefore, there is risk of thermal injury at the beam periphery when there is a mismatch between the skin protected by CSC and that exposed to laser irradiation. For the cooling and irradiation sequences considered herein, heat extraction provided by a 60 milliseconds spurt/30 milliseconds delay correctly matches the heating profile of a 10 mm diameter beam.