Background and Objective: Skin cooling is used to protect the epidermis in a variety of laser dermatology procedures, including leg vein treatment, hair removal, and port wine stain removal. Spray and contact cooling are the two most popular methods, but similarities and differences of these techniques are not well understood. Study Design/Materials and Methods: A theoretical model of skin cooling is presented for two different regimens: "soft" cooling in which freezing of the skin is not permitted and "hard" cooling in which the skin can be frozen to a given depth. Spray and contact cooling were also compared experimentally using an in vitro model. Results: For a fixed skin surface temperature, spray and contact cooling theoretically produce the same cooling profile in the skin. Anatomic depth of cooling depends on the time for which either the spray or contact is applied. In vitro experiments caused temperature at the simulated basal layer to be between −5 and +5°C for both spray (tetrafluoroethane, boiling point −26°C) and contact (−27°C sapphire plate) cooling. The theoretical precooling analysis shows hard mode to be faster and more selective than soft mode; however, cooling time for hard mode must be carefully controlled to prevent irreversible epidermal damage caused by freezing. Conclusions: Both spray and contact cooling provide efficient skin cooling. The choice of cooling method depends on other factors such as the target depth, cost, safety, and ergonomic factors.

Based on this new theory, the treatment pulsewidth for non-uniformly pigmented targets is significantly longer than the target thermal relaxation time (TRT). The theory provides new recommendations for photoepilation and photosclerotherapy parameters.

Skin precooling can be used to reduce epidermal thermal damage in laser procedures (such as hair removal) where the target structures are located up to several millimetres below the skin surface. We have developed and experimentally verified a computational model that describes contact precooling of a multilayered skin structure prior to laser irradiation. The skin surface is assumed to be brought into thermal contact with a cold plate made of material with a high thermal conductivity. The approximate analytical solution for the skin temperature is obtained by considering the plate as a local heat sink. The time evolution of temperature (in both the skin and the plate) is simulated numerically to yield the optimal cooling parameters. To experimentally verify the numerical results of the model, we performed direct measurements of skin temperature for contact cooling with a sapphire plate held at several different temperatures in the range +10 to -30 degrees C.

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PURPOSE: To use histological techniques to assess and compare the ablation depth, local damage, and surface quality of corneal ablations by a Q-switched Er:YAG laser, an optical parametric oscillator laser at 2.94 µm, a long pulse Er:YAG laser, and a 193-nm excimer laser. METHODS: Human cadaver eyes and in vivo cat eyes were treated with a 6.0-mm diameter, 30-jrai-deep phototherapeutic keratectomy ablation and a 6.0-mm diameter, -5.0O-D photorefractive keratectomy ablation. Human cadaver eyes were also treated with a 5.0-mm diameter, -5.00-D laser in situ keratomileusis (LASHi) ablation. Fluences and pulse widths used were 200 mJ/cm p 2 and 70 ns for the Q-switched Er:YAG, 150 mJ/cm p 2 and 7 ns for the optical parametric oscillator laser (OPO), 500 mJ/cm p 2 and 50 microseconds for the long pulse Er:YAG, and 160 mj/cm p 2 and 20 ns for the excimer laser. In the ablation rate study, 12 porcine eyes were ablated by the OPO laser with a range of layers and at different fluences ranging from 60 to 150 mJ/cm p 2 , all using a 1.5-mm spot on the eye. The ablation depth of these acute ablations was evaluated by light microscopy examination. RESULTS: In the acute damage study, light microscopy showed a thin surface layer in all samples with minimal thermal damage except on the long pulse Er:YAG corneas. Transmission electron microscopy revealed less than 0.3-µm surface damage for all specimens of both the optical parametric oscillator and the excimer laser samples with no evidence of collagen shrinkage. Transmission electron microscopy showed damage layers of 0.5 to 3 µm for Q-switched Er:YAG and 3 to 10 µm for long pulse Er:YAG. Scanning electron microscopy showed smooth surfaces in all eyes, although the excimer was the roughest. In the porcine eye study, ablations were produced in both PTK and PRK modes with the ablation rate per layer increasing with the fluence. At 120 mJ/cm p 2 , the average ablation rate was 1.9 µm per layer. CONCLUSIONS: The histology from the short pulse mid-infrared optical parametric oscillator laser at 2.94 µm was comparable to the 193-nm excimer with a smooth, damage-free, ablation zone when performing PRK and LASLK. [J Refract Surg 2000;16:40-501