Importance of cutaneous cooling during photothermal epilation: theoretical and practical considerations

Sapphire contact cooling is widely used to prevent non-specific epidermal injury from occurring during selective laser treatment of unwanted hair and vascular lesions. This small study was performed to examine the clinical response of the skin to 800-nm laser irradiation with varying extent of cutaneous sapphire contact cooling. Observed clinical responses are compared to those predicted by theoretical analysis in an attempt to construct a more complete picture of sapphire contact cooling and its role in preventing non-targeted tissue devitalization during laser treatment of the skin using a wavelength, pulse duration, and radiant exposure commonly used for laser hair removal.

Three subjects each received a total of three pulses of laser light (800 nm) of equal radiant exposure (60 J/cm(2)), pulse duration (30 msec), and spot size (9 mm x 9 mm), but with varying extent of cutaneous cooling. One site was pre-cooled and heat-sinked with a chilled (5 degrees C) sapphire window in contact with the skin; another site was heat-sinked only with a room-temperature (20 degrees C) sapphire window in contact with the skin; and a third site received no pre-cooling or heat-sinking. Each site was examined immediately after treatment and at intervals throughout a 3-month period. The thermal response of the tissue was calculated in each case using a Monte Carlo model for light transport in multi-layered tissues coupled with an axisymmetric finite-difference heat diffusion model. Thermal injury was modeled as a first-order kinetic rate process using an Arrhenius expression.

In all three subjects, the sites that were pre-cooled and heat-sinked showed no evidence of epidermal or dermal devitalization. The three sites that were heat-sinked only had a few patches of perifollicular epidermal devitalization and subsequent desquamation without any permanent epidermal or dermal injury, as would be evidenced by pigmentary alteration or textural change. In each subject, the site that received no pre-cooling or heat-sinking sustained epidermal and dermal devitalization, appearing as ulceration and resulting in sustained erythema and textural alteration. Clinical responses predicted by theoretical analysis agree with the clinical observations and show that the dominant effect of sapphire contact cooling for pulse durations of 30 msec or less is the reduction of fluence within the epidermis resulting from index matching at the skin surface.

The results of this small study suggest that by judiciously selecting the laser pulse duration and pre-cooling and heat-sinking the epidermis in a manner that provides index matching and compression of the skin, epidermal damage can be avoided while administering the highest, most effective radiant exposures.