Temperature calculations on a model of a port-wine stain during argon laser coagulation have been performed. The model consists of four plane parallel layers representing the epidermis, the upper ...dermis, the superficial capillary blood plexus, and the deeper dermis. The light distribution inside the skin is calculated on the basis of the Kubelka-Munk theory. The numerical calculations have been performed by an 'alternating-direction-implicit' finite difference method. For the usual combination of exposure time and beam radius of the argon laser (between 0.1 and 0.6 s, and 0.5 mm or 1 mm) the shortest exposure time and largest beam radius yields the most efficient plexus coagulation. The model confirms that plexus coagulation implies destruction of epidermis and stratum papillare. Heat summation effects at the laser spot boundaries can be avoided when a distance of 2.5 times the beam radius is used between adjacent spots (centre-to-centre). An optimal argon laser treatment technique is suggested yielding full plexus coagulation with minimal or no scarring. Two simple physical requirements are given for the ideal laser to coagulate port-wine stains in an efficient and selective manner, suggesting application of pulsed dye lasers, emitting either at 418 nm or at 577 nm.
