Spatial temperature distribution in human hairy and glabrous skin after infrared CO₂ laser radiation

Background: CO(2) lasers have been used for several decades as an experimental non-touching pain stimulator. The laser energy is absorbed by the water content in the most superficial layers of the skin. The deeper located nociceptors are activated by passive conduction of heat from superficial to deeper skin layers.

Methods: In the current study, a 2D axial finite element model was developed and validated to describe the spatial temperature distribution in the skin after infrared CO(2) laser stimulation. The geometry of the model was based on high resolution ultrasound scans. The simulations were compared to the subjective pain intensity ratings from 16 subjects and to the surface skin temperature distributions measured by an infrared camera.

Results: The stimulations were sensed significantly slower and less intense in glabrous skin than they were in hairy skin (MANOVA, p < 0.001). The model simulations of superficial temperature correlated with the measured skin surface temperature (r > 0.90, p < 0.001). Of the 16 subjects tested; eight subjects reported pricking pain in the hairy skin following a stimulus of 0.6 J/cm(2) (5 W, 0.12 s, d1/e(2) = 11.4 mm) only two reported pain to glabrous skin stimulation using the same stimulus intensity. The temperature at the epidermal-dermal junction (depth 50 mu m in hairy and depth 133 mu m in glabrous skin) was estimated to 46 degrees C for hairy skin stimulation and 39 degrees C for glabrous skin stimulation.

Conclusions: As compared to previous one dimensional heat distribution models, the current two dimensional model provides new possibilities for detailed studies regarding CO(2) laser stimulation intensity, temperature levels and nociceptor activation.