gms | German Medical Science

Ultra-high power subpicosecond pulses can ablate practically transparent materials, possibly by non-linear processes. The advantages of ultrashort laser pulses in the range of 300 fs for processing of human cornea has been previously demonstrated (Kautek et al. Appl.Phys. A58,1994, 513). The safety aspects of lasers with 30 fs at 800nm and 1 kHz are assessed in the framework of the SAFEST-Project subsidized by the Federal Ministry for Education and Technology.

Human corneas not usable for transplantation and skin samples, both kept in tissue culture medium, were used for the studies. They were ablated with a high-energy femtosecond-laser (30 fs) at 1kHz with a fluence between 0,017 and 0,094 J/cm² and a duration of exposure of 1 to 400 seconds. Immediatly after treatment the samples were frozen and stored until histological examination with a histochemical method was performed (nitroblue-tetrazolium, Neuman et al., J.Invest.Derm.,99,1992,160). Both necrotic areas and ablation zones were microscopically measured and statistically analysed.

Threshold values could be measured both for tissue necrosis and ablation for human cornea and skin samples as previously described for homogenous materials. They were found to be lower for increasing pulse numbers and are in the range of 0.042-0.143 J/cm² (necrosis) and 0.068-0.203 J/cm² (ablation), respectively.

The visible lesions and maximum permissible exposure threshold values for all types of relevant human tissue (cornea and skin) are being studied. In spite of the difficulties encountered with the inhomogeneity of tissue samples and the histological analysis (as compared to the standard materials) as well as inherent shifts of laser fluence they do provide data for the extension of existing laser safety regulations and standards (e.g. maximum permissible exposure threshold values, MPE).