Abstract

SA.08.10

Characteristics of femtosecond laser effects in corneal tissue

Vossmerbaeumer U., Jonas J. B.
Department of Ophthalmology, Mannheim Medical Faculty, University of Heidelberg

Objective: Femtosecond laser devices make use of physical phenomena of ultrashort laser pulses that arise in the sub-picosecond range of duration. The thermal relaxation of electrons within several picoseconds induces an energy transfer to the heavier ions. This sudden temperature rise in the spot results in an explosive expansion of the plasma and a shock wave which is attenuated to an acoustic wave immediately when travelling in the tissue. Due to its mechanical inertia the material follows the expansion and the tissue is disrupted in the focus. We investigate the energy characteristics in correlation to size and shape of stromal effects.
Methods: An experimental prototype of a femtosecond laser machine was used to apply linear pulse series of 2-15 µJ to human and porcine corneal tissue in vitro. Following histologic processing the specimen were subjected to quantitative and qualitative analysis of the visible effects. Single cavitation bubbles were assessed for shape and position in correlation to laser energy. 500 pulse effects were measured. Statistical analysis of the data was performed to test the hypothesis of a correlation between tissue effects and laser energy.
Results: The morphology of the laser effects was similar in various layers of the corneal tissue with a uniform distribution of circular and ovoid bubbles. For the different energy levels tested, weak positive correlations with position and morphology could be established. The phenomenon of intrastromal tissue bridges was observed between all types of effects with a more homogeneous alignment along the corneal curvature than in trephination pulse lines.
Conclusions: Fs laser devices in ophthalmic microsurgery deliver pulse series for targeted tissue disruption, without however producing contiguous cuts. Analysis of single effects reveals major variability as to form and dimensions in relationship to applied energy. The smoother effect lines of subsurface-parallel pulses compared to penetrating patterns may be due to the microarchitecture of the tissue.

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