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RPE Damage Thresholds and Mechanisms for Laser Exposure in the µs to ms Time Regimen
1Rumohr M., 1Schüle G., 1Hüttmann G., 2Brinkmann R.,
1Medizinische Universität zu Lübeck, Medizinisches Laserzentrum (Lübeck)
2Medizinisches Laserzentrum Lübeck GmbH (Lübeck)
Introduction: Laser induced damage mechanisms supposingly change when pulse durations are shortened from ms to µs. Using ms, thermal denaturation of the RPE takes place, while probably microbubbles around the strong absorbing melanosomes thermomechanically disrupt the RPE cells. Subject of this study is the determination of the damage mechanism and thresholds induced by laser pulse duration between 5µs and 3ms.
Material and Methods: A cw Argon laser at 514nm is externally modulated by an Acousto Optic Modulator (AOM) and the light was coupled to a 50µm diameter ophthalmic slit lamp fiber. Variable laser pulse durations from 100ns up to cw can thus be generated. Freshly prepared porcine RPE samples were used. The samples were irradiated with single pulses of 5µs, 50µs, 500µs and 3ms duration and afterwards stained with CalzeinAM as life/death marker. To prove the existence of microbubbles around the melanosomes during irradiation, the emitted bubble formation and collapse acoustic transients were recorded with a hydrophone.
Results: At 5µs laser pulse duration, the threshold of microbubble formation (222mJ/cm²) was below the RPE damage threshold radiant exposure (250mJ/cm²). If cells are damaged, acoustic bubble transients revealing microbubble formation were found. Also microbubble formation transients have been detected if no RPE cells were damaged. In this case probably the RPE cell was flexible enough to compensate for microbubble induced volume expansion. At 50µs pulse duration, the RPE damage threshold (439mJ/cm²) was below the microbubble onset (483mJ/cm²). At this pulse duration three different kinds of events happen: 1. Microbubble formation without RPE damage; 2. Microbubble formation with RPE damage; 3. Cell damage without microbubble formation. At longer pulse durations (500µs, 3ms) all RPE cells are thermally denaturated at radiant exposure 2-3 times below the threshold for microbubble formation.
Conclusions: A thermo mechanical damage mechanism by microbubbles was found as primary RPE damage mechanism for single 5µs laser pulses. At 50µs laser pulse duration a pure thermal damage as well as a thermo mechanical damage mechanism is possible. A pure thermal damage is given for 500µs or longer laser pulse durations.