Thermal and mechanical mechanisms in laser damage to the retina.

The mechanisms thought to be responsible for the laser-induced reaction in biological tissues are thermal effects, mechanical effects, and electromagnetic field effects. In the retina all damage is associated with a disturbance of the pigmented layer, the pigment epithelium, and no evidence is found of primary damage in the neural layers. This indicates that the photonabsorption mechanism is a single photon process, and suggests that field effects are absent or negligible when compared to thermal processes. The limited heat transfer in the retina has been demonstrated by a radioautographic method, showing the peak temperature distribution throughout the lesion site. The results show that thermal damage is confined to the pigment epithelium and the receptor cells. The damage to the inner retinal layers, and that peripheral to the center of coagulation, arises secondarily from mechanical disturbances. The mechanical damage component is greater in Q-switched lesions, where the short pulse duration gives rise to explosive phase changes and the generation of a high-pressure front. The passage of this front induces rupture of cells and shearing due to bulk physical displacement of tissue. No evidence is found of cavitation as a result of tissue resonance induced by ultrasound.