A model for predicting the site of paraxial retinal lesions secondary to "coaxial" operating microscope illumination systems.
We measured a 15 +/- 2-mm displacement of the illuminating element of the coaxial operating microscope from the true optical center of the microscope. We further calculated the displacement of the image of the illuminating element of the operating microscope on the retina of the simplified schematic eye and found that the "coaxial" operating microscope would focus the light between 1.3 and 1.8 mm superior to the fovea of the eye, depending on the theoretical refractive status of the simplified eye. Actual measurements on a simulated retina (35-mm camera) confirmed the calculated displacements. The deviation varied with eye tilt and microscope position. We also found that the maneuvers commonly used by anterior segment surgeons of tilting the microscope and rotating the eye downward tended to displace the image of the illuminating element closer to the fovea, increasing the chance for symptomatic retinal damage. A combination of such maneuvers may reduce the potential for clinically significant retinal phototoxicity by displacing the light further inferior to the fovea.