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AF Scheuerle, H Specht, S Bueltmann, K Rohrschneider; Foveal Topography And Its Interaction With Ophthalmic Wave Front Analysis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2050.
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Purpose: Aberrometry has become a potential tool to improve the results of refractive surgery of the cornea. Tscherning and Hartmann-Shack are the physical principles most widely used for the detection of corneal aberrometry. Both methods are based on a defined pattern of light that is projected onto the fovea, reflected and analysed. Tscherning aberrometry systems illuminate a foveal area of about 1.0 mm in diameter, the reflecting spot of Hartmann-Shack measures only 10 micron. We investigated the foveal surface topography by means of the Heidelberg retina tomograph (HRT II) with respect to the area of individual fixation located by scanning laser ophthalmoscopy. Methods: Topometric images of the retina, centered around the fovea, were obtained in 12 eyes of 6 healthy subjects (refraction + 1.0 to - 5.0D). 3 high quality scans of each eye were acquired by HRT II (15°-field). A scanning laser ophthalmoscope (SLO) was used of detect the center of fixation of each eye in order to match it with the retinal topography. Results: All subjects archieved a stable fixation. The center of fixation differed up to 50 micron from the anatomic center of the fovea. The topography of the surrounding foveal region revealed remarkable interindividual and intraindividual differences in surface quality. In two cases the retinal nerve fiber layer profile of the fovea was unilaterally tilted up to 10°. In one case retinal vessels produced an uneven surface within a radius of 500 micron to the center of fixation. Conclusion: The reflecting area around the center of fixation does not correspond to an ideal parabolic mirror. An ideal parabolic surface is a prerequisite for aberrometry. The irregularities of the the human fovea can cause distortions of the light reflected and can be a major source of error in the calculation of an exact wave front aberration map.
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