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Daniel R Neal, Thomas D Raymond, Wei Xiong, Richard James Copland; Laser refractive surgery centration: visual axis or line-of-sight?. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3914.
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Since the eye is not a centered optical system, there has been a debate over the correct location for centering a laser refractive surgical treatment. The cornea has a well-defined axis and center and yet it is not co-located with the pupil. It has been observed in some cases that aberrations may be induced for a decentered pupil.
Aberrations were modeled centered on the cornea with different amounts of induced aberrations. These were compared to published literature results for induced aberrations for four different laser platforms.
The models indicate that there is a significant coupling between spherical aberration and coma for off-axis pupils if the spherical aberration (SA) is assumed to be centered on the cornea. This matches the clinical results for the cornea centered laser systems, but not the results for the pupil centered systems. For a typical (6 mm pupil) -0.37 um RMS corneal spherical aberration induced by a conventional LASIK treatment, the resulting coma is 0.37 um for a centered pupil verses 0.62 um for a decentered pupil. Using only the spherical aberration term and centering the treatment on the corneal vertex, the simulation predicted 0.35 um RMS coma, in excellent agreement with the clinical results.
The desired centration is more a property of the design of the laser delivery system than of the eye itself. While the eye may have a significant coupling between aberrations on the cornea and on the pupil, the individual laser system may take these effects into account. Residual errors may be due, in part, to remodeling of the epithelial layer after healing, and hence be somewhat unpredictable.
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