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N.J. Coletta, H. Han; Simulation of Increased Spherical Aberration Associated With Corneal Swelling: Relationship to Posterior Corneal Surface Shape . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2011.
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Purpose: After contact lens removal, soft contact lens (SCL) wearers show greater positive spherical aberration (Z4,0) than non–contact lens wearers (Coletta and Moskowitz, J. Vision, 2003; Vol. 3,12, p.37a). This effect is greater in the overall ocular aberrations than in aberrations derived from the anterior corneal surface, implying that increased spherical aberration after SCL wear is partially due to changes in the internal optics of the eye. Studies of SCL–induced hypoxia indicate that the cornea swells in the posterior direction (Erickson et al., OPO, 1999) and swelling flattens the posterior corneal surface (Moezzi et al., OVS, 2004). We used a four–surface schematic eye (Navarro et al., JOSA A, 1985) to determine whether changes in the posterior corneal shape could increase the eye's spherical aberration. Methods: The schematic eye model had aspheric surfaces for the anterior cornea, anterior lens and posterior lens, and these surfaces were kept constant for all simulations; the posterior corneal surface was spherical in the baseline model. Corneal swelling was modelled as a 16% corneal thickness increase, which could be accompanied by either a 0.4% decrease in corneal index, a 2.3% increase in posterior corneal spherical radius, a change in posterior corneal asphericity from Q = 0 to Q = –0.4, or combinations of these parameters. Chosen parameter values for thickness, index and posterior radius were based on previous studies of corneal swelling. Wavefront aberrations for a 6 mm pupil were calculated with Zemax software. Results: Positive spherical aberration of the model eye increased by 0.36% with an increase in thickness alone, and by 3.11%, 2.45% or 17.11% when the thickness increase was accompanied, respectively, by decreased index, increased posterior radius, or the asphericity change of the posterior surface. However changes in thickness, index and posterior radius all resulted in greater changes in defocus than in spherical aberration. Only asphericity increased spherical aberration more than defocus, by a factor of 2.7. Combinations of parameters decreased the absolute effect of asphericity but increased the ratio of spherical aberration to defocus. Conclusions: Changes in the posterior corneal surface shape could be a source of the increase in positive spherical aberration associated with corneal swelling after SCL wear. The increase in spherical aberration probably involves a change in the asphericity of the posterior corneal surface. This finding has implications in the study of the role of aberrations in refractive error since many subjects with refractive error are SCL wearers.
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