CORNEAL ABERRATIONS IN KERATOCONIC EYES: INFLUENCE OF PUPIL SIZE AND CENTERING

SILVIA A COMASTRI; LILIANA I PEREZ; GERVASIO D PEREZ; GABRIEL MARTIN; ARTURO BIANCHETTI

Lima, Perú

Congreso; VII Reunión Iberoamericana de Óptica (RIAO) y X Encuentro Latinoamericano de Óptica, Láseres y Aplicaciones (OPTILAS); 2010

Pontificia Universidad Católica del Perú

Ocular wavefront aberration varies among subjects and under different conditions and is usually expanded in Zernike polynomials. Expansion coefficients can be determined for the whole eye (employing aberrometers) and for the front corneal surface (using data supplied bytopographers). The pupils taken into account can have arbitrary sizes in both cases while their centers are on the line of sight in the first case and on the keratometric axis in the latter. Since aberrations affecting retinal image in everyday vision correspond to the ocular pupil under natural conditions (photopic or not), to analyze this image, a transformation of coefficients associated to a pupil into those associated to another is often required. Various authors have addressed this problem and, considering scaling and decentering of circular pupils and aberrations of up to 7th order, we have previously developed an analytical method enabling this transformation. On the other hand,aberrations of up to 2nd order can be compensated with conventional ophthalmic or contact lenses (also affecting higher-order aberrations) while the customized correction of higher-order aberrations may yield considerable visual benefit in old or abnormal eyes (keratoconus, post-Lasik, etc.). In the present article we apply our method to transform the coefficients corresponding to 20 corneas of 10 keratoconic subjects. First, using the SN CT1000 topographer, we determine original aberration coefficients for a 6mm pupil diameter and also ocular natural photopic pupil data ((260±20) lx). The higher-order root mean square wavefront error is RMSHO<5.77µm, RMSHO is related to index KISA% (sometimes used to quantify keratoconus) only for large pupils and, as expected, the dominant aberrations are 2nd order astigmatism and 3rd order coma. The mean natural pupil diameter is 3.6mm and the ocular pupil centre is above the keratometric axis, the mean shift being 0.3mm. Finally, using the original coefficients and our transformation formulas, we evaluate new coefficients corresponding to the diameter of each eye´s photopic natural pupil. Coefficients computed considering coordinates origin at the ocular pupil centre can be greater or smaller than those evaluated assuming the origin on the keratometric axis. If RMSHO is greater than approximately 1.00µm then the difference between these coefficients is large so, as stated by other authors, it is convenient to shift the coordinates origin to the ocular pupil centre and use transformation formulas but if RMSHO is smaller then this difference is lesser than 0.12µm so coefficients could be directly determined with tolerable error using a topographer.