Correlation between the dioptric power, astigmatism and surface shape of the anterior and posterior corneal surfaces

A knowledge of the shape of the cornea is of major importance for the planning and monitoring of surgery, and for the correct diagnosis of corneal diseases. Many authors have studied the geometry of the second corneal surface in the central region and it has been stated that there is a high correlation between the central radii of curvature and asphericities of the two corneal surfaces. In this work we extend this study to a larger, central, 6 mm diameter of the cornea. Surface height data, obtained with an Oculus Pentacam from 42 eyes of 21 subjects, were analysed to yield surface power vectors. Corneal heights of both surfaces were also decomposed into low order Zernike polynomials and the correlations between each of the power vectors and low-order Zernike coefficients for the two surfaces were studied. There was not only a strong correlation between spherical powers and Zernike defocus coefficients, but also between the astigmatic components. The correspondence between the astigmatism in both surfaces found here can be of the utmost importance in planning optical surgery, since perfect spherical ablation of the first surface does not assure total correction of corneal astigmatism.This work has been partially supported by the Spanish Ministry of Education and Science through the project nr. FIS2005-05053. Julian Espinosa and Jorge Perez acknowledge financial support from the University of Alicante through the project GRE07-7P. Henryk Kasprzak acknowledges the University of Alicante for its economical support through the project Senior 08/07

Corneal primary aberrations compensation by oblique light incidence

The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10) 3274–3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight (~0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.David Mas acknowledges the support of the Generalitat Valenciana through the project BEST-2008-148. Henryk T. Kasprzak acknowledges the University of Alicante for its financial support through the project Senior 08/07

Adaptación y calibración de un topógrafo corneal para medidas dinámicas

El ojo humano es un sistema dinámico que sufre pequeñas rotaciones y deformaciones. Normalmente estos cambios son promediados por los aparatos de medición comercial, a pesar de que proporcionan información importante acerca de la fisiología del ojo y el proceso visual. En este sentido, proponemos la recalibración y construcción de un topógrafo dinámico a partir de una cabeza de proyección de anillos de Plácido y una cámara de video (CCD). En este trabajo se presentan los métodos y algoritmos utilizados para la obtención de la matriz de radios de curvatura y reconstrucción topográfica. Con este sistema, los parámetros de la CCD y los algoritmos se pueden manipular a voluntad, de modo que es posible adaptar los algoritmos de reconstrucción o las propiedades de la cámara. Si la cámara se adapta para obtener un flujo continuo de imágenes, es posible obtener información acerca de la dinámica ocular

Corneal astigmatism compensation by oblique light incidence

Trabajo presentado en el IV European Meeting in Visual & Physiological Optics, Heraklion, Creta (Grecia), 31/08/2008 al 01/09/2008.Astigmatism is the most common corneal deformation. Many studies have confirmed that almost half the population has some corneal astigmatism and majority of young adults has the steeper meridian in the vertical direction (with-the-rule astigmatism). This astigmatism is usually small and has little effect on the retinal image. The eye is not a centred system but line of sight is deviated from the optical axis. Recent works have shown that aside of optical balance between aberrations, the angle of incidence of light on the cornea also helps to compensate ocular aberrations. This angle makes that light incidence on the cornea is not normal but tilted to the temporal part thus inducing off axis aberrations. Tabernero et al. have studied the mechanism of compensation of ocular aberrations and have determined that crystalline horizontal tilt generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Oblique light incidence induces two main aberrations: coma and oblique astigmatism. As we said in the previous paragraph coma is cancelled by the internal optics of the eye. In this presentation we state that tilted light incidence may also play a role in compensating residual corneal astigmatism. In our study we have proposed a theoretical Kooijman eye model with a slight with the rule astigmatism. Light rays at different angles are considered and light patterns at different axial distances are obtained. We have analyzed the maximum of the peaks and the secondary momentum with respect to the PSF centroid. Since a good PSF peak must fulfil both criterions, we have also explored the combination of both of them in a simple merit function. With these quality parameters we have determined the plane and the angle where the best PSF is obtained. Our results show that the best point spread function is obtained for an oblique light incidence with a tilt that is compatible with mean values of the angle kappa thus suggesting a mechanism of passive compensation of astigmatism.Spanish Ministry of Science through project FIS2005-05053

Ultrasonic in vivo measurement of ocular surface expansion

Our aim was to ascertain whether the ultrasonic measurement of longitudinal corneal apex displacements carried out in a proper headrest is a credible method of ocular pulse (OP) detection. To distinguish between longitudinal movements of the eye globe treated as a rigid body and ocular surface expansion caused by the variations of the eye-globe volume, two ultrasound distance sensors were applied to noninvasively measure displacements of cornea and sclera. The same sensors were used to examine the influence of the anterio–posterior movements of a fixed head on the registration of corneal apex pulsation. In both experiments, ECG signals were synchronically recorded. Time, spectral, and coherence analyses obtained for four healthy subjects showed that the ocular surface expansion due to pulsatile ocular blood flow (POBF) is the main component of longitudinal corneal displacement. Ocular surface pulsation is always affected by the head movement. However, there exist some unique properties of signals, which help to distinguish between head and eye movements. A rigid headrest and a bite bar are required to stabilize the head during OP measurement. Ultrasonic technique enables noninvasive and accurate in vivo measurement of corneal pulsation, which could be of interest for indirectly estimating intraocular pressure propagation and POBF component.This work was supported in part by the Polish Ministry of Education under Grant N N518 423336 and in part by the Spanish Ministry of Education under Grant PR2009-0377

Accuracy of Hartmann-Shack aberrometry for eye dynamics measurement

Presentación realizada en el "International Winter Workshop on Experimental Eye Research" celebrado en Jugów (Polonia) del 15 al 17 de Enero de 2010