Assessing keratoconus using corneal topography

Apr 01, 2009

Dr Sami El Hage outlines a technique for early detection of corneal ectasia using four different displays: the normal average cornea, Zernike polynomials, Fourier harmonics and elevation plots.

Key Points

Many instruments can detect advanced stages of keratoconus, for example, a slit lamp biomicroscopy. In order to make accurate observations, the pupils of the suspected corneal ectasia patient must be dilated. Then, using a cobalt filter, Fleisher's ring (a gray circle) can be observed. Through a cross-sectional view, it is possible to detect the presence of Vogt's striae. In addition, a retinoscope is used to observe an abnormal pupillary reflex, and a pachymeter employed to measure the thickness of the cornea. Finally, a keratometer can also be used to observe any distortion of the keratometric mires and to measure the central curvature of the cornea. Many indices have been proposed to detect keratoconus, and most of the calculations are based on keratometric readings. However, corneal topography and aberrometry remain the best and most comprehensive means of detecting the early stage of keractesia.

Corneal Topographer: what it comprises

In corneal topography, the configuration of the rings and the optical working distance are determining factors in the accuracy of the measurements and how representative the data yielded from the topographer are regarding the shape of the cornea. The closer in distance the configuration of the rings, the less extrapolation in the calculations, so the more representative the data is of the corneal shape. The greater the optical distance between the corneal vertex plane and the keratoscopic rings, the smaller the amount of error at the periphery of the cornea.

Four different displays

We propose a technique for detecting corneal ectasia using corneal topography, as well as corneal aberrometry/wave-front technology, using four different displays: the Normal Average Cornea (NAC), Zernike polynomial, Fourier harmonics, and elevation plots. Due to the topographer's number of rings, their configuration, and their coverage of the quasi-totality of the cornea, the extrapolation is minimized, thus allowing a better assessment of corneal surface irregularity.

For the keratoconus detection, the software has four special displays to check and balance each other. The first display shows the Normal Average Cornea (NAC), an average dioptric plot of several normal corneas, resulting in a Normal Average Corneal shape, with a central steepening and gradual peripheral flattening. The shape factor of the NAC is SF 0.442.

The second display shows Zernike polynomials and their aberrations. The first order of Zernike polynomial is the piston. The second is the defocus and astigmatism. The third order is trefoil, the vertical and horizontal comas and the fourth order is tetrafoil, secondary astigmatism and the spherical aberration.

The third display shows three Fourier harmonics. The first harmonic shows the degree of tilt, the second harmonic shows the degree of astigmatism and the third harmonic shows the higher order aberrations.

The fourth display shows the corneal elevations and the differences between the cornea and the osculatory circle.

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