Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

usefulness of both topography and wavefront aberrometry; and the use of topography in cataract surgery.

As topography analysis is becoming an increasingly important aspect of the anterior segment evaluation, this textbook will assist ophthalmologists in their analysis for years to come.

Elizabeth A Davis MD FACS

Director

Minnesota Eye Laser and Surgery Center

Minnesota

Eye Consultants

Adjunct Clinical Assistant Professor

University of Minnesota

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CONTENTS

Jorge L Alio, José I Belda Sanchis,

Ahmad MM Shalaby

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Keratometry and corneal topography with placido discs systems were originally invented to measure anterior corneal curvature. Computer analysis of the more complete data acquired by the latter in recent years has been increasingly more valuable in the practice of refractive surgery. The problem in the placido disc systems is that one cannot perform a slit scan topography of the cornea. This has been solved by an instrument called the Orbscan that combines both slit scan and placido images to give a very good composite picture for topographic analysis. Bausch and Lomb manufacture this.

Spectacle correction of sight is designed only to eliminate defocus errors and astigmatism. These are the only optical aberrations that can be handled by the simplest theory of imaging, known as paraxial optics, which excludes all light rays finitely distant from a central ray or power axis. Ignoring the majority of rays entering the pupil, paraxial optics examines only a narrow thread-like region surrounding the power axis. The shape of any smoothly rounded surface within this narrow region is always circular in cross-section. Thus from the paraxial viewpoint, surface shape is toric at most: only its radius may vary with meridional angle. As a toric optical surface has sufficient flexibility to null defocus and astigmatism, only paraxial optics is needed to specify corrective lenses for normal eyes.

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CHAPTER 1: ORBSCAN

Paraxial optics is used in keratometers and twodimensional topographic machines.

RAYTRACE OR GEOMETRIC OPTICS

The initial objective of refractive surgery was to build the necessary paraxial correction in to the cornea. When outcomes are less than perfect, it is not just because defocus correction is inadequate. Typically, other aberrations (astigmatism, spherical aberration, coma, etc) are introduced by the surgery. These may be caused by decentered ablation, asymmetric healing, biomechanical response, poor surgical planning, and inadequate or misinformation. To assess the aberrations in the retinal image all the light rays entering the pupil must properly be taken in account using raytrace (or geometric) optics. Paraxial optics and its hypothetical toric surfaces must be abandoned as inadequate, which eliminates the need to measure surface curvature. Raytrace optics does not require surface curvature, but depends on elevation and especially surface slope. The Orbscan uses raytrace or geometric Optics.

ELEVATION

Orbscan measure elevation, which is not possible in other topographic machines. Elevation is especially important because it is the only complete scalar measure of surface

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