Lensometro

SURVEY OF OPHTHALMOLOGY

VOLUME 55  NUMBER 5  SEPTEMBER–OCTOBER 2010

HISTORY OF OPHTHALMOLOGY
MICHAEL MARMOR, EDITOR

Origins of the Keratometer and its Evolving Role in Ophthalmology
Ron Gutmark, MD,1 and David L. Guyton, MD2
The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA; and 2The Wilmer Eye Institute, The Johns Hopkins Hospital, Baltimore, Maryland, USA1

Abstract. The keratometer, or ophthalmometer as it was originally known, had its origins in the attempt to discover the seat of accommodation in the eye. Since that early beginning, it has been reinvented a number of times, with improvements and modifications made in the original principles of its design for new applications that arose as ophthalmology advanced. The cornea is not onlyresponsible for the majority of the refraction in the eye, but is also readily accessible for measurement and modification. The keratometer’s ability to measure the cornea has allowed it to play a central role in critical advances in ophthalmic history. This review describes the origins and principles of this instrument, the novel applications that led to the keratometer’s continued resurgences over itsnearly 250-year history, and the modern devices that have borrowed its basic principles and are beginning to replace it in common clinical practice. (Surv Ophthalmol 55:481--497, 2010. Ó 2010 Elsevier Inc. All rights reserved.) Key words. astigmatism  contact lens  cornea power calculation  keratometer  ophthalmic surgery


doubling mechanism  history  IOL devices  ophthalmometer refractive

Keratometer Development and Origins
As early as the late 1700s, scientists attempted to develop techniques of measuring the cornea’s curvature because of their interest in determining the mechanism of visual accommodation. Jesse Ramsden and Everard Home were among those who proposed that accommodation occurred primarily from changes in the cornea. To prove their theory, Ramsden and Homeattempted to measure its curvature. In 1779, after trying several designs, they settled on one that consisted of a telescope that examined a doubled, reflected image in the cornea.88 This enabled them to measure whether the curvature of the cornea changed during
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Ó 2010 by Elsevier Inc. All rights reserved.

accommodation.68 Eventually, they concluded that no significant changes occurred, butmaintained this notion as one of three mechanisms acting concurrently to allow accommodation of the eye: (1) change in the cornea’s radius, (2) change in the distance between the crystalline lens and the retina, and (3) change in the shape of the crystalline lens.68 In 1801, Thomas Young described experiments that he conducted on himself in an attempt to clarify which of the three mechanisms wasactually occurring during accommodation. He wrote: I shall take the range of my own eye, as being probably about the medium, and inquire what changes will be necessary in order to produce

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Surv Ophthalmol 55 (5) September--October 2010

GUTMARK AND GUYTON

it [accommodation]; whether we suppose the radius of thecornea to be diminished, or the distance of the lens from the retina to be increased, or these two causes to act conjointly, or the figure of the lens itself to undergo an alteration.126

Basic Principles and Early Designs
Early attempts at measuring the cornea relied on rulers and compasses,122,126 but the accuracy of these methods was not sufficient. The first important step that led to thecreation of the modern keratometer was the realization that reflections of objects in the eye could be utilized as an accurate way to measure the corneal curvature. By treating the cornea as a spherical convex mirror, one can easily determine the radius of curvature of this ‘‘mirror’’ by employing the laws that govern reflections and the geometric relations of similar triangles (Details in Appendix A)....