SURVEY OF OPHTHALMOLOGY VOLUME 22 l NUMBER 6 l MAY-JUNE 1976

PERSPECTIVES IN REFRACTION MELVIN L. RUBIN, EDITOR

Increasing the Range of the

Keratometer

WARREN MANNING AND DAVID MILLER, M.D.

Department of Ophthalmology, Beth Israel Hospital, Boston, Massachusetts

Abstract. How can the ophthalmologist perform keratometry on a patient whose cor- neal curvature exceeds the limits of the keratometer? A technique for extending the range of the keratometer to test such patients, e.g., those suffering from keratoconus or cornea1 plana, is presented. (Surv Ophthalmol 22:413-414, 1978)

Key words. cornea1 curvature - keratometry -

W hen you come right down to it, precise- ly determining the cornea1 curvature

with a non-contact method was a rather significant scientific feat. What is more sur- prising is that it was first accomplished back in 1619, by Father Christopher Scheiner.3 He had noticed that shiny balls of different radii produced reflected images of different sizes. This prompted him to make a series of balls of progressively larger curvature. To perform keratometry, he would seat a subject in front of a window bathed in sunlight, and match the size of the image of the window frame reflected from the cornea with the one produced by one of the calibrated balls.

The next major advance in keratometry was introduced by Ramsden. He added a greater precision of image measurement by using a magnification system. However, precisely measuring an enlarged reflected im- age from the constantly moving eye could try the patience; therefore, Ramsden also in- troduced the doubling device into the keratometer. With such a system one simply matches the cornea1 reflection of the moment

to itself and no movement is perceived. Thus, the present day keratometer allows

the operator to measure the size of the reflected image. The machine then converts image size to cornea1 radius using the relationship,

r _ 2ayI Y

where r = anterior cornea1 radius, a = distance from mire to cornea (75

mm in keratometer), y1 = image size, and y = mire size (64 mm in keratometer).

Then, the machine converts front cornea1 radius into refracting power, F, using the relationship,

F = 337.5 r(mm)’

The Bausch and Lomb keratometer in use today enables the user to directly determine cornea1 curvatures in the range of 36.0 to 52.0 diopters. While these limits are sufficient for most of the cornea1 curvatures encountered, they do not meet all known situations. For in-

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414 Surv Ophthalmol 22 (6) May-June 1978 MANNING, MILLER

stance, patients suffering from keratoconus or cornea1 plana have curvature values out- side these limits.

The technique used to extend the range of the keratometer to include these special groups is a simple one. A spherical test lens is mounted over the central aperture of the keratometer mire. The engraved side of the lens mount should face the mire. With the auxiliary 1,ens mounted on the keratometer, one measures the cornea in the usual way. To convert the drum reading to the true reading, one could consult a pamphlet published by Bausch and Lomb entitled Extended Range of Keratometer Reading. l The pamphlet con- tains tables showing the extended range using two specific lenses: + 1.25D for steep corneas, and - 1,OOD for flat corneas.

While these tables are useful, there is nothing sacred about a + 1.25D and - 1 .OOD lens. Since auxiliary lenses serve to magnify or minify the cornea1 image (yl) in order to get it back into the range of the doubling device, the true reading should merely be a product of the artificial value and some con- stant, C, to account for the change induced by the auxiliary lens. All lenses will extend the range of the keratometer. We need only to determine the conversion factor, C, unique to each lens.

We decided to run a simple experiment, in which five steel balls of various radii were used in conjunction with A.O. Tillyer Spherical Test Lenses. Readings were taken with lens increments of +.25D from -2.50D to +4.50D.

The results indicated that our hypothesis was correct. The new relationship was as follows: (1) True curvature = C X Keratometer reading

(diopters) (diopters) The unique “C value” corresponding to

each lens can be found from Equation 2 for plus, and Equation 3 for minus lenses.

[$ C~,, = 1.00 + .145P C,,, = 1.00 + .116P

The variable P is the power of the auxiliary lens. You will note that the equations are of straight lines. Thus, the deviation caused by a +4.00D lens is twice that of a +2.OOD lens. This also fits in well with the concept of a lens as a source of magnification. The difference in slopes for the two lines can be attributed to the magnification produced by the different thickness of the auxiliary lens itself.

For example, in the case of a conical cor- nea, in which a +2.00 auxiliary lens yields a reading of 47.25 diopters, the true power is 60.95 (using equation 2).

In the instance of a very flat cornea in which a -2.00 auxiliary lens yields a reading of 42.5 diopters, the true power is 32.65 (using equation 3).

While these specific equations are to be used only with A.O. Tillyer Spherical Test lenses, similar relationships would be ex- pected with other manufacturers’ lenses. The accuracy of Equation 1 used in conjunction with Equations 2 and 3 is f0.25D.

References 1. Extended Range of Keratometer Reading,

Bausch & Lomb, Rochester, NY 2. Levene JR: Clinical Refraction and Visual

Science. London, Butterworth, 1977, p 128 3. Scheiner C: Occlusive Fundamentum Opticum.

Insbruck. 1619

Reprint requests should be addressed to: David Miller, M.D., Department of Ophthalmology, Beth Israel Hospital, 330 Brookline Avenue, Boston, MA 02215.