letters

Evaluating Videokeratoscopes

W e are responding to the article evaluating cur­

rently available videokeratoscopes. 1 The article

reported the capabiliry of these instruments to image test

objects that purportedly simulate clinical settings. The

general ophthalmologist reading this article would think

that reflective keratoscopes cannot tell the truth abour

corneal shape. We have a reflective keratoscope that de­

picts local curvature and clearly shows corneal curvature

outside the photo refractive keratectomy (PRK) knees,

the contour of central islands, the localized effects of

sutures, and the steepening over and flattening anterior

to pterygiums. The results of keratoscope evaluations depend on

the units of measurement and the design of the simula­

tions. In the Discussion, Dr. Belin notes that axial power

measures spheres and the cornea is an asphere. However,

this issue is more than a question of minor experimental

technique. Axial power is a poor descriptor of shape,

whose measurement is the purpose of corneal topogra­

phy. It tends to be unstable near the apex normal, where

the angle berween the surface normal and the instru­

ment axis approaches zero. By reporting only axial

power, the study fails to reward instruments that accu­

rately measure the most useful shape descriptors, the

height and local power of aspheric profiles. Dr. Belin's test surfaces that simulated ablations

were bicurves whose curvatures were centered on the

keratoscope axis. Along radials, such surfaces will have

discontinuous tangents that cause some rings to disap­

pear and other rings to double falsely. Confronted with

a false ring pattern from an artificial surface that the

instruments were not designed to measure, the kerato­

scopes either gave no reports or reported the surface

erroneously. We specifically disagree with Dr. Belin's

statement that sudden transitions "may be important in

analyzing the effect of PRK at the edge of the treatment

zone." In 75 PRK cases, we have never seen a ring omis­

sion or a ring doubling using the Optikon Keratron. We

do not believe that the surfaces used in this study simu­

lated corneas, even after surgery.

Our previously published evaluations of the Kera­tron found less than 0.50 diopters of error in the central region of aspherical test objects? It is difficult to know why Dr. Belin found much higher error. Unlike the

clinical evaluation of a patient who can look at a target,

the results of measuring a test object depend on its po­sitioning in front of the keratoscope. Since only one meridian was measured, the more sensitive keratoscopes may be accurately measuring the surface as positioned,

and some of the error may be artifactual. Because of the above limitations, Dr. Belin's con­

clusions should be viewed with extreme caution. To se­

lect an instrument for purchase or to understand the attributes of an instrument, the clinician should con­sider the study in perspective relative to other instru­ment evaluations in the literature.

References

KENNETH L. COHEN, MD NANCY K. TRIPOLI

Chapel Hill, North Carolina

l. Belin MW, Ratliff CD. Evaluating data acqUisition and smoothing functions of currently available videokeratoscopes. J Cataract Refract Surg 1996; 22:421-426

2. Tripoli NK, Cohen KL, Holmgren DE, Coggins JM. Assess­ment of radial aspheres by the arc-step algorithm as imple­mented by the Keratron keratoscope. Am J Ophthalmol 1995; 120:658-664

Reply: Kenneth L. Cohen, MD, and Nancy Tripoli sug­gest that my paper implies that "reflective keratoscopes cannot tell the truth about corneal shape." While this state­ment mayor may not be true, it was not germane to my study. The paper was specifically designed to compare the axial solution of all available videokeratoscopes. At the time of the testing, seven Placido-based videokerato­scopes were commercially available. All seven systems offered axial-based solutions. Two systems (Humphrey MasterVue and Optikon Keratron) used the arc-step recon­struction method. Additional systems (Alcon EyeMap, Top­con CM-1000, EyeSys CAS) offered tangential or local curvature maps. The axial curvature maps were the only maps offered on all systems and allowed for direct com­parisons of the different units. The objects were specially designed and the testing specifically performed to allow for unit-to-unit comparisons. The study was unique in evalu­ating all available videokeratoscopes and for looking at individual point accuracy as opposed to averaging a large

J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996 871