The posterior corneal curvature has a greater effect on the total corneal power in keratoconus eyes than previously realized.
This article was reviewed by Wallace Chamon, MD
A relationship exists between the posterior and anterior surfaces of the cornea, such that the posterior surface diminishes the power of the total cornea.
A ratio is believed to exist between the anterior and posterior powers of the cornea that varies from 0.8 to 0.9 in normal eyes. In keratoconus, a progressive eye disease, the normally round cornea thins and begins to bulge into a cone-like shape. The cone shape of the cornea deflects light as it enters the eye on its way to the retina, causing distorted vision in patients.
Wallace Chamon, MD, explained that the ratio is used to estimate an artificial refractive index for the cornea that compensates for the posterior surface of the cornea with its real refractive index of 1.376. Smaller indices are used to compensate for the negative power of the posterior that ranges from 5 to 7 D.
However, in order to calculate the ratio in the refractive indices, some presumptions must come into play, namely, that the factor between the two surfaces is a constant factor in all kinds of eyes, regardless of whether they are normal or not and the cornea has a constant thickness, said Dr. Chamon, adjunct professor of ophthalmology, Department of Ophthalmology and Visual Sciences, Paulista School of Medicine, Federal University of São Paulo, São Paulo, Brazil, and volunteer clinical faculty, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago.
“With these assumptions, there is good accuracy in determining the total corneal power by measuring only the anterior surface of the cornea,” he said.
While that last statement has been a consistently held belief, Dr. Chamon and his colleagues challenged both presumptions. He was joined in his research by Rafael Kobayashi, MD, Felipe M. C. Taguchi, MD, and Ibraim V. Vieira, MD.
Regarding keratoconic eyes, Dr. Chamon pointed out that the progression of thinning of the cornea differs from that in normal eyes and that progression is more aggressive toward the apex or the thinnest part of the cone.
According to Dr. Chamon, when comparing maps of thinning progression, ophthalmologists can see clearly that a keratoconic eye does not follow the progression of thinning in normal eyes.
“In a normal cornea, the ratio between the anterior and posterior surfaces can be very predictable,” he explained. “However, if we assume that the keratoconus begins with the thinning of the cornea and leads to progression to a steeper cornea, we have to assume that the ratio has changed because with corneal thinning, the posterior curvature will increase.”