Corneal Asphericity Changes after Implantation of Intrastromal Corneal Ring Segments in Keratoconus

Keratoconus is a progressive corneal thinning of unknown cause in which the cornea assumes a conical shape, with progressive irregular astigmatism and deterioration of visual acuity. The normal anterior corneal surface is prolate, and it could be described as conic (flattening of the radius of curvature from the apex toward the periphery)1. In keratoconus corneas, the steepening of the central cornea leads to an increase in cornea asphericity (Q). The expression «aspherical surface» simply means a surface that is not spherical. The outer surface of the human cornea is physiologically not spherical but rather like a conoid. On average, the central part of the cornea has a stronger curvature than the periphery. The typical corneal section is a prolate ellipse, consisting of a more curved central part, the apex , with a progressive flattening towards the periphery. In the inverse profile, i.e. when the cornea is flattened in its center and becomes steeper towards the periphery, the term cornea oblate is used to define this condition. The asphericity of the cornea is usually defined by determining the asphericity of the coniconoid which best fits the portion of the cornea to be studied. The physiologic asphericity of the cornea shows a significant individual variation ranging from mild oblate to moderate prolate2,3. Most studies agree that the human cornea Q (asphericity) values ranges from -0.01 to -0.801,4,5. Currently, the most commonly accepted value in a young adult population is approximately -0.23 ± 0.086, measured at a 4.5 mm optical zone. This is the first study in the literature that shows the Q-values range in keratoconus patients, stratified by the evolution grade of the conus, and the changes induced in corneal asphericity by the implantation of the Ferrara intrastromal corneal ring segments (ICRS).