Abstract
This study was aimed to assess the simulated keratometry (Sim K) and the total corneal refractive power (TCRP) in eyes with keratoconus with respect to the Amsler-Krumeich classification. We enrolled 100 eyes of 100 keratoconic patients and 25 age-matched normal eyes. The Sim K and TCRP were measured with a rotating Scheimpflug system (Pentacam HR, Oculus). The differences between Sim K and TCRP in the keratoconus group were significantly larger than those in the control group (p < 0.001). The differences between Sim K and TCRP became larger in the progressive stages of the disease (p = 0.191 for stage 1, p = 0.008 for stage 2, p < 0.001 for stage 3, p < 0.001 for stage 4). We found a significant correlation of Sim K with the differences between Sim K and TCRP in keratoconic patients (r = 0.497, p < 0.001). The differences between Sim K and TCRP for keratoconus were significantly larger than those for normal eyes, and the differences between Sim K and TCRP tended to become larger in the progressive stages of the disease. It is suggested that the Sim K readings overestimate the TCRP, especially in advanced keratoconus, and that this discrepancy is a possible source of a hyperopic refractive error after cataract surgery.
Highlights
P-value keratometry, calculated by using the standard keratometric index (1.3375) and the radius of anterior corneal curvature, and the total corneal refractive power (TCRP) is determined as the total refractive power, calculated by ray tracing through the anterior and posterior corneal surfaces according to Snell’s law
We found no significant differences between simulated keratometry (Sim K) and TCRP between the control and grade 1 keratoconus groups, the differences became larger with the progressive stages of the disease
We found no significant correlation of Sim K with the differences between Sim K and TCRP in the control group (Pearson correlation coefficient r = −0.109, p = 0.606), but a significant correlation in the keratoconus group (r = 0.497, p < 0.001) (Fig. 2)
Summary
P-value keratometry, calculated by using the standard keratometric index (1.3375) and the radius of anterior corneal curvature, and the total corneal refractive power (TCRP) is determined as the total refractive power, calculated by ray tracing through the anterior and posterior corneal surfaces according to Snell’s law. The detailed analysis of the differences between the Sim K and TCRP in healthy and keratoconic subjects may provide further insights on understanding of the etiology of refractive error, and on the precise IOL power calculation for keratoconic patients with cataract. The goal of the present study is twofold; to retrospectively assess these corneal power differences between healthy and keratoconic subjects, and to compare the Sim K and TCRP, with respect to clinical stage of the disease, in a cohort of keratoconic subjects
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