Correlation between Refractive Error, Corneal Power, and Thickness in a Large Population with a Wide Range of Ametropia

Abstract

To determine the correlations between mean keratometry (KM), central corneal thicknesses (CCT), and cycloplegic spherical equivalent (SE) in patients with a wide range of ametropia. Retrospective analysis of the excimer laser surgery database at the University of Ottawa Eye Institute between 1993 and 2008 was performed. This study included 3395 eyes from 1858 subjects. The refractive error ranged from +6.75 to -14.00 D. CCT was obtained either by ultrasound pachymetry or anterior segment tomography. Keratometry was determined using an autokeratorefractometer. In the myopic group, the SE was observed to be inversely proportional to the KM (correlation coefficient, -0.18; P < 0.01). The KM and CCT were also inversely proportional (-0.11; P < 0.01). In hyperopes, a correlation between the cycloplegic SE and KM was also found (-0.25; P < 0.01), but the CCT did not correlate with either of these metrics. A direct correlation for the myopic group was found between KM and the difference in power of the principal meridians (keratometric astigmatism [KA]) (0.08; P < 0.01). This relationship was not observed for the hyperopic group. Within the myopic group the SE correlated with the refractive astigmatism (RA) (-0.04; P = 0 0.04). In all groups, a strong correlation was observed between RA and KA (0.78; P < 0.01). In the myopia group, the KM showed close correspondence with KA and an inverse relationship with SE and CCT. In hyperopes, an inverse correlation between the KM and SE was found, but no correlation with CCT was evident.