The importance of centration and zone diameter in keratometry of irregular corneas

Session Details

Session Title: Imaging II

Session Date/Time: Tuesday 13/09/2016 | 14:00-16:00

Paper Time: 14:00

Venue: Hall C1

First Author: : A.Fredriksson SWEDEN

Co Author(s): :    A. Behndig                    

Abstract Details

Purpose:

The aim of this study was to investigate the central and eccentric astigmatism in keratoconus(KC) in comparison to healthy eyes and to highlight the importance of factors such as centration and measurement zone diameter in keratometry of irregular corneas.

Setting:

Department of Clinical Sciences/Ophthalmology, Umeå University. University eye clinic.

Methods:

From a material of 68 patients with KC, 28 right eyes from 28 patients with an inferotemporal cone were selected according to specified criteria from Oculus Pentacam HR® measurements, and were matched with corresponding healthy control eyes. The flat (K1), steep (K2) and peak keratometry readings and the 65% mean equivalent K-readings were registered from the “True Net Power” and axial/sagittal front and back surface displays. Astigmatic power vectors KP0 and KP45 were calculated. Data were analysed for a 6-mm and two 3-mm zones centered on the corneal apex or the pupil, and for 8 paracentral 3-mm zones.

Results:

The irregular KC corneas generally showed higher astigmatisms and keratometric powers. Compared to a standard 3-mm apex-centered measurement, many astigmatic values in KC corneas differed significantly when employing a 6-mm measurement zone, a zone centered on the pupil or decentered measurement zones. In healthy corneas, the keratometry readings generally differed much less between different measurement zones. For posterior astigmatism, however, differences in keratometric readings between different zones were seen also in healthy corneas.

Conclusions:

In irregular KC corneas variations in keratometry centration and measurement zone diameter can have a large impact on the keratometric readings. These factors are highly relevant for example when keratometry readings are used as an aid in subjective refraction or particularly in toric/spherical IOL power calculation in KC.

Financial Disclosure:

NONE