Conservation of Arclength in Keratoconic and Normal Corneas with Air Puff Induced Deformation

Abstract

Purpose: : It has been reported that surface area is conserved between normal (NL) and keratoconic (KCN) corneas, except those with advanced disease, indicating redistribution of surface area rather than bulging in keratoconus. The purpose of the current study was to evaluate corneal arclength prior to and during deformation in KCN and NL corneas.

Methods: : A total of 16 eyes of 10 NL subjects and 15 eyes of 14 subjects with moderate KCN were recruited. All subjects underwent examination with the CorVis ST (Oculus, Wetzlar, Germany), with intraocular pressure (IOP) measured by the PASCAL Dynamic Contour Tonometry (DCT) (Ziemer, Port, Switzerland), as well as IOP with corneal compensation (IOPcc) measured using the Ocular Response Analyzer (ORA)(Reichert, Buffalo, NY). Corneal arclength was determined in the central 6mm of the horizontal meridian in the Scheimpflug images of the CorVis ST in ~140 images over a 30ms airpuff. The maximum and minimum arclengths were extracted from these images, as well as the initial and final arclengths. Δarclength was calculated as the difference between maximum and minimum values. Groups were statistically compared with two sample t-tests for all parameters.

Results: : All eyes decreased in arclength from initial state to applanation, and then continued to decrease to maximum deformation. Maximum arclength corresponded to initial state and minimum arclength corresponded to maximum deformation. No difference was found in maximum (p=0.42) or minimum (p=0.96) arclength during deformation between NL and KCN groups. Δarclength was statistically greater (p < 0.032 using the Satterthwaite method for unequal variances) in NL (0.124 ± 0.050mm) than KCN (0.093 ± 0.019mm). IOP was not different between groups using DCT (p=0.24) or ORA (p=0.10).

Conclusions: : A consistent decrease in arclength during deformation within the central 6mm of the cornea in both NL and KCN is consistent with conservation of arclength across the cornea, rather than central compression. The greater change in central arclength in normal corneas compared with KCN indicates that corneas with keratoconus had a greater compensatory change in arclength in the annulus outside of 6mm, which is consistent with peripheral location of the cone. Results imply a redistribution of corneal mass in both normal corneas and those with keratoconus, without measureable central corneal stretching under the applied load investigated. This is consistent with conservation of surface area and a bending vs bulging theory of shape change in keratoconus.

Keywords: keratoconus • cornea: clinical science • imaging/image analysis: clinical