Comparison of two methods of determining central corneal vault under a scleral lens: estimation by slit lamp biomicroscope and anterior segment OCT.

Purpose: The aim of this study was to ascertain if there is agreement in the
measurement of central corneal vault (CCV) under a scleral lens, between
estimations made using a slit lamp biomicroscope (SLB) and measurements
made using anterior segment opticalcoherence tomography (AS-OCT).
Method: 30 images were taken of CCV under various scleral lenses using a SLB
and an AS-OCT. Estimations of CCV from SLB photographs were made using known thicknesses of the scleral lenses used and imagej software. The in-built measurement callipers was used to measure CCV on the AS-OCT. Right eyes were imaged on the AS-OCT first and the SLB second and vice versa for left eyes.
Results: Bland-Altman analysis of scleral lens thicknesses as measured with a radiuscope thickness callipers and measured with the AS-OCT showed fair agreement (mean difference 18.6 microns and lower and upper limits of agreement were -14.72 and 52 microns respectively) and so it was assumed that the AS-OCT was making reasonably accurate measurements of the CCV. Bland Altman analysis of the agreement between the two methods of measuring CCV showed a mean difference of 128 microns and the lower and upper limits of agreement were -47.7 and 303.6 microns respectively.
Conclusions: There is a huge variation in the estimation of CCV as made by SLB when compared to measurements taken by AS-OCT. This variation cannot be
explained by; differences in corneal curvature, magnification of the scleral lens, angle at which the illumination is at for the SLB estimation, differences in corneal location between the two measurements or lens settling. It is recommended that CCV should not be estimated using a SLB as these estimations appear to be highly unreliable.