# Repeatability of Corneal Astigmatism and Equivalent Power with the MS-39 Tomographer Derived from Model Surface Fitting in a Cataractous Population

## TL;DR

This study shows that using advanced tomography to measure corneal surfaces is reliable for determining astigmatism and power in cataract patients.

## Contribution

The study introduces a robust method for extracting corneal surface characteristics using model surface fitting in a cataractous population.

## Key findings

- Surface characteristics extracted from corneal epithelium, stroma, and endothelium are highly repeatable using model surface fitting.
- Surface asphericity measurements require a larger region of interest to ensure robustness.
- Equivalent power and astigmatism can be reliably determined using all data points within a defined region.

## Abstract

What are the main findings?
Modern high-resolution anterior segment tomographers are capable of extracting surface height data from the corneal front and back surfaces and from the epithelium–stroma interface.The higher refractive index of the corneal epithelium suggests that the cornea should be considered as a dual-layer structure to account for potential inhomogeneity in the epithelial thickness.

Modern high-resolution anterior segment tomographers are capable of extracting surface height data from the corneal front and back surfaces and from the epithelium–stroma interface.

The higher refractive index of the corneal epithelium suggests that the cornea should be considered as a dual-layer structure to account for potential inhomogeneity in the epithelial thickness.

What is the implication of the main finding?
Model surfaces, such as floating best-fit spheres or conoids, could be fitted to the height map data within a specific region of interest to determine relevant surface characteristics such as curvatures, asphericities, and apex positions.Based on a dataset with bilateral repeat measurements in a cataractous population, we were able to confirm that the extracted surface characteristics seem to be very robust. However, surface asphericity should be extracted from a larger region of interest to ensure more robust data.

Model surfaces, such as floating best-fit spheres or conoids, could be fitted to the height map data within a specific region of interest to determine relevant surface characteristics such as curvatures, asphericities, and apex positions.

Based on a dataset with bilateral repeat measurements in a cataractous population, we were able to confirm that the extracted surface characteristics seem to be very robust. However, surface asphericity should be extracted from a larger region of interest to ensure more robust data.

We investigated the repeatability of the MS-39 in determining power vector components—the spherical equivalent (SEQ) and astigmatic powers (C0 and C45) and asphericity (Q)—of corneal epithelium, stroma, and endothelium in a large patient cohort. In this retrospective cross-sectional single-centre study, we evaluated a dataset containing 600 MS-39 anterior segment tomography measurements from 200 eyes (three repeat measurements each) taken prior to cataract surgery. The exported measurements included height map data for the epithelium, stroma, and endothelium surface. Model surfaces (spherocylinder (SphCyl), cylindrical conoid (CylConoid), and biconic (Biconic), all in the 3/6 mm zone) were fitted using nonlinear iterative optimisation, minimising the height difference between the measurement and model. The mean (MEAN) and standard deviation (SD) for each sequence of measurements were derived and analysed. In the 3 mm and 6 mm zone, the MEAN SEQ was 53.47/53.56/53.57 and 53.21/53.54/53.54 D for SphCyl/CylConoid/Biconic for the epithelium, −4.47/−4.51/−4.51 and −4.45/−4.50/−4.50 D for the stroma, and −6.23/−6.26/−6.26 and −6.18/−6.29/−6.30 D for the endothelium. With the three surface models and the 3/6 mm zone, the SD for SEQ/C0/C45 was in the range of 0.04 to 0.11/0.05 to 0.13/0.04 to 0.11 D for epithelium; 0.01 to 0.02/0.01 to 0.05/0.01 to 0.06 D for stroma; and 0.01 to 0.02/0.02 to 0.07/0.03 to 0.07 D for endothelium. Fitting floating model surfaces with astigmatism to map data of the corneal epithelium, stroma, and endothelium seems to be a robust and reliable method for extracting equivalent power and astigmatism using all the datapoints within a region of interest.