Central and peripheral corneal thickness measured with the TOPCON specular microscope SP-2000P

Modern refractive surgery and follow up relies on a knowledge of corneal thickness (CT) and shape, and the reliability of modern instrumentation providing such data is important. This study sought to determine the performance of the TOPCON SP-2000P specular microscope in measuring CT. The aims of this study were: (a) to determine if there is any difference between the CT results obtained from the first image, the clearest of three images and the mean of measurements from three images; (b) to determine the correlation between central and peripheral CT and (c) to investigate the reliability (repeatability and reproducibility) of the SP-2000P in the determination of central and peripheral CT.

The central and peripheral CT measurements shown on the first (f), the clearest (c) of three images and the mean (m) CT shown in three images captured with the SP-2000P of 43 subjects were compared. All images were captured by the same examiner. Nineteen of the subjects (male) returned on another day and measurements were taken by two examiners.

fCT, cCT and mCT were not significantly different from each other. The intraclass correlation coefficients (ICC) between the three values were> 0.9 for all corneal locations measured. However, the variability in the differences between fCT and cCT was relatively greater for peripheral CT (except temporal). The peripheral CT values obtained were all significantly greater than the central CT, and statistically significant correlations were found between the central and each of the peripheral thickness. There was no statistically significant between-visit or between-examiner differences in the central or peripheral CT. The ICC values for between-visit differences for central and temporal measurements were > 0.9 but for the other corneal locations, the ICC values were 0.81 to 0.88. For between-examiner differences, the ICC value was 0.82 for inferior CT and > 0.9 for the other four CT.

Differences in CT measurements obtained from the first and clearest images captured by the SP-2000P were not statistically significant but can be clinically significant for peripheral CT. We therefore suggest the use of the CT value obtained from the clearest of three images. For every unit change in central CT, there is an approximately equivalent change in peripheral CT. Based on the ICC values, the SP-2000P showed very good repeatability and reproducibility in the determination of central CT. For the determination of peripheral CT, the repeatability and reproducibility of the SP-2000P were good and reasonably good respectively. These findings have relevance to the measurement of the cornea before and after refractive surgery.