The Repeatability of Corneal Power Measurements with SD-OCT

Abstract

Purpose : To study the repeatability of corneal power measured by spectral domain optical coherence tomography (SD-OCT). Corneal power measurements are important for planning intra-ocular lens placements.

Methods : A CIRRUS™ HD-OCT (Carl Zeiss Meditec, Inc., Dublin, CA) system with Anterior Segment Premier Module was used to scan normal eyes, post-LASIK eyes and eyes with corneal pathology, using a scan pattern with 24 redial scans over 9 mm diameter with 1024 samples per B-scan centered at the vertex with an A-scan acquisition rate of 27 kHz. Three repeated scans were acquired. Scans with poor quality were detected by a scan quality algorithm and excluded from the repeatability calculations. The corneal power measurement software calculates the anterior and posterior corneal powers by segmenting the anterior and posterior surfaces followed by robust parabolic fitting in the 3mm diameter central zone. The anterior (k_a = (n₁-n₀)/R_a), posterior (k_p = (n₂-n₁)/R_p), and net (k = k_a + k_p - D x k_a x k_p /n₁) corneal powers are calculated for each meridian [1]. Overall anterior, posterior and net powers are determined by averaging over selected meridians.
The repeatability standard deviation of the corneal power measurements was evaluated by calculating the variance of all measurements from a single visit and taking the square root of the average of all variances. The coefficient of repeatability was also calculated as the ratio of the repeatability standard deviation to the mean.

Results : The following table shows the repeatability standard deviation (SD) of corneal power measured for the normal, post-LASIK, and pathology (including Keratoconus).

Conclusions : Our study indicates that a corneal power repeatability SD of less than 0.25 diopter can be achieved for normal and post-LASIK cases. Because SD-OCT directly measures the anterior and posterior surfaces simultaneously when the eye is in the natural and relaxed condition without any external intervention, it may yield more consistent results than standard keratometry in post-LASIK and keratoconic eyes in which the anterior–posterior corneal curvature ratios are altered by surgery or disease.

References:
[1] Tang M, Chen A, Li Y, Huang D, Corneal power measurement with Fourier-domain optical coherence tomography, J Cataract Refract Surg 2010; 36(12):2115–22

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Table 1