Microscope-Integrated OCT-Based Topography Maps for Corneal Astigmatism Assessment

Abstract

Purpose : Assessing corneal astigmatism intraoperatively is key to providing good post-surgical refractive outcomes. As an example, there are limited tools to assess astigmatism in corneal transplants intraoperatively, and hence after penetrating keratoplasty, 15-31% of these patients have astigmatism greater than 5 diopters (D) [Feize S, et al. J Ophthalmol. 2011]. To help address this issue, we developed Microscope-Integrated OCT (MIOCT)-based topography with the potential to give surgeons intraoperative, quantitative corneal astigmatism information. In previous work, we described the assessment of astigmatism ≥ 6 D [Tian J, et al. ARVO E-Abstract 2018]. We describe here the detection of astigmatism below 6 D in reference phantom corneas.

Methods : Plastic, custom-printed phantom corneas with known astigmatisms spanning 0.0 – 6.0 D were imaged using MIOCT consisting of a research 200 kHz 1040 nm swept source engine integrated into a commercial surgical microscope (Leica Proveo 8). The phantoms were designed to have curvatures spanning physiologic corneal curvatures. Each radial OCT volume was 1444 x 700 x 36 (depth x Ascan x Bscan). After an initial manual pilot segmentation of the first frame, each Bscan was then automatically segmented to detect epithelial boundaries, resampled to a 6^th order Zernike function, and corrected for system-specific distortions. Axial curvatures were calculated according to ANSI standards. Curvatures within the 3-mm optical zone were then used to calculate the magnitude of astigmatism for comparison with the known references.

Results : The mean absolute paired differences in astigmatism between our method and the known reference values was 0.80 D. Differences were tested using the Wilcoxon Signed-Rank Test and found to not be statistically significant (p=0.5781). A sample corneal topography map created from MIOCT data is shown in Fig 1.

Conclusions : We assessed our MIOCT-based corneal topography on custom-printed reference cornea phantoms and were able to achieve accuracy within 1 D. This system has potential for intraoperative astigmatism management, particularly for full-thickness corneal transplantations.

This is a 2020 ARVO Annual Meeting abstract.

 

Figure 1: A) Corneal phantoms spanning 0.0-6.0 diopters. B) Sample topography map of a 3-diopter astigmatic phantom cornea.