Changes in Central Corneal Thickness With Air-Puff-Induced Corneal Deformation Using a Method to Correct Scheimpflug and Refractive Distortion

Abstract

Purpose: To establish a method to determine central corneal thickness (CCT) and anterior chamber depth (ACD) of an air-puff-deformed cornea at the highest concavity (HC) state.

Methods: The Fink method for refractive correction of Scheimpflug images of a convex pre-deformed (PRE) cornea was implemented for 155 eyes of 155 participants imaged with the Corvis ST (Oculus Optikgeräte GmbH). This method was subsequently modified for the HC state of deformation. The tracked edges of each participant's cornea were exported at the PRE and HC states. Ten participants who had a visible crystalline lens in the image were selected to determine ACD in both states. The center points on the corneal tracked edges and lens were used to determine uncorrected CCT and ACD, respectively.

Results: Average undeformed CCTPRE was significantly lower than deformed CCT_HC (584 ± 31 and 626 ± 34 µm, respectively) (P < .0001). No significant difference was found for the corrected ACD between the two states. Corrected CCT and ACD were significantly greater than the corresponding uncorrected values for both deformation states (P < .0001). Percent change in CCT was found to be correlated to change in arc length at HC (P < .0001).

Conclusions: Distortion in Corvis ST images at the HC state can be corrected using a modified Fink method. CCT was found to increase in the HC state, compared to the PRE state. The CCT change during deformation may be important in the study of the compressive response of the cornea. [J Refract Surg. 2021;37(6):422-428.].

Figure 1.

(A) An example of using the iris as a landmark for determining the location of lens in the pre-deformation state. The pink Xs are the points used to calculate the anterior chamber depth after the refraction at the anterior corneal surface. (B) The entire light path has been shown for the corrected edge of the anterior lens surface in the pre-deformation concavity state.

Figure 2.

A mathematical representation of the cornea reflected about its horizontal axis to apply the Fink method in a concave state. The Corvis ST is manufactured by Oculus Optikgeräte GmbH. ACS = anterior corneal surface; PCS = posterior corneal surface

Figure 3.

(A) An example showing the use of the iris as a landmark for determining the location of the lens in the highest concavity state for the same participant from Figure 1. The pink Xs are the points used to calculate the anterior chamber depth after the first refraction at the anterior chamber surface. (B) The entire light path for the corrected anterior lens surface edge in the highest concavity state.

Figure 4.

The anterior (red) and corrected posterior corneal surface (green) was overlaid onto the raw Corvis ST (Oculus Optikgeräte GmbH) image in the pre-deformation (top) and highest concavity state (bottom). The pink X indicates the point where the uncorrected anterior chamber depth was measured. The center point on the corrected anterior lens surface would be out of the image.

Figure 5.

A summary of the means and standard deviations of the uncorrected and corrected (A) central corneal thickness (CCT) and (B) anterior chamber depth (ACD) in 155 and 10 participants, respectively. Paired t tests were conducted (P < .05). The Corvis ST is manufactured by Oculus Optikgeräte GmbH. NS = not significant

Figure 6.

The relationship between the change in central corneal thickness (CCT) and the change in arc length at highest concavity was found to be significantly correlated. (R² = 0.1323, P < .0001).