Corneal thinning during air puff indentation

Abstract

 

Purpose

 

To monitor the changes of central corneal thickness (CCT) during air puff indentation using a customized software program.

 

Methods

 

Forty-two healthy subjects aged 14 to 36 years were recruited. Air puff indentations were performed using a Scheimflug-based non-contact tonometer (Corvis ST). During an air puff indentation, the Scheimpflug camera captured a complete movement of the cornea along the horizontal meridian and generated 140 image frames (Figure 1). The Corvis ST provided pachymetry reading (Corvis CCT) by averaging CCT from 5 image frames at pre-indentation stage. A software program, with manual and automated modes, was developed to monitor the CCT changes during air puff indentation. In the manual mode, the outer reference of the CCT was set at the center of the presumed corneal epithelium. In the automated mode, the interface between air and cornea was the outer reference. The inner reference was the interface between cornea and anterior chamber in the two modes. “Baseline CCT” was obtained from 18 to 19 image frames at pre-indentation stage. CCT at maximum deformation / highest concavity (HC-CCT) was also determined using the software (Figure 2).

 

Results

 

The mean ± SD of the Corvis CCT was 563±33µm. The baseline CCTs using the manual and automated modes were 556±36µm and 599±38µm, respectively; while the HC-CCTs were 521±35µm and 536±27µm, respectively. There was a significant difference in baseline CCT between the manual and automated modes (paired t-test: t=-27.08, p<0.01). The HC-CCT was significantly thinner than the baseline CCT (paired t-test: t=10.90, p<0.01 for manual mode; paired t-test: t=21.83, p<0.01 for automated mode), by an average of 6.3% and 10.5% in the manual and automated modes, respectively.

 

Conclusions

 

The Corvis ST used 5 image frames to derive the CCT. Our software program utilized 18 to 19 image frames to generate a more precise CCT at the pre-indentation stage. CCT measured using the automated mode was thicker than the manual mode due to different outer references. Corneal thinning was demonstrated in all subjects during air puff indentation. It may be a useful parameter reflecting some corneal biomechanical properties.

 

 

Figure 1. A typical image frame of the cornea from a subject. The white line on the top is the presumed corneal epithelium.

 

 

Figure 2. A typical image frame of the cornea from the same subject during maximum deformation / highest concavity.

 

Keywords: 479 cornea: clinical science • 480 cornea: basic science