Surface tilt measured with the EyeSys videokeratoscope: influence on corneal asymmetry

To investigate whether the apparent corneal asymmetry in the EyeSys videokeratoscope (VK EyeSys Laboratories, Houston, TX) image is a result of the cornea being tilted in relation to the instrument and to examine the possibility of deducing this tilt from a single captured image.

Videokeratoscopic images were captured with and without a front surface conicoidal contact lens (experiment 1). An image was captured with central fixation followed by image capture with fixation 10 degrees off center. These two images were used to calculate the angle of tilt with central fixation. The approximate tilt of the cornea derived from a single captured image was determined by the use of a mathematical model applied to some of the ring images (experiment 2). Twenty-four subjects were used in each of the above experiments.

The mean tilt for the first group of subjects with the contact lens on the cornea was 2.8 degrees, whereas the tilt for the cornea alone was 3.2 degrees (experiment 1). The corneal tilt for the second group of subjects was 3.3 degrees, and the approximate tilt derived from a single captured VK image, using the equation, was 3.2 degrees (experiment 2).

The similarity of the tilt angle with and without the contact lens in place suggests that the apparent asymmetry seen in the VK images of human corneas may be largely an artifact arising from corneal tilt and not nasal/temporal asymmetry. The agreement of the corneal tilt derived from two VK images and the approximate tilt derived from a single image indicates that the latter may offer a quick and convenient way to determine the fixation adjustment required to eliminate corneal tilt.