SURFACE TILT MEASURED WITH THE EYESYS VIDEOKERATOSCOPE - INFLUENCE ONCORNEAL ASYMMETRY

PURPOSE. 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 captur ed image. METHODS. Videokeratoscopic images were captured with and wit hout a front surface conicoidal contact lens (experiment 1). An image was captured with central fixation followed by image capture with fixa tion 10 degrees off center. These two images were used to calculate th e angle of tilt with central fixation. The approximate tilt of the cor nea 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.RESU LTS. The mean tilt for the first group of subjects with the contact le ns on the cornea was 2.8 degrees, whereas the tilt for the cornea alon e was 3.2 degrees (experiment 1). The corneal tilt for the second grou p of subjects was 3.3 degrees, and the approximate tilt derived from a single captured VK image, using the equation, was 3.2 degrees (experi ment 2). CONCLUSIONS. The similarity of the tilt angle with and withou t the contact lens in place suggests that the apparent asymmetry seen in the VK images of human corneas may be largely an artifact arising f rom corneal tilt and not nasal/temporal asymmetry. The agreement of th e corneal tilt derived from two VK images and the approximate tilt der ived from a single image indicates that the latter may offer a quick a nd convenient way to determine the fixation adjustment required to eli minate corneal tilt.