To develop a screening test based on the difference (Diff) between the anterior corneal surface and the anterior best fit sphere in the central region of the Orbscan IIz topography as a way of etecting previous myopic photorefractive surgery.

From 1623 patients who had no refractive surgery and no corneal disease, 3132 topographies were defined as normal. From 120 patients who had Orbscan IIz topography after myopic photorefractive surgery, 238 topographies were defined as eyes that had undergone refractive surgery. The first objective was to determine the difference (Diff) between the anterior corneal surface and the anterior best fit sphere in the central region. The second objective was to classify the anterior elevation map of Orbscan IIz topography.

The Diff value of the center of the anterior cornea surface averaged 0.008±0.003 mm in normal eyes, and all values were over 0 mm. However, in eyes that had previous refractive surgery, the average was 0.014±0.009 mm, and all values in this group were less than 0. The specificity and sensitivity was 100 % in both groups. The spherical equivalent of the degree of myopic correction and the Diff value of the center of the anterior cornea surface showed a linear relationship. Consequently, we could derive a formula to determine the degree of myopic correction with a known Diff value of the center of the anterior corneal surface.

The screening test, based on the Diff value of Orbscan IIz topography, is quite useful in determining whether an eye has undergone previous myopic photorefractive surgery.