Corneal topography and optical zone location in photorefractive keratectomy

Optical zone diameters in photorefractive keratectomy are small in order to minimize the depth of stromal tissue excision and, possible, the resultant stromal scarring. A small optical zone demands accurate placement on the corneal surface. This article reviews the principles that are important to location and measurement of the optical zone in photorefractive keratectomy. The ablated area should be centered on the cornea overlying the entrance pupil while the patient is fixating coaxially with the surgeon. An optical zone that is too small or decentered may decrease acuity, lessen contrast sensitivity, or produce glare, either on the fovea or on the perifoveal retina. The relationship between optical zone size and location and quality of vision is not yet known. Studies of this relationship will require videokeratography to locate the optical zone and its relationship to the entrance pupil, a capability that current videokeratoscopes do not possess.