Instrument error was first described by Koch in 1989. The instruments used by ophthalmologists to measure corneal power (keratometers and corneal topographers) cannot obtain accurate measurements in eyes that have undergone corneal refractive surgery. These instruments often miss the central, flatter zone of effective corneal power. The flatter the cornea is, the larger the zone of measurement is, and the greater the error. Placido disk topography is susceptible to similar errors and usually overestimates the central corneal power, leading to a postoperative hyperopic refractive surprise in myopic eyes. Emerging technologies based on direct anatomical analysis of the cornea (Scheimpflug and computer modeling techniques) may offer a truer measure of corneal central power.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.