Pitfalls in the determination of intraocular lens power without ultrasound.

The rule is simple: An 18 diopter intraocular lens tends to restore the basic refraction. For each diopter of basic refraction to be corrected add or subtract 1.25 diopters. To determine the basic refraction a careful history and an examination of old glasses and past refraction records is necessary. When the basic refraction cannot be ascertained, capital errors may occur. Given the basic refraction, errors or several diopters are still apt to occur. Calculation of intraocular lens power based on K-readings and ultrasonic measurements is therefore always advisable and a must when there is doubt about the basic refraction.