Error correction of intraocular lens (IOL) power calculation

Intraocular lens (IOL) power depends mainly on the corneal power and the axial length of the eye. There is a significant error in the estimation of axial length by the usual methods due to the assumption that the ultrasonic speed has a constant value of 1,540 m/sec in the different media of the eye. In-vitro measurements have shown that sound speed in the cataractous lens may vary between 1,580 and 1,690 m/sec. Unlike the original method of accommodation that involves registrations of the eye echography for a period exceeding 30 seconds, modification proposed in this study allows in-vivo estimation of the sound velocity in the lens during only 2 to 3 seconds. Consequently, this modification allows measurements to be performed on subjects with minimal training and cooperation. Using regression analysis, actual axial length of the eye is estimated and the proper IOL power is calculated. Several normal and cataractous cases are examined to test the validity of the method. The resultant estimated IOL power turned out to be less than that obtained by the customary method of 0.4 to 1.0 diopters. It is therefore concluded that improved estimation of the axial length increases the efficacy of those formulas used for calculating the IOL power at different vision states.