Ultrasound axial length measurement in biphakic eyes.

To determine the calculations to correct the error in ultrasound measurement of a phakic eye with a phakic intraocular lens (IOL) (biphakic eye) to increase the accuracy of IOL power calculation should cataract surgery become necessary. Mathematical derivations were performed to calculate the error caused by a phakic IOL while measuring an eye using an average sound velocity. A simple model was developed to allow correction of the axial length (AL). If the eye is measured at an average velocity of 1555 meters/second, the following calculation can be used depending on the material of the phakic IOL: AL(corrected) = AL(1555) + X x T, where T is the central thickness of the phakic IOL and X = +0.42 (0.41 - 0.42) for poly(methyl methacrylate), -0.59 (0.56 - 0.59) for silicone, +0.11 (0.10 - 0.12) for Collamer, and +0.23 (0.23 - 0.24) for acrylic. By obtaining the dioptric power of the phakic IOL and central thickness from the manufacturer and knowing the material of the lens, a mathematical correction can be made to arrive at the true ultrasound AL in a biphakic eye.