Ultrasound axial length measurement in biphakic eyes
Accepted 9 October 2002.
Abstract
Purpose
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.
Methods
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).
Results
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: ALcorrected = AL1555 + 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.
Conclusion
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.
aJules Stein Eye Institute, University of California, Los Angeles, USA
bSt Mary’s Eye Center, Santa Monica, California, USA
Reprint requests to Kenneth J. Hoffer, MD, St. Mary’s Eye Center, 1441 Broadway, Santa Monica, California 90404, USA.
1 The author has no financial or proprietary interest in any material or method mentioned.
ABSTRACT