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Surgical consistency a factor in IOL calculation accuracy
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Determining the perfect power IOL for an eye depends on multiple factors, but the primary ones are biometry and effective lens position. Even first-year residents understand the importance of accurate biometry: The better the measurements, the more accurate the results.
 Overlap of the capsulorrhexis over the optic edge helps to ensure consistent IOL positioning. |
The axial length measurement was improved with the move to immersion A-scan and then further with the optical method used by the IOLMaster (Carl Zeiss Meditec). For an average eye, for every 1 mm variance in the axial length, the IOL power changes by about 3 D. The K values have a nearly one-to-one ratio with the IOL power selection. In my practice, I may have four sets of preop K values to analyze: the manual Ks, the auto-refractor Ks, the topography Ks and the IOLMaster Ks. To minimize the risk of hyperopic surprise postop, choosing the lowest K values is helpful but may not be the most accurate.
But multiple factors determine the effective lens position, including the capsulorrhexis. For most IOLs, we aim to have a slight overlap of the capsulorrhexis edge over the edge of the IOL optic.
This ensures a consistent placement of the IOL. If the rhexis is too large, then the optic may prolapse slightly forward, resulting in a myopic surprise. If the rhexis is too small and phimosis ensues, the optic may be pushed posteriorly, resulting in a hyperopic shift. When implanting IOLs with a 6-mm optic, aiming for a consistent 5- to 5.5-mm capsulorrhexis can actually improve postop accuracy.