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Optimising IOL Power Calculations
New approaches to improve prediction of effective lens position. Dermot McGrath reports from the 39th Congress of the ESCRS in Amsterdam
New approaches for assessing estimated lens position (ELP) with better intraocular lens (IOL) position prediction and axial length changes in cataract surgery should be considered to optimise IOL power calculations, a new study suggests.
“With the introduction of highly precise optical biometers, the main limiting factor for IOL power calculations is [estimating] the effective lens position, which is key in minimising residual refractive errors,” David Pablo Piñero Llorens PhD said.
Significant discrepancies between predicted and actual ELP often result in refractive surprise, he noted.
“Something that has been suggested but never studied in detail is the potential anatomical changes occurring after cataract surgery and the possible changes in axial length due to the volumetric changes related to the surgery. Several studies have looked at this, but one of the main criticisms is there are potential artifacts with optical biometers, which [do] not allow [for] a precise measurement of the axial length in pseudophakic eyes,” he said.
Dr Piñero Llorens’s prospective study set out to optimise the ELP calculation of two different monofocal IOLs using measurements obtained with an optical biometer in terms of the position of the IOL and potential ocular globe changes after cataract surgery. The team assessed post-surgery anatomical changes using an optical low-coherence interferometry system (OLCI) previously validated in pseudophakic eyes.
A total of 472 eyes from 280 patients with a mean age of 73.5 years undergoing cataract surgery were divided into two groups according to the IOL implanted: one group of 330 eyes with AcrySof® IQ SN60WF (Alcon), and the other group of 142 eyes with Akreos® MI60L (Bausch & Lomb). Refractive and biometric changes were evaluated during a six-month followup period with an optical biometer taking account of potential measurement artifacts.
The team also made a comparison between the ELP estimated with the SRK-T formula (ELP SRK-T) and ELP calculated considering clinical real data (ELP AXL-corrected clinical).
As well as significant changes in refraction, they detected a significant increase in anterior chamber depth (ACD) and a significant reduction in the axial length (AXL). The mean difference between ELP SRK-T and ELP AXL-corrected clinical was 0.17 (±0.39) and -0.23 (±0.43 mm) in groups one and two, respectively.
Mean differences between the IOL power implanted and IOL power calculated using the SRK-T formula and ELP AXLcorrected clinical were -0.39±0.90 D (p<0.001) and 0.39 D±0.87 D (p<0.001) in groups one and two, respectively. “A strong and statistically significant correlation of these differences with the difference between predicted and clinically obtained spherical equivalent was found in both groups,” Dr Piñero Llorens said.
Summing up the results, Dr Piñero Llorens said potential anatomical changes occurring after cataract surgery could explain the level of refractive predictability obtained using the vergence formula SRK-T.
“This supports the hypothesis that these anatomical changes are real and not only due to an artifact from optical biometers. New approaches for estimating ELP with better predictions of the position of the IOL and AXL changes with surgery should be considered to optimise IOL power calculations. These calculations could also be implemented for other types of monofocal and multifocal IOLs,” he concluded.
Dr Piñero Llorens is a researcher at the Department of Optics, Pharmacology, and Anatomy at the University of Alicante, Spain. david.pinyero@ua.es Dr Hideki Fukumitsu is a PhD student at the Department of Optics, Pharmacology, and Anatomy at the University of Alicante, Spain.
