Measuring IOL Power

To choose the power for intraocular lenses (IOL) before 1975, Sir Harold Ridley devised a simple formula: 18.0 dioptres plus 1.25 times the preoperative prescription. “Surprisingly, about 50% of his patients were still within plus or minus one dioptre, but he had some extreme refractive surprises of nine dioptres or more,” Oliver Findl MD told the American Society of Cataract and Refractive Surgery in the 2018 Cornelius Binkhorst Lecture in Washington DC, USA. Advances in biometry and increasingly sophisticated formulae, including one developed by Richard Binkhorst, brother of Cornelius, and SRK II and SRK/T have progressively narrowed that outcome range to about one-half dioptre, said Dr Findl, of the Vienna Institute for Research in Ocular Surgery, and Hanusch Hospital, Vienna, Austria. Indeed, analysis of 2.3 million cataract surgeries in the ESCRS European Registry of Quality Outcomes for Cataract and Refractive Surgery (EUREQUO) shows that in the current era of optical biometry, 73% of patients are within 0.5D of their target refraction. “But that means that more than one-quarter are actually outside of that,” Dr Findl noted. Most of the biggest misses come with short or very long eyes, he added. Today’s patients demand better, especially with multifocal or toric lenses and clear lenses exchange, Dr Findl said. Postoperative refraction is the main factor for patient satisfaction. He reviewed sources of error in IOL power calculation, and existing and emerging technologies that can further improve refractive outcomes. SWEPT-SOURCE OCT BIOMETRY As recently as 15 years ago, axial length measurement was a significant source of IOL power errors mostly because ultrasound was still used, Dr Findl said. Partial coherence interferometry (PCI) optical biometry changed that, nearly eliminating axial length measurement error in 95% of cases. He noted that optical biometry was born in his hometown Vienna, Austria, and that he had the opportunity to perform the first trials using this technology in the prototype setting. The latest swept-source OCT biometry better penetrates eyes with dense cataracts, in one study successfully imaging more than 90% of eyes that failed PCI biometry (Hirnschall N et al. Ophthalmol Ther 2018), Dr Findl said. Essentially only eyes with white cataracts could not be measured. Other than variation in postoperative refraction, which cannot be addressed preoperatively, anterior chamber depth prediction is the major remaining factor in IOL power errors. Single-piece IOLs with non-angulated haptics help because they respond less to capsule fibrosis than three-piece designs (Wirtitsch MG et al. JCRS 2004), Dr Findl noted. Current IOL designs also are stable even with imperfect capsulorhexes (Findl et al. JCRS 2017). With a modern IOL the effect of the rhexis, even if it is too large or too small, is negligible for refractive outcome, he noted. NEURAL NETWORK IOL CALCULATIONS Theoretically, machine learning networks have the potential to improve the accuracy of IOL power calculations over fixed formulae, though their black-box nature inhibits their use in understanding any underlying physiological basis for variation, Dr Findl said. However, recent tests show that they are no better than the best current formulae – at least not yet (Kane JX et al. JCRS 2017). Measuring refraction on the operating table is another way to reduce refraction error, but it is complicated by corneal changes induced by surgery, Dr Findl noted. While intraoperative aberrometry may be useful for eyes that have undergone previous corneal surgery, it does not predict postoperative IOL position. Intraoperative OCT, where the capsule position is measured in the aphakic condition, can help better predict IOL power, but cannot predict post-op shifts from capsule fibrosis, Dr Findl said. It can be used to verify or adapt IOL choice and is especially useful for short or irregularly shaped eyes. TORIC LENSES Measuring of posterior corneal curvature is a key step for improving toric IOL outcomes, Dr Findl said. Automated devices that help align lenses at surgery may also help. “My take-home message is: Use optical biometry and optimised IOL constants. For corneal measurements, use at least two devices if you can,” Dr Findl said. He expects better measurements to improve power calculation formulae further, and that changing IOL power in the eye may one day be possible. Oliver Findl: oliver@findl.at

Tags: Oliver-Findl, ASCRS 2017