Bright path ahead

Examining six requirements for reducing errors and eliminating 
‘refractive surprise’

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Photo of Howard Larkin

Even under optimal conditions, using the best measurement tools and corneal power formulas available, post-cataract surgery refractions are still off by more than 0.5D in one out of 11 patients with uncomplicated corneas – and in many more cases among patients who have had corneal refractive surgery, keratoconus or keratoplasty, Douglas D Koch MD told the American Academy of Ophthalmology (AAO) 2016 Annual Meeting in Chicago, USA. Further improvement in cataract surgery refractive outcomes will require better corneal measurement instruments, Dr Koch said in the Jackson Memorial Lecture at the AAO opening session. These will permit calculation of intraocular lens (IOL) power based on individual patients’ observed anterior and posterior corneal curvature rather than on constants for the presumed relationship derived from population studies, eliminating a significant source of error. “Regression approaches have maxed out; we are not going to get formulas that do much better. We need to be able to accurately measure all our key parameters, preferably preoperatively,” said Dr Koch, of Baylor University, Houston, USA. Measurement is especially important in aberrated corneas, where posterior topography cannot be reliably extrapolated from anterior topography. In the meantime, however, careful patient preparation and biometry can minimise the chance of “refractive surprise”, Dr Koch noted. He reviewed six requirements for reducing and managing corneal measurement errors before, during and after surgery, as well as emerging technologies that will further improve performance. HEALTHY CORNEAS AND TEARS Corneal curvature and astigmatism measurements can easily vary 0.5D or more from day to day using the same machine, and even more among different modalities, Dr Koch noted. Poor tear film quality is frequently the major cause of variability. Dr Koch emphasised stabilising tear film and corneal surface disease as much as possible before measuring for cataract surgery. WELL-TRAINED TECHNICIANS Consistent measurement processes are essential to generate reliable corneal measurements. Technicians must be trained in properly preparing patients and conducting tests. SCEPTICAL SURGEONS Surgeons should critically evaluate measurement data, and obtain multiple readings from more than one device, 
Dr Koch said. “We as clinicians have to look at the quality of the refractive data, whether we look at the reflected mires or numerical values such as standard deviations, and we need to ask why variations occur and address potential causes,” he said. BETTER DEVICES Devices that better measure posterior corneal surfaces and aberrated anterior corneas are needed. Already Scheimpflug and OCT devices using elevation data, and a new instrument using coloured LED lights to measure the second Purkinje image reflected off the posterior corneal surface, can provide posterior curvature data. Formulas using this data support more accurate IOL power calculations for post-myopic LASIK and photorefractive keratectomy (PRK) patients, with up to 68% within 0.5D of target (Wang L et al. Ophthalmology. 2015;122:2443-9). Dr Koch expects this will improve rapidly. Posterior corneal data also is helpful for calculating toric IOL power, even in uncomplicated corneas, because the orientation of posterior astigmatism varies and can add or subtract up to 1.0 D, leading to over or under correction and even flipping the astigmatism axis. INTRAOPERATIVE ABERROMETRY Studies have found up to 74% of post-myopic refractive surgery patients within 0.5D of target using this technology (Fram NR et al. Ophthalmology. 2015;122:1096-101), Dr Koch noted. POST-OP IOL POWER ADJUSTMENT Technologies that can change IOL power in the eye after surgery are also promising, Dr Koch said. These include the Calhoun Vision lens, which reshapes a special lens material by selectively polymerising monomers, and a femtosecond laser device that alters the refractive index of a small region of conventional acrylic IOLs, imparting a pattern that changes lens power. “We are blessed with a wonderful collaboration among physicians, scientists and industry. I am very confident that with the work we are doing we will have great success. There is a bright path ahead toward better outcomes for our patients,” Dr Koch concluded. Douglas D Koch: dkoch@bcm.edu