At a Glance
- Surgeons currently have a variety of IOL power formulae at their disposal, but even so, unexpected surgical results are not uncommon
- While working on ray tracing, we were inspired to develop an ocular biometer, Mirricon, that measures every refractive surface in the eye
- Mirricon can calculate lens position and IOL power required without resorting to IOL power formulae
- Our device has just completed an independent and prospective 114-eye trial that has shown it to provide equal or better performance compared with current optical biometers
In the early days of cataract surgery, before the days of A-scan ultrasound axial length biometry, ophthalmologists used a standard 18.0 D prepupillary intraocular lens (IOL) to replace the cloudy crystalline lens they had just extracted – and patients were expected to have the same degree of refractive error after surgery as they had beforehand. But in the 1970s, surgeons began calculating the power of the IOLs they inserted to achieve better vision, based on biometric measurements of the eye – principally, the axial length and keratometry. Many even required “A-constants” – theoretical values specific to the design and placement of individual IOLs. As IOL types diversified and procedures improved, patients began expecting better results from their cataract surgeries. Today many patients demand good vision at both near and far distances, and spectacle independence is the order of the day.
Assumptions and estimations
To accomplish this, surgeons have a variety of different formulae at their disposal to estimate appropriate IOL power. A key part of this estimation is the effective lens position (ELP), which currently relies on the accurate measurement of anterior chamber depth and corneal refractive power, as measured by corneal keratometry or topography. But there are a number of factors and assumptions that can confound this process: each IOL has its own constant that needs to be plugged into the formula, and assumptions are made about the curvature of the posterior corneal surface. Furthermore, if prior refractive surgery has been performed, then IOL power calculators (like the one available online at ASCRS.org) won’t produce a single power recommendation – they will present you with a wide range of options… which is less than ideal. Even without prior refractive surgery, you can make your measurements, follow the rules, use the calculator, and your patient can still experience a “refractive surprise.” Despite the fact that laser refractive surgery results in vision within 0.5 D of the intended target up to 92 percent of the time, fewer than 60 percent of IOL implantations after cataract surgery achieve this goal. Sometimes, there are additional problems like eyes that can’t be measured with the current generation of ocular biometers because of dense cataract, or errors in data entry or transcription.