Reduced need for adjustments
There are a number of reasons for the enhanced precision of the new aberrometry system. First is the higher-resolution wavefront sensor, which measures more than 1,200 spots over a 7.0-mm pupil.
In addition to the aberrometry, the system simultaneously captures refraction, topography, keratometry, and pupillometry, and ensures that all these measurements can be spatially registered to each other. This means that accurate Ks, calculated from topometric data, can be factored into the treatment design to adjust for the cosine effect without any manual data entry.
The iDesign aberrometer now builds in a correction for chromatic aberration (CA) from the laser light. In the past, physicians used nomogram adjustments to bring the wavefront refraction in line with the manifest. The built-in CA adjustment has brought the wavefront refraction closer to the true refraction and reduced the need for nomogram adjustments.
In a review of my past 50 eyes treated, there was a 48% reduction in nomogram adjustments needed to match the wavefront and manifest refractions.
We learned in the past that when patients had very small amounts of residual error, they were happier if that error was slightly on the hyperopic side rather than on the myopic side.
For this reason, many of us aimed for a refractive target of +0.25 D instead of 0.00 D. By simply correcting for infinity (instead of correcting for a 20-foot exam lane), the iDesign system builds in that slight boost, making it possible now to aim for superlative distance vision.
These changes to the algorithms mean that it is important for surgeons who start using the new aberrometry system to ensure that patients aren’t accommodating, and to avoid using previous nomogram adjustments until they have evaluated 50 cases or so without adjustments.