The idea is amazing: use a high-tech device during cataract surgery to measure the eye and determine the perfect power lens implant to achieve excellent vision without glasses. The classic teaching is that the cataract prevents us from achieving the best biometric data at the time of the preoperative consultation. The hope is that an aphakic refraction, done in the operating room after the cataract is removed and before the lens implant is inserted, will give us a critical piece of data that will make the lens power calculation perfect.
Let me break your heart. It just doesn’t work that way, at least not now. I have used all of the widely available intraoperative aberrometers and I can tell you with certainty that my lens power calculations are more accurate than those from these devices. And I’m not alone. Remember that the lens power determination is a combination of the refractive vergence calculation and the determination of the final effective lens position (ELP) in the eye.
To understand the issue, let’s first look at the list of the most important parameters in the determination of the final lens implant power. The 2 most important ones are the axial length of the eye and the power of the cornea. Next (in order) would be the anterior chamber depth and perhaps the effect of the posterior cornea. Lower down on the list are other factors, such as the white-to-white measurement, the refraction (both phakic and aphakic), and variables such as lens thickness, patient age, and perhaps even patient sex or demographics.
The only measurement that the aberrometer is taking is the aphakic refraction, which of course relates to the axial length and corneal power, both anterior and posterior. We can and already do measure the axial length and the anterior and posterior cornea at our preoperative visit. So what further dimension does having the aphakic refraction provide?
It is telling that these intraoperative aberrometers are unable to give us a lens power unless we input a large amount of preoperative data. For comparison, the auto-refractor in my clinic will measure the refraction of an eye without me entering supporting data, such as previous prescription. And even the bathroom scale in my home doesn’t require me to enter my height, waist size, or previous weight. These devices just measure and give their values. Not so with these aberrometers, because we must still enter the axial length, the corneal power, the anterior chamber depth, and even the lens power that we calculated before coming to the operating room.
With toric lenses the aberrometers may fare well, particularly in the hands of surgeons who don’t routinely measure the posterior corneal power. The aphakic refraction will take both anterior and posterior corneal curvature into consideration and that could help. Note, however, that the axis of a small degree of astigmatism is much more difficult to determine than in the case with a large amount of astigmatism.
Adding the one extra bit of information, the aphakic refraction, is not enough to perfect the lens power calculation because this still gives us no further insight into the issue of the ELP. We cannot determine exactly where in the eye the lens implant will settle once the capsular bag has contracted a month after surgery. If you really trust the aberrometer, will you then explant the intraocular lens the moment when it tells you that the pseudophakic refraction is ametropic? I would not. As such, I ask myself if it even makes sense to measure the pseudophakic refraction on the operating room table.
I dream of the day that we have such an amazing aberrometer and biometer attached to our surgical microscopes that absolutely no preoperative measurements would be warranted. This magical device would measure the axial length, the anterior and posterior corneal powers, the anterior chamber depth, and it would even image the entire anterior segment. The built in real-time topographers would give a heads-up overlay and the software algorithms would nail down the exact ELP. This machine is coming, but it may take years.
In the meantime, the companies that make the intraoperative aberrometers are working on their own formulas, calculations, and databases to enhance their results, which makes their software more valuable than their hardware. In my own hands, I have found that it is certainly possible to achieve about 90% accuracy of target +/- 0.5 diopters, and that beats the accuracy of relying on an aberrometer.