March 2019


Refractive editor’s corner of the world
IOL power choice in advanced cataract surgery

by Michelle Stephenson EyeWorld Contributing Writer

Intraoperative aberrometry used to refine the IOL power
Source: John Berdahl, MD


Thanks to the implant, cataract surgery is the most powerful form of refractive surgery. Whether a patient is comfortable wearing glasses after cataract surgery or desires spectacle independence, modern day implant power formulas have made cataract surgeons more accurate. There are variables, though, that despite our best efforts, limit our refractive accuracy. Advanced formulas have helped tremendously. Understanding how these newer formulas calculate lens power can help cataract surgeons increase accuracy significantly.
Knowing how a cataract surgeon can take advantage of a formula for a particular situation can help our accuracy even more. In this column, my partner John Berdahl, MD, and colleagues Douglas Koch, MD, and Bryan Lee, MD, share their wisdom on how to achieve a fundamental goal of cataract surgery: optimization of the refractive outcome to achieve a patient’s visual hopes and expectations. Thank you to Drs. Berdahl, Koch, and Lee for expanding our knowledge in the advancing field of intraocular lens implant calculation formulas.

Vance Thompson, MD,
Refractive editor

Technologies, formulas surgeons are using to calculate IOL power

According to John Berdahl, MD, Sioux Falls, South Dakota, now that immersion A-scans are providing consistent axial lengths and biometry has transitioned to providing more accurate Ks and measuring anterior chamber depth or lens thickness, surgeons are including topography and retinal thickness when evaluating potential lenses. “We get our K values from the biometer, but we also get a topography to make sure that is regular because our biometry does not give us topography. This is helping people get the right lens in the eye,” Dr. Berdahl said. Dr. Berdahl uses the LENSTAR biometer (Haag-Streit, Koniz, Switzerland), but said that the IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) does a great job, too. “Additionally, new swept source OCTs that are coming out to do optical biometry will be great as well,” he said.
Douglas Koch, MD, Houston, agreed. “I use both the IOLMaster 700 and LENSTAR. I prefer using two because I sometimes find disparities in the two measurements, and those can give me a clue to go back and reevaluate the ocular surface or remeasure. In terms of in-the-clinic use, our team prefers the IOLMaster 700 because it’s easy and fast to use, and it’s robust about getting good axial length measurements through dense cataracts. I also like the fact that it has what’s called telecentric keratometry, which means that it measures corneal curvature relatively independent of the distance from the device to the cornea. That seems to give a bit of a boost to the accuracy. A nice advantage of the LENSTAR is that is has the Hill-RBF formula, which is one of our favorites,” he said.


Dr. Berdahl’s go-to formula is the Hill-RBF. “I use that in every situation that is within bounds. If it’s out of bounds, I use the Barrett Universal formula. Those two formulas usually line up well. In extreme cases, I’ll use aberrometry as well, and in post-refractive situations, I use the ASCRS Post-Refractive Calculator,” he said.
Dr. Koch uses the Barrett, the Hill-RBF, and the Holladay 1 for all cases. In long eyes, he uses the Holladay 1 with a Wang-Koch axial length modification. “I still find this to be my most accurate way to calculate IOL power in axial myopes over 26 mm. In short eyes, I add the Holladay 2, which I think is helpful. Although in short eyes, we have found that none of the formulas are as accurate as we’d hoped, so I sometimes will try to do an average of them in order to get the best result,” he said.
Bryan Lee, MD, JD, Altos Eye Physicians, Los Altos, California, is using Hoffer Q, SRK/T, Holladay 1, Barrett, and Haigis for every patient. He often uses Holladay 2 as well.

Making adjustments

Although surgeons are still analyzing their outcomes data and making adjustments based on outcomes, they make fewer adjustments than they have in the past. “This is because we’ve made them along the way,” Dr. Berdahl said. “We don’t make adjustments to our A-constants frequently because we’ve done all of that work along the way. If we make adjustments to the lens at the time of surgery, it’s primarily because of aberrometry.”
For a starting point, Dr. Lee uses the User Group for Laser Interference Biometry (ULIB), which is an online database of other surgeons’ outcomes that helps to generate A-constants that are more accurate than the manufacturers’ lens constant. “At the same time, it’s still important to do your own analysis of your outcomes because the individual surgeon may be different from the pool of the data. It is important to analyze your outcomes because, for instance, the ZCB00 IOL [Johnson & Johnson Vision, Santa Ana, California] is listed as having an IOL constant of 119.3, but for me, it’s more like 119.5. That ends up making a difference cumulatively. It’s important to track your outcomes,” he said.

Intraoperative aberrometry

Dr. Berdahl only uses intraoperative aberrometry in two situations: for patients who desire spectacle independence and in extreme situations, such as those eyes that are not in the middle of the bell curve and for which additional data points are needed to improve the outcome.
Dr. Berdahl noted that aberrometry is important in his decision of which implant power to use. “There are three scenarios: virgin eyes, those requiring toric lenses, and post-refractive eyes. With the advent of the Hill-RBF, we are rarely changing the spherical power in a virgin eye. It’s probably less than 5% of the time now. When implanting toric lenses, I used to change the power or the orientation about 50% of the time, and it’s about 33% of the time now because of Barrett’s good work on his astigmatism toric planner. On post-refractive eyes, I’m changing it 25–30% of the time. I think that aberrometry still plays a critical role in decreasing enhancement rates for us, especially in toric patients,” he said.
Dr. Koch added that his practice looked in-depth at their data and found that using aberrometry did not help them. “In fact, had we not used aberrometry, we would have had slightly better results. However, even though it doesn’t make me more accurate, aberrometry sometimes helps me identify an outlier. In those situations, I find that if I use aberrometry and nudge the IOL calculation in that direction, it can be helpful. That said, some surgeons find aberrometry to be invaluable,” he said.
Dr. Koch thinks that aberrometry might gradually be replaced. “Our preoperative measurements will continue to improve, as will the formulas, and there will always be the limitation of aberrometry that the eye is modified by the time you’re doing those measurements. The cornea’s been doused with all kinds of drops and fluid, and there may be some edema,” he said.

The future

Dr. Koch is interested to see how much further surgeons can go with their accuracy. “The current stumbling blocks for the best calculations are effective lens position and corneal power. There are new devices for measuring posterior corneal power, but that’s still evolving and not as accurate as we might want. Considering all the sources of error, we are going to be lucky if we hit 90% within 0.5 D. In terms of new technology, I’m intrigued by the concept of looking at intraoperative OCT and using that to help modify the formula for effective lens position, but that’s an expensive and time-consuming way to do calculations. I would hope that we could find some equivalent of that in the preoperative measurement as well. However, even if that comes to fruition, the IOL position shifts in some eyes postoperatively,” he said.

Editors’ note: Dr. Berdahl has financial interests with Alcon (Fort Worth, Texas). Dr. Koch has financial interests with Alcon, Carl Zeiss Meditec, Johnson & Johnson Vision, and Perfect Lens (Irvine, California). Dr. Lee has no financial interests related to his comments.

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