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Range of options available for IOL calculations

Issue: July 25, 2014

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CEDARS Debates is a monthly feature in Ocular Surgery News. CEDARS — Cornea, External Disease, and Refractive Surgery Society — is a group of cornea, cataract and refractive surgery specialists, here to discuss some of the latest hot topics in ophthalmology.

With laser refractive surgery becoming so widespread over the past 20 years, there is now an ever-increasing number of post-refractive patients who require cataract surgery. IOL calculations are much less accurate with traditional methods after refractive surgery, yet post-refractive patients seem to have a particularly high demand for the best refractive result possible. How to calculate IOL power for these patients has become a major problem for cataract surgeons. This month, David Goldman, Parag Majmudar and Tal Raviv discuss various options for IOL calculations, including the ASCRS online calculator, OCT-based corneal measurements and intraoperative aberrometry. We hope you enjoy this discussion.

Kenneth A. Beckman, MD, FACS
OSN CEDARS Debates Editor

ASCRS IOL calculator

David A. Goldman

Some of the most challenging cataract surgery patients can be post-LASIK patients. This is not because the surgery itself is more challenging, but because the prediction of postop refractive outcome is less accurate. Furthermore, post-LASIK patients are used to spectacle freedom, and the concept of refractive error is often unacceptable to them. In addition, they often feel any post-cataract refractive procedure is because of a complication, regardless of any preoperative discussion.

While there are several methods to improve postoperative refractive accuracy, my personal favorite is the Post-Refractive IOL Calculator available on the American Society of Cataract and Refractive Surgery website. Designed by Warren Hill, Li Wang and Douglas Koch, the site has been optimized to handle patients with prior myopic LASIK/PRK, hyperopic LASIK/PRK or RK. It has also evolved to allow input from multiple topography/optical biometry devices.

One of the beautiful features of this online software is that you can input as much data as you have and it will work with what is provided. More information will yield more results, but oftentimes information such as pre-LASIK refractions are unavailable. Once all data forms are entered, IOL powers are presented from different methods, including clinical history, Feiz-Mannis, corneal bypass, adjusted EffRP, adjusted Atlas 0-3, Masket formula, modified Masket, Wang-Koch-Maloney, Shammas, Haigis-L and Galilei.

While intraoperative aberrometry devices give instant feedback to the operating surgeon, it is important to remember that aberrometry is measuring dynamic variables. That is to say, during cataract surgery, the values of effective lens position, anterior chamber depth and corneal power are continuously changing and may not be at an ideal steady state when measurements are taken. We have all had the experience in which a fold is seen in the posterior capsule, and upon wound hydration, the fold disappears. Aberrometry may provide outstanding accuracy but can only do so when the intraoperative variables can be stabilized to truly match postoperative anatomy. The features measured for the ASCRS calculator are static and represent what postoperative measurements will resemble. Therefore, it can provide excellent reliability.

While sometimes the different formulas of the post-refractive IOL calculator yield large differences (particularly the historical methods), in my experience this is a small occurrence. In my hands, using data from the Zeiss Atlas 9000 and Haag-Streit Lenstar, the outcomes are within 0.5 D more than 90% of the time. The software is easily accessible, free of charge and a welcome addition to the practice of ophthalmology.    

• David A. Goldman, MD, can be reached at Goldman Eye, 3401 PGA Blvd., Suite 440, Palm Beach Gardens, FL 33410; 561-630-7120; email: david@goldmaneye.com.
• Disclosure: Goldman has no relevant financial disclosures.

OCT-based corneal measurements

Parag A. Majmudar

In the late 1990s and early 2000s, millions of patients underwent excimer laser refractive surgery. In the coming years, it is expected that many of these patients will present for cataract surgery. We are well aware of the limitations of conventional corneal topography and standard IOL predictive formulae in achieving a reasonable degree of refractive accuracy after cataract surgery. This is due to the fact that the corneal curvature, index of refraction and, therefore, corneal power are altered after laser vision correction. Various modalities have been developed to try to achieve greater success in these patients, who typically are motivated to have “perfect” outcomes after cataract surgery.

A number of physicians and scientists have developed modifications in IOL calculation formulae. The Haigis-L formula appears to be one of the more accurate formulae and uses a correction curve to convert the measured corneal radius to derive an effective equivalent corneal power. However, there are no less than 10 to 12 formulae at the disposal of the ophthalmologist. As you can imagine, evaluating each of these can be a time-consuming process. With two of my fellows, Dianna Seldomridge and Dennis Goldsberry, I developed a spreadsheet in 2004 that enabled us to enter the patient information once, and all known formulae at that time were automatically analyzed and an IOL power determined. The American Society of Cataract and Refractive Surgery soon thereafter developed a fantastic calculator on its website, which is free for all to use. It has proven to be immensely valuable. However, it still requires knowledge of pre-LASIK measurements, and at times this is not readily available. It also requires some data entry, which can be time consuming.

Intraoperative aberrometry is poised to make a big splash. There are now several companies already in the market or developing a product for general use. However, the downside to this technology is that it is not readily available everywhere, and in its infancy, it is cumbersome to use. The measurements must be taken after lens removal and before implantation of the IOL, and current devices require several steps to ensure reliable readings, which adds to the OR time. In addition, the results, while better than guessing, are still subject to some variability.

The original concept was to measure the power of the cornea and select an IOL based on that value. When LASIK and PRK altered the power of the cornea, conventional topographers were not able to accurately perform this task. Enter optical coherence tomography. Most ophthalmologists are familiar with this technology because they use it in their clinical practices nearly daily. At the moment, I am aware of one system that uses OCT to calculate the true power of the cornea, and that is the RTVue system from Optovue. OCT has the optimal spatial resolution and range to map the cornea at 5 µm, compared with the Pentacam (Oculus) and Galilei (Ziemer) devices, which have a resolution of 15 µm. Given a longer scan time with the Pentacam, this difference in resolution means that if there is a fixation loss of 1 mm, there can be a greater chance of measurement error. In fact, Pentacam measurements were removed from the ASCRS calculator due to variability.

In a study published in 2010, Tang et al described the power calculation formula based on OCT. They found that in normal eyes, the repeatability for RTVue corneal power is high, with a small degree of variability. Baikoff et al presented data at the European Society of Cataract and Refractive Surgeons summer meeting in 2011 that showed excellent repeatability of the OCT in determining corneal power in patients who had undergone LASIK or PRK. At the Cullen Eye Institute, a study of 24 eyes showed that the mean absolute error when using the Haigis-L to calculate post-LASIK IOL power was 0.63 D. In contrast, using OCT, the mean absolute error was 0.52 D. Furthermore, seven of 12 eyes in each group were within 0.5 D of the predicted postoperative refraction.

The group at Doheny Eye Institute compared contact lens over-refraction, clinical history, Haigis-L (IOLMaster, Carl Zeiss Meditec), double-K Holladay 2 (Orbscan II, Bausch + Lomb), and OCT in 16 eyes and found that the mean absolute error was lowest in the OCT group (0.65 D), followed by Haigis-L (0.67 D). The highest mean absolute error was in the contact lens over-refraction group (1.59 D) and clinical history group (1.32 D).

I performed a study with 20 consecutive eyes that underwent cataract surgery and IOL implantation that had previously undergone LASIK. Seventeen eyes had undergone myopic LASIK, and three had undergone hyperopic LASIK. In this series, the mean absolute error for the OCT was 0.64 D vs. a mean absolute error of 0.72 D with the ASCRS calculator using the Haigis-L formula. I found that by using the OCT, 45% of eyes would have been within 0.5 D of predicted vs. only 30% with the ASCRS calculator. In addition, the percentage of eyes that would have been off by greater than 1 D was 15% using the OCT and 30% using the ASCRS calculator/Haigis-L.

What surgeons want is an easy, repeatable way of measuring corneal power, without crunching a thousand numbers, without interrupting the flow of surgery as with intraoperative aberrometry and by going back to the good old days of having a device that can reliably measure the important parameters in seconds. The OCT results compare favorably with the accepted gold standard, and it appears that this device might just fit that bill.    

Intraoperative aberrometry

Tal Raviv

We all face and appreciate the challenges of post-refractive IOL selection. Fortunately, there has been incremental progress on this front over the last decade, most recently with optical coherence tomography-based corneal measurement and intraoperative aberrometry. Historically, most of us have taken the everything-under-the-sun approach — using all available historical and actual measurements and applying all available formulas, which the American Society of Cataract and Refractive Surgery calculator has allowed us to do with ease. But most recently, attention has focused on the no-history formulas, such as the Haigis-L and Shammas, as well as topographically measured actual keratometry readings, whether by reflection or scanning topography.

Most of the post-LASIK ASCRS calculator formulas attempt to arrive at a corrected keratometry reading to be used in our standard third- and fourth-generation IOL formulas. The problem with that approach is that even in non-refractive IOL calculations, our ability to hit the refractive target within 0.5 D is still woefully poor (estimated at 40% to 60%) compared with our results with LASIK (more than 90% within 0.5 D). Early results with corneal OCT-based measurements appear to be promising due to the resolution of the OCT. Nonetheless, even with perfect keratometry readings, we are still faced with the unpredictability of the effective lens position and therefore flawed refractive results, until we can solve this problem.

ORA intraoperative aberrometry (WaveTec) allows us to measure and calculate an IOL power by a completely different method than our regression-based third- and fourth-generation formulas. By performing an aphakic refraction and applying the vergence formula (plus a proprietary regression adjustment for presumed effective lens position), ORA can attain highly accurate results without relying on correctly measuring the keratometry.

I use ORA on all my post-refractive cases and have been pleased with the improved accuracy. The literature has borne this out. Ianchulev found that intraoperative aberrometry was significantly more accurate in post-LASIK eyes than Haigis-L and Shammas, achieving a median absolute error of 0.35 D vs. 0.53 D for Haigis-L and 0.51 D for Shammas. Using aberrometry, 67% of post-LASIK eyes were within 0.5 D of what was predicted. Brint found similar results in 1,104 post-myopic LASIK eyes, with 67% of eyes within 0.5 D compared with 48% and 50% with the Haigis-L and Shammas, respectively.

Furthermore, it is not uncommon to have residual astigmatism after LASIK, and recent studies have shown the superiority of intraoperative aberrometry in reducing astigmatism with toric IOLs — another notch in ORA’s belt.

Nevertheless, all formulas and even ORA are still fallible. I therefore always use the ASCRS calculator to bring in my range of IOLs and fine-tune with ORA. In the rare cases in which ORA readings are not possible (for instance, due to povidone-iodine epitheliopathy or inadvertent intraoperative corneal abrasion), I fall back to the ASCRS calculator.

Fortunately, we are getting better with our post-LASIK IOL measurements, and as ORA improves — look at the progress made from the first ORange unit — we will hopefully continue this trend to afford our patients consistently superior outcomes.