Precision Biometry

Here's what you need to know to succeed as a refractive cataract surgeon.

BY FARRELL C. TYSON II, MD, FACS

I practice in southwest Florida where the population primarily consists of Medicare patients, and the environment is highly competitive. It's interesting to note that even though we're in a blue-collar area and our cataract surgery volume recently has dropped, the number of premium IOLs we're implanting actually has increased. Patients still want better optics and a better lifestyle in spite of the current financial crisis.

To provide patients with their best possible vision, I make use of the entire range of IOL options, which has expanded greatly over the years. Achieving good results was relatively easy when we were dealing with only standard monofocal lenses. We just needed the postoperative refraction to be in the ballpark. Now, we have aspheric lenses and the ability to tailor correction of spherical aberration. We have multifocals, which means we're not only targeting good distance vision but near as well. If we're off a foot or two at 20 feet, it can really move the near point in or out, which can make or break the results. It can be the difference between a happy patient and an unhappy patient. We also have toric lenses, for which each company is using its own calculation formula. Therefore, if we don't input the correct values right from the start, we could have more IOL surprises with the toric lenses than we would with standard monofocals.

All of the new lenses have different technical features that we need to take into account. So to hit our target outcomes, we need to think like refractive surgeons. Our mindset must be that good is not good enough. We need to utilize all of the available tools in pursuit of perfection. Each surgical plan should be customized for the patient's personality type, his lifestyle and his eyes.

To customize appropriately, we have to perfect the variables we can control. Otherwise, the new lenses put us on a path to failure and frustration. Chief among the variables under our control are keratometry, biometry and IOL calculations.

Aim for Reproducible Keratometry

Primarily, keratometry drives our IOL calculations. Measuring K values accurately is nice, but since the ultimate goal is to optimize, it's best to use a method that's reproducible. Avoid using corneal topography values for some patients, keratometer values for others and IOL Master values for others. Use the same method so it's reproducible. For example, IOL Master Ks may not always be accurate, but they're reproducible. Therefore, as long as you optimize those Ks, you'll hit your target every time.

It's important to understand how a keratometer works. It measures the corneal curvature over a 3-mm ring, not over an area. Therefore, if the eye has symmetrical astigmatism, the instrument is finding a maximum and a minimum point. But if the eye has asymmetrical astigmatism, the instrument doesn't know. It attempts to find an average and produces a "best fit," which isn't necessarily the correct maximum and minimum points. Fortunately, most of the IOL formulas are based on an average.

It's also important to realize that all brands of keratometers aren't the same. They use different corneal refractive indices. When I began practicing here, we had an IOL Master just sitting in the corner, because its K values didn't match the ones being measured with our Haag Streit keratometer. That was because the IOL Master had a different corneal index of refraction in its setup menu. You need to make sure you're measuring apples to apples and oranges to oranges. Most corneal topography units also allow you to set the corneal refractive index to match the keratometers in your practice.

Use Latest Technology for Axial Length Measurement

Axial length is the other major variable used in IOL formulas. Because of measurement variability, you shouldn't use contact biometry to determine axial length. Instead, you should use immersion ultrasound, which is the gold standard, or the IOL Master.

Immersion ultrasound, which I perform with the Accutome unit, can be performed quickly, especially if you use an immersion shell. The Accutome unit provides good anterior chamber depth (ACD) measurements. ACD is being used more often in the newer IOL calculation formulas, such as the Haigis and the Holladay II. I've encountered a few IOL surprises when I've used the IOL Master to measure ACD for the Haigis formula.

When used with the newer formulas, a reliable ACD measurement adds accuracy for eyes that fall outside normal parameters. We need increased accuracy for eyes that have undergone corneal refractive procedures, for example. The Accutome unit has two different built-in formulas for postrefractive patients. It also has multiple formulas with optimization built in. Another feature of the Accutome unit is portability, which provides a costeffective way to use the technology in multiple satellite offices.

The other option for biometry is the IOL Master, which utilizes laser interferometry. It goes through an algorithm to produce an axial length number equivalent to immersion A-scan. The algorithms recently have been upgraded, and the unit now uses a higher intensity bulb to enable readings through denser cataracts. It's supposed to be effective approximately 95% of the time. In my mind, that means that 5% of the time it doesn't work, especially in the presence of posterior subcapsular cataracts. So in those cases, it's necessary to use the ultrasound.

The Hoffer, Holladay I, SRKT and Haigis IOL formulas are built into the IOL Master.

Optimize Your Optimization

We know we can use different third-generation IOL power calculation formulas for eyes of different sizes. In a standard population, the Hoffer Q works well for small eyes. The Holladay I is good for intermediate-normal eyes, and the SRKT is best for larger eyes.

With today's computer technology, you can quickly optimize the third-generation formulas for your own patient population. Most immersion ultrasound systems and the IOL Master allow optimization after approximately 25 cases. However, simple optimization brings any of these three formulas to your population average. There is a regression to the mean. So how do you take this into account? Ideally, you should optimize separately for the different axial length groups.

You do that by optimizing a group of medium axial length patients for the Holladay I, a group of small axial length patients for the Hoffer Q, and a group of large axial length patients for the SRKT (Figures 1-3). This allows you and your staff to choose a formula based on axial length that's much more accurate at the extremes.

Figure 1. Unoptimized formulas for IOL calculations provide accuracy across a limited range of axial lengths.

Figure 2. Simple optimization of IOL calculation formulas results in a regression to the mean. Accuracy is lost in the extremes of axial length.

Figure 3. Multiple optimizations, based on axial length, increase accuracy across a larger axial length range.

Alternatively, you can use a fourth-generation formula, the Haigis or the Holladay II. The goal of the fourth-generation formulas is to expand the third-generation ranges to be more accurate and make it possible to use one formula for every patient. While the use of the third-generation formulas is necessary to achieve good results, the fourth-generation formulas are preferable. Keep in mind that the Holladay II requires seven points of information, which means there are seven chances for input error. Also be aware that the Haigis acts as a third-generation formula in most of the optimizations that are built into the computer, because it's only optimizing one of the three surgeon constants. To increase accuracy by optimizing all three surgeon constants, you have to run 100-150 eyes through a spreadsheet.

No Room for Refractive Surprises

Precise keratometry and biometry and careful optimization of the IOL power calculation formulas are necessary components of refractive cataract surgery. No matter how good we are at surgery or how much hand-holding we provide, we can't achieve the results our patients seek if we don't get all three of these components right. ■

Dr. Tyson is a refractive cataract/glaucoma surgeon at the Cape Coral Eye Center in Florida.