Article Date: 12/1/2004

Pre- and Post-op Diagnoses
Here's how wavefront technology reveals the causes of some previously confounding vision issues after corneal laser treatment and IOL implants.
By Louis J. Catania, O.D., F.A.A.O.

The uses of wavefront aberrometry for corneal laser vision correction are widely known. They include:

One unique feature of Marco's 3-D Wave is its ability to analyze the natural lens.

Why Distortion?

The Marco 3-D Wave is helping clinicians get to the bottom of difficult cases in the practice of Peter Van Hoven, O.D.

"Some patients who've had IOLs implanted experience distortion," he says. "They can't put their finger on it, but we can see the distortion with the 3-D Wave postoperatively."

A recent case was a classic example. "A pseudophakic patient had been complaining of nonspecific visual distortion in her right eye for several months," Dr. Van Hoven says. "Her best corrected visual acuity was 20/25­3, and the eye was otherwise healthy. The Internal OPD of the 3-D Wave revealed torsion on the IOL secondary to capsular fibrosis. The patient returned to her surgeon with a copy of the Internal OPD. The problem was fixed and the patient is now happy with her vision." 

Corneal vs. lens aberrations

If we take the total root mean square (RMS) value of the eye and subtract topography elevation's RMS value, we're left with the internal power (mostly from the lens) of the eye. And now we're using instruments to identify lens aberrations. This is an essential measurement for any patient who's thinking about LASIK and has a total RMS greater than 0.4 or 0.5. If a significant portion of that RMS total is coming from the lens, LASIK -- even custom LASIK -- will leave the patient with clinically significant higher-order aberrations.

The 3-D Wave differentiates corneal from lens aberrations, automatically calculating even complex cases where a cylinder and axis require vector analysis for accuracy. It gives us a final "blended" cylinder that we're looking at on that lens, offering results in diopters as well as RMS value.

The 3-D Wave also can detect early lens changes before they can be seen as cataracts with a slit lamp.

Managing IOLs and cataracts

The 3-D Wave assesses cataracts and clear lens symptoms, as well as pre- and postoperative capsular opacification. We implant a lot of multifocal IOLs in our practice. Whenever we start getting capsular fibrosis in the periphery -- even though we can't see it with a slit lamp -- the patient is saying, "My near vision is going."

What's happening is that peripheral fibrosis of the capsule causes the silicone or acrylic material to distort or bend. We don't see that with the slit lamp, of course, but the patient is losing vision. A monofocal or multifocal patient can lose two or three lines of distance and near vision. But after YAG laser capsulotomy, the lens reestablishes its optical integrity again. It's just another part of the post-care management of these patients.

Decentrations and other IOL issues present challenges, as well. If I move the corneal crosshair on the topography map a millimeter or two and it shows no power variation, while the corresponding registered crosshair on the lens map shows a 3.00D shift, there's a problem. The lens is either tilted or decentered. This feature of the 3-D Wave allows us to measure what's happening internally.

 

The 3-D Wave differentiates corneal from lens aberrations and calculates even complex cases requiring vector analysis.

 

Daily applications

Sometimes, refractive surgeons cannot achieve the best correction on the cornea, so we're seeing more lens surgeries. Once the lens is replaced, there is greater potential to begin to correct higher-order aberrations.


The 3-D Wave can detect decentered or tilted IOLs.

Intraocular lenses are made from several different materials, such as silicone and acrylic. New synthetic lens materials can be made with anterior prolate surfaces. With just a prolate shape, an IOL can reduce a lot of spherical aberration. Then, we can also begin to build higher-order aberration correction onto that surface as well.

For a practice that does a lot of post-LASIK care or cataract and clear lens work, the 3-D Wave is a useful device. It enables us to diagnose complex, previously undetected problems before surgery. We also can determine the causes of vision problems after surgery and help patients reach a higher level of satisfaction. The value of the 3-D Wave in this area is growing.

 

 

 

How Do We Detect Aberrations on the Lens?

There are three different ways to "see" aberrations:

  • Zernike coefficients, where perfect is flat
  • Point-spread function, where perfect is a point of light on a black background
  • Retinal image from the Snellen chart.

Patients often can't relate to our questions regarding the Zernike coefficients. (Root mean square values are mathematically converted to two-dimensional color maps and three-dimensional models of a specific aberration.) But when we talk about aberrations -- coma, spherical aberration, trefoil and the like -- the point-spread function is something patients can describe. They might say, "I see a little tail of light coming off of it" or "I see a double image."

However, the three measurements are interconnected. All three are needed for a clear picture. For example, a patient with a perfect point-spread function and image might show a "cornflake" effect with 0.2 on the Zernike. At 0.4 or greater, patients begin to observe the aberrations. For example, they'll tell you, "I see double -- two overlapping images." This is vertical coma, and the patient can detect the distortion.

As the RMS values increase, the patient's perception becomes more dramatic. Not only do letters begin to blur, but with spherical aberrations, contrast sensitivity degrades. And most of this problem is coming from the lens. Most third-order problems like coma and trefoil come from the cornea, but most fourth-order problems like spherical aberrations come from the lens.1 Therefore, the lens becomes critically important.

When we wait for the lens to become a cataract, we're subjecting patients to vision degradation from the time they're 30 or 40 years old until they actually start losing vision from the cataract. The Marco 3-D Wave plays an important role in detecting these problems. We can measure the second-order Zernikes with a traditional phoropter or autorefractor, but only wavefront technology can measure third, fourth and higher-order aberrations.

1. Sachdev N, Ormonde SE, Sherwin T, McGhee CN. Higher-order aberrations of lenticular opacities. J Cataract Refract Surg. 2004;30:1642-1648.


The 3-D Wave displays Zernike coefficients, point spread function and RMS values, offering a total of 13 mapping options.

 

 



Optometric Management, Issue: December 2004