EyeNet Magazine


 
Quest for the Best Vision
By Lori Baker Schena

 
 

From new IOLs to applications beyond refractive surgery, wavefront isn’t just for custom ablation anymore. An overview.

Blame it on the Baby Boomers. These trailblazers who so enthusiastically embraced refractive surgery during the past decade are now demanding that their IOLs also be a cut above those of their parents’ generation. And this pursuit of the best vision possible through the most advanced technology available is one of the driving forces behind the trend to expand the applications of wavefront science.

More Than Custom Ablation
Wavefront technology has changed the nature of refractive surgery, allowing surgeons to perform customized ablations that, in theory, enhance the quality of vision achieved. While this technology has been officially available for only a little more than two years, its potential has already caught the attention of researchers and manufacturers.

For instance, three new IOLs—all designed to reduce spherical aberrations along the same theory as wavefront technology—are now on the market. Traditional eye examinations and current spectacle and contact lens technology are being rethought. And wavefront aberrometry is being applied in areas other than refractive surgery.

Scott M. MacRae, MD, attributes this enthusiasm for wavefront technology to the ongoing quest for “super vision.”

“The advent of wavefront has caused a revolution in thought,” he said. “I am very impressed with the new strategies currently being developed based on wavefront technology—strategies that I hadn’t even envisioned. It’s all pretty exciting.”

Innovation: Wavefront Eye Exam
When Daniel S. Durrie, MD, first began performing refractive surgery techniques, approximately 3 percent to 5 percent of his patients experienced quality of vision problems. While his patients commented on these problems, the traditional phoropter could not detect the error.

“Once we had a wavefront device capable of measuring higher-order aberrations, we could finally detect problems, make enhancements and improve their vision,” Dr. Durrie said. “In the last few years, I have upgraded 200 eyes with this technology, correcting the higher-order aberrations and thus improving the quality of vision.

“The phoropter only measures sphere and cylinder, and also makes an assumption that everyone’s cylinder is symmetric,” Dr. Durrie explained. “Yet when we study people’s eyes, we discover that the refractive error is different, and most astigmatism is a form of coma. We have oversimplified the refractive error in eyes for years. Now that we are using wavefront routinely in advanced vision practices, we are able to better understand why patients have certain conditions such as astigmatism.”

Dr. Durrie argues that this ability to detect higher-order aberrations will spell the end for the ophthalmologist’s traditional diagnostic tools. In fact, he predicted that in 25 years, there will be a wavefront aberrometer in every ophthalmologist’s office. The question, “Which looks better, 1 … or 2?” will be a “turn-of-the-century” axiom and the phoropter will go the way of the eight-track player.

And the future is now in Dr. Durrie’s practice, where most of his patients undergo an “advanced vision exam” that involves wavefront aberrometry, topography studies and tear film analysis. He has even reconfigured his office to incorporate Nidek’s “all in one” OPD scan/wavefront analyzer/ corneal topography and autorefractor and keratometer device into the regular examination room. “This device has made it much easier for the patient to undergo multiple tests in the same floor space,” he said. “Eye exams will never be the same.”

Inovation: Spectacles and Contact Lenses
Despite its prominent role in refractive surgery, wavefront has not bypassed spectacles and contact lenses. Ophthonix offers the Z-View Aberrometer, which, the company says, is the first wavefront aberrometer specifically designed to meet the needs of the “dispensing eye care practitioner.”

The device measures third- to sixth-order aberrations as well as sphere, cylinder and axis to quickly provide a fully optimized prescription. The company asserts that the machine provides objective wavefront measurements via through-the-lens optics, “virtually eliminating instrument myopia.”

The company also offers the iZon Lens, a customized spectacle lens that matches “the optical fingerprint of the eye.” With this system, the patient’s prescription is taken by the company’s aberrometer and transmitted to the Ophthonix programmer, which writes the prescription onto the lens in a process that is analogous to burning information on a CD. The end result is a lens that optimizes the correction of the measured aberrations of the patient’s eye.

“Large contact lens companies and small design houses are also working on wavefront-guided contact lenses that will create a lens based on wavefront error rather than the standard parameters,” Dr. Durrie noted.

One substantial obstacle facing wavefront spectacles and contacts is decentration. “The distortion seen by a high myope when gazing anywhere other than the optical center of a conventional spectacle is bad enough; the problem may be heightened with wavefront spectacles,” said Jonathan M. Davidorf, MD. “Worse yet, what can we expect after the new wavefront spectacles have been inadvertently used as a seat cushion or pillow and no longer rest properly on the patient’s face? The patients with the most to gain—highest wavefront error—will also be the ones with the most to lose.”

Innovation: New IOLs
As for IOLs, “manufacturers have recognized the advantage of making a lens that corrects for the positive spherical lens of the cornea,” noted Samuel Masket, MD.

While some in the field refer to these new IOLs as “wavefront-adapted,” Dr. Masket warned that this term can be misleading. “While it is true that wavefront analysis makes it obvious that spherical aberration [a higher-order optical aberration] may be induced after cataract surgery, and we can prevent it with an aspheric IOL, my sense that the ‘wavefront’ term is used with these IOLs because it has market appeal.

“However, it must be recognized that these IOLs aren’t customized for an individual based on his or her wavefront analysis [as can be the case with customized wavefront LASIK], and they are not specifically designed for each patient,” Dr. Masket added. “The IOL manufacturers have cleverly taken the information that we have garnered from wavefront laser treatment with regard to induced spherical aberration and have manufactured aspheric IOLs to overcome the tendency for increased spherical aberration with conventional IOLs after cataract surgery.”

Whether these “wavefront” IOLs genuinely improve vision or are simply used as a marketing ploy—and the jury is still out—many believe that they will eventually replace the current lenses on the market. “My sense is that within the next few years, most IOLs will be aspheric in their design,” Dr. Masket said. “In my view, there is good science to the hype, and more products will follow this trend. However, a certain small percentage of cases, based on corneal topography, will be disadvantaged with aspheric IOLs.”

Here’s a rundown of the wavefront-adapted IOLs:

1. Tecnis IOL. The trend began with the Tecnis IOL, originally designed by Pharmacia and now distributed by Advanced Medical Optics. According to Dr. Masket, the lens, which has been on the market for two years, has a three-piece silicone optic that was modified from the original model to correct for spherical aberration. “There have been peer-reviewed literature reports of the advantages of the Tecnis IOL with regard to improved contrast sensitivity with the aspheric optic, which has also been noted to yield improved vision for nighttime driving,” he said.

The manufacturer has been careful to point out that this is the first FDA-approved IOL lens to “significantly reduce spherical aberrations for improved functional vision”—a claim supported by clinical trials using wavefront analysis, hence another connection with wavefront verbiage.

AMO explained the rationale for the lens by noting that in the youthful eye, negative spherical aberration of the lens compensates for positive spherical aberration of the cornea. However, in the aging eye, the lens gradually grows and changes its shape, losing its ability to compensate for the cornea, and functional vision declines. The Tecnis IOL was designed to address this problem.

Richard Lindstrom, MD, explained that the modified prolate aspheric shape of the lens is designed to counteract the typical average spherical aberration of the cornea. However, he added, one of the significant concerns with the lens is the problem with decentration, which can induce coma. A decentration of the Tecnis lens by as much as 0.4 mm or 7 degrees of tilt might induce new aberrations. “This is significant,” said Dr. Lindstrom, “because when you review the literature and look at the average decentration of IOLs, approximately 10 percent of patients will experience decentration, and with this lens, the results could be a degradation of the vision—particularly in the larger pupil.”

Added Dr. MacRae, “Studies have shown that if you displace the lens by 300 micrometers, you achieve only 50 percent of the benefit associated with wavefront-adapted IOLs.” This is one of the major limitations of wavefront-guided IOLs.

2. Acrysof HOA Model SN60WF. Dr. Davidorf started implanting wavefront-adapted IOLs as soon as they were available because he “believed in the science behind them.”

He admitted that while in practice the experience with the lenses has been positive, “it is difficult to determine the specific advantage for any given patient. We are implanting them in cataract patients, who would be happy with any lens on the market, so the incremental improvement in the clinical setting may be difficult to ascertain. However, as we implant these in larger numbers of patients, we will start to better understand their advantages.”

Dr. Davidorf explained that the Tecnis lens decreases the spherical aberration by increasing the thickness of the peripheral portion of the IOL. In contrast, Alcon’s Acrysof HOA Model SN60WF, another wavefront-adapted lens, compensates for spherical aberration by addressing over-refraction at the periphery with no increase in edge thickness. In fact, it has the same thin lens profile as the AcrySof Single-Piece IOL.

“By decreasing the posterior curvature, the marginal light rays in the periphery are focused on the retina,” Dr. Davidorf said. “In essence, it is like an aspheric surface on the posterior curvature. The marginal and para-axial rays are focused on the proper spot on the retina, and the image quality improves, minimizing spherical aberration.”

Another difference between the two lenses is the “natural lens” concept of the AcrySof lens, which approximates the light transmission of a 25-year-old human crystalline lens. The company claims that the Natural IOL filters high-energy blue light, providing a protective effect to retinal pigment epithelium cells.

“Not only may there be some advantages from a safety standpoint with this lens, which is yellow in color, but there also may be other advantages from a visual quality standpoint,” Dr. Davidorf pointed out. Studies involving Modulation Transfer Function (MTF), which is a highly accurate optical measurement designed to determine the lens system’s ability to generate quality vision (the higher the contrast, the better the image generated by the lens), have shown that the Acrysof HOA Model SN60WF had a 50 percent increase in the MTF value in the higher spatial frequencies under mesopic conditions compared to the Tecnis.

Another advantage, noted Dr. Davidorf, is that the design of the single piece Acrysof IOL provides steady axial position, which is considered a major parameter influencing postoperative refractive change.

3. SofPort Lens System. Bausch & Lomb introduced its entry into the wavefront-adapted IOL market in October at the Joint Meeting in New Orleans. Unlike the other two aspheric lenses on the market, the SofPort Advanced Optics Aspheric Lens System is designed to be aberration free, which leaves a small amount of residual spherical aberration due to the cornea.

Bausch & Lomb maintains that this residual spherical aberration can enhance vision in the pseudophakic eye with increased depth of field. Additionally, optical performance is unaffected by pupil size or location of the optic due to uniform center-to-edge power.

“The Bausch & Lomb concept is more forgiving of decentration of tilt and in my mind is a safer lens to use because you won’t have degradation of the vision. This is my preferred lens for the typical cataract patient, although there are select cases where the Tecnis lens makes sense—for example, for those patients who drive a lot at night,” Dr. Lindstrom said.

He added, “The good news is that we now have lots of choices for our patients. Whether we want a lens with posi- tive spherical aberration, negative spherical aberration or no spherical aberration, we need to pay close attention to which lens would benefit the patient most. Surgeons need to understand the pluses and minuses of each.”

Innovation: Adjustable IOL
Even newer is the Calhoun Laser Adjustable Lens (Calhoun Vision Inc.). “This IOL can theoretically adjust the wavefront error after it is implanted in the eye by the radiation of a specific frequency of blue light on a silicone-based material with monomers that can migrate within the lens according to differences in the concentration gradient,” said Dr. MacRae.

“With this technology, higher-order aberrations can be customized postinsertion by placement of the polymerizing light to the specific areas required for customization to improve wavefront error,” he added.

Innovation: Diagnostic Role in Cataract Surgery
And in another innovative twist, wavefront aberrometry is now being used to document vision problems from early stage cataracts. 

This potential use for wavefront technology was reported by a New Zealand research group that demonstrated—through wavefront aberrometry—that coma and tetrafoil were higher in eyes with cortical cataract, and spherical aberration and tetrafoil were increased in nuclear cataract.1 The findings suggest that wavefront could help give ophthalmologists an objective means to confirm the need for surgery in patients with early opacities.

Dr. Durrie cited an example from his own practice, with a patient who was experiencing problems with her vision. With his traditional tools, he had a difficult time determining whether an early cataract was causing the problems. However, wavefront analysis revealed higher-order spherical aberrations, which indicated that her lens was the culprit. Thus, she was a candidate for cataract surgery, although it was difficult to tell from a routine exam.

Future Innovations: Looking Ahead

“The field has changed so dramatically it has been difficult to keep up,” Dr. MacRae noted. In his first book on wavefront technology, he and his coauthors focused on customized corneal ablation. “But with the wavefront IOLs, wavefront contact lenses and other devices coming down the road, we had to change focus.2 Without question, wavefront is the biggest revolution in ophthalmic optics in the last 200 years.”

For Dr. Masket, wavefront technology is positioned to become an established part of the field. “The market will be there and the science will be there,” he said. “The science is logical—and the market will demand it.”

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1 Sachdev, N. et al. J Cataract Refract Surg 2004;30(8):1642–1648.
2 Wavefront Customized Visual Corrections: The Quest for Super Vision II (Thorofare, N.J.: Slack Publications, 2004).

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Wavefront IOLs

Wavefront IOLsAMO
Technis Z9000 and Z9001
6-mm optic; overall length of 12 mm (Z9000) and 13 mm (Z9001). Available in +5 D to +10 D in 1-D increments and +10.5 D to +30 D in 0.5-D increments. Three-piece foldable, square edge.
Alcon
Acrysof  HOA SN60WF
6-mm optic. Available in +16 D to +26 D in 0.5-D increments (range to be expanded). Single-piece, aspheric, blue-light filtering.

Bausch & Lomb
SofPort AO

6-mm asymmetrical bioconvex optic and 13 mm overall. Available in 0 D to +4 D in whole diopters and in +5 D to +30 D in 0.5-D increments. Multipiece, aspheric, modified “C” loop, 5-degree angulation. Square edge.
Calhoun Vision
Laser Adjustable Lens 300
6-mm optic and 13 mm overall. Modified “C” loop, 10-degree angulation. Biconvex, square edge.
Note: Chart information provided by the companies.

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Meet the Experts

Jonathan M. Davidorf, MD Director of the Davidorf Eye Group in West Hills, Calif., and assistant clinical professor of ophthalmology at the University of California, Los Angeles. Financial interests: Has consulted for Alcon.

Daniel S. Durrie, MD Director of refractive surgery at Durrie Vision in Overland Park, Kan. Financial interests: Is a consultant for Nidek but does not have any financial interests in the “all in one” device.

Richard Lindstrom, MD Founder and managing partner of Minnesota Eye Consultants and adjunct professor emeritus of ophthalmology at the University of Minnesota. Financial interests: Is a consultant for AMO and Bausch & Lomb.

Scott M. MacRae, MD Professor of ophthalmology and visual science at the University of Rochester. Financial interests: Is a consultant for Bausch & Lomb.

Samuel Masket, MD Clinical professor of ophthalmology at the University of California, Los Angeles, and in private practice at Advanced Vision Care in Century City. Financial interests: Is a consultant for Alcon and AMO.

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