Barriers to super-vision still remain

Beyond the hype, experts face the technological challenges that remain on the road to better vision.

From its inception a decade ago to its current commercial development, wavefront technology has brought excitement and anticipation to the refractive surgery community. But amid the sound and the fury of hype, experts note that there are limits to the capabilities of the first-generation clinical models of wavefront-guided ablation that are or soon will be available.

The idea that 20/10 super-vision is attainable for most patients with wavefront-guided ablation is “simplistic” and “cartoonish,” said George O. Waring III, MD.

“Although it is undeniable that wavefront technology is significantly improving the outcomes of excimer laser corneal surgery and the designs of IOLs and contact lenses, the details of how to apply aberrometry in clinical practice — especially for the correction of higher-order optical aberrations — are still being worked out,” Dr. Waring said in a recent issue of Journal of Refractive Surgery.

Dr. Waring is editor-in-chief of the Journal of Refractive Surgery, published by Slack Inc., the publisher of Ocular Surgery News.

Can excimer laser technology achieve an improvement in best corrected visual acuity for every patient?

“The answer is clear: not yet,” said Isaac Lipshitz, MD, of the Ophthalmic Health Care Center in Tel-Aviv, Israel, in an article in the same issue. “… even though excimer laser technology brought to our attention many new aspects in optics, it cannot bring us to the promised land of super-vision,” he said.

Dr. Waring cautioned surgeons against accepting the steady barrage of information implying that “wavefront analysis will lead us to the holy grail of super-vision for our patients, most seeing 20/10 without correction.”

He said his purpose in raising this issue was not to be negative about the technology, but to balance any “excessively positive and rosy view of aberrometry as our salvation” and to “pose challenges to the clinical and research vision communities.”

Surgical, not optical

Noel A. Alpins, MD, commented in the same issue about wavefront technology and corneal topography.

“We now possess two, highly sensitive tools for the objective measurement of the eye. Our challenge is to reconcile them toward the ultimate goal: the best possible vision for our patients” he said.

However, he added, there is a flawed concept at the heart of wavefront-guided refractive surgery.

“The suggestion that wavefront analysis is a comprehensive treatment solution … is based on the principle that all optical aberrations in the optical system of the eye, from the anterior surface of the cornea back to the retina, can be corrected on the surface of the cornea,” Dr. Alpins said.

He noted that the eye has been likened to an optical instrument like the Hubble Space Telescope, which can be modified to correct for lower- and higher-order aberrations. But equating the characteristics of a mechanical device to those of the human eye is a large leap, he said.

“Assigning to the living eye-brain system a concept that might hold true for an optical instrument – with corneal surgical principles playing a secondary role to optical priorities – is contrary to surgical intuition and understanding developed from past experience. The process of laser vision correction should be regarded as a surgical science as much as an optical science,” Dr. Alpins said.

Optical challenges

In that surgical science, there are some caveats that clinicians must keep in mind, Dr. Lipshitz noted in his article.

“There are inherent physiological differences and changes in optical and corneal biomechanical properties of different corneas that bedevil the goal of super-vision,” he said.

Aging is one of these factors.

“Wavefront measurement of the eye alters with age because of changes in the lens and cornea that occur over time. Even if we achieve super-vision at a certain time, new aberrations will appear subsequently with age,” Dr. Lipshitz said.

Dr. Alpins echoed this statement.

“Any change in the crystalline lens will also complicate the long-term usefulness of wavefront-driven changes,” he said.

There are also changes in the eye during accommodation, Dr. Lipshitz noted.

“When the eye accommodates, the lens changes shape and that induces changes in higher-order aberrations,” he said. “Even if we achieve super-vision for distance vision, when looking at near, newly induced higher-order aberrations may occur.”

Changes in wavefront can also occur during pupil constriction.

“Change of pupil size in light, dark, accommodation and convergence dramatically affects vision,” Dr. Lipshitz said.

Constricting the pupil avoids midperipheral higher-order aberrations, increases depth of focus and even corrects lower-order aberrations. Consequently, he said, a patient may have aberration-free vision with a very small pupil, but once the pupil enlarges vision may be highly aberrated.

Corneal challenges

Dr. Lipshitz also said the biomechanical effect of an excimer laser on the cornea is not precisely predictable due to biological variations among patients.

The thickness, strength, hydration and collagen properties of the cornea, as well as flap biomechanics and tear film properties, can all affect the wavefront measurement of an eye, he said.

In addition to these variables, Dr. Alpins noted that the refractive and corneal astigmatism values are rarely equivalent.

“Simple arithmetic analysis shows that applying astigmatism treatment exclusively on the refractive cylinder axis can leave an excessive amount of corneal astigmatism,” he reported.

He said failure to align the maximum ablation closer to the flattest corneal meridian results in off-axis loss of effect in reducing corneal astigmatism. As a result, lower-order astigmatic aberrations and coma would not be minimized.

Variations in the healing process, including corneal epithelium wound healing and corneal collagen wound healing, can also produce variations in outcomes, Dr. Lipshitz said.

Preoperative challenges

There are also technological challenges to achieving an optimal result with wavefront-guided ablation, Dr. Lipshitz said. He noted optical axis alignment, refraction, topography and wavefront testing are all performed when a patient is sitting upright. The exact head position is often not fixed for these measurements.

Consequently, when a patient lies down for wavefront-guided ablation, “one can never be sure that the supine head and eye positions are exactly the same as they were when measurements were taken in the upright sitting position,” Dr. Lipshitz said. “… for accurate placement of wavefront-guided ablation, the various instruments must all be in register.”

Also, he said, after these measurements are taken, the laser can be aligned on the geometric, pupillary or optical axes. These differences, however minor, will affect the correction of higher-order aberrations, especially coma, he said.

Technological challenges

Dr. Lipshitz also noted the paradox of detecting wavefront aberrations as a plane but correcting the aberrations on a curved cornea.

“Wavefront is measured as a planar surface at the exit pupil, but the laser correction is done on the curved corneal surface, which is aspheric and different for every eye,” he said.

The microkeratome is another technical factor of which surgeons should be aware. Dr. Lipshitz noted that microkeratomes labeled to cut flaps at a specific thickness can in practice have considerable variation.

Flap thickness depends on many variables, including diameter of the incision, blade quality, microkeratome design, corneal curvature and hydration, surgical technique and also environmental factors. All of these parameters can contribute to higher-order aberrations.

Environmental factors such as room humidity and temperature can influence the amount of laser energy delivered to the cornea with each pulse. Dr. Lipshitz said the consistency of laser pulses is constantly changing.

“The laser pulse energy changes all the time, so the instrument calculates and uses an average energy. When we finish a procedure, the laser works slightly differently than at the onset, and it changes even more when doing multiple procedures,” he said.

Therefore, requiring exact energy doses to the cornea to correct higher-order aberrations is a difficult standard to meet.

Additional technical challenges include the accuracy of the laser beam tracker and the accuracy of the wavefront sensors when used repetitiously or in conjunction with other technology.

Dr. Lipshitz reminded surgeons that even when using the same brands of components for wavefront-guided ablation, instruments can differ greatly.

“We have two identical models that work differently and we use different nomograms for each one,” he said.

Measurement conversion

In practical terms, in the clinic, there are also challenges to using wavefront-guided technology. One of these is the new set of nomenclature: Zernike polynomials, trefoil, root mean square and so on. (For more on this, see part 2 of this series in the February issue.)

In addition to these new terms, familiar refractive errors are described in a different scale. Instead of being expressed in diopters, errors are expressed in microns. Surgeons must decipher these figures on a wavefront map, in Zernike format, to understand how much corneal tissue will be ablated during surgery.

The method of representing aberrations in microns, instead of diopters, has created difficulty for practicing ophthalmologists, said Jack T. Holladay, MD.

“Topography maps in diopters have taken people 20 years to learn. We are comfortable with it,” Dr. Holladay told Ocular Surgery News. “While wavefront maps look just like topography maps, they are in microns rather than diopters. This increases the learning curve and makes the transition to wavefront a little more difficult.”

Another problem is surgeons’ lack of guideposts with the new terminology. There is a lack of normative values to which surgeons can compare their patients’ results.

“One of the issues with all the wavefront units right now is that they don’t give normative values,” said Marguerite B. McDonald, MD. “The other thing is, the machine should tell us when errors might cancel each other out or lessen their individual impact on vision when added together. This would be very useful.”

Learning curve

Both Drs. Holladay and McDonald said wavefront systems should provide simple-to-interpret readouts to make the new technology less daunting.

“There is no reason to make wavefront maps complicated,” Dr. Holladay said. “Optical scientists are trying to convince ophthalmologists to throw away their knowledge of topography and diopters and start new with wavefront and microns. If we can just take the wavefront maps and put them in diopters, it would be a lot easier for us.”

The developers of wavefront technology, who were mostly optical scientists, not surgeons or clinicians, configured wavefront sensors to calculate aberrations in microns, Dr. Holladay said. He said these optical “purists” insisted that microns be used because this is the most exact way to represent the aberration profile.

“But from a clinical standpoint, this doesn’t correlate with the rest of the equipment in the surgeon’s room, which is all in diopters,” he said. “This forces ophthalmologists to do the math in their heads. The PhDs are trying to convince clinicians to throw away their knowledge of topography and diopters and start new with wavefront and microns. If we can just take the wavefront map and make it in diopters like topography, it would be a lot easier.”

“It’s much better to lose some accuracy in the noise, so clinicians can use the last 20 years of their topography map experience, rather than relearn it,” he added.

Dr. McDonald agreed.

“This should all be made much more easy for the clinician. Steve Klyce, PhD, and Luke Smolek, PhD, both of the department of ophthalmology, LSU, New Orleans, are making great strides in this area,” she said.

Super-vision a reality?

What lies ahead in the wavefront race for better vision?

The experts interviewed for this article believe that time is the only factor holding clinicians back from offering their patients vision that exceeds their BCVA.

With steady research and application of knowledge to clinical situations, Dr. Waring said that a reasonable aphorism to cling to is “20/10 by 2010.”

Other surgeons said that with current limitations that exist for wavefront technology — ophthalmic, optical, technological, functional —much work still lies ahead in the next decade and beyond.

Dr. Alpins commented that inclusion of corneal parameters into the refractive plan is necessary to achieve a consistent improvement in BCVA, so-called super-vision. This technique avoids leaving excess regular or irregular astigmatism on the cornea that can occur with wavefront technology alone.

Dr. Lipshitz said some of the potential complications discussed in his article may not negatively affect all wavefront-guided ablation surgeries, but their total impact is considerable.

“Current excimer laser technology and foreseeable developments will not allow us to consistently correct our patients’ vision better than that which can be achieved with spectacles or contact lenses. … Refractive surgery – especially LASIK – must overcome many challenges to reach this goal,” Dr. Lipshitz said.

For Your Information:

• George O. Waring III, MD, can be reached at Emory Vision Correction Center, Atlanta, GA U.S.A.; +(1) 404-250-9700; fax: +(1) 404-250-9006; e-mail: georgewaring@emoryvision.com.
• Noel A. Alpins, MD, can be reached at 7 Chesterville Road, Cheltenham, VIC 3192, Australia; +(61) 3-9584-6122; fax: +(61) 3-9585-0995; e-mail: alpins@newvisionclinics.com.au.
• Isaac Lipshitz, MD, can be reached at Ophthalmic Health Center, 40 Einstein St., Tel Aviv 46101, Israel; e-mail: lipshitz@netvision.net.il.
• Jack Holladay, MD, MSEE, can be reached at 5420 Dashwood St., Suite 207, Houston, TX 77081 U.S.A.; +(1) 713-668-7337; +(1) 713-668-7336; e-mail:docholladay@docholladay.com. Web site: www.docholladay.com.
• Marguerite B. McDonald, MD, can be reached at the Southern Vision Institute, 2820 Napoleon Ave., Suite 750, New Orleans, LA 70115 U.S.A.; +(1) 504-896-1240; fax: +(1) 504-896-1251; e-mail: mbm2626@aol.com.

References:

• Waring GO. Challenges to wavefront correction: introduction. J Refract Surg. 2002;18:737.
• Alpins NA. Wavefront technology: A new advance that fails to answer old questions on corneal vs. refractive astigmatism correction. J Refract Surg. 2002;18:737-739.
• Lipshitz I. Thirty-four challenges to meet before excimer laser technology can achieve super-vision. J Refract Surg. 2002;18:740-743.