Targeting Emmetropia

Experts explain how wavefront mapping can help refractive surgeons and their patients hit the bull's eye.

BY LESLIE GOLDBERG, ASSOCIATE EDITOR

LASIK's Evolution

BY JACK T. HOLLADAY, M.D., M.S.E.E., F.A.C.S.

Performing standard LASIK procedures began in earnest after the procedure gained FDA approval in October 1995. By 1999, we had discovered that the effective optical zone shrank as the amount of myopia treatment increased. While the energy applied in the center of the cornea was correct, it became insufficient toward the periphery, resulting in a smaller optical zone than that programmed into the laser.

I mentioned this to Michael Mrochen, Ph.D., a specialist in corneal laser surgery and physiological optics working at WaveLight (at the time owned by WaveLight Laser Technologie AG, Erlangen, Germany) at the first Wavefront Congress in Santa Fe, N.M., in February of 2000. Michael and Theo Seiler, M.D., Ph.D., then published the theory and empirical results that came to be implemented in the WaveLight laser — the origin of wavefront-optimized LASIK (WFO). WaveLight had developed a radial compensation function to provide enough energy in the periphery by accounting for the curvature changes such that the intended and actual optical zones were the same.

WFO LASIK

WFO simply means that the laser puts onto the cornea the energy level that was intended, by increasing the energy in the periphery to compensate for loss of energy as the beam moves out from the center. As we learned more about the technology, we found that shrinking the optical zone actually induces positive spherical aberration in the cornea. The WaveLight was one of the first lasers that did not induce as much significant positive spherical aberration with its standard treatment.

As WFO matured, the VISX laser (formerly owned by VISX, Inc., Santa Clara, Calif.) gained the ability to measure aberrations of the eye and improved its radial compensation function accordingly; the result was greater predictability in surgical outcomes. Once we were able to acquire wavefront measurements, we could measure patients' spherical aberrations, as well as higher-order aberrations (HOAs) such as coma and trefoil. We could now design a treatment that took into account all aberrations.

In WFO treatment, the surgical nomogram was still based on sphere, cylinder and axis of astigmatism; it did not do anything to correct positive spherical aberrations. If the patient had positive spherical aberrations — which 90% of patients do before refractive surgery — WFO was not adding any additional aberrations, but by the same token was not doing anything to reduce them.

With WFO protocols, if you had +0.3 μm of positive spherical aberration, which is not that uncommon, it would still be present postoperatively. By contrast, the wavefront-guided LASIK (WFG) procedure would measure the aberration and then try to reduce it, as well as the HOAs, to zero.

Three Tiers of LASIK

With standard LASIK, the optical zone shrinks toward the periphery and positive spherical aberrations are induced. With WFO LASIK, the surgical correction is similar to that of a standard treatment but the procedure does not induce significant positive spherical aberrations. WFG LASIK also does not induce spherical aberrations and furthermore attempts to reduce pre-existing aberrations to zero, giving the patient a better optical system than what was possible with corrective lenses.

To compare these surgical options in our practice, we performed both WFO and WFG LASIK in 2006 and 2007, giving patients the choice of either the WaveLight Allegretto laser (Alcon, Fort Worth, Tex.) or the VISX with CustomVue platform (Advanced Medical Optics, Santa Ana, Calif.). Just over 100 patients ended up in each group. We reviewed cases by analyzing the spherical and higher-order aberrations, best-corrected acuities and uncorrected acuities. We found that WFG improved the patients' optical system by reducing and customizing the aberrations, while WFO did not improve aberrations and sometimes increased the amount of coma. In terms of corneal aberrations, WFG did a better job.¹

Our study also found that lowering HOAs improves visual acuity and low-contrast sensitivity. Previously, Steven C. Schallhorn, M.D., had published a study relating aberrations as measured by wavefront to visual performance. He found that "top gun" pilots have fewer aberrations, much better acuity and better contrast sensitivity than clinic patients. Second, he noticed a correlation between lower aberrations and better visual performance.

As seems obvious, there is no question that aberrations are "bad." The lower that surgeons can make them, the better the performance of our patient's visual system. When we correlated the performance of patients' vision in low contrast, we found that reducing HOAs improved the quality of their visual system. Consequently, in my opinion the WFG procedure is a better modality for most patients.

Disclosure: Dr. Holladay is a consultant to AMO.

Reference

1. Eyeworld Symposium 2007, Stockholm and ESCRS Refractive Surgery Symposium. Sweden, Sept. 8-12, 2007.

Designing a Better Ablation

BY GUY KEZIRIAN, M.D., F.A.C.S. AND
KARL G. STONECIPHER, M.D.

Douglas Adams, author of the popular radio series and book The Hitchhiker's Guide to the Galaxy, could have been describing refractive surgery when he said, "I may not have gone where I intended to go, but I think I have ended up where I intended to be." The same might be said for the journey through the realm of wavefront-guided ablations that led back to phoropter-based treatments using wavefront-optimized ablations.

When Theo Seiler, M.D., Ph.D., treated the first human eye with an aberrometer-based LASIK in 1999, he anticipated that the benefit would be the creation of an aberration-free optical system. Along with his collaborator, Michael Mrochen, Ph.D., and others, Dr. Seiler soon realized that the real benefit of using aberrometers came from the ability to design treatments that did not induce new aberrations. Most eyes don't have symptomatic higher-order aberrations — most visual complaints are caused by refractive errors, which dwarf higher-order aberrations in magnitude. We now understand that treating higher-order aberrations is not necessary in the vast majority of eyes.

Early lasers did induce large amounts of higher-order aberrations, particularly spherical aberration. Spherical aberration is easy to conceptualize. Imagine a cornea with a -6 D refractive error treated with a small optical zone laser (such as the ones we used in the mid 1990s) that has a perfect refractive outcome, so that the refractive error in the central cornea is zero (plano). Now imagine that the treatment "transitions" from the central treatment zone to the edge of the treatment, where no correction is performed. At the treatment edge, the refractive error is still -6 D. Between the central area and the edge there is a gradient of refractive errors that progress from zero centrally to -6 at the edge of the treatment. Because this is radially symmetric, it is called spherical aberration. If it were asymmetric it would be called coma, with possible other components as well.

Because spherical aberration increases from the center to the periphery, it is easy to understand why pupil size is so important to post-LASIK vision in eyes with old-style, small optical zones. At night, pupils dilate to let light in through the part of the cornea with the gradient of refractive errors. The resultant spherical aberration causes visually significant problems with glare, particularly with oncoming traffic lights. For more on this topic, I refer you to a paper written by Kezirian and Stonecipher.¹

Early in the course of producing wavefront-guided treatments, Seiler and his co-workers realized that they could avoid most of the visual problems after LASIK by designing a profile that doesn't induce spherical aberration. The underlying principle was to extend the refractive treatment further out from the center so that light passing through the entire central and mid-peripheral cornea is in focus. The result is a larger postoperative optical zone that is clearly visible on topography maps of the cornea (Figure 1).

Figure 1. A post-LASIK topography in an eye that underwent a 6 D wavefront-optimized correction using the WaveLight Allegretto laser. Note the extension of the refractive correction to the full 6.5 mm diameter of the optical zone.

The ablations that Seiler and Mrochen designed have become known as wavefront-optimized ablations, because they were improved based on wavefront data. Seiler and Mrochen did most of their work using the Allegretto laser made by WaveLight, AG of Erlangen, Germany. WaveLight has incorporated wavefront-optimized ablations into all its ablation profiles.

Underlying the success of wavefront-optimized ablations is the ability to retain preoperative corneal asphericity after treatment. Corneal asphericity can be considered as the ratio of the corneal curvatures at two points. For refractive surgeons, curvatures outside the ablation zone are uncontrollable, so the current approach is to use ablations that extend the refractive correction as far to the edge of the optical zone as possible. For example, an eye with a central curvature of 44 D and an "edge of the optical zone" curvature of 42 D that undergoes a 4 D correction would have corresponding approximate curvatures of 40 D and 38 D after surgery. This approach leaves the ratio of central and mid-peripheral curvatures unchanged and is the underlying principle behind "wavefront-optimized" ablations.

The success of wavefront-optimized ablations was demonstrated in the United States FDA studies of the Allegretto Wave excimer laser system. Along with Maureen O’Connell, I designed and conducted these studies through my company, SurgiVision Regulatory Consultants, Inc.² The studies demonstrated several key points:

(1) Wavefront-optimized ablations do not induce significant amounts of spherical aberration. Over a 7 D range, the mean spherical aberration change was only 0.1 μm — nearly an order of magnitude less than that of prior studies.³

(2) Corneal asphericity was unchanged by treatments using wavefront-optimized ablations. The concept that WFO ablations produce a "prolate" cornea are unfounded.

(3) The WaveLight system effectively reduced higher-order aberrations (HOAs) in eyes with significant amounts of preoperative aberrations, but most eyes did not have enough preoperative HOAs to benefit. An apt analogy is giving Tylenol to reduce fevers in people who don't have a fever. The studies showed that 81% of eyes did as well or better with wavefront-optimized treatments as with wavefront-guided treatments, and only 4% of eyes had a clear need for wavefront-guided treatments. The study did confirm that, in those eyes with significant HOAs, treating with wavefront guided treatments resulted in improve outcomes, as long as those HOAs were in fact related to the cornea and not other parts of the optical system, such as the lens.

(4) Eyes that have lenticular aberrations should not have wavefront-guided treatment. If the topography is symmetric and the aberrometry shows significant aberrations, then the aberrations originate in the lens. Treating these eyes with wavefront-guided ablations puts irregular aberrations on the cornea and can worsen vision.

In another 5 years, it is conceivable that the primary use of an aberrometer will be to follow the progression of early cataracts and not to design ablation profiles for laser vision correction. Lasers that currently have not "optimized" their ablation profiles will likely adopt them and the treatment of choice for eyes with significant corneal aberrations may be with topography-guided, not aberrometry-guided, ablations.

Ablation profiles for laser refractive corrections are a compromise between optics and physiology. Corneal thickness, biomechanics, pupil size, refractive error and the need to work with the multiple lens-system of the eye create challenges that limit the optical results of surgery. However, over the past decade advances in both understanding and technology have greatly improved the quality of visual results delivered by laser refractive treatments.

WaveLight is not the only company to have learned these lessons over the years. Other laser manufacturers have followed suit and have implemented these concepts into their platforms and have made other contributions to improve visual quality after refractive surgery. Like Douglas Adams, we didn't intend to set out on this course when the concepts of wavefront-guided ablations were introduced, but we do find ourselves where we wanted to be — with better lasers and better options for our patients.

Disclosures: Dr. Kezirian is the owner of SurgiVision Consultants, Inc., based in Scottsdale, Ariz. His company conducted the FDA clinical trials of the WaveLight Allegretto Laser. Dr. Kezirian also administered the original trials that led to the first LASIK approvals for the VISX and Summit lasers and is a consultant to several ophthalmic companies. Dr. Stonecipher has participated as a clinical investigator in several FDA trials for ophthalmic lasers.

References

1. Kezirian GM, Stonecipher KG. Subjective Assessment of Mesopic Visual Function after LASIK. Ophthalmology Clinics of North America 2004.
2. Stonecipher KG, Kezirian GM. Wavefront-optimized versus wavefront-guided LASIK for myopic astigmatism with the Allegretto Wave: three-month results of a prospective FDA trial. J Refract Surg. 2008 Apr;24:S424-30.
3. Yoon G, MacRae S, Williams DR, Cox IG. Causes of spherical aberration induced by laser refractive surgery. J Cataract Refract Surg. 2005 Jan;31:127-35.

Selecting the Right Platform

BY KARL STONECIPHER, M.D.

Refractive surgeons are fortunate to have a number of surgical modalities available to address each patient's unique refractive needs. In recent years, a key question has been whether laser profiles that take advantage of wavefront mapping should supplant traditional LASIK protocols. I use both the VISX CustomVue and the WaveLight Allegretto systems, and discuss below the factors I consider in choosing between conventional LASIK and wavefront-optimized (WFO) or wavefront-guided (WFG) LASIK.

With the VISX platform, I think that every LASIK case should make use of wavefront mapping unless the patient has some kind of lenticular anomaly or their corneal thickness precludes a custom ablation because it would weaken the cornea structurally. The only option other than the CustomVue profile is a conventional procedure. I customize every case possible when using this device because I think it's much better than conventional LASIK for creating an ablation profile, especially since conventional laser profiles with this platform don't have a WFO option (the efforts at optimization of conventional LASIK profiles went into the CustomVue WFG platform).

With the WaveLight Allegretto, probably 85% to 90% of of surgeons who have this laser at their disposal use WFO LASIK initially because of faster throughput compared to WFG procedures, in which success is dependent upon image capture techniques that can be time consuming. For this laser, the WFG platform is appropriate if the root mean square higher-order aberration (RMSh) error is above a certain threshold, typically 0.3 μm to 0.4 μm. WFG treat ments can produce better outcomes in such cases. However, if the aberration originates in the lens, it cannot be corrected by a corneal procedure. In some patients with lenticular aberrations, WFG treatment can actually produce results inferior to standard LASIK. Also, if corneal anatomy precludes a WFG procedure (e.g., insufficient tissue volume as measured by pachymetry), a WFO procedure is an option.

Less than 5% of LASIK patients in my practice require a custom treatment. I screen new patients using the OPD-Scan (Nidek, Fremont, Calif.), which has the ability to calculate the internal wavefront, enabling the surgeon to distinguish between corneal, lenticular and total aberrations. This feature allows me to identify patients with lenticular wavefront anomalies more easily. I am able to quickly determine if the aberrations do in fact arise from the patient's cornea with this device. Again, with the WFG option we only want to treat those higher aberrations that arise from the cornea.

For my practice, WFO is a better option in the majority of patients. When properly selected, patients can achieve refractive outcomes and satisfaction levels comparable to WFG procedures. WFO treatments also allow for the potential to retreat with WFG profiles if enhancement is necessary, provided there is sufficient corneal tissue available. I tell some patients with borderline or thin pachymetry readings that I plan to perform a conventional treatment — a transepithelial PRK with the VISX laser. The idea behind this option is to remove the least amount of tissue possible. The patient will then return and we will customize him/her later if needed, based on patient complaints and if the corneal thickness allows. In my experience, this additional treatment is a pretty rare occurrence.

HOA Treatment

It is important to remember that the WFO protocol is a pure refractive change — it does not treat HOAs; rather, it reduces the amount of spherical aberrations induced by older surgical protocols used in conventional LASIK. Think of the cornea in terms of treatment profiles — some regions are untouched by the laser while others are fully treated. WaveLight designed a better transition zone by increasing the energy in the periphery to reduce the spherical aberrations that were induced by conventional treatments. In some patients treated with LASIK prior to 1999, we can now measure the type and extent of aberrations present. Wavefront analyzers were designed with this need in mind. In patients with appropriate residual corneal thickness and subjective complaints, a WFG option for enhancement is now available with either the VISX or WaveLight laser. I have found this successful as an enhancement option in some select patients; in others, purely treating the residual refractive error, especially residual astigmatism, can solve the issues as well.

It takes about 1 to 3 months postop to accurately assess patient response to surgery. We found in our study¹ that the WFO and WFG platforms performed similarly in most eyes for vision and refraction. The WFG procedure performed better in patients with HOAs of greater than 0.3 to 0.4 μm, while the WFO performed better for patients with very low HOAs preoperatively. Both methods preserved contrast sensitivity and low contrast acuity. Lastly, my retreatment rates are comparable: WFO at 0.88% and custom treatments at 1.27%. Figure 1 presents a decision-tree for procedure selection derived from our study findings.

Disclosure: Dr. Stonecipher has participated as a clinical investigator in several FDA trials for ophthalmic lasers.

Reference

1. Stonecipher KG, Kezirian GM. Wavefront-optimized versus wavefront-guided LASIK for myopic astigmatism with the Allegretto Wave: three-month results of a prospective FDA trial. J Refract Surg. 2008 Apr;24(4):S424-30.

LASIK: The Next Generation

BY SCOTT MACRAE, M.D.

For refractive surgeons, the transition from conventional to wavefront LASIK represented a quantum leap forward in our abilities. Having access to wavefront data allowed us to customize our surgical nomograms, improving refractive correction in the peripheral optic zone and reducing spherical aberrations. Recall for a moment Plato's allegory of cave dwellers who see only shadows at the lowest levels of the cave and gain clarity as they move upward each level toward the sunlight. Refractive surgeons have traced a similar path: from the relative darkness of early conventional LASIK to the greater precision of wavefront-optimized and then wavefront-guided procedures, at each new level we attain crucial information that brings our knowledge — and our patients' vision — into sharper focus.

My colleagues and I at the University of Rochester developed refinements that we feel bring the capabilities of refractive surgeons to an even higher level. We studied in detail the data from the FDA trials that led to approval of Bausch & Lomb's Zyoptix laser.¹ First, we found that increasing the optical zone of the ablation to 6.5mm or larger neutralizes the increase or change in spherical aberration. When analyzing the level of postoperative sphere in the study subjects, we found that patients with substantial preoperative spherical aberration tended to be overcorrected after surgery. This is particularly true for retreatments, and has been noted with all laser platforms. Additionally, if the patient has preoperative coma or trefoil, overcorrection is more likely to occur. Patients with preoperative negative spherical aberration will tend to be undercorrected.

Manifest Destiny

The Rochester Nomogram (also known as the Advanced Nomogram, marketed by B&L under the name "Advanced Personalized Technologies") created at the University of Rochester Eye Institute was designed to improve wavefront accuracy. This nomogram achieves HOA correction as well as spherical correction without sacrificing spherical accuracy. How? By taking into account the aberration interactions noted above and the patient's manifest refraction. The Rochester nomogram factors the preoperative manifest sphere and astigmatic power into the equation, allowing us to increase or decrease the correction as needed.

Each time we omitted the manifest refraction from the equation, we got slightly inferior results; when we included it, accuracy improved. Eventually, we realized that manifest refraction is the brain's interpretation of the wavefront and is just as powerful a metric as either Zernike or Fourier interpretations, particularly in virgin eyes. Understandably so — manifest refraction has been the gold standard for clinicians in optics for the past 200 years.

If I had to recommend just one factor — either a wavefront or a manifest refraction — for clinicians to rely on in a noncomplicated case, I feel the manifest refraction is the more powerful predictor of patient satisfaction postoperatively. This is why most manufacturers do not recommend custom LASIK if there is a large discrepancy between the manifest and the wave front refractions. Using the manifest refraction in conjunction with preoperative Zernike wavefront data and the HOA information creates a level of synergy not available in older surgical protocols, and allows us to achieve even greater accuracy. Other nomogram developers are beginning to work with this concept and validate its effect as well.

On retreatments, aberration interaction becomes even more critical. It's well known that patients who previously underwent conventional, pre-wavefront LASIK for myopia correction tend to have significant amounts of positive spherical aberration. If you try to retreat that patient with the intent of providing only minimal additional spherical correction but substantial reduction of spherical aberration, you'll get a farsighted or hyperopic overcorrection, just as we predicted with the Rochester nomogram. In virgin eyes the phenomenon is very subtle, but can be revealed in data sets on the order of 300 to 400 eyes. That's why the FDA study was so helpful in identifying this elusive but important effect.

A Brighter Future

The Rochester Nomogram represents the second-generation refinement in custom LASIK. It's my belief that excimer manufacturers will begin to integrate aberration interaction in a systematic way in the future. But it is also important to anatomically customize treatment based on low mesopic pupil size, corneal thickness, age and optical zone size to avoid removing too much tissue and yet provide an adequate optical zone. Increased sophistication in flap design and creation is another component of anatomical customization, and efforts are underway to improve our techniques on that front.

I believe that wavefront sensors will continue to improve and will eventually be combined with topography to give us corneal, lenticular and whole-eye wavefront readings that provide a comprehensive view of the source of ocular aberration. If one is removing a cataract, one does not want to treat the lenticular aberrations; however, one does want to correct the corneal aberrations since these will persist despite a perfectly executed cataract extraction.

Such an example illustrates the importance of aberration localization, or determining where the aberration originates. Having sophisticated diagnostic tools capable of pinpointing the origin of the wavefront abnormality and compensating for aberration interaction is going to be a real advantage in the next generation of excimer technology. Future systems should also have greater dynamic range and sensitivity, allowing us to achieve refractive correction in ever more challenging cases. Like Plato's cave dwellers, we hope that one day we'll be able to exit the cave once and for all, and bask in the enriching daylight. OM

Disclosure: Dr. MacRae is a consultant to B&L.

Reference

1. USFDA submission by Bausch & Lomb. Technolas 217z Zyoptix System for Personalized Vision Correction. Nov. 24, 2003.