At Issue: custom corneal ablations

At Issue posed the following question to a panel of experts: “Have custom corneal ablations improved LASIK results?”

Optimistic but marginal improvement

Noel Alpins MD, FACS: Customized corneal ablations have recently been brought to notice with the highly publicized advent of wavefront technology. In practice most treatments using the LASIK procedure can already be “customized” or tailored to the individual patient’s parameters in order to obtain enhanced outcomes. Scrutinizing recent results employing preoperative wavefront analysis shows optimistic but marginal improvement, difficult to differentiate from the normal biological spread of outcomes that occur in refractive procedures.

An important consideration preceding all refractive surgery is the question of what measurement modality to employ as a basis for treatment. The answer becomes more complicated when astigmatism is involved, with the difficulty compounded by the commonly occurring difference that exists between the eye’s refractive and topographical properties. Emphasizing one aspect in excess of another introduces the potential for a suboptimal visual outcome. Improved results can predictably be obtained when both refractive and topographical parameters are integrated in the one treatment process.

Vector planning customizes these two individual characteristics by apportioning how much each should influence the treatment. This technique is valuable for symmetrical treatments, currently the most commonly performed for both hyperopic and myopic astigmatism. This could readily be extended to asymmetrical treatments where naturally occurring irregularity is present.

LASIK and photorefractive keratectomy (PRK) procedures are now capable of treating irregularities in corneal shape. Such customized topographically driven ablations have demonstrated improved best-corrected visual acuity (BCVA) of patients debilitated by the symptoms associated with this uncommon form of irregular astigmatism.

Wavefront-guided refractive treatment primarily addresses the aspect of visual degradation and aberrations secondary to spherical and astigmatic error. Its potential is promising in the reduction of spherical aberration with the maintenance of a cornea closer to the ideal prolate shape. This will assist the reduction of untoward symptoms of halos and ghosting, particularly associated with night vision. The benefits in the treatment of wavefront-observed aberrations associated with astigmatism are less clear, considering the likelihood exists that excess to the minimum corneal astigmatism remains on an uneven corneal surface after treatment.

Noel Alpins, MD, FACS, can be reached at 7 Chesterville Rd., Cheltenham, VIC 3192, Australia; 61) 3-9584- 6122; fax: 61) 3-9585-0995; e-mail: alpins@ newvisionclinics.com.au.

Benefits astigmatism, corneal abnormalities

Richard J. Duffey, MD: Let me take the perspective of a refractive surgeon who has done no custom corneal ablations to date, and then break down an answer derived from two separate perspectives — theory and practice.

In theory, it makes sense to eliminate not only spherical and cylindrical refractive errors but also higher-order aberrations that ultimately affect visual acuity. Custom ablations have the ability to do this, but the practical application over the corneal surface is where the rubber will meet the road. I suspect the average LASIK patient will see minimal or no significant clinical advantage of custom ablation of an otherwise “normal” cornea.

Using myself as an example (I have 20/20 uncorrected visual acuity (UCVA)), by wavefront analysis, I have mild to moderate high-order aberrations, which when “corrected” with the Visx WavePrint customized lens provided no clinical improvement in my visual acuity, at least under the environmental and lighting conditions in which I was tested.

However, for the patient who suffers significant irregular astigmatism, I feel custom ablation will provide a significant improvement in this patient’s visual system. If the majority of the corneal irregularities that lead to irregular astigmatism can be eliminated, certainly the patient will notice a significant clinical improvement in his or her overall visual acuity.

That brings me to two pertinent practical questions. Can custom corneal ablations be physically coupled to the corneal surface with PRK, LASIK or laser epithelial keratomileusis (LASEK), and can wound healing be sufficiently regulated to prevent the “undoing” of the custom ablation?

If the answers are affirmative, then with these improvements in coupling technology, surgical technique and wound modulation, I feel customized corneal ablations will be most helpful for patients suffering significant irregular astigmatism and corneal contour abnormalities and of less benefit to the average patient seeking refractive surgery who has minimal or “normal” visual system aberrations to begin with.

Richard J. Duffey, MD, can be reached at 2880 Dauphin St., Mobile, AL 36606; (334) 470-8928; fax: (334) 470-8924; e-mail: drrduffey@hotmail.com.

Remains to be seen

Peter S. Hersh, MD: Because clinical trials of wavefront-guided customized ablations have only recently been started and relatively few patients treated, it is difficult to quantify the potential impact on LASIK results. Certainly, preliminary results suggest that higher-order aberrations indeed might be both diagnosed and treated using wavefront technology. This is particularly exciting for the treatment of the patient suffering adverse optical sequelae after LASIK.

However, there are a number of pitfalls that may need to be overcome in the quest for the perfect customized LASIK procedure.

1. Wavefront diagnostic information must be validated and reproducible. For instance, are there tear film effects, centration effects? Is the dynamic range of diagnostic instrumentation sufficient in the clinical setting?
2. Wavefront information must be properly registered on the cornea during ablation. Small decentrations or cyclotorsions possibly could mitigate the small ablation asymmetries designed to correct higher-order aberrations.
3. What are the effects of the LASIK flap? A number of recently completed studies suggest that the LASIK flap influences postoperative corneal topography. Although this may impact our current treatments, it may be especially important when considering wavefront-guided ablations.

First, in order to isolate and study the impact of flap biomechanics on corneal topography in recent work, we mapped sagittal height and power changes using differential topography maps on eyes. Using a nasal hinge, we found that areas of the cornea closer to the hinge tended to have a higher topography than areas further from the hinge.

From this, we inferred that the corneal flap in LASIK may retract asymmetrically toward the hinge, producing a tendency for axial asymmetry in postop topography with respect to the hinge axis. This tendency potentially could cause the flap to introduce iatrogenic optical aberrations of its own, thus confounding our treatment of existing optical aberrations via wavefront-guided ablations.

Next, other evidence suggests an indirect influence of the LASIK flap on corneal topography. In a multicenter, prospective, randomized trial of LASIK versus PRK patterns, we found that LASIK topography patterns, in general, were more regular than PRK patterns.

Two effects could be hypothesized to account for these differences.

First, there may be a masking effect secondary to the elastic modulus of the flap; that is, the flap does not deform to exactly transmit the ablation topography to the corneal surface and simply covers up minor underlying irregularities in the ablation bed.

Second, wound healing could account for the differences found between PRK and LASIK.

Thus there is a growing body of evidence that the LASIK flap per se may influence the ultimate corneal topography. How might this impact the development and success of wavefront- guided treatments?

First, the masking and retraction effects might be small compared to the aberrations we are attempting to correct, leaving us with good clinical results.

Second, a two-stage procedure may be preferable. For instance, first the flap is cut and then allowed to heal. After stability is achieved, a wavefront measurement is taken, incorporating the contribution of the flap. Only then would the flap be relifted and ablation performed.

Third, future research may give us a more predictive understanding of the effects of the LASIK flap, and thus we may be better able to incorporate these into new treatment algorithms. This might be facilitated by the development of microkeratomes that create more reproducible flaps or by new technologies of flap production such as the femtosecond laser.

Or, finally, we indeed may find that the flap’s topography effects overwhelm the aberration-correcting ablation asymmetries derived from wavefront analysis, substantially negating the desired optical effects. In such a case, maybe PRK or the evolving LASEK procedure, rather than LASIK, may be the procedure of choice to optimize the effect of wavefront-guided treatments.

4. It will be important to define which aberrations we want to correct as well as the visual processing implications of customized ablations. For instance, how will depth of field be affected? Will some patients actually be amblyopic to 20/8 visual acuity?
5. What are the effects of a nonstatic visual environment over both the short and long term? Pupil size will affect an eye’s aberration profile. So too will accommodation. In addition, aberrations contributed by the crystalline lens will change with time. Thus, over the course of the day as well as over the course of a patient’s life, the aberration profile will change. The ultimate implications for wavefront-guided treatments remain to be learned.

Peter S. Hersh, MD, is a professor of ophthalmology with the New Jersey Medical School and director of the Cornea and Laser Eye Institute, Hackensack University Medical Center. He can be reached at Glenpointe Centre East, 300 Frank W. Burr Blvd., Teaneck, NJ 07666; (201) 883-0505; fax: (201) 692-9646; e-mail: phersh@vision-institute.com; Web site: www.vision-institute.com.

Fascinating development

Thomas Neuhann, MD: I have personal experience with two forms of customized ablations. Topolink is the topography-based version and Zyoptix is the wavefront-based version with which I have worked. Compared to the standard algorithm of the Technolas 217 laser (Planoscan), Topolink was clearly superior in predictability (defined as achieved versus attempted refractive error) and in efficacy (defined as UCVA postop versus BCVA preop): 80% of Topolink eyes were within ±0.5 D and 94% within ±1 D compared to 67% and 84% of Planoscan eyes respectively; 82% of Topolink were uncorrected postop within one line of best corrected preop vision, as compared to 71% for Planoscan.

With Zyoptix, results are recent and can therefore be judged only provisionally, pending more detailed evaluation. After 3 months, refractive results are as good as, but not better than, Topolink. Postop increases in BCVA compared to preop do occur, but at least not significantly more than with Topolink or even Planoscan. As the main — and important — advantage of this technique, it seems to evolve that less tissue is ablated as compared to Planoscan; a comparison with Topolink has not yet been possible. This enables us to obtain larger optical zones. This, together with the optically better-shaped surface, is a clear advantage in low-contrast and night vision.

In summary, customized ablation is certainly the way to go. But it cannot be overemphasized that we are at the early beginning of a fascinating development but by no means have a ready-to-shoot procedure in our hands yet.

Thomas Neuhann, MD, can be reached at Helene-Weber-Allee 19, D-80637 Munich, Germany; (49) 189-159-31339; fax: (49) 189- 1578394; e-mail: ThomasNeuhann@compuserve.com.

Technology more challenging

Olivia N. Serderavic, MD: Customization of corneal ablations has been improving LASIK results for many years. From the time when corneal ablation procedures were first performed, surgeons have been modifying to whatever extent was permitted by available technology and regulatory constraints their choice of procedure, laser ablation parameters and microkeratome use based on each individual patient’s age, visual requirements, refraction and corneal parameters.

In addition, surgeons also have been customizing initial procedures on second eyes by factoring in the patient’s ablation and wound healing responses when surgery has not been performed consecutively on the same day.

As refractive results improved, surgeons and patients were still frequently dissatisfied with postop quality of vision. Industry became responsive to the necessity of maintaining optical quality more satisfactorily after LASIK. Laser ablation profiles were improved to create smoother corneal curvatures with more constant dioptric gradients over the ablation area. Additional improvements in ablation strategies allowed further customization by enabling surgeons to maintain patients’ preop asphericities more closely while factoring in pupil size more accurately.

These improvements, which were not based on wavefront technology, have been shown with wavefront analyzers to have produced fewer optical aberrations. When results with these newer ablation profiles and nomograms are compared with previous LASIK results, improvements in visual acuity and quality are frequently cited incorrectly as resulting solely from the use of “wavefront-guided” nomograms.

Wavefront technology, nonetheless, will allow surgeons to further improve customized ablations. Spatially re solved refractions are starting to improve refractive results. The recent availability of more accurate measurements of more corneal parameters and their inclusion in nomogram calculations also will improve results. The determination of optimal functional vision for individual patients and the optimal integration of wavefront information regarding higher-order aberrations with laser profiling will be more challenging.

Olivia N. Serderavic, MD, can be reached at 1036 Park Ave., New York, NY 10028; (212) 794-8691; fax: (212) 734-2307.

References:

• Ginsberg NE, Hersh PS. Effect of lamellar flap location on corneal topography after laser in situ keratomileusis. J Cataract Refract Surg. 2000;26:992-1000.
• Hersh PS, Scher KS, Irani R, Summit PRK-LASIK Study Group. Corneal topography of photorefractive keratectomy versus laser in situ keratomileusis. Ophthalmology. 1998;105:612-619.
• Abbas U, Hersh PS. Early corneal topography after excimer laser photorefractive keratectomy. Ophthalmology. 1998;15:22-26.
• Abbas U, Hersh PS. Natural history of corneal topography after excimer laser photorefactive keratectomy. Ophthalmology. 1998;15:2212-2224.
• Hersh PS, Abbassi R, Summit PRK-LASIK Study Group. Surgically induced astigmatism after photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg. 1999:389-398.