Reflections on Refractive Surgery

Corneal recontouring by excimer laser ablation of the exposed corneal surface still dominates refractive surgery.

Continued progress in laser refractive diagnostics, technologies, and techniques presented at the XX congress of the ESCRS.

Can we claim yet that wavefront technology directly produced these advances?

Far-ultraviolet laser large area corneal recontouring is still the predominant refractive surgical procedure performed globally in virgin, non cataractous eyes. This dominance was highlighted once again at the XX congress of the ESCRS in Nice. Presentations reviewed the continuing improvement in safety and efficacy of photoablation of the exposed corneal surface after mechanical or laser removal of the epithelium ( PRK) or lifting of a stromal (LASIK) or epithelial (LASEK) flap.

It was interesting that wavefront technology was not listed this year in the ESCRS programme as the topic of any of the scientific sessions or symposia on corneal refractive surgery. Has there been an increased realization that the main sources to date of improved refractive and functional visual results after corneal refractive surgery are refined nomograms, better contour profiles, enlarged optical and blend zones, corneal tissue sparing and more accurate tracking and centration - and not wavefront?

Most speakers and attendees understood that application of wavefront technology is still in its infancy.
Wavefront technology has not yet been the direct source of most of our improvements. In fact, critical analysis of results reveals similar uncorrected and best-corrected visual acuities after laser ablation in virgin corneas using the refinements listed above with or without wavefront-guidance. For example, the percentage of eyes with uncorrected visual acuity of 20/20 or better at 6 months postoperatively in the US FDA clinical trial using the Visx WavePrint wavefront ablation system - in a study cohort of 315 eyes with preoperative spherical equivalents up to -6.0 D - was 95%, exactly the same percentage as Steve Schallhorn, MD obtained using the conventional Visx S3 laser system without wavefront-guidance - in a study of 517 eyes with preoperative SE up to -7 D.

What then has been the basis for some claims of 'super vision' resulting from incorporation of wavefront-guidance of ablation? We must be sure that comparative studies of both primary procedures and re-treatments clearly differentiate between visual function improvements related to wavefront and those related to other laser system refinements.

In the US Visx WavePrint wavefront ablation system clinical trial, almost half of the eyes demonstrated total higher aberrations values within .1um of the pre-operative values and almost one-third of eyes had more than .1um of worsening total higher order aberrations.

What should we consider successful results? Should we always be aiming for the lowest total higher aberration value? Is it better to end up with a lower total RMS value even in cases where a higher value can provide a patient with better visual performance? Cases exist for which functional vision can be improved by adding one type of higher order aberration to another type - resulting in a higher RMS value than if the patient had only one of these aberrations.

Which procedure provides the best quality of vision - LASIK, LASEK or PRK? Which of those procedures will offer the best wavefront-guided results? There are still no definite answers. Niels Ehlers, MD presented data demonstrating no significant differences in total higher order aberration values at one year following PRK and LASIK, but more spherical aberration after LASIK and more coma after PRK. Further studies will clarify differences in optical quality related to biomechanics and wound healing. Future improvements in technique may provide LASEK with wound healing benefits - which currently do not exist as demonstrated by Jorge Alio MD in confocal microscopic studies of PRK and LASEK.

Although we are still waiting for real advantages to wavefront guided-PRK, -LASEK, and -LASIK, it is clear that wavefront and ray tracing analysers have increased our diagnostic armamentarium for more complete evaluation of factors affecting surgical results and have provided additional proof of causes of visual complaints.

It must be emphasized that we are now just learning about the functional significance of the different orders of optical aberrations as defined by mathematical equations. We are now just learning about the adequacies and inadequacies of these existing mathematical equations. We are now just learning about which patients would benefit most from successful wavefront guided treatment. We are now just learning about what data should be integrated into out treatment plans. This whole process is very much a deja-vu for those of us involved in development of corneal topography analysis and topography-guided surgery.

We are still waiting for topography - and wavefront-guided laser systems to allow us to treat reliably very irregular corneas resulting from complications of previous surgery.

Studies and cases presented in Nice continue to show that masking techniques can help with small surface irregularities and that localised topography-guided custom-contoured ablations can improve vision if the affected area is large and regular. No system to date has demonstrated reliable laser treatment of irregularly irregular astigmatism. Wavefront aberrometers cannot yet even measure reliably large amounts of irregularity. We must incorporate topographic data for treatment.

Several surgeons, engineers and I have been emphasizing for many years that neither topography-guided or wavefront-guided laser ablations would be reliably successful without proper rotational alignment. In this same column in 1998, I wrote that "once accurate raw data is obtained, we have to ensure that alignment at the time of data gathering is the same as that at the time of laser ablation" and "ablation requires recognition of conjunctival vessels or iris crypts or other means to check and maintain alignment". Why has it taken so many years for laser companies to acknowledge the necessity of completely controlling alignment and to start incorporating systems using natural features of the eye for torsional registration?

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