Excimer laser refractive surgery is widely practiced and, in most cases, has proved to be safe and predictable. The various techniques include PRK, LASIK and LASEK.
LASIK is by far the most common refractive procedure in use today, owing to the advantages of quicker visual rehabilatation and less postoperative pain. The aim of ideal refractive surgery treatment is to achieve an accurate and predictable postoperative refraction, as well as maintaining long-term stability in the absence of complications. In addition, one aims to keep problems with glare and nighttime vision to an absolute minimum. Preoperative examination that includes a measure of the scotopic pupil diameter and corneal topography is essential. Also important is an understanding of ocular abberations and their potential impact on vision.
In addition to a variety of excimer lasers, there are many treatment algorithms from which to choose. The main approaches in use today are conventional, wavefront-guided and topography-guided treatments. The purpose of this article is to discuss wavefront-guided customized ablation versus topography-guided customized ablation with particular regard to retreatments.
One of the most common problems following refractive surgery is under- or overcorrection. Other problems include small and decentred optical zones and irregular ablations. All can give rise to debilitating symptoms.
In the past, our options for treating these corneas were limited; however, technological advances have now increased our armamentarium. Two main customized approaches, wavefront- and corneal topography-guided treatments, are now available. The question is: Which treatment is best, and which should we use in any given case? This remains a healthy debate.
Several differences are noted between wavefront- and topography-guided treatment approaches. All elements of the optical system (ie, tear film, anterior corneal surface, corneal stroma, posterior corneal surface, crystalline lens, vitreous and retina) are taken into consideration with the wavefront system. In theory, most (if not all) aberrations of the eye can be corrected by customized corneal ablation based on ocular wavefront measurements. As the derived information is converted into mathematical data (ie, Zernike polynomials), we can classify as well as quantify the aberrations.
In practice, there are limitations to this technology. The wavefront measurements are taken at a certain point in time, and hence, are essentially static measurements. The eye, however, is a dynamic optical system. In certain lighting conditions and with corresponding pupil diameters, all the aberrations may be correctable. This may not be the case in other situations (eg, changes in light intensity and accommodative states). Furthermore, if we add in factors such as wound healing, ablation predictability, flap induced aberrations and changes in the tear film, we begin to see that wavefront-guided treatment may not be as accurate as we would like. It is interesting to note that there is no conclusive evidence, as of yet, favoring a wavefront-guided approach over a conventional guided approach for the majority of virgin eyes, possibly for reasons stated above.
Topography-guided treatments, on the other hand, use information derived solely from corneal topography. The aim is to reshape the cornea into an ideal curve, also taking into account the current spherocylindrical correction.
The cornea is the primary refractive structure of the eye, and if corneal aberrations are corrected, the whole optical system should dramatically improve. Topography-guided treatment does have advantages over wavefront-guided treatments in that it can be used in cases with (1) corneal scarring or other media opacities and (2) highly irregular corneas where wavefront measurements are not possible. Furthermore, one can incorporate a Q-value into the treatment, thereby maintaining or improving the natural aspheric shape of the cornea. The major drawback, however, is that the rest of the ocular refractive media are ignored. Potentially, unpredicatable refractive outcomes — despite a corrected corneal profile — may arise.
It is important to note that over approximately 4 years, a promising approach has been developed that takes information from the topographer and converts it into a wavefront map using Zernike polynomials. With this advancement, it is possible to look at aberrations derived mainly from the cornea and compare them with those arising from the ocular system as a whole. This technology has been termed corneal wavefront customized treatment and has been developed by Schwind eye-tech solutions (Kleinostheim, Germany).
Most current wavefront-guided treatments are guided by information derived from the entire optical system. This does not allow for differentiation of aberrations arising solely from the cornea.
Retreatments with wavefront-guided and topography-guided procedures have produced variable results. Knorz and Jendritza5 used topography-guided LASIK in eyes with previous refractive treatment and/or eyes with previous trauma. They found a significant improvement in UCVA, a significant reduction of corrective cylinder and a more regular topography. Drawbacks included undercorrection and difficulty centering the treatments. In other studies, good results have been shown with topography-guided treatments for decentered ablations as well as postkeratoplasty astigmatism.6-8
Alio et al9 concluded that topography-guided LASIK for irregular astigmatism — where a defined pattern was observed — was useful, however, it was not effective for undefined irregular astigmatism. Wavefront-guided treatment, they suggested, may be a better option for these patients. More recently, Jankov et al10 showed that topography-guided LASIK and PRK procedures resulted in a significant reduction of refractive cylinder and increase in UCVA in eyes with irregular astigmatism. Similar to findings from other studies, they also experienced problems with undercorrection.
Alio et al11 attained superior wavefront-guided treatment results compared with conventional treatment results for eyes that needed enhancement following previous refractive surgery. Although all eyes achieved a satisfactory visual outcome, those that underwent wavefront-guided treatment had no change or a reduction in higher order aberrations and contrast sensitivity did not change from preoperative values.
Mrochen et al12 showed that wavefront-guided treatment can be a useful treatment for decentered ablations; the investigators also commented that in some cases the aberrations were too large for wavefront sensing. Castanera et al13 also reported good results, but importantly noted that wavefront-guided treatment with the Zyoptix technique (Bausch & Lomb, Rochester, NY) required almost twice the ablation depth versus standard planoscan retreatment. The latter is an important consideration, especially for retreatments where there may not be much available stroma to stay within safe limits.
Whatever technique is used, results will vary depending on the complexity of the problems to be treated with irregular astigmatism — decentered ablations and central islands being more challenging situations. Ablation depth will also vary depending on the treatment approach. Topography-guided ablation is an elevation-based strategy and hence highly tissue consuming. Wavefront-guided ablation is a Zernike-based ablation strategy and is more tissue consuming when compared with a standard treatment, but usually less so compared with a topography-guided treatment. The amount of tissue consumption also depends on the laser.
Currently, we favor corneal wavefront-guided treatments for eyes that previously underwent refractive surgery. We use our recently acquired Esiris platform (Schwind eye-tech solutions) and incorporate the Optimized Refractive Keratectomy-Custom Ablation Manager (ORK-CAM). The Schwind system (Corwave) essentially uses a topography assisted wavefront algorithm, so it is possible to see how much aberration is caused by the cornea itself as compared with the total wavefront (ie, Ocular wavefront) from all the ocular media.
We can selectively simulate the effect of these aberrations on vision, allowing for individual treatment planning — particularly when dealing with decentrations and retreatments). The corneal wavefront-guided ablation, being a slope-based (ie, optical refractive errors) ablation strategy, tends to be more tissue saving.
Results to date have been satisfactory, and we have recently evaluated nine retreatments using this corneal wavefront-guided approach. Mean preoperative sphere was -0.375 ±2.271 (range -2.00 to +4.75); mean preoperative cylinder was -0.950 ±0.643 (range 0.00 to -2.00); and unaided vision was 0.471 ±0.125 (range 0.30 to 0.70 decimal acuity). All retreatments were performed using LASIK; flap lifts were performed in seven patients and new flaps were created in two patients. We used the M2 Microkeratome (Moria, Antony, France) for flap creation. At 3 months postoperative, mean unaided vision was 0.90 ±0.046 (range 0.70 to 1.2 decimal acuity); mean sphere was -0.028 ±0.088 (range -0.50 to 0.25) and mean cylinder was -0.306 ±0.325 (range -0.75 to 0.00). All patients were within 0.50 D of target refraction and 78% were within 0.25 D of target. No adverse events were recorded, and no patients lost BCVA.
Topography-guided customized treatments are favored mainly for virgin eyes with asymmetric astigmatism. We use the Allegretto 400Q platform (Wavelight Laser Technologie AG, Erlangen, Germany) for these treatments. Topographies are obtained with the Wavelight Topolyzer, and this integrates with the excimer laser via the company's Topography-Guided Customized Ablation Treatment (T-CAT) software. We are considering Schwind eye-tech solutions corneal wavefront customized treatment for these eyes as well, and we are organizing a study to see which technique is better and more predictable (ie, corneal wavefront vs topography-guided). We rarely use topography-guided procedures for retreatments, as we feel corneal wavefront treatment using the Schwind platform gives a more favorable and predictable refractive outcome with relatively less ablation depth.
Treatment should be individualized to every patient. Many factors — type of previous treatment, corneal topography, scotopic pupil diameter and available stroma — need to be taken into consideration when planning the best approach. Undoubtedly, further technological advancements will achieve even more accurate, predictable and sustainable results.
Ioannis M. Aslanides MD, PhD, MBA, is director and consultant ophthalmologist at Emmetropia Mediterranean Eye Institute, in Crete, Greece. Dr. Aslanides states that he has no financial interest in the products or companies mentioned. He may be reached at i.aslanides@emmetropia.gr; +30 2810 226198 (telephone); or +30 2810 343436 (fax).
Ali A. Mearza, FRCOphth, MBBS, is a refractive surgery fellow at the Emmetropia Mediterranean Eye Institute, in Crete, Greece.
Up Front | May 2006
Wavefront-Guided vs Topography-Guided
These two procedures are the main refractive surgery approaches used today.
Ioannis M. Aslanides, MD, PhD, MBA, and Ali A. Mearza, FRCOphth, MBBS
