Techniques for Advanced Surface Ablation
Raymond Stein, MD, FRCS(C)

Advanced surface ablation, which has overcome many of the problems that plagued PRK, opens up new vistas in the refractive landscape, including the possibility of laser vision correction up to -12 D.

Despite the popularity of LASIK, surface ablation remains an attractive procedure. Surface ablation's appeal derives from two unique attributes. first, for an equivalent correction, surface ablation leaves a structurally stronger cornea than LASIK. Thus, the incidence of corneal ectasia is lower with surface ablation. From a corneal integrity standpoint, surface ablation is probably a better procedure than LASIK for high myopia or patients with atypical topography, such as inferior steeping, a steep central cornea (>48D), or high oblique astigmatism. In these cases, surface ablation offers a more conservative option than LASIK.

Optical Issues
Surface ablation may also provide superior optical outcomes. Let me start with an observation. Over the years I have had occasion to use surface ablation to enhance more than 20 post-LASIK patients. (These patients had stromal beds too thin for further ablation.) In 80% of these patients, multiple wrinkles or folds were clearly present in Bowmans membrane, an observation that is consistent with other reports.

On slit lamp examination prior to enhancement, all of these patients had very smooth corneas, with no obvious striae. All had excellent best corrected visual acuity. I believe that this wrinkling of Bowmans membrane is an inherent problem in myopic LASIK, especially high corrections, that derives from the geometry of the flap and bed. That is, if one cuts a flap and no laser ablation is performed, the flap will fit back on the bed perfectly; however, a myopic ablation removes tissue and decreases the surface area of the bed. the result is a flap that is now slightly too large for the bed. In a high percentage of patents, the result is wrinkling of Bowmans membrane.

Fortunately the epithelium undergoes hyperplasia and/or hypoplasia to smooth the anterior surface, and although best corrected visual acuity is preserved, the question remains whether the wrinkling of Bowmans membrane produces more subtle effects, such as the induction of higher order aberration. I  suspect that there is subtle effect on vision, the size and nature of which are unpredictable. In FDA studies of wavefront-customized ablation, the PRK outcomes were often superior to the LASIK outcomes in comparable eyes, perhaps for the reasons just outlined.

Back to the top.

Postoperative Pain
On the other side of the coin, surface ablation has had problems in four primary areas: pain, haze, delayed recovery, and regression. In advanced surface ablation steps are taken to control each of these.

We take several steps to minimize patient discomfort. First, we apply ice immediately after the laser ablation. The ice is in the form of a frozen Merocel® sponge that has been soaked with BSS®. We apply the frozen sponge to the cornea for 10 seconds, then remove it for 5 seconds and repeat the cycle three or four times.

Another step that appears to reduce postoperative discomfort involves making sure that the bandage contact used until the patient reepithelializes fits relatively tightly. The lens should move less than 0.5 mm on the eye. Flatter lenses move more and appear to increase discomfort.

We are also fairly liberal with pain-control medication. We have surface ablation patients use a topical non-steroidal anti-inflammatory drug (NSAID) 2-3 times per day for 24-48 hours after surgery. For this purpose we use both diclofenac (Voltaren®; Novartis) and ketorolac tromethamine (Acular®; Allergan), and we find both effective. Surface ablation patients are also given a tetracaine minim, which is nonpreserved. The patient is told to use it once or twice over the first 24 hours as needed for discomfort. Finally, we also prescribe an oral analgesic, such as Tylenol® II, for patients who need it.

These measures are quite effective, and 85% f our surface ablation patients report a comfort level equivalent to our LASIK patients. However, there remains the 15% who still have some pain and photophobia, making this an issue that requires our continued attention.

Back to the top.

Haze and Mitomycin-C
We use mitomycin-C in all surface ablation cases in which there is a significant risk of haze. The specific regimen is based on the patient's correction, as follows:

• Under -5 D: no prophylaxis for haze
• -5 to -8 D: mitomycin-C applied for 30 seconds
• -8 to -10 D: mitomycin-C applied for 45 seconds
• -10 to -12 D: mitomycin-C applied one minute

Mitomycin-C 0.02% (0.2 mg/mL) is applied after the cornea has been cooled with frozen BSS as described. Before applying the mitomycin-C, a slightly moist Merocel sponge is used to remove excess fluid from the surface of the cornea. (A relatively dry corneal surface is important to prevent dilution of the mitomycin-C.) We apply the mitomycin-C by means of a circular corneal light shield that has been soaked with mitomycin-C and then placed on the cornea for 30 seconds to 1 minute, as appropriate. We remove the light shield and irrigate the ocular surface with BSS.

Since we began using mitomycin-C, we have not experienced a case of clinically significant haze, even with corrections up to 12 D. At 2-4 month after surgery, a very small percentage of patients have trace haze, but this isn't visually significant for the patient and resolves with time. Haze has ceased to be a significant problem.

Back to the top.

Delayed Recovery and Regression
Recovery times following surface ablation have been greatly improved, thanks in large part to the smooth post-operative bed left by advanced flying spot lasers, such as the WaveLight Allegretto Wave excimer laser that I use the overwhelming majority of my surface ablations. Today, most surface ablation patients are able to drive within 5-7 days after surgery. Although still not comparable to the visual recovery following LASIK (where patients feel they can drive the next day), visual recovery from surface ablation is much faster than in the past.

Prior to ablation, I use the Amoils rotary brush to remove the epithelium, removing 8.5-9.0 mm of epithelium, depending on the transition zone to be used. The brush creates a very smooth epithelial edge that, I believe, contributes to faster healing. Regression, which was a significant problem in the early days of surface ablation, is greatly diminished as a risk factor. I attribute this to the larger optical zones that we can now make. For example, 6.0 x 5.0 mm was an extremely popular optical zone, and it still is used by some lasers today. By contrast, the Allegretto laser that I now use can treat pure astigmatism (eg. plano -3.00 x 180) with either a 6.5-mm or 7.5-mm x 9-mm optical zone, and the incidence of regression is significantly lower. The same is true with spherical treatments: the incidence of regression has diminished as the 5.5- and 6.0-mm optical zones that were used for years gave way to 6.5- or 7.0mm zones.

Back to the top.

Results
My results performing surface ablation over the last 2 years with the Allegretto laser have been very satisfying (Table I). Of the approximately 4,750 corneal laser procedures I have performed in the period, 730 (15%) were advanced surface ablation. Outcomes have been very good, even in the very high corrections -- for example, among corrections between -9 and -12 D, 92% were 20/20 or better without enhancement.

It is not surprising that, with these results, we have performed very few enhancements on advanced surface ablation patients over the last 2 years. In fact, our enhancement rate with surface ablation is now lower then with LASIK. In addition, the induction of higher order aberration has been less with advanced surface ablation than LASIK, and this is consistent with reports in the literature using a variety of lasers.

Back to the top.

The Refractive Surgery Landscape
A viable surface ablation technique changes the over-all refractive landscape. Until recently, I believed that the safe upper limit for laser vision correction was in the vicinity of -8 D. for higher corrections, it looked as if the best solutions were intraocular, most often with a phakic intraocular lens (IOL). However, with advances in laser technology and the use of mitomycin-C, I now feel comfortable performing surface ablation up to -12 D, using either a wavefront-guided or a wavefront-optimized approach.

Where until recently I would have used a phakic IOL between -8 and -12 D, I am now fairly comfortable with surface ablation in that range. If laser vision correction becomes the norm up to, say, - 12 D, that has the potential to significantly diminish the phakic IOL market - the number of candidates with corrections greater than -12 D is very small, especially compared to the number of potential patients between -8 and -12 D.

Back to the top.

The Future of Laser Vision Correction
Outcomes with advanced surface ablation have improved with advances in laser technology and pharmaceutical management, but surface ablation still lags behind LASIK in the areas of comfort and speed of visual recovery. One development that holds great potential is the epithelial separator that would allow us to make an epithelial flap without alcohol ("epi-LASIK").

At this point, it looks as if the future of corneal refractive surgery is a race between epi-LASIK and LASIK. If surface ablation is to challenge LASIK in a meaningful way, however, we will have to find a way to deal with the pain and recovery-time issues. If epi-LASIK can reduce pain and speed recovery to the point where it rivals LASIK, then it has a bright future. However, the jury is still out.

Back to the top.

THE BOTTOM LINE

Although surface ablation has advantages in terms of safety and visual outcome, it has been associated with some significant problems, including haze, pain, slow visual recovery, and regression. The combination of frozen BSS to chill the cornea after surgery, a tight bandage contact lens, and aggressive pharmaceutical management has reduced the incidence of pain to the point where 85% of surface ablation patients have roughly the same comfort level as LASIK patients. Use of mitomycin-C has removed significant haze as a complication of surface ablation. The smooth ablation surface that current flying spot lasers deliver speeds recovery following surface ablation, although the pace of visual recovery is still significantly slower than with LASIK. Larger optical zones have greatly diminished the incidence of regression following surface ablation, to the point where enhancement rates for surface ablation are lower than for LASIK. Visual outcomes have been stellar. Advanced surface ablation opens up the possibility of extending the range of laser vision correction to -12D.

Raymond Stein, MD, FRCS(C), is medical director of the Bochner Eye Institute, chief of ophthalmology, Scarborough Hospital, Scarborough, Ontario, and immediate past-president of the Canadian Society of Cataract & Refractive Surgery. He is a consultant to WaveLight Technologies.

Back to the top.

Call for a FREE consultation!
260-486-0065 or toll free 1-800-644-6393!

Copyright © 2004-2008 Shealy Eye Laser Center
6036 Trier Road Fort Wayne, IN 46815
Tel: (260) 486-0065 or (800) 644-6393  Fax: (260) 486-3437
email: shealy@shealyeye.com

Privacy Policy     Terms & Conditions/Disclaimer