Customized Ablation #4: The Wave Moves Forward -- But Consider LASEK!
Customized Ablation: the Wave Moves Forward – but Keep an Eye on a Newly Developing Technique – LASEK!
Spotlight on Ophthallmic Lasers
Irving J. Arons
As I have written over the past two years (most recently in the February 15th and August 1st, 2000 issues of Ocular Surgery News (OSN)), the outlook for performing customized ablation, based on wavefront or ray tracing diagnostic technologies, continues to be bright. At this year's ASCRS, we saw the first indications that the diagnostic technology was moving forward, and now, at the recent ISRS (International Society of Refractive Surgery) and RSIG (Refractive Surgery Interest Group) pre-meetings held in conjunction with this year's AAO meeting, we learned of the results of the first customized treatments and the approaches each company was taking.
One thing appears evident. Correction of higher order aberrations is not going to be easy. There are still many unknowns along the path to achieving "SuperVision", before customized ablations becomes a reality. Although getting closer yet -- as I have written, we are probably still 1½ to 2 years from the reality of customized ablation being used in general practice.
On first blush, and based on early reports, PRK customized treatments appear to provide better results than those obtained from customized LASIK -- see the Summit Autonomous clinical results reported below. If this continues to be true -- primarily because of the influence of the flap, and its unknown (yet) effect on final outcomes, a relatively new procedure, LASEK, or the use of an epithelial flap, followed by PRK, first performed by Dr. Massimo Camellin of Rovigo, Italy, but now being used by several surgeons around the globe, may become the preferred method for producing customized ablations. From what little that has been presented to date, the healing response and restoration of visual stability of LASEK is about seven days, compared to the 1-2 months for standard PRK, and 1-2 days for LASIK. Thus the combination of wavefront (or ray tracing) diagnostic and customized PRK using LASEK could eliminate the unknown effect created by a microkeratome-created flap, and might just prove to be the right ticket to the next advance in refractive surgery.
There is, as with most things, an alternative approach -- two step LASIK. In order to alleviate the effect of the flap, a LASIK procedure would be done in the normal manner first, and then, about a week later, a wavefront diagnostic performed to determine the higher order aberrations yet to be corrected, followed by a "touch up" or enhancement with LASIK to provide the final correction.
It will be up to the marketplace to decide which technique the public would accept -- a one-step customized PRK/LASEK, or a two-step LASIK.
So just what is LASEK. As first described in OSN in the March 1, 1999 issue, LASEK, or laser epithelial keratomileusis, is based on the detachment of an epithelial flap via the use of an alcoholic solution that softens the epithelium and allows it to be "rolled" back into a flap, and which can then be repositioned over the ablation following PRK. In use, a pre-incision of the corneal epithelium is made using a trephine with an 80 mm blade, with a blunt portion of about 100° at the 12 o'clock position to circumscribe the flap area. A rotation of about 10° is made to form the incision. Two to three drops of 18%-20% alcohol solution is placed onto the cornea within the marker and left in place for 30 seconds. The area is then dried and thoroughly washed with water and the pre-cut margin lifted with a modified Desmarres spatula and the epithelial flap gently detached, gathered, and folded up at the 12 o'clock position. Epithelial trephination is designed to leave a hinge of about 80-100° at the 12-o'clock position. (A final irrigation with antihistamine is used to reduce any initial release of histamine induced by the alcohol.) Following a traditional PRK treatment, with light smoothing at its conclusion, the epithelial flap is then repositioned with a small spatula.
According to Dr. Camellin, "The epithelial flap is very elastic. It goes back into place easily and smoothly." Following the treatment, a soft contact bandage lens is applied for 3-4 days to keep the flap in place and allow for re-epithelization. Dr. Camellin noted, "The day after, the epithelium is still perfectly transparent and the patients complain of no more than a slight discomfort, comparable, in my experience, to the effects of LASIK." He reported that after 24 hours, the flap was perfectly sealed around the edge in all of his patients. He explained that at this stage, the epithelial flap has a tectonic rather than a visual function. The new epithelium, which regenerated within seven days, enables the eye to reach 100% of visual acuity. After four days after removal of the bandage contact lens, his patients had visual acuities, on average, better than those with traditional PRK, and comparable with LASIK. In his opinion, because of the high dilution of the alcoholic solution, only part of the epithelial cells die and on the epithelium, on the whole, remains vital.
In the July 15, 2000 issue of OSN, Dr. Camellin reported on over one-year's experience with the LASEK technique. He reported on the treating of 249 patients with the technique, of which 204 were myopic, 41 hyperopic, and 29 retreated after PRK haze, radial keratotomy, keratoplasty or LASEK. The longest followup was 14 months. Intraoperative flap management was easy in 60% of the cases, average in 28%, and difficult in 12%. All of the difficult cases involved strong adherence of the epithelium to Bowman's membrane. He found that retreatments were inevitably difficult, although the flap was easily detachable within 3 months, almost as in a primary operation. He believes that this confirms that the presence of a basal membrane reduces scar exuberance and excessive proliferation of new collagen. Postoperative pain appears limited, if any, to the first 24 hours after surgery, with no pain experienced by 44% of patients, some discomfort by 42%, and pain by 14%. He attributes the postoperative pain partly due to the contamination of the conjunctiva by the alcohol solution. He further noted that almost 90% of his patients achieved 80% of their preoperative best corrected visual acuity by day ten following surgery.
At this year's combined meeting, there were at least two papers on LASEK and one poster. At the WRSS meeting, Dr. Sunil Shah of Birmingham England presented on "Alcohol Delamination of the Epithelial Basement Membrane" and Dr. Paolo Vinciguerra of Italy provided, "Laser Epithelial Keratomileusis (LASEK): One-Year Results of a New Excimer Refractive Procedure", while Dr. Chao-Chien Chu had a poster entitled, "Comparison of Laser Epithelium Keratomileusis (LASEK) and Photorefractive Keratectomy (PRK)".
According to his paper, Dr. Shah provided details wherein he treated 178 eyes of 89 patients, with one eye using alcohol debridements and the second eye having the epithelial flap. His results showed that the eye with the flap had a higher UCVA Snellen fraction post-op for the first six weeks following surgery, and then both eyes were equal out for the remainder of the one year followup. In post-op BCVA Snellen fraction, the epithelial flap eye proved better than the PRK eye with debridement after the first week and out for a year. The flap eye also had significantly less haze than did the debrided eye.
In Dr. Vinciguerra's talk, he reported on 432 eyes. According to his abstract (I was not able to attend his talk), 89% of his patient achieved refractive stability within 1-2 weeks, and at 12 months, the Snellen Equivalent was -0.10D ± 0.7D. Haze did not exceed trace and the patients reported only postoperative "grittiness". He concluded that LASEK provides significantly quicker visual recovery and refractive stability than standard PRK, produces practically no haze, and eliminated post-PRK pain.
Dr. Hu's poster compared performing LASEK on 19 patients, and PRK on the fellow eye. The LASEK eyes achieved best UCVA within one month, while PRK eyes did that in about two weeks. LASEK eyes achieved steady-state refraction in one month, and there was no difference in terms of post-operative pain. He concluded that the visual recovery was somewhat slower in the LASEK group, which might have resulted from the disproportional central islands he found in the early post-operative period. (This is surprising as Dr. Camellin has noted that in his experience, the laser should be set at lower values than those normally indicated, as the almost total absence of regression entails a higher correction than that obtained with normal PRK...it is advisable to reduce preset values by 10% when correcting up to 10D of myopia, and 20% for myopia between 10D and 20D.)
Wavefront Clinical Results -- From the "Catch the Wave" WRSS (ISRS) Presentations
Alcon Summit Autonomous
Summit Autonomous appears to be in the lead in terms of U.S. clinicals (although Asclepion-Meditec, WaveLight and Schwind continue to make progress outside of the U.S.). At the Academy, Dr. Marguerite McDonald had treated some 59 patients in two groups. In the first group of 23 eyes, treated beginning in October, 1999, 23 patients were given bilateral LASIK and 13 patients bilateral PRK. Each patient had one eye treated according to the CustomCornea diagnostic, while the other eye was treated according to standard refraction. With six month followup, the myopic LASIK group showed that 85% of the customized eyes achieved UCVA of 20/25 or better, compared to 92% who had the standard LASIK treatment. Similarly, in the hyperopic group, 90% of the conventionally treated eyes achieved 20/40 or better, compared to only 80% of the eyes given custom LASIK.
In the second group of 13 patients given either customized or conventional PRK, 85% of the customized eyes achieved UCVA of 20/20 or better compared to only 65% of the standard PRK eyes. In a third group of 23 patients treated with customized and conventional LASIK, after the algorithms had been adjusted, but with only one week followup, 70% of the customized eyes achieved 20/20 or better, compared to 78% of the conventional LASIK eyes.
Thus, it appears, even with very early data, that there is a decrease in higher order aberrations with customized PRK (46% decrease) compared to only 26% decrease with customized LASIK. Could this lead to a PRK comeback? Especially, when combined with LASEK, as described above.
At the ISRS/WRSS meeting, Keith Williams, speaking for VISX, presented on four patients who had had previous unsuccessful refractive surgeries, and had undergone wavefront analysis to correct the problems incurred. All four got markedly improved acuity results, albeit after only one week of followup. The patients were first refracted with conventional manifest refractions, and then again with VISX's WaveScan wavefront diagnostic. The surgeon then ablated a plastic lens, the PreView lens, with which the patient could "test" his/her new refraction to determine if it provided better acuity. If the wavefront method was preferable, the patient was treated with a second LASIK, using the VISX Star S3 variable spot scanning laser. Of the four re-treated patients, ranging in pre-op UCVA from 20/30 to 20/200, all achieved improved results, with three achieving 20/15 one week post-op, and the 20/200 patient improving to 20/30, and all were corrected to their best pre-op BCVA. The only negative was that contrast sensitivity was slightly worse post-op. (It should be noted that the PreView lens used by VISX, and shown on the Academy floor, was first used by Asclepion-Meditec, whose AWACS (Asclepion Wavefront Aberration Correction Simulator) process, whereby a wavefront imprint is placed onto an optical plastic slide for previewing a patient's wavefront guided correction. According to company sources from both companies, Asclepion came up with the idea first.)
Bausch & Lomb
Stephen Slade made the case for use of Bausch's Zyoptix system, which incorporates its Zywave wavefront aberrometer and the Technolas 217Z laser. According to Slade, more than 400 patients have been treated to date in Europe, but results on only 32 patients enrolled in a prospective pilot study in Germany were presented. One eye received standard LASIK, while the other received a wavefront-guided ablation. The results showed on average, a one line increase in BCVA in eyes treated with the wavefront-guided ablation. Of the 32 patients, 7 had no difference between the two eyes; 4 had worse BCVA in the custom eye; and the remaining 21 had better BCVA in the custom eye. These early results are promising, but only compared BCVA and not UCVA, to allow a comparison to results with both the Autonomous and VISX systems. B&L hopes to begin its U.S. clinical trials in December.
Jack Holladay described what LaserSight was doing in custom ablation. It turns out that the company is planning to do all of its work with its AstraMax system, which is similar to B&L's Orbscan device, aiming to achieve prolate corneas, with less tissue removal. The AstraMax only provides stereographic topographic diagnostic information, and does not (yet) have wavescan ability.
Professor Dr. Theo Seiler made the case for WaveLight, stating that "SuperVision" was not the main goal, but that correcting almost everyone to 20/15 was probably good enough, while the use of wavefront was most useful for correction of problematic eyes. Wavefront corrects both refraction and aberration. Some of the problems remaining are to achieve centration of the treatment -- aligning the measurement and the treatment, while adjusting for cyclotorsion -- the difference from measuring while sitting and treating while the patient is lying down. He reported on his patients with at least three months followup, achieving no loss of more than 1 line of vision, but getting an increase in RMS error on average of 1.44±0.74, a 44% increase, a suboptimal result. (Is this due to the effect of the LASIK flap?) Standard LASIK results in a 5-25 times increase. What was accomplished, 15% of the patients treated with waveguided ablation achieved 20/10, compared to only 5% of standard LASIK patients; 45% were between 20/16-20/12, compared to 30% of LASIK treated; and 30% achieved 20/20 compared to 50% with LASIK alone. Thus, 80% achieved 20/20 or better with wavefront, while 85% got that correction with standard LASIK. Dr. Seiler reiterated that, at least at this stage in its development, wavefront guided ablation is best used to clean up previous bad results; that it has not yet achieved its potential, but correlates well with BCVA; and, super normal vision may yet be achievable.
The Road to SuperVision -- from the RSIG Meeting
Jack Holladay led off the discussions with his "prolate is best" speech, wherein LaserSight, as noted above, plans to use its stereographic AstraMax analyzer to achieve prolate customized ablations. Michael Knorz next described the Bausch & Lomb approach of using its ZyWave and Technolas 217Z system. Marc Odrich than discussed VISX's CAP (topographic -- using the Zeiss MasterVue topographer) method, comparing it with wavefront, with only limited experience. Marguerite McDonald then presented her previously reported results, which showed that PRK waveguided results appear to be better than those achieved with LASIK. And finally, Scott MacRae discussed Nidek's OPD Scan approach, which combines topography with a scanning slit autorefractor/retinoscopy to give wavefront-like results.
But the best was left for last, a roundtable discussion where five leading refractive surgeons, Rubens Belfort, Daniel Epstein, Jose Guell, Ronald Krueger, and Michael Lawless debated the merits of wavefront versus traditional corneal topography. Some of the panelists thought that topographical analysis was more relevant and easier to understand, while other thought that wavefront was the future, with topography as an adjunct. Most agreed that wavefront presented more information, but the question was, could this information be used with today's laser systems to provide better refractive solutions?
Some of the comments:
On the subject of wavefront versus topography:
-- topography may be valuable for planning ablation
-- gather the topography, but treat with the wavefront
-- wavefront doesn't give the location of the aberration,
need the topography to find out where it is
-- topography is probably quite good, but wavefront is
the future and will be done.
Can wavefront be used to create new contact lenses to compensate for higher
-- possibly, but topographic analysis is good too
-- could make a wavefront CL, but it would ride/move on the corneal surface
-- easy to make the CL (both VISX and Asclepion are making trial lenses)
but the problem is keeping it centered on the eye
What about Hartman Schack aberrometers versus Tschenring?
-- clinical evidence isn't in yet
-- no comparative data yet, as to which provides better information
-- HS has been used for gathering astronomical data with excellent results.
All in all a very interesting and productive two days of discussion about the future of customized refractive surgery.