Customized Ablation #7: ASCRS Report: What's New in Customized Ablation and A New Treatment for Hyperopia and More
ASCRS Report: What's New in Customized Ablation and A New Treatment for Hyperopia and More
Irving J. Arons
Much has happened since the last time I addressed the subject of customized ablations in January (my AAO reports in the January 1st and 15th issues of OSN). More thought has been given to the inherent problems -- some of which are now starting to be addressed -- and more data has been collected by several of the companies involved in wavefront-guided custom ablation. As I wrote in January, the results with "classic" LASIK have gotten much better thanks to improved lasers, trackers, and keratomes. Most patients today can obtain uncorrected visual acuity close to or better than 20/20. But wavefront-guided ablations appears to offer even more -- once the problems are solved.
In a recent article ( "Wavefront-guided ablation: What Are the Obstacles to Implementation?", Refractive Eyecare for Ophthalmologists, April 2002), several ophthalmologists addressed some of the issues remaining to be solved. Ron Krueger listed some of these: 1) proper registration of the wavefront information to the laser and its tracking system; 2) wound healing issues -- will the process of healing obliterate the lasers' fine corrections? 3) biomechanical changes in the cornea, especially due to flapmaking in LASIK; 4) the reliability and reproducibility of the wavefront information itself; and 5) defining ablation patterns that truly correct the wavefront pattern.
In ablating the cornea to correct aberrations using wavefront guidance, we are dealing with minute corrections. Such things as the angle at which the pulse strikes the cornea may make a difference in the effect. And, there are hardware and software issues to be addressed. Dr. Krueger believes that there is no single obstacle, but an array of challenges that need to be addressed one step at a time.
In the same article, Marguerite McDonald discussed the unpredictable biomechanical changes induced by the microkeratome cut, that in turn alters the wavefront map in an unpredictable way. That is one of the reasons driving the renewed interest in PRK and LASEK, and why some experts are discussing the possibility of a two-step LASIK procedure, in which first the flap is made, and several weeks later the wavefront diagnostic is taken and the ablation performed.
Registration of the wavefront data was discussed by Dan Durrie (and Marguerite in one of her ASCRS presentations ["Importance of Tracking, Centration, and Registration in Custom Ablations," Marguerite McDonald, MD, ASCRS, Philadelphia, June 2002]). Dr. Durrie noted that the wavefront diagnostic was done on a seated patient, and then was attempted to be transferred to a supine patient some time later. As he discussed, it has been challenging just to center the ablation properly and to maintain orientation in the face of cyclotorsion from the patient's change in posture. With custom ablation the challenge is to position the ablation perfectly, time, after time, after time.
With both the Alcon LadarVision system and the B&L Zyoptix with which he works, he has nothing to confirm that he is in the right place when he starts the ablation. What he would like to see is technology that not only confirms the accuracy of the registration, but that actually performs the alignment and locks it into place. As he noted, this is a quantum jump from current eye tracking.
"What's needed for custom ablation is a failsafe device that will align the ablation pattern with the eye and not let the laser operate unless and until the two are perfectly registered," he said.
Another note of caution was provided by Omar Hakim. He discussed the changing basis of the wavefront, stating that the wavefront was dynamic, changing almost on a moment-to-moment basis. It changes with pupil size and accommodation, and it evolves in other ways over longer periods of time as the eye matures and ages. Thus, a depiction at a point in time may not ideally depict what may happen at a later date.
"The wavefront we are looking at is the product of multiple variables that will differ over time. It is a static representation of a dynamic phenomenon." He suggests that the use of an adjustable corneal inlay in tandem with an adaptive technology, such as liquid crystal chemistry, might be able to create a dynamic wavefront correction.
Dick Lindstrom addressed the biomechanical changes and wound healing processes that occur during LASIK. He noted that it might be difficult to correct the 1-5 micron higher order aberrations when the microkeratome induces 6-12 microns of aberration into each cornea in an unpredictable fashion. And, the final arbiter of corneal wound healing is the corneal epithelial cell -- which is 6-8 microns thick.
As he noted, "One epithelial cell in the wrong place induces more aberration than we are aiming to correct." But as he further noted, "We should not be too pessimistic. This technology is still in its infancy."
Marguerite McDonald, MD, in her ASCRS presentation noted that as the complexity of the correction increases, the requirements for centration, registration and tracking become more stringent. With just a 10° misalignment with cyclotorsion, a second order cylinder correction results in a 30% residual error. The same misalignment for a sixth order correction leaves a residual correction of 100%, with no impact on the desired correction.
She concluded, "While centration, registration and tracking are important for all procedures, they become even more so for customized treatments, especially when complex higher order corrections are attempted."
Manufacturers Addressing Problems
With all of that said, where are we today? Many of the problems noted above are being addressed by the laser manufacturers. Several have already addressed the problems of registration and cyclotorsion, some are also beginning to address the problems induced by the flap with intraoperative pachymetry which accurately measures the real thickness of the flap and the depth of ablation in real time.
In his opening remarks at the VISX/EyeWorld Symposium on "Wavefront Diagnostics, Outcomes and New Technologies, held during the ASCRS meeting, Douglas Koch, MD, put it into perspective, "20/20 was the minority outcome during the early days of PRK/LASIK in the 1990s, today 20/20 outcomes are the majority, while in the future, with the advent of wavefront guided ablations, 20/20 outcomes will be the minimum standard!"
In the future, ophthalmic surgeons can look forward to some exciting innovations. Companies such as Asclepion-Meditec (soon to be part of Zeiss Ophthalmics and renamed Carl Zeiss-Meditec) and WaveLight have begun incorporating tissue saving and prolate optimization algorithms into their computer programs driving their lasers. Asclepion introduced its CRS-Master product at ASCRS. This product combines topography and wavefront in an all inclusive workstation with a new sophisticated software package which can be directly linked to the Meditec excimer laser. It allows the surgeon to integrate flap data, corneal thickness, pupil size, topography and wavefront data to define the ideal ablation profile for each eye. The program includes a tissue saving mode, and a prolate and night vision optimization mode, all built into its algorithms.
Nidek, in its newest incarnation of the EC-5000, the EC-5000CXII, (to be introduced this fall) has added an improved, faster responding four-beam infrared eye tracker, along with precision alignment and pupil registration of the undilated eye, a new cyclotorsion error correction algorithm, and the multispot ablation (called MultiPoint by Nidek) that was announced at last fall's AAO meeting, all combined with its combination wavefront and topographical diagnostic, the OPD scan.
VISX has also developed an iris registration cyclotorsion feature for their Star S3 laser tracker. And, as noted above, Schwind is among the first excimer laser producers to link online pachymetry using optical coherence tomography (OCT) with their laser, to intraoperatively visualize the actual thickness of the flap (rather than the theoretical thickness), the depth of ablation, and residual corneal thickness. This enables the surgeon to determine how much tissue has been ablated, and how much residual stromal/corneal thickness remains. The Sirius pachymeter, linked to the ESIRIS laser, allows real-time online visualization of the various stages of the LASIK treatment. (It should be noted that Haag-Streit also produces a corneal pachymetry system, the HS Pachymeter, which can be used in a similar fashion to the Schwind Sirius pachymeter (which I believe comes from Zeiss Humphrey), for inline, real-time optical low coherence reflectometry, to measure corneal thickness to a precision of 1 micron. Haag-Streit said it was working with at least two laser manufacturers who are evaluating its system (believed to be WaveLight and perhaps VISX).
VISX also announced just prior to the ASCRS meeting that it had signed an agreement with Tracey Technologies to market the Tracey Visual Function Analyzer (VFA) ray tracing device, along with the 20/10 Perfect Vision WaveScan aberrometer, to enable refractive surgeons to evaluate both normal eyes (with the WaveScan) and abnormal eyes (with the Tracey VFA), increasing the range of eyes that can be evaluated and treated. VISX has also licensed the Ruiz multifocal patents for the treatment of presbyopia, using its Contoured Ablation Pattern (CAP) procedure, already available on its Star S3 laser for treating astigmatism. Initial trial data on 23 eyes was reported at the meeting. The ablation pattern forms an aspheric cornea giving good vision throughout the visual spectrum (near, intermediate and at distance).
Competition for "Bladeless" flap production
A new entrant showed its wares in the race for "bladeless" flap refractive surgery. 20/10 Perfect Vision displayed its new femtosecond laser in competition with IntraLase. The difference between the two laser systems appears to be the fact that 20/10 Perfect Vision intends to couple its laser with its wavescan aberrometer. In the race for bladeless flaps, 20/10's laser is programmed to produce curved/contoured flaps (to preserve the natural shape of the cornea). The femtosecond laser is capable of both stromal lenticle formation and removal, and of intrastromal ablation, with cutting open the stroma. Because of gas formation within the stroma, the refractive effect may be limited to custom treatments of about two diopters of correction. For higher corrections, it may require a separate treatment sometime after the first treatment has healed. Intrastromal corrections are expected to be done for myopia, hyperopia and for presbyopia (via monovision).
Advances with LASEK
In another attempt to alleviate the effects of the microkeratome-created flap, there were several new LASEK techniques discussed at the meeting. These were in addition to Marguerite McDonald's gel-assisted LASEK technique, which I wrote about in my AAO roundup in January. The new techniques, again to preserve the epithelial tissue included hydrodissection (Richard Rashid, MD) with either BSS solution or the use of Genteal (or even Genteal Gel, in some cases), LASEK viscodissection (David Langerman), and Ioannis Pallikaris's new sub-epithelial separator device, the SES. This is an automated microkeratome-based device for use with his sub-epithelial Lasik procedure (or EpiLasik, as others have called it). The device, along with a suction ring, creates an epithelial flap without the need for using alcohol, that allegedly results in faster healing and less pain for the patient. The SES is not currently available in the U.S., but is expected to be on the market international by Ciba Vision in early 2003. Dan Durrie has even suggested a new name for LASEK, to avoid its confusion with LASIK. He advocates calling LASEK procedures "Advanced Surface Ablation".
A New Treatment for Hyperopia (and beyond)
With its recent FDA marketing approval, Refractec held a symposium to introduce conductive keratoplasty (CK) to ophthalmologists. At the symposium, held in conjunction with Ophthalmology Management, Dan Durrie, Robert Maloney, and Marguerite McDonald provided some very interesting information about the use of CK to treat astigmatism (Durrie), presbyopia (Maloney), and hyperopia -- the U.S. clinical trial results (McDonald). Dan Durrie led of the session discussing his experience in using CK for treating astigmatism. What he found was that he could actually adjust the amount of correction intraoperatively by viewing the reflected ring formed by the cornea as the probe was applied in different quadrants.
Robert Maloney discussed how the ViewPoint device could be used to provide near vision via monovision to correct for presbyopia. With the patients he has treated, the effect looked good out to 3 years. Although the device had only been approved for hyperopia without astigmatism, it is being used successfully off-label for treating astigmatism, presbyopia, and over and under corrected LASIKs. Dr. Maloney noted that there was a very fast learning curve and was fool proof to use. In his presbyopia trials, after nine months, 79% of his patients were seeing 20/20 J2; 83% 20/25 J3; 87% 20/32 J5; and 91% 20/40 J6. Stability was achieved in within 2-3 weeks.
Dr. Maloney also described how the heating effect of CK was different from what was achieved with LTK (using the Sunrise laser). In a thermal effects study done by A.J. Welch's group at the University of Texas, the heating zone profile produced by the ViewPoint needle probe was more uniform and, thinner, and deeper (a column of heating 250 microns wide and 80% of the cornea deep), while the LTK laser pulses produced a much wider heating zone that concentrated in the upper stroma. That might explain the anticipated less regression with the CK device compared to the LTK. And, in fact, when Dr. McDonald was asked about the "temporary" label given to Refractec by the FDA, she said it was because of their previous experience, and it would probably be removed when Refractec provided its two-year data to the agency.
Dr. McDonald discussed the FDA trial data that led to the .75 to +3.00 diopters of hyperopia approval. At 12 months, 54% of patients achieved 20/20; 74% were 20/25; and 91% were 20/40 or better. As for stability, there was 0.9 of a diopter loss in the first 3-6 months, 0.03 diopters in the 9-12 month period, and only 0.04 diopters during months 12-24.
There is, however, one caveat to consider about the use of ViewPoint CK for treating hyperopia. Most low hyperopes are comfortable with their vision. It is only when they become presbyopic that they begin looking for alternatives. Thus, the real problem to be solved with CK is presbyopia, not low to moderate hyperopia, along with some degree of astigmatism (and over/under corrected LASIKs). But that said, because of the low price of the device, the ease of its use, and its safety record, once the "temporary" label is removed, I would expect to see widespread use of this device as an alternative to hyperopic LASIK.