Customized Ablation Platforms: Limitations and Advantages

In an effort to compare several of the current customized ablation excimer laser platforms, I contacted refractive surgeons with a knowledge of multiple systems and asked them to share their opinions and experience by responding to a simple questionnaire I designed. The responses included in this article provide important information on the differences, advantages, and limitations of these laser systems.

WHAT IS YOUR PREFERRED EYE TRACKER AND WHY?
Centration and Saccadic Movement
Scott MacRae, MD, of Rochester, New York, values accuracy over speed by far. He prefers an eye tracker that does not require pupil dilation and allows him to see exactly where the eye is centering throughout the procedure. Dr. MacRae refers to a study in which he participated on how fast a system must be based on the natural history of ocular movement during laser ablation.¹

?We sampled 1,500 to 3,000 times during each treatment of nine eyes,? he reports. ?All of us were surprised that we only needed a closed-loop tracking system of 1 Hz to capture 96% of the eye movements during ablation. A similar study conducted by VISX, Inc. [Santa Clara, CA], had comparable findings. With a quick, large saccade, the surgeon should take his foot off the pedal, be patient, and then realign the eye.?

Dr. MacRae would not trust any laser to treat a large saccade well, because he says that the ablation would be more oblique, even if the tracker were able to track an eye in a situation such as this one.

?Most trackers have a passive shutoff if the patient moves more than 1 mm,? he notes. ?It is much more important that one center very accurately at the beginning of the ablation. If you lock on and you're 0.75 mm decentered, you have introduced a systematic error, which the tracker will only maintain. It is worth taking the time to make sure one is in the center of the pupil or wherever one wants to center the ablation.?

Laser Versus Video-Based Systems
Michael Gordon, MD, of San Diego prefers LADARVision (Alcon Laboratories, Inc., Fort Worth, TX).

?I believe it provides the best tracking and spot delivery,? he explains. ?The space-stabilized image provided to the laser ? produces excellent results with a flying-spot system. I do not believe that video-based systems—particularly the slower ones such as VISX's STAR S4 Excimer Laser System—can produce the accurate placement with a small spot we will need for custom treatments.?

Minneapolis surgeon David Hardten, however, likes the ActiveTrak 3-D active eye tracker (VISX, Inc.) best, because he finds it versatile, as well as straightforward and easy to use. He adds that this type of tracker works well with most eyes, but he notes that, if ocular anatomy renders tracking difficult, the treatment may be continued without tracking. He asserts that the tracker is rapid enough to track all but the fastest saccades, in which case it pauses for safety. He comments that, because the eye is not fixating during these large ocular movements, it would be inappropriate to treat the eye when it is not fixating due to the eccentricity of the treatment applied. Additionally, Dr. Hardten notes that the versatility of the VISX system allows for the tracking of patients with extreme amounts of eye movement due to the loss of fixation using fixation devices such as the Occuguard (eyeFix, Inc., Minneapolis), eyeFix Speculum System (eyeFix, Inc.), and Thornton Fixation Ring (Duckworth & Kent Ltd., Hertfordshire, England).

WHAT SAFETY FEATURES DOES YOUR FAVORITE TRACKER POSSESS, AND HOW DOES IT (OR YOU) ENSURE ON-AXIS ABLATION?
Ronald Krueger, MD, of Cleveland notes that the LADARVision tracker will automatically shut off the laser if it moves outside the tracking zone, and Brian Boxer Wachler, MD, of Beverly Hills, California, appreciates that the tracker on this system can register the treatment.

“There is a horizontal reference line that is aligned with reference marks placed by the surgeon,” Dr. Gordon explains further. “This ensures accuracy and minimizes the effect of going from the sitting to reclining position, which produces the greatest cyclotorsion effect. The LADARVision platform also has a virtual hinge protector, which helps to minimize the effect of possibly ablating the undersurface of the hinge.”

Arturo Chayet, MD, of Tijuana, Mexico, prefers the eye tracker used by the NAVEX laser system (NIDEK, Inc., Fremont, CA), because the tracker halts laser ablation when the eye drifts ±250 µm from the pupil's center. Because Dr. MacRae can ascertain where the treatment is centered at any time during the procedure, he values the Zyoptix system's eye tracker (Bausch & Lomb Surgical, San Dimas, CA). He finds that precise visualization is reassuring and creates a more surgeon-friendly environment.

For his part, Dr. Hardten likes that the STAR S4 Excimer Laser System has active and passive tracker capabilities.

?Certainly, no eye tracker yet verifies patient fixation,? he says. ?This is still the surgeon's job, but, in the near future, the VISX system should have iris registration verification to the preoperative WaveScan image that should help verify alignment on the wavefront image, detect cyclotorsion, and provide better verification of patient fixation,? he says.

DOES YOUR LASER SYSTEM TAKE CYCLOTORSION INTO ACCOUNT? IF NOT, HOW DO YOU COMPENSATE FOR THIS INTRAOPERATIVELY?
Dr. Hardten notes that the STAR S4 does not currently take cyclotorsion into account, but he refers to his earlier comments about VISX's efforts to use iris detail to detect this phenomenon. He observes that the amount of cyclotorsion can change intraoperatively in some patients, and he will pause the treatment in these cases until the fluctuation ceases and the patient realigns fixation. Dr. Hardten says that good results and previous studies' poor ability to detect differences in results in most eyes leads him to believe that shifting cyclotorsion is inconsequential in most cases.² He asserts that the phenomenon will assume importance as surgeons fine-tune their results, however.

Although the NAVEX laser system does not yet account for cyclotorsion, Dr. Chayet states that the forthcoming Nidek Torsion Error Detector will compare an OPD-Scan image (NIDEK, Inc.) (measured while the patient is seated) with an eye-tracker image (measured while the patient lies under the excimer laser microscope). The detector will then display the error axis. According to Dr. MacRae, Bausch & Lomb is also working on the problem of cyclotorsion. He says that the company will introduce an iris-tracking system this year that can track x-axis, y-axis, and rotational movement of an eye with a dilated or undilated pupil. Presently, he deals with cyclotorsion by marking the limbus at the 3- and 6-o'clock positions while the patient is at the slit lamp. He then confirms positioning when the patient is lying down and again intraoperatively if he suspects cyclotorsion. As described earlier, Drs. Gordon, Krueger, and Boxer Wachler rely on a horizontal reference line, against which they align the limbal marks they have made.

HOW DOES THE Z-AXIS AFFECT YOUR LASER'S ABILITY TO TREAT PATIENTS? AT WHAT POINT DOES IT MATTER?
Dr. Hardten comments that the VISX S4 system tracks the z-axis and has a passive shut-off mechanism, but he notes that “it is important only if you are significantly off in the z-axis to where you are out of focus.” Dr. Krueger, meanwhile, remarks that z-axis error is not a concern with LADARVision, which uses nondivergent beams to ensure proper focus.

Dr. Chayet believes that ablation size and shape are the key factors. In the point of precise ablation, he thinks it is important that the NIDEK cross lights are able to focus easily. Moreover, he says this system can help the surgeon detect slight misalignment.

By contrast, Dr. MacRae asserts that the subtle importance of the z-axis becomes evident when using the Zyoptix system: “The z-axis is subtly important, and one can see this directly with the Bausch & Lomb system. It has a green laser light, which adjusts the z-axis. This should be superimposed with its red fixation light at the time of surgery. If one is out of focus and readjusts the z-axis, the tracking centration shifts slightly, indicating that the z-axis focus does have a subtle effect (30 to 60 µm) on the tracker positioning. The tracker can easily be recentered if this is noted.”

WHAT DOES THE FUTURE HOLD REGARDING REGISTRATION?
Dr. Gordon comments that registration performed prior to dilation allows the surgeon to capture the pupil/limbal relationship in an undilated state. He states that, in the future with CustomCornea (Alcon Laboratories, Inc.), this step will occur at the time of wavefront measurements. He believes this timing is important to the treatment of higher-order aberrations, because small changes in axis significantly decrease the treatment's effect at the level of third-order aberrations and higher. Dr. Boxer Wachler asserts that the registration process is critical to ensuring that the treatment is not decentered, especially hyperopic treatments, which can induce higher-order aberrations.

Several of the respondents named iris registration as the next step in customized ablation. Another point Dr. MacRae emphasizes is the need for identical registration between the wavefront analyzer and the laser tracker.

WHAT MAY WE EXPECT IN THE FUTURE REGARDING CENTRATION?
“Centration will significantly improve when the optical system is centered based on the pupil and the wavefront,” states Dr. Hardten, who believes “a system that is geared toward the optics as seen through the pupil … should be better than any other way we currently have of determining centration.”

Dr. Gordon considers the fact that surgeons currently base centration on the center of the pupil or on the Purkinje image to be “a weak point in all systems, as we do not know what the correct reference point is.” He comments that studies are underway to determine if the reference point is over the entrance pupil or if it is better to center on the light reflex of the cornea. He himself prefers to center over the entrance pupil.

For his part, Dr. Krueger suggests that tracking the fovea would be an improvement in both tracking and centration, but he is not aware of any efforts in this area to date.

WHICH COMPONENTS AVAILABLE ON EXISTING, FDA-APPROVED SYSTEMS WOULD COMPOSE YOUR DREAM MACHINE?
Although Dr. Hardten chooses the STAR S4, he desires the addition of three more oblique lights superiorly so that he can see the fluid pattern as a treatment evolves. Dr. Chayet is interested in using the NAVEX system for his dream machine's base, the OPD-Scan as the aberrometer, and the LADARVision eye tracker for flying-spot ablation.

Dr. MacRae favors NIDEK's two-light slit lamp system, which he says gives an excellent, glare-free view of the pupil and facilitates the discernment of z-axis focus. He also likes the NIDEK system's ergonomics best. For superb optics, Dr. MacRae would incorporate the microscope (Leica Microsystems, Inc., Wetzlar, Germany) featured on the STAR S4. He asserts that Zyoptix has the best eye tracker and overall system. Nevertheless, Dr. MacRae notes that no current laser platform has solved the problem of plume control, so he would include the Dell PlumeSafe System (Buffalo Filter, Buffalo, NY) developed by Steven Dell, MD, of Austin.

Dr. Boxer Wachler wished that he could use LADARVision without taking predilation photographs. By centering all his laser treatments on the coaxially sighted corneal light reflex, which approximates the visual axis, he has eliminated the step and made his dream a reality. He now dilates patients immediately after they complete their paperwork, performs the cycloplegic and fundus examinations, captures the centration photographs with a dilated pupil, and then performs surgery. The single remaining component for Dr. Boxer Wachler's dream machine would be an ergonomically designed patient bed.

Dr. Krueger opts exclusively for an Alcon system, although he would also consider including the WaveLight Laser System's flying spot scanning delivery system (WaveLight Laser Technologie AG, Erlangen, Germany) and a COAS Wavefront System (Wavefront Sciences Inc., Albuquerque, NM). Regarding the Autonomous system, he notes that, “Although many don't like dilating patients, I like the fact that I can mark the patient's limbus, capture the wavefront centration scan, dilate the patient, and then capture the wavefront measurement at the maximum pupil dilation. Following these steps, I can take the patient straight to the laser and treat, orienting on the limbal markings as reference points.” He adds that “a chair is a chair is a chair.”

Dr. Gordon's dream machine would offer preoperative eye registration and a repetition rate of 200 Hz. It would include radar-based tracking that produces a space-stabilized image and a flying spot of less than 1 mm in size with Gaussian energy distribution (both available on the LADARVision system). Dr. Gordon favors the robustness of the STAR S4 system, which he says requires minimal maintenance. He also likes its vacuum and video capture, the latter of which he says is easy to access and of good quality. For versatility, he would use the VISX chair but modify it with the LADARVision's hand controls so that the unit could be swung out for use with an INTRALASE FS laser (IntraLase Corp., Irvine, CA) next to it. Finally, he would meld the working distance of the LADARVision4000 with the microscope and comfort of the STAR S4.

Brian S. Boxer Wachler, MD, is Director of the Boxer Wachler Vision Institute in Beverly Hills, California, and he is a faculty member at the University of California, Los Angeles. He is a consultant for Alcon Laboratories, Inc. Dr. Boxer Wachler may be reached at (310) 860-1900; bbw@boxerwachler.com.

Arturo S. Chayet, MD, is Director of the Codet-Aris Vision Institute in Tijuana, Mexico. He is an international investigator for NIDEK, Inc. Dr. Chayet may be reached at +52 66 4683 57 23; arturo.chayet@arisvision.com.mx.

Michael Gordon, MD, is a partner at the Gordon Binder Vision Institute in San Diego. He is a consultant for Alcon Laboratories, Inc., and an investigator for WaveLight Laser Technologie AG. Dr. Gordon may be reached at (858)455-6800; mgordon786@aol.com.

David R. Hardten, MD, is Director of Refractive Surgery for the Minnesota Eye Consultants in Minneapolis and Clinical Associate Professor of Ophthalmology at the University of Minnesota in Minneapolis. He is a consultant for VISX, Inc., and TLCVision, Inc., and has conducted research for VISX, Inc., Bausch & Lomb, Alcon Laboratories, Inc., Allergan, Inc. (Irvine, CA), and Refractec, Inc. (Irvine, CA). Dr. Hardten may be reached at (612) 813-3632; drhardten@mneye.com.

Ronald R. Krueger, MD, is Medical Director of the Department of Refractive Surgery, Cole Eye Institute, Cleveland Clinic Foundation, Cleveland. He receives research funds and travel support from Alcon Laboratories, Inc. Dr. Krueger may be reached at (216) 444-8158; krueger@ccf.org.

Scott M. MacRae, MD, is Professor of Ophthalmology and Professor of Visual Science at the University of Rochester in Rochester, New York. He is a consultant to Bausch & Lomb Surgical and NIDEK, Inc. Dr. MacRae may be reached at (585) 273-2020; scott_macrae@urmc.rochester.edu.

Karl G. Stonecipher, MD, is Director of Refractive Surgery at Southeastern Eye Center in Greensboro, North Carolina. He does not hold a financial interest in any product mentioned herein. Dr. Stonecipher may be reached at (800) 632-0428; stonenc@aol.com.

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