REGISTRATION IS AN IMPORTANT and developing issue for wavefront-guided LASIK treatment. Systems today use varying methods of registration, such as limbus and iris registration. "It is important to have some type of registration performed," says Brian S. Boxer Wachler, MD, an assistant professor at UCLA's Jules Stein Institute. "When patients have their wavefront scan acquired they are in an upright sitting position, yet when they lie down for treatment, the eye may have some cyclorotation.
"If a surgeon can not or does not compensate for that cyclorotation by also knowing where to rotate the treatment (so it is in the same position on the eye as when the patient was sitting upright), then the treatment will be off by varying degrees because of the rotational component," he says. "A lot of times, the wavefront aberrations can be affected if the rotation application of the treatment is off from the true position," he explains.
Dr. Boxer Wachler says Alcon was the first to define the term "registration" and it was essentially the first to publicize the importance of controlling this parameter. Other companies, such as Visx Inc. (Santa Clara, Calif.), have also developed their own form of registration, known as iris registration.
Dr. Boxer Wachler primarily uses the Alcon LADARVision system, which employs limbus registration. However, he has used the Visx laser as well. Limbus Registration
Ronald R. Krueger, MD, also uses Alcon's LADARVision system. "Limbus registration is very effective," he says. "Now Alcon has auto-registration, which automatically identifies the structure so you can get better predictability and uniformity of types of capture. They are also working on scleral-vessel registration so the rotational aspects can be compensated for," says Dr. Krueger, medical director at the Cleveland Clinic Foundation Hospital.
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| Error induced from cyclotorsional movements in an eye with 1 D of cylinder. |
"The real test of good registration is how well you do on custom retreatments, because those are the really aberrated eyes, and those aberrations are so sensitive that if you are off a little bit you are going to get lots of new aberrations somewhere else. So far in my hands it has worked well, I have been able to help some people who have had problems before," Dr. Krueger says. The autoregistration uses the same principles as the limbus registration. "The system 'sees' where the limbus is, and then will over lay it," says Dr. Krueger. "Surgeons still have an opportunity to adjust for rotation and registration if necessary on the first of the five captures with the wavefront device," says Dr. Krueger. He notes that this form of registration is supposed to increase the accuracy of alignment by fourfold. "Automatic limbus registration makes things faster and a little easier. Scleral vessel registration may make it even better in the future," he says. Iris Registration
Visx identified the need for iris registration and developed its technology, which is currently in use internationally. The company hopes for approval for iris registration in the first quarter of 2005.
"Alignment and registration are very important because lack of cyclotorsional registration will induce errors," explains Julian Stevens, Moorfields Eye Hospital, London.
Iris image registration permits compensation for cyclorotational errors and permits verification of patient ID and re-registration for intraoperative cyclotorsional movement, he notes.
Jeffrey Machat, MD, of TLC Vision in Toronto, also has experience with Visx's automated iris registration. He says, "In pharmacologically dilated eyes, cyclorotation can be 15 degrees or more. Upright wavefront assessment but supine laser ablation can cause cyclo-misalignment and patient movement also can produce cyclotorsion."
Error induced from cyclotorsional movements in an eye with 1 D of cylinder can be significant. For registration of ablations, WaveScan exams are acquired with the patient in the upright position. The laser ablation is performed with the patient in the supine position and with one eye covered.
The iris registration technique calculates the torsional angles from multiple measurements. "Sophisticated algorithms calculate ablation torsional angle," says Dr. Stevens, who notes that the algorithms compensate for pupil distortion, axial misalignments, changes in pupil size, and reflections. For example, the algorithm compensates for pupil center shift by using the iris outer boundary as a static reference.
"For iris registration, the system 'unwraps' the iris in both images and selects multiple reference points," says Dr. Machat. "Because the iris pattern is unique to each patient, the iris regulation system can find most prominent iris features for each eye."
Iris registration challenges include the need to account for different camera appearances of an iris with the WaveScan versus Visx laser, as well as different lighting, pupil size and shape, angle, and flap or epithelial debridement.
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As part of a five-step process, the Visx system "flattens out" the iris image to reveal unique reference points. |
Pupil Centroid Shift
Wavefront-guided treatments require more attention to centration because the pupil centroid also shifts between wavefront acquisition and the laser treatment, says Marguerite B. McDonald, MD, of the Southern Vision Institute, in New Orleans.
"Ignoring the pupil centroid shift may lead to sub-optimal results in wavefront-driven refractive surgery," she explains. "Pupil centroid shift can occur in different lighting environments. As the pupil changes size, its centroid may not remain stationary, relative to the outer iris boundary."
Dr. McDonald notes that the larger the pupil size difference between the wavefront and laser capture, the larger the centroid shift. "Therefore, it is critical to compensate for differences in pupil size between wavefront capture and laser delivery," she says.
The CustomVue, which has outer iris boundary registration, compensates for pupil center shift. "Outer iris boundary registration means that no matter where the laser pupil centroid is located, the wavefront ablation will be consistently centered over the wavefront pupil centroid," Dr. McDonald notes.
Dr. Stevens adds, "The next generation of technology will include cyclotorsional tracking, which tracks cyclotorsional movements intraoperatively. This uses similar technology to that used for cyclotorsional registration."
One Thousand Points
Nidek has introduced iris registration for its systems internationally and pending FDA approvals in the United States. The technology essentially imprints and calculates thousands of points on the iris and uses that for registration accuracy. (The Nidek EC-5000 Excimer Laser System approved in the United States does not employ iris registration.)
"The patient gets the iris biometry scan during the wavefront measurement, so it is not an extra step, says Rich Bains, Nidek's director of clinical education. "The capture of the image of the iris is a normal step while we are capturing the wavefront and topography with our OPD scan and then it is compared to the state of the iris when the patient is laying down underneath the laser."
Because each person's iris has a unique imprint, like a fingerprint, thousands of points on the iris can be used for registration and calculation as the ablation is taking place, says Kuntal Joshi, Nidek's director of global marketing.
He cites three distinct advantages to Nidek's iris registration. First, the thousands of points of measurement ensure accuracy. Second, there is no need to dilate the eye prior to measurement, which is required with some other iris registration on other wavefront-guided LASIK systems. "With registration that requires dilation, the efficiency of the technology in a clinical practice is very difficult and it adds a complexity to the actual procedure time," he says. Third, increased accuracy improves outcomes. "Off-center and decentered ablations are reduced," he says.
Iris Recognition
The Zyoptix 100 system, sold outside the United States by Bausch & Lomb, features iris recognition. The new Zyoptix 100 iris recognition technology maps the entire iris, generating a unique digital "ZY-ID" for each patient. Data is used to automatically correct cyclotorsion and pupil shift information in the diagnostic phase. Greater accuracy of the laser ablation is possible in the treatment phase. The "ZY-ID" is stored with the patient's treatment file and used for automatic recognition at the laser ensuring that the correct eye is treated each time.
"This technology allows for rapid patient identification and ensures the correct patient and eye are treated. It also minimizes the need for data input. In addition, automatic patient recognition enhances treatment safety," says Kristine Morrill, communications director for B&L, Europe, Middle East & Africa.
With Zy-ID, the patient's details are automatically displayed, saving time and increasing safety. The system also has a multi-dimensional eyetracker, which is designed to improve the accuracy of treatment placement.
"The eyetracker has a 6.6 msec reaction time, and the iris recognition feature corrects for intraoperative eye movements and potential misalignment of ablation," says Ms. Morrill. "It also corrects for rotational movement and rotational differences between the diagnostic process and laser treatment can be up to 10 degrees. A rotational misalignment of 5 degrees can lead to an under-correction of approximately 17 percent."
The Zyoptix 100 Multi-dimensional Eyetracker can determine the difference in rotational angle before treatment and automatically compensates on a variety of pupil sizes. It also corrects for pupil center shift between diagnostic process (dilated pupil) and laser treatment (undilated pupil), which can be as great as 500 µm. "This can have a significant impact on refractive error correction, especially when addressing higher-order aberrations," she says.
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| Alcon's LADARVision system employs limbus registration. |
The system also corrects for movements in the third (or z dimension). For example, the pre-adjusted treatment plane might be altered if the patient accidentally presses into the headrest or the bed is moved. This could result in defocus of the ablation surface.
Saccadic eye movements and involuntary transversal movements in the x/y plane are also corrected, as the system's eyetracker incorporates active, dynamic X-Y tracking.
The Outlook
Registration is a key to the successful and precise outcomes with wavefront-guided LASIK ablations. In the future, new technologies, such as scleral-vessel registration or updated versions of current limbus or iris registration techniques will help ensure the precision needed for optimal outcomes.
"The iris registration technology optimizes clinical outcomes simply through alignment of wavefront map and laser ablation in all axes," says Dr. Machat. "Just as X-Y eyetracking has become the standard today, so will automated iris registration tomorrow."
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