Side-by-Side Systems

Currently, a couple of manufacturers offer users the option of captureing cor­neal topography and wavefront data from separate instruments. Usu­ally, users can then plan an ablation using a wavefront-guided ap­proach, or, in some cases, a topography-guided method. Here is a rundown of them:

• Bausch & Lomb’s Zyoptix Diagnostic Workstation. This system allows the surgeon or tech first to image a patient’s eye using the Orb­scan IIz Anterior Segment Analysis System. The Orbscan provides topographic information about the anterior and posterior cornea, cor­neal pachymetry and the depth of the anterior chamber. If the physician also has equipped the workstation with the Zywave II Wavefront Aberrometer, it will also accept its information. The aber­rometer measures the aberrations and is the primary means by which the custom treatment is planned. The Orbscan data supports the wavefront plan­ning in the form of pachymetry, K readings and asphericity values.

 • WaveLight Allegro Analyzer and Allegro Topolyzer. WaveLight offers the option of custom wavefront-guided or custom topography-guided treatments with its Allegretto Wave laser, depending on which of its diagnostic de­vices you use. Both are currently only available for custom procedures outside the United States. Of course, surgeons can also use the company’s wavefront-optimized treatment for primary cases in normal eyes that don’t require custom treatments.

CustomVis' experimental ZCAD software combines topography and wavefront to provide a useful treatment plan for patients that excludes lenticular aberrations. CustomVis

The Allegro Analyzer is an aberrometer that uses the Tscherning principle of aberrometry, which projects an image of the individual measurement spots onto the retina. If the patient falls into the small group of patients that needs more higher-order aberration treatment than can be provided by the Allegretto’s wavefront-optimized treatment, the surgeon can use the Allegro Analyzer’s wavefront data to plan a custom ablation.

For patients whose corneas are very irregular due to causes such as previous laser vision correction or radial keratotomy with very small optical zones, decentered ablations, an irregular microkeratome cut or a corneal transplant, WaveLight has a topography-based treatment using its Topolyzer placido-disk based topographer.

This spring, the company plans to expand its topography-based treatment by offering the Oculus Pentacam anterior-segment imaging device linked to the Allegretto Wave laser system with software called OcuLink. The device will be marketed by WaveLight as the Oculyzer, and the company says it will provide a wider range of diagnostic information than the Topolyzer. Neither system, however, currently merges its topographic information with the Allegro Analyzer’s wavefront data to yield a separate, merged map for custom-treatment planning.

Combo and Near-Combo Systems

There are also several laser systems that attempt to bridge the gap between aberrometry-based systems and topography-guided ones. Two new products appear to have actually merged the technologies. Here’s a look at these hybrids:

 • Nidek’s OPD-Scan. Out for several years now, the Optical Path Difference Scan uses a technology called dy­namic skiascopy, a variant of retinoscopy, to acquire 1,440 measurement points in about 0.4 seconds. These points communicate the aberration power map of the eye, corneal topography and autorefraction. The instrument can provide standard to­po­graphy maps, refraction maps and power maps.

Aleksandar Stojanovich, MD, a Norwegian surgeon whose practice is charged with managing the country’s patients with irregular astigmatism, has looked into many systems aimed at managing complex corneas, including Nidek’s OPD-Scan.

“The OPD-Scan will give you the necessary topography and aberration information, but doesn’t offer a merger of the information,” he says. “You use either one or the other based on a patient’s presentation. For example, for a pa­tient with irregular astigmatism, you would use the system’s topography information. For a virgin eye, on the other hand, Nidek has an optimized ablation, rather than one that tries to fix aberrations that aren’t really bothering the pa­tient.”

Specifically, the surgeon can choose to use one of the following: Optimized Aspheric Treatment Zone ablations to prevent spherical aberration that can cause night vision problems, glare and halo; Customized Aspheric Treatment Zone ablations that are topography-guided for corneas that are irregular after decentered ablations or have central islands; or OPD-Guided Customized Aspheric Treatments for those eyes that need their total aberrations reduced by a wavefront-guided treatment. In a study of OPDCAT treatments, 86 percent of patients gained at least a line of best-corrected acuity, 86 percent were within 0.5 D of their in­tended refraction and no one lost any contrast sensitivity as of three months postop.

 • Schwind’s ESIRIS with the ORK Custom Ablation Manager. Though the ORK-CAM also keeps the topographic (called “corneal wavefront” in Schwind parlance) and aberrometer data separate for ablation planning purposes, what makes it intriguing from a surgeon’s point of view is that the system can separate out corneal aberrations from those of the rest of the eye. This means a “topography-guided” ablation can actually tackle aberrations that are cornea-based, while leaving the rest of the visual system undisturbed.

The Scout corneal wavefront data (left) is used by Schwind's ORK-CAM system to devise an ablation plan (right) for a patient. Maria Clara Arbelaez, MD

The system uses the Optikon (Rome) Keratron Scout topographer for the corneal wavefront treatments, and Wavefront Sciences’ (Albu­quer­que, N.M.) wavefront analyzer for the aberrometry. Schwind provides the software, ORK-CAM, that im­ports the data from the diagnostic systems, al­lows the input of the subjective re­fraction, displays the data in terms of Zernike coefficients in order to plan ablation profiles and exports the information on ablation volume to the laser.

Maria Clara Arbelaez, MD, medical director of the Muscat Eye Laser Center in Muscat, Oman, explains how the Optikon Scout calculates the cor­neal aberrations.

“The corneal wavefront is based on corneal topography,” she says. “The corneal surface is measured by a topographic system and compared to a theoretical model eye, calculating any deviation between the measured cor­nea and the theoretical optimal cor­neal surface. This topography-based wavefront error is called the corneal wavefront and is described in terms of Zernike coefficients.”

Dr. Arbelaez says this approach uses corneal optical errors rather than corneal elevation as other systems do. She says this approach may offer an advantage for treatment since it’s not dependent on pupil size.

For the aberrometry-based treatment, the system uses the eye’s higher-order aberrations at certain optical zones, in addition to sphere and cylinder. Both the corneal-wavefront based treatment and the aberrometry-based treatment, however, take a physiologic element into account in the form of asphericity.

“We believe that customized corrections should be aspheric and not simply wavefront-driven, to pre-compensate for corneal biomechanical effects, flap cut, intraocular pressure, etc.,” says Dr. Arbelaez. “This maximizes outcomes and minimizes the tissue removal requirements.”

In a study of the aspheric ESIRIS ablations (not topography or wavefront guided) in 226 eyes with up to
-10.25 D (mean: -4.1 D), 90 percent were within 0.5 D of the target refraction, and approximately a third gained a line of vision, 5 percent gained two lines, 56 percent stayed the same and 5 percent lost a line. No one lost more than two lines.

Though the ESIRIS with the ORK-CAM can do some new things with diagnosis and treatment, U.S. surgeons may not be able to use the system soon. A representative of Schwind says the company doesn’t fore­see a trial for the devices in the United States in the near future.

 • Carl Zeiss Meditec MEL-80 with the CRS-Master II. According to London surgeon Dan Reinstein, the CRS-Master II combines topography information with wavefront data into an advanced treatment plan for each individual eye. The CRS-Master 2 is still being tested in Europe, but should be available for sale, at least outside of the United States, in 2006. The device uses the high-resolution WASCA aberrometer and the Zeiss Humphrey Atlas topographer to gather its data. According to Dr. Reinstein, though the devices use two measurement heads, the wave­front registration is based on the position of the visual axis relative to iris features, the limbus and conjunctival vessels, and topography is registered by the visual axis. “There­fore,” he says, “both are registered to each other.”

“The CRS Master II software integrates the wavefront, topography, pa­tient refraction, corneal curvature, cor­neal thickness and LASIK flap thickness all into a complex calculation that’s designed to, in the first place, produce a refractive correction without inducing aberrations, and, second, reduce the aberrations that are inherent to the eye before sur­gery,” explains Dr. Reinstein.

Dr. Reinstein says the CRS-Master software also takes into account what’s occurring in the cornea biomechanically.

Armed with the energy correction functions originally described by Michael Mrochen, PhD, of Zurich’s Institut für Biomedizinische Technik, and modified by Carl Zeiss Meditec’s Jesus Cabeza, PhD, the CRS-Master II optimizes the energy delivery to the cornea. Dr. Rein­stein has provided Carl Zeiss with valid data on the biomechanical response of the cornea ob­tained by detailed analysis of stromal thickness profile changes from Artemis 3D VHF digital ultrasound (Ultralink LLC, St. Petersburg, Fla.). “This research has enabled us to in­cor­porate some very sophisticated, highly non-linear energy correction functions,” he says. “This means we can treat extremely high myopia, in the range of -8 D to -13 D, without real concerns of producing night-vision problems or loss of contrast.”

Though the topography and the wavefront may seem separate from corneal biomechanics, Dr. Reinstein says the three are linked.

“To be able to do a topography-wavefront guided treatment, you have to be able to cut out the tissue you in­tend to cut,” he explains. “If you don’t have a clear compensation for fluence or biomechanical issues, as well as epithelial healing, then you’re not going to be able to perform an optimal  to­pography- or wavefront-guided treatment.”

In a pilot study of the system in complicated corneas with such problems as decentrations, small optical zones and/or large amounts of spherical aberration, Dr. Reinstein says all patients were within ±0.75 D of the intended refraction, which he thinks is impressive given the nature of the patients. He says that, for primary cases, he feels the system gives him better refractive results because, “in a way, it’s customizing the treatment to another level.”

 • The CustomVis ZCAD. CustomVis (Perth, Australia) is currently testing a combined topography and wavefront imaging system for planning ablations with its Pulzar Z1 solid-state laser.

Here, Carl Zeiss Meditec's CRS-Master II displays the topography map (left) and ablation plan (right) for a -2.5 D myope with 0.5 D of astigmatism. Carl Zeiss Meditec

Company founder Paul Van Sarloos, PhD, says the combined system helps surgeons tailor the treatment to the individual patient. “It uses topography for the higher-order aberrations, and wavefront for the lower-order aberrations,” he says.

The system uses Tracey Tech­nologies (Houston) equipment to capture the topography and wavefront data, but then uses its own algorithms to reconstruct the topography. To make sure the laser is treating where the topography and wavefront found aberrations, the ZCAD registers both sets of data using the corneal vertex and the limbus. “We also get registration for the eye tracker so it’s positioned on the correct part of the cor­nea,” explains Dr. Van Sarloos. The topography and the wavefront images are taken sequentially by the same de­vice, with the patient staying in the same position.

Dr. Van Sarloos says using the merged diagnostic information can help in tricky cases. “Because lenticular aberrations change with accommodation and age, there are certain higher-order aberrations measured with wavefront that I wouldn’t recommend treating in a permanent fashion,” he says. “So, this is one of the main reasons to merge with topography: to verify that these higher-order aberrations exist on the cornea before treating them there.”

The system can be used on primary refractive surgery cases as well as complex corneas that are more irregular, but only up to a point. “When you get to really aberrated eyes, like some with irregular astigmatism after penetrating keratoplasty, you use only topography because the wavefront would be very hard to get,” Dr. Van Sarloos explains. “With ZCAD, the user can choose between a merger of topography and wavefront or topography only. When you take a wavefront image in a highly aberrated eye, you’ll get a warning that there are a lot of bad measurement points.”

Dr. Van Sarloos says CustomVis is just beginning to use ZCAD clinically. “We haven’t done huge numbers of patients, yet,” he says. “We’ve done a small trial, and are just releasing a second generation of ZCAD that incorporates what we’ve learned from the first version. How­ever, it’s extremely promising. The vision has been good in the ZCAD patients, and we think we can make it even better.”