Improved topography systems enhance clinical judgment, may build refractive market

Issue: November 1997

While laser vision correction chains in the United States and Europe work to cultivate consumer interest in refractive surgery, and excimer laser companies court ophthalmologists who may be interested in the subspecialty, sustained growth in the market may depend more on the latest generation of corneal topography systems. Better topographers provide surgeons with more information at the beginning of surgery on which to base decisions. That can improve predictability and outcomes and lead to fewer return visits for flat-fee patients, decreased malpractice risk, higher profit margins and heightened consumer confidence.

"Some of the new modalities that these new topography devices are introducing should help us to utilize corneal topography much more in our clinical practices," said Ronald R. Krueger, MD, a corneal specialist at the St. Louis University Eye Institute. "They provide extra information and should expand our understanding of what we are doing, what we can expect from refractive surgery and our overall ability to manage cases."

C-Scan

Technomed Technology's (Huntington Beach, Calif.) C-Scan system is an advanced Placido-based topography machine that uses its color capabilities to increase the level of corneal surface information available to the surgeon, providing topography based on 10,800 measuring points. The system uses a 35-mm cone, with 15 color rings providing 30 measurement zones. A coaxial LED serves as the system's fixation light. The system is driven by an IBM-compatible personal computer with a 166 MHz microprocessor with 1 GB of hard drive space.

Color is what differentiates the C-Scan from other Placido-based topography systems. Most Placido topography systems use alternating rings of black and white inside the Placido cone. But on irregular corneas, these rings often merge into one another or overlap, which can skew results. The C-Scan system uses colored rings, which its designers say allows the system to clearly distinguish rings, even in the case of overlapping and partial breaks due to thin or absent lacrimal film on an irregular or highly eroded cornea.

Color coding, according to Technomed, ensures the exact outline of each ring, and they are assigned to the correct calibration data. Black and white systems may show an incorrect number of rings if there are areas of overlap. C-Scan's color Placido analysis is possible because the cone is partnered with a high-resolution color video camera, the PAL system.

Captive field focusing

So that the C-Scan's findings are not compromised by operators' varying ability to focus on the cornea, the system uses a captive-field focusing system that mathematically compensates for defocusing with Z-axis, or distance axis, correlation, based on triangulation calculation. Without the Z-axis correlation, the modified ring widths that generally occur in topography as a result of defocusing, distort the results, requiring each operator to locate his or her own optimal focusing distance. Usually, the eye moves too quickly for the operator to locate the best focusing point, but by triangulating the distance, the C-Scan can mathematically correct each defocused point caused by the user.

"The system aims a diode laser at the cornea, which should be anywhere inside a defined range (captive field), giving the user the correct distance range from the subject's eye," explained Herbert von Wallfeld, president of Technomed. "The data is compared with calibration data in the computer."

Localized line of sight

Because the C-Scan system can localize the line of sight inside the smallest central measuring ring with a radius of 0.2 mm, detail from this region, which usually is ascertained through interpolation, is directly available and fairly detailed. A fixation light aimed at the patient's eye during the measuring process is recognized by the system as a reflection on the lacrimal film of the corneal surface, which marks the outline of the line of sight. The C-Scan can determine the exact position of this point and build topometry from it. This point, according to Technomed, seldom corresponds to the geometric center of the first ring, which is usually the jump-off point for measurements, so the system's truer reading enhances surgical outcomes.

C-Scan's capabilities

C-Scan technology permits fairly advanced corneal analysis. The system's astigmatism analysis mode enables the comparison of preop and postop examinations of the cornea and provides a detailed analysis of the development of astigmatism. After completing a differential calculation for preop and postop examinations selected by the investigator, the C-Scan allows the investigator to view and compare topometric data and simulated keratometric data from four optical zones, 0 to infinity, 0 to 3 mm, 3 mm to 5 mm and 5 mm to 7 mm. Additionally, the astigmatism analysis program allows the calculation and display of data per semi-meridian, with values calculated for both the flat and steep meridians. Induced astigmatism is calculated using six methods: magnitude subtraction, vector analysis, vector decomposition, Cravy's vertical vectors, Naesser's polar values and algebraic.

The C-Scan can create a three-dimensional picture of the cornea on the basis of patient data selected by the user. Calculation pitch values are carried out at a reproducible accuracy of 2 µm.

"This is unique, because we still find people who think that Placido systems cannot measure in 3 D," Mr. von Wallfeld said. "C-Scan can because we can measure the center point, and because of the Z-axis correlation. With its geometry of the system, we can calculate a variety of 3-D measurements."

Virtual surgery

C-Scan's ray-tracing program allows the investigator to simulate the projection of any object through optical elements, permitting a virtual glance through a patient's eye.

"Ray tracing, for the first time, gives clinicians an objective overview of the visual function of the cornea," Mr. von Wallfeld said. "You can look through the cornea and see its irregularities, how the rays are broken and how they are refracted onto the retina. This is a qualitative and mathematical way of looking at the retina."

When this is combined with the planned Topolink feature, which is still in development, surgeons will be provided with a postop picture of a cornea before an incision is ever made, according to the manufacturer. Surgeons will be able to test the cornea for refraction and function and see the final outcome before the procedure is performed, they say.

With the Topolink system, the investigator will record preop data, then indicate the required curvature for perfect or near-perfect uncorrected visual acuity. That data will be input into the excimer laser to enable the excimer's algorithm to affect a virtual photoablation. The data will then be transferred back to the C-Scan unit, where the Topolink software will use its ray-tracing feature to test the accuracy of the surgeon's intended operation.

"This gives the doctor a much better idea of how surgery is going to turn out," Mr. von Wallfeld said. "Some doctors are using ray tracing in their informed consent agreement with patients and therefore decreasing their liability."

Eventually, systems like C-Scan may be able to provide real-time corneal analysis, which will allow scanning lasers to perform refractive correction based on input received directly from topography, Dr. Krueger said.

Depending on how well the system is equipped, C-Scan ranges in price from $19,900 to $28,000. The unit's color camera, according to Mr. von Wallfeld, adds to the system's overall cost.

Orbscan

The Orbscan system from Orbtek Inc. (Salt Lake City) is a non-Placido, slit-lamp based topography system that provides anterior surface measurements, posterior segment measurements and full corneal pachymetry. Triangulation defines the three-dimensional location of every data point on each corneal surface. Anterior and posterior corneal measurements comprise 16,000 discrete data points, while anterior chamber measurements are taken at 5,000 points, for a total of more than 20,000 points in each tomographic map.

The Orbscan system uses light generated at the slit lamp and diffusely reflected from corneal surfaces, the iris and the lens to build a map of the cornea. The light images are acquired by a video camera. Data points are then translated by Orbscan software into maps or three-dimensional models. The Orbscan system is powered by a mini-tower, IBM-compatible Pentium Pro 200 computer. Included with the system are 64 MB of RAM and a 1.2 GB hard drive. A 266 MHz computer is available as an option.

"The system measures the anterior surface of the cornea and the posterior surface of the cornea, and with that comes the corneal thickness," explained Jack Savage, president of Orbtek. "We also measure the iris plane as well as the anterior surface of the lens, which then allows us to calculate anterior chamber depth, eye rotation and an estimate of angle kappa. So as far as capability goes, we measure in a direct 3-D fashion all of those surfaces. From those surfaces we can then calculate almost anything we want."

Variety of maps can be made

The Orbscan system's Pacscan 2.0 software package can create pan-corneal pachymetry maps, anterior and posterior corneal surface maps, radial keratotomy and astigmatic keratotomy overlays, a laser in situ keratomileusis (LASIK) safety check nomogram and refractive power maps. It also has an acoustic equivalence option for the translation of Pacscan to ultrasonic values for surgical nomograms and a corneal optical axis identification mode for improving contact lens fitting.

The system's Fitscan elevation contact lens fitting software uses three-dimensional data and user-input information for size, and style and clearance to generate spherical and toric contact lens designs. Because the fitting technique is based on direct measure elevation, fitting irregular corneal surfaces with lenses is easier than with other topography methods that rely on curvature measurements. Because the measurements are independent of tear film, there are no inaccuracies due to pooling or dry eyes.

Orbtek is working with most of the major excimer laser manufacturers, according to the company, and has developed "LaserLink" software modules to allow the surgeon to design optimal ablation patterns to be developed and downloaded to the laser itself. A difference map may then be viewed to establish the need for further surgical enhancements.

Better technology?

Representatives of Orbtek, which also manufactures Placido-ring-based systems, said non-Placido technology is better because it is the only topography system designed in tandem with the evolution of excimer lasers. Most topography systems currently available are based on ideas that predate excimer laser technology, they said.

"We believe that slit-lamp topography is better because it was designed with the needs of the refractive surgeon in mind," Mr. Savage said. "The main difference between slit-lamp technology and Placido technology is slit-lamp directly measures the three-dimensional shape of the eye, without the assumptions that many Placido topography systems are forced to make."

The system's pachymetry feature, which can calculate thickness in about 1.5 seconds, is helpful to LASIK surgeons who need to know corneal thickness before surgery. Orbscan's LASIK Safety Nomogram program, contained in the Pacscan software, is based on the Casebeer nomogram for corneal thickness. This program considers the desired amount of correction in combination with its measurement of the thinnest point on the cornea and indicates the risk of corneal ectasia.

Detailed information

Because the system does not use a Placido cone, it can provide detailed information from the center of the cornea, a feature which, save the C-Scan system, is not possible with other Placido-based systems, the manufacturer said.

Depending on how it is equipped, the Orbscan ranges in price from the Orbshot portable, 3-lb. unit at $8,000 to approximately $30,000 for a fully loaded system with a range of software and free future upgrades.

Perhaps more valuable than Orbscan's enhanced topography is its ability to offer physicians peace of mind, company representatives said.

Saving time and money

"When a patient is paying a flat fee for a procedure, first-time results are critical for the physician," said Lorraine Berry, in Orbtek's marketing department. "Every time a patient comes in for an enhancement that is necessary because the doctor did not have the proper clinical tools to support his original diagnosis, that doctor is losing time and money."

The need for surgical enhancements following the original operation also brings up the issue of malpractice.

"With better information up front, doctors can protect their interests, get more accurate information and improve outcomes," said Alan Bunting, vice president of marketing for Orbtek. "This lowers their liability and encourages more surgery."

Because of their ability to provide physicians and patients with a clear idea of what to expect in terms of results, systems like the C-Scan and Orbscan may be critical to growth in the refractive surgery market.

"In any elective surgery, most patients have false expectations," Mr. Bunting said. "By showing patients what they can expect, we can build confidence in refractive procedures, which will grow the market."

For Your Information:

Jack Savage is the president and CEO of Orbtek, Inc. He can be reached at 3030 South Main St., Suite 600, Salt Lake City, UT 84115; (801) 484-8777; fax: (801) 484-3972; e-mail: shilo@orbtek.com.; URL: http://www.orbtek.com.

Herbert von Wallfeld is the president of Technomed Technology. He can be reached at 5132 Bolsa Ave., Suite 104, Huntington Beach, CA 92649; (714) 899-2133; fax: (714) 899-2131.

Ronald R. Krueger, MD, can be reached at St. Louis University Eye Institute, 1755 Grand Blvd., St. Louis, MO 63104; (314) 577-8656; fax: (314) 771-0596; e-mail: kruegemr@slu.edu. Dr. Krueger does not have a direct financial interest in the products mentioned in this article. He has been a paid speaker for Technomed Technology.

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