Corneal topography: Clinicians weigh in on what they want

In the past, the question debated in practices nationwide was: "Should we invest in a corneal topographer?" In more recent years, however, the question is more likely to be: "When are going to get that topographer?"

With this question come the corollary considerations: Which one? How much can I or should I spend? Will I see a return on my technologically based investment? How do I balance the need for technological expansion of my practice against projected patient traffic in the coming year?

Last month, Primary Care Optometry News featured interviews with product managers, marketing specialists and others representing the manufacturers of corneal topographers used in this country and offered a quick guide to the latest in technology.

Here, we speak to clinicians who use the technology on a daily basis either in private practice or in research. While they discussed the technological aspects of the instruments, they also took time to address issues of diagnosis, practice management and the future of corneal topography.

Technology in practice

Richard Clompus, OD, of Clompus & Reto Vision Associates in West Chester, Pa., uses Humphrey System's Atlas corneal topographer model 990 in precise fitting of disposable and rigid gas-permeable (RGP) contact lenses, as well as in diagnosing keratoconus.

"The nice part about [the Atlas] is that it fits in our pretest area on a rotating table," Dr. Clompus said. "This permits us to use it more as a screening instrument, which is the best way to use it. It's used every day between six and eight times a day. We have a protocol set up, because we've been doing this now for close to 4 years.

"We consider [the corneal topographer] a pretest instrument to be used on all contact lens patients once a year," Dr. Clompus continued.

Important in refractive surgery

Dr. Clompus feels corneal topography is also very important for refractive surgery patients. "The only way you can comanage a patient - especially for refractive surgery - is to have a preop corneal topography done, and you really can't provide postoperative care without it," he said.

For Perry Rosenthal, MD, of Boston Eye Associates, corneal topography is an integral part of his everyday lens fitting and approach to diagnosis, as well as his ongoing research. Dr. Rosenthal currently uses the Euclid ET-800 exclusively in his clinical research.

"The cornea requires a high-quality reflective surface, without which the placido ring topographer would be rendered useless," Dr. Rosenthal said. "The Euclid topographer directly measures the projected image, which is not dependent on smooth surfaces. Placido-based technology measures slopes, the resulting data of which are then converted into sagittal heights. Processing these data requires corneal shape assumptions, which are often incorrect. Additionally, we understand that the more the corneal shape deviates from those assumptions, the greater the chance of corneal toricity, asymmetry and irregularities. Peripheral measurements are especially inaccurate [with placido-based images].

"Measuring heights directly, as the Euclid does, requires no prior assumptions about corneal shape," continued Dr. Rosenthal. "This instrument automatically re-centers data around the corneal apex as a reference point. The instrument does not measure the center or periphery of the cornea, but rather measures the whole cornea from limbus to limbus and beyond, and does not extrapolate."

"Being in a university environment, I have four different systems," said Robert Mandell, OD, a professor emeritus from the University of California School of Optometry, Berkeley. Dr. Mandell has the EyeSys System 2000, the Dicon CT-200, the Tomey TMS-2, the Humphrey Atlas Model 991 and the Alliance Keratron.

"These instruments were really developed for following the cornea before and after surgery," Dr. Mandell said. "After the ophthalmology market became saturated, the manufacturers began to look at the needs of optometry. However, if optometrists are comanaging patients, they're going to want the same thing as the ophthalmologists."

"What we are now witnessing in the corneal topography field is the confusion surrounding a new development," he continued. "There are all sorts of claims being made that simply can't be backed up. New instruments are being introduced to the field with much fanfare and no real research. We have already seen several instruments that were shown at conventions a few years ago disappear from the scene. Why is this so? Because in spite of what some of the manufacturers are saying it is difficult to beat the overall advantages of placido disc (ring target) technology. Past objections to placido disc instruments have been overcome but are still brought up over and over again by those developing other systems."

Mitchell M. Loftin, OD, is in full-time practice and is conducting clinical research at the Mason Eye Institute at the University of Missouri at Columbia.

"In our department, we have the EyeSys corneal topographer, but we're also doing research with PAR Vision on their rasterstereophotogrammetry technology, and we also have limited time on Orbtek's Orbscan II here," Dr. Loftin said. "We're doing some comparison work on that."

Decide what data you want

"Right now, I'm leaning toward the PAR (Accugrid corneal topographer)," Dr. Loftin continued. "Obviously, you have to take into account the kind of data you're looking for. If you're looking for elevation data, then my experience has been that the PAR is the most accurate for giving data that correlate with rigid lens fitting and with the location of the cones in keratoconus patients. If you are trying to diagnose early stages of keratoconus, I've seen the Orbscan pick up on some posterior corneal changes that correlate with some of the other changes we're seeing in the cornea before other units can."

Describing the Atlas as "extremely accurate," Dr. Clompus said one feature that sets the instrument apart is a unique software program called the arc-step algorithm, the function of which Dr. Clompus described as "almost like laying a net over the surface of the cornea. And by doing so, can also give you ring location, slope information and elevation data. Most placido-based corneal topography instruments can give you curvature, but can't give you elevation. Elevation is important because it gives you a precise and detailed way of viewing the cornea and, therefore, gives you another option of using that information to help fit RGP lenses or diagnose pathology.

"The other part, which is unique, is that the Atlas has a software program that will design for you a bi-toric gas permeable lens. You can map the patient, input his or her spectacle Rx, push `fit' and it will come up with a very sophisticated bi-toric design that otherwise could take probably the better part of a half an hour to set down on paper and design."

Used on all contact lens patients

Dr. Rosenthal explained that, in the course of collecting data for his research, he screens nearly every patient with corneal topography. "I use it on all of my contact lens patients, because we want to correlate the fit of the RGP lenses with true corneal topography so that eventually we can customize the lens design to the individual eye," Dr. Rosenthal said.

He added that the latest technology available from Euclid is "considerably more accurate than any other currently available on the market."

"When you measure corneal height directly, you don't have to make any assumptions about the shape," he said. "This instrument gives us a 14 mm, area of coverage - the whole surface of the conrea and limbus - and no other instrument does that. I think it would be nice if we didn't have to use fluorescein, but that's really a minor issue."

Patient comfort

Dr. Loftin said that as topographers evolve in their mapping and memory capabilities the issue of patient comfort will also be addressed.

"I believe the comfort level for patients is improving," he said, "but you do have to take everything into account. First of all, the illumination levels of both the PAR and the Orbscan are much lower than the EyeSys and some of the other placido disk devices. So, the photophobia is greatly decreased.

"Also, you have to take into account whether your patient is sensitive to - with regard to the other two instruments [PAR and Orbscan] - having eye drops put into the eye," Dr. Loftin continued. "With the PAR unit, I have not had a problem with instilling fluorescein dye in a patient and the patient complaining about it hurting, but I know that in the literature other doctors have complained that this might be a hindrance. Mine is an RGP lens population, so I'm using fluorescein dye on almost every patient anyway. So, for me, it's not a major hindrance - but for someone else it might be."

Dr. Loftin said none of his patients have complained about the illumination with the Orbscan. "But if you have a patient who has a nystagmus, you've got a bit of a problem, especially a Down syndrome patient who has keratoconus," he said. "If you have excessive eye movement, you'll have some real difficulties getting a usable map. With the PAR, that's not a problem because the photograph is instantaneous, but you don't get the data on the posterior corneal surface. So, again, it depends on what you're trying to get as to which instrument is best for you."

Placido's advantage

According to Dr. Mandell, placido-based technology continues to grow in accuracy and efficacy while maintaining current high levels of dependability.

"The main advantage of the placido disc videokeratograph is that it provides a magnification of the ring image from the cornea in such a way that even the smallest corneal defect distorts the camera image and can be detected," Dr. Mandell said. "This magnification cannot be achieved by any of the other measurement methods. The radius of curvature measurements can be converted to height measurements with good accuracy. On the other hand instruments that measure surface height directly encounter large errors when a conversion is made to radius of curvature. I discovered this problem 40 years ago when doing my doctoral thesis on corneal topography.

"A significant development in videokeratography is the use of offset (peripheral) fixation points for peripheral corneal coverage," Dr. Mandell continued. "More importantly, this allows for very accurate measurements of the central region or optical zone of the cornea. This has been made possible by new algorithms that can be used to interpret the offset fixations. These and other advancements promise to make videokeratography the system for others to try and beat for some time to come."

Choosing a topographer

"I would say placido disk instruments are good screening units," Dr. Loftin said. "But if you want to quantify and qualify the type of lesion you've got, you must have an instrument that takes direct elevation measurements. It depends on how much information you want to get and how much money you want to spend."

When choosing a topographer, deciding the type of data you want to collect is only one consideration. Dr. Clompus also advises practitioners new to the field or those just setting up offices to carefully consider issues of efficacy, speed and space as they wrangle with the decision of which topographer will fit best into their office environment.

"You certainly want one that you can use efficiently in the practice," Dr. Clompus said. "That means not selecting one that goes in a separate room with a visual field instrument. It truly belongs in pretest so that it is efficient, easy and also cost effective. Keep in mind that the first unit we ever had was $28,000. The most recent Atlas we purchased cost approximately $14,000. The cost came down by 50%, and it's a better product than the other one.

"You will also need some method of storing the data, probably externally, and the most common technique that I'm aware of is the Zip Drive," Dr. Clompus continued. "It's a 100 megabyte floppy disk that is very inexpensive. No matter how big the hard drive in your mapper is, you will eventually fill it up. The Zip Drive is a very economical way to back up and archive all of your patients. In my case, for example, I can fit almost 1,000 maps on one Zip Drive."

Factoring in cost and efficacy, Dr. Loftin suggests that young ODs enter the world of corneal topography with a dependable, inexpensive model - or one that can grow with their practice.

"If I were a young OD with less than $10,000 to spend, I would strongly look at Orbtek's Orbshot, which I think sells for around $8,000," Dr. Loftin said. "The Orbshot ties into the Orbscan, which allows you to compare the data back and forth between the two units. In the $10,000 to $20,000 range, the PAR CTS system is the unit I would recommend. For those interested in research and refractive surgery in high myopia, the Orbscan provides the most comprehensive data. Placido disc based units are less accurate but have excellent contact lens software programs."

Developing a protocol

The management of patients and the daily use of a corneal topographer require some planning and no small amount of communication with office staff who must be educated not only with regard to the new technology, but also in the development of instrumentational protocol.

"It's very important that practices develop a protocol concerning when they're going to recommend the procedure," Dr. Clompus said. "When we first got topography, we thought we would use it for unusual prescriptions. We thought we would use it if we suspected that a patient had keratoconus. And we attached a relatively large fee to corneal topography. Then we began to realize that there are so many patients who can benefit by this, that we have to look at this not as being like a threshold field, but rather more like a screening visual field. It's cheaper; it's faster.

"We do a lot of what we call screening maps on large numbers of patients every day," Dr. Clompus continued. "And these are all patients who wear contact lenses or may have a difficult refraction or may be here for a preop refractive surgical evaluation. But we keep the cost low. And I would prefer to do a test ten times a day that we charge $15 for rather than charge $100 and do one a month. And most of time - we should keep in mind - patients are paying for this test out of pocket."

Evolving use of the technology

As for the evolution of corneal topography usage, Dr. Mandell said that, for the most part, existing topography systems have been developed primarily for screening keratoconus and other conditions that might be of importance to the surgeon. Various conditions require different data from the topographers.

"For example, after refractive surgery, especially photorefractive keratectomy, surgeons are looking for things like central islands," said Dr. Mandell. "So, there's been quite a debate about whether one instrument or the other is capable of detecting that condition. Concerning the detection of keratoconus, how sensitive is one instrument compared to the other? There are no really good data on that to allow a practitioner to make that comparison."

Dr. Mandell said most available topography systems are based on the same techniques that optometrists use routinely in their offices. Because of this, optometrists have been hesitant to invest in an instrument that mimics their current capabilities. He added though, that innovations in speed, efficiency and accuracy have made topographers increasingly more attractive to optometry practices.

Dr. Mandell also issued some words of caution for clinicians shopping for a topographer: "Before you put your money down, be cautious of any new device that hasn't been out on the market for at least a year and hasn't been well proven in the practitioner's office," he said. "Anybody who puts his or her money down on a device because they see a lot of bells and whistles at a convention is making a mistake."

Looking forward

Dr. Clompus said one of the forward looking aspects of the Atlas design is the option for the clinician to indicate lens preferences to the computer. The technician can map the patient, then ask the computer to suggest a specific lens and lens design. "We know what the power is for the trial lens - it will even tell you what the expected over-refraction would be," he said. "The instrument can actually help the technician fit the product - or at least get the lens on the patient's eye before I even see the patient. That's a big time saver.

"The instruments are going to continue to improve, both in processing time and in the ability to gain more information from each corneal map," Dr. Clompus continued. "The computers keep getting quicker every year, the storage capacities keep getting larger and it will be an incremental improvement until the technology we use (placido disk) may eventually be surpassed by scanning technology that will be completely different."

Dr. Rosenthal questions the accuracy of the computer's lens suggestion capability. "With the Euclid, we are involved in using the corneal topography to customize a design for each eye," he said. "In other words, we're not saying: Here are the designs. What should the best parameters be for these specific designs? We're saying: This is the corneal topography, create a new design for us. That's the difference. It is called Design To Fit. Ultimately our expectation is that the software will be able to analyze the three-dimensional corneal representation. It directly measures the elevation."

Dr. Loftin added that a doctor in the University of California's optometry program (Brian Barsky, PhD, at Berkeley) is attempting to correlate topographical measurements and tie them in with a manufacturing procedure such as automated computerized lathing techniques. (See Contact Lens Spectrum, April 1996, p. 39).

"I think that is where corneal topography can really pay off for patients who have corneal disease in the future," he said.

For Your Information:

• Richard Clompus, OD, in private practice, can be reached at Clompus & Reto Vision Associates, 1450 East Boot Rd., West Chester, PA 19380; (610) 696-1368; fax: (610) 430-2079. Dr. Clompus has no direct financial interest in any of the products mentioned in this article. He is a paid consultant for Humphrey Systems.
• Mitchell M. Loftin, OD, is in full-time practice and is a clinical assistant professor at the Mason Eye Institute at the University of Missouri - Columbia. He may be reached at The Mason Eye Institute, 1 Hospital Drive, Room #EC149, Columbia, MO 65212; (573) 882-1027; fax: (573) 882-8474; email: mitchell_loftin@muccmail.missouri.edu. Dr. Loftin has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Robert Mandell, OD, is professor emeritus at the University of California School of Optometry, Berkeley, CA 94720; (510) 642-9679; fax: (510) 376-4242. He is a paid consultant for Dicon.
• Perry Rosenthal, MD, in private practice, can be reached at Boston Eye Associates, 1244 Boylston Street, Chestnut Hill, MA 02167; (617) 735-8810; fax: (617) 735-8814. Dr. Rosenthal has a direct financial interest in the Euclid ET-800 and is a paid consultant for Euclid Systems Corp.

• The Accugrid is available from PAR Vision, 8383 Seneca Turnpike, New Hartford, NY 13413; (315) 738-0600; fax: (315) 738-0562.
• The Atlas Model 991 and Atlas Eclipse Model 992 are available from Humphrey Systems, 2992 Alvarado St., San Leandro, CA 94577-0700; (800) 227-1508; fax: (510) 483-8025.
• The CT-200 is available from Dicon, 10373 Roselle Street, San Diego, CA 92121; (800) 426-0493; fax: (619) 554-0332.
• The EyeSys System 2000 and the EyeSys Vista (hand-held corneal topographer) are available from EyeSys Technologies, 3 Morgan Drive, Irvine, CA 92618; (714) 859-0656; fax: (719) 951-7218.
• The ET-800 is available from Euclid Systems Corp., 2810 Towerview Road, Herndon, VA 20171; (888) 3EUCLID; fax: (201) 773-6334.
• The Keratron Corneal Analyzer is available from Alliance Medical Marketing Inc. (Distributor for Optikon) 2250 Third Street South, Jacksonville Beach, FL 32250; (800) EYE-TOPO; fax: (904) 247-3133.
• The Orbshot and the Orbscan II are available form Orbtek, 3030 South Main St., Ste. 600, Salt Lake City, UT 84115-3554; (801) 484-8777; fax: (801) 484-3982.
• The Tomey AutoTopographer is available from Tomey Corporation, 300 Second Ave., Waltham, MA 02154; (800) 358-6639; fax: (781) 290-5885.