Corneal measurement technologies for refractive surgery advance; validation needed

New technologies reflect the importance of corneal thickness measurements for refractive surgeons.

As corneal thickness measurements have become more important to refractive surgeons, the technologies used to make these measurements have evolved.

Although these new technologies can deliver reproducible measurements for a range of applications, ultrasonic pachymetry still remains the gold standard for corneal thickness measurement for many refractive surgeons, said Perry S. Binder, MD.

“Ultrasound will ultimately be supplanted by future technologies,” Dr. Binder told Ocular Surgery News. “It is just going to take time for us to take a look and see what these can and cannot do. ”

Currently, some of the new technologies for measuring corneal thickness show differences from one another when used to measure the same corneas. (See accompanying article below.) Dr. Binder said it will be some time before ophthalmologists learn the best ways to use each of these technologies in clinical practice.

“There are lots of reasons why these technologies do not agree with each other, because they are all different in the way they analyze their data,” he said.

“They [the new technologies] make for great images, and I think there are other applications that we need them for, in addition to LASIK,” he continued. “We are putting plastics in the cornea, and it would be nice to be able to quantitatively measure how deep we actually placed them. As with any new technology, we do not know all of their capabilities. There may be other applications we can use them for that we currently are not aware of.”

Dr. Binder said he currently uses 50 MHz ultrasonic pachymetry for corneal thickness measurement and has recently begun using the Oculus Pentacam. He said he is studying these and other technologies to determine which might be the most accurate and useful diagnostic tools. Dr. Binder reviewed some of the available technologies for measuring corneal thickness in an interview with Ocular Surgery News.

Different devices record different corneal measurements in same eyes LAS VEGAS — Several diagnostic devices reported different corneal measurements from the same patients, according to a presentation here. Canan A. Utine, MD, and colleagues compared pachymetric and keratometric measurements taken by the Oculus Pentacam, the Bausch & Lomb Orbscan and the Carl Zeis Meditec IOLMaster to measurements using ultrasonic pachymetry and standard keratometry. They presented the results of their study in a poster presentation at the American Academy of Ophthalmology meeting. The researchers compared central corneal thickness (CCT), thinnest corneal thickness (TCT) and keratometry in five patient groups: 49 myopic eyes, 29 keratoconic eyes, 24 emmetropic eyes, six hyperopic eyes and seven eyes that had undergone LASIK. In keratoconic eyes, the Orbscan reported CCT and TCT as significantly thinner than ultrasonic pachymetry and Pentacam (P < .005).="" in="" myopic="" and="" hyperopic="" eyes,="" cct="" and="" ctc="" measured="" by="" orbscan="" were="" thicker="" than="" pentacam.=""> In myopic, keratoconic and emmetropic eyes, the Pentacam reported keratometry as lower than the IOLMaster. In emmetropic and myopic eyes, the Pentacam reported keratometry as higher than the Orbscan (P < .005).="" the="" iolmaster="" consistently="" reported="" the="" steepest="" keratometry.=""> Pentacam and Orbscan measurements were comparable in keratoconic eyes, and there were no significant differences in eyes with previous LASIK among all machines, according to the poster. The authors suggested that the differences in measurements could be due to the way the devices take their measurements: the Orbscan scans the central cornea twice with a scanning slit technique, whereas the Pentacam scans 360· circularly. “The knowledge about the study principles of the devices, in association with the clinical findings of the patients, will be helpful in the correct evaluation of the measurement results,” the authors said.

Ultrasound

Ultrasonic pachymetry uses sound waves to measure the thickness of the cornea, Dr. Binder explained.

The device sends out a wave of sound that bounces back when it encounters a change in substance, such as from solid to liquid. A probe picks up the signal and the device computes the difference.

“It does a differential measurement by measuring several hundred times a second to give you an average of those readings,” Dr. Binder said. “I will take two or three readings until I come up with one that is within 2 µm of the previous measurement, and I use that one.”

Ultrasound is probably the most widely used device for measuring corneal thickness, Dr. Binder said, but it has some drawbacks.

“One shortcoming is that you have to put the probe back in the same spot to get your readings,” he said.

Scheimpflug technology

The Pentacam uses a rotating Scheimpflug camera to capture images in three dimensions and compute the thickness of the cornea as well as other structures in the eye.

“It projects a slit beam across the cornea and then looks at the two sides of the beam and the difference in the width of the beam in several areas,” Dr. Binder explained. “Unlike ultrasound, which only can measure one spot, this measures across an 8 mm or 9 mm area.”

The device computes thickness at several spots on the cornea, including central, superior, inferior, nasal and temporal, and it ultimately creates a color printout that shows a diagram of the cornea from limbus to limbus. The printout is color coded for corneal depth — for instance, greens and blues indicate thinner areas and reds and oranges are thick.

“It gives the surgeon an overall appreciation for the thickness of the cornea in all areas,” Dr. Binder said, “whereas with ultrasound, you have to physically put the probe in these different areas.”

Corneal topography

Most corneal topographers map only the front surface of the cornea, but one topographer, the Bausch & Lomb Orbscan, estimates the back surface of the cornea by increasing its readings by 8%, Dr. Binder said. This gives an approximation of the readings that ultrasound technology produces, he said.

Confocal microscopy

Another technology that can be used to measure corneal thickness is confocal microscopy.

“This uses a high magnification lens, like a giant microscope that you focus through the tissue,” Dr. Binder said. “The operator first looks at the endothelium of the cornea and changes the depth of focus of the confocal microscope until it hits the front of the cornea, the epithelium.”

The difference between those two points is the corneal thickness, he said.

“That has been compared to ultrasound, and there have been differences as well, with that measuring thicker than what ultrasound tends to read,” he said.

High-frequency ultrasound/laser

The Ultrasound BioMicroscope (UBM, Paradigm Medical Industries), uses high-frequency ultrasound to create a cross-section image of the tissues in the anterior segment, Dr. Binder said.

“The surgeon looks at the cross image of the cornea and puts a bar where he thinks the epithelium is and another bar where he thinks the endothelium is, and the program calculates what the thickness is,” he said. “The advantage there is, like the Scheimpflug, you can get multiple thickness readings across the cornea in a given meridian. The drawback is that the surgeon has to place the cursor where he thinks the thickness is. The machine just does not necessarily do it automatically, so there is potential for error.”

Similar images can be created with laser technology rather than ultrasound using the Visante OCT (Carl Zeiss Meditec), a version of optical coherence tomography that was specifically designed for imaging the anterior segment. The Visante uses a different frequency of laser from Zeiss’ Stratus OCT, which is used to create retinal images and measure retinal thickness.

Very-high-frequency ultrasound

The Artemis very-high-frequency ultrasound, from Ultralink, is an even more accurate and high-speed probe that uses an immersion water bath to interface with the cornea.

“This gives some very nice high-resolution images of the cornea,” Dr. Binder said. “The negatives are that if the same operator puts his dots on the same spot several times, and comes back again and does it the next day on the same image, you get a pretty wide variation of 10 µm or 20 µm, which is not sensitive enough for a lot of our needs.”

Dr. Binder recommended that surgeons in private practice may want to wait until tests comparing these technologies have been published in peer-reviewed journals before investing in one or another.

For more information:

• Perry S. Binder, MD, can be reached at the GordonBinder Vision Institute, 8910 University Center Lane, Suite 800, San Diego, CA 92122; 858-455-6800; fax: 858-455-0244; e-mail: garrett23@aol.com. Ocular Surgery News was unable to determine whether Dr. Binder has a direct financial interest in the products discussed in this article or if he is a paid consultant for any companies mentioned.

• Oculus Inc., maker of the Pentacam, can be reached at 2125 196th St. SW, Suite 112, Lynwood, WA 98036; 888-284-8004; fax: 425-670-0742; Web site: www.oculususa.com. Paradigm Medical Industries, maker of the Ultrasound BioMicroscope, can be reached at 2355 South 1070 West, Salt Lake City, UT 84119; 801-977-8970; fax: 801-977-8973; Web site: www.paradigm-medical.com. Ultralink LLC, maker of the Artemis VHF, can be reached at 2083 Hawaii Ave., NE, St. Petersburg, FL 33703; 727-527-1277; fax: 727-527-3634; Web site: www.arcscan.com. Ziemer Ophthalmic Systems AG, can be reached at Allmendstrasse 11, CH-2562 Port, Switzerland; 41-32-332-70-50; Web site: www.ziemer-ophthalmics.com. Bausch & Lomb, maker of the Orbscan corneal topographer, can be reached at 180 East Via Verde, San Dimas, CA 91773; Web site: www.bausch.com. Carl Zeiss Meditec, maker of the Visante OCT, can be reached at 5160 Hacienda Drive, Dublin, CA 94568-7562; Web site: www.meditec.zeiss.com.

• Daniele Cruz is an OSN Staff Writer who covers all aspects of ophthalmology.