Refractive Surgery
Edited by Arturo Chayet, MD
These two technologies may allow surgeons to measure the cornea more accurately than ever before.
Walter Bethke Senior Editor
There are a couple of different methods for looking at the cornea's microscopic features, as well as its thickness. Here's what researchers who use these technologies are learning about the cornea and how surgeons can better manipulate it, especially in LASIK.
VHF digital ultrasound can capture the entire angerior segment
in one scan, allowing angle-to-angle and sulcus-to-sulcus measurements
for accurate phakic IOL placement.
Daniel Reinstein, MD
Very High-frequency Digital Ultrasound
This technology is on the cusp of U.S. FDA 510K approval.
The Very High-frequency (VHF) Ultrasound device, called the Artemis, works by sweeping a 50-MHz transducer in several arcs over the cornea. Then, a computer digitizes the signal, and 3-D software interprets it, displaying such information as overall pachymetry, stromal thickness and residual stromal thickness post-LASIK down to a resolution of 1 µm.
One of the device's inventors, Daniel Reinstein, MD, has used it to study LASIK's effects on the cornea, and he is currently in a study that mates the Artemis with a wavefront analysis system.
Dr. Reinstein believes that neither wavefront nor topography-linked ablations can work effectively for abnormal corneas unless the actual, physical epithelial profile is taken into account.
"This is because the epithelium changes when the stromal surface is uneven," he says. "If the stroma is ablated incorrectly in LASIK, the epithelium compensates, albeit inadequately, leaving the patient with irregular astigmatism. Even though the epithelium fixes half the bent surface, there's still a bend off to the side." He says the VHF ultrasound will show the epithelial changes that cause the bend while topography will miss them due to the smoothing effect of the epithelium.
He says that working with the device in 6,000 eyes pre- and post-LASIK has shown him that surgeons should try to become as adroit as possible with handling thin, 130-µm flaps. This is because the thinner the flap, the more tissue surgeons will leave after LASIK. This decreases the probability, however small, of ectasia.
"We tested the chances of getting ectasia with flaps that were usually cut 130 µm, 160 µm or 180 µm thick," Dr. Reinstein says. "We found that the ectasia rate was three times higher if the surgeon preferentially used thicker flaps."
The Artemis also shows promise with the proper sizing of posterior chamber phakic lenses, which rely on accurate sulcus-to-sulcus measurements to work safely. VHF ultrasound can "get behind" the iris in vivo, yielding the exact measurement, something Dr. Reinstein says optical systems can't do. It may also be helpful with angle-supported anterior chamber lenses as it can measure the angle exactly, allowing the surgeon to customize the lens to the appropriate width.
U.S. approval may come as early as May. "It's in the process of 510K right now," says Dr. Reinstein. "And because it's on the coattails of previous, similar devices, we expect approval by the ASCRS Meeting."
Corneal guttata as viewed with the Confoscan 2 confocal
microscope.
Carlos Carillo, MD
Confocal Microscopy
This technology has made it possible to view cells and corneal features that would otherwise be too difficult to see under normal magnified viewing.
"Since the cornea is transparent, getting good images from its various layers can be difficult, because of limitations related to resolution, depth of focus and contrast," says Carlos Carillo, MD, who works with a confocal microscope at the Codet-Aris Vision Institute in Tijuana, Mexico. He says that, since conventional microscopes collect all of the light reflected back through the imaged object, "out-of-focal-plane" information above and below areas of interest creates noisy and unsatisfactory images in all but the thinnest of specimens when viewed under high magnification.
Another stumbling block of normal magnification is that to use it, you must mechanically fix, embed or stain specimens. This makes it difficult or impossible to view undisturbed physiologic processes, and also introduces artifact into the subject due to the handling process.
Confocal microscopy avoids these problems. The confocal microscope gives the clinician the ability to optically section living or in vitro tissues noninvasively over time. It does so by focusing the incident light in the same plane as the objective lens of the microscope. In effect, both the condenser lens and the objective lens are focused at the same point, hence the term "confocal."
Dr. Carillo is working with the Confoscan 2 from Fortune Technolo-gies America (Greensboro, N.C.) (For other available units, see above box). He and his colleagues' research is in the initial stages. They're using the device to study the thickness of the corneal flap, analyzing the LASIK interface and counting endothelial cells before and after cataract surgery. They can also determine the residual stromal thickness to help guide them during refractive enhancement procedures.
The microscope can also be used to count endothelial cells, either automatically or manually. "It counts them by detecting cell edges and forms," says Dr. Carillo. "It takes about 10 minutes to do automatically."
Working with the device can be challenging, though.
"When trying to measure the tissue remaining after LASIK, it's not always easy," says Dr. Carillo. "You have to get used to interpreting the images, knowing when you're in a certain layer of the cornea. LASIK patients are different because you get used to viewing a normal density of keratocytes, and there are fewer in the LASIK interface."
The mechanics of using the machine can be challenging, as well. "When the patient sees the optical part of the microscope coming toward his eye, he starts blinking," says Dr. Carillo. "You must be patient and start the exam when you can." He says patients also become nervous when he uses the microscope to measure a part other than the center of their eyes, since this necessitates their looking to one side or the other. "They become a little nervous when the microscope is coming at them from a direction that they can't really see," he says.
Though challenges remain, he and his colleagues get better with the device every day. He says they're using it effectively in patients "in whom we suspect problems with endothelial cell count for cataract surgery and for ensuring precise flap thicknesses with our microkeratome."
Confocal Microscope Makers
| Hund Microscopes (Wetzlar, Germany) |
www.hund.de |
| Tomey Europe | www.clinilab.fi/tomey.htm#order |
| Fortune Technologies America (Greensboro, N.C.) |
1 (888) FTA-5064 |
| Meridian Instruments (Okemos, Mich.) |
1 (800) 247-8084 |