Versatile corneal, anterior segment imaging technologies improve accuracy

An eclectic array of anterior segment testing and measurement technologies enable cataract and refractive surgeons to accurately assess anatomic features to optimize clinical and visual outcomes.

Anterior segment optical coherence tomography, very high-frequency ultrasound biomicroscopy and Scheimpflug photography help surgeons assess corneal thickness and topography, anterior chamber depth and axial length, and iris and ciliary body status.

Used alone or in combination, these modalities also help surgeons manage complications.

Although all anterior segment measurement devices have strengths and weaknesses for various indications, they must be rapid and user-friendly to be included in the busy surgeon’s arsenal, Michael W. Belin, MD, told Ocular Surgery News.

“Probably the most important thing for devices that are used, especially for screening patients, is that they are rapid and easy. Otherwise, you don’t use them,” he said.

Assessment for cataract surgery

The IOLMaster (Carl Zeiss Meditec) is gaining momentum as the standard measurement tool for cataract surgeons because it offers speed, efficiency and consistency, Dr. Belin said.

“Not that it offers accuracy advantages over ultrasound, but [it] may become [the] standard of care for a number of reasons,” he said. “The IOLMaster is technician- and office-friendly. So, as far as cataract surgery, that would be my answer.”

Surgeon Roxana Ursea, MD, favors high-resolution ultrasound biomicroscopy (Quantel Medical) for optimizing IOL location because it offers superior angle-to-angle measurements.

In addition, the Quantel ultrasound biomicroscopy device does not require the use of a scleral shell, allowing the patient to sit upright rather than lie supine, she said.

In cataract surgery, a device that combines the capabilities of the IOLMaster with autorefraction and other functions would be a major benefit, Dr. Belin said.

“I think that’s very doable,” he said. “We’re seeing multifunction devices coming out. I think in the future we’ll probably see other multifunction devices, both for cataract surgery and for refractive surgery.”

Femtosecond laser-based cataract procedures such as lens fragmentation, anterior capsulotomy and corneal incision creation call for intraoperative guidance, and such monitoring is being done with OCT or Scheimpflug imaging, David Huang, MD, said.

“I think these intraoperative monitoring devices will gradually become more common as the technology becomes better understood and the economy of scale sets in,” Dr. Huang said.

Testing for refractive surgery

OCT offers high resolution and versatility in assessing corneal opacity, epithelial thickness, LASIK flap thickness and residual stromal bed thickness, Dr. Huang said.

“Preop and postop, looking at flaps, OCT is definitely the way to go,” he said. “Before doing an enhancement, I always use OCT to look at the residual stromal bed thickness and whether it’s really regression or there’s actually some sign of ectasia. You can sort of differentiate them by looking at the epithelial thickness, overall thickness and posterior curvature. And also looking at the original flap thickness, obviously. So, I think that’s very valuable.”

The Pentacam (Oculus) and Galilei dual-Scheimpflug camera (Ziemer Ophthalmic Systems), in addition to OCT, are useful in mapping the anterior and posterior corneal surfaces to gauge corneal refractive power, Dr. Huang said.

“There are a few circumstances where mapping the posterior surface is also useful, for example, in post-LASIK cataract surgery intraocular lens calculations, where measuring the anterior surface alone is not sufficient to really accurately measure the net corneal power,” he said.

Scheimpflug photography offers the most thorough preoperative assessment for refractive surgery, Dr. Belin said.

“I think they’ve pretty much supplanted the Placido devices, which really can only give you one bit of information, which is the anterior corneal surface,” Dr. Belin said. “The Scheimpflug devices are very easy to use, rapid and very technician-friendly.”

Dr. Ursea said she prefers using the Visante anterior segment OCT (Carl Zeiss Meditec) to assess patients before and after LASIK. However, she favors the Artemis very high-frequency ultrasound device (ArcScan) in complicated cases.

“Many times the Visante anterior segment OCT doesn’t visualize as well within the cornea, especially within the stroma,” Dr. Ursea said. “The Artemis can give you epithelial mapping and stromal mapping, so it kind of divides the layers and can tell you what’s happening at different depths within the cornea.”

Maximizing photographic quality

OCT and Scheimpflug photography are effective for different stages of refractive surgery, Dr. Belin said.

“It really depends on what the application is,” he said. “If you want to look at a flap creation, while both of them can do that, OCT will probably give you a better image at the moment. These are all evolving. If you want to be measuring the corneal thickness, the curvature, the elevation, then Scheimpflug technology does a better job. They all have their strengths and weaknesses.”

Dr. Belin pointed out that good resolution does not necessarily mean good image quality.

“An A-scan may be very accurate, but it doesn’t give you a pretty picture to look at,” he said. “Resolution, accuracy and ‘imaging’ are very different. OCT is nice for imaging, but I don’t believe it has the resolution that the Scheimpflug devices have.”

A combination of anterior segment OCT and ultrasound biomicroscopy would be ideal, Dr. Ursea said.

“The advantage of the UBM is deeper penetration, while the anterior segment OCT offers a better image with higher resolution of the cornea,” she said. “The technology used by these two instruments is different. One is using light and the other sound.”

A device combining corneal staining and confocal microscopy would be a boon to the assessment of ocular surface infections, Dr. Huang said.

“Confocal microscopy of the cornea has been around for a long time but it still doesn’t have a lot of specificity in looking at microorganisms,” Dr. Huang said. “Some sort of in vivo staining or fluorescence that allows us to really distinguish infections or microorganisms with confocal imaging would be great.” – by Matt Hasson

• Michael W. Belin, MD, can be reached at Department of Ophthalmology, University of Arizona, 655 N. Alvernon Way, Suite 108, Tucson, AZ 85711-1824; e-mail: mwbelin@aol.com. Dr. Belin serves as a consultant to Oculus.
• David Huang, MD, PhD, can be reached at Casey Eye Institute, 3375 SW Terwilliger Blvd., Portland, OR 97239-4197; e-mail: osn@dr-huang.net. Dr. Huang receives royalties from a patent on OCT licensed to Carl Zeiss Meditec as well as grant support, stock options and travel support from Optovue.
• Roxana Ursea, MD, can be reached at Department of Ophthalmology, University of Arizona, 655 North Alvernon Way, Suite 108, Tucson, AZ 85711-1824; 520-322-3800, ext. 204; fax: 510-321-3665; e-mail: rursea@eyes.arizona.edu.