OCTs ushering new era in complex contact lens fittings
Complex contact lenses, including scleral lenses, have unique fitting considerations that may be augmented with additional optic scans, says Jeffrey Sonsino, OD, FAAO, of Nashville, TN, at SECO 2017.
Atlanta-Innovative uses of optical coherence tomographer (OCT) scans are ushering a new era in contact lens fitting, according to one expert in the field.
Complex contact lenses, including scleral lenses, have unique fitting considerations that may be augmented with additional optic scans, says Jeffrey Sonsino, OD, FAAO, of Nashville, TN, at SECO 2017.
Dr. Sonsino cited the physical dimensions of complex lenses that make them challenging to fit, arguing that OCTs have proven to be effective in augmenting eye topography analyses when fitting complex contact lenses because such lenses are designed to avoid contact with the central cornea.
“The reason OCT is powerful in scleral and hybrid lenses is that these are vaulted lenses,” he says.
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The number one application of OCT in lens fitting is in measuring the corneal bulge, which allows for superior accuracy to the traditionally used slip beam estimation method. While standard measures of success with patients allow for some wiggle room in measurements-in some cases, even up to 800 µm, according to his research-that wisdom is coming into question following recent studies that looked deeper into the cornea-oxygen relationship.
One study found that the difference between a 200 µm vault and a 400 µm vault resulted in a 30 percent reduction in oxygen levels after just five minutes. This is especially important in patients who have oxygen problems with very low endothelial cell count. It’s especially important to fit these patients with thin lenses no thicker than 300 µm to allow for better oxygen permeation.
“Now all of a sudden, we have criteria that tells us we have to be a lot more accurate with our scleral lens evaluations and fittings,” Dr. Sonsino says.
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Rise of OCT in eye care
Although ophthalmologic use of OCTs had been established in the 1990s, imaging technology had not advanced far enough to show image resolution in great detail. However, Dr. Sonsino says this trend has changed.
“When you compare modern-day OCTs to that,” he says, “the brand doesn’t matter-the resolution is so much better, and you can see so much more detail.
Originally developed to help visualize posterior eye tissue, OCTs are now used in the diagnosis of a myriad of retinal, anterior segment, and optic nerve diseases such as macular degeneration and glaucoma.
But in recent years, usage of OCTs has expanded. Today, OCTs are being adopted by more and more eyecare professionals looking to properly fit patients with complex contact lenses.
Choosing the right OCT method
For those interested in adopting OCT, it’s important to note there are different kinds of scans available, depending on the device used.
• Line scans. The most common is the line scan, which images one “slice” of the interior segment. The operator can control where the scan happens, and most models allow for beam rotation to image specific cross sections.
• Crossline scan. Crossline scans create both a vertical and horizontal scan simultaneously, offering twice the amount of information provided by line scans. Dr. Sonsino recommends following up crossline scans with multiple line scans.
• Pachymetry scans. In addition to being used on the naked eye, pachymetry scans can also be used on patients wearing their contact lenses because the scan images slices throughout the cornea. The downside, Dr. Sonsino cautions, is that it images only about 6 mm, providing a diminished field of view.
However, Dr. Sonsino cautions against over-reliance on the technology and replacing other, proven tools, such as topography.
“It’s not to the point where you can replace some of the existing technology when you’re talking about monitoring the progression of keratoconus or diagnosing pellucid marginal degeneration vs. keratoconus," he says.
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OCT and lens design
The increasing usefulness of OCT in complex lens fitting has raised the question of whether they could be used in the lens design process of scleral lenses.
The process is already undergoing initial experimentation, some in the field pioneering proprietary lens-designing software that utilizes OCT. Already, one joint pilot study with the Boston Foundation for Sight, Oregon University of Health & Science, and Harvard Medical School used OCT to guide the design of a prosthetic device meant to treat corneal disease.
However, these efforts are not yet far enough along to offer any significant benefits beyond what’s already available to optometrists using conventional tools, Dr. Sonsino says.
“Whether or not this is something that’s better than the technology we have, I’m not convinced. The technology that all of us have access to is probably just as good as [OCT-based lens designing] technology,” Dr. Sonsino says.
