Corneal Inlays and Corneal Onlays
Corneal inlays and onlays are tiny lenses or other optical devices inserted into the cornea to reshape the front surface of the eye to improve vision. In fact, some of these devices resemble very small contact lenses.
The primary use of current corneal inlays (the term "corneal onlays" is used less frequently these days) is to improve near vision and reduce the need for reading glasses in older adults who have presbyopia.
Because corneal inlays are implanted within the tissue of the cornea, this type of refractive surgery is less invasive than phakic IOL procedures that involve larger implantable lenses that are placed deeper in the eye, in front of or behind the pupil.
Through use of corneal inlays and onlays for vision correction, eye surgeons may avoid complications sometimes associated with procedures such as LASIK and PRK because no corneal tissue is removed. And these devices may have fewer risks than implantable lenses because the surgery takes place within the cornea, not inside the eye.
Kamra Corneal Inlay
While the technology is not yet FDA-approved, clinical trials began in early 2006 for one device created by AcuFocus, which has formed a business alliance with Bausch + Lomb.
The Kamra corneal inlay (left) is used for presbyopia correction. Here it is shown next to a typical contact lens. (Image: AcuFocus)
Previously called the ACI 7000 and now known as the Kamra, this corneal inlay is designed to correct presbyopia. During the Kamra procedure, a thin flap is created in the center of the cornea where the device is inserted. The flap then is replaced over the inlay to hold it in place.
The procedure takes less than 15 minutes and can be performed in the eye surgeon's office. Sutures are not required, and only topical anesthesia in the form of eye drops is used.
According to Bausch + Lomb, the Kamra is designed to block certain light rays reflecting from near objects that ordinarily would not be focused correctly by the presbyopic eye.
"Its optical principle is the pinhole phenomenon, which when used in photography (in terms of a small aperture), creates greater depth-of-focus and thus allows clear vision at all distances near, intermediate and distance," said Vance Thompson, MD, who is participating in clinical trials for the device.
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Thompson, a member of AllAboutVision.com's editorial board, said it's important to remember that the Kamra is still in investigational phases, but that early results are very promising.
"Having worked in the field of presbyopia correction for more than 18 years, I find it is such a pleasure to use a device like the AcuFocus corneal inlay," Thompson said. "It provides our patients who have needed reading glasses with good near vision, while still maintaining good distance vision."
Thompson said the Kamra is just 3.8 millimeters in diameter and only 10 microns thick (about 1/10th the thickness of a piece of paper). The device is made of an opaque biocompatible polymer called Kynar, a thermoplastic material that softens in heat and hardens in cooler conditions.
Raindrop Near Vision Corneal Inlay
Researchers from Mexico presented promising early results from surgical implantation of the Raindrop Near Vision Inlay (ReVision Optics) during a 2009 American Society of Cataract and Refractive Surgery conference.
Formerly known as PresbyLens in the U.S. and Vue+ in Europe, the tiny (2mm diameter) inlay is made of medical-grade hydrogel plastic similar to that used for soft contact lenses and has optical characteristics that are almost identical to the human cornea, according to the company.
The inlay improves both near and intermediate vision, as demonstrated by a study involving 38 people who received the Raindrop Near Vision implant in their non-dominant eye:
- All people enrolled in the study ranged in age from 45-56.
- Six months after receiving the implant, most people could perform near vision tasks such as reading a newspaper without the need for eyeglasses.
- Most subjects also did not require eyeglasses for intermediate tasks such as reading a computer screen.
- All study participants had 20/25 or better distance vision in both eyes.
- Only one adverse event was reported, when an inlay needed to be repositioned.
The Raindrop Near Vision corneal inlay is placed within the cornea under a LASIK-style flap. When in position, the inlay changes the curvature of the cornea so the front of the eye acts much like a multifocal contact lens.
In June 2012, ReVision Optics announced the Raindrop Near Vision inlay advanced into the final phase of its U.S. clinical trial required for FDA approval.
In earlier phases of the trial, patients who had the device implanted in the cornea of their non-dominant eye had an average improvement of five lines of near visual acuity and one to two lines of intermediate visual acuity on a standard eye chart, while maintaining binocular distance vision of 20/20.
Phase 3 of the U.S. clinical trial will be conducted at a number of specialized ophthalmology centers across the country. If you are interested in participating in the trial and want to find out if you are eligible, you can learn more at the ReVision Optics website.
The Flexivue Microlens (Presbia Cooperatief U.A., Amsterdam) uses a laser, which creates a tiny "pocket" just below the eye's surface. Eye surgeons use a specially developed instrument to then insert the microlens for correction of presbyopia. The pocket seals itself to hold the lens in place.
The lens is made of hydrophilic polymer, a highly wettable synthetic substance often used in intraocular lenses that permanently replace the eye's natural lens in cataract surgery. However, the Flexivue Microlens is permanent but can be easily removed and replaced if a stronger prescription is needed.
The tiny lens is 3 mm in diameter and only 20 microns thick at the edges about a quarter of the width of a human hair.
This lens is being researched outside the U.S.
W. Bruce Jackson, MD, University of Ottawa professor and chairman of ophthalmology, has helped pioneer another minimally invasive vision correction technology known as the corneal onlay. These devices are made of mostly liquid, genetically engineered collagen-like material.
The corneal onlay is made up of artificially produced tissue similar to the eye's surface or cornea and can be used to reshape the eye. (Image provided by Dr. W. Bruce Jackson)
Corneal onlays do not require a thin flap, as occurs with inlays, but instead are placed in an artificially created pocket under the outer layer of the cornea (epithelium). This pocket then holds the onlay in place until the disturbed epithelial cells grow back to cover the device.
Jackson told AllAboutVision.com that laser energy ultimately might be applied to the onlay to sculpt it for vision correction, just as the eye's natural surface is now reshaped in LASIK and PRK.
The corneal onlay technology described by Jackson is still in experimental stages.
Other researchers are investigating techniques such as using biocompatible adhesives to attach an onlay temporarily to the outside of the cornea long enough to allow epithelial cells to grow over and secure the implant.
Reversibility is a major advantage of corneal inlays and onlays, which can be removed at any time.
About the Author: Vance Thompson, MD, FACS, is the director of refractive surgery at Vance Thompson Vision in Sioux Falls, S.D. He also is an assistant professor of ophthalmology at the Sanford USD School of Medicine, a leading researcher in technologies for laser and implant vision correction and a member of All About Vision's editorial advisory board.
Original version of this article was by Marilyn Haddrill.
[Page updated March 2014]
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