Corneal Inlays and Corneal Onlays
As their names suggest, corneal inlays and onlays differ in where they are implanted within the cornea: corneal onlays are placed near the surface of the eye, directly under the thin outer layer of the cornea called the epithelium; inlays are placed deeper in the cornea in the thicker middle layer called the stroma.
Corneal inlays are placed within the central stromal layer of the cornea; corneal onlays are positioned just beneath the outer epithelium.
Because they are implanted deeper in the cornea, corneal inlays may be more stable and deliver more predictable and long-lasting results than corneal onlays. For these reasons, it appears corneal inlays will likely be the preferred device for doctors and patients interested in this refractive surgery option.
Corneal inlay surgery sometimes can be combined with LASIK surgery to correct both presbyopia and nearsightedness, farsightedness and/or astigmatism. (This is considered an "off-label" use of the procedure in the United States, meaning the U.S. Food and Drug Administration (FDA) will allow an eye surgeon to perform the combined procedure, even though it has not yet been officially approved by the FDA.)
Corneal inlay surgery is less invasive and may have fewer risks than phakic IOL procedures, which involve implanting lenses further back inside the eye, either directly behind or in front of the pupil.
Also, because no corneal tissue is removed during inlay surgery, this procedure may be a good alternative for people whose corneas are too thin for LASIK or PRK.
Kamra Corneal Inlay
Developed by AcuFocus, the Kamra corneal inlay received FDA approval for use in the United States in April 2015. The device also has received a CE mark for the European Economic Area, allowing it to be marketed in over 30 countries across Europe.
The Kamra inlay is designed to reduce or eliminate the need for reading glasses among people over age 40 who have good distance vision without glasses but have problems seeing up close due to presbyopia.
The Kamra corneal inlay (left) is used for presbyopia correction. Here it is shown next to a typical contact lens. (Image: AcuFocus)
The Kamra device is very small and thin just 3.8 millimeters (about 0.15 inch) in diameter and 6 microns thick (about half the thickness of plastic wrap used to store food). It consists of an opaque outer ring and a tiny (1.6 mm) central opening that is placed directly in front of the center of the pupil. This creates a pinhole camera effect that extends the natural range of vision, making near objects appear clearer without significant loss of distance vision.
The Kamra inlay typically is implanted in the non-dominant eye, so the distance vision of the patient's dominant eye is completely unaffected. The procedure takes less than 15 minutes and can be performed in the eye surgeon's office. The eye heals quickly without the need for stitches.
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Raindrop Near Vision Inlay
The Raindrop Near Vision Inlay is a corneal inlay for presbyopia being developed by Revision Optics.
Formerly known as PresbyLens in the U.S. and Vue+ in Europe, the tiny (2.0 mm 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 Raindrop Near Vision Inlay typically is placed within the cornea of the non-dominant eye under a LASIK-style flap created with a femtosecond laser. 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 a recent study of 38 people (ages 45 to 56) who underwent the Raindrop Near Vision Inlay procedure:
- 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.
Though ReVision Optics states on their website that the company is actively pursuing regulatory approvals and market opportunities for the Raindrop Near Vision Inlay worldwide, the product is still considered an investigational device in the United States and has not yet received FDA approval for routine use.
Presbia Flexivue Microlens
Another innovative corneal inlay designed for the correction of presbyopia is the Presbia Flexivue Microlens, being developed by Ireland-based Presbia PLC.
This device resembles a tiny bifocal intraocular lens like those used in cataract surgery. It measures just 3.2 mm in diameter and is available in a range of powers, depending on the patient's near vision needs.
During the Presbia Flexivue Microlens procedure, small pocket is created in the corneal stroma with a femtosecond laser, and the inlay is inserted into this pocket with a special device. The pocket seals itself, and the entire procedure typically takes less than 10 minutes.
The corneal inlay can be removed and replaced with a higher or lower power lens if needed.
The Presbia Flexivue Microlens is not yet FDA-approved for use in the United States, but it has received a CE mark for the European Economic Area.
In a 2012 study, patients in Italy and Greece who underwent the Flexivue Microlens procedure reported very high satisfaction rates one year after implantation, with 97 percent saying their uncorrected near vision in the implanted eye was either good or excellent, and 97 percent reporting that their uncorrected distance vision with both eyes open was either good or excellent.
A clinical trial required for approval of the Presbia Flexivue Microlens in the U.S. commenced in 2014.
Corneal onlays are similar to corneal inlays, but are designed to be placed closer to the front surface of the cornea, just under the outer layer called the epithelium.
The corneal epithelium comprises about 10 percent of the overall thickness of the cornea and acts as a protective barrier to keep bacteria, dust and other foreign substances from penetrating the eye. The epithelium also absorbs oxygen and nutrients from the tear film so they can nourish the rest of the cornea.
Though some research in developing corneal onlays is ongoing in Europe, it appears that corneal inlays have become the preferred technology for corneal implants designed for vision correction, according to W. Bruce Jackson, MD, professor and chairman of ophthalmology at the University of Ottawa (Canada) and one of the early investigators of corneal onlays.
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 associate 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.
Gary Heiting, OD, also contributed to this article. Original version of this article was by Marilyn Haddrill.
[Page updated August 2015]