By Fred Gebhart
SAN FRANCISCO -- October 27, 2009 -- A new wave of interest in corneal inlays, each made of different material, having a different mechanism of action to correct presbyopia, and distinctive in size, were discussed at the 2009 Joint Meeting of the American Academy of Ophthalmology and the Pan-American Association of Ophthalmology (AAO-PAAO).
Growing concerns about quality of life issues among aging baby boomers have helped rekindle interest in corneal inlays for the treatment of presbyopia. The current generation of inlays offers the same advantages considered important more than a decade ago, when inlays were being developed to treat myopia and hyperopia. They remain removable, reversible, and repeatable, unlike more familiar corneal subtraction procedures such as laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).
Several corneal inlays are undergoing clinical trials, noted Francisco Sanchez Leon, MD, Instituto Novo Vision, Mexico City, Mexico, in his October 23 presentation "Clinical Performance of Clinical Inlays." Current entrants include AcuFocus, InVue, and PresbyLens. None are approved for marketing in the United States, and all are implanted in the nondominant eye, but they have few other similarities.
The AcuFocus corneal inlay (AcuFocus, Irvine, California) is 2.8 mm in diameter and 5 microns thick and has a 1.6-mm pinhole in the centre. The opaque inlay is designed to function like a camera lens with a small aperture that increases depth of field. In bright light, the pinhole effect enhances visual acuity, Dr. Leon explained; whereas, in dim light, the pupil dilates well beyond the opaque ring, allowing adequate light to enter from the sides while preserving central visual acuity. It is implanted either in a corneal pocket or under a flap. Micropores 5-11 microns in diameter aid corneal circulation and transport.
The device was originally made to be placed under a corneal flap created by either a microkeratome or femtosecond laser, Dr. Leon continued, but he recommended implantation in a pocket. A pocket can be carved into the middle stroma, deeper than a flap placement. Placing the inlay deeper alleviates concerns about anterior nutrition and neutrophic corneal disease. A pocket also holds the inlay more securely because there is little room for movement or slippage.
InVue (BioVision AG, Bruggs, Switzerland) is a water-permeable hydrogel lens 3 mm in diameter and 10 microns thick at the edge. Centre thickness depends on the add power of the lens in an annular ring surrounding a central clear area. The lens transforms an emmetropic cornea into a multifocal cornea. The central area is for reading, the peripheral zone for distance, and the mid-periphery for middle distance tasks. A multifocal cornea tends to provide significantly improved reading ability but at the cost of a slight decrease in contrast sensitivity, which degrades distance vision, Dr. Leon explained. Like other multifocal optics, a multifocal cornea can induce night-vision symptoms, particularly haloes.
The PresbyLens corneal inlay (ReVision, Lake Forest, California) is a polymer lens 1.5 mm in diameter. The lens is similar to a soft contact lens but sits beneath a corneal flap. Placing the lens just beneath the corneal surface changes the corneal topography and optical profile to create a multifocal system to enhance near vision while leaving distance vision almost unchanged.
"This is an additive effect," explained Stephen Slade, MD, Laser Center of Houston, Humble, Texas, in his presentation entitled "A Study of the Efficacy of the PresbyLens Corneal Inlay." "It just adds to the central area of the cornea."
Dr. Leon noted that the manufacturer is developing a pocket placement technique.
[Presentation title: Refractive Surgery Section V: Introstromal Implants for Refractive Surgery. Friday, October 23, 2009]
E-Mail this DGDispatch to a colleague
