Inlays, Onlays, Rings & Things
Irving J. Arons
As pointed out by Bill Link at the recent ASCRS meeting in Los Angeles (1990), there is a surge of interest in various surgical approaches to correcting impaired vision. From a mild renewed interest in RK, probably brought on by the intense interest in excimer laser wide area ablation and the inability of a large number of surgeons to participate in the human clinical trials limited by the FDA, to the renewed interest in other means of changing refractive correction by changing the shape of the cornea.
Under investigation -- some in human clinical trials -- are differing forms of epikeratophakia using onlays of both human tissue and synthetic lenticle materials; inlays and rings using both hard and soft plastics; and standard and multifocal IOLs for correcting myopia in phakic eyes. In addition to the implantation of rigid or soft IOLs, some researchers are still investigating the possibility of injecting either a hydrogel or silicone into a cleaned-out capsule to provide corrected vision with some degree of accommodation.
In addition to traditional keratophakia, where recipient cornea is cut, frozen and reshaped on a cryolathe, synthetic collagen and hydrogels are being used to replace the removed corneal tissue. Lypholized donor corneal tissue (Kerato-Lens and Kerato-Patch) is still processed for some researchers by Allergan Medical Optics (AMO), along with its hydrogel material (Kerato-Gel). Human trials with the latter for both aphakic and myopic correction are underway. Alcon Surgical is also sponsoring clinical trials with its proprietary hydrogel material.
In addition, the Choyce polysulfone inlay program is still going strong. The newest version is microperforated, having thousands of 2 micron holes that allow nutrient and gas permeation. The trials are sponsored by Surgidev and FDA approval to expand the availability of the inlays is hoped for late in 1990.
Still another inlay program is being sponsored by Optical Radiation. ORC has developed a hydrogel Fresnel intracorneal lens which is flat and thus of relatively uniform thickness. Since the degree of correction is controlled by the Fresnel design, the lens can be placed into a narrow pocket dissection.
The other inlay trial, still in pre-clinical study, is with the Keravision intra-corneal ring (ICR). This PMMA 6-8mm ring is inserted in a channel in the peripheral cornea and is tightened to steepen the cornea for correcting hyperopia and loosened to flatten the cornea for correcting myopia. Much work still needs to be done to optimize the device and procedures before human clinicals start.
Several companies, including Chiron Ophthalmics and G.E. Medical Systems are sponsoring programs to place either hydrogels or collagen onlays onto a de-epithelized cornea and have the epithelium regrow and hold the onlay in place. The Chiron program, with a proprietary hydrogel, is in limited pilot testing, with two non-seeing eyes outside of the U.S. having been tested to date. The G.E. Medical Systems' collagen onlay program, called LASE (Laser Adjustable Synthetic Epikeratoplasty) is underway at Emory University, under the auspices of Keith Thompson and Khalil Hanna. In this program, the collagen onlay (supplied by Domilens) can be reshaped as needed using an excimer laser, prior to re-epithelization.
As noted in our February 1990 column, work continues on perfecting technology on an injectable IOL with the hope of retaining some accommadation. Allergan Medical Optics continues a low level effort on the original program in this field, initiated in the early 1980s by its then subsidiary Innovative Surgical Products. ISP had planned to inject an enzyme into the capsule to dissolve the cataractous lens and then follow this with a liquid polymerizable silicone polymer. As I recall experiments with monkeys had taken place but with many unresolved problems. In France, Domilens has a research program underway using a collegan material, while another research program announced by Vision Technology a year or two ago has apparently been halted. We expect other programs in this area are quietly underway at other research institutes.
So the field of surgical vision correction is humming. Lasers may have the spotlight, but a lot is going on both in the footlights and backstage.