The Vision Membrane

The increasing evidence of keratorefractive surgery's limitations is producing a gradual shift in the surgical approach to correcting refractive error. No amount of customized corneal ablation can reduce the amount of significant aberrations caused by correcting moderate-to-severe ametropia. Additionally, all efforts to correct presbyopia by operating on the corneal surface are doomed to failure, because creating a bifocal cornea distorts distance vision too severely.

Only refractive IOLs with an adequate optical diameter enable aberration-free refractive surgery in moderate-to-severe ametropia. The VISION MEMBRANE (VISION MEMBRANE TECHNOLOGIES, Inc., Carlsbad, CA) employs a radically new approach to correcting moderate-to-severe ametropia and presbyopia.

BACKGROUND
Factors such as prolonged healing times, irregular astigmatism, nighttime visual symptoms, and the expense and effort entailed in the purchase and maintenance of an excimer laser have encouraged the continued development of IOLs for refractive surgery. Moreover, phakic IOLs are capable of providing a quality of vision superior to that attained with LASIK or PRK.

Nevertheless, various factors have limited the use of phakic IOLs. Iris-fixated lenses can provide excellent optical results, but they can be difficult to implant and can become significantly decentered. The true incidence of cataract formation induced by phakic posterior chamber lenses has yet to be determined. The thickness of current anterior chamber IOLs has necessitated a smaller-diameter optic in order to eliminate endothelial touch. These lenses are centered on the cornea's geometric center, rather than the pupil's. The disparity in centration produces a small effective optical zone and a large degree of glare as the pupil increases in diameter.

Additionally, many physicians consider the risks associated with removing a clear crystalline lens to be excessive due to imprecise refractive results and the need for extensive intraocular surgery.

DESIGN
The VISION MEMBRANE is constructed entirely of medical-grade silicone. When implanted in the anterior chamber of the eye, this thin, vaulted membrane can correct both presbyopia and refractive errors including myopia, hyperopia, and astigmatism. Unlike standard IOLs, which generally have a thickness of 700 to 1,000 µm, the VISION MEMBRANE is 200 µm thick, regardless of the dioptric power (Figure 1). Furthermore, the foldable device's dimensions and vaulted shape create an excellent blend of stability, flexibility, and implantability through an incision less than 2.0 mm wide (Figure 2).

ADVANTAGES
Compared with an IOL with a flat optic, the VISION MEMBRANE's curved optic produces a greater amount of space between it and the delicate corneal endothelium. Unlike the 4.5-mm optic of the pioneering Baikoff IOLs (NuVita lens, Bausch & Lomb Surgical, San Dimas, CA), the optic of the VISION MEMBRANE is 6.5 mm in diameter, a size that improves the pupil-optic relationship and thereby eliminates halos and glare in almost all cases. Also, the vaulted design of the device prevents pupillary block and thus avoids the need for a peripheral iridotomy, which is necessary with various phakic IOLs.

Another advantage of the VISION MEMBRANE is that its hydrophobic silicone material and broad haptic design prevent the formation of anterior synechiae. Angle-fixation makes implantation simpler, because it eliminates the need for excessive surgical manipulation. Due to the VISION MEMBRANE's extreme flexibility and variable vaulting in the anterior chamber, one size of the device fits nearly all eyes, although two sizes will be available.

The device also employs modern diffractive optics in order to focus incoming light, and the quality of vision that they produce is equal to that created by an IOL with refractive optics. Modern diffractive optics effectively correct ametropia, and ongoing European and US trials of pseudophakic bifocal IOLs indicate the potential advantage of diffractive over refractive optics for the correction of presbyopia. Unlike standard lenses that use refractive optics, the VISION MEMBRANE's diffractive optics do not rely on the lens material's index of refraction in order to gain the desired refractive effect.

INTENDED USE
All patients must be 18 years of age or older with a generally stable refraction to undergo implantation of the VISION MEMBRANE. The device will be available in two forms. The first VISION MEMBRANE design is single-powered for the correction of -2.00 to -15.00 D of myopia in 0.50-D steps, and +2.00 to +6.00 D of hyperopia in 0.50-D increments. The bifocal form of the VISION MEMBRANE device is designed to correct presbyopia and either myopia or hyperopia. This version may be used in presbyopes, as well as in patients who have already received a posterior chamber IOL after cataract extraction but who have limited reading vision.

Of course, the quality of results attained in patients combined with surgeon opinion will determine the success of the VISION MEMBRANE. It has been implanted in trial patients outside of the US, and further trials outside of the US will commence within the next 1 to 2 months. European and US trials may begin in 5 to
8 months.

CONCLUSION
The VISION MEMBRANE possesses a combination of advantages not found in any existing IOL. These benefits include flexibility, a large optic, the ability to correct both presbyopia and refractive error, and a greater degree of safety of the delicate structures of the anterior chamber.

In the near future, we will see a tremendous increase in the use of anterior chamber IOLs, although LASIK and PRK will continue to be important modalities for correcting low ametropia and refining the results with pseudophakic IOLs. I believe that anterior chamber devices will provide more predictable outcomes, offer a lesser incidence of complications, and have the potential to correct presbyopia. As a result, these devices will attract ophthalmologists with cataract and IOL surgical skills to the refractive-surgery arena. A major revolution in this field is upon us.

Lee T. Nordan, MD, is the director of Nordan Eye Laser Medical Group in Carlsbad, California. He is President and CEO of VISION MEMBRANE TECHNOLOGIES, Inc. Dr. Nordan may be reached at (760) 930-9696; laserltn@aol.com.