A vast revolution in eye care has occurred during the last 2 decades as excimer laser keratorefractive surgery has become an accepted and routine procedure. Just as the excimer laser greatly improved upon the results of RK, the stage is now set for phakic IOLs to build upon the results obtained with LASIK and PRK.
LIMITATIONS OF LASIK AND PRK
Although LASIK and PRK enhance visual function in most cases, these procedures have important limitations. The thin cornea allows for only an optical zone diameter in the range of 6 mm, a situation that induces significant optical aberrations at the junction of the optical zone and the untreated cornea. Postoperative dry eye and fluctuating vision are not uncommon in patients. Additionally, surgeons cannot adequately treat presbyopia at the corneal level, because these procedures result in an unacceptable loss of contrast sensitivity. All of these problems are worse in the higher versus lower ranges of preoperative ametropia.
THE DESIRABLE PHAKIC IOL
The ideal phakic IOL will possess a combination of qualities that current phakic IOLs lack. They include
(1) a thinness of 350 µm or less for all refractive powers,
(2) an optic diameter of at least 6.00 mm,
(3) a foldable lens material that may be implanted through an incision sized 3 mm or smaller,
(4) a lens design that allows safe and easy insertion of the IOL into the anterior chamber,
(5) an anterior chamber design (the safest IOL location),
(6) a design that permits explantation, and
(7) the capability of correcting ametropia and presbyopia simultaneously without causing excessive glare.
THE VISION MEMBRANE
Design Details
The VISION MEMBRANE (Vision Membrane Technologies, Inc., Carlsbad, CA) is a silicone, anterior chamber phakic IOL that employs a combination of refractive optics and sophisticated diffractive optics to maintain a constant thinness in the range of 200 to 300 µm for all refractive powers. In addition, the VISION MEMBRANE is the first IOL to possess a vaulted optic (Figure 1), which provides enough clearance from the corneal endothelium to accommodate an optic of greater than 6 mm in diameter. This larger optic is critical to eliminating the halos that have been inherent in phakic IOLs with optic diameters of 4.5 mm. Moreover, the vaulted optic precludes the need for a peripheral iridectomy to prevent pupillary block and angle-closure glaucoma.
The sophisticated diffractive optics of the VISION MEMBRANE provide a portion of the bifocal correction by means of an increased depth of field, rather than two disparate images created by refractive optics of two different powers. As a result, an eye implanted with the IOL should achieve distance and near visual function with high contrast at each distance.
Implantation
The foldable VISION MEMBRANE can be implanted by means of an injector through a clear corneal incision of less than 3 mm. The implantation technique for this lens is similar to that used for a posterior chamber pseudophakic IOL after cataract extraction. Preoperatively, instilled pilocarpine 1% creates a miotic pupil. The surgeon loads the VISION MEMBRANE into the lubricated injector cartridge, creates a sideport incision, and injects viscoelastic into the anterior chamber. The IOL is injected into the anterior chamber through a 2.8-mm clear corneal incision (Figure 2). The surgeon engages the inferior haptics of the VISION MEMBRANE into the inferior angle before removing the cartridge tip from the anterior chamber. Bimanual I/A removes all viscoelastic from the anterior chamber, and the surgeon uses the I/A instruments to adjust the position of the lens, if necessary. The anterior chamber is inflated to a normal pressure with BSS (Alcon Laboratories, Inc., Fort Worth, TX), and the wound is checked. Finally, the surgeon places a bandage contact lens and a drop of ZYMAR (Allergan, Inc., Irvine, CA) on the eye.
Explantation
The surgeon may explant the VISION MEMBRANE by grasping its superior haptic with a forceps through a 2.8-mm incision and then externalizing the entire IOL by means of gentle traction. The wound will remain watertight whether or not the surgeon implants a new lens.
THE FUTURE
Vision Membrane Technologies, Inc., expects phakic IOLs to play an ever-increasing role in refractive surgery, especially for the correction of moderate-to-high ametropia and presbyopia. I believe that the typical cataract/IOL surgeon's practice will quickly transition into a refractive practice as well. Phakic IOLs' high-quality, accurate, and stable correction of refractive errors will attract cataract surgeons to the technology.
The VISION MEMBRANE offers a unique combination of highly desirable characteristics not currently available with any other phakic IOL. This lens will soon offer surgeons an opportunity to correct refractive error and presbyopia without inducing halos. Moreover, the surgical implantation technique will be familiar to all cataract surgeons. Trials of the VISION MEMBRANE are ongoing in Mexico, and they should commence in Europe and the US within 6 to 9 months.
Phakic IOLs will most likely become the dominant form of refractive surgery worldwide within the next 5 to 8 years. Keratorefractive surgery and phakic IOLs will coexist, but phakic IOLs surely represent the next major stage of the ongoing refractive surgery revolution.
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.
