Achieve accommodation, eliminate spherical aberration and other errors, and minimize incision size. These three ends together represent the grand dream of IOL development. Many promising lenses are in the pipeline, and each puts us one step closer to the perfect lens implant. Here, we review three of the newest efforts.
Design. The Tetraflex IOL (Lenstec) is a small-incision, single-optic accommodative IOL. Made from hydroxymethylmethacrylate, it has a 26 percent water content. This one-piece lens has a 5.75- millimeter equiconvex optic, Tetraflex haptics and a square-edged design to inhibit posterior capsular opacification, said Mark Packer, MD, FACS, clinical assistant professor of ophthalmology, Oregon Health & Science University, and in private practice in Eugene, Ore.
Delivery. The lens is implanted through a 2.5-mm incision, and Robert E. Kellan, MD, who invented the Tetraflex, noted, “The contoured haptic design with an anterior vault allows the Tetraflex to take advantage of all forces induced during accommodation.” Dr. Kellan is director, Kellan Eye Center, Boston, assistant professor of ophthalmology, Boston University, and associate staff, ophthalmology, Tufts-New England Medical Center.
Dynamics. Questions remain about exactly how natural accommodation works and how to measure it objectively in any IOL. Theoretically, two forces are at play during accommodation: anterior movement of the vitreous face and contraction of the ciliary muscle. “The Tetraflex was designed to use these forces to provide anterior axial movement of the lens. The anterior angle orientation (5-degree contoured haptics) and optic are designed to move as a unit, without any hinge action. The haptic design allows the lens to move with the entire capsular bag. It does not require postoperative cycloplegia,” Dr. Packer said.
“With accommodative effort, the entire lens moves, and the optic changes its radius very slightly to provide good vision without spectacles across the range needed for 95 percent of daily activities,” Dr. Kellan said. He noted that Lenstec is using the iTrace Aberrometer to study aberration induced at the time of near fixation and far fixation. In the FDA trials, 95.6 percent of 91 reported cases had accommodative amplitude of more than 1 D, 69.2 percent could accommodate more than 2 D and 19.8 percent more than 3 D, he said. He uses the analogy of a handshake to describe the accommodation process: “Some handshakes are firm and others are soft. The people who can squeeze firmly would get good results. This suggests that ‘exercises’ improve outcomes over time. It also appears anecdotally that aggressive readers get better results. Perhaps, someone who does not read a lot would not be so motivated to ‘use’ their near vision and exercise to improve.”
Development. The Tetraflex has been approved in Europe, Australia and the Middle East since 2003, with about 6,000 lenses implanted so far, according to Dr. Kellan. U.S. trials are beginning, with about 50 lenses implanted to date. It should be ready for U.S. sale by 2008.
Design. The SmartIOL (Medennium) is an injectable, adjustable, accommodative IOL. The SmartIOL concept is based on this hypothesis: “An IOL that fills the capsular bag would allow the ciliary muscle to resume control of lens shape alteration, thereby producing focus along the entire accommodative range,” explained David F. Chang, MD, clinical professor, University of California, San Francisco, and in private practice, Los Altos, Calif.
“Many believe that the best way to achieve accommodation is to fill the capsular bag with a flexible gel,” Dr. Chang said, “but this approach must surmount many obstacles. One must determine how much gel to inject and how to predictably control the resulting lens power. The capsulorhexis must be small, so that it can be sealed to confine the gel. This may require new surgical technologies, which must be sufficient to remove a brunescent cataract. Finally, how does one control the critical optical interface in the central location of the capsulorhexis?” said Dr. Chang.
This lens has unique thermoplastic properties that permit a temperature-induced change in shape, Dr. Packer explained. Because wax is chemically bonded to the acrylic polymer, the material remains in a solid state at room temperature. “Each acrylic unit of the lens material has a 16-18 carbon side chain. Varying the number of side chains can change the material’s thermodynamic properties. In its current composition, the chemically bonded wax accounts for 75 percent of the material’s weight.” Dr. Packer said.
Delivery. He explained that the lens is heated and compressed into a thin, 50-mm long rod, then cooled. “This rod-shaped lens can be implanted through a small incision [with the current design, about 3.5 mm in width], through a standard capsulorhexis.”
Dynamics. Then, Dr. Packer continued, “Because the wax component melts at body temperature, adjusting the percentage of wax content produces a semisoft gelatinous polymer once the lens is in the eye. As the rod warms to body temperature, it changes back to a pliable lens measuring 10 mm in diameter and 3.5 mm in thickness. The lens fills the capsular bag as it recovers its predetermined shape and dioptric power.” Dr. Chang noted that with this technology, the surgeon does not have to close the capsulorhexis, nor alter standard phaco technique.
The SmartIOL may have other benefits, too. Pressure of the optic against the lens capsule may inhibit lens epithelial cell (LEC) metaplasia and migration, thus preventing anterior and/or posterior capsular opacification, Dr. Packer explained. “If the capsule does become cloudy, one could safely perform a YAG capsulotomy without leakage of the lens material. Because the lens completely fills the bag, it may eliminate anterior movement of the vitreous body and the attendant risks of retinal detachment in high myopia,” he reported.
Development. Using an injector system, Samuel Masket, MD, has successfully implanted SmartIOL prototypes into human cadaver capsular bags. Dr. Masket is in private practice in Los Angeles and is clinical professor of ophthalomogy, University of California, Los Angeles. In vitro testing has shown that the lens is flexible enough to allow compression by forces on the scale of those that act on the capsular bag, Dr. Packer reported. “The next step is to determine the right amount of lens elasticity to allow just enough shape change in response to ciliary muscle contraction,” Dr. Chang said.
Further development of this lens technology is on hold until the company secures funding for an FDA trial, explained Dr. Masket.
Design. The UltraChoice 1.0 (ThinOptX) is a plate-haptic, hydrophilic acrylic IOL. It is so thin that “it can be rolled into a small cylinder and implanted/injected through an ultra-small incision no larger than 1.5 mm,” said Kenneth J. Hoffer, MD, chairman of the board of ThinOptX and clinical professor of ophthalmology, University of California, Los Angeles.
Delivery. In fact, some researchers are implanting the UltraChoice through an incision less than 1 mm long, according to Dr. Packer. He added that it is one of the best current IOL candidates for microincision surgery. “For the past three years, I have performed all of my cataract and lens extractions through two paracentesis incisions of 1.2 mm width. IOL insertion has, of course, required the construction of a third incision,” Dr. Packer said.
Dynamics. This lens is designed to eliminate spherical aberration. Dr. Packer explained that one surface of the lens has a continuous curvature, as with traditional lenses. The second surface contains a series of steps, each 50 micrometers in height. Each step has a unique surface curve, a slightly different radius and can be focused on a single focal point. He pointed out that this “ensures a constant dioptric power with reference to the aperture size or distance from the lens axis. “
The lens should be aspheric, as are all lenses used in optical equipment, from telescopes to the 20 D lens used with the indirect ophthalmoscope,” he said. Most pseudophakic IOLs are designed as a portion of a sphere, he said. Lens thickness also causes aberrations, because light rays travel a greater distance in the thicker portion of the lens, Dr. Packer explained. With a central axis thickness of 50 µm for a meniscus lens and 300 µm for a biconvex optic, the ThinOptX lens eliminates this type of error as well, he said.
Development. FDA studies began in July 2004. This lens was approved in Europe in 2002, and European studies are ongoing.
Dr. Chang is on the medical advisory board for Medennium, but has no financial interest in the SmartIOL.
Dr. Hoffer is chairman of the board of ThinOptX, and his family owns shares in the company.
Dr. Kellan is inventor of the Tetraflex lenses, sold his patents for this technology and holds no current financial interest in this IOL.
Dr. Masket is a consultant for Alcon, Medennium and Visiogen. He resigned his post as medical director for Medennium last year.
Dr. Packer is a consultant for Advanced Medical Optics, Advanced Vision Science, Visiogen, Carl Zeiss Meditec and Bausch & Lomb. He has received honoraria, travel and research funds from Eyeonics, Staar Surgical and Alcon Laboratories.