San Diego, CA—Stare at this magazine.
Now gaze out the window at that tree across the street.
The human eye can focus on objects at any distance, simply by changing the lens shape when eye muscles pull on attached fibers. But when the eye is not healthy, these fibers—called the Zonula of Zinn (I'm not kidding)—can't do their job. That's why a million people every year have cataract surgery to replace cloudy lenses, and others use laser surgery to reshape the cornea.
Cataract patients can focus at any distance by changing the distance between these two artificial lenses.
The downside of these methods is that laser surgery is irreversible, and lens replacement provides only a fixed focal length (patients can see movement clearly, but still need glasses). So ophthalmologist Mona F. Sarfarazi has developed another method.
Called the Sarfarazi Innovative Elliptical Accommodative Intraocular Lens (EAIOL), the patent-pending device is expected to go under FDA testing in Q4, and to be on the market by 2004. In the Sarfarazi EAIOL, your natural eye muscles do the focusing, when they pull and release the haptics—an artificial membrane that controls the distance between two artificial lenses. As the distance between lenses changes, the eye focuses on objects at different depths.
"The lens is folded up and inserted through a 2-3 mm incision, which is too small to be seen by the eye," explained Sarfarazi. "The surgery time is only 15-20 minutes. If the patient is not happy with it, you take it out. There is no damage to the eye and another lens can be inserted."
Since it works with the brain and the eye muscles, it provides a long range of vision. And it can be performed on patients of any age, as opposed to adults-only laser surgery. Potential patients include those suffering from cataracts, presbyopia, myopia, congenital cataracts, and macula degeneration.
Sarfarazi did most of her research as a glaucoma specialist at the Massachusetts Eye and Ear Infirmary, a Harvard University-affiliated hospital in Boston. To produce and market her creation, she moved to California earlier this year.
Once she had the general idea, Sarfarazi had to calculate the details, including the best size, shape, and material for a range of focal lengths. She also had to satisfy constraints like manufacturability, and the lens' ability to be folded (nested) for surgical implantation.
She went to MSC.Software for help. Their consulting services department decided to use MSC.Nastran and MSC.Patran finite element analysis (FEA) simulation software to optimize the design. First, the 2D and 3D EAIOL geometry was created using MSC.Patran, then boundaries, restraints, and loads were modeled in MSC.Nastran, which also produced analysis and post processing results.
A natural lens changes shape when it's stretech by the Zonula of Zinn -- allowing the eye to focus.
"Now we have three parts called haptics, which is like a membrane, and two optics or lenses," says Sarfarazi. "We started with five haptics and along the way, we tested three and six haptics to find the most stable combination. Simulation of this system inside the bag had proven that the lens system with three haptics has the best possible design mechanically."
From optics analysis, they knew they needed about 2 mm of relative motion between the lenses, to achieve an acceptable range of focal lengths. Another design constraint was that the assembly had to fit into the bag of the natural lens. Finally, they created a vented design, to release pressure from trapped fluids like the aqueous and vitreous humors.
"Because of design changes made as a result of simulation, manufacturing is made much easier," Sarfarazi says. For materials, they chose PMMA (polymethyl methacrylate, the hard plastic also used in Plexiglass), for the lenses, as well as a proprietary, softer material for the haptics. Manufacturing involves a combination of injection molding and hand-assembling these components.
Overall, simulation allowed the team to halve mechanical design cycle time to just six months. They also shaved manufacturing time and production costs, she says. The lens will be sold under the name of Sarfarazi's firm, Shenasa Medical LLC.
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