The medical establishment already has a very clear picture of cataracts and how to treat them. That’s encouraging, given the fact that by age 80, more than 50% of all Americans will have developed a cataract, and every year more than three million will undergo eye surgery to correct it. What is also encouraging is the surgical outcome: the success rate is 95%, with vision typically restored within a 20/20 (normal) to 20/40 (good) range. Those are excellent results, especially given how far treatment has progressed in such a short time.

Modern cataract surgery was first performed in the late 1960s, enabled by the development of an ultrasound technology that emulsified the eye’s diseased natural lens, along with the discovery of a suitable replacement-lens biomaterial, polymethylmethacrylate (PMMA). Since the first prosthetic intraocular lens (IOL) was rigid, however, the incision required to insert it into the eye was large (encircling roughly half the cornea), required sutures, and made recovery long and outcomes variable.

When deformable materials, such as hydrophobic acrylic and silicone, replaced PMMA in the early 1990s, incision size decreased dramatically due to the new materials’ ability to be rolled, folded, and bent during insertion. As IOL materials evolved, the insertion process did too, shifting from forceps to a tapering tube (similar to a syringe) that pushed the lens into the eye. Lenses are now being delivered through increasingly smaller incisions ranging from 1.8 to 2.8 mm.

With the size of the incision directly related to post-surgical aberrations in vision, engineers at Bausch + Lomb, Rochester, NY, , continue to look for improvements and have recently set an ambitious 1 mm incision goal. To achieve this, ongoing research and development is focused on new lens materials, improved IOL geometry, and better inserter designs. That’s where finite element analysis (FEA), with its capability to realistically simulate a wide variety of physical phenomena, enters the picture.

Simulation sees what can’t be measured

Engineers at Bausch + Lomb have been using Abaqus FEA, the Dassault Systèmes brand for realistic simulation, in biomedical applications for about 10 years. It was first employed to model the conformation and deformation of contact lenses on the cornea; this helped evaluate lens performance, including optical properties. Other applications have included improving cataract surgery tools and modeling manufacturing procedures.

“We use FEA in our iterative design process to shorten development time by analyzing each design or by developing design rules-ofthumb,” explains Robert Stupplebeen, design engineer and analyst at Bausch + Lomb.

In general, to create its FEA models, the Bausch + Lomb team first builds 3D CAD models in SolidWorks and then uses the software’s Associative Interface to import the model into Abaqus. From there, simulations are often coupled with other programs, such as SigFit, an optomechanical pre- and post-processor (developed by Abaqus Integration Program member Sigmadyne), and ZEMAX, a comprehensive optical design software package.

When starting a new product design project, “getting sufficient biological test data can be problematic,” says Stupplebeen. “With just about any biomedical product or process development, there are a lot of assumptions that need to be made.”

In the case of cataract surgery, the Bausch + Lomb product development team is focused on two primary modeling issues that can be confirmed by testing: the insertion force required to implant the IOL, and the geometry of the lens as it emerges from the inserter. But they also are interested in what can’t be measured in real life, such as the geometry and internal stresses of the lens when it’s in the inserter.

“We validate our model on the things we do know and then utilize the rest of what the model tells us to gain a better understanding of the physical behavior,” says Stupplebeen. “Without FEA, all of these things are just unknowns.”

The cataract surgery simulation setup requirements are stringent, says Stupplebeen. “The analysis is highly non-linear with large deformations, difficult self-contact, sliding contact, and hyper-elastic material properties. To handle all this in one model, we chose Abaqus/Explicit.”

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