Visualizing Eye Surgery

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					custoMEr spotlight

       Eye Surgery
        Abaqus FEA brings prosthetic
        lens insertion into focus
        for Bausch + Lomb

    The medical establishment already has a                                                                     better inserter designs. That’s where finite
    very clear picture of cataracts and how to                                                                  element analysis (FEA), with its capability
    treat them. That’s encouraging, given the                                                                   to realistically simulate a wide variety of
    fact that by age 80, more than 50 percent of                                                                physical phenomena, enters the picture.
    all Americans will have developed a cataract,
    and every year more than three million will                                                                  Simulation sees what
    undergo eye surgery to correct it. What is                                                                   can’t be measured
    also encouraging is the surgical outcome:                                                                   Engineers at Bausch + Lomb have
    the success rate is 95 percent, with vision                                                                 been using Abaqus FEA, in biomedical
    typically restored within a 20/20 (normal)                                                                  applications for about ten years. It was
    to 20/40 (good) range. Those are excellent                                                                  first employed to model the conformation
    results, especially given how far treatment                                                                 and deformation of contact lenses on
                                                      Geometry of a typical replacement lens (IOL) used
    has progressed in such a short time.                                                                        the cornea; this helped evaluate lens
                                                      in cataract surgery. Standard dimensions for the
                                                      acrylic lens are 6 mm diameter with a 1 mm center         performance, including optical properties.
    Modern cataract surgery was first performed       thickness. The lens needs to be compressed inside         Other applications have included improving
    in the late 1960s, enabled by the development     an inserter tip that delivers it into the eye through a   cataract surgery tools and modeling
    of an ultrasound technology that emulsified       corneal incision that averages 2.8 mm.                    manufacturing procedures.
    the eye’s diseased natural lens, along with
    the discovery of a suitable replacement-          evolved, the insertion process did too,                   “We use FEA in our iterative design process
    lens biomaterial, polymethylmethacrylate          shifting from forceps to a tapering tube                   to shorten development time by analyzing
    (PMMA). Since the first prosthetic                (similar to a syringe) that pushed the lens                each design or by developing design rules-
    intraocular lens (IOL) was rigid, however,        into the eye. Lenses are now being delivered               of-thumb," explains Robert Stupplebeen,
    the incision required to insert it into the eye   through increasingly smaller incisions                     design engineer and analyst at Bausch +
    was large (encircling roughly half the cornea),   ranging from 1.8 to 2.8 mm.                                Lomb.
    required sutures, and made recovery long and
    outcomes variable.                                With the size of the incision directly related             In general, to create its FEA models, the
                                                      to post-surgical aberrations in vision,                    Bausch + Lomb team first builds 3D
    When deformable materials, such as                engineers at Bausch + Lomb in Rochester,                   CAD models in SolidWorks and then uses
    hydrophobic acrylic and silicone, replaced        NY continue to look for improvements                       the software’s Associative Interface to
    PMMA in the early 1990s, incision size            and have recently set an ambitious 1 mm                    import the model into Abaqus. From there,
    decreased dramatically due to the new             incision goal. To achieve this, ongoing                    simulations are often coupled with other
    materials’ ability to be rolled, folded, and      research and development is focused on new                 programs, such as SigFit, an optomechanical
    bent during insertion. As IOL materials           lens materials, improved IOL geometry, and                 pre- and post-processor (developed

8     INSIGHTS September/October 2010                                                                                              
  by Abaqus Integration Program member                                                                          quarter of a million elements with over
  Sigmadyne), and ZEMAX, a comprehensive                                                                        100K increments. To run the five-hour
  optical design software package.                                                                              analysis, the team used a Cray CX1 with
                                                                                                                Windows HP Server 2008.
  When starting a new product design project,
 “getting sufficient biological test data can be      Lens strain is illustrated as the IOL is being pushed      Validating lens strain and
  problematic,” says Stupplebeen. “With just          by the plunger inside the inserter during a cataract       inserter forces
  about any biomedical product or process             surgery simulation. Rare lens tear has been                By using Abaqus, the team was able to
  development, there are a lot of assumptions         observed to occur at points of stress where the
                                                      plunger contacts the IOL or on the trailing haptics.
                                                                                                                 calculate the force applied on the inserter
  that need to be made.”                                                                                         versus the displacement experienced by the
                                                                                                                 lens and then compare it with test data. The
  In the case of cataract surgery, the Bausch +
                                                                                                                 analysis yielded results that correlated well
  Lomb product development team is focused
                                                                                                                 with the tests. The team was also able to
  on two primary modeling issues that can
                                                                                                                 measure strain on the lens while visualizing
  be confirmed by testing: the insertion force
                                                                                                                 its deformation as it traveled through the
  required to implant the IOL, and the geometry
                                                                                                                 inserter. These peak strain measurements
  of the lens as it emerges from the inserter.
                                                                                                                 correlated well with extremely rare failure
  But they also are interested in what can’t be
                                                                                                                 modes (tip fracture, lens tear, and lens
  measured in real life, such as the geometry
                                                      Strain on the lens is shown as the IOL emerges from        scratches) and were also found to occur in
  and internal stresses of the lens when it’s in      the tip of the inserter. The areas of highest stress       the same locations where past real-world
  the inserter.                                       correlated with the location of rare lens scratching.      failures had occurred.
                                                      Values represent 0-60% strain.
 “We validate our model on the things we do                                                                     “Given the agreement between simulation
  know and then utilize the rest of what the                                                                     results, physical tests, and observations, the
  model tells us to gain a better understanding                                                                  validated model is being used to reduce the
  of the physical behavior,” says Stupplebeen.                                                                   likelihood of failure modes, reduce insertion
 “Without FEA, all of these things are just                                                                      force, and develop the next generation of
  unknowns.”                                                                                                     IOLs and inserters,” says Stupplebeen. “As
                                                                                                                 surgeons strive for smaller incisions, we
  The cataract surgery simulation setup
                                                                                                                 have to develop a more compressible
  requirements are stringent, says Stupplebeen.
                                                                                                                 material, thinner lenses, and/or new inserter
 “The analysis is highly non-linear with large
  deformations, difficult self-contact, sliding       Section view of the FEA model used to simulate
  contact, and hyper-elastic material properties.     lens (IOL) insertion during cataract surgery. The

  To handle all this in one model, we chose
                                                      IOL (green) is being pushed by the plunger (purple)        FEA benefits are clearly visible
                                                      and is about to enter the inserter tip (blue) through     Whatever the product development direction
  Abaqus/Explicit.”                                   which it is delivered into the eye. The lens, tip, and
                                                                                                                (and Bausch + Lomb is pursuing them
                                                      plunger are modeled using deformable elements;
                                                                                                                all), Abaqus FEA is helping their designers
 The model: lens, inserter, incision                  the loading door (brown) and loading area (dark
                                                                                                                and engineers make predictions about
                                                      green) are modeled using rigid elements.
  From an ophthalmologist’s point of view,
                                                                                                                what will work and what will not. Since
  the cataract surgical procedure is relatively
                                                      To simulate the lens insertion process,                   cataract surgery product design cycle
  simple: Take a standard IOL, which consists of
                                                      the Bausch + Lomb team modeled an                         time is typically about a year and a half,
  a circular lens with four appendages (haptics)
                                                      acrylic lens, with average lens and inserter              with an additional year for clinical trials,
  that stabilize the lens in the eye; load the lens
                                                      dimensions, and applied the following                     accelerating prototyping with realistic
  in the inserter and fill the inserter tube with a
                                                      parameters to the model: hyper-elastic                    simulation provides tremendous bottom-line
  viscoelastic lubricant; make a small corneal
                                                      Neo Hooke material properties; Rayleigh                   benefit.
  incision and remove the damaged crystalline
  lens using an ultrasonic device; then place the     damping to reduce low frequency                           “We recognize the significant return gained
  inserter in the incision and push the plunger,      oscillations; general contact with zero                    from continued investments in simulation,”
  inserting the IOL. The surgery is outpatient        friction (because of the smooth surfaces and               says Stupplebeen. “Without a doubt, it
  and typically takes under 10 minutes.               lubricant); a nonlinear pressure-overclosure               has helped us shorten our time to market,
                                                      relationship to reduce contact penetration;                decrease our development costs, and
  From an engineering perspective, however,           and mass scaling to reduce solve time by a                 improve our product performance.”
  the procedure is quite challenging given the        factor of 10.
  geometry: an industry-standard precision lens
  has a 6 mm diameter, a center thickness of 1        The loading area of the inserter was treated
  mm, and four haptics; an average incision is        as rigid and modeled using R3D4 elements,
  2.8 mm.                                             while the lens, plunger, and tip were all
                                                      treated as deformable using C3D8R and
 “It’s like trying to suck a Frisbee through a        C3D4 elements. The model of the lens and                     For More Information
  vacuum,” says Stupplebeen. “During the              inserter, which are designed in tandem             
  insertion, the lens can experience strains in       because of their close interrelatedness, is        
  excess of 60 percent.”                              highly complex with approximately a                                                                                                INSIGHTS       September/October 2010              9