Medical device design presents specific challenges for finite element analysis (FEA) engineers, who are tasked with simulating the mechanical behavior of medical devices or their components under a variety of conditions. These simulations help designers optimize the device and/or its component design prior to prototyping and physical testing.
Rarely are finite element analysts able to properly simulate the mechanical behavior of medical devices such as medical implants with a linear static analysis. Instead, more complex nonlinear analyses must be performed, often involving surface contact and nonlinear materials. Each medical device has its own set of unique FEA demands, such as Nitinol material simulation for vascular stents or screw-thread interactions for orthopedic bone supports. Improvements in ophthalmic implants are being made by using these powerful FEA tools to simulate advanced materials for an intraocular lens (IOL). However, producing an optimal IOL design using FEA requires advanced planning to overcome modeling challenges.
An intraocular lens is most commonly used to replace a crystalline lens that has been clouded over by cataracts. The first IOL was implanted over 50 years ago, but only recently has this technology made great strides in more closely mimicking the human eye. Traditional IOLs are made of an inflexible material (PMMA) and have a fixed focus for distance viewing. While this is a great improvement over the untreated condition, it lacks the ability of the natural eye to focus at varying distances, reducing the implant recipient's quality of life.
Designers are responding to the need for a more advanced, improved IOL by producing a new type of lens called an accommodating IOL. Unlike traditional IOLs, this design incorporates a new flexible, gel-like material that connects to the ciliary muscle and makes it possible for the implant recipient to focus on distant and nearby objects. The range of focus of accommodating IOLs falls short of mimicking the healthy human eye. The designer’s challenge is to increase the dioptric power while minimizing product development time and material costs for these next-generation IOLs.
Intraocular Lens - The gel-like material of the accommodating IOL is nearly incompressible (Poisson's ratio between 0.48 and 0.49). There is a piston around the outside of the soft material that pushes it inward. Due to the incompressibility of the gel, as the piston pushes the outside of the gel inward, the center bulges outward to maintain overall volume — it is this bulge that determines the optical properties of the lens, and ultimately, its dioptric power." class="caption" align="left">Attempting to measure the exact deformation of an IOL is difficult and generating various materials with suitable properties is costly and time consuming. These design challenges make FEA an ideal application to test various prototypes and materials in order to rapidly converge on an optimal design at an optimal cost. NEi Software (Westminster, CA) has worked with several medical optics companies to refine and improve their designs by using NEi Nastran FEA software to simulate the accommodating IOLs. Many lessons learned through this experience are applicable not only to IOLs, but also to other complex assemblies involving finite element analysis.
Some of the challenges faced during IOL simulations were controlling element distortions encountered due to the large strains involved. This was handled by careful attention to mesh quality and size; elements too large could cause inaccuracies, while elements too small could unnecessarily increase model size and run time. The challenge of finding the optimal mesh size was overcome by building models of various element densities to better understand mesh sensitivity. After the FEA results were obtained, the next step was to take the deformation results into an optical analysis program to determine the optical performance of the resulting deformed shape. Once this result was obtained, an iterative process proceeded on improving the design and creating a final prototype.
Modeling this complex interaction in a virtual environment requires attentive planning. When simulating an analysis such as an IOL assembly, answering seven key questions will help define the model to enable a successful analysis (at left). Nearly all these questions are applicable to any complex finite element analysis and should play a role in pre-analysis product development best practices.
This article was written by Jonas Dalidd, Application Engineering Manager at NEi Software in Westminster, CA. For more information, visit http://info.hotims.com/28055-158.