Once their models were set up, the group validated the contact algorithms, using pressure data physically recorded inside actual joints of cadaver knees, against their MRI/FEA model predictions.

Shape and size matter

SCORE researchers next turned their attention to shape. “It turns out you can't just stick any C-shaped meniscal tissue into a knee. Even current techniques using x-rays of donors and patients to try to get as close a match as possible come up short: the criteria used to clinically select menisci from cadavers include length and width of the bones, but not the height (e.g. variation of thickness) of the meniscus, which turns out to be critical.

“Small changes in dimension, even just 10% mess things up,” says D'Lima. “If the outer edge of the meniscus is too thick or too thin, when you run the FEA analysis you see excessive stress creep in. Nature gets it right during development because everything — bones, ligaments, and cartilage — grows to fit each individual.”

Evaluating best fixes

The third research challenge for the SCORE group was the question of how best to fix a replacement meniscus in place in its new knee environment. Surgeons currently favor two methods for allografts: One is to implant a cadaver meniscus, complete with accompanying bone blocks at its edges, directly into holes drilled into the recipient's own leg bones. This process requires complicated surgery with significant after-pain and rehabilitation.

Another method is to stitch the horns of the cadaver meniscus to small holes in the recipient's bone, which involves a surgeon viewing the site through an arthroscope and working with tiny incisions. The SCORE group researched all commercially available suture materials to get strength and stiffness data and incorporated ‘virtual stitches’ into their FEA knee models to study the contact stresses. They determined that a suture stiffness of about 50 Newtons per millimeter approached the performance of bone plugs. “So you can get the same mechanical fixation with less invasive surgery,” says D'Lima.

To generate and explore the algorithms that best describe the ‘perfect’ meniscus for a single patient, D'Lima's group is employing SIMULIA's Isight for simulation process automation and design optimization.

“We're using it to optimize the material properties and shape of the meniscus,” says D'Lima. “With our experimental data, we can keep changing the characteristics of our finite element model until we identify that particular complex material model that satisfies all our conditions.”

Smart FEA looks promising

As for the final question about future wear and damage of any meniscus, original or replacement, D'Lima is looking to apply Abaqus FEA to “mechanobiology,” the study of how biological tissues respond to mechanical forces. “We have an entire laboratory looking at how cartilage cells respond to mechanical stimulus,” he says. Earlier work demonstrated that, at lower stresses (such as walking), cells produce new tissue-forming proteins. At higher levels (damage in a car accident or perhaps even too much golf) the cells shut down and start secreting proteins that actually break down the tissue.

“We want to predict how your meniscus will behave, and how its cells change properties, under different stresses,” says D'Lima. “To model such processes, we are hoping to work with SIMULIA to develop ‘smart’ FEA elements that would both ‘sense’ stresses and change their mechanical properties as a result.” In the meantime, D'Lima says, it appears that exercise — but not overexercise — is the best way to keep knee tissues healthy. For those who will need a meniscus, or even a whole knee replaced, today's technologies are leading to the most realistic spare parts possible.