Even before Tiger Woods withdrew from the 2008 golf season after hobbling to a win at the U.S. Open, certain researchers into biomechanics were concerned about the health of his left knee.

Woods’s injury occurred around the same time that the Orthopedic Research Laboratories at the Shiley Center for Orthopaedic Research & Education (SCORE) at Scripps Clinic in California published a study of knee replacement patients who had tiny computer chip implants added at the time of surgery. The chips sent radio telemetric data to receivers that recorded the stresses on the knee joint during various activities. “Out on the golf course, the force we measured in our patients—who were nowhere close to Tiger’s skill level—was 4½ times body weight on the leading knee when they were hitting a drive,” says Darryl D’Lima, laboratory director. 

“People think jogging or climbing stairs is harder—but the twisting in golf is much tougher on the knee. Given the speed and dynamics of Woods’s swing, his injury came as no surprise to us.”  The researchers are now monitoring the same implant patients as they ski.  “It is our goal to study the effects of a whole range of movements on knee health,” says D’Lima. 

Even if you’ve never played any sport, or felt a single twinge of pain, your knees are at risk.  “Mother Nature designed the human knee to last about 30 years,” points out D’Lima.  “But the human lifespan has expanded much further than that, and evolution hasn’t caught up.” 

Tiger Woods’s knee injury (reportedly to the anterior cruciate ligament, or ACL, which stabilizes the inside of the knee joint) responded positively to microsurgery and physical therapy. But many people do not fare so well if they sustain damage to a critical cartilage deeper inside the knee: the meniscus. And it doesn’t have to be the result of a sports injury; it can just be the effects of time. In your 30s you may not have any symptoms, since the cartilage that lines and insulates your knee joints has no nerves, but degeneration has already begun. “It’s like the rubber soles of your favorite shoes,” says D’Lima.  “It doesn’t affect you as they slowly wear down—you only notice when your feet suddenly start slipping.”

The meniscus is made up of two, C-shaped pads of cartilage tissue, located between the joints formed by the bottom of the thigh bone (femur) and the top of the shin bone (tibia).  It was first thought of as a body part like the appendix—not critical for normal health, and even likely to cause trouble when diseased.  When a meniscus is torn, or wears out, the knee can lock up, making walking impossible. Because the meniscus has a very poor blood supply, it does not heal well on its own.

Fifty years ago surgeons solved the problem by removing the entire damaged meniscus because they thought it didn’t serve any purpose. Patients walked out the hospital door, but five years after meniscus removal they were back—with osteoarthritis (OA).  Surgeons then decided to remove only those parts of the meniscus that were damaged. The result?  Patients were fine for 15 years—and then developed OA.

“If only we had finite element analysis (FEA) back then, surgeons would have known that tissue removal was the wrong way to go,” says D’Lima. “Removing it takes away key biomechanical support of the knee.” The meniscus turns out to function as both a spacer and a shock absorber, explains D'Lima. “It provides load sharing, contact stress amelioration, and stability—all of which we can now study with FEA.” 

D’Lima’s SCORE research team is using Abaqus FEA software from Providence, RI- based SIMULIA, the Dassault Systèmes’ brand for realistic simulation, to make increasingly complex ‘virtual’ computer models of human knee components on which they can test a variety of potential replacement parts and surgical techniques.

Some of the data used to set up the FEA models comes from those earlier implant patients who golfed and skied while sending out radio telemetry. “The sensors in our patients’ knees provided us with force measurements that we were able to use as load inputs for our FEA analyses of the meniscus,” D’Lima says. Although his team’s first attempts to model meniscal function began in 2000, D’Lima says, “I’ve only been able to solve the complex material and contact problem to my satisfaction in the last couple of years since I started using Abaqus.”

Whatever the materials being proposed for meniscus replacement, SCORE has identified four problems that need to be solved in order to achieve optimum knee function: 

• Size and shape of replacement must match the “original.”

• Properties must be duplicated by replacement materials.

• How to attach it must be determined.

• Ability to withstand wear over the lifetime of the patient must be engineered in.

“For each of these challenges, we are finding that FEA, combined with magnetic resonance imaging (MRI), provides the tools we need to study the alternatives,” says D’Lima.

The pairing of MRI and FEA has greatly benefitted medical R&D in recent years for accurate modeling of human body parts. Design engineers can now convert two- dimensional MRI “slices” into stacked, 3D models; SCORE used Mimics software from Materialise, Leuven, Belgium, for its knee work. The resulting CAD (Rhino3D) models detailed bone, articular cartilage (lining the end surfaces of thigh and shin bones), other soft tissues (like the ACL), and meniscal cartilage. SCORE next employed Altair’s HyperMesh by Altair, Aukland, New Zealand, to mesh the contact areas between these components in preparation for FEA analysis with Abaqus. Again, the golfing, skiing knee-replacement patients were useful, this time providing data for boundary conditions.

The toughest modeling challenge was representing the material properties of the meniscus accurately. “One of the reasons it’s difficult to study biological tissues, especially the meniscus, is that every possible complexity exists within the same material,” says D’Lima.  A meniscal model needs to be not just elastic, but nonlinear elastic. It must have plasticity and, to describe how the material properties change with time, an added viscoelastic component. The meniscus also behaves differently in tension than it does in compression, so it’s important to show in which direction the stresses are being applied. The tissue is anisotropic and inhomogenous as well. “Abaqus FEA can represent every one of these characteristics and it provides the advantage of being able to stack all of the material properties into the same model,” says D’Lima. 

When testing their models’ material properties, the group found that there is no substitute for complexity as far as the meniscus is concerned. When they modeled the meniscus using simple, linear properties, they got menisci that were either too soft or too stiff.  “Our research has shown that repetitive contact stresses over about two megapascals (MPa) causes stress that actually starts killing meniscal cells,” says D’Lima. A meniscus that is too soft transmits forces over two MPa to the nearby articular cartilage, while one that is too stiff directly absorbs the brunt of all applied forces so that its cells begin to die. “There is no sweet spot with a simple material,” says D’Lima. “It has to be complex.”

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.

SCORE researchers next turned their attention to shape.  “It turns out you can’t just stick any C-shaped meniscal tissue into a knee,” says D’Lima.  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 stresses creep in. Nature gets it right during development because everything— bones, ligaments, and cartilage—grow to fit each individual.”

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—a process that 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.”

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 over exercise—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.