Scripps Health researchers use Abaqus FEA to optimize new designs and explore surgical alternatives
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’ injury occurred around the same time that the Orthopedic Research Laboratories at the Shiley Center for Orthopaedic Research & Education 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 four and a half times body weight on the leading knee when they were hitting a drive,” says the laboratory director, Darryl D’Lima, M.D. PhD.
“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’ 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.
Recovered patient’s knee (left) outfitted with radio telemetric receiver that records data from computer chip implanted during surgery on replacement knee. Patient is tracked while skiing (right) to measure stresses on the knee joint; this data provides input for computer models used for simulations.
Knees are nature’s shock absorbers
Even if you’ve never played any sport, or felt a single twinge of pain, your knees are at risk for damage or arthritis over time because of something that everyone does: grow older. “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’ 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. “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. Indeed, 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 we’d only 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 have a very important function as both a spacer and a shock absorber, D’Lima says. “It provides load sharing, contact stress amelioration and stability-all of which we can now study with FEA.”
Two-dimensional MRI imagery of the knee joint (left) is transformed into a 3D CAD model (center) which is then meshed for FEA (right).
FEA models the knee
D’Lima’s research team, at Scripps’ Shiley Center for Orthopaedic Research and Education (SCORE), is using Abaqus FEA software from 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.”
The SCORE group has had a lot to solve. Meniscal replacements are the holy grail of a number of research projects, at Scripps and elsewhere, that aim to help patients with damaged menisci avoid knee arthritis entirely by implanting allografts (from cadavers), artificial biomaterials, or even tissue engineered from the patient’s own cells.
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: “You’ve got to match the size and shape of the replacement to the patient, you need to duplicate its complex material properties, you’ve got to figure out how to attach it in place, and it has to survive wear and damage over the lifetime of the patient,” D’Lima says. “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.”
The pairing of MRI and FEA has greatly benefited 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 Materialize 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 to mesh the contact areas between these components in preparation for FEA analysis with Abaqus.
Materials matter: The first, and 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 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.
Abaqus FEA models of knee menisci demonstrate the importance of dimension (size and shape) to optimal stress reduction in the knee. Top image shows loading on a meniscus of normal dimensions. A thicker outer edge generated high stresses in the meniscus (bottom left), while a thinner outer edge transferred stresses to the tibial cartilage (bottom middle). Reducing the width of the meniscus generated high stresses in both the meniscus and tibial cartilage (bottom right).
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.”
Size (and shape): SCORE next turned its attention to shape. “Can you just stick any C-shaped meniscal tissue that fits into a knee?” wondered D’Lima. It turns out you can’t. Even current techniques of 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 ten percent, 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.”
FEA evaluates alternative
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. Here again FEA provided a useful analysis tool: 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.
“Now that we have the design pipeline in place, we can essentially begin optimizing knee replacement to each person who needs it,” says D’Lima. “We can identify what shape is best for a particular individual, what are the material properties that will work best in that person’s knee, and make recommendations about securing the implant surgically.”
To generate and explore the algorithms that best describe the ‘perfect’ meniscus for a single patient, D’Lima’s group has recently begun employing SIMULIA’s Isight software for simulation process automation and design optimization. “Isight is a very useful tool for customization,” says D’Lima. “We’re using it to optimize the material properties and shape of the meniscus. With our experimental data in hand, we can keep changing the characteristics of our finite element model until we identify that particular complex material model that satisfies all our conditions.”
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