Case Studies

Replacing Nature's Shock Absorbers: Realistic Simulation Probes Biomechanics of Knees
Scripps Health researchers use Abaqus FEA from SIMULIA to optimize new designs and explore surgical alternatives

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.

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. Ph.D.

"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.

Knees are the body's Achilles heel
Even if you've never played any sport, or felt a single twinge of pain, your knees are at risk for damage and/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. 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 human knee is a complex body part that can wear out long before its owner does. The meniscus (in blue), which serves as both a spacer and a shock absorber, is particularly susceptible to wear and tear over time.

The meniscus: more important than surgeons thought
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."

Finite element analysis 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. Such projects have had varying degrees of success (see sidebar 1).

Two-dimensional MRI imagery of the knee joint (left) is transformed into a 3D CAD model (center) which is then meshed for FEA (right).

Sidebar 1: a directory of spare parts for knees
A human cadaver allograft doesn't have to be as fresh as, say, a heart transplant, because the meniscus doesn't need much blood supply; its cells can stay fresh in a bath of nutrients for a few days. Nor is rejection as big an issue with allografts as with organ transplantation, because the patient's immune system, which works through the blood, doesn't 'see' the implanted foreign meniscus well due to its poor circulation. Nevertheless, an allograft must match the size of the recipient closely, which is can be difficult to achieve. Some researchers are experimenting with using animal tissue cut to fit, but menisci taken from tissues like cow Achilles tendons don't recapitulate the complex material properties of human ones.

Replacement menisci made from artificial biomaterials (usually some form of polymer, e.g. polyester, polytetrafluoroethylene (Teflon), or polyurethane) are also currently under study, but one of the problems with plastic is that it degrades over time. The beauty of a living meniscus is that its cells continuously renew themselves, producing new cartilage (fastest in a younger person). Neither an allograft nor an artificial material can do that.

So how about growing your own? Engineered tissue is still largely a future dream for researchers and knee patients alike. "You can grow some tissue that looks like a meniscus from a person's own cells, but the cells don't multiply well and the strength is not the same as the original," says D'Lima. "Stem cells are another possible alternative because they divide rapidly, but they may have more issues with rejection and tumor generation."

MRI and FEA team up to study knee function
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 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 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. Again, the golfing, skiing knee-replacement patients came in useful, this time providing data for boundary conditions (see sidebar 2).

Sidebar 2: Supercomputer puts high-density models through their paces
The full resolution model was run on a supercomputer, at the DataStar, San Diego Supercomputer Center at UCSD, with 2528 processors running at 15.6 teraflops (OS= IBM AIX). As many as four, 8-way 1.5 GHz IBM p655 nodes were used, each with 16 GB memory. The supercomputer model had a high mesh density (average element size ~0.5 mm), including bones and ligaments as deformable bodies with 2.8M degrees of freedom and took an average of 8 hours to run. This high-density model was used to validate the results of smaller, lower-resolution models that ran the various conditions on a desktop computer: A 32-bit Windows XP, Dell Precision 490 Workstation, with Dual Processor Motherboard with 2 Dual Core Intel Xeon 5150 2.66 GHz CPU and 4GB RAM. The models run on the desktop had an average element size of ~1.2 mm, with the bones treated as rigid bodies, and ligaments replaced with nonlinear springs. The desktop models ranged from 50K to 200K degrees of freedom and took 2 to 6 hours to run. Hexahedral and pentahedral elements were used to model the orthotropic material properties and various zones of cartilage so as to be able to "morph" the models during shape analysis.

It's all about the materials
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 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 (D'Lima: "the entire knee is a problem of contact") using pressure data physically recorded inside actual joints of cadaver knees, against their MRI/FEA model predictions.

Pressure data recorded from inside a cadaver knee (left) is used to validate the contact algorithms for Abaqus FEA model of the meniscus (lower right). Pressure sensor is pictured above right.

Size (and shape) do matter
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."

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).

FEA helps evaluate alternative surgical techniques
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."

Contact stress data for normal knee (green), knee with no meniscus (red), and various types of surgical replacement (blue). The bars at right show that a stitched-in meniscal replacement (with a suture stiffness of 50 Newtons per millimeter) approaches the performance of more complex bone-block surgery.

Customizing meniscal replacements with optimization software"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 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."

Smart FEA looks promising for future 'mechanobiology' research
As to the final question about future wear and damage of any meniscus, original or replacement, D'Lima next 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 has 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 then start secreting proteins that actually break down the tissue.

"We want to be able 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 your knee tissues healthy. With the pace of research accelerated by FEA and optimization, should you eventually need a meniscus, or even a whole knee replaced, the technology is on the way to provide you with the most realistic spare parts possible.