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.
Select figure to enlarge.

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 four-and-a-half times their body weight on the leading knee when they were hitting a drive,” says the laboratory director, 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' 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 heels

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.”

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. 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 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.”