Of the more than 7,500 parts in the human body, the knee's meniscus may be the most vulnerable.

The crescent-shaped cushions of rubbery cartilage—two in each knee—act as shock absorbers as people walk, run, pivot and bend. Sudden stops and twisting motions can cause the meniscus to rip or experience a more gradual tear.

Jeremy Lin is the latest pro athlete to fall victim. The popular New York Knicks point guard will have surgery this week on a small, chronic tear in his left knee and miss the rest of the season, the team announced Saturday. Last month, Red Bulls power forward Juan Agudelo, Atlanta Braves third baseman Chipper Jones and Kansas City Royals catcher Salvador Perez all had meniscus surgery.

Many skiers, cyclists, joggers, golfers and other weekend warriors also damage their menisci, as do a growing number of teens and adolescents who play sports.

It isn't just athletes who are at risk. Cartilage weakens and frays naturally with age, so older people can tear a meniscus just walking or rising from a chair. Excess weight also places extra stress on joints and wears down cartilage faster.

"A lot of tears are due to chronic degeneration," said Dr. Frederick Azar, chief of staff at the Campbell Clinic in Germantown, Tenn., and a spokesman for the American Academy of Orthopaedic Surgeons, or AAOS. "People may attribute them to a sudden movement, but usually the trouble has been brewing for a long time."

Not surprisingly, knee injuries are rising with the aging population and the obesity epidemic. More than four million Americans visited physicians for meniscus tears in 2009, more than double the number of 2000, according to the AAOS.

Not every torn meniscus needs to be fixed. In a landmark study in the New England Journal of Medicine in 2008, researchers randomly selected 991 people aged 50 to 90 to undergo MRIs of the right knee. Overall, 30 percent of the women and 42 percent of the men were found to have a tear or other meniscus damage. Of those, 61 percent said they hadn't experienced any pain or disability in the knee during the previous month, meaning a torn meniscus can often go unnoticed.

That's why orthopedists often say, "Treat the patient, not the MRI."

Surgery is usually recommended in the case of a sudden, severe meniscus tear or in a young athlete with a long playing career ahead. Surgery is also warranted if the knee makes a "popping" or "clicking" sound or catches when bending, which often means that a piece of meniscus has come loose inside the joint.

But if the tear is the result of long-term degeneration and ostearthritis has set in, several studies show that patients do just as well with physical therapy as they do with surgery.

"If the MRI shows a meniscus tear, but the patient isn't experiencing catching or locking [and] their X-rays show early arthritis, I don't think they'd be a surgical candidate," said Dr. Michael Stuart, professor of orthopedics at the Mayo Clinic in Rochester, Minn. "But we would help with their pain. We'd suggest weight loss, activity modification, anti-inflammatory medications, maybe an injection of a local anesthetic and orthotics in their shoes."

Much also depends on the size and location of the tear. While surgeons can suture a small tear on the periphery of the meniscus, the inner portion of the meniscus doesn't have its own blood supply, so repairs there seldom heal. Surgeons remove the damaged portion instead.

As late as 1971, surgeons frequently removed the entire meniscus if part of it was damaged. Now they leave as much of the meniscus in place as possible. Nearly 700,000 such "partial meniscectomies" were performed in 2006, usually with arthroscopy, in which surgeons using tiny incisions and a lighted scope that lets them both see and sculpt the tissue.

Many patients report significant relief from the procedure, which is usually done on an outpatient basis.

"I wish I'd had the surgery sooner," said Haralee Weintraub of Portland, Ore., who hurt her right knee skiing and put up with pain for 10 years before she had the procedure in 2003 at age 50.

But having only a partial meniscus does alter the way the knee joint handles body movements and raises the risk of osteoarthritis later. Whether that's inevitable depends on the patient's age, health, genes and activity levels.

Physical therapy and strength training before and after the surgery can be crucial. But patients are often told to avoid high-impact sports, or they run the risk of needing a knee replacement later.

By the time he was 18, Anthony Baldinelli had torn his menisci three times playing soccer and had much of the cartilage removed. Now he gets a flare-up of arthritis when he plays sports more than once a week. Doctors want to avoid a knee replacement because he's only 23. "On X-rays, my right knee looks like an old man's," said Baldinelli, a publicist in Raleigh, N.C.

When a meniscus tear is very large or complex, a transplant may be an option. The tissue is taken from cadavers that have been frozen and screened for infections. The risk of rejection is minimal with cartilage; matches are based on size instead.

Follow-up studies have found that about 80 percent of transplant recipients find significant pain relief. But a donor meniscus isn't as good as an original. "After about 10 years, 40 percent of them will have torn and require additional surgery or partial or total removal," said Dr. Stuart.

Mike Schwartz, 43, who runs an Internet apparel company in Santa Monica, Calif., damaged his knees as a gymnast in his teens and had a meniscus transplant in 2007. He is grateful that his knee no longer swells up easily, but it is still stiff and sore if he leaves it unbent too long. "My knee still pretty much hurts all the time," he says.

In many cases, physical therapy can stave off the need for meniscus surgery for a few years—or indefinitely. The training strengthens and retrains leg muscles to put less strain on damaged tissue.

Some clinics are trying a variety of techniques aimed at coaxing the body to heal itself, with or without surgery. Injections of platelet-rich plasma, or PRP, are popular among pro athletes. Doctors withdraw several ounces of the patient's own blood, spin it and separate out the platelets, which secrete natural growth factors, then inject them back into the site of injury, where they theoretically stimulate healing. But there have been few randomized trials to date and results have been mixed.

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