Wounded knees

Tom Brady's misery has plenty of company as millions of Americans every year endure torn ligaments and shredded cartilage

On second thought, maybe she should have just gone ahead and barreled into the pitcher.

It was a sunny Sunday in May - Mother's Day, in fact - and Lindsay Jordan blasted an inside-the-park homerun playing softball in Dedham. As she hurtled toward home, the pitcher blocked the plate. Rather than crash into him, Jordan attempted to dodge past.

"My knee just buckled. What scared me was when I heard it pop," said Jordan, a 29-year-old lawyer and inveterate athlete. "I've never heard anything pop or tear like that before."

Tom Brady, Lindsay Jordan - diagnosis: torn anterior cruciate ligament - feels your pain. And so do millions of other Americans, hobbled by knees gone bad. The culprits range from traumatic sports injuries, such as that suffered by the New England Patriots quarterback, to damage done over a lifetime that reduces the once super-slick tissue covering the knee into something more closely resembling Swiss cheese.

And the toll is sure to rise.

We're asking our knees to do more than ever before: At one end of the age spectrum, girls are participating in sports in unprecedented numbers - and sustaining all the injuries that come with it. At the other end, many older Americans are no longer content to surrender to aches, pains, and immobility - or sacrifice their golf and tennis games. And at the same time, the nation is in the midst of an obesity epidemic, forcing knees to carry substantially more weight than they're designed to.

Cranky, creaky knees generate upward of 20 million visits to the doctor each year in the United States, and they're the single most common reason that patients see orthopedic surgeons. Researchers estimate that by 2030, the number of total knee replacements - used in patients with severe degenerative arthritis - will soar to 3.5 million, a sixfold increase.

In many respects, knees and their treatment are a metaphor for all of medicine: There's luck and genetics, old beliefs trumped by new science, and, on the frontier of research, gee-whiz advances that coax the body to heal itself, with materials as diverse as collagen and silk and bone marrow stem cells being studied for their potential to repair wounded knees.

"The knee is an amazing piece of engineering," said Dr. Martha Murray, a Children's Hospital Boston researcher developing better ways of fixing broken knees. "It can tolerate stress year after year after year, and for the most part, it continues to function extremely well through 100 years of life.

"That said, we do ask a lot of our knees."

The knee is a joint. But classifying it as just another of the body's joints would be akin to calling Beethoven just another musician.

In truth, the knee is a marvel of bone, tissue (those are the ligaments), shock absorbers (the meniscus), and smooth, shiny padding (that's the cartilage) capable of supporting hundreds of pounds of weight - while flexing. It's like a flesh-and-bone drawbridge, with the knee at the apex of the body's two longest bones, the femur and the tibia. Compare that to the relatively humble ball-and-socket contraption that is the hip.

To understand the unremitting stress that a knee must endure, consider this: If you gain an extra 10 pounds, your knee interprets that as an additional 50 to 70 pounds because of the biomechanical demands placed on it.

"So it's a very vulnerable joint, whether to arthritis or to injuries," said Dr. Nicholas DiNubile, an orthopedic surgeon in the Philadelphia suburbs and representative of the American Academy of Orthopaedic Surgeons. "You land funny, you twist funny, and you create really high forces across this structure for a lifetime. It's one of the reasons we see so many knee problems."

Sometimes, they happen in the flash of a desperate tackle. Just ask Brady, who reportedly sustained two torn ligaments - the anterior cruciate, known as the ACL, and the medial collateral, MCL - earlier this month when the shoulder of a Kansas City Chiefs defender clipped the knee of the Patriots' star player.

The knee's four ligaments function like rubber bands, providing stability and elasticity. Lose one, and there's a good chance the damage won't stop there.

"It's as if you have a boat tied up and one line gets cut," said Dr. Stephen Murphy, an orthopedic surgeon at New England Baptist Hospital. "Then the other lines have to pick up stress." Sometimes, the stress simply becomes too much.

Ligaments can tear when they're asked to do something they're not really designed to handle. "In terms of the most dangerous thing that the knee has to do," said Dr. Jennifer Baima, a physical rehabilitation specialist at Brigham and Women's Hospital, "it's quick pivot-shift activity." That means fast-moving games like basketball and soccer.

Not that the risk is shared evenly. Gender plays a role: Depending on the sport, women can be as much as five times more likely to snap a ligament. Different researchers point to different reasons for that, but the suspects include hormones and the fact that women have wider hips and a narrower notch in the knee, which can act like a guillotine.

Genetics also appear to influence chances of shearing a ligament. DiNubile treated a brother and sister who, within three months of each other, tore their ACLs.

If the knee ligaments are like the cables that give a suspension bridge flexibility, then the articular cartilage is like the pavement that allows a smooth ride. It is the slick, glistening material attached to the end of bones in the knees.

Dr. Brian McKeon, a New England Baptist orthopedic surgeon, refers to cartilage as the "holy grail" of the knee, prized because it is 10 times slicker than ice, allowing the knee's bones to move pain-free - assuming the cartilage remains intact.

"You can have a gorgeous ligament, but if you don't have cartilage it's worthless," McKeon said. "It's like you can have a beautiful lake, beautiful skates, but if the ice ain't there, you ain't skating."

Cartilage degrades over the years in the presence of arthritis. Sometimes, arthritis is triggered by genetics. Sometimes, by obesity. And, sometimes, it's the legacy of an injury sustained decades earlier, on the high-school football field or the college basketball court.

And once the cascade of damage begins, it is not easily reversed.

"It becomes a much more serious issue because unlike other parts of the body that have blood supply and can repair themselves, the articular cartilage is unable to do that," said Dr. David Ayers, chairman of orthopedics at the University of Massachusetts Medical School.

The result, in the worst cases: bone grinding against bone. Pain. Mobility compromised. "It makes you cringe," said Dr. Andrew Pearle, an orthopedic surgeon at New York's Hospital for Special Surgery. "Their bones creak."

In the most severe cases of cartilage disintegration, patients undergo knee replacement surgery.

If the damage is more isolated to select spots where cartilage has worn away, doctors can create a tiny fracture in the underlying bone - which does have a blood supply, unlike the cartilage - in the hope that as it heals, material that mimics cartilage will fill the hole.

In another method for repairing cartilage pioneered by Cambridge biotechnology corporation Genzyme, surgeons harvest a small swatch of a patient's cartilage and send it to the company. There, it's multiplied into 12 million cells, which are shipped back and injected into the patient's cratered cartilage.

Sitting just beyond the horizon are novel experiments designed, essentially, to make patient's ailing knees heal on their own by causing new ACLs to sprout.

Murray, the Children's Hospital scientist, uses platelets and plasma from the patient's blood, mixing them with collagen to create a slurry that is then injected through tiny holes. An encouraging study in pigs has given Murray hope that the method will stimulate growth of new ACLs in humans.

Another approach, invented by doctors at New England Baptist and Tufts University, relies on silk from worms in Brazil. The material is biochemically modified in such a way that the patient's severed ACL is tricked into thinking the silk scaffold is actually ligament cells, coaxing the damaged ACL to start growing around the decoy. The technique worked in goats, and preliminary studies have been conducted in humans in Germany with more planned in Europe.

In Lindsay Jordan's case, the surgeon replaced her torn ACL with a graft from her hamstring. Her recovery, aided by physical therapy, is proceeding well.

Soon enough, she vowed, she will be back playing sports, including softball, trying to slam homeruns.

"But now I joke, anytime that happens, even if it's over the fence," she said, "I'm stopping at third base."

Stephen Smith can be reached at stsmith@globe.com.