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Meniscal damage is another common way for good knees to go bad. The menisci are semicircular pads of rubbery cartilage that sit at the junction where the thighbone meets the shinbone; orthopedists like to compare them to shock absorbers. Injuries and wear can cause this tissue to rip and fray in a number of ways, and when that happens, the results can include pain, stiffness, and instability. Doctors have learned that if they trim away the frayed or torn edges of a meniscus, the knee will often feel good as new, though if they take the whole thing out (which of course they tried for a while), bad things happen. Over the past 20 years, it’s become common to effect meniscal repairs arthroscopically, that is, through small incisions using chopstick-sized arthroscopes and special surgical instruments. With more than a million and a half knee arthroscopies performed annually, arthroscopic meniscal repair is the most common operation done today in America, according to Dr. James Tasto, a local past president of the Arthroscopy Association of North America.

Yet another bad thing that can happen to knees is that the bones that meet there can break. That’s what happened to Meghan McShannic, with whom I talked as she was recovering from knee surgery at UCSD’s Thornton Hospital. McShannic, a youthful 45, worked as the controller for a Bay Area dot-com. She lived in Walnut Creek but had come to San Diego to have her knee surgery, and she shared with me the sad prelude to that decision. Ten or 15 years ago, she and her husband had become avid skiers, she said. They’d traveled throughout the West to indulge their passion and had even bought a house near Tahoe. Toward the end of April 2001, they were out at Alpine Meadows. “It was my 22nd day of skiing that season,” she recalled. “There was two feet of heavy new spring snow, and I caught an edge, and — I don’t know. It was sort of a freak thing. I fall all the time, and there was nothing overly dramatic about this fall. Until I landed.”

McShannic said it was obvious on the slopes that she had broken something, but in the emergency room in Truckee, an X-ray revealed the extent of the bad news. The top of her tibia (the shinbone) had shattered. McShannic had her first surgery in the Bay Area in April of 2001. Her surgeon at that time inserted three large titanium screws to pin the pieces of her bone back together. By August she was walking again, but it was clear that her leg had healed with a deformity. “It basically made me knock-kneed,” she explained. In November of 2001, her Bay Area doctor had examined the interior of the knee with an arthroscope. “That actually improved things quite a bit,” McShannic said. “ ’Cause it cleaned up a bunch of the scar tissue.” But it also revealed more bad news. “All the places where the bone had shattered, I had lost cartilage.”

McShannic wasn’t talking about meniscal cartilage, but rather the other type of cartilage found in knees, so-called articular or surface cartilage. This is the slippery white tissue that covers the ends of the thigh and shinbones. One of its functions is to spread out the loads that are put on those bones as they hold us upright and move us through our activities. Articular cartilage also helps the two bones to glide smoothly over one another. Researchers say the friction in a healthy knee is close to zero. “Cartilage slides against cartilage at least ten times better than ice on ice,” one told me. “And ice on ice is an order of magnitude better than a lot of other materials.”

Another amazing thing about articular cartilage is its wear-resistance. “It’s a remarkable, remarkable material,” exclaimed Dr. William Bugbee, an assistant professor of orthopedics at UCSD’s Medical School. “It can withstand these loads that are upwards of three and four and seven and ten times body weight. Millions and millions of cycles a year for the average person. You think about the loads! No mechanical device on earth can withstand that.” Still, despite its toughness, articular cartilage can be damaged by accidents like the one McShannic suffered. When that happens, yet another remarkable characteristic comes to the fore: its ineptitude at healing.

In this regard, it’s instructive to contrast articular cartilage with skin or bone. Rip into your skin, and within seconds blood starts bringing repair materials to the site. Inflammatory cells and other blood-borne substances trigger a series of reactions that rebuild the damaged tissue; before long the site can look as good as new. Bone is even better at fixing itself, according to Bugbee. When it is injured, it repairs itself with normal bone rather than with some kind of scar tissue.

For centuries people have known that cartilage falls at the other end of the healing spectrum. Writing in 1743, a medical researcher named Hunter noted that “it is universally allowed that ulcerated cartilage is a troublesome thing and that once damaged, it is not repaired.” A couple of factors explain this, Bugbee told me. For one thing, articular knee cartilage has no blood supply. Instead it’s nourished by synovial fluid, a clear, viscous liquid that functions like a sort of futuristic engine oil — one that not only squishes between the moving parts but also absorbs into their surfaces and helps to make them slippery. For all its mechanical elegance, synovial fluid doesn’t contain any of the healing agents carried by blood, such as cells and platelets.

Another reason cartilage heals so poorly may have to do with its structure. “Cartilage is unique,” Bugbee said. Compared to other parts of the human body, the tissue matrix contains a paucity of cells. “It’s sort of like a Jell-O mold,” the surgeon said. “Ever done Jell-Os with fruit inside? Maybe you have a grape every three inches. That’s what cartilage is like. Each grape is a cell, and the Jell-O is the matrix.” That matrix is a complex framework of collagen studded with large, specialized molecules, water, and the cartilage cells. Designed to be smooth and lubricating and withstand huge forces, the matrix is “the white stuff at the end of the chicken bone,” Bugbee said.

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