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Liz Scarpelli's postural orientation is at the moment horizontal. Her leg is elevated in a surgical sling as Scott Dye, an orthopedic surgeon atCaliforniaPacificMedicalCenter, examines her knee with an arthroscope. The ghostly image of the joint—femur, tibia, and patella—appear magnified on a flat screen above the gurney. An athletic woman of 51, a former gymnast and skier, Scarpelli is a physical therapist who works with patients to rehabilitate their joints after surgery. While demonstrating to one patient a technique for leg-strengthening knee squats, Scarpelli blew out her own knee for the third time. Dye's arthroscopic camera shows healthy bone and ligaments, but large chunks of cartilage float about like icebergs in the fluid spaces around the joint. Dye expertly scrapes up the pieces and sucks them out before sewing up the holes and moving on to the next five surgeries scheduled for the day.
To hear Scott Dye speak of it, the knee joint is among the greatest of nature's inventions, "a 360-million-year-old structure beautifully designed to do its job of transferring load between limbs." But it is also among the most easily injured joints in the human body; medical procedures involving knees total a million a year in theUnited States.
"In standing upright, we have imposed unprecedented forces on the knee, ankle, and foot," Bruce Latimer says. When we walk quickly or run, the forces absorbed by our lower limbs may approach several multiples of our own body weight. Moreover, our pelvic anatomy exerts so-called lateral pressure on our lower joints. Because of the breadth of our pelvis, our thighbone is angled inward toward the knee, rather than straight up and down, as it is in the chimp and other apes. This carrying angle ensures that the knee is brought well under the body to make us more stable.
"But nothing is free in evolution," Latimer says. "This peculiar angle means that there are forces on the knee threatening to destabilize it. In women, the angle is greater because of their wider pelvis, which explains why they are slower runners—the increased angle means that they're wasting maybe ten percent of their energy—and also why they tend to suffer more knee injuries."
